Zircon as a Recorder of Trace Element Changes during High-Grade Metamorphism of Neoarchean Lower Crust, Shevaroy Block, Eastern Dharwar Craton, India

Abstract Systematic changes in whole-rock chemistry, mineralogy, mineral textures, and chemistry are seen along a ca. 95-km traverse of late Archean granitoid orthogneisses the Shevaroy Block, Eastern Dharwar Craton, southern India. The passes from amphibolite-grade gneisses north to granulite-grade rocks (charnockite) south. Changes include depletion Rb, Cs, Th, U granulite grade as relative amphibolite gneisses, oxidation trends regionally highly oxidised granulite-facies near magnetite–haematite buffer relatively reduced amphibolite-facies below fayalite-magnetite-quartz. Rare earth elements show limited mobility hosted variety minerals whose presence is dependent on metamorphic ranging titanite allanite monazite vicinity orthopyroxene-in isograd apatite charnockite. Cathodoluminescence back-scattered electron sub-grain imaging sensitive high-resolution ion microprobe analysis zircon 29 samples dioritic, tonalitic, granitic orthogneiss reveals magmatic cores that record emplacement protoliths mostly about 2580 2550 Ma, with few older mid tonalites. Protolith was modified during metamorphism by overgrowth and/or replacement. Relative igneous cores, U-enriched zircon, dominant formed at 2530 predating retrograde growth 2500 Ma. Uranium-depleted mantles grew between Ma south isograd. In some these samples, U-depleted preceded U-undepleted indicating progression increasingly depleted compositions With increasing (from granulite) state, disappear assemblage became Th. Whole-rock U-Th decoupled relict compositions, reflecting development chemical differences were produced metamorphism, rather than just dioritic vs protoliths. Although situ anatexis melt extraction may have played role, trace can be explained action externally derived, oxidising, low-H2O activity hypersaline fluids migrating up through lower crust. Fluids element migration end result subduction related processes cumulated collision concatenation island arcs continental blocks. These tectonic assembled Craton Archean.