Estimation of emplacement pressure for 2350 Ma high-Mg tholeiite dykes, Vestfold Hills, Antarctica
European Journal of Mineralogy Volume 6 Number 2 (1994), p. 195 - 208
published: Mar 31, 1994
manuscript accepted: Dec 7, 1993
manuscript received: Dec 4, 1992
ArtNo. ESP147050602004, Price: 29.00 €
Abstract 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 depth of emplacement has been estimated using two different approaches: (l) Geobarometry based on aluminium exchange between clinopyroxene and plagioclase and (2) examination of high pressure liquidus mineralogy of the high-Mg tholeiites. Geobarometry applied to groundmass assemblages (clinopyroxene + plagioclase) indicates across-dyke variations. Samples from the chilled margins give higher pressure estimates (max. 7 kbar) than more slowly cooled samples from the dyke interior (1 to max. 5 kbar, corresponding to a depth of ~ 3 - 16 km). The variations of the liquidus mineralogy (opx + ol) from chilled margins and from dyke interior samples are consistent with the results derived from the Al-exchange geobarometry. It is inferred that pressure estimates from the dyke interior (1 to max. 5 kbar) represent the ambient emplacement pressures, whereas pressure estimates derived from phenocryst assemblages (ol + opx) in chilled margins may represent crystallization pressures prior to dyke emplacement. The results suggest a much shallower emplacement depth (1 - max. 5 kbar) of the 2350 Ma old tholeiites than previously thought ( 7 -8 kbar).