Tectonic Setting

Shillong plateau in northeast India is very significant with regard to its tectonic evolution. Rb vs. (Y+ Nb) and Nb vs. Y relationships show that the Mikir Hills Massif granitoids have geochemical characteristics similar to those of continental arc as the granitoids plot in the Volcanic Arc Granite (VAG) field (Fig. 11). Further high-K calc-alkaline-shosonite, perluminous and I-type characteristics of the granitoids indicate that these rocks were emplaced in a convergent-margin setting or derived from a source previously affected by subduction. Mikir Hills Granitoids exhibit fractioned LILE/HFSE as well as fractionated LREE/HREE patterns (Fig. 9, 10). The fractionated LILE/HFSE pattern is generally recognized as a distinct feature of convergent margin magmatism (Winter, 2001; and references therein). These geochemical characteristics of the granitoids thus favour magmatism in a convergent margin tectonic setting within Shillong plateau.

Figure 11. Tectonic discriminant diagrams.

Tectonic discriminant diagrams.

(A) Rb vs. Y+Nb and (B) Nb vs. Y tectonic discriminant diagram for the granitoids of Mikir Hill massif of Shillong plateau. VAG, volcanic arc granite, Syn-COLG syn-collisional granite, WPG within plate granite, ORG ocean ridge granite fields are after Pearce et al., (1984).