Early Cretaceous Vein-Related Garnet Granulite in Fiordland, Southwest New Zealand: A Case for Infiltration of Mantle-Derived CO₂-Rich Fluids

Abstract

Regionally extensive two-pyroxene granulite facies orthogneisses of Early Cretaceous age in Fiordland, southwest New Zealand, are criss-crossed by garnet-bearing feldspathic veins (and dikes) having associated marginal reaction zones of garnet granulite. The two-pyroxene granulites resulted from fluid-absent meta-morphism of a suite of synkinematic primary anhydrous intrusions. Subsequent restricted formation of garnet granulite in feldspathic compositions, and locally eclogite in ultramafic compositions, proceeded chiefly via reactions involving hornblende breakdown, and occurred in response to sharply increased load pressure and local lowering of water activity. The restricted occurrence of carbonate scapolite and $$CO_{2}$$-rich fluid inclusions in the vein areas suggests that water activity was lowered by infiltrating carbonic fluids. Infiltration occurred along pre-existing fracture systems, many of which were already filled with plagio-clase-rich veins and dikes. Published carbon isotope data for $$CO_{2}$$ in scapolite within the veins indicates a mantle source. Open system behavior accompanying infiltration favored garnet stability by lowering bulk rock ferric/ferrous ratio and $$Na_{2}O$$ content. Fluid infiltration occurred at or near peak metamorphic pressure (~12 kbar at 650-700°C). Granulite metamorphism was of short duration (< 20 m.y.) and accompanied tectonic thickening in a subduction-related magmatic arc.