ZIRCON TRACE ELEMENT CHEMISTRY AS A FUNCTION OF METAMORPHIC GRADE ALONG A TRAVERSE OF LOWER ARCHEAN CRUST, EASTERN DHARWAR CRATON, TAMIL NADU, SOUTH INDIA

Systematic changes in whole-rock and mineral chemistry are observed along a 95 km traverse of lower Archean crust in the Eastern Dharwar Craton (Hansen and Harlov 2007 J Pet 48, 1641). From north (amphibolite-facies) to south (granulite-facies), chemical trends include bulk depletion of Rb, Cs, Th, and U; enrichment in Ti and F and depletion in Fe and Mn in Bt and Amph; increasing Al with decreasing Mn in Opx; and F enrichment with Cl depletion in FAp. In the northernmost portion of the traverse the principal REE-bearing minerals are titanite and allanite. Near the Opx-in isograd, Mnz is the major REE and Th-bearing phase, occurring as discrete grains. Further south in the charnockites, Mnz is restricted to tiny Th-poor inclusions in REE-rich FAp.

SIMS analysis of zircon (Zrn) grains along the traverse reveals domains of magmatic zoning with ca 2550 Ma ages, recording the emplacement of (mostly) granitic protoliths. Magmatic Zrn was modified during metamorphism in two distinct ways: (i) replacement by variably U-Hf enriched, Th-Y-HREE depleted Zrn along cracks, magmatic growth zones, and margins; and (ii) dissolution, reprecipitation, and overgrowth by faceted, U-Th-Y-HREE depleted Zrn. Type (i) Zrn is dominant in northern gneisses and formed at ca 2530Ma, predating retrograde titanite at ca 2500Ma. Zrn modification may reflect re-equilibration with other accessory phases at amphibolite- to lower granulite-facies conditions. Type (ii) Zrn appears near the Opx-in isograd and increasingly consumes protolith Zrn southwards. Type (ii) ages spread from 2530 to 2500Ma. With increasing charnockitisation of granitic protoliths southwards, whole-rock compositions become decoupled from magmatic protolith Zrn, and coupled with U-Th-Y-HREE depleted type (ii) Zrn, demonstrating that chemical changes were produced in an open system during a 2530-2500Ma metamorphic event.

Mineralogical and whole-rock compositional changes can be accounted for by progressive dehydration and oxidation reactions. Although in situ anatexis and melt extraction may play a role, whole rock and Zrn trace element depletion data are best explained by the action of externally-derived low-H2O activity brines migrating up through the mid-to lower crust during orogenesis similar to that described by (Newton and Manning, 2010, Geofluids, 10, 58).