Fractured-controlled fluid flow and metasomatism in the contact aureole of the Marulan Batholith (New South Wales, Australia)

Abstract

The Late Silurian Bungonia Limestone was contact metamorphosed adjacent to the Early Devonian Glenrock Granodiorite at 500–580 °C and 0.7–2.2 kbar. At >60 m from the granodiorite contact the limestone is pure calcite marble. Closer to the contact it was extensively metasomatised to form metre-scale dykes of garnet and clinopyroxene, massive wollastonite skarns, decimetre-thick veins dominated by vesuvianite, and distinct breccia zones that contain millimetre-wide veinlets of vesuvianite±garnet. These skarns and veins represent channel ways for aqueous Si–Mg–Fe–Al-bearing metasomatic fluids of magmatic origin. The δ18O values of the marble are typically little reset (>20‰), even within millimetres of the skarn breccias, and only within tens of centimetres of the thicker skarns. The pattern of metasomatism and isotope resetting observed around and within the skarn zones does not accord with simple flow models that assume fluid flow from the granodiorite into marble that initially contained a connected, fluid-filled porosity. Instead, the data are most compatible with hydrofracture of corrosive, metasomatic fluids into initially dry rocks in short-duration flow events.