Petrogenesis of the Borjuri diorite pluton in the Mikir Massif of Northeast India: implications for post-collisional intermediate magmatism during the Pan-African orogeny

The Assam-Meghalaya Gneissic Complex (AMGC) of northeast India contains numerous Pan-African granitic bodies that have been attributed to post-collisional rift-related magmatism. The present study is concerned with the first appraisal of intermediate magmatism (diorite, monzonite, and monzodiorite) found in the Borjuri Pluton of Mikir Massif, which is the eastern extension of AMGC. The diorites are strongly metaluminous and exhibit enriched LREE ([La/Yb]N = 1.63–7.37) with respect to HREE ([Gd/Yb]N = 1.95–2.27). The studied rocks do not show any mineralogical or textural indication of metamorphism. Tectonic discrimination diagrams indicate that these rocks originated in a within-plate tectonic setting. The lower Mg# (33.49–38.69), low Cr (below detection limit), and Ni (27–41 ppm) contents along with elemental ratios such as Rb/Sr (0.32–0.95), La/Nb (0.49–4.21), and Nb/Ce (0.11–0.64) suggest a crustal source for the diorites. Discrimination diagrams coupled with elemental ratios suggest that these rocks originated due to partial melting of mafic components in the crust with possible contribution from mantle materials. The P–T conditions of diorite emplacement (7.4 kbar, 688 °C) were calculated using the amphibole-plagioclase geothermobarometer. Geochemical and geochronological data of the Pan-African felsic plutons reported from the AMGC indicate that these rocks were emplaced in a post-collisional extensional regime. The Borjuri Pluton is in close proximity with the Kathalguri Pluton, which has been reported as a product of Pan-African magmatism. In view of the numerous extensional Pan-African felsic magmatism reported from the AMGC and based on the close vicinity of the Borjuri diorites with the Kathalguri granites, we speculate that the Borjuri diorites are products of the Pan-African post-collisional magmatism.