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Abstract

Archean granitoids of the Bastar Craton mainly occur as gneisses (3.56, 3.50 Ga) and undeformed granitoids (c. 2.5–2.48 Ga). Based on detailed geochemical characteristics two compositional types of gneisses: tonalite–trondhjemite–granodiorite (TTG) and transitional TTG (t-TTG) have been identified. The TTG rocks are further classified into low-HREE (heavy rare earth element) type and high-HREE type. It is proposed that melting of a thick enriched oceanic plateau basalt at deeper level may have generated the low-HREE TTG, whereas melting at shallower depth of the thick plateau can explain the geochemical signatures of the high-HREE TTG. The t-TTG was formed as a result of reworking of the older TTG crust. These two gneisses were probably formed at different time at 3.56 and 3.50 Ga as manifested from the age of the gneisses. The granitoids were formed at a later stage (c. 2.5–2.48 Ga) by reworking of the pre-existing gneissic crust consisting of TTG and t-TTG. Presence of a small 3.58 Ga undeformed K-rich granitoid from the northern part of the craton might indicate yet another earlier crustal reworking event.

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