Geochemical characteristics of orthopyroxene-bearing metamorphosed lithologies in the northern Chilka lake area, Eastern Ghats belt, India: Insights into the original nature and petrogenesis of precursors

Joel George 

The northern Chilka Lake area stone quarries expose a high-grade terrane comprising a metapelitic suite interbanded with orthopyroxene-bearing gneisses and garnet-biotite leucogneisses (leptynites). This study presents bulk-rock and mineral chemistry data of the orthopyroxene-bearing units to evaluate the true nature of their protoliths and to comment on their petrogenesis. Field, petrographic, and geochemical observations reveal three distinct variants: Enderbitic paragneisses (EP), Garnetiferous Charnockites (G-Ch), and Non-Garnetiferous Charnockites (NG-Ch). Orthopyroxenes in each unit are texturally and compositionally distinct, and highly likely of metamorphic origin. A detailed study of the EPs suggests that their field and chemical characteristics are more consistent with a depositional origin rather than an intrusive one. Based on bulk rock geochemical aspects of non-migmatitic samples, a provenance involving both felsic and mafic-ultramafic rocks is speculated for the unit. Under this interpretation, the ‘ultramafic two-pyroxene granulite xenoliths’ previously reported to be within this unit possibly represent lithic fragments from an ultramafic source, thereby likely reflecting the very low transport distance of some of the additions to the unit. In contrast, the G-Ch likely represent disequilibrium melts derived from the metapelitic suite in the region, with their slightly higher Fe, Ca, Mg, Nb, Ti, Eu, and Sr contents compared to leptynites possibly linked to restite entrainment. As G-Ch rocks rarely exist with the prevalent metapelitic suite as wallrocks, it is deduced from field evidence that the melts migrated away from source region just like the leptynite precursors. The NG-Ch rocks, with their characteristic HREE+Y depletion, negative Nb, Ti anomalies and high Sr are most likely products of partial melting of predominantly granodioritic component of a lower crustal TTG gneiss. This study therefore highlights the complexity of the metasedimentary units in the Chilka lake area and reaffirms earlier interpretations regarding the presence of a metagreywacke unit, which may be as predominant as the already recognized metapelite unit. Along with that, this study also suggests the presence of two distinct granitoids of igneous origin, which, although undated, are interpreted to be products of crustal reworking during the Grenvillian orogeny.