A petrologic and fluid inclusion study of charnockites from the Lützow-Holm Bay region, East Antarctica: Evidence for fluid-rich metamorphism in the lower crust

Abstract

The Proterozoic terrain of Lützow-Holm Bay in East Antarctica comprises an amphibolite- to granulite-facies progression from north to south, exposing an oblique section of the lower crust. The granulite terrain is represented dominantly by orthopyroxene-bearing anhydrous granulites (charnockites), garnet- and biotite-bearing gneisses (leptynites), and garnet, sillimanite and graphite bearing aluminous metapelites (khondalites). Thus it is petrographically identical with its Gondwanian counterparts in Sri Lanka and southern India. Charnockite formation is characterized by the consumption of garnet, biotite and quartz to produce orthopyroxene, alkali feldspar and plagioclase under isothermal conditions and at reduced water activities. A number of mineral-reaction textures, including orthopyroxene-plagioclase symplectites around clinopyroxene relics in intermediate charnockites and spinel rims between adjacent garnet and sillimanite in khondalites, suggest near-isothermal conditions of equilibration. Mineral phase equilibria barometry traverse across the granulite terrain indicates an increase in pressure from c. 5 kbar in the north to c. 8–9 kbar in the south. The temperature gradient of c. 100°C is less pronounced. Fluid inclusion studies indicate abundant CO 2 trapped within the charnockite minerals. The systematic variation in fluid densities with a garnet→feldspar→quartz succession within individual samples argues against post-metamorphic fluid infiltration and fits a scenario of mineral equilibration during near-isothermal uplift following a clockwise pressure-temperature-time path. Along the pressure gradient, the fluid densities show an increase from c. 0.98 to c. 1.10 g/cm 3. This strengthens other evidence of a synmetamorphic entrapment of CO 2. Preliminary carbon isotope measurements of CO 2 extracted from inclusions yield δ 13C values of c. −5 to −6‰, suggesting a sub-continental mantle source. The Gondwanan Lützow-Holm Bay terrain had undergone substantial extension during the late Proterozoic, with displacement along megafaults and emplacement of several alkaline-subalkaline granite and syenite plutons, and also K-basalts. It is therefore envisaged that the abundant CO 2 in the charnockites could have been derived from an attenuated continental lithosphere, and transferred through magmatic conduits.