PALEO - MESOARCHEAN CRUSTAL EVOLUTION IN SOUTHERN INDIA – EVIDENCE FROM TRACE ELEMENT GEOCHEMISTRY OF SARGUR ULTRAMAFIC ROCKS

The Sargur ultramafic rocks provide possible evidence for Archean plate tectonic events that led to the emplacement of early formed oceanic crust on the Dharwar continental crust due to collisional tectonics. The narrow, linear belts that are not continuous but dismembered represent the deeper crust that is seen exposed as a result of juxtaposition of the oceanic komatitic basalts and komatites with the younger continental crust. New trace element data from relatively fresher outcrops of ultramafic rocks sampled from deeper parts of some active magnesite mines are indicative of a mixed signature for these komatitic rocks. The ultramafic rocks are compositionally peridotitic to dunitic with the primary olivine and pyroxene altered to serpentine in most samples. However, some samples are not showing extensive serpentization with some of the primary olivine preserved with insignificant alteration.

Preliminary trace element study of ultramafic samples from Karya, Doddakanya and Thaluru areas of Sargur supracrustal belt show more or less a flat REE pattern with negative Eu anomaly. While the Karya and Doddakanya samples have depleted REE values with respect to chondrite the Thaluru ultramafic samples are showing slight enrichment. The trace elements Nb, Sr and Zr show significant depletion when normalized to Primitive Mantle. The sub-chondritic behavior of trace elements, particularly REE, are significant in understanding the evolution of the ultramafic magmas. The depletion of these elements, assuming they were not lost to post-crystallization events are suggestive of the source being more primitive with less differentiation. The presence of negative Eu anomaly is indicative of extraction of plagioclase feldspar from the source melts during early differentiation processes that had preferentially leached Eu out of the system without significantly altering the other trace elements. The negative anomaly of Nb could be indicative of an environment that was not entirely represented by an extensional oceanic setting but an interaction of extensional with subduction-type setting. Nd isotope data is awaited to substantiate the possibility of obduction-type emplacement of oceanic crust on the continental crust.