Stable isotopic evidence for fluid infiltration during contact metamorphism in a multiply‐metamorphosed terrane: the Reynolds Range, Arunta Block, central Australia

Abstract

The Lander Rock Beds form the local basement of the Reynolds Range in the Arunta Inlier of central Australia. These dominantly quartzose and pelitic lithologies underwent low‐grade (c. 400 °C) regional metamorphism prior to contact metamorphism (c. 2.5 kbar) around S‐type megacrystic granitoids at 1820–1800 Ma. The Lander Rock Beds are overlain by metasediments of the Reynolds Range Group, which were subsequently intruded by granitoids at c. 1780 Ma. Regional metamorphism at 1590–1580 Ma produced grades varying from greenschist (400 °C at 4–5 kbar) to granulite (750–800 °C at 4–5 kbar) from north‐west to south‐east along the length of the Reynolds Range. Oxygen isotope ratios of the Lander Rock Beds were reset from 13.4±0.8 to as low as 6.7 adjacent to the contacts of the larger plutons, and to 10.3±1.1 around the smaller plutons. Biotite in all the major rock types found in the aureoles has δD values between −52 and −69, probably reflecting resetting by a cooling igneous+metamorphic fluid near the plutons. Sapphirine‐bearing and other Mg‐ and Al‐rich rock types have low δ18O values (4.0±0.7). The precursors to these rocks were probably low‐temperature (c. 200 °C) diagenetic–hydrothermal deposits of Mg‐rich chlorite, analogous to those in Proterozoic stratiform precious metal and uranium deposits that form by the infiltration of basin brines or seawater. As in the overlying Reynolds Range Group, regional metamorphism involved little fluid–rock interaction and isotopic resetting.