Journal of Conference Abstracts

Pluton Growth by Dike Transport of Granitoid Magmas: A Geochemical Study of the Wiley Glacier Complex, Antarctic Peninsula

Chris Wareham (c.wareham@bas.ac.uk), Alan Vaughan (a.vaughan@ bas.ac.uk) & Ian Millar (i.millar@ bas.ac.uk)

British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, U.K.

Early Cretaceous gabbro, quartz-diorite­granodiorite, and tonalite­granodiorite plutons of the Wiley Glacier Complex, Antarctic Peninsula magmatic arc, were emplaced in a zone of syn-magmatic extensional shearing. The oldest component pluton is the Creswick Gap quartz-diorite­granodiorite, which is cut by the slightly younger Moore Point gabbro. Together, these plutons form the wall rock to the deformed Burns Bluff pluton. This pluton comprises tonalite­granodiorite sheets which, at its margins, are interleaved with screens of sheared wall rock.

The Moore Point pluton has a predominantly mantle origin, with epsilonNd₁₄₁ of ca +4 and epsilonSr₁₄₁ of ca +1. The Creswick Gap pluton contains both mantle and crustal components and has epsilonNd₁₄₁ between ca +4 (quartz-diorite) and ca -1 (granodiorite) and epsilonSr₁₄₁ from ca -1 (quartz-diorite) to ca +22 (granodiorite). The quartz-diorite facies probably fractionated from a similar parent magma to that of the Moore Point gabbro, whilst the granodiorite probably fractionated from a gabbro­quartz-diorite magma during continued crustal assimilation. Geochemical variations within the Creswick Gap pluton cannot be generated by AFC models. Some of the granodiorite samples have chemical characteristics which typify partial melts of amphibole ± garnet-bearing crust. We suggest that the pluton comprises fractionates of basaltic magma and crustal partial melt. The Burns Bluff tonalite sheets have epsilonNd₁₄₁ between ca +4 and ca -2 and epsilonSr₁₄₁ from ca +5 to ca +31. Their isotopic and chemical compositions suggest that some are fractionates of basaltic magma, others are predominantly partial melts of amphibole ± garnet-bearing crust, and some are mixtures of these two magma types. The chemical and isotopic compositions of the Burns Bluff tonalite sheets and its dike-like internal structure suggest that it grew incrementally via the addition of melt batches from a variety of crustal and mantle sources, a consequence of the dike growth mechanism.

An XRD Approach to Studying Phyllosilicate Microstructure in the Otago Schist (Lake Hawea Section) of Southern New Zealand

L. N. Warr¹ (hq0@ix.urz.uni-heidelberg.de) & S. C. Cox² (100400.3245@CompuServe.COM)

¹ Geol.- Pal. Inst. Ruprecht-Karls Universität, INF 234, 69120, Heidelberg, Germany.

² Geology Department, University of Otago, P.O. Box 56, New Zealand.

The shape of muscovite and chlorite XRD peaks is used to characterise phyllosilicate microstructure within Otago Schist, mineral veins, and associated selvages from the Lake Hawea section of Southern New Zealand. This area presents an ideal opportunity to study changes in microstructure within a prograde metamorphic sequence (pumpellyite-actinolite to greenschist facies) which has undergone a range of polyphase deformational conditions (Textural zone 11B to Textural Zone IV). Simple plots presenting Full-width-at-half-maximum (FWHM) data of muscovite against chlorite are presented which are used to characterise the X-ray scattering domain-size in the crystallographic c*-direction. The range of FWHM values and the slope of best fit regression curves reveal basic differences in defect concentration and microstructural heterogeneity which can be related to the interaction of intracrystalline deformation and recrystallisation. The main features are summarised as follows. a) Schists contain shallow regression slopes showing muscovites to be generally more defective than chlorites, with moderate heterogeneity. Although microlithons have higher defect concentrations, and are more heterogenous in microstructure than adjacent cleavage domains, both fall along the same regression curve. b) Selvages are more defective and heterogenous than the schist, characterised by steeper, near 1:1 regression slopes reflecting an increase in defect concentration within the chlorites. c) Veins contain the least defective chlorites which are more homogenous than the schist and selvage components. This relatively cheap and quick approach to assessing microstructural heterogeneities is seen as an important stage before the application of HRTEM methods.