STABLE ISOTOPIC EVIDENCE FOR FLUID INFILTRATION DURING CONTACT-METAMORPHISM IN A MULTIPLY-METAMORPHOSED TERRANE - THE REYNOLDS RANGE, ARUNTA BLOCK, CENTRAL AUSTRALIA

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 degrees C) regional m etamorphism prior to contact metamorphism (c. 2.5 kbar) around S-type megacrystic granitoids at 1820-1800 Ma. The Lander Rock Beds are overl ain by metasediments of the Reynolds Range Group, which were subsequen tly intruded by granitoids at c. 1780 Ma. Regional metamorphism at 159 0-1580 Ma produced grades varying from greenschist (400 degrees C at 4 -5 kbar) to granulite (750-800 degrees C at 4-5 kbar) from north-west to south-east along the length of the Reynolds Range. Oxygen isotope r atios of the Lander Rock Beds were reset from 13.4 +/- 0.8 parts per t housand to as low as 6.7 parts per thousand adjacent to the contacts o f the larger plutons, and to 10.3 +/- 1.1 parts per thousand around th e smaller plutons. Biotite in all the major rock types found in the au reoles has delta D values between -52 and -69 parts per thousand, prob ably reflecting resetting by a cooling igneous + metamorphic fluid nea r the plutons. Sapphirine-bearing and other Mg- and Al-rich rock types have low delta(18)O values (4.0 +/- 0.7 parts per thousand). The prec ursors to these rocks were probably low-temperature (c. 200 degrees C) diagenetic-hydro therm al deposits of M g-rich chlorite, analogous to those in Proterozoic stratiform precious metal and uranium deposits t hat form by the infiltration of basin brines or seawater. As in the ov erlying Reynolds Range Group, regional metamorphism involved little fl uid-rock interaction and isotopic resetting.