ESTIMATION OF EMPLACEMENT PRESSURE FOR 2350 MA HIGH-MG THOLEIITE DYKES, VESTFOLD HILLS, ANTARCTICA

Emplacement pressures of the 2350 Ma old, high-Mg tholeiite dyke suite in the Vestfold Hills block have been previously estimated to be 7 - 8 kbar. New data suggest that previous estimates are too high. The dep th of emplacement has been estimated using two different approaches: ( 1) Geobarometry based on aluminium exchange between clinopyroxene and plagioclase and (2) examination of high pressure liquidus mineralogy o f the high-Mg tholeiites. Geobarometry applied to groundmass assemblag es (clinopyroxene + plagioclase) indicates across-dyke variations. Sam ples from the chilled margins give higher pressure estimates (max. 7 k bar) than more slowly cooled samples from the dyke interior (1 to max. 5 kbar, corresponding to a depth of approximately 3 - 16 km). The var iations of the liquidus mineralogy (opx + ol) from chilled margins and from dyke interior samples are consistent with the results derived fr om the Al-exchange geobarometry. It is inferred that pressure estimate s from the dyke interior (1 to max. 5 kbar) represent the ambient empl acement pressures, whereas pressure estimates derived from phenocryst assemblages (ol + opx) in chilled margins may represent crystallizatio n pressures prior to dyke emplacement. The results suggest a much shal lower emplacement depth (1 max. 5 kbar) of the 2350 Ma old tholeiites than previously thought (7 - 8 kbar).