Journal of Conference Abstracts

The Cluanie Granite of Northwest Scotland: A Hybrid Pluton?

J. W. Pembroke & R. S. D'Lemos

Geology and Cartography Division, Oxford Brookes University, Oxford.

The Cluanie Granite of Northwest Scotland displays a number of features consistent with magma mixing and associated intense magmatic overturning. Whilst the pluton is homogeneous on a large scale, displaying few internal contacts, many inhomogeneities occur at outcrop and in thin section. The absence of obvious facies and striking contacts within the granite may suggest that the pluton has had a simple emplacement history. However, it is postulated here that such features were eradicated through intense mixing during batch construction of the pluton. Evidence for such activity includes widespread disequilibria microtextures such as spike zones, discontinuous zoning patterns and cellular cores in plagioclase and inclusion trails in alkali feldspar megacrysts. X-ray mapping and EDS Scanning Electron Microprobe analysis were used to study disequilibria textures. Textural heterogeneity involving petrogenetically contrasting microtextures in close proximity is also consistent with a magma mixing model as is the random and clustered distributions of mafic minerals at thin section scale. The distribution of minerals was investigated using nearest neighbour analysis by digitising their spatial configuration directly in to INFO-MAP. The occurrence of intense magmatic overturning, required to facilitate widespread mixing, is supported by elaborate yet common flow related features including megacryst clustering and complex mafic 'schlieren'. This data shows that the Cluanie Granite has experienced extensive magma mixing during its construction history. Emplacement of magma batches of varying composition at rates which outstrip crystallisation creates an environment conducive to mixing. Our studies of other plutons reveal that the Clean Granite is typical rather than an isolated case. This indicates that magma mixing is a key process in pluton assembly and provides a 'missing link' between ascent and emplacement. It can therefore be stated that those plutons which exhibit no obvious evidence for batch emplacement and magma interaction may be the most mixed granites of all.

Age and Origin of Southern Patagonian Flood Basalts, Chile Chico Region (46°45'S)

N. Petford¹ (n.pet@king.ac.uk), M. Cheadle², A. Cadman¹, B. Barreiro³ & T. Brewer⁴

¹ School of Geology, Kingston University, Kingston KT1 2EE, U.K.

² Department of Earth Sciences, University of Liverpool, Liverpool L69 3BX, U.K.

³ NIGL, Keyworth, Nottingham, NG12 5G, U.K.

⁴ Department of Mineral Resources Engineering, University Park, Nottingham NG7 2RD.

New chemical and age data from the Chile Chico region (ca. 46°S) of the current Andean volcanic arc gap are reported from a detailed traverse through a lower plateau basalt sequence of Hy-normative olivine tholeiites. ⁴⁰Ar/³⁹Ar ages of 51.7 ± 0.7 to 51.8 ± 0.9 imply an eruption time of ca. 0.1 Ma and confirm the similarity in age of these rocks with the neighboring Posadas basalts in Argentina. Ages of <10 Ma in an upper sequence of Ne-normative alkali olivine basalts confirm the wide range in ages reported in earlier K-Ar reconnaissance studies of the region, and show that flood basalt magmatism occurred throughout most of the Tertiary (ca. 50 Ma) in this region of Patagonia. The minimum estimated volume of the lower sequence and surrounding Miocene basalts exposed between 46-49°S is ca. 2 x 10⁴ km³, giving an average magma eruption rate of 0.2 km³ yr, comparable with the Parana lavas.

The basalts are OIB-like (Ba/La <15, La/Nb <1.6) with Mg# to 67, epsilon_Nd of +6 to +2 and ⁸⁷Sr/⁸⁶Sr of 0.7030-0.7045. Correlation between La/Nb, TiO₂ and isotopic composition between the lower and upper sequence basalts suggest a switch in source region from predominantly asthenospheric to lithospheric mantle with time.

The new ⁴⁰Ar-³⁹Ar ages for the Posadas basalts at Chile Chico (51.8 Ma) coincide nicely with the predicted onset of Eocene ridge-trench collision. Major and trace element modelling suggests that the Eocene age Chile Chico magmas originated by ca. 5% melting of an anomalously hot mantle source at between 80-90 km depth, at a mantle potential temperature of 1400-1450°C. Although the subducting slab normally prohibits hot mantle from rising to melting depths, slab windows within the subducting oceanic lithosphere may provide temporal and spatial opportunities for hot, relatively depleted to OIB-like subcontinental mantle, to rise to more shallow depths where melting can occur.