Laurentia is the largest of the many continental fragments that have been postulated to form the late Mesoproterozoic - early Neoproterozoic supercontinent, Rodinia. To rigorously test Rodinia reconstructions and investigate its assembly and breakup, it is important to establish a set of key (well-defined and precisely dated) paleomagnetic poles for Laurentia with which to compare paleopoles of similar quality from the other fragments. We review the 1.7-0.7 Ga paleopole database from Laurentia and two of the other larger fragments which are thought to have formed part of Rodinia, namely Baltica and Siberia, with a view to assessing published reconstructions.
Owing to large age uncertainties, often hundreds of millions of years, most paleopoles from Laurentia are of limited use in paleocontinental reconstructions. Paleopoles are not well dated when rock units have poor age constraints or their remanences are not established as primary. However, ten key paleopoles, mostly based on coordinated paleomagnetism and U-Pb geochronology, are available for Laurentia in the 1.48-1.42, 1.30-1.23, 1.14-1.09 and 0.78-0.72 Ga intervals. The drift of Laurentia is very poorly constrained in the long gaps between these intervals.
Numerous reconstructions have placed Baltica and Siberia adjacent to Laurentia during the Mesoproterozoic and early Neoproterozoic, although precise locations and orientations vary considerably. They can be tested by comparing paleopoles for Baltica and Siberia with those of similar age from Laurentia. Although the Baltica paleomagnetic data set is less robust than that from Laurentia, there are three key paleopoles between 1.7 and 0.7 Ga. The earliest are at 1.63 and 1.56 Ga, but have no counterparts from Laurentia. However, an important key paleopole match at 1.27 Ga between Baltica and Laurentia is consistent with reconstructions in which Baltica is located adjacent to the present-day eastern Greenland coast of Laurentia with Gothian and Labradorian belts aligned. On the other hand, the postulated breakup, relative rotation and re-amalgamation of Baltica and Laurentia between 1.27 Ga and 1.00 Ga are difficult to confirm from the existing paleomagnetic record because of poor age control.
In the case of Siberia, paleomagnetic tests of reconstructions are difficult because paleopoles are poorly dated and their ages do not match those from the best paleomagnetic data of Laurentia or Baltica. Nevertheless, a preliminary comparison of late Mesoproterozoic data for Siberia and Laurentia suggests that a location of Siberia adjacent to the Arctic Islands of North America, as shown in recently published reconstructions, is permissible.
Post-migmatization (1.17-1.16; 1.09-1.07; 1.06 Ga) intrusive suites in western Grenville, Québec, represent regional markers that reveal the polyphased build-up of eastern Laurentia in Mesoproterozoic times. Felsic and mafic 1.17-1.16 Ga magmas intruded through rheologically contrasting marble, quartzite and quartzofeldspathic gneiss lithotectonic domains. Intrusion style and extent of deformation vary systematically across the orogen, showing a link with tectonic boundaries and crustal rheology. In the mechanically strong gneiss complexes, mafic dykes are non deformed. Their host gneiss, a series of 1.4 Ga island arcs now at granulite facies, comprises the sole record of 1.20 Ga high-P - high-T metamorphic assemblages. In high-strain zones along tectonic boundaries, 1.16 Ga dykes are locally strongly deformed and monzonite-gabbro bodies form trains of sheet-like intrusions concordant with the boundary-zone fabric. The concordant and elongate shape of these bodies results from emplacement, not deformation. As such, the style and site of emplacement requires the tectonic boundaries to exist, and the domains/terranes to be stiched to Proto- Laurentia, at the time of intrusion. Collision of Grenvillian and pre-Grenvillian terranes with proto-Laurentia started at ca. 1.22 Ga, and led to doubling of the crust and high-grade regional metamorphism at an early stage of the Grenvillian orogenic cycle (Elzevirian orogeny). Following orogenic collapse (ca. 1.18 Ga), mafic and felsic magmas ponded and underplated the crust. The crustal architecture of western Grenville in Québec was by that time largely formed. Ascent of magmas though the crust and emplacement of the Morin anorthosite-mangerite-charnockite and Chevreuil gabbronorite-monzonite suites were triggered by and took place during a second, 1.17-1.16 Ga orogenic pulse (Shawinigan) which involved further westward thrusting of accreted terranes. Mid-P - mid-T overprinting associated with this contractional, intraplate reactivation event is strongly partitioned across the orogen and resulted in complex tectono-metamorphic patterns marked by major N-S - trending high-strain zones and associated NE-SW - NW-SE conjugate shears. Extension with reactivation of pre-existing structures followed. The imprint of the ca. 1.06 Ga Ottawan orogeny is only minor as illustrated by the largely pristine nature of 1.09-1.07 Ga K-rich alkaline and 1.06 Ga granite intrusive bodies.
Rodinia was one of the earliest supercontinents, being formed at 1.3-1.0 Ga, and subsequently fragmented at 750-600 Ma. The Siberian craton is thought to have been part of Rodinia, but the evolutionary history of the supercontinent from accretion to dispersion is poorly documented within it. We present new geologic and geochronological evidence of the timing of formation and breakup of Rodinia from the Sharyzhalgai block, which represents the SW margin of the Siberian craton. The Sharyzhalgai block is composed of Proterozoic metamorphic (granulite and amphibolite facies) and magmatic complexes. Although the timing is not yet fully constrained by our data, the main orogenic events which led to the assembly of the block appear to have occurred before 1.4-1.6 Ga.
Mafic dyke swarms are prominent indicators of extensional events in the Neoproterozoic. Diabase dykes range from 1 to 50 m in thickness, and can be traced up to 10-15 km along strike. Trends are sub-meridianal to NW, oblique to the present margin of the Siberian craton. Dyke distribution is highly variable, from rare (<5 dykes/km) to abandant (10 dykes/100 m). Inclination also varies, from gently dipping (conformable with the foliation of the gneisses in which they are enclosed), to subvertical.
The diabases correspond to sub-alkaline basalts geochemically, and are comparable with the mafic volcanics typical of intracontinental rift zones (e.g. Red Sea or Rio Grande rift). At present the age of the dykes is constrained only by geological evidence. Some dykes cut volcanic-sedimentary sequences of the Early Riphean Karagass suite; these dykes and their host rocks are in turn unconformably overlain by Vendian platform cover sediments. Because the dykes have suffered only an extremely low grade degree of sencond alteration, future geochronological and paleomagnetic investigation will allow the timing of dike intrusion and extension to be more precisely constrained.
Currently available geological and geochronological data on the Precambrian complexes of the Arctic structures, as well as the paleoreconstructions of localization of the continents and ancient oceans (Dalziel, 1991; Powell et al., 1993; Condie, Rosen, 1994; Patrick, McClelland, 1995) suggest that in Neoproterozoic there existed the collision-accretion belts of Grenvillian or slightly younger age between the Lavrentia with Greenland, Siberia, Baltica and Barentsia. The presence of such Grenvillian belt between the Eastern Greenland, northern part of the Ellesmere Island from one side, and Baltica with Svalbard, from the other hand, was repeatedly described but its extension on the periphery of Siberia (in a direction of the Angara fold belt on the one hand and the Franklinian fold belt - on the other) either was not discussed because of the absence of data on the Siberian belts or was only anticipated. Three zones are distinguished within the Taimyr folded area: South-, Central-, and North-Taimyr, whose structural and compositional features suggest various geodynamic conditions of their origin. The composition of the South-Taimyr zone sediments reflects the setting of passive margin of the Siberian continent. The North-Taimyr zone is a slope and foot of the Kara continental block, and the Central-Taimyr zone, located between them, has an accretional nature. The northern Precambrian part of Taimyr is considered by some authors as a block which together with similar structures of Novosibirsk Islands, Chukotka Peninsula, Sjuard Peninsula, Canadian Archipelago and North Greenland composed Arctida paleocontinent (Zonenshain, Natapov, 1987). The Central Taimyr accretionary zone formed in Neoproterozoic as a result of the collision of the island arc with blocks of continental masses. This zone includes the Mamont-Shrenk and Faddey gneissic terranes with 850 Ma granites (model Sm-Nd age is 1800-1900 Ma), and island arc units, and ophiolites with 740 Ma plagiogranites (model Sm-Nd age is 785-850 Ma). Neoproterozoic ophiolitic belts are interesting in the contexts of their correlation with similar complexes of the south-west framing of the Siberian Craton and the Arctics. These ophiolites possibly record the opening of the Paleopacific Ocean and the breakup of Neoproterozoic Rodinia supercontinent.The Kara continental block occupies the northern zone of Taimyr. It may be the part of Barentsia (or Arctida, after Zonenshain & Natapov, 1987), which have had collision with Siberian paleocontinent in the Late Paleozoic. Sincollision (autochthonous) granites originated at the boundary of 300 Ma at the expense of continental crust of Grenwillian age - T Nd (DM)=1170 - 1080 Ma.
Dalziel IWD, Geology, 19, 598-601, (1991).
Powell CMcA, Li ZX, McElhinny MW, Geology, 21, 889-892, (1993).
Condie KC & Rosen OM, Geology, 222, 168-170, (1994).
Patrick BE & McClelland WS, Geology, 23, 81-84, (1995).
Zonenshain LP & Natapov LM, Actual problems of continent and ocean tectonics, Moscow, Nauka, 31-57, (1987).
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 a ca. 1450 Magmatic arc 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 Terrain, which includes volcano-sedimentary belts, felsic plutonic-gneiss belts, and intrusive granitoids having TDM of ca. 1.80 Ga (Van Schmus et al., 1999). Southern RSIP in western SWMT includes the Santa Helena Terrain, where undeformed 1.0 Ga granites intrude a Mesoproterozoic basement complex that is locally overlain by sedimentary rocks of 1.3 to 1.0 Ga Aguapei group. The region was deformed during formation of the Sunsas-Aguapei orogenic belt to the west.
Santa Helena Terrain is dominated by the Santa Helena batholith that is bordered to the north and northeast by granodioritic to tonalitic gneisses and to the west by tonalite and metavolcanic rocks. Four granites in the batholith yield U/Pb zircon ages of 1476±35 to 1423±06 Ma. TDM ranges from 1.70 to 1.52 Ga, indicating some older crustal component. Gneisses to the NE yield U-Pb zircon ages of 1450±13 Ma with TDM from 1.56 to 1.49 Ga, indicating that protolith for this gneiss was not RJNP basement. Tonalites to the west yield U/Pb ages of 1488±11 to 1463±04 Ma with TDM from 1.53 to 1.50 Ga. These indicate that their magma was derived from a source containing a very little, if any, older crust. We conclude that the granites, orthogneisses, and tonalites are all components of a 1.45 Ga NW-trending volcano-plutonic arc (Santa Helena Arc). The relatively low TDM for Santa Helena complex suggest that much of it is juvenile crust with variable, often minor, contributions from RJNP to the east. We suggest this arc was developed along a continental margin comprised of the RJNP. Correlation of the Santa Helena arc with terrains in Laurentia is premature, but it is interesting to note that it is similar in age to the Pinwarian terrane in Labrador (Gower, 1996) and to juvenile granitic provinces in the east-central U.S. (Van Schmus et al., 1996).
Gower CF, Geol. Soc. Spec. Pub, 112, 197-218, (1996).
Sadowski GR & Bettencourt JS, Precam. Res, 76, 213-227, (1996).
VanSchmus WR, Bickford ME & Turek A, Geol. Soc. Amer. Spec. Paper, 308, 7-32, (1996).
VanSchmus WR, Geraldes MC, Fetter AH, Ruiz A, Matos J, Tassinari CCG & Teixeira W, J. Conf. Abstr., 4, (1999).
The Borborema Province of NE Brazil comprises the west-central part of a wide Pan-African - Brasiliano continental collision belt that formed during ca. 600 Ma assembly of West Gondwana. Erosion levels to depths of 15 km or more and good exposures offer an excellent opportunity to study the pre-collisional history of this belt. Our geochronologic results over the past decade, along with recent work by others, demonstrate events that created or modified crustal domains at 3.45, 3.2, 2.7, 2.1, 1.7, 1.0, 0.8, and 0.6 Ga. Although NE Brazil contains small Archean domains within Transamazonian basement, the first major event was formation of extensive continental crust about 2.35 to 2.1 Ga, before and during the Transamazonian orogeny, in conjunction with formation of the Paeloproterozoic supercontinent Atlantica. Incipient breakup of Atlantica occurred in NE Brazil about 1.8 to 1.7 Ga, resulting in sedimentation and bimodal volcanism in intracratonic basins.
A more successful breakup occurred before or about 1.1 Ga, resulting in separation of parts of Atlantica from each other, including separation of the Ceará-Rio Grande do Norte (CE/RN) craton from the São Francisco-Congo (SF/C) craton and formation of extensional basins along the northern part of the latter. By 1000 to 970 Ma the CE/RN craton and SF/C craton were reconverging, with development of a magmatic arc more than 700 km long (Cariris Velhos orogen) along the NW leading edge of the SF/C craton and subsequent rejoining of the CE/RN and SF/C cratons. Eastward extension of the 1.0 Ga Cariris Velhos orogen into West Africa is presently unknown. The CE/RN/SF/C land mass may have been part of Rodinia, but its location is not certain at present.
About 850 to 700 Ma several extensional basins developed in NE Brazil, with intraplate bimodal volcanism and clastic sedimentation. This extension may have been part of the global breakup of Rodinia. From 700 to 600 Ma plate convergence among the CE/RN/SF/C craton, Amazonian craton, West African craton, and several other microcontinents (e.g., Nigerian block) culminated in formation of West Gondwana. Accretion of pre-Brasiliano (900 to 700 Ma) juvenile oceanic arcs occurred elsewhere in Brazil and West Africa, but such terranes have not been found in NE Brazil. The continental collisions began about 640 to 630 Ma and peaked about 610-590 Ma in NE Brazil. Collisional tectonism, including transcurrent faulting associated with escape tectonics, continued through much of the Cambrian.
Present debate concerning the late Mesoproterozoic assembly of the hypothetical supercontinent that has come to be referred to as Rodinia, began in the mid-1980's with the suggestion that Laurentia must have broken out of a late Precambrian supercontinent. Key elements of the 1991 papers by Moores, Dalziel, and Hoffman were that the Pacific margins of Laurentia and East Antarctica + Australia might have been juxtaposed prior to the opening of the Pacific Ocean basin, and that opening of the Pacific might have been balanced (on a globe of constant radius) by closing a hypothetical Mozambique ocean basin between East and West Gondwanaland and other Pan African/Brazilide ocean basins within West Gondwanaland. The most notable line of evidence in support of the South-West United States - East Antarctic ("SWEAT") fit put forward by Moores and supported by Dalziel, was that the 1.8-1.6 Ma Yavapai and Mazatzal orogens of Arizona and New Mexico, and 1.2-1.0 Llano orogen of Texas might have counterparts in East Antarctica, and have been truncated by the opening of the Pacific Ocean basin in the Neoproterozoic. Juxtaposition of Laurentia and East Antarctica in early Neoproterozoic times seemed broadly compatible with the existing paleomagnetic data, as was demonstrated by Powell et al. (1993).
Eight years and much debate later, it seems likely to me that the Yavapai-Mazatzal-Llano orogen does indeed have an Antarctic equivalent. The geological story is more complex than initially conceived, but more revealing. Geochemical data emerging from basement rocks collected in the Shackleton Range suggest that a former connection to Laurentia in the vicinity of Arizona and New Mexico is indeed possible (Helper et al., 1996). The Llano orogen, moreover, may have a continuation along the margin of the Weddell Sea in the Maudheim belt that has been mapped by German and South African workers. This now appears, however, to have been a continuation of a Namaqua-Llano orogen resulting from collision between the Kalahari craton of southern Africa and the present southern margin of Laurentia between 1150 and 1120 Ma (Dalziel et al., 1998). The Coats Land nunataks, initially thought to be part of that orogen because of their "Grenvillian" age, were part of a separate crustal block that collided with Kalahari at about the same time. Both Kalahari and the Coats Land block were caught up in the collisional zone that united East and West Gondwanaland at the close of the Neoproterozic, possibly into a Pannotia supercontinent including Laurentia (Dalziel, 1992).
Thus geologic connections between the southwestern United States and East Antarctica still constitute one of the strongest lines of evidence linking Laurentia and the present southern continents in a supercontinent assembled at the close of the Mesoproterozoic.
Dalziel, IWD, GSA Today, (1992).
Dalziel, I, Mosher, S, & Gahagan, L, Geological Society of America, Abstracts with Programs, (1998).
Helper, Met al, Geological Society of America, Abstracts with Programs, (1998).
Powell, C, Li, Z, McElhinny, M, Meert, J, & Park, J, Geology, (1993).
The about 500 km long coastal stretch of central Dronning Maud Land (DML), East Antarctica, is critical for understanding both Gondwana and Rodinia assembly. We report the first extensive geochronological study of magmatic and metamorphic rocks from the area (Jacobs et al., 1998). These new U-Pb SHRIMP zircon and Sm-Nd-data of rocks sampled during the German international GeoMaud 1995/96 expedition indicate that the oldest rocks in central DML are Mesoproterozoic in age. Oldest rocks are a sequence of sedimentary rocks, consisting of metapelites, paragneisses, carbonates and quartzites that are intercalated with metavolcanic rocks. The crystallisation ages of felsic gneisses interpreted as metavolcanic rocks were determined at c. 1130 Ma. Inherited zircon cores are not older then c. 1200 Ma. Syn-tectonic granite sheets and plutons intrude the supracrustal sequence. Zircons from such meta-igneous rocks give crystallisation ages of c. 1080 Ma. Sm-Nd model ages < c. 1.8 Ma indicate that no major amounts of older crust were involved during crust formation. Some rocks record a Grenville-age metamorphism at c. 1080 Ma. The break-up of Rodinia has thus far not been recorded in the study area. Central Dronning Maud Land was strongly overprinted in Pan-African times during collision between E- and W-Gondwana. This event has caused polyphase metamorphism between c. 580 and 515 Ma, preceded by a phase of anorogenic AMCG-type magmatism at c. 600 Ma. In our view central Dronning Maud Land is part of the southern continuation of the Mozambique Belt into East Antarctica.
Jacobs J et al., J. Geol., 106, 385-406, (1998).
The Precambrian basement of central Madagascar underwent a major imprint during the Pan-African Orogeny that was responsible for Gondwana assembly. In a Gondwana reconstruction at 550 Ma, Madagascar appears as part of the East African Orogen between the Indian and Tanzanian cratons. A tectonic synthesis is attempted on the basis of recent structural, petrological and geochronological results. A first event was responsible for the building of a layered middle crust over >120000 km2. This occurred in HT-LP conditions contemporaneous with the emplacement of the conspicuous "stratoid" (sheetlike) granites, that share a similar setting and alkaline nature everywhere, whatever their local names (Emberger, 1958; Nédélec et al., 1995). The country rocks are mainly tonalitic to granodioritic gneisses 800 Ma in age (Ashwal & Tucker, 1997) of arc affinities that question a possible link of northwestern central Madagascar with the juvenile crust of the Arabian-Nubian Shield. Besides, relations of this granitic and gneissic middle crust with overlying supracrustal rocks and nearby mafic complexes will also be discussed at the light of new structural data. Paquette & Nédélec (1998) obtained U/Pb zircon ages of 630 Ma from the stratoid granites north of Antananarivo. This is the age of the tectono-magmatic event (Di) responsible for the very consistent structural pattern of the central Madagascar basement, with foliations dipping moderately to the west and lineations shallowly plunging to the WSW. They suggested that this could result from post-collisional extension in the Mozambique Belt, assuming an earlier collision between Madagascar (at least pro parte) and the Tanzanian Craton.The Di structural pattern has been modified by major shear zones. The Antananarivo "virgation" (flexure) zone is an E-W sinistral shear zone sealed by the emplacement of the Ambatomiranty dykes at 560 Ma (Paquette & Nédélec, 1998). South of Fianarantsoa, the Di structures are changed into steep foliations and NNE trending subhorizontal lineations as an echo of the submeridian strike-slip tectonics recognized in southern Madagascar by Martelat et al. (1995).The main shear zone is the Angavo belt, 50 km in width, that runs broadly N-S along 1000 km in the eastern part of Madagascar. East of Antananarivo, steep foliations and lineations shallowly plunging to the north point to its transcurrent nature and syntectonic charnockites are indicative of HT-LP conditions. All these N-S shear zones are consequent to a major Di+1 event at 570-550 Ma. This strike-slip tectonics reworked central Madagascar during the final stage of Gondwana assembly.
Ashwal LJ & Tucker RD, Terra Nova Abs, 9, 163-164, (1997).
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Paquette JL & Nédélec A, Earth Planet Sci. Lett, 155, 45-56, (1998).
Current models suggest the Rodinia supercontinent to have formed about 1000-1200 Ma ago and to have disintegrated again at about 750 Ma, with most of the continental fragments surrounding Laurentia reassembling at about 550 Ma to form Gondwana (Dalziel, 1997). We have investigated high-grade terrains constituting the Mozambique belt in southeastern Africa (Tanzania, Malawi, Mozambique) and Madagascar, that were apparently situated at the outer margin of Rodinia prior to breakup, to test this simplistic model.Except for northern Mozambique, none of these terrains contain any significant proportion of 1000-1200 Ma (Kibaran/Grenvillian) igneous rocks, raising doubts about their role in the formation of Rodinia. In Tanzania, the peak of granulite-facies metamorphism occurred between 625 and 640 Ma (Möller et al., 1996 and own data), whereas farther S, in Mozambique, peak metamorphic conditions were reached 615 Ma ago (Kröner et al., 1997a). In contrast, high-grade metamorphism in southern Malawi occurred at 550-580 Ma, and in central and southern Madagascar at ~560-565 Ma (Kröner et al., 1997b). These high-grade episodes are generally interpreted to reflect continental collision and amalgamation of Eastern Gondwana during the Pan-African event, although the granulites of Tanzania seem incompatible with such a scenario (Appel et al. 1998). In southern Malawi and Madagascar, abundant calc-alkaline early Pan-African granitoids were emplaced 650-800 Ma ago, and there is no evidence of rifting during this time, as would be expected if these terrains had begun separating from Rodinia. We suspect that most of southeastern Africa and Madagascar was not marginal to Rodinia at 1000-1200 Ma, perhaps not even part of it. Most of the continental fragments now constituting the Mozambique belt probably drifted across the Mozambique ocean in the late Neoproterozoic, thereby developing Andean-type active margins, before becoming involved in collisional events between 640 and 550 Ma ago.
Appel P, Möller A, & Schenk V, J. Metam. Geol., 16, 491-509, (1998).
Dalziel IVD, Geol. Soc. America Bull., 109, 16-42, (1997).
Kröner A, Sacchi R, Jaeckel P & Costa MJ, J. Afr. Earth Sci., 25, 467-484, (1997a).
Kröner A, Pidgeon RJ, Sacchi R & Windley BF, Terra Nova, 9, Abstr. Suppl. 1, 163, (1997b).
Möller A, Mezger K & Schenk V, Eur. J. Min., 8, Beiheft 1, 191, (1996).
The Neoproterozoic geology of the Seychelles is dominated by 750-755 Ma granitoids. Basaltic dolerite dykes are also present, and from one dyke (Takamana) we were able to recover zircons from a coarse grained granophyre in the dyke centre. Four U-Pb isotope dilution analyses give concordant ages and with a weighted mean of 750.2±2.5 Ma. Palaeomagnetic data from the dykes are characterized by northerly declinations with intermediate to steep positive inclinations (Mean declination=011.6, inclination=44.2, a95=6.4; VGP: 57.3N, 075E). These magnetizations partly match earlier reported palaeomagnetic data from the host 750-755 Ma granites, and hence some dolerite dykes and the granitoids appear broadly coeval on both isotopic and paleomagnetic grounds.
In previous studies of the Seychelles rocks, paleomagnetic data were 'rotated' back to Madagascar and then back to Africa placing the Seychelles adjacent to Somalia in a 'traditional' Late Precambrian Gondwana reconstruction. Such reconstruction were motivated by an apparent match between the Seychelles poles and a Late Precambrian pole from Namibia. However, the intrusion age of the Seychelles granitoids (c. 750-755 Ma) and dykes (<=750 Ma) are considerably older than the Namibian pole and clearly demonstrate that the Seychelles granitoids and dolerite dykes pre-date Gondwana assembly (c. 550 Ma). As a starting point we tested some published Seychelles-India euler poles in order to compare the Seychelles data with the 730±10 Ma Malani dolerite pole from NE India. With the existing euler data it is quite clear that the Seychelles dolerite dyke pole plots near the Malani pole, and along the oldest segment of the c. 720-530 Ma East Gondwana apparent polar wander path. This lends support to the proposed Neoproterozoic magnetization age of the Seychelles dykes, but if we accept an approximate, similar magnetization age for the Malani and Seychelles dolerites (750-730 Ma), the match is considerably improved with an euler pole of latitude=25.8 and longitude=7.8, and a rotation angle of angle of 32.1.
During the Neoproterozoic most continents have been suggested to exist as part of the Rodinia Supercontinent, which may have formed at c. 1100-1000 Ma. We place Seychelles-India-Madagascar at 15-30N, and as outboard continental terranes of West Rodinia at c. 750 Ma. The 750 Ma palaeoposition of Seychelles marks the incipient birth of this microcontinent.
Neoproterozoic dyke swarms in Australia and mafic intrusives in western North America have been interpreted as part of a giant plume-related radiating dyke swarm formed just before the breakup of supercontinent Rodinia, with the plume centre located just east of the Adelaide Fold Belt (Park et al., 1995). However, as pointed out by Wingate et al. (1997), the age difference between the Australian dyke swarms (827±6 Ma and 824±4 Ma for the Gairdner and Amata swarms, respectively) and the ca. 780 Ma mafic intrusives in North America makes them unlikely to be part of the same magmatic episode. On the other hand, if the Australian dyke swarms indeed represent a mantle plume, and if South China was located just east of the Tasman Line, as postulated in the Rodinia configuration of Li et al. (1995), South China would have been very close to the plume centre, and thus one would expect to find ca. 825 Ma mafic intrusives there.
There are widespread ca. 825 Ma granites around the Yangtze craton of South China Block, but few mafic rocks of that age have been reported previously. We present here SHRIMP zircon ages of four mafic intrusives in the northern Guangxi region that co-exist with ultramafic intrusives. The mafic/ultramafic intrusives appear as WNW-trending dykes which intruded the Mesoproterozoic Sibao Group, but which are truncated by both ca. 825 Ma granites and overlying Neoproterozoic volcanoclastic rift successions. Euhedral magmatic zircon grains from all four dykes give concordant U-Pb ages of 830--820 Ma, with an overall mean of 828±7 Ma. North-south trending dykes of possibly similar age occur in western Yangtze craton but these have not yet been precisely dated.
The 828±7 Ma age for the mafic/ultramafic intrusives in South China is identical to the 827±6 Ma and 824±4 Ma ages from the Australian dyke swarms, thus making the South China dykes strong contenders for the missing arms of a ca. 825 Ma giant plume-related radiating dyke swarm that formed just before the breakup of Rodinia. The widespread ca. 825 Ma granites are possibly co-magmatic with the mafic/ultramafic intrusives, although they often intrude at a slightly later time, and might reflect extensive crustal melting at the plume-head. The erosion that exposed some of the mafic/felsic intrusives prior to the onset of rift deposition could reflect doming above the plume-head and/or uplift of the rift shoulders.
If this interpretation is correct, it would give further support to the proposed South China-Australia connection in Rodinia, and would also constrain the starting time for the rifting that eventually led to the breakup of Rodinia to be ~825 Ma.
Li ZX, Zhang L & Powell CMcA, Geology, 23, 407-410, (1995).
Perk JK, Buchan KL & Harlan SS, Earth Planet. Sci. Lett, 132, 129-139, (1995).
Wingate MTD, Campbell IH, Compston W & Gibson GM, Precamb. Res, 87, 135-159, (1998).
Located north of Gondwanaland in a Pangean reconstruction, Armorica is one of the 18 pieces, actually recognized, which detached from this continent in the Lower Paleozoic. After a tentative delineation of its original boundaries using geological and geophysical data its migration during Paleozoic time is discussed. Generally speaking, all these pieces drifted " northward" across the Iapetus Ocean. The study of the Gondwanan Paleozoic rotation poles also suggest that this continent suffered, in addition to its well known "northward" migration, a 50° clockwise rotation around its own vertical axis located in the Gulf of Guinea. As a consequence, the pieces detached from Northern Gondwana (such as Armorica) between the fragmentation episode (Early Ordovician) and the final re-amalgamation stage to Laurentia (Devono-Carboniferous time) suffered a cinematic history which is different from that of Gondwana. This rotation explains why there are so many difficulties to correlate the Panafrican-Avalonian-Cadomian structures of the drifted blocks, with these of Gondwana in a Pangean configuration. A Gondwanan mother country for Armorica, is proposed. After this restoration, the Panafrican-Avalonian-Cadomian structural trends known in western Europe, Eastern Canada and Northern Africa accurately link each other. These correlations are reinforced by the Upper Proterozoic geochronological data actually published. Our reconstruction is also in agreement with the location of the threshold which separated during the Lower Ordovician, the Arabic-Chinese-Iranian shallow platform from the deeper Armorican shelf. This restoration shows why the Chinese trilobites partly colonized Armorica and the Northern part of Gondwana but not its Northwestern border. There are consequently no reason to try to correlate, faunas, structures, radiometric ages, and paleomagnetic data between Armorica and Northern Gondwanaland in the usual Pangean reconstruction.Starting with this Upper Proterozoic reconstruction one can observe that it exists large Pre-Panafrican cratons now squeezed inside the restored Cadomian-Avalonian-Panafrican network which are completely ignored in the usual Rodinia reconstructions. This is mainly because they are covered by Upper-`Proterozoic or Paleozoic formations and don't crop out. These pieces which resulted from the fragmentation of Rodinia must be incorporated in any Rodinia reconstruction. Some of these hidden Rodinia pieces are only known because they geometrically predate the Panafrican orogens but others have been indentified in rare and isolated boreholes .
The Mesoproterozoic Tsodilo Hills Group is a new lithostratigraphic unit in NW Botswana (Key, 1998) identified in the area where only Neoproterozoic sediments, correlatives of the Damara belt of Namibia, were previously recognised (Carney et al., 1994). The deposits form a fining-upwards succession: from quartzite-dominated complex to siltstone with ironstone to carbonates. The quartzitic interval meets the citeria required to define it as a formation, which prompted the present author to propose the Male Hill outcrop as a stratotype of a new unit, the Male Hill Formation.
The Male Hill Formation, ca. 500 metres thick, is affected by minor thrust faults, shear zones and a pronounced trust plane with indicators of tectonic transport toward SW. The sedimentary features are well preserved there where foliation is less penetrative. The succession is characterised by light grey to whitish quartzites enriched in heavy minerals, micaceous quartzites and mica schists an association tentatively correlated with the Irumide belt clastic units. Their protoliths are interpreted as: (1) well sorted quartz sandstones, (2) sandstones interbedded with shales and muddy sandstones, and (3) mudstones. Conglomeraties are subordinate. The sedimentary facies associations and geometry of lithosomes suggest that the deposition occurred on tidally influenced shelf as a result of aggradation of offshore sand ridges. The ongoing deposition was interrupted by three regressive events. Two of these are marked by two complexes, each less than 2 metres thick, with layers of red sandstone and siltstone, mud-draped mega-ripples, mudstone and sedimentary breccias. Their enrichment in phosphorous minerals (Wendorff & Vink, in prep.) is here considered as a result of upwelling of deep marine waters, related to the sea-level changes. The third regression was associated with deposition of thin but widespread conglomeratic complex due to an increased supply in the terrigenous material, and with a change in the palaeocurrents pattern. The subsequent pulse of subsidence counterbalanced this forced regression and re-established the previous conditions.
The above observations not only document the results of interplay between tectonics, sedimentation and upwelling in the Mesoproterozoic marine basin opening at the margin of the Kalahari-Zimbabwe Craton, but also provide sequence stratigraphy criteria for regional correlation of its sedimentary record.
This paper is a contribution to IGCP Projects No 418 and 419.
Carney NJ, Aldiss DT & Lock NP, The Geology of Botswana. Geological Survey of Botswana Bulletin, 37, 113, (1994).
Key RM, Geological Map of the Republic of Botswana (1:1,500,000), Geological Survey of Botswana, (1998).
Proterozoic lithospheric evolution of southern Africa was characterised by the accretion of orogenic mobile belts around Archean cratonic nuclei (Congo and Kalahari Cratons). Major tectonothermal and/or magmatic events resulted in the formation of the Eburnian Kheis Belt (Paleoproterozoic), the Kibaran Namaqua Natal Belt and Sinclair Sequence (Mesoproterozoic), and the Panafrican Damara Belt (Neoproterozoic). Whereas the Paleo- and Neoproterozoic events of this region are relatively well constrained, the genesis of the late Mesoproterozoic Sinclair Sequence is still under debate. Two contrasting plate tectonic models for the evolution of the Sinclair Sequence have been proposed: i) a continental rift model, based on structural, sedimentological and geochemical evidence (Kröner, 1977; Borg, 1988), and ii) a collision-related model, based on geochemical signatures and isotopic age data (Watters, 1976; Ziegler & Stössel, 1993; Hoal & Heamann, 1995).
The current abstract presents results from a geochemical study of felsic intrusive and extrusive suites, exposed north of the Naukluft Mountains in central Namibia. These igneous suites are considered to be equivalent to the Sinclair Sequence. The studied units include the Piksteel Granodiorite Suite, syeno- to alkali-feldspar-granites of the Gamsberg Granite Suite, felsic volcanics of the Nückopf Formation, and felsic dikes which have intruded the latter two units.
The rocks from these magmatic suites display distinct enrichment of LILE, as well as low Nb/Th- and Ta/Th-ratios in MORB-normalised diagrams, which strongly suggest the formation in a magmatic arc setting. Parental magmas have probably been derived from wet partial melting of either subducted oceanic crust or a peridotitic mantle wedge and have been subjected to subsequent fractional crystallisation. The Piksteel granodiorite suite, with its comparatively least evolved geochemical signatures, has to be attributed to this early stage. Trace element and REE patterns suggest a continuous magmatic evolution from the relatively primitive Piksteel granodiorites to the more evolved Gamsberg granites, and finally to the highly evolved Nückopf volcanics and felsic dikes. There are no known andesitic, volcanic equivalents to the Piksteel granodioritic stage but such rocks might have been removed due to the subsequent erosion of higher crustal levels.
Borg G, Prec Res, 38, 75-80, (1988).
Hoal BG & Heamann LM, Communs Geol Surv Namibia, 10, 83-91, (1995).
Kröner A, Prec Res, 4, 163-213, (1977).
Watters BR, Nature, 257, 471-473, (1976).
Ziegler URF & Stössel GFU, Geol Surv Namibia Mem, 14, 106 p, (1993).
Five biotite granite plutons, namely Rwentobo, Kamwezi, Ntugamo, Chitwe and Chabakonzo in SW Uganda have investigated to determine the precise crystallisation age of the intrusions in order to reconstruct the magmatic evolution of this area. These granites occur in the study area within a terrane of highgrade metamorphic gneisses and migmatites, overlain by low-grade metapelites. The granites belong to the Karagwe - Ankolean (K-A) System. The K-A System is a sedimentary succession with a thickness of ~8000 m and which deposited during an extensional phase (ca. 1400 - 1300 Ma) while the deformations occurred during a compressive phase resulting from a possible collision event outside the study area. Two differnt granite types can be distinguished: (1) biotite granites which had been intruded mostly syntectonically and (2) pegmatite granites and associated tin-bearing veins which developed post-tectonic signature. The biotite granites have a similar major and trace element compositions reflecting a pure by calcalcaline character. The SiO2 content varies between 65 and 77 wt.%. Based on the classification of De La Roche (1980) the granitoid rocks span a field between granodiorites and predominantly monzo- and syenogranites. In using the usual discrimination parameters, all the collected samples are peraluminous and of I-type character. In order to develop more precise ages, single zircon dating had been used for the reconstruction of the mineralization history. For this purpose, long prismatic, brown to light brown, transparent zircon grains with a c-axes length of up to 250 µ m and a length/width ratio of 2.0 to 2.5 have been selected for dating by single grain evaporation method of Kober (1986). Cathodenlominicense has shown that all grains are without any cores and inclusions. The obtained ages range between 1365 and 1299 Ma. They can be interpreted to be representative for the age of the main phase of the Kibaran orogeny. Additionally, a much younger age was obtained being as young as 450 Ma. This age most likely reflects new crystallization of zircons caused by hydrothermal activity in this area and falls within Pan-African activity.
De la Roche et al., Chem. Geol., 29, 183-210, (1980).
Kober, Mineral. Petrol., 93, 482-490, (1986).
The assembly of Rodinia involved most of the world's continental masses and undoubtedly had a profound effect on the global exogenic system. Recent and ongoing studies of secular change in carbonate geochemistry indicate that Mesoproterozoic orogenesis significantly affected the carbon cycle, crustal weathering patterns, and perhaps the oxidation state of the atmosphere and biological evolution, effectively ending a long period of apparent global biogeochemical stasis.
New data from Mesoproterozoic sucessions in Laurentia and Siberia allow construction of a radiometrically calibrated carbon isotopic curve that extends back 1600 m.y. Early Mesoproterozoic successions worldwide typically record relatively stable C-isotopic compositions near 0‰, suggesting global biogeochemical stasis. Marked transitions to more positive carbon isotopic compositions occured at ~1250 Ma (to +3.5‰) and again at ~800 Ma (to >+5‰). Rather than a short-lived excursions, these transitions appear to represent prolonged changes in marine isotopic compositions and can be temporally linked to Rodinian orogenic events via marine Sr isotopic compositions. 87Sr/86Sr was exceptionally low in the early Mesoproterozoic, rose briefly to values >0.7070 between 1250-1300 Ma, again fell to values ~0.7055 by 1200 Ma. Marine Sr isotopic values remained non-radiogenic through much of the rest of the Mesoprotoerozoic, suggesting a significant involvement of juvenile crust during Rodinian assembly and/or high crustal production rates.
Tectonically driven changes in Mesoproterozoic carbon cycling may have had profound implications for the Earth's biogeochemical evolution. The shift to moderately positive 13C values indicates an increased flux of organic carbon burial, which may have resulted in an increase in atmospheric pO2. Notably, the timing of these events corresponds closely with the first apprearance of acanthomorph acritarchs and multicellular algae in the fossil record, suggesting that the eukaryotic "big bang" began in the Mesoproterozoic and may have ultimately been driven by supercontinent assembly.
Studies of precambrian structures of southwestern margin of Siberian platform performed in northwestern part of East Sayan indicate the presence of complexes related to the different stages of oceanic subduction system evolution:
1. Young island arcs, which reflect initial stages of ensimatic arc development and consist mainly of island-arc tholeiites and tonalite-trondhjemites (south part of Shumikha complex and east segment of Arzybej block).
2. Evolved island arc complex formed on primitive sialic basement and made up of differentiated calc-alkaline series and greywackes. (west segment of Arzybej block).
3. Fragment of back-arc basin includes high-Ti ferrotholeiites and oceanic plagiogranites (north part of Shumikha complex).
Strong domination of tonalite-trondhjemites on calc-alcaline rocks with K-enrichment trend is distinctive feature of young island arc complexes. Almost all acidic rocks found there are strongly enriched in Na, Al, Sr, depleted in Rb, Th, Y and have elevated La/Yb. Petrologic estimations of melting conditions yield P>16 Kbar for Shumikha and P>22 Kbar for Arzybej tonalite-trondjemites and T~1000-1020oC. As noted by Drummond and Defant (1990), such magmas are most likely produced by melting of relatively hot and young (<20 Ma) subducting slab. Thus, Shumikha and Arzybej island arcs might form in the vicinity of spreading center (<450 km for Sumikha and <600 km for Arzybej arc; estimated using Peacock, 1990). This, in turn, probably reflects the relatively small dimensions of oceanic basin.
High-Ti ferrotholeiites are the only metabasalts distinguished in reconstructed back-arc basin. The generation of such melts is usually related to diffused spreading of oceanic crust, i.e. to initial stages of rift evolution. Since Mg-basalts, which follows ferrotholeiites as rift evolves, are not found in Shumikha back-arc basin, the lifetime of back-arc spreading center could not be long. Also taking into account the extremely rare occurrences of K-enriched calc-alcaline rocks in related island-arc complex, the quick termination of subduction system activity can be proposed.
The minor isotopic Pb-Pb and U-Pb data indicate that last metamorphic event occurred 605 Ma ago in Shumikha block and 870 Ma ago in Arzybej block. The age of formation of complexes is thought to be not older than 1800 Ma. The data shown above suggest that there were probably a series of short-lived oceanic island arcs at the southwestern margin of the Siberian craton in the Mezoproterozoic which developed in relatively small oceanic basins. These arcs could than serve as a primitive sialic basement for more evolved blocks such Derbina microcontinent (Turkina, 1997) or ensialic arcs.
Drummond MS, Defant MJ, J. Geophys. Res, 95, 21503-21521, (1990).
Peacock SM, Science, 248, 329-337, (1990).
Turkina OM(in Russian), Geologiya and Geophysika, 38, 1192-1201, (1997).
The nunataks of Jutulsessen (72°00', S 2°40' E) in Gjelsvikfjella consist mainly of migmatitic and banded gneisses which are locally orthopyroxene bearing. This gneiss complex is intruded by numerous granitic dykes of which a swarm of aplites is the latest phase. The aplite dykes are alkali granitic and are calcic according to Peacock's alkali-lime index. They are characterised by LREE enrichment (230 ppm La) and by a La/Yb ratio of 150.
In addition, the gneiss complex is intruded by a plutonic suite including the calc alkaline Stabben syenite and a body of melagabbro. The Stabben syenite is a cylindrical intrusion with a diameter of 0.3 km that rises as a tower several hundred meters above the ice cap. Phenocrysts of alkali feldspar with minor amounts of plagioclase and interstitial quartz make up the felsic phases. The mafic minerals are biotite, calcic amphibole and locally clinopyroxene. REE rich minerals including chevkinite, monazite and allanite are present. It is LREE enriched (200 ppm La) and has a La/Yb ratio of 50. According to Shand's index the syenite is metaluminous. The syenite intrusion has according to Whalen (1987) an A-type signature and plot between volcanic arc granites and within-plate granites in the discriminant diagrams of Pearce et al. (1984). Due to calc-alkaline composition, the metaluminous signature and the calcic amphiboles, the Stabben intrusion is suggested to have intruded through strongly attenuated continental crust.
U-Pb zircon age determination by the single grain evaporation method on the aplite dykes and the Stabben syenite yield ages of 495±15 Ma and 501±19 Ma, respectively. These ages date the emplacement of the dykes and the pluton and provides a minimum age of the granulite facies migmatic event and simultaneous deformation.
The aplite dyke and the gneiss complex have different protoliths. <epsilon>Nd data suggests that the source of the aplite dykes and the gneiss complex separated from a depleted mantel source at 1600 Ma and 1100 Ma, respectively.
The Stabben syenite form the western part of a granitoid igneous suite that include fayalite granites. This suite has been mapped eastwards from Gjelsvikfjella via Mulighoffmann fjella to Filchnerfjella for a distance of 200 km. The Rakekniven syenite in Filchnerfjell has yielded a U-Pb single zircon evaporation age of 521±8 Ma. This suggest that the whole granitoid suite is Pan-African in age and that this part of Dronning Maud land is part of the East Antarctica Mobile Belt and a possible continuation of the Mozambique Belt as suggested by Jacobs et al. (1998).
Jacobs J, Fanning MC, Henjes-Kunst F, Olesch M. & Paech H-J., The Journal of Geology, 106, 385-406, (1998).
Pearce JA, harris NB & Tindle AG., Journal of Petrology, 25, 956-983, (1984).
Whalen JB, Currie KL & Chappell BW, Contribution to Minaralogy and Petrology, 95, 407-419, (1987).
Backstripping tectonic subsidence analyses of the Australian interior basins, was applied to unravel the timing of Neoproterozoic breakup of a Proterozoic supercontinent (Lindsay et al., 1987). The same approach in conjunction with Proterozoic petroleum occurrences, can also be applied to Paleoproterozoic supercontinental fragmentation and amalgamation. The evolution of Purana Basins of India and McArthur Basin of Australia, was related to an episodic breakup of a paleoproterozoic supercontinent(?). The Purana Basins of India are almost undeformed and unmetamorphosed Proterozoic supracrustal sediments, occurring in seven independent basins, the Cuddapah (1.8 Ga), Vindhyan, Chattishgarh, Bastar, Pranhita-Godavari, Bhima and Kaladgi Basins, which altogether occupy more than 20% area of Precambrian exposures in the Indian Peninsular Shield. The aggregate sedimentary thickness in each basin vary from 0.5 to 9 km, and contain predominantly orthoquartzite-shale-carbonate-evaporite suites. Prolific stromatolitic and microbial organic life was recorded from Cuddapah and Vindhyan Basins (Kale and Phansalkar, 1991). Gaseous hydrocarbons from Vindhyan Basin and bitumens from Cuddapah Basin are also reported from Riphean strata (Chandra et al., 1998). Interestingly, the oldest live oil reported is from Barney Creek Formation (1.7 Ga) of the McArthur Basin, Australia (Jackson et al., 1986).
Organic life evolution especially microfossils document a major transition in the biological world near Meso-Neoproterozoic (Riphean) boundary (Knoll and Sergeev, 1995). This change is most evident in the diversification of eukaryotic fossils (Knoll, 1992), but a shift in Prokaryotic assemblages is also observed (Sergeev et al., 1995). The rapid changes in the evolution of organic life resulted in the accumulation of large masses of bacterial/biogenic organic matter in sediments. The Precambrian source rocks thus developed in Australia, China and India(?), have high oil-generative potential. The lacustrine Barney Creek Formation and marine Velkerri Formation of McArthur Basin are the most ancient oil source rocks.
The burial of organic matter, source rock development and maturation events are considered as global events. Hence, the Proterozoic hydrocarbon occurrences may be explained based on global palaeo- supercontinental cycles. According to the recent Proterozoic supercontinental reconstruction, there was a supercontinent around 1.9 Ga, consisted of Northwest Laurentia, East Antarctica, Australia and India (Hoffman, 1991; Myers, 1990). The major trend of Earth's Riphean history is represented by the breakup of this supercontinent and amalgamation of another supercontinent (Rodinia) at 1.1 Ga. The breakup of the Pre-Rodinian and Rodinia supercontinents produced large intra platformal depressions and interior cratonic basins by riftogenesis during Paleo- Mesoproterozoic. This process was wide spread in Australia and India. The Cuddapah Basin and the McArthur Basin are one of the ancient Riphean sedimentary basins developed during supercontinental fragmentation and amalgamation. Thus detailed studies on Purana Basins of India and McArthur and other Proterozoic basins of Australia, may improve our understanding on the evolution of Paleoproterozoic supercontinent.
It has recently been demonstrated that the 1.1 Ga mobile belt of southern Africa (the Namaqua-Natal Metamorphic Province), continued eastwards in Gondwana through the Falkland microplate, on (probably via the Haag Nunatak block) into Western Dronning Maud Land, East Antarctica (the Maud Province) and thence northwards to the southern part of the Mozambique Belt (Lurio-Manama belt). This Namaqua-Natal-Falkland-Haag-Maud-Southern Mozambique belt makes up a major (~2000 x 400 km) segment of the network of 1.1 Ga (Grenvillian) mobile belts which constitutes one of Earth's most extensive orogens. The Grenvillian belts represent the collision suture zones which resulted from the amalgamation of the Mesoproterozoic supercontinent of Rodinia, the configuration of which remains a subject of intense debate.
In the original SWEAT reconstruction of Rodinia, the Grenville Front in Laurentia (in Mexico) continues into East Antarctica, thus forming a possible piercing point between Laurentia and East Antarctica. New palaeomagnetic data from Coats Land (East Antarctica) show that the Namaqua-Natal and Maud Provinces were coherent at 1.1 Ga. This means that the Maud Province became part of the Kalahari plate and did not become part of East Antarctica until Lower Palaeozoic times (c.f. the original SWEAT reconstructions which placed the Maud Province near SW Laurentia). Recent Rodinia reconstructions position the evolving Kalahari Craton on the opposite (SE) flank of the Grenville Orogen. Consequently, the Namaqua-Natal-Falkland-Haag-Maud-southern Mozambique belt should also exhibit many features in common with the Grenville Province. In central Africa, the Kibaran-Irumide intracratonic belts, which did not even appear on the early Rodinia reconstructions, were evolving at approximately the same time. As an aid to understanding the assembly processes and configuration of Rodinia therefore, we compare the geological features of the NNFMM belt with that of the Grenville Province on the one hand and the Kibaran orogen on the other.
Reconstructions of the eastern Laurentian margin of Rodinia at the end of the Mesoproterozoic suggest a northern extension of the Grenville-Sveconorwegian orogenic belt through the East Greenland Caledonides (Park, 1992). Until recently only limited isotopic evidence for late Mesoproterozoic/early Neoproterozoic collision between Baltica and Laurentia had been recorded in east Greenland due to extensive reworking of the belt during the 450-400 Ma Caledonian Orogeny. New U-Pb SIMS zircon ages from the Krummedal supracrustal sequence of the East Greenland Caledonides presented here, combined with similar ages from elsewhere in East Greenland (Strachan et al., 1995) and Svalbard (Gee et al., 1995), now verify the existence of a major tectonothermal event on the eastern margin of Laurentia at 950-890 Ma.
The late Mesoproterozoic Krummedal supracrustal sequence comprises a thick succesion of variably deformed and metamorphosed metasediments and contains abundant deformed granitic sheets and plutons. Whereas the majority of these plutons are Caledonian in age, several deformed Neoproterozoic bodies have been recognised. Krummedal metasediments give SIMS 207Pb/206Pb zircon ages of 938±39 Ma (Forsblad Fjord) and 943±15 Ma (Eremitdal), while an augen gneiss from Bartholin Nunatak gives an age of 934±9 Ma. A kyanite- and garnet-rich migmatite from Jaettedalen, Louise Boyd Land contains both Neoproterozoic (891±8 Ma) and Caledonian (206Pb/238U ages - 437±11 Ma) zircon rims on rounded Mesoproterozoic detrital zircon cores. We suggest that these 940-890 Ma ages, coupled with petrographic evidence for a pre-Caledonian low pressure/high temperature extensional event preserved in this migmatite, record melting and metamorphism occurring in response to post-orogenic collapse rather than to crustal thickening.
The Neoproterozoic ages presented here postdate all Grenville-age tectonothermal events in N. America, Canada and Labrador, but are synchronous with post-collisional collapse, high grade metamorphism and post tectonic granite emplacement in the Sveconorwegian orogenic belt which occurred in response to rifting of the S. American plate from southern Baltica (Park, 1992). If the early Neoproterozoic ages in the East Greenland Caledonides date post-orogenic collapse of the northern extension of the Grenville/Sveconorwegian belt then Baltica must have rifted from northeastern Laurentia at c. 950-890 Ma, approximately the same time as South America rifted away from southern Baltica. A period of Rodinian dispersal may therefore have occurred in early Neoproterozoic times, prior to the development of orogen-parallel sinistral motion of Baltica northwards along the eastern Laurentian margin. Neoproterozoic ages from the Scottish Moine suggest that Baltica and Laurentia had reconverged by 820 Ma (Rogers et al., 1998), or even earlier if the c. 870 Ma age from the Sgurr Beag Nappe (Friend et al, 1997) represents renewed convergence rather than extension.
Friend CRL, Kinny PD, Rogers G, Strachan RA & Paterson BA, Contrib. Mineral. Petrol, 128, 101-113, (1997).
Gee DG et al, Precamb. Res, 70, 215-234, (1995).
Park RG, Geology, 20, 725-728, (1992).
Rogers G, Hyslop EK, Strachan RA, Paterson BA & Holdsworth, RE, J. Geol. Soc. Lond, 155, 685-696, (1998).
Strachan RA, Nutman AP & Friderichsen, JD, J. Geol. Soc. Lond, 152, 779-784, (1995).
In the framework of the Volga-Urals region (VUR) in the Early Riphean was formed shallow sedimetary basin with asymmetrical alloction of the facial belts. The form and dimensions of this basin were similar probably to present contours of the VUR. The lowermost levels of the Lower Riphean are represented here by the silicoclastic sublittoral, littoral and subterrestrial deposits, whereas midle and upper levels combine shallow and moderately deep marine terrigenous-carbonate sediments. The Middle Riphead deposits comine terrigenous shallow marine and subterrestrial depostis on the western zone of the basin, whereas for its eastern parts shallow marine carbonate rocks are more characteristic. The Upper Riphead sedimentary sequence is represented in the lower part by alluvial sandstones and conglomerates; its middle and upper levels comine shallow marine terrigenous rocks with glauconite and carbonate deposits.
On the Midle and North Urals only Upper Riphean sedimentary complexes are known. It combine predominantly shallow marine terrigenous associations. On the Timan-Pechora region (TPR) for the end of the Middle Riphead two main zones of sedimentation were reconstructed - shelf basin with the shallow marine sedimentation of the west and continental slope one on the east. This sonation was rather characterisitic for the Late Riphead also. marine sedimentation on the west and continental slope one on the east. This zonation was rather characteristic for the Late Riphean also.
On the Kola peninsular the Lower and Middle Riphean deposits are located in small riftogenous basins. The Upper Riphean and Vendian deposits have got more wide distribution on the north part of the peninsular. During the Middle Riphean the sedimentation took place in the intracratonic basin. The Late Riphean basin had pericratonic character and was connected on the west with Varanger basin (?) and the so-called "Timan basin" on the east (?). It was supposed (Siedlecka, 1985; Norwegian-Russian..., 1993) that on all this territory two principally different types of the sedimentary basins are contacted: shallow marine shelf basin on the south-west and turbidite continental slope basin on the north-east.
So, the Early and Middle Riphean sedimentary sequences of the western megazone of the Urals and adjacent parts of the Russian platform were formed in isolated basins with different paleogeographic and paleotectonic styles. This conclusion is in a rather good agreement with the ideas of the predominance during the Early and Middle Riphean the processes of the within(intra)plate tectonics (Khain, Bozhko, 1988). Only at the beginning of the Late Riphean the great pericratonic sedimentary basin was formed on the eastern, north-eastern and north parts of the EEP.
This investigation was founded by RBRF (grant 97-05-65107).
Siedlecka A., Geol. Surv. Finl. Bull., 331,, 175-185, (1985).
Khain V, Bozhko N., Historical geotectonics. Precambrian. Moscow, Nedra., 382, (1988).
The Prince Charles Mountains (PCM) straddle the boundary between two major tectonic provinces identified in the Lambert-Amery Glacier area: the Archaean to Palaeoproterozoic Ruker Terrain in the south, and the Meso- to Neoproterozoic mobile belt in the north. Their relationship remains obscured by the lack of comprehensive studies of that relationship in the transitional area of the central PCM. We present new field data, geochemical and geochronological data which provide an insight into the geological structure and evolution of this part of East Antarctica.
The central PCM are mainly composed of metamorphosed and deformed volcanic sequences and intrusive bodies, with smaller volumes of post-tectonic granites and mafic dykes. Six tectono-thermal episodes have been identified by U-Pb zircon, Sm-Nd and Rb-Sr data: 1) formation of mafic to felsic volcanic rocks derived from subduction-modified mantle sources, depleted MORB-like or enriched subcontinental mantle sources, layered gabbro and tonalite to granodiorite intrusions at ca. 1300 Ma; 2) deformations, granite intrusion, and Alpine-type peridotite at ca. 1200 Ma; 3) amphibolite-facies metamorphism at ca. 1110-1115 Ma, 4) granites and thermal overprint, possibly amphibolite-facies recrystallization at ca. 1000-1050 Ma, 5) thermal overprint at ca. 810 Ma, 6) thermal overprint at 500-650 Ma. Most metavolcanic and early igneous rocks were derived in oceanic arc or continental arc environments at an active continental margin. The central PCM probably forms a major suture between two crustal blocks (Ruker Terrain + Vestfold Block and Enderby Land + India + Proterozoic terrains) which appeared during the closure of an ocean basin which probably existed between the two blocks during the Mesoproterozoic until ca 1200 Ma, when the effects of a continent-continent collision were felt in the northern PCM terrain.
The age of volcanic activity in the PCM corresponds with mafic dyke emplacement ages reported from the Vestfold Block. Generally the NE-SW direction of volcanic rock strikes coincides with the youngest Vestfold Block dykes. Thus, compression and extension vectors in the central PCM and Vestfold Block, respectively, should have been roughly coeval and co-linear. Assuming the N-S or NNE-SSW continental margin trend roughly coincided with the contemporary strike of the Lambert - Amery glacier rift system, we suggest that a NE-SW trending mid-ocean ridge was submerged beneath the northern portion of continental block, while oblique subduction occurred in the central PCM at ca. 1400-1300 Ma. The on-going movements caused anticlockwise rotation of the Vestfold Block, reflected in the present-day dyke swarm strikes. The Neoproterozoic thermal overprints and granite formation may reflect other collisional events or magmatic underplating within a compressional metamorphic belt. Thus it is most likely that in this part the Rodinia supercontinent must have been formed by ca. 1200 Ma and experienced mostly compressional tectonics at its dispersal in the Neoproterozoic.
Aeromagnetic studies have a profound impact on our knowledge of the Precambrian provinces of the East Antarctic Shield. The dominant sources of magnetic anomalies within the shield are the Precambrian basement rocks. These basement rocks are difficult to study by direct methods due to ice-cover and sparse outcrops. However, these regions of Precambrian crystalline-rock outcrop are critical because they provide benchmarks from which geological information can be extrapolated using regional magnetic anomaly data. On the basis of characteristic anomaly wavelength and amplitude a number of magnetic patterns attributed to different crustal units are distinguished. A magnetic low over the Princess Martha Coast and linear short-wavelength magnetic anomalies over the Ahlmannryggen-Borgmassivet area correspond to the Archaen to Mid-Proterozoic Grunehogna Province.
A broad linear belt of magnetic highs and lows observed over the H.U. Sverdrupfjella and Kirwanveggen is associated with a Middle- to Late Proterozoic metamorphic belt (Maudheim Province). The Coats Land (CL) area exhibits a broken anomaly pattern with isolated short-wavelength highs. In the south it is bounded by the prominent Shackleton Range (SR) magnetic anomaly and on the west by the Druzhnaya Anomaly. Our interpretation contradicts the suggestion that the CL area is a continuation of the Mazatal-Yavapai-Grenville provinces of North America (Dalziel, 1991) due to the inconsistent structural pattern of magnetic anomalies within the proposed orogenic belt. Results of our study neither contradict nor support a postulated connection of the Kaapvaal-Zimbabwe Province into western Dronning Maud Land (WDML) but certainly indicate that the CL crustal block has never been part of the African craton. It appears instead to be typical of the East Antarctic shield. Bounding character of magnetic anomalies in the neighboring terranes (Maudheim Province and SR) relative to the above mentioned crustal block strongly indicate existence of ancient cratonic fragment in Coats Land. Magnetic anomalies related to the Ross orogenic event are very limited in WDML and not discernible in the SR area. This, together with the geologic observations, allow to admit that Ross-age suture resulting from Gondwana coalescence is not presented in WDML and CL. It must lies southwards of the current exposures of the SR or at least in the Shackleton Range. The tectonic evolution of WDML from Late Proterozoic to Cambrian time does not appears to reflect a period of major continental reorganization in that area, but a less reactivation along the structures arisen during the preceding Grenville orogeny at 1100 Ma. Magnetic anomalies associated with fragmentation of Gondwana during the Mesozoic are widespread and varied in form, size and length. The causative sources are predominantly igneous rocks. Structurally, they are depicted within the ancient Grunehogna cratonic fragment as isolated intrusions and linear chain of mafic intrusions or deep-seated fault; apparently they inherited some ancient inhomogenity in the Precambrian crust and are recognized within the Proterozoic mobile belt as intrusions of different composition but predominantly alkalic in nature. Three well-known intrusions are located in the eastern shoulder of the Jurassic Jutulstraumen-Pencksökket failed rift whose axis is associated with negative anomalies eventually suggesting that axial intrusions were not developed along the axis of the rift as a precursor to crustal splitting.
Dalziel, IWD Pacific margins of Laurentia and East Antarctica-Australia as conjugate rift pair: evidence and implications for an Eocambrian supercontinent, Geology, 19, 598-601, (1991).
It was the end of the Neoproterozoic, it was the beginning of the Phanerozoic, it was the age of Rodinia, it was the age of Gondwana, it was the epoch of global glaciation, it was the epoch of true polar wander. It was the season of breakup, it was the season of amalgamation, it was the spring of the metazoans, it was the winter of the Ediacarans, we had Pangea before us, we had Panottia behind us, the continents were all going to the equator, the continents were all going the other wayin short, the period was so far unlike the present period, that some of its noisiest authorities insisted on its being received, for better or worse, in the superlative degree of comparison only. The Neoproterozoic to Cambrian interval represents on of the most puzzling and intriguing transitions in Earth history. It marks the time of significant metazoan evolutionary development, of major fluctuations in seawater and atmospheric chemistry, of rapid plate reorganization and supercontinental breakup along with a possible change from global icehouse climate to a greenhouse climate. All of these changes may have taken place over relatively short intervals of geologic time giving rise to speculation about cause and effect between the observed changes. For example, did continental breakup lead to the end of the icehouse climate (snowball earth) and concomitant rise in 13C and 87Sr/86Sr isotopic ratios? Did this climatic change in turn trigger the rise of the metazoans? Was the rise of the metazoans stimulated by the changes in the oceanic environment due to the rapid redistribution of landmasses? Indeed, this period in earth history may represent more than a simple time marker, it may represent a fundamental shift in the modus operandi of the earth's climatological and geodynamic systems. These changes appear to broadly coincide with the assembly of the Gondwana continent following the breakup of the earlier supercontinent Rodinia. This talk will outline the current status of tectonic models during the interval from 600-515 Ma and the Dickens of a time we face unraveling its mysteries.
Relative positions of pre-existing cratonic blocks in the Mesoproterozoic continental assembly and subsequent break-up of the Rodinia supercontinent remain controversial. This paper focusses on two areas, located respectively between the Sao Francisco and Congo cratons (Mayumbian and Zadinian Supergroups), and the Congo and Tanzania cratons (Kibaran belt). Recent SHRIMP emplacement ages of magmatic rocks and other isotopic measurements in progress (Sr, Nd, Ar) shed new light on their Mesoproterozoic history.
The Sao Francisco and Congo cratons are separated by the West Congolian (Pan African) belt, formed during Western Gondwana aggregation. In its internal thrust and fold belt domain, the eastwards verging, inverted, oldest Kimezian Supergroup (polycyclic Palaeoproterozoic) rests on the Zadinian Supergroup, itself thrusted onto the Mayumbian Supergroup. The Mayumbian is in contact with the youngest West Congolian (Neoproterozoic) Supergroup, in which open folds progressively die out and give rise to tabular foreland deposits, resting unconformably on the Archaean Congo craton. Using limited bulk zircon dating, the bimodal supracrustal sequences of Mayumbian and Zadinian Supergroups were assigned a Mesoproterozoic age and are sometimes considered to be segments of the Kibaran belt at the western edge of the Congo craton. SHRIMP data show that the acidic volcano-plutonic Mayumbian Supergroup was emplaced at c. 920 Ma. Some arguments point to only slightly older Neoproterozoic ages for the underlying Zadinian Supergroup (tholeiitic volcanics and various metasediments). This suggests that between the Sao Francisco and Congo cratons no Mesoproterozoic geodynamic activity was recorded, indicating a united and single behaviour of both cratons in the Rodinia configuration. This situation (Palaeoproterozoic and Pan African belts without Mesoproterozoic belts) also occurs at the northern edge of the Congo craton and in West Africa. The Mayumbian and Zadinian Supergroups are evidence for an early Neoproterozoic rifting phase, which preceded deposition of the West Congolian Supergroup sedimentary sequences followed by the Pan African orogeny.
The Congo and Tanzania cratons are separated by the Mesoproterozoic, Kibaran type belt. From poor time constraints (based mainly on Rb-Sr systematics), various geodynamic models have been proposed for its evolution, although episodic structural reactivation and isotopic resetting occur throughout the belt. Apart from Sn-granites, emplaced at 1 Ga under poorly understood geodynamic conditions, SHRIMP data for various crustal anatectic granites from its northeasternmost segment point to a single period of emplacement at c. 1375 Ma, dating the Kibaran orogenic phase. This is in contradiction with previous ages (mainly Rb-Sr), spanning 200 Ma. For this segment an intraplate evolution has been proposed, excluding any Mesoproterozoic oceanic crust. The major part of the belt, however, is exposed in Congo, where only limited recent work is available and no ensimatic evolution is documented. A united and single behaviour of the Congo and Tanzania cratons in Mesoproterozoic times is, therefore, also favoured.
The transition between continental and oceanic crust marking the onset of seafloor spreading is normally not accessible for direct study, but hidden under thick covers of sediments and seawater. In a few places - e.g. East Greenland, the Red Sea - this transition is exposed in situ. In one compressive orogen - the Scandinavian Caledonides - a passive margin, complete with the continent-ocean transition is excellently preserved in the allochthon (Andreasson, 1993; Svenningsen, 1994a).
The Sarek Dyke Swarm (SDS) occurs in the Sarektjåkkå Nappe (SN) of the Seve-Kalak Superterrane in the northern Swedish Caledonides. The SN has two main components; 1) a 4-5 km thick sequence of rift-related sedimentary rocks which is intruded by a suite of tholeiitic dykes - the SDS - constituting 70-80% of the nappe (Svenningsen, 1994a & b. The nappe was subjected to deformation during Caledonian thrusting, which rotated dykes and sediments into variable orientations. However, this rotation is the only evidence of Caledonian deformation in the interior of the eastern parts of the SN, and both sedimentary rocks and dykes are preserved in a pristine state.
The tholeiitic dykes of the SDS frequently occur in sheeted dyke complexes, and up to eleven successive generations can be identified based on cross-cutting relations. The sedimentary rocks are characteristic of continental shallow-water depositional environments (Svenningsen, 1994b), and the SN is interpreted as a part of the fossil continent-ocean transition between the Baltic craton and the Iapetus Ocean, marking the initiation of seafloor spreading (Svenningsen, 1994a, 1996).
In the sheeted dyke complexes of the SDS, irregular, bubble-shaped pods and veinlets of fine to medium grained diorite occur in increasing numbers in successively younger dykes, whereas they are absent in the oldest dykes. The rapid successive emplacement of tholeiitic magma raised the ambient temperature in the dyke complex, so that crystallisation in the youngest dykes mimicked similar processes in gabbro plutons. The diorite pods are equivalent of gabbro pegmatites (Larsen & Brooks, 1994), crystallised from a late interstitial melt, and they are thus co-genetic and co-eval with the dykes.
Six zircon fractions, including two single grain analyses, from the diorite were analysed. The data yielded a linear array of points that are 0.4-0.8% normally discordant, indicating a crystallisation age of 608 ±1 Ma. This age is inferred to precisely date the onset of seafloor spreading in the Iapetus Ocean.
Andréasson, PG, Tectonophysics, 231, 1-32, (1994).
Larsen, RB & Brooks, CK, J Petrol, 35, 1651-1679, (1994).
Svenningsen, OM, Geol Jour, 29, 323-354, (1994a).
Svenningsen, OM, Tectonophysics, 231, 33-44, (1994b).
Svenningsen, OM, GFF, 118, A41-42, (1996).
Rift related magmatism along the eastern margin of Laurentia ranges in age from 760 Ma to 550 Ma and is associated with a protracted history of Neoproterozoic breakup of Rodinia and opening of the Iapetus ocean. Early rift related magmatism is peralkaline in character, ranges in age from 760 Ma to around 700 Ma, and has been documented in the southern and central Appalachians. Younger magmatic activity extends from 620 Ma to 550 Ma and is widespread in the northeastern Appalachians. This latter phase can be divided into two distinct pulses; an earlier one (620 Ma to 590 Ma) of largely tholeiitic character and a later pulse (570 Ma to 550 Ma) of predominantly transitional to alkaline composition.
The youngest igneous activity occurs in western Newfoundland. An ankaramite flow from the Skinner Cove thrust slice of the Humber Arm Allochthon yielded a U/Pb zircon age of 550.5 +3/-2 Ma. Zircons from the tonalitic gneiss of the Fleurs de Lys Belt yielded a U/Pb age for igneous crystallization of 555 +3/-5 Ma and were slightly discordant along a mixing line with an upper intercept age of 1400 +220/-200 Ma. Basement in a nearby outcrop with a U/Pb zircon age of 1510 ±6 Ma provides a likely source for the inheritance. Oldest drift related strata in this region are around 530 Ma.
Earlier magmatic activity in the south-central Appalachians is associated with intra-continental rifts that did not proceed to continental breakup, but is contemporary with the ~760-700 Ma rifting of East Gondwana (Australia, Antarctica, India) from western Laurentia. The 620 Ma to 590 Ma magmatic pulse overlaps with rift related magmatism in Baltica and probably relates to the opening of this arm of the Iapetus Ocean. The 570 Ma 550 Ma pulse corresponds with final opening of Iapetus between Laurentia and Rio de la Plata/Amazonia.
Integrated U-Pb geochronology and paleomagnetic studies of the Mundine Well dyke swarm (MDS) of the Pilbara Craton, Western Australia, are employed to test the hypothesis that Australia and Laurentia were joined as part of the Rodinia supercontinent. Paleomagnetic results and ion microprobe U-Pb dating of zircon and baddeleyite indicate that the MDS was emplaced at 755 ± 3 Ma and is equivalent to dykes of the Northampton Inlier. Combining new paleomagnetic data for seven MDS dykes with previous results for six Northampton dykes (Embleton and Schmidt, 1985) yields the first precisely-dated primary paleopole (135°E, 46°N, A95 = 4°) for the Neoproterozoic of Australia. The MDS pole falls 30° away from the 780 to 740 Ma APW path segment for Laurentia, indicating that a Rodinian connection between Australia and Laurentia (± South China?) may not have existed at 755 Ma. If the Rodinia configuration is correct for earlier times, breakup between Australia and Laurentia occurred before 755 Ma. It has been suggested that the Sturtian glaciation in Australia preceded or accompanied breakup along eastern Australia (Powell et al., 1994). If so, the Sturtian rocks should be older than 755 Ma and hence may not be coeval with glaciogenic rocks of the post-755 Ma Rapitan Group in Laurentia. An older limit for breakup along eastern Australia is provided by a U-Pb zircon age of 777 ± 7 Ma (W. Preiss, pers. comm., 1998) for the Beaucaut Volcanics that underlie the Sturtian rocks.
Embleton BJJ & Schmidt PW, Aust. J. Earth. Sci, 32, 279-286, (1985).
Powell CMcA, Preiss WV, Gatehouse CG, Krapez Band Li Z-X, Tectonophysics, 237, 113-140, (1994).
The argument on tectonic position of Chinese cratonic blocks (CCBs) in reconstruction of Rodinia is due to inadequent tectonic constraint and correlation of CCBs with other cratons. Tectonic analysis of CCBs on available data indicates that the Mesoproterozoic episodes are associated with extensional or collisional tectonic regimes respectively,and there are several Grenville-age orogenic belts in China.
In early Mesoproterozoic (1.85-140Ga), North China Craton (NCC) could be subdivided into followed distinct tectonic unites. Aulacogen system (1.85-140Ga) overlaid metamorphic basement. Mafic dyke swarms mainly intruded within Archean high-grade metamorphic basement (AHMB). Nw-trending Taihang mafic dyke swarm (1.77 Ga) extends over thousand kilometer along its strike, almost across the whole NCC. Anorogenic plutonism (1.74-1.68 Ga) is relevant to branches of aulocogen. Moreover, many age data of 1.8-1.9 Ga have been documented with AHMB, interpreted as the ages of uplifting and cooling events. Thus, 1.80-190 Ga episodes are widespread in NCC and associated with extensional setting.
Grenville-age orogenic belts are also recognized in the margin of NCC and Yangtze Craton (YC). North Qinling orogenic belt along southern margin of NCC, mainly consist of back-arc basin assemblages, island arc con~lex, and Mesoproterozoic ophiolite relics (1.0 Ga) ,to its southern side it is reworked and truncated by South Qinling-Dabie Phanerozoic orogen. Its tectonic history could be summarized as rifting of southern margin of NCC (1.55 Ga), followed by active continental marginal collision between NCC and an island-arc (1.0 Ga). It is an accretionary orogen along southern margin of NCC, rather than multiphase collisional orogen between YC and NCC. Along northern margin of NCC there is also Mesoproterozoic orogenic records (1.00-1.l0 Ga).The Mesoproterozoic marginal orogenies exert a strong influence on cratonic basin, leading to the occurrence of a major unconformity (1.00 Ga) within cratonic cover sequences, widespread marine regression, and tectonic difference of NCC from west to east.
Jiangnan orogenic belt located in southeast margin of YC, could be subdivided into ophiolite (1.00 Ga) ,back-arc turbidite (1.10- 0.90 Ga), island-arc volcanics ~granites(0.96-0.93 Ga), and s-type granites (0.90-0.96 Ga). Its tectonic evolution could be interpreted as subduction along SE margin of YC (1.30-120 Ga), followed by collision of YC-island arcs- Cathaysia. Cathaysia records at least three major Precazi~rian tectono-thermal events (1.0 Ga, 1.8-1.9 Ga, 2.4-2.7 Ga), among which 1.0 Ga event is clearly associated with collision of TC with Cathaysia. Moreover, Mesoproterozoic orogenic records (1.0-l.2 Ga) and ophiolite (1.OGa) have been also recognized along western margin of YC, resulted from active continental margin. The delineation of Mesoproterozoic tectonic unites and relevant episodes, suggest that CCBs should be ancient fragmented cratonic blocks, rather than isolate continent nuclei's. The tectonic correlation of CCBs with other cratons of the world on Mesoproterozoic tectonics will provide important constraints for reconstruction of Rodinia.
Three lithotectonic and metamorphic units are exposed in the Bernhardi Heigths-Högbom Outcrops area of the eastern Herberts Mountains (northern Shackleton Range). From N to S, and structurally from top to bottom, they include: i) an high-grade gneiss-migmatite complex containing garnet-bearing meta-intrusives of dioritic to tonalitic composition ("Upper High Grade Gneiss Complex"); ii) a metamorphic "Ophiolitic Complex" consisting of medium-grade mafic-ultramafic rocks and metasedimentary cover rocks, and iii) an high-grade gneiss complex made up of migmatitic gneisses and rare amphibolites ("Lower High Grade Gneiss Complex").
Prominent rock types in the Ophiolitic Complex are amphibolites of N-type MORB to OIB geochemical and Nd isotope affinity, metagabbros interpreted as metacumulate rocks having been formed in a MOR-type setting and metaperidotites. Single-stage Nd-model ages for the OIB-type amphibolites of around 1.0 Ga constrain a maximum formation age for the igneous protoliths of the ophiolitic unit. The polyphase metamorphic evolution of this complex included an early Barrovian-type metamorphic stage (550°C, 7-8 kbar), followed by decompression with concomitant T increase (up to 650°C, 5-6 kbar) and a late retrogression under greenschist facies conditions. K-Ar amphibole dates for amphibolites which are in the range of 500-490 Ma provide evidence for a late-Pan-African (Ross-age) age of metamorphism.
Relict felsic granulites from the Upper High Grade Gneiss Complex record an early high-P granulite-facies event (675-750°C, 8.4-11.3 kbar), followed by migmatization under low-P upper-amphibolite-facies conditions (730-850°C, 5-7 kbar). This early metamorphic event possibly reflects a Proterozoic tectonometamorphic cycle. The subsequent metamorphic stages span from intermediate-P amphibolite-facies to greenschist-facies conditions. They document the tectonometamorphic reactivation of the High Grade Gneiss Complex as a consequence of the Ross-age thickening and exhumation history of the northern Shackleton Range crystalline rocks.
The overall geotectonic setting which arises from the Bernhardi Heights section may be related to subduction-collisional processes involving a Neo-Proterozoic oceanic crust and adjacent high grade continental margins. Tectonic stacking of both oceanic and continental rock units occurred within a thrust-fold orogenic belt which formed through a major phase of transpressional thrust tectonics followed by a late phase of post-thickening orogenic collapse. The most likely time span for this tectonic evolution (about 520-490 Ma) overlaps that of the Ross Orogen within the Transantarctic Mountains (e.g. Stump, 1995; Tessensohn, 1997), generally interpreted a result of subduction-accretion at the paleo-Pacific margin of Eastern Gondwana. In spite of the age similarity, the fact that the newly discovered ophiolitic complex is bounded by high grade basement complexes on either sides clearly indicate that the "Ross" orogen of the Shackleton Range is a distinct tectonic element, namely a collisional mobile belt.
Stump E, The Ross Orogen of the Transantarctic Mountains, Cambridge University Press, 249 p, (1995).
Tessensohn F, The Antarctic region: geological evolution and process, Terra Antartica Publication, Siena, 5-12, (1997).
Recent mapping, petrological and geochronological data highlight the particular history of the East Antarctic Shield on the 135-145°E region. Along a 250 km shore line transect, the basement is composed of massive blocks separated by vertical shear zones of several kilometres in thickness. Therefore, the basement corresponds to a collage of various crustal segments during a transcurrent syn-metamorphic tectonics dated at around 1.7 - 1.5 Ga. For the moment, two different types of crustal blocks have been distinguished: some blocks consist of minor archean gneissic country rocks occuring as screens and enclaves within dominant intrusives of granodioritic to granitic compositions. This prominent plutonic event is dated at 2.44 Ga and gneisses up to 2.8 Ga (U-Pb zircons). Gneisses display high grade, amphibolitic to granulitic, metamorphic imprint. Younger ages (1.7 Ga) have been found only in very localized shear zones and in granitic and pegmatitic dikes. These blocks mainly outcrop in the eastern areas, from Port Martin (141.5°E), to Cape Denison and at least the Mertz Glacier (144°E). some other blocks involve mainly metasedimentary formations recrystalllized in greenschists (Cape Hunter, ) and amphibolite (Pointe Géologie, 140°E, Cap Jules) conditions. In particular, in the Pointe Géologie area, the metamorphic HT-LP imprint and the related anatexis are thought to be associated with a major lithospheric stretching event. The sources of metasediments are constrained by Nd and Sr isotopes and by inherited zircons ages; they indicates pelitic protoliths from both archean (2.6-2.8 Ga) and paleoproterozoic (1.72-1.76 Ga) ages. The timing of deposition is set by the youngest inherited ages (1.72) and 1.69 Ga which corresponds to the peak metamorphism marked by newly formed zircons and monazite In summary, the heterogeneous basement of TA and GVL suffered two post-archean major thermal events: the earlier is characterized by a huge plutonic event (Cape Denison-Port Martin intrusives) at 2.44 Ga and the second is marked either by the metamorphic transformation of paleoproterozoic volcanic and sedimentary series (Cape Hunter, Pointe Géologie, Cap Jules) and by only very local remobilizations and recrystallizations along shear zones, within the archean blocks. No later, panafrican and/or grenvillian, metamorphic imprints or thermal resettings were observed in that region. Therefore, the crystalline basement of Terre Adélie and George V Land is part of the Mawson block, stable since 1.5 Ga and represents a preserved piece of the Rodinia surpercontinent
The Torridonian supracrustal succession, comprising Stoer, Sleat and Torridon Groups, unconformably overlies the Lewisian gneiss complex (LGC). Ages were obtained from over 200 single detrital zircon grains, representing four Stoer Gp. formations (basal Stoer, Bay of Stoer, Stac Fada, Meall Dearg) and two Torridon Gp. formations (Applecross and Aultbea). SHRIMP II's high spatial sensitivity permits detailed investigation of core/overgrowth relationships within detrital grains exhibiting complex growth histories. Coupled with high resolution cathodoluminescence and/or backscatter imaging, multiple levels of information may be recovered, enabling more unique solutions to provenance questions in complex populations.
Stoer Gp. is a ~2 km-thick sequence of siliciclastic fluvial and lacustrine syn-rift sediments with both easterly and westerly provenance. Concordant 207Pb/206Pb ages of Stoer Gp. zircons range from 2.98-1.75 Ga, with >80% falling between 2.92-2.68 Ga. Cathodoluminescence patterns (oscillatory zoned cores, truncated by brightly luminescent, low-U, rims) resemble Lewisian gneiss zircons (Kinny and Friend, 1997). Most Stoer Gp zircons with such overgrowths yield ages between ~2.55-2.45 Ga, coincident with a Scourie region high-grade metamorphic event (Corfu et al., 1994). Few Stoer Gp zircons have ages overlapping with inferred Lewisian central region TTG protoliths (~3.03-2.96 Ga; Kinny and Friend, 1997); rather, Gruinard Bay TTG protoliths (~2.85-2.75 Ga; Whitehouse et al., 1997) or northern region TTGs (~2.84-2.80 Ga) are suitable sources. Each Stoer Gp. unit has minor 1.91-1.75 Ga zircons, likely from Paleoproterozoic arc components (Ard gneisses?), or from Laxfordian sources. Meall Dearg Fm. has rare ~2.00 Ga grains, possibly derived from nearby Loch Maree Gp. supracrustals. The Stoer Group was probably derived from local sources and deposited in closed basins.
Torridon Gp., which unconformably overlies the Stoer Gp., comprises 5-6 km of mostly westerly-derived, red alluvial sandstones. Contrary to findings of Rogers et al. (1990), ~1.75-1.85 Ga and pre-3.00 Ga grains represent significant components of the detrital budget for Applecross Fm. Torridon Group zircons display weak age clusters at 3.09-3.04 Ga and 2.86-2.65 Ga but also show age modes at 1.80 Ga, 1.66 Ga and 1.10 Ga, possibly representing sources of Ketilidian, Labradorian and Grenvillian affinities. Aultbea Formation contains rare 1.50 Ga grains (Pinwarian?). The Torridon Group was probably derived, in part, from the eastern margin of Laurentia. Many Laurentian Proterozoic lithotectonic domains have age equivalents in Baltica but paleocurrent data from the Torridon Gp. sandstones and the lack of paleomagnetic evidence for substantial post-depositional rotation of the Lewisian block relative to the rest of Laurentia, argue against significant provenance from sources in that region.
Kinny PD, Friend CRL, Contribs. Mineral Petrol, 129, 326-340, (1997).
Corfu F, Heaman LM & Rogers G, Contribs. Mineral Petrol, 117, 215-228, (1994).
Whitehouse MJ, Claesson S, Sunde T & Vestin J, Geochim. Cosmochim. Acta, 61, 4429-4438, (1997).
Rogers G, Krogh TE, Bluck BJ & Kwok YY, Geol. Soc. Australia Abs, 27, 84, (1990).
The original hypothesis of a Late Mesoproterozoic/Early Neoproterozoic supercontinent, Rodinia, in which Laurentia is at the core, relied heavily on the apparent continuity of the late Mesoproterozoic Grenville orogenic belt from Laurentia into Coates Land in East Antarctica, and thence between East Antarctica and India to Australia. The Rodinian fit was supported by palaeomagnetic data then available, which suggested that Rodinia could have existed from late Mesoproterozoic (ca. 1075 Ma) to mid-Neoproterozoic (ca. 725 Ma). Subsequent work has shown that the Grenville-aged belt in Coates Land does not link Laurentia to the rest of Antactica and Australia, and that other geological links proposed (e.g., matching stratigraphy, radiating mafic dykes) are equivocal. The palaeomagnetic constraints still appear robust in the 1100 to 1000 Ma interval, but are complicated at the older end of the interval by rapid apparent polar wander implied by the Grenville Loop. Analysis of the possible fits of parts of Rodinia shows that there are three main large continental masses, viz. 1: Laurentia, 2: Australia and East Antarctica, possibly with India and NE Madagascar, and 3: the Kalahari-Tanzania-Congo-San Franciso. Other continental blocks are possibly attached to either Laurentia (Siberia, Baltica and Amazonia) or Australia-India (North and South China, Tarim and Cimmeria). IGCP 440 proposes to examine the evidence relating to the assembly and breakup of Rodinia, with the aim of producing a GIS-linked map(s) showing its geodynamic evolution.
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