The Measurement of the Thermal History around the Grassy Granodiorite, King Island, Tasmania by Use of Fluid Inclusion Data

Measurements of homogenization temperatures of fluid inclusions relative to their location in colored and chemically zoned garnet crystals collected along a 500 m traverse from a granodiorite contact, King Island, Tasmania, Australia give a thermal history of the cooling pluton. Homogenization temperature vs. distance curves are essentially linear. The gradients are approximately 385°C at the contact to 305°C at 500 m from the inner core; 420°C to 305°C, outer core; 740°C to 425°C, inner midsection; 800°C to 470°C, central midsec-tion; 680°C to 420°C, outer midsection; 540°C to 305°C, inner edge and 470°C to 305°C, outer edge. The thermal model suggested is that relatively small amounts of hotter saline aqueous solutions permeated large quantities of cooler contact rocks (at approximately 350°), already having a thermal gradient, caused the crystallization of andradite-core garnets. The solutions initially were not in thermal equilibrium with the contact rocks but ultimately the resultant temperature recorded in the inclusions did reach thermal equilibrium. As convective heat-loss from the pluton continued the isotherms moved out from the granodiorite during crystallization. Due to decarbonization reactions, overpressures exceeding the breaking strength of the overlying rocks were produced resulting in faulting with the aqueous fluid entering a 2 phase area (boiling) and garnet cores generally being fractured. With the waining of pluton-produced heat, lower temperature inclusions were trapped in the now unfractured grossular-rich garnet rims. These represent the return of the isotherms back to the pluton's contact.