Integrated palaeomagnetic, isotopic and geochemical studies on mafic dykes and their Archaean host rocks in the eastern Karelian Province of the Baltic Shield have revealed an extensive dyke swarm which yields U-Pb (baddeleyite) and Sm-Nd ages between 2349 and 2476 Ma. The NW-SE/ E-W trending gabbronoritic to Fe-rich tholeiitic dykes are petrographically well preserved and yield stable remanent magnetizations with multicomponent nature. We have isolated four different remanence components which we believe to have geological importance. The most dominating remanence component A represents an extensive ca. 1.84 Ga old Svecofennian overprint. Component B is inferred to be ca. 1.75 Ga old, acquired at the waning stage of the Svecofennian orogeny. All dyke rocks have been partially remagnetized during the Svecofennian orogeny, but some granulite grade Archaean rocks have preserved from the remagnetization. Component D' was isolated from such a well-preserved Archaean host rock at the vicinity of the dykes. Because component D' occurs in the highest unblocking temperatures, we suggest that it may represent the primary, ca. 2.44 Ga old remanent magnetization of the dykes. However, another high temperature component, D, isolated both in the dykes and in the unbaked Archaean basement, is also thought to be about the same age. The primary nature of component D is supported by its occurrence in all other 2.44 Ga old formations elsewhere in the Baltic Shield. If component D is regarded as ca. 2.4 Ga old, we suggest that the Archaean basement was regionally reheated and remagnetized due to the emplacement of coeval 2.44 Ga old K-granites, mafic dykes, layered intrusions and volcanites in the area. Component D may thus represent a ca. 2.4 Ga old thermochemical remanence, acquired at deep crustal levels, shortly after the dykes were emplaced. Alternatively, component D is of secondary origin, acquired in later geological processes.
We have tested continental reconstructions based on pole D and on pole D'. Reconstruction based on pole D and on a well-defined pole from the 2.45 Ga Matachewan dykes of the Superior craton of Laurentia, places the Kola Peninsula adjacent to the present-day SE Greenland, thus spatially far from the Matachewan dykes. Reconstruction based on pole D' places the Karelian dykes straight eastward to the Matachewan dykes and thus implies that the dykes had a common origin and, consequently, Laurentia and Baltica formed parts of the same supercontinent. Implications of these reconstructions are discussed.
The paleomagnetism has provided significant information in the understanding of the geodynamic history of the Earth. However, most of the available data (77%) were obtained on Phanerozoic rocks (IAGA global paleomagnetic database). Only 7% concern the Palaeoproterozoic period which occupies 20% of the Earth life. Moreover, less than 0,1% of them were obtained on Palaeoproterozoic rocks from the Amazonian craton.
In order to improve our understanding of the geodynamic history of this craton during this period, a paleomagnetic study has been carried out along of the Oyapok river. This work, asociated with geological and geochronological approaches, was done on the BRGM's geological mapping program of French Guiana with the association of the University of Orléans. All rocks range in age between 2.0 and 2.2 Ga. Twenty-two sites (150 oriented cores) were collected from granites (8 sites), diorites and gabbros (3 sites), amphibolites (7 sites), graywackes (4 sites). In order to identify the magnetic mineral composition and its relationship with associated minerals, mineralogical analyses have firstly been carried out on several samples (SEM, EMP, IRM, Curie temperature, AMS).
The mineralogical observations show that magnetite and/or poor titanomagnetite are the main magnetic carriers in granitic and dioritic rocks and that ilmenite is the dominant magnetic mineral in amphibolites and metamorphic graywackes. Both thermal and AF demagnetization techniques were applied to our collection. The amphibolites and metamorphic graywackes present unstable magnetic behavior due to the lack of ferromagnetic minerals which implied that no paleomagnetic direction could be isolated. However, two stable directions were isolated from diorites and gabbros (A component, DA=146°, IA=60, kA=38.2, a95A=6.3° with nA=17 samples, 3 sites) and granites (B component, DB=92°, IB=2°, kB=15.3, a95B=11.5° with nB=15 samples, 8 sites).
Although based on a small sample collection, we obtained successful reversal and baked contact test. Two virtual paleomagnetic poles were computed from these mean directions with latA=-36°N, longA=339°E, (dp=7.2° and dm=9.5°), latB=-2°N, longB = 37°E, (dp=6.6° and dm=12.2°). These results are significantly different one each other and distinguishable from existing paleomagnetic Palaeoproterozoic data from the same tectonic unit of the Amazonian craton which are younger than our studied rocks. This shows a possible important mobility of this tectonic unit during 2.2 to 2.0 Ga. Using already published reconstruction model of the Amazonian craton relative to the West African craton, we show that our results are compatible with those from West Africa during the same time period confirming that they probably belonged to the same tectonic mega-unit.
By early Palaeoproterozoic, 2.6-2.5 Ga ago, crust of the Fennoscandian Shield was stabilized and brittle deformations became predominate. However, the main late Archaean structural elements generally survived then: (1) vast areas of uplift and extension with the mantle-derived magmatism (Karelian and Kola cratons with magmatism of the siliceous high-Mg series (SHMS) in form of rift-like volcanosedimentary belts, dyke swarms and large layered intrusions); (2) zone of submergence and compression between them (Lapland-Umba granulite belt of medium pressure with crustal-derived enderbite-charnockite magmatism which was similar to the late Archaean Kola-Norwegian granulite belt), and (3) transitional zones of tectonic flowage occurred between low- and high-grade terranes, which evolved in extensional regime; the better preserved example is the Belomorian Mobile Belt (BMB). A particular feature of the BMB is the presence of widespread small (up to hundred metres long) rootless intrusions of basic and ultramafic rocks, locally termed the drusite complex. According to xenoliths data, the lower crust of the BMB (and, probably, all Kola-Karelian domain) was formed as a result of underplating of the early Palaeoproterozoic SHMS magmas. These main structural provinces formed a symmetric regional structural-metamorphic zonation, which was characterized by gradually increase of deformations and metamorphism from greenschist facies through amphibolite facies to granulite facies from extension areas toward the compression zone. This type of tectonics was rather differed from plate-tectonics and could be described in terms of plume-tectonics. Geological evidence for plate-tectonic activity (ophiolite complexes, suture zones, back-arc basins, etc.) in the region are fixed only from 2.2-2.0 Ga ago, practically simultaneously with the first appearance of a new type of magmas - geochemically enriched Fe-Ti picrites and basalts of different alkalinity, similar to the Phanerozoic within-plate magmas. From this time the further evolution of the shield could be characterized in terms of plate-tectonics.
Igneous activity occurred in the Early Palaeoproterozoic (2.5-2.4 Ga) on all Precambrian shields, including the Baltic Shield. In the Belomorian Mobile Belt (BMB) peculiar intrusive mafic-ultramafic rocks, known as drusites, are widespread. Their typical feature is a coronary (drusitic) texture, which was formed at the late magmatic stage and suggests more than 25 km depth of rock formation (Stepanov, 1981). Results by S. Bogdanova, (1996) obtained on the 2434±7 Ma (Bogdanova, Bibikova, 1993) drusites at Tolstik Peninsula also point to a deep-seated character (about 12 Kb) of their metamorphism. At least three drusite rock associations are distinguished, with a lherzolite-gabbronorite association (LGA) being most common (Stepanov,1981). Recent U-Pb dating of the Kovdozero basic-ultrabasic massif (Efimov, Kaulina, 1997) gave 2440±10 Ma. However, the massif is different in some of its features from the typical LGA, thus demanding for further geochronological studies on LGA rocks. According to geological, petrographical, and petrogeochemical data, the Shobozero massif is a typical representative of a LGA in BMB. To date the formation of LGA, the the Shobozero gabbronorite was sampled. One fraction of long prismatic, transparent zircon and one fraction of titanite were analysed by the conventional U-Pb technique (Krogh, 1973). This type of zircons is regarded as being of late-magmatic origin. A nearly concordant 207Pb/206Pb age of 2425.3 ± 2.1 Ma was obtained. If a regression line is drawn through this point and the titanite point which is concordant at 1780 Ma, the upper intercept age is 2432 ± 10 Ma. Thus we accept the age of 2430 ± 5 Ma as the best estimate for the age of the late magmatic stage in the formation of the gabbronorite massif. Dating of individual zircons from this sample by the stepwise lead evaporation method (Kober, 1986, 1987) shows that all grains, except one, display no significant variations in age from one step to another thus forming a "plateau", which is usually interpreted as the true crystallisation age. Slightly younger ages in the beginning of analyses suggest partial Pb-loss. The plateau ages obtained vary from 2440 to 2410 Ma. Weighted average on 18 steps out of 31 yields age of 2433±3 Ma. In the eastern Baltic Shield, Early Palaeoproterozoic (2.5-2.4 Ga) magmatism indicates rifting. The axial part of the NW-SE rift system is marked by the Vetreny Belt-Paanajarvi-Kuolajarvi synclinorium with its Sumian-Sariolian bimodal volcanic complex. At its NE flank within the BMB the Earth's crust was more than 50-60 km thick (Slabunov & Stepanov, 1996), and intrusions of LGA and charnockite (Topozero-type) crystallised at great depth. At the SW flank of the rift (i. e. within the Archean Karelian craton) layered mafic-ultramafic intrusions (Olanga and Koillismaa Groups), shallow-depth potassic granitoids (Nuorunen-type), and basic dyke swarms occur.
It was suggested that form 2.45 Ga onwards the Archaean crust of the Baltic Shield was extensively rifted and partly dispersed. P-T and resent geochronological studies, however, give evidence of a more complicated history. Revealed P-T-t path of some plutonic and supracrustal rocks developed at the N-E (Kolvitsa-Umba area) and S-W (Pon'goma area) margins of the Belomorian-Lapland Belt (BLB) indicate an event of crust thickening occurred during Early Palaeoproterozoic-"Seletsk"-period (2.43-2.40 Ga). This developed substantial NW-SE strike - slip faulting and high-T, high-P metamorphism. The studies show that at 2.43-2.40 Ga both NE and SW margins of the BLB must have been at the same depth corresponding to P=10-11 kbar. Emplacement of ca 2.4 Ga felsic plutones and dense mafic dyke swarms at great depth accompanied by decompression cooling of the rocks suggest the deep-level extension of thickened crust. High-P, high to moderate P metamorphism and strike-slip faulting were also typical during ca 1.9 Ga Svekofennian event which is commonly interpreted in terms of collision orogeny. However, kinematic indicators suggest NW-SE major movements at the NE boundary of the BLB and NE-SW direction of the tectonic movements at its SW margin. Despite the limited data, P-T studies show that at 1.9-1.8 Ga the NE part of the Belomorian belt might have been at greater depth than its SW margin. That may indicate the various ratio or selective uplift of the different part of the Belomorian Lapland Belt.
Tectonic models explaining the earliest Palaeoproterozoic evolution of the Kola region, north-eastern Baltic Shield, are usually identified as (i) extensional or (ii) combination of extension and compression zones. The basic point of all models is a fact that the Pechenga-Imandra-Varzuga palaeorift was originated in extensional setting 2.4-2.5 Ga ago. In contrast, such components of the Kolvitsa belt as the 2.45-2.46 Ga old Kolvitsa garnet-bearing gabbro-anorthosite massif and ca. 2.4 Ga old intermediate to mafic granulites are decisive for models suggesting that the emplacement of the gabbro-anorthosites and 2.4 Ga old high-grade metamorphism took place in compressional setting. Models of this kind are based on ideas about the formation of the Kolvitsa gabbro-anorthosite and granulites only in compressional environment rather than on studies of concrete structural elements that can be observed in field.
We argue that a ca. 2.44 Ga old mafic dyke complex of the Kolvitsa gabbro-anorthosite massif was formed and immediately deformed in extensional setting. Most mafic dykes are sheeted dykes, and some were formed by emplacement in a single dyke chamber of, at least, three batches of mafic melts that gave rise to three lithologically different mafic rocks. Mafic dykes intruded into both sheared and non-sheared gabbro-anorthosites, and each dyke was metamorphosed and sheared parallel to its contacts before the emplacement of new batch of mafic melts. Automagmatic breccias are characteristic of the mafic dykes. In these breccias, fragments of the host rocks are not squeezed into each other and they were displaced, as a rule, as far as a few centimetres. The developments of high-T and high-P metamorphic mineral assemblages in these igneous rocks may be explained by crystallisation in extensional setting, for instance, at deep crustal levels (Bridgwater et al., 1994). Orientation and morphology of dykes that intruded through over the north-eastern Baltic Shield 2.4-2.5 Ga ago also suggest that their emplacement occurred in extensional setting.
It is quite possible that extension and compression zones did exist contemporary with each other in the earliest Palaeoproterozoic, 2.4 to 2.5 Ga, in the Kola region, but this conclusion needs to be supported by strong evidence. Available structural data combined with isotopic ages make extensional models more preferable so far.
Bridgwater D., Glebovitsky V.A., Sedova I., Miller J., Alexejev N, Bogdanova M.N., Yefimov M.M., Chekulaev V.P., Arestova N.A., Lobach-Zhuchenko S., Terra Abs, 2, 4, (1994).
In the northern Baltic Shield extensive reworking of the Archean basement occurred between 2.45 and 1.8 Ga. The earliest episodes of reworking were associated with rifting and formation of volcanic-sedimentary basins, whereas, the later stages related to accretion of juvenile Svecofennian terranes in the south and collisional orogeny in the north in the Lapland-Kola belt. Early rifting at 2.45 Ga is recorded by a variety of mafic and anorthositic intrusive complexes, followed by extensive mafic dykes and volcanism. The Polmak-Pavisk-Pechenga-Imandra/Varzuga-Ust Ponoy Belt (PPPIB) is one of the largest structures developed as a result of the rifting event.
The Pechenga complex lies at the north-western end of the PPPIB and has been previously divided into the Pechenga and South Pechenga Series, which are separated by a major fault zone. The Pechenga series forms a southerly dipping asymmetric synclinorium ca. 70 km long by 30 km wide and contains in excess of 8 km of volcano-sedimentary rocks. The key features of these rocks are: (1) The sequence rests with a marked angular unconformity on Archean metamorphic basement and 2.45 Ga layered mafic intrusions; (2) a variety of volcanic rocks are preserved through the stratigraphy, within which there are marked compositional differences with age; (3) poor geochronological control, published dates indicate an age range of 2.33 - 1.97 Ga and (4) marked stratigraphic variations in trace element and Nd isotopic compositions within mafic rocks.
Previously the lithological and geochemical signatures have been used to interpret the sequences as forming in a variety of often conflicting tectonic settings (e.g. island arcs to ocean floor settings). However recently models involving mantle plumes as a driving force for the volcanism have arisen. A major problem in such models concerns the age range of the volcanism. Existing imprecise age estimates for the Pechenga sequence are open to interpretation but appear to suggest that volcanic activity lasted ca. 500 Ma. Clearly this time period is excessively long to invoke a single plume source.
Here we present new whole rock geochemical data (major, trace, REE and Nd-isotopes) to illustrate the importance of processes such as crustal contamination, fractionational crystallisation and variable mantle melting to explain the compositional diversity of the volcanics. Using these data as a base, together with new geochronological studies a model for the volcanism is developed.
Mafic dyke swarms are exceptional geological time markers that often punctuate major episodes of crustal rifting. A knowledge of the timing of dyke emplacement is essential for understanding the tectonic evolution of rift-related environments and for the regional correlation of igneous activity. Dyke swarms are extremely important in continental environments as they are often the only surviving evidence of quite considerable geological events (e.g. rifting, mantle plumes, plate subductions, or crustal "break-up") and can be used to monitor the geological history of the continents over long periods of time. Dyke swarms are also ideal indicators for use in the reconstruction of Precambrian crustal blocks.
Several dyke swarms comprising a voluminous number of dykes are visible in the eastern and northern parts of the Fennoscandian Shield, and thus Finland, Russian Karelia and the Kola Peninsula provide a good area for research into the formation of Palaeoproterozoic dykes in particular. Integrated studies of the geochronology, geochemistry and palaeomagnetism of Palaeoproterozoic dyke swarms are aimed at identifying various dyking events in the eastern Fennoscandian Shield and their relationship to economically important layered intrusions and ophiolites, and at establishing the earliest part of the Proterozoic apparent polar wander path for Fennoscandia. Dykes were sampled in regions of the Karelian and Kola Province (Taivalkoski and Suomussalmi areas of eastern Finland, Suoperä, Lake Pääjärvi, near the Burakovsky Intrusion in Russian Karelia and the mid-Kola Peninsula) where overprinting by the 1.8 Ga Svecofennian orogen is thought to be a minimum. Together with previous zircon ages (summarized in Vuollo, 1994), the results indicate that there were several dyke intrusion events in the eastern Fennoscandian Shield between 2.5 Ga and 1.97 Ga. It is suggested that these dykes and sills can be divided into at least six main groups in terms of their geochemical composition, absolute age criteria and occurrence (2.5, 2.45, 2.33, 2.2, 2.1 and 1.97 Ga).
Dyke swarm U-Pb Sm-Nd Trend
1 (Gabbronorite) 2555±65 NE
(Kola)a
2 (Gabbronorite) 2446+5/-4 2421±30 NW
(Karelian) (Karelian)
(Fe-tholeiite) 2476±30 NW
(Karelian)
(Bon-norite) 2395 (min.) 2370±77 ENE
(Karelian) (Karelian)
(Tholeiite) 2378 (min.) 2450±150 NW
(Karelian)
3 (Tholeiite) 2331+19/-3 E-W
(Karelian)
4 (Layered sill) 2220±11b
(Layered sill) 2203±49
5 (Fe-tholeiite) 2113±5c NW
6 (Fe-tholeiite) 1981+5/-4 1997±43 NW
(Karelian) (Karelian)
(Gabbro-peridotite) 1956±20 NW
(Kola)a
a) Fedotov G, 1994
b) Tyrväinen A, 1983
c) Pekkarinen et al., 1991
Vuollo JI, Acta Univ. Ouluensis,Ser. A, 250, 47, (1994).
Fedotov G, Geol. of the Kola Peninsula. ed. Mitrofanov F, 53-56, (1994).
Pekkarinen LJ, Lukkarinen H, Geol Bull Finland, 357, 30, (1991).
Tyrväinen A, Map-sheet areas 3713 and 3714 Geol Surv Finland, (1983).
This study examines a suite of ca 2.0 Ga mafic dykes located at 66°-66°30' N in southern East Greenland The dykes show geochemical signatures suggesting extension related magmatism and they may collectively be taken to define a brief period of rifting prior to the collisional event that formed the Nagssugtoqidian orogen. The dykes crystallized from evolved basaltic magmas, are enriched in LREE and have relatively high LILE contents. Quantitative modelling of major and trace elements show evidence for high pressure fractionation and assimilation processes with garnet in the fractionating mineral assemblage. Isotope data supports a model of extensive crustal assimilation coupled with high pressure fractionation. Initial 87Sr/86Sr values are ca 0.703, <epsilon>Nd 2.0 range between +1 and -8 and Pb isotopes are within the range found in the basement gneisses.
The findings suggest a Paleoproterozoic evolution of this region marked by basaltic-ultramafic underplating presumably accompanied by rifting. Differentiation of magmas at deep crustal levels resulted in addition of garnet-pyroxenitic cumulates to the base of the continental crust. Relatively low Ni values (137 - 8 ppm) in the studied dykes indicate that extensive amounts of olivine removal occured in the initial stages of the evolution of these magmas. Therefore the evolved nature of the dykes suggests that significant amount of a high pressure Mg-rich cumulate was produced prior to the onset of garnet-pyroxenite fractionation.
High pressure metamorphism affected parts of the basic dyke swarm. This suggests that they were emplaced prior to or in the initial stages of the Nagssugtoqidian orogeny, rather than in an environment of extensional collapse of overthickened crust at the termination of the orogeny. This setting in relation to the Nagssugtoqidian orogeny is consistent with the geochemical variations observed in the dykes and emphasizes that dunitic and garnet-pyroxenitic cumulates were added to the base of the crust in Paleoproterozoic time in this region.
U-Pb data on detrital zircon from six clastic metasedimentary units in the Sveconorwegian Province of S Norway were acquired with a Cameca IMS1270 ion microprobe. Sampling was realized along an E-W transect in the northern part of the Rogaland-Hardangervidda, Telemark and Kongsberg terranes. In Hardangervidda, the supracrustal sequence divides into the Festningsnut and the Hettefjords Groups, the relative time relationships between the two being unclear from field relationships. The upper age limit for deposition of a conglomerate in the Festningsnut Group is estimated at 1572 ± 10 Ma, from the age of the youngest detrital zircon (n = 15), and for deposition of a quartzite in the Hettefjords Group at ± 24 Ma (n = 35). In the Telemark terrane, the supracrustal sequence divides into the Rjukan (1.51 Ga), Seljord, Heddal and Bandak groups. The Eggedal formation is commonly attributed to the Seljord Group. The deposition age of a quartzite of this formation is younger than 1714 ± 32 Ma (n = 37). The lack of ca. 1.50-1.60 zircon in this sediment suggests that it belongs of a generation of sediments older than the Rjukan Group. A metasandstone of the Heddal Group and a metasandstone-arkose in the top of the Bandak Group yield deposition ages younger than 1112 ± 20 Ma (n = 36) and 1054 ± 22 Ma (n = 43) respectively. These data support the interpretation that the Heddal and Bandak Groups developed in Sveconorwegian intra-orogenic basins. The Bandak sample was collected in the hanging wall of the Mandal-Ustaoset fault zone, which makes the terrane boundary between Rogaland-Hardangervidda and Telemark; there, deposition of the immature and locally conglomeratic Bandak Group is probably associated with active shearing along this zone at ca. 1.05 Ga or after. In the Kongsberg terrane, the Modum complex displays several quartzite units. One of those has a deposition age younger than 1475 ± 20 Ma (n = 37), similar to the ones of the Seljord Group quartzites in Telemark and a quartzite at Kragerø in Bamble. These three units probably correlate across terrane boundaries.
Ages distribution of all samples shows a minor zircon population at 2.80-2.65 Ga and a major population at 1.95-1.75 Ga, indicating the occurrence of limited Archean and major Sveconfennian evolved crust in the catchment regions. A rare 3.25-3.10 Ga Mesoarchaean component is detected in two samples of the oldest generation of sediments in Hardangervidda and Telemark. Large populations in the ranges 1.65-1.45 Ga and 1.20-1.05 Ga occur in the Heddal and Bandak sediments, pointing to significant Gothian and Sveconorwegian accretion of felsic to intermediate crust in the SW Baltic shield.
Detrital zircons in Svecofennian metasediments from Finland and Sweden are dominated by Paleoproterozoic, c. 1.9-2.1 Ga crystals with an abundance peak at 2.0 Ga. Similar ages, up to 2.2 Ga, have been reported from several other Palaeoproterozoic metasediments in the North Atlantic region, including samples from northern Finland, the Kola Peninsula, Svalbard, Norway, NW Scotland, and Greenland. This reflects the age distribution in the Proterozoic, differentiated crust which was eroded to deliver zircon to the sediments. Rocks which were up to 0.2 Ga older than any known, presently exposed major Proterozoic crustal components in the region therefore appear to have dominated the source area(s) for Svecofennian metasediments. Such rocks have also been important sources of sediment in adjoining parts of the Baltic and Laurentian Shields.
Most of these metasediments are interpreted to have been deposited in magmatic arc environments, and they can in extreme cases have been transported thousands of kilometres before deposition. It appears, however, most unlikely that the bulk of these enormous quantities of sediment should be derived from outside the region. Minor 1.90-1.95 Ga old Svecofennian magmatic rocks are known from Finland, and recently have similar ages been reported from north-central Sweden. Limited evidence from the Lapland Granulite Belt indicates that this structure, which may have been an important source of Svecofennian detritus, also includes Paleoproterozoic magmatic rocks older than 1.9 Ga. Nd TDM model ages of up to 2.1 Ga from much of the Svecofennian Domain have commonly been interpreted as due to a minor contribution of Archaean material, or to a less than average depleted mantle source. However, the Nd data also allows for the existence of juvenile 'protosvecofennian' crust of such age, which may be largely buried by younger rocks or strongly reworked during the Svecofennian orogeny, and therefore has escaped recognition. The deliberate selection of well-preserved rocks for geochronological work may also have resulted in some sampling bias. Large portions of the North Atlantic and European Paleoproterozoic crust are covered by younger sediments or ice, and are rather poorly known. Recent work on drill cores from the Baltic states and Belarus has demonstrated that Paleoproterozoic crust extends from the Svecofennian Domain towards the SE under the East European Platform sediments, and that it includes magmatic rocks which are up to 2.0 Ga old. Slightly older magmatic rock ages, up to 2.1 Ga, are found farther to the SE, in the northwestern part of the Ukrainian Shield. Other possible distant sources of 1.9-2.1 Ga old sedimentary detritus may be found in northern South America, which in plate reconstructions has a position 'south' of Baltica-Laurentia.
Collisional suture zones are first order discontinuities in the continental lithosphere originating as the sites of rifting, sea floor spreading and later subduction. Criteria for their recognition include linear belts of high strain and high grade metamorphism, ophiolites especially at higher crustal levels, clockwise PTt paths, arc magmatism and associated juvenile isotopic signatures both in magmatic and sedimentary protoliths. Beside their importance in understanding lithospheric history and architecture, suture zones can facilitate large-scale correlation of orgenic belts. Deeply eroded collisional sutures may lack ophiolites. Instread belts of well-dated subduction-related juvenile crust (identified using isotope geochemcal criteria) may define suture zones especially if geophysical data can demonstrate structural continuity to mantle depths.
The Lapland-Kola Orogen (one of the targets of the Europrobe Svekalapko Project) constitutes an oblique section through a major Palaeoproterozoic suture zone (centred on the Lapland Granlite Belt) within the northern Fennoscandian Shield. Here, evidence for juvenile crust comes from Palaeoproterozoic metasedimentary and metaigneous rocks with positive initial <epsilon>Nd values over a strike length of 600 km. Adjacent Archaean terranes, with negative <epsilon>Nd signatures, contributed little detritus, suggesting a basin of great areal extent. The Lapland Granulite Belt probably initiated during rifting of older crust at c. 2.5 Ga to 2.4 Ga as suggested by layered mafic and anorthositic intrusions developed throughout the northernmost Fennoscandian Shield at this time. Crustal separation followed by subduction of the resulting ocean led to arc magmatism dated by the NORDSIM ion probe at c. 1.93 Ga in the Tersk terrane in the southern Kola Peninsula. This correlates with arc magmatism in the Inari terrane to the northwest dated at c. 1.94 -1.91 Ga (Barling et al. 1997; Tuisku and Huhma 1998). Deep burial during collision to high pressure granulite- or even eclogite-facies conditions (Tuisku and Huhma 1998) and subsequent exhumation and cooling took place between 1.90 and 1.87 Ga based on Sm-Nd, U-Pb (Tuisku and Huhma 1998) and new Ar-Ar data.
Shallow seismic reflection data across the Lapland Granulite Belt reveal strong dipping reflectors parallel to near-surface structures. Refraction data, e.g. from the POLAR profile, have been interpreted to show that the Lapland Granulite Belt is a superficial structure consistent with gravity modelling. However variations in deep crustal velocity and Vp/Vs ratio (Walther and Flueh 1993) together with reflections traversing the entire crust revealed by reprocessing the Polar Profile data (Pilipenko et al. in prep.), may be interpreted to image a trans-crustal structure - possibly a fossilized subduction zone - supporting an arc origin for the protoliths of the Lapland Granulite Belt and the location there of a major lithospheric suture.
Barling J, Marker, M & Brewer, T, Abstract Supplement No. 1 to Terra Nova, 9, 129, (1997).
Pilipenko VN, Pavlenkova NI & Luosto U, Tectonophysics, (1999).
Tuisku P & Huhma H, Geological Survey of Finland, Special Paper, 26, 61, (1998).
Walther C & Fleuh ER, Precambrian Research, 64, 153-168, (1993).
In the Quebec/Baffin Island segment of Trans-Hudson Orogen, four orogen-scale assemblages are preserved within a crustal-scale thrust stack which is obliquely exposed in several >20 km natural depth sections. At lowest structural levels (Level 1), the orogenic lower plate comprises parautochthonous crystalline basement of the Archean Superior Province (3.22-2.74 Ga) and it's imbricated Paleoproterozoic rift-margin (2.04-1.92 Ga). At intermediate structural levels (Level 2), the orogenic upper plate is composed of allochthonous Paleoproterozoic crustal assemblages including an ophiolite (2.00 Ga), a fore-arc clastic apron, a magmatic arc (1.86-1.82 Ga), and arc-derived detritus. At higher structural levels (Level 3), the upper plate contains allochthonous Paleoproterozoic north-facing platformal strata (ca. 1.93 Ga) and depositional basement (ca. 1.95 Ga), foreland basin deposits, and a granitic batholith (1.86-1.85 Ga). At the highest structural levels (Level 4), an upper plate domain of Archean (ca. 2.92-2.80 Ga) orthogneiss and Paleoproterozoic supracrustal and metaplutonic rocks is interpreted as the western edge of the North Atlantic craton.
Three Paleoproterozoic mafic dykes swarms form a radiating pattern within the crystalline basement of Level 1 and may represent one quadrant of a giant radiating dyke swarm, whose focus is interpreted to mark the location of a mantle plume at ca. 2.22 Ga ('Ungava hotspot'; Buchan et al., in press). In northern Quebec, the absence of a Paleoproterozoic continental margin sedimentary prism and the presence of a voluminous (> 5 km thick) flood basalt sequence within Level 1 suggests that the northern margin of the Superior Province may have been a 'hotspot margin' leading to complete continental rifting between 2.04 and 1.92 Ga. The northern margin was overthrust between ca. 1.80-1.79 Ga by the ophiolite and magmatic arc terrane of Level 2 and by the carbonate platform-to-basin sequence of Level 3. In contrast, the western (Hudson Bay) margin of the Superior Province is characterized by west-dipping Paleoproterozoic transgressive carbonate platform-to-basin strata stratigraphically intercalated with ca. 1.96 Ga rift margin tholeiitic basalts. The basalt sequence is interpreted as recording partial (but not complete) rifting of the passive margin. The apparent absence of a continental margin sedimentary prism along the northern margin of the Superior Province is interpreted as a consequence of rifting of the continental margin and removal of the original passive margin sequence (now preserved in Level 3). Complete rifting along the northern margin (in contrast to the western margin) is ascribed to an landward 'jump' in the active ridge from an oceanic spreading axis onto the northern margin of the Superior Province following the northwest migration (present coordinates) of the Ungava hotspot (cf. Gaina and Müller, 1998).
Buchan KL, Mortensen JK, Card KD & Percival JA, Can. J Earth Sci, 35, in press, (1998).
Gaina C & Müller RD, GSA Abswt. Prog., 30, A354, (1998).
The New Quebec, Ungava, Torngat, and Nagssugtoqidian orogens in northeastern Laurentia are a geometrically complex, spatially and temporally linked system of Paleoproterozoic orogens separating a collage of Archean cratons. The orogens formed as a result of relative northward movement and sequential collision of Archean North Atlantic and Superior cratons, and attached Paleoproterozoic supracrustal sequences, with an Archean craton which resided to the north (present day). Also involved in the collisions were Archean rocks of suspect parentage that are now confined to the intervening regions between the intact Archean cratons.
The 300 km-wide north-trending segment of this orogenic system, defined as the Southeastern Churchill Province (SECP), separates the Superior and North Atlantic cratons. The SECP has a broadly tripartite character consisting of: 1) a west-verging fold-and-thrust belt (New Quebec Orogen) developed in 2.17-1.86 Ga sedimentary and volcanic cover rocks, and Superior craton basement 2) a medial, composite terrane (the Core Zone) having Archean and Paleoproterozoic components, and 3) a doubly-verging, fan-shaped wedge (Torngat Orogen) developed primarily in juvenile (<1.95 Ga) Paleoproterozoic sediments and inferred to represent an accretionary complex along the suture between the Core Zone and the North Atlantic craton. Dextral (west) and sinistral (east) transcurrent shear zones, which are synchronous- to post- tectonic with respect to thrusting, separate the bordering orogens from the Core Zone.
The Core Zone is itself a mosaic of variably reworked Archean crustal blocks, ca. 2.3 Ga and <1.95 Ga supracrustal rocks, and notably, 1.84-1.81 Ga granitoid rocks belonging to the De Pas batholith. Core Zone collision with the North Atlantic craton commenced at ca. 1.86 Ga and with the Superior craton <1.84 Ga. Detailed U-Pb geochronology from the southern part of the Core Zone indicates that constituent blocks were joined and shared a common tectonothermal history by 1.81 Ga. Subsequent to Core Zone amalgamation, the Core Zone and bordering orogens were overprinted by transcurrent shearing, which persisted locally to 1.74 Ga.
Affinity of Archean crust in the Core Zone is an outstanding problem having significant implications for developing Paleoproterozoic tectonic models for the region. A possible model for evolution of the Core Zone proposes that a significant component of the Core Zone was part of the Superior craton prior to 2.2 Ga. There are no compelling geological or geophysical data to suggest the Core Zone includes elements of North Atlantic craton crust, or that it contains a suture between fundamentally different Archean blocks.
First-order similarities between the rocks and deformation events in the eastern Trans-Hudson (Canada) and Nagssugtoqidian (West Greenland) orogenic belts have long been recognized. Recent multidisciplinary field and analytical studies have delineated detailed orogenic chronologies and kinematically compatible tectonic episodes on both sides of Davis Strait. The present challenge is to derive a model for the tectonic evolution of the entire region; here we provide a testable scenario and some consequent far-field implications. The overall tectonic framework of the greater orogenic system appears to be one of accretion of local and exotic lithologic elements (terranes) between three colliding Archean cratons: the Superior, Nain/ North Atlantic (southern Greenland) and "northern" (northern Greenland/ north Baffin). The ~1.92-1.87 Ga Burwell and Sisimiut/ Arfersiorfik calcalkaline rocks intrude the Nain/ North Atlantic craton; they are interpreted as a consequence of east/ southeast-dipping subduction, establishing the Nain/ North Atlantic as the uppermost plate in the orogenic system. Termination of this magmatism is interpreted as the cessation of subduction consequent to collision with the "northern" craton (Nagssugtoqidian Orogen), while in Labrador, accretion of the composite terrane that comprises the 1.94-1.90 Ga shelf/trench Lake Harbour Group and Tasiuyak metasedimentary rocks cut by the Cumberland monzogranite batholith occurred <1.85 Ga (Torngat Orogen). Meanwhile, offshore to the west, development of the 1.86-1.82 Ga Narsajuaq/ Lomier/ dePas arc was underway, interpreted to be the result of north/ northeast-dipping subduction. Sinistral strike-slip deformation along the western edge of the Nain/ North Atlantic craton (Abloviak shear zone) was active from ~1.84-1.82 Ga. This is synchronous and kinematically compatible with thick-skinned folding of imbricated supracrustal units and basement in the Nagssugtoqidian orogen. Collision of the Narsajuaq/ Lomier/ dePas arc with the overriding composite Lake Harbour/ Tasiuyak/ Cumberland terrane occurred <1.82 Ga, followed by their southward accretion, along with the 2.00 Ga Purtuniq ophiolite, onto the lower plate Superior craton at ~1.80-1.79 Ga. Thick-skinned folding of accreted Paleoproterozoic units and the Superior craton occurred >1.76 Ga. Relatively late reactivation along earlier tectonic boundaries occurred ~1.79-1.71 Ga: in Labrador, the partially subducted but buoyant eastern end of the still-young Narsajuaq/ Lomier/ dePas arc was uplifted relative to the Nain/ North Atlantic craton along the Komaktorvik shear zone, while in West Greenland, development of the Nordre Stromfjord and Nordre Isortoq steep belts and attendant sinistral strike-slip movement occurred. The Paleoproterozoic Piling (central Baffin) and Karrat groups (northern West Greenland) are predicted to comprise a north-verging thrust belt that flanks the southern margin of the "northern" craton. Initiation of thrusting may be diachronous; in the eastern Karrat Group as a result of the Nagssugtoqidian collision (~1.86 Ga), and in the Piling Group as a result of collision with the Narsajuaq/ Lomier arc (<1.82 Ga).
The Paleoproterozoic Torngat Orogen forms the western flank of the Archean North Atlantic craton (NAC) in Labrador and is part of the larger Trans-Hudson - Nagssugtoqidian orogenic belt. Development began ca. 1.9 Ga with subduction under the NAC from west and north. This was followed ca. 1.86 - 1.85 Ga by head-on collision of NAC with an unnamed Archean block to the north to form the Nagssugtoqidian Orogen, and oblique collision with juvenile arc terranes and another Archean block to the west to form the Torngat Orogen. Continued northwards indentation of the NAC led to development of a broad frontal thrust belt in the Nagssugtoqidian Orogen and sideswipe transpressional tectonics in the Torngat Orogen. As a result Torngat Orogen is a narrow, doubly-vergent orogen characterized by a series of medial sinistral shear zones that allowed NAC to decouple from terranes to the west during its indentation. Initial collision ca 1.86 Ga was followed by sinistral shear between 1.84 and 1.82 Ga under granulite facies conditions at pressures of ca. 8 to 10 kbar. Granulite facies conditions were thus maintained for a period of about 40 my. Sinistral shear was coeval with periods of west- and east-directed thrusting in a transpressional environment. East-directed thrusting removed the 1.9 Ga magmatic arc along most of the NAC margin, possibly by the process of plug uplift predicted by geodynamic modeling. This thrusting also buried the NAC margin and induced static, high-pressure granulite facies metamorphism under pressures of up to 14 kbar. The development of Torngat Orogen was terminated ca 1.8 Ga following collision of the Superior craton from the west (1.82 - 1.8 Ga) and renewed east-directed overthrusting onto the NAC resulting in exhumation and cooling of the internal zone of the orogen in the period 1.8 - 1.71 Ga. Seismic refraction studies (Funck and Louden, 1998) have revealed a prominent crustal root under the northern Torngat Orogen, perhaps formed by attempted subduction of NAC crust late (ca. 1.8 Ga?) in the evolution of the orogen. The seismic data also indicate a prominent sub-vertical discontinuity extending to mid-crustal depths that may mark the medial shear zones being carried on late, east-verging thrusts. The crustal root does not appear to extend to southern Torngat Orogen where an original, east-dipping collisional boundary seems to be preserved. The preservation of the root indicates minimal lower crustal heating and reworking, a feature that is consistent with the lack of post-orogenic granites and which suggests that large- scale, post-collisional crustal melting as a result of delamination or other processes did not occur. This may be a feature common to other components of the NE Trans-Hudson - Nagssugtoqidian belt and may be connected to the fact that the belt is underlain largely by refractory(?) Archean lithosphere.
Funck, T & Louden, K, LITHOPROBE Report, University of British Columbia, Can, 61, 77-91, (1997).
The Palaeoproterozoic Nagssugtoqidian Orogen in West Greenland hosts a number of dismembered, partially melted supracrustal suites in upper amphibolite to granulite facies. Single zircon ion-microprobe U-Pb and LAM-ICP-MS 207Pb/206Pb dating, and whole rock Sm-Nd data show that most metasediments were deposited in Palaeoproterozoic time and contain detritus from both Proterozoic and Archaean sources. The Nagssugtoqidian Orogen is divided into a southern (SNO), central (CNO) and northern (NNO) segment. The SNO represents the deformed southern Archaean foreland juxtaposed to the CNO during thrusting. The Nordre Strømfjord steep belt (NSSB) separates CNO from NNO and has lithologies similar to the northern CNO.
Two distinct supracrustal suites were recognised in the SNO. The Maligiaq suite occurs thrust interleaved with Archaean gneisses along the northern margin of the SNO. Its metasediments show T(DM)= 2.82-2.76 Ga and detrital zircons of 2.85-2.1 Ga suggesting that clastic sediments derived from mainly Archaean sources were deposited in a continental basin or margin in Palaeoproterozoic time. The Ikertooq suite further south occurs tightly folded with Archaean gneisses and is dominated by mafic lithologies. A metasediment with T(DM)=2.41 Ga and 2.4-1.9 Ga detrital zircon ages indicates that also this suite is Palaeoproterozoic.
The CNO contains two major supracrustal sequences. The Nordre Isortoq suite in central CNO occurs as a ENE-trending steep belt of clastic metasediments. Isotope data (T(DM)=2.9 Ga, 3.4-2.4 Ga 207Pb/206Pb zircon ages) from metasediments in the west of the Nordre Isortoq suite suggest that these rocks were deposited in the Archean while those in the east have ages that are similar to those of the Nordre Strömfjord suite (see below), deposited in the Palaeoproterozoic. The two suites occurring in this central steep belt cannot be distinguished on field criteria. The sediment-dominated Nordre Strömfjord suite in the northern CNO and the NSSB is spatially related to 1.90-1.94 Ga arc quartz-diorites and is tectonically interleaved with Archaean gneisses with mantle peridotite pods along the contacts. Detrital zircons have predominantly 2.20-1.95 Ga ages and the metasediments are interpreted as deposited from a now eroded island arc complex. T(DM) ages of 2.38-2.26 Ga agrees with this conclusion. Lithological similarities and the age isotopic evidense suggest that the eastern Nordre Isortoq suite is part of the Nordre Strömfjord suite.
These data indicate that the Nagssugtoqidian Orogen hosts several Paleoproterozoic supracrustal suites that represent two distinct tectonic settings, one in the SNO, resembling a continental margin setting, while the one in the CNO is likely to represent an accreted terrane. Two of the suites are presently located at major tectonic boundaries in the orogen. The Maligiaq suite is located at the SNO/CNO thrust boundary, while the Nordre Strömfjord suite traces a proposed crustal suture.
The Nagssugtoqidian orogen of West Greenland represents a NNE-trending segment of the mosaic of Paleoporoterozoic orogenic belts in the North Atlantic realm. It lies between the North Atlantic craton to the south and lesser known Archean gneisses of the Rinkian orogen to the north and is presumed to link coeval orogens in eastern Canada and northern Europe. This superbly exposed orogen offers a unique insight into mid-deep crustal tectonic processes and is important in understanding the tectonic development and plate reconstructions for the North Atlantic region in the Paleoproterozoic.
The orogen is subdivided into three lithotectonic segments, representing respectively the parautochthonous reworked southern foreland (SNO - Archean orthogneisses and Paleoproterozoic mafic dykes), a central collision zone (CNO - interleaved Archean and Paleoproterozoic ortho- and paragneisses) and a northern segment (NNO - dominated by Archean orthogneisses) that forms the transition to the Rinkian orogen to the north. Lithotypes and the timing and style of magmatism, metamorphism and deformation within the CNO collectively suggest an intercontinental collisional setting for the orogen, our preferred interpretation in spite of a lack of evidence for distinct crustal cratonic blocks within the orogen.
Paleoproterozoic orogenic activity begins at ca. 2.04 Ga with the intrusion of the Kangâmiut mafic dyke swarm in the southern foreland and SNO, an event tentatively associated with the opening of an oceanic basin. Paleoproterozoic sediments were deposited after 1.95 Ma, the age of the youngest detrital zircons, and before the start of arc magmatism.
Juvenile calc-alkaline arc magmatism between 1.92 to 1.87 Ga is preserved in CNO and represents a period of subduction. It is the earliest evidence for Paleoproterozoic convergence in the orogen. Termination of magmatism marks the start of collision and northwest-directed thrust imbrication that interleaved Archean and Proterozoic lithotectonic units in the CNO and caused upper amphibolite to granulite peak metamorphism around 1.86-1.84 Ga. At near-peak metamorphic conditions, thrusting gave way to fold-dominated, N-S convergence and top-to-the-east extensional low-angle shear zones that formed as late as ca. 1.825 Ga.
ENE-trending steep belts contain non-linked high-strain zones that overprinted all earlier fabrics at ca. 1.77 Ga and record overall sinistral strike-slip movement with restricted displacements. The late folding and steep belts dominate the ENE-trending structural grain of the orogen.
We favour a tectonic model in which south-dipping subduction of oceanic lithosphere beneath a southern continent causes 1.92-1.87 Ga arc magmatism and leads to the eventual collision by 1.86 Ga of the northern and southern forelands.
The Ketilidian orogen of South Greenland is a well-preserved example of a Palaeoproterozoic Cordilleran orogen that has escaped the effects of continent-continent collision. From NW to SE it comprises: (1) the Border Zone characterised by Palaeoproterozoic metasedimentary and volcanic rocks deposited on an Archaean basement; (2) a 100-150 km-wide, Cordilleran-type batholith emplaced during sinistral transpression and now exposed at mid-crustal levels (the Julianehåb Batholith); (3) the Psammite and Pelite Zones, representing a deformed forearc basin, in which mainly epiclastic sedimentary rocks, largely derived by erosion of the batholith during uplift shortly after emplacement, were subsequently intensely deformed and underwent extreme high-temperature/low pressure metamorphism, anatexis and emplacement of S-type granitic rocks.
Deformation of the Ketilidian forearc was characterised by a strong component of arc-parallel extension, a feature typical of modern forearcs such as Sumatra and the Aleutians. The earliest phases of intense ductile deformation (D1, D2) produced arc-parallel, NE-trending stretching lineations, a flat-lying intense planar fabric and consistent, top-NE displacements revealed by asymmetric elements of LS fabrics viewed on XZ sections. Concordant, tabular intrusions of hornblende and biotite granodiorite, diorite and gabbro, representing late phases of the Julianehåb Batholith, were intruded syn- to late-D2 into already strongly deformed rocks at the batholith-forearc margin and within the forearc. Later phases of deformation refolded the D1/D2 flat-lying structure.
We evaluate two models to explain margin-parallel stretching in the Ketilidian forearc rocks: (1) transpression and (2) vertical decoupling. Transpression will produce margin-parallel fabrics but these will generally be steeply-dipping S to LS fabrics. Only if the vertical shortening component during transpression is intense, will flat-lying planar fabrics be produced. An additional problem is that consistent top-NE displacements in the forearc are not easily reconciled with transpression, even assuming a strong component of vertical shortening. In the vertical decoupling model, intense, flat-lying LS fabrics in the middle crust of the Ketilidian forearc were formed in a low-angle detachment zone. This crustal-scale ductile shear zone accommodated transport of the forearc towards the NE, along the batholith margin, probably in response to oblique convergence at the subduction boundary. The existence of such mid-crustal detachments is implicit in many accounts of modern forearc deformation where, at high crustal levels, margin-parallel transport of the forearc on steeply-dipping, strike-slip faults is accompanied by vertical axis rotation of large-scale crustal blocks.
Accretion of voluminous juvenile, Paleoproterozoic lithosphere along the south margin of the Archean North Atlantic craton took place largely in an arc terrane during sinistral oblique convergence. In S Greenland, the Ketilidian Orogen comprises several tectonostratigraphic elements from N to S: 1) a northwestern Border Zone of reworked autochthonous Archean orthogneisses and locally overthrust parautochthonous Paleoproterozoic supracrustal cover (Vallen, Sortis Groups and equivalents, 2) the voluminous composite Julianehåb Batholith - the plutonic component of a juvenile arc covering more than half of the exposed orogen - emplaced contemporaneously with sinistral transpressive shear largely between 1855-1800 Ma, 3) a fore-arc fluvial to shallow marine deltaic clastic apron of the Psammite Zone and 4) deeper-water metagreywackes of the Pelite Zone.
Precise U-Pb single-grain zircon dating of Psammite Zone clastic sediments shows that they are erosional products sourced directly from the adjacent uplifted Julianehåb Batholith. SHRIMP U-Pb dating of larger populations of grains from these rocks and from Pelite Zone members not only support this conclusion but also reveals a significant proportion of older Paleoproterozoic (1850-1970 Ma, 2100 Ma, 2300-2325 Ma) as well as Archean (c.2700-2800 Ma) detritus. Detrital zircons c.2100-2300 Ma are a ubiquitous but minor component of most Paleoproterozoic orogens in northeast Laurentia (e.g. Torngat, Nagssugtoquidian, Rinkian) although their source remains enigmatic.
In the forearc sediments, constrained to have been deposited rapidly after 1792 Ma, ages of synkinematic magmatic rocks tightly restrict the timing of three distinct phases of deformation, culminating in peak HT/LP metamorphism and S-type granite development via sediment anatexis, between c.1790-1785 Ma (zircon, monazite). Post-kinematic rapakivi granite suite intrusions, emplaced between c.1755-1725 Ma, decisively post-date peak metamorphic conditions reached in the fore-arc.
Recent work within the Kobberminebugt shear at the NW margin of the Julianehåb Batholith, shows a complex non-coaxial deformation history with at least two shear-sense reversals during successive emplacement of batholith components. Earliest-recognized synkinematic plutonic members were intruded at c.1845 Ma, while the latest shear history is recorded at c.1805 Ma. North of the Kobberminebugt shear zone, a deformed granite clast within polymict conglomerate yields a magmatic age of 1880 ± 2 Ma (U-Pb, zircon; Qipisaqqu) and attests to a distinct, but previously unrecognized, earlier phase of Ketilidian arc(?) magmatism. The Ketilidian metasedimentary host and associated supracrustal package (Vallen & Sortis Groups) are constrained to be younger than this but older than an intrusive augen granite that is synchronous with an early phase of deformation at c.1848 Ma (U-Pb, zircon; Qoornoq). Certain components of the arc batholith are therefore interpreted to have developed astride the Archean continental margin; subsequent convergence resulted in strain partitioning focussed along several transpressive shear zones both within and near the boundaries of the accreting arc batholith.
The late Archaean Lewisian Complex of north-west Scotland was extensively reworked during the Palaeoproterozoic (locally termed Laxfordian). Correlation with Palaeoproterozoic collisional orogens throughout the Laurentia - Fennoscandia region (e.g. Whitehouse et al., 1997), however, is complicated by a paucity of appropriate ages in the Lewisian literature. In one of the earliest detailed studies of the Lewisian of the Outer Hebrides, Dearnley (1962) recognised a post-Scourie dyke (i.e. < 2.0 - 2.4 Ga) granulite facies event which he termed "early" Laxfordian, followed by "late" Laxfordian retrogression and formation of granite - migmatite complexes which are particularly well-developed in north Harris and south-west Lewis (Myers, 1971). The latter are dated at ca. 1.7 Ga (van Breemen et al., 1971) and are broadly correlated with granites of the mainland Lewisian in the type Laxfordian (Loch Laxford). Recently, granulite facies metamorphism in South Harris has been constrained to > 1.827 ± 0.016 Ga from an Sm-Nd mineral isochron (Cliff et al., 1998). To date, however, geochronological confirmation of the regional nature of early Laxfordian events has been based upon relatively imprecise and model dependent Sm-Nd whole-rock model ages (Whitehouse, 1990a) or Pb-isotope evolution arguments (Whitehouse, 1990b), with other chronometers (e.g. Ar-Ar, Cliff et al., 1998) responding to late Laxfordian events.
Ion-microprobe U-Th-Pb data are presented from a number of localities throughout the northern Outer Hebrides. Cathodoluminescence imaging (CL) of zircons from a diorite at the Butt of Lewis reveals a complex polyphase history starting at ca. 2.85 Ga. Most grains have concordant, low-Th/U euhedral tip overgrowths (metamorphic?) which yield a weighted average 207Pb/206Pb age of 1860 ± 10 Ma (2<epsilon>). Zircons from a tonalite at Borve, South Harris show oscillatory zoning in CL and crystallised from a melt at 1881 ± 13 Ma (2<epsilon>, weighted average 207Pb/206Pb age), with rare late Archaean cores suggesting some interaction with older continental material. Detrital grains from a Langavat supracrustal sample yield concordant 207Pb/206Pb ages in the range 1.83 - 2.15 Ga, with a strong peak at 1874 ± 6 Ma (2<epsilon>, weighted average of 13 analyses). These data provide the first unambiguous evidence for regional ca. 1.87 Ga events in the northern Outer Hebrides, with development of a calc-alkaline continental marginal arc and contemporaneous, probably arc derived, sedimentation, only a few tens of Ma prior to granulite facies metamorphism.
Whitehouse MJ, Bridgwater D & Park RG, Terra Nova, 9, 260-263, (1997).
Myers J, Scott J. Geol., 7, 234-284, (1971).
van Breemen O, Aftalion M & Pidgeon RT, Scott J. Geol., 7, 139-152, (1971).
Cliff RA, Rex DC & Guise PG, Precamb. Res, 91, 401-418, (1998).
Whitehouse MJ, Scott J. Geol., 26, 131-136, (1990a).
Whitehouse MJ, Isotope. Geosci., 86, 1-20, (1990b).
The Transamazonian orogeny of South America (= Eburnian of Africa) was first recognized in a belt that transected the Amazon craton, where investigators found rocks that yielded radiometric dates in the range of 2.1 - 1.9 Ga (Paleoproterozoic). Did the event also remobilize Archean crust, or accrete juvenile crust, elsewhere in Brazil? Yes. Studies now track a Transamazonian-age orogen across the northeastern lobe of the hook-shaped São Francisco craton. And recently, we have documented the presence of a belt of Transamazonian deformation in the Quadrilátero Ferrífero, at the southern end of the São Francisco craton's southern lobe (Alkmim and Marshak, 1998). In this belt, the Transamazonian event yielded a northwest-verging fold-thrust belt, which underwent extensional collapse to form a dome-and-keel province. In this province, domes which exclusively contain Archean gneiss and migmatite, were juxtaposed against pre-deformed Paleoproterozoic supracrustal rocks along steep shear zones at 2.1 Ga (Marshak et al., 1997). We have also obtained results from Sm-Nd dating in the Araçuaí and Ribeira belts, two Brasiliano (= Pan African) orogens that lie between the southern lobe of the São Francisco craton and the Atlantic coast, which suggest that, while crust within the Araçuaí belt has Archean ancestry, crust of the Ribeira belt along the coast has Transamazonian ancestry (Brueckner et al., in review). Other researchers have found evidence of Transamazonian arc rocks in the Ribeira belt southeast of the São Francisco craton. Taken together, these observations indicate that a west-verging Transamazonian collisional orogen, responsible for the accretion of an arc as well as exotic crustal fragments, was responsible for significant growth of eastern Brazil during the Paleoproterozoic. This orogen may have been responsible for the initial stitching of the Congo craton to the São Francisco craton. Notably, part of the crust that was affected by the Transamazonian event became incorporated in the São Francisco craton, while part was later pervasively remobilized by Brasiliano tectonism.
Alkmim FF & Marshak S, Precambrian Research, 90, 29-58, (1998).
Marshak S, Tinkham D, Alkmim FF, Brueckner H & Bornhorst T, Geology, 25, 415-418, (1997).
The SW Amazon craton (SWAC) is often shown joined to Laurentia-Baltica during assembly of Rodinia ca. 1000 Ma (Sadowski & Bettencourt, 1996). Detailed knowledge of the SWAC is essential for evaluating this hypothesis. Here we report new results that define more precisely a largely juvenile 1750-1800 Ma volcano-plutonic terrane in SWAC. The Amazon craton has an Archean core bordered by 2.7 to 1.0 Ga mobile belts; in the SWAC these are Ventuari-Tapajós province; Rio Negro-Juruena province (RNJP); Rondonia-San Ignacio province (RSIP); and Sunsas-Aguapei province (SAP). SW Mato Grosso (SWMT) includes southernmost parts of RNJP and RSIP plus sedimentary cover, deformation, and plutons associated with SAP to the west. The RNJP in eastern SWMT consists of the 1.80 to 1.75 Ga Jauru Terrane, which includes volcano-sedimentary belts, felsic plutonic-gneiss belts, and intrusive granitoids. Southern RSIP in western SWMT includes the Santa Helena Terrain, where undeformed 1.0 Ga granites intrude a ca. 1.45 Ga arc complex (Geraldes et al., 1999) that is locally overlain by sedimentary rocks of 1.3 to 1.0 Ga Aguapei group. The western part of the region was deformed during formation of the 1.0 Ga Sunsas orogenic belt to the west.
Volcanic rocks, plutons, and gneisses of the Jauru terrane yield U/Pb ages from 1795±10 to 1747±13 Ma. TDM ranges from 1926 to 1868 Ma, indicating that the original magmas may have been contaminated by small amounts of older crust. The Jauru terrane probably represents essentially juvenile material accreted to the SWAC about 1.75 Ga. Several deformed, tonalite to granite plutons intruded into the Jauru terrane yield U/Pb ages from 1567±06 to 1536±11 Ma with TDM from 1883 to 1773 Ma, indicating that their magmas were probably derived mainly from crust of the host terrane. These plutons are coeval with the Serra de Providência rapakivi suite in Rondonia (Tosdal et al., 1996), although rapakivi features have not been reported in SWMT. The Jauru terrane also includes several undeformed, crustally derived plutons ranging in age from 1.5 to 1.4 Ga. The Jauru terrane, including the ca. 1.55 Ga plutons, represents SE extension of the RNJP from Rondonia (Tassinari et al., 1996). The combination of ca. 1.55 Ga rapakivi affinity plutons and ca. 1.80-1.75 Ga basement in the the Jauru terrane is not compatible with relationships in either Baltica (Ramo & Haapala, 1995) or Laurentia (Gower, 1996; Van Schmus et al, 1996), so that correlation of RNJP terranes with either Laurentia or Baltica is still uncertain.
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Tosdal RM, Bettencourt JS, Leite WBJr, & Payolla BL, XXXIX Cong. Bras. Geol. Anais., 6, 591-594, (1996).
Van Schmus WR, Bickford ME & Turek A, Geol. Soc. Amer. Spec. Paper308, 7-32, (1996).
Terranes, made up of arc-related igneous rocks and associated sediments, were accreted onto the continental margin of southwestern North America from 1.9 to 1.6 Ga. They formed the Yavapai-Mazatzal province in Arizona, of which the Pinal terrane represents the southeasternmost part (Karlstrom and Bowring, 1988). U-Pb analyses on magmatic and detrital zircons were performed on 21 samples to constrain timing of geologic events in this area, and 25 Sm-Nd whole rock analyses were made to infer sources of sedimentary- and igneous rocks.
Two sedimentary facies and three distinct igneous events can be defined: the 1.7 Ga St. Catalinas volcanic arc, the 1.66-1.65 Ga eugeoclinal Pinal basin facies, 1.66-1.64 Ga granitoid intrusions and the 1.64-1.62 Ga Dos Cabezas volcanic arc with an associated shelf facies. A suture zone, interpreted as remnants of a subduction zone (Swift, 1987) separates the Pinal basin and the Dos Cabezas volcanic arc into two tectonic blocks. Dating the igneous cycle in the volcanic arc reveals similarities to modern active plate margins: first intermediate to felsic volcanics erupt, followed by late- to post-tectonic granitoids and subsequently post-tectonic basalts. Quartzite-rhyolite associations constitute the uppermost, mature rock sequence of the growing island arc.
<epsilon>Nd(t)-values of metapelites and graywackes from the Pinal basin range from -0.34 to 2.58. Values increase from NW to SE. This trend represents decreasing input of recycled material from the young continental margin to the NW. Two analysed samples from the St. Catalinas volcanic arc have <epsilon>Nd(1700) = 2.03 and 2.49, respectively, values close to the depleted mantle model. Quartzites and volcanic rocks from the Dos Cabezas arc show a tight cluster of <epsilon>Nd(t)-values from 2.72 to 4.03. The volcanic rocks are juvenile additions to the crust; sedimentary rocks were recycled rapidly from young igneous rocks. Posttectonic basalts with <epsilon>Nd(t) = 4.46 and 4.54 are interpreted as being derived from a depleted upper mantle source.
The lifespan of the basin was about 20 Ma, calculated from the age of the youngest detrital grain to the age of intrusion of late- to post-tectonic granites. Magmatic activity in the Dos Cabezas volcanic arc covers a time interval of 37 Ma. Geologic events (sedimentation, accretion, volcanism, intrusion) rapidly followed each other along the outward-growing active continental margin, where large volumes of mature crust were differentiated from the mantle in a brief time-interval.
Karlstrom, K E & Bowring, S A, J. Geol., 96, 561-576, (1988).
Swift, P N, unpublished PhD thesis, University of Arizona, (1987).
Tectonic analysis of high grade gneiss terrains is often complicated by a complex poly-cyclic history of the rocks and by a lack of a decipherable stratigraphy. In such geological settings, dyke swarms can be useful relative time markers and can be used as sensitive monitors of post-emplacement tectonism and metamorphism. Ramberg (1949) used the progressive structural and metamorphic reworking of the mafic Kangâmiut dykes in West Greenland to separate Archean basement in the south from its reworked equivalents in the Paleoproterozoic Nagssugtoqidian Orogen (= NO) to the north. Within the NO dykes lost their igneous texture and are transposed into parallelism with the gneissic foliation. In the foreland, the dykes preserved their ophitic texture, which is, however, variably overgrown by metamorphic phases. The cause of metamorphism of the dykes in the foreland has been interpreted by some workers as syn-intrusive ("autometamorpism", e.g. Jack, 1978), while others (e.g. Mengel et al., 1997) argued for static metamorphism caused by tectonic loading during the Nagssugtoqidian orogeny. The tectonic setting in which the Kangâmiut dykes were emplaced has been a long lasting controversy. Windley (1970) suggested that the shear fabrics seen in the central part of a minority of the dykes is the result of syn-kinematic intrusion in a compressive environment. Others (e.g. Kalsbeek et al., 1987) consider intrusion in a rift-setting more likely. This study shows that pristine and metamorphosed Kangâmiut dykes exposed in the foreland of the Nagssugtoqidian Orogen preserved a pre-Nagssugtoqidian 40Ar/39Ar signature. Metamorphism of the dykes in the foreland occurred shortly after their emplacement, within the resolution of the argon system. We suggest that crustal heating during dyke emplacement or heat derived from the individual dykes itself caused the development of metamorphic minerals and textures. Internal shearing of the Kangâmiut dykes in the foreland post-dates dyke emplacement. Shear fabrics observed in the dykes are therefore unrelated to dyke emplacement. Inside the orogen reworked equivalents of the Kangâmiut dykes record parts of the post-Nagssugtoqidian cooling path.Two suites of Kangâmiut dykes have been identified (ca. 2.5 Ga and ca. 2.04 Ga). Internal shearing of the dykes in the foreland occurred as late as 1.74 Ga, contemporaneous with closure of the argon system in hornblende inside the orogen, following peak Nagssugtoqidian metamorphism at ca. 1.85 Ga. The duration of thermotectonic activity during Nagssugtoqidian orogeny is bracketed between intrusion of the youngest Kangâmiut dykes at ca. 2.04 Ga and post peak-metamorphic cooling of the orogen at 1.74 Ga.
Jack, S. M. B., Unpublished Ph.D. thesis, University of Liverpool., 391 pp., (1978).
Kalsbeek, F., Pidgeon, R. T. and Taylor, P. N., Earth Planetary Science Letters, 85, 365-385, (1987).
Mengel, FC, Bridgwater, Dand Hageskov, B, Terra Nova, 9, 355, (1997).
Ramberg, H, Meddelelser fra Dansk Geologisk Forening, 11, 312-327, (1949).
Windley, B, Geological Journal Special Issue, 2, 79-92, (1970).
The Paleoproterozoic Lapland-Kola Orogen (LKO) principally comprises late Archean gneisses that were variably reworked during the Paleoproterozoic, concomitant with the juxtaposition of the different terranes of the orogen. Our 40Ar/39Ar mineral data indicate that the LKO experienced a pervasive post-tectonic thermal overprinting, but that in spite of this distinct tectonic phases can still be recognised. Young plateau ages of 1.7 Ga (colourless mica) in the Central Kola Composite Terrane (CKCT) and 1.77 Ga (hornblende) from hornfelse-like rocks, that contain kyanite-staurolite-cordierite assemblages, point to thermal resetting. The minerals overgrow the main tectonic foliation or grow parallel to the walls of undeformed quartz veins that cross cut the foliation. Furthermore, they overgrow retrograde muscovite-chlorite aggregates, which development is related to a steep shear zone that forms the northern limit of the CKCT. These observations point to a late-stage, post-tectonic reheating phase. This event essentially affected the entire LKO and has reset the Ar-isotope system of muscovite, as shown by 1.75-1.7 Ga plateau ages along the 250 km long profile. Biotite Ar/Ar ages are often in this range, but occasional elevated ages indicate the uptake of excess argon. This implies that the temperature at the sampled erosion level of the LKO during the Paleoproterozoic was in excess of 350°C, until about 1.7 Ga.The main tectono-metamorphic phase in the LKO is probably 1.87-1.78 Ga old, implied by hornblende plateau ages of the Belomorian Terrane. Rocks of this terrane do not contain relics of a Late Archean K-Ar system, pointing to a complete thermo-tectonic resetting or neoformation of hornblende during the Paleoproterozoic. However, hornblendes from rocks of the CKCT, although mostly yielding ages below 1.87 Ga, occasionally retain a late Archean 40Ar/39Ar signal. On the other hand, hornblendes from the Murmansk Terrane - that forms the northern foreland of the LKO - are only partially reset, as shown by age spectra with apparent ages that increase from about 1.9 Ga to 2.65 Ga. Taken together the hornblende age spectra point to a northwards decreasing thermal and tectonic resetting of the late Archean rocks during the Paleoproterozoic LKO and, hence, decreasing paleotemperatures. 1.9 Ga hornblende K-Ar ages from the northern part of the Karelia province - the southern foreland of the LKO - point to a more important resetting than the Murmansk Block. 1.88 Ga 40Ar/39Ar and Rb-Sr mineral ages of the Umba Granite, that cross-cuts the fabric in the Umba Granulite, suggest that the granulite metamorphism is older than the main tectono-metamorphic phase of the LKO. Furthermore, a hornblende from orthogneisses to the South of the Imandra-Varzuga Greenstone belt, yielding a 1.90 Ga plateau age, point to older Paleoproterozoic tectono-metamorphic phases in the LKO.
This study focuses on the tectonothermal history of south-central Sweden and presents the results of 40Ar/39Ar analyses and U-Pb ion-microprobe (NORDSIM) ages. Samples were taken along and betwen major shear zones to assess the timing of migmatizastion, cooling and/or shearing in the particular regions and to correlate the tectonothermal history obtained with larger tectonic processes affecting the Svecofennian Domain of the Baltic Shield.The eastern part of south-central Sweden is situated in the western region of the Svecofennian orogen. The area is dominated by supracrustal rocks and an early intrusive suite of granitoids and associated mafic rocks (Suite 1-c. 1.9-1.85 Ga). These were deformed and metamorphosed under predominantly medium- high grade conditions (Dx). The area is bordered to the south and west by the Transscandinavian Igneous Belt (TIB). This suite yields ages in 2 distinct groups from c. 1.85 to 1.77 Ga and from 1.71 to 1.65 Ga. Igneous activity in the area was completed by the intrusion of small bodies of granite and syenite with ages of c. 1.5 Ga and a swarm of Mesoproterozoic mafic dykes. These rocks have been included in the rapakivi group which elsewhere in the Fennoscandian Shield range in age from 1.65 to 1.53 Ga. There are three groups of meta-sedimentary rocks, which successively from south to north include: 1) Quartzites, metarkoses and subordinate metapelites of the Västervik synclinorium, 2) A group of metavolcanic rocks north of Loftahammar peninsula, and 3) North of Söderköping these metavolcanic rocks give way progressively to metagreywackes and other siliclastic rocks, intercalated by marbles and felsic and mafic metavolcanic rocks. The entire sedimentary-igneous complex has been subject to Svecofennian deformation. The metasedimentary and metavolcanic rocks have undergone an early, probably syn-intrusive and pre-deformational stage of HT peak metamorphism and migmatization (metamorphic stage M1). A network of NW and W-striking ductile shear zones transects the Svecofennides of south-central Sweden from the Sveconorwegian deformation front to the Baltic coast. Schematically four major deformation zones are shown. These shear zones (Dx+1,x+2) fold and rotate the earlier Dx regional fabric. All of the major and minor Dx+1 and Dx+2 deformations in the region can be explained by a NS oriented main compressive stress field. They formed fabrics perpendicular to sigma1, oblique conjugates, or various structures resulting from strain partitioning along the major oblique strike slip shear zone. The older Dx phase developed in a different stress field. Our geochronological results document a multi-stage and prolonged thermal history of the area from c. 1850-1450 Ma. These results will be compared with recent results obtained from the Baltic states to better understand the larger scale implications for the geologic history of the Peri-Baltic region.
Available data on seismic modelling of the lower crust and the zone transitional to mantle, on a positive gravity anomaly in the zone of high grade metamorphism, and specific features of the deep structures of the boundary zone between the Archaean Fenno-Karelian craton and the Palaeoproterozoic Svecofennian accretion orogen are described. The data on a stratigraphy of the supracrustal units provide the correlation them through the Raahe-Ladoga suture that may be argued by geological and geochronological data. There are evidence that the basins developed near and in connection with volcanic arcs existed only during short time, not more than 30 Ma because the age of the initial volcanic rock in the sequences under consideration is of 1.92 Ga, and the most ancient plutonic rocks cutting metasediments were emplaced 1.89 Ga ago. There exist new geological, petrologic, geochemical and geochronological data on the main sinmetamorphic granitoid complexes which include norite-enderbite (Kurkijoki complex, 1871±6 Ma), diorite-tonalite-trondhjemite (Lauvatsaari-Impiniemi complex, 1864±13 Ma) and tonalite-granodiorite (Tamhanko complex, ca 1860 Ma) assemblages and compose the new crust generated at the active continental margin. During the late stage of the metamorphic cycle the Tervus K-granite complexes was formed (1856±7 Ma). Systematics of the available Sm-Nd isotope data and calculation the TDM model ages which are of 2.3-2.4 Ga provide the conclusion that magmas of all these complexes were emplaced from juvenile Proterozoic sources and contaminated by metasedimentary rocks contained the Archaean clastic material. SE Finland and N Ladoga region together are a classic terrain of andalusite-sillimanite type zonality where the metamorphic grade varies from greenschist to granulite facies (Korsman et al., 1984; Glebovitsky et al, 1985). Some features of the mineral assemblages and mineralogical thermo-barometers allow the PT conditions to be estimated and the prograde path of evolution to be inferred, which is supported by study of fluid and melt inclusions in the minerals. The initial anatectic migmatites appear in the garnet-biotite-sillimanite-muscovite subfacies, and the partial melting become more and more extensive in granulite facies. The homogenisation method, i.e. melt inclusions in a leucosome of the early migmatites, was used to determine temperatures in the zone where migmatization had commenced. The lowest temperature was ca 680°C and the highest reached up to 770°C, which is in agreement with estimates obtained using mineralogical thermometers. For the pressure estimation a criometry of CO2 inclusions was used. If to take into account only the most dense inclusions a pressure in the metamorphic zonality increases from 3.5 to 5.5 kb that is in a good agreement with the data obtained from independ methods. Pressure measurements of the retrograde stage indicate that temperature did not fall significantly during decompression, as the melt was at least preserved and likely appeared in the migmatites.
Zircons from the low-pressure, high temperature K-poor leucosomes migmatites from Tampere area (Southern Finland) display a complex zoning structures using SEM based CL-imaging. The common feature in most grains is the presence of a distinct core which clearly appears to be a remnant of an original grain. This core is overgrown by one to two thin outer rims, which suggests as many stages of zircon crystallization. The cores display various types and styles of zoning although an oscillatory zoning is the dominant feature. The rims of the grains are characterized by two different structures: i) weakly oscillatory zoning, which is considered as typical of magmatic recrystallization, and ii) unzoned outer rim overgrowth of about 10 to 25 µ on an older broken oscillatory zoned grain, which is considered as growth of new zircon during the metamorphic event. Ion probe data on cores and rims from all the samples yielded a slightly discordant 207Pb/206Pb age. The cores retain an age group of 2866-2002 Ma, which is interpreted as the protolith age. Rims yielded two age groups with respect to their internal structure: the weakly oscillatory zoned rims yield an intermediate age of 1959-1904 Ma, whereas the unzoned rims gave an age of 1872-1886 Ma. This difference in age and internal structures of the rims can be interpreted as two different geological events within the Svceofennian orogeny. The youngest age group is clearly related to the peak metamorphic event and migmatisation, consistent with the previous conventional Sm-Nd data on garnet and U-Pb on monazite. Whereas the magmatic rim age group remains unclear, this age might be related to one of the magmatic events known within the Svecofennian domain, in the Primitive Island Arc Complex of the Savo schist belt, the Arc Complexes of Central Finland or the Jormua ophiolite complex.
The Palaeoproterozoic Svecofennian Orogen was formed by accretion of juvenile island arcs and sedimentary basins, 1.92-1.88 Ga in age, onto the Archaean craton. Accretional processes and magmatic underplating thickened the crust so that its thickness is still up to 65 km in central Finland and 50 km in southern Finland. In central Finland convergence ceased before the intrusion of post-tectonic 1.87 Ga granites. In southern Finland, however, there is a 100 km wide E-W trending zone, characterized by late-orogenic 1.83 Ga granites and migmatites associated with high T-low P metamorphism, where deformation has continued at least until ca. 1.80 Ga. Two post-orogenic magmatic stages also occur in this zone: ca. 1.80 Ga monzonite-granite magmatism and ca. 1.60 Ga anorthosite-rapakivi granite magmatism. Compressional (Ehlers et al., 1993) and extensional (Korja & Heikkinen, 1995) tectonic settings for the zone have been proposed.
In the Turku area the subvertical limbs of regional upright or northwestwards overturned NE-SW trending D3 folds are commonly strongly sheared, intensely migmatized and intruded by garnet and cordierite bearing granites. Shearing and folding and were coeval with granulite facies metamorphism at ca. 1830 Ma. These structures are cut by subvertical, N-S to NNE-SSW trending shear zones with E-side down movement. In West Uusimaa similarly oriented several hundred metres wide shear zones deform the late-orogenic granites and kinematic indicators show a consistent E-side up movement, explaining the metamorphic jump from andalusite-cordierite rocks in the west to garnet-cordierite granulites in the east. These NNE zones are bound both to the north and south by E-W trending steep shear zones. The southern one is a prominent right-lateral strike-slip zone with a minimum displacement of 20 km (Ehlers et al., 1993). The northern one shows a right-lateral movement in the west and dip-slip movement in the east.
The late-orogenic structural evolution of the Svecofennian orogeny in SW Finland can be explained in a two stage model. During stage 1, the NW-SE crustal shortening produced the regional D3 folds during the peak of metamorphism. During continued shortening deformation became localized in the shear zones. NNE trending zones are considered reverse, while the E-W zones served as lateral strike-slip faults. During stage 2, they where reactivated as extensional faults. As a working hypothesis, we propose an oblique NW-SE convergence followed by extensional collapse on a minor scale, as indicated by the thick crust remaining. The second stage could have taken place during the intrusion of the post-orogenic granitoids.
Ehlers C, Lindroos A & Selonen O, Precambrian Research, 64, 295-309, (1993).
Korja A & Heikkinen P, Tectonics, 14, 504-517, (1995).
The main ouline of the Precambrian in northern and eastern Europe has been summarized by Gorbatschev & Bogdanova (1993). Detailed correlations between the Fennoscandian and Ukrainian Shields were, however, hampered by limited information on the Precambrian crust in the Baltic Sea region, which to a large extent is hidden under Phanerozoic platform sediments. Information on the Precambrian crust in this region has recently become available as part of oil prospecting activities on the island of Gotland and in adjacent offshore areas. In this investigation, which forms part of the Eurobridge project, fourteen drill cores penetrating the Palaeoozoic platform sediments on Gotland have been studied. Based on their petrological, geochemical and isotopic character, combined with geophysical data (Gyllencreutz 1998), four zones (I-IV) can be distinguished. Each of these zones may be correlated with crustal units adjacent to the Baltic Sea, i.e. the Fennoscandian Shield in Sweden and Finland as well as the concealed Precambrian in Estonia, Latvia and Lithuania.
I. The Fårö zone forms part of a major Mid to Late Proterozoic anorogenic structure, which extends from Landsortsdjupet to Riga. It is characterized by Jotnian sandstones and the Riga rapakivi batholith.
II. The Visby zone is dominated by highly metamorphosed crust with orthogneisses, paragneisses, felsic meta-volcanics and post-metamorphic granites. This zone is correlated with the Early Proterozoic (Svecofennian) ore-bearing continental margin volcanic environment of Bergslagen (Sweden), as well as ore-bearing successions in Jussarö-Orijärvi (southwestern Finland), Sakkusaare-Johvi (Estonia) and Staicele (Latvia). The Bergslagen terrane thus encircles the Landsortsdjupet-Riga anorogenic structure.
III. The Viklau zone is composed of amphibolites with primitive geochemical and Sm-Nd isotope characters, which are identical with the Svecofennian amphibolites in the Valdemarsvik and Tiveden areas, which form the southwestern border of the Bergslagen terrane in Sweden. This zone is parallel with the LLDZ shear zone in Sweden and the Nemunas Fault Zone in Lithuania. It is suggested that the border between the Visby and Viklau zones (as well as the LLDZ and Nemunas structures) correspond to the southwards decrease in Moho depth, that has been recognized in the Baltic Sea by Korja et al. (1993).
IV. The Hoburgen zone is characterized by amphibolites and granitoids. The amphibolites are probably discontinuous fragments of the Viklau zone and may be correlated with granulite facies mafic crustal fragments in Lithuania (e.g. at Darius). Most granitoids in this zone are petrologically distinct from the granitoids of the Transscandinavian Igneous Belt (TIB) and their correlations are uncertain. However, a coarse-grained granitoid in the southwesternmost part of Gotland (Kvarne) may possibly be correlated with TIB.
Korja, A., Korja, T., Luosto, U & Heikkinen, P., Tectonophysics, 219, 129-152, (1993).
Gorbatschev, R. & Bogdanova, S., Precambrian Research, 64, 3-21, (1993).
Gyllencreutz, R, M. Sc. thesis, Stockholm University, 29 pp, (1998).
A magnetometric ground survey of central Gotland has, together with re-interpretation of earlier geophysical data, provided new information on the Precambrian crystalline basement in the Baltic Sea region. This region has previously attracted geophysical interest from, among others, oil prospectors and the major deep seismic profiling programme within the European Geotraverse. From the latter, it is known that significant variations in the Moho depth exist in the Gotland area. The purpose of the present study has been to characterize the geophysical properties of the continental crust in this region, in order to improve the correlations between individual crustal units around the Baltic Sea. The work has been carried out within the Eurobridge programme. The following results have been obtained:
Northernmost Gotland shows a pronounced and extensive gravity low. This anomaly correlates well with satellite altimetry low-gravity anomalies in the adjacent off shore areas, which has been interpreted to represent rapakivi granites connected to the Riga Massif (Wannäs & Hayling, 1993). No magnetic data are available from this part of the island.
On central-western Gotland, the new magnetic anomaly pattern reveals a positive anomaly of 1000-2000 nT, traversing the island in a general E-W direction from Klintehamn to Buttle. The same area also shows a positive gravity anomaly. The gravity and magnetic anomaly patterns in the region, show that the extent and magnitude of the Klintehamn-Buttle anomaly may be correlated with a similar positive anomaly near Valdemarsvik on the Swedish mainland.
On southern Gotland, magnetic and gravity lows are present in a relatively narrow WNW-ESE-trending zone. This zone is correlated with the Loftahammar-Linköping Deformational Zone on the Swedish mainland, and the Nemunas Zone in Lithuania. An extensive high-magnetic zone is located in the offshore areas immediately south-west of Gotland. This anomaly may correspond to the high-magnetic granitoids of the Transscandinavian Igneous Belt.
Wannäs KO & Hayling KL, Precambrian Research, 64, 311-317, (1993).
The East European Craton (EEC) consists of three crustal segments, Fennoscandia, Volgo-Uralia, and Sarmatia (e.g. Gorbatschev & Bogdanova, 1993). These segments are separated by Neo- to Mesoproterozoic (Riphean) aulacogens. The Fennoscandian segment comprises an Archaean core in the northeast, while its major part is composed of Palaeoproterozoic juvenile crust.
The earliest event in the creation of the EEC was the collision between Archaean Sarmatia and Volgo-Uralia which were united into a single continent ca. 2.1-2.0 Ga ago.
New EUROBRIDGE research demonstrates that in the area between the Baltic and the Ukrainian Shields, the Palaeoproterozoic formation of new continental crust proceeded outwards from two growth nuclei. One of these was Sarmatia with adjoining Volgo-Uralia, the other the Archaean core of Fennoscandia.
Accretion of new portions of juvenile crust to the northwestern margin of the combined Sarmatian/Volgo-Uralian continent occurred between 2.0 and 1.9 Ga (e.g. Bibikova et al., 1995). Closest to the Archaean core, the Osnitsk-Mikashevichi Igneous Belt was formed in an Andean-type setting. Farther outboards, the precursors of the Cenral Belarus, Belarus-Baltic and East Lithuanian belts developed in oceanic-type arcs and basins.
At an angle with this system of accretionary belts, a different belt of Palaeoproterozoic juvenile crust grew outwards from the SW margin of the Archaean core of Fennoscandia. In that region, the Svecofennian Domain of the Baltic Shield and its continuation in the basement of Estonia, Latvia and western Lithuania were formed between 1.9 and 1.8 Ga by the accretion of several magmatic arcs and intervening depositional basins (e.g. Gaál & Gorbatschev, 1987; Nironen, 1997).
There is a notable absence of close correlation between the accretionary events in the two Palaeoproterozoic belts growing outwards from Sarmatia/Volgo-Uralia and Archaean Fennoscandia, respectively. In the light of our new results, this appears natural since the two belt systems developed independently.
According to palaeomagnetic data (e. g. Elming et al., 1998), the docking of the Sarmatian/Volgo-Uralian and Fennoscandian continents with each other occurred between 1.8 and 1.7 Ga. Subsequently, they shared a common history. This change was marked by the development of the ca. 1.8-1.65 Ga Transcandinavian Igneous Belt along the western margin of the Svecofennian Domain and by the formation of Gothian magmatic arcs between 1.69 and 1.58 Ga ago (e.g. Åhäll and Gower, 1997).
Later in the Proterozoic, the crust of the EEC was rifted and Riphean aulacogens were formed. These developed at the sites of Palaeoproterozoic collisional (the Central Russian and Pachelma Aulacogens) and accretionary (the Volhyn-Orsha Aulacogen) crustal-segment interfaces.
Åhäll & Gower Ch.F, GFF, 119, 181-191, (1997).
Bibikova EV, Bogdanova SV, Gorbatschev R, Claesson S & Kirnozova TI, Stratigraphy and Geological Correlation, 6, 591-601, (1995).
Elming S, Mikhailova NP & Kravchenko SN, Geophysical Journal, 20, 71-74, (1998).
Gaál G & Gorbatschev R, Precambrian Research, 35, 15-52, (1987).
Gorbatschev R & Bogdanova S, Precambrian Research, 64, 3-21, (1993).
Nironen M, Precambrian Research, 86, 21-44, (1997).
The reconstruction of Proterozoic supercontinents has revealed that 2.1-1.8 Ga orogens occur on nearly every continent and thus is considered to represent a global orogenic event, which produced a large number of linear mobile belts welding the Archean continental nuclei, e.g. the Transamazonian orogens of South America, the Eburnian and Limpopo orogens of Africa, the Trans-Hudsonian, Penokean, Wopmay and Ketilidian oregens of North America, the Svecofennian and Kola-Karelian orogens of North Europe, Nagssugtoqidian orogen of Greenland, the Capricorn orogen of Western Australia, the Eastern Ghats orogen of India, Trans-North China orogen of China. Lithological, srtuctural and metamorphic studies show that most 2.1-1.8 Ga orogens developed as a result of the collision between two Archean continents or between Archean continent and Paleoproterozoic arc. Correlations of lithological, structural, metamorphic and geochronological data and paleomagnetic reconstruction for these collisional orogens and the welded Archean cratons strongly suggest the existence of two pre-Rodinia supercontinents: one comprising the major Archean cratons in circum-South Altantic provinces, including West Africa, Congo, Guiana-Amazon, São Luis, São Francico and Rio De la Plata cratons, referred to herein as South Atlantic (SA) Supercontnent; and the other consisting mostly of the major Archean-Paleoproterozoic cratons in circum North Atlantic provinces (North America, Baltic and Greenland cratons) and Siberia, West Australia, Antarctic, Noth China and possibly Indian cratons, referred to herein as North Atlantic (NA) Supercontnent. The preliminary configurations of these two supercontinents have been made based on the available geological and paleomagnetic data. The final amalgamation of the two pre-Rodinia supercontinents during Late Mesoproterozoic (Grenvillian age) resulted in the assembly of the Rodinia supercontinent.
The terms orogenic and anorogenic have long characterised ideas of continental rock-forming processes. While there is general consensus regarding first-order issues associated with orogenic processes (such as convergent-margin magmatism, arc accretion and related crustal growth), our understanding of rocks and processes described as "anorogenic" has proven more challenging. In particular, the question remains whether such rocks and processes are strictly anorogenic (i.e. unrelated to orogenic processes) or whether an obscure link actually exists between orogenic processes and rocks traditionally considered as anorogenic? In recent compilations, the 1.90-1.55 Ga growth of the Baltic Shield has been satisfactorily explained by distinct westward steps where successively younger crustal terranes were progressively amalgamated to the continent. This prolonged period of growth along the southwestern edge of Baltica was accompanied by episodic intra-cratonic magmatism in the Svecofennian Domain, well east of the active margin, that included: 1) ca. 1.81-1.76 Ga granites, 2) ca. 1.70 Ga granites, and 3) three distinct episodes of rapakivi suites in the 1.66-1.50 Ga interval. After stabilisation of the Gothian orogen at 1.55 Ga, intra-cratonic magmatism stepped westwards producing at least five bimodal magmatic episodes between 1.50-1.20 Ga, all represented in the 200 km wide Gothian segment of Baltica.New whole rock geochemistry and U-Pb data from SW Sweden demonstrate that new Gothian crust formed from magmatic events in distinct, west-stepping N-S trending belts at 1.69-1.65, 1.62-1.58 and 1.56-1.55 Ga. Each of these pulses is apparently answered by rapakivi magmatism at 1.66-1.62, 1.59-1.54 and 1.53-1.50 Ga, starting ca. 600 km east of the active margin. The present data set suggests that each peak of the rapakivi magmatism occurred approximately 30 m.y after a major magmatic event associated with convergence along the active margin. This correlation is strengthened by the sympathetic westward younging of the three rapakivi suites. Emerging data therefore favours a tectonic link between three discrete orogenic stages of Gothian growth and three suites of rapakivi magmatism in the Svecofennian Domain distal from the active margin. Given the comprehensive magmatic record in Baltica and the episodic character of the prolonged intra-cratonic magmatism, this region provides a suitable area to test models linking orogenic processes with distal, inboard magmatism that has traditionally been considered anorogenic in origin.
Integrated studies of systematic aeromagnetic, gravity, and petrophysical data are effective in interpreting the structural geology and the tectonic history of Precambrian basement areas. Additional information on the effect of erosion on various lithologies is provided by digital elevation data. The dense sampling interval of the aeromagnetic data allows the regional scale studies to be focused to outcrop scale. By benefitting petrophysical measurements as the link between geology and geophysics, the observation scale becomes even more detailed. The tectonic analysis of southeastern Finnish Lapland in the northern Fennoscandian shield, was based on these kind of data sets. The geological interpretation of this area, composed of Precambrian high grade gneisses and granitoids, is presented with a particular emphasis on the magnetic method. The results stress the importance of detailed petrophysical analysis incorporated into aeromagnetic interpretations. The demonstrated geophysical marks, shown as block boundaries and deformational styles, are applicable also to other Precambrian shield areas.
The structural framework of the study area was constructed during the late Archaean, and repeatedly reworked during the Palaeoproterozoic. As interpreted from the regional data sets, the same weakness zones were kinematically reactivated during various tectonic episodes. The beginning of the fragmentation of the Archaean Karelian craton was a turnover in the tectonic history of the northern Fennoscandian shield. The continental rifting period at 2.5-2.1 Ga ago produced major block boundaries, which emerge as extensive gravity gradients, and parallel weakness and fault zones within the fragmented crustal blocks. After that, the Svecokarelian continental collision and the associated deformation at 1.9-1.8 Ga ago reworked the magnetic image. Wide areas of highly magnetic granites were produced, associated with an intense regional positive anomaly. Inside this 200 km x 80 km wide regional granitoid anomaly, there are marks of metamorphic zoning and banded patterns due to relics of gneisses and supracrustals. The Svecokarelian influence was so overwhelming, that the effects of the earlier late Archaean structural events were attenuated and are hard to recognize at present.
A study of supracrustal formations is particularly important for the understanding of the Precambrian. An important task of this study is to reconstruct the primary nature of metamorphic rocks and the environments in which their protoliths were formed. Because metamorphic processes, especially in the granulite facies, commonly lead to the loss of relics of primary structures and textures, petrogeochemical techniques have a significant place in solving this task.
The methods elaborated earlier with author's participation are based on a systematic approach to an analysis of geologic-geochemical properties of rocks, allowing for a study of specific geological cross-sections and identification of the primary nature of metamorphosed Precambrian supracrustal formations.
This approach includes the use of original techniques that make it possible to perform an integrated geologic-geochemical analysis, to compare data on the Precambrian and Phanerozoic, to identify the extent to which the compositional changes of rock assemblages in real cross-sections comply with the tendencies typical of volcanic and sedimentary sequences, and to determine probable conditions of the formation of protoliths for rock associations.
The problem of the protolith's nature of metamorphic rocks from high-pressure granulite belts has been investigated by the aforementioned systematic geologic-petrogeochemical method. It is not ruled out that the granulite might have been formed in the course of Precambrian volcanism and sedimentation, or that it may have an infracrustal or intrusive genesis. Features of successive bedding of formations in geological sections have been identified, and the relationships of the succession with the rock composition and tectonics have been studied; new information on the pre-metamorphic history of high-pressure granulite belts have been thus obtained.
These studies were focused on the Lapland granulite, and were conducted in comparison with the material on various structures of Eurasia - granulites of the Anabar, the Southern Aldan and the Ukrainian Shields, granulites of the Near-Baikal region, North-Eastern Asia, Southern and South-Eastern India, and Northern China. These rock complexes are found to be polychronous and heterogeneous, and their geological sections contain rocks of various genesis, including a fairly large amount of supracrustal rocks.
In the composition of supracrustal rocks, the granulite belts are most similar to volcanic-sedimentary formations of young Phanerozoic arcs, which is supported by the graywacke type of sediment genesis. The Lapland granulite rock composition and metamorphism show as well a relationship with the various evolution stages of the convergent boundaries of the Kola and Belomorian protoplates.
The available methods used to reconstruct the environments in which protoliths of Precambrian metamorphic complexes were formed are generally based on a direct comparison of chemical composition parameters of Precambrian and Phanerozoic rocks. These methods are possible to use with the provision that certain conditions are fulfilled. One of the conditions is that there are no considerable evolutional distinctions in the chemistry of the environments of one and the same type.
Investigations have shown that chemical composition parameters of rock assemblages change at the Phanerozoic-Precambrian transition (Kozlov, Martynov, 1995).
To estimate the influence of these changes on the resulting reconstructions, we have examined granulite belts of Eurasia, whose protoliths had formed in environments similar to those of island-arcs (Kozlov, 1995; etc.), and metamorphic rocks of the Pechenga-Varzuga Belt, which are interpreted to originate as protorifts (Smolkin, 1992; etc.). It turned out that some structures of the Pechenga-Varzuga Belt fall into the field of young arcs; this suggests that the method of direct analogies is inappropriate for Precambrian structures.
The shifts of petrochemical characteristics of Precambrian rocks relative to Phanerozoic are found to be non-linear, at least in case of basic migmatites (Martynov, 1997). The volume of the attribute space occupied by the Precambrian rocks is smaller than that of the Phanerozoic rocks. This fact implies that the possibility to reconstruct environments is limited by a certain time interval, and it is not correct to use a direct correlation of par