176 Metamorphic Petrology III

Thursday, 8 November 2001
1:30 PM - 5:30 PM

PETROLOGIC, ISOTOPIC AND TRACE ELEMENT GEOCHEMICAL EVIDENCE FOR A RECYCLED OROGENIC PROVENANCE FOR THE ARCHEAN SANDSTONES, EASTERN INDIAN CRATON

Paper number: 176-13
Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

Petrologic and Geochemical analyses of little metamorphosed middle-late Archean sandstones from the Eastern Indian Craton show that the sediment sources are dominantly the 3.3-Ga old amphibolites of the Older Metamorphic Group (OMG) and tonalites of the Older Metamorphic Tonalite Group (OMTG), the two oldest lithologic groups of the Eastern Indian Craton. Although the modal QFL (Quartz-Feldspar-Lithic) plot of the sandstones indicates a continental block provenance, the QmFLt (monocrystalline Quartz- Feldspar- total Lithics) data further show that the sandstones were derived from the OMG and OMTG plutons in a recycled orogenic setting. Rare Earth Element (REE) patterns of the sandstones show a light REE-enriched signature, (La/Sm)N varying from 5.2 to 6.7, with no Eu anomaly, while the heavy REE patterns display flat (Gd/Lu)N values of 0.9 to 1.6. Incompatible trace element spider plots demonstrate an overall similarity with global Proterozoic-Archean sandstones, including strong Nb-Ta negative anomalies but with a higher Ba/Th and a less conspicuous Sr depletion than the ancient sandstones. Ce/Pb of the sandstones vary between 1 and 4 with a positive correlation with Ce contents; the latter also depends on the feldspar content of the sandstones. Depleted mantle Nd model ages (TDM) range from ~2.8Ga to 4.0 Ga, with the majority falling between 3.2-3.5 Ga. In ƒ(Sm/Nd) vs. 143Nd/144Nd plot, the sandstones plot precisely in the void between the regional amphibolites and tonalites.

A definite inference from the REE and the ƒ(Sm/Nd) vs. 143Nd/144Nd plots is that the sandstones represent a bimodal mechanical mixture of OMG and OMTG. The high feldspar content reflects the high Ba/Th ratios and less negative Sr depletion. The QmFLt plot and the strong Nb-Ta depletion suggest the protoliths of the sandstones to be in an arc setting, probably in a fore-arc basin. The average Nd model ages are almost identical to the Sm-Nd crystallization ages of the OMG and OMTG at ~3.3 Ga; the higher TDM ages of ~4.0 Ga suggests the existence of older continental crust in this Eastern Indian Craton. The low Ce/Pb ratios (1-4) varying with the feldspar content in the sandstones indicate a variably Pb-enriched Archean crust and that the Ce/Pb ratios of continental crust are not constant as commonly believed.