Geochronology and petrogenesis of the Miocene S-type Sumayar granite in the Karakoram Block, north Pakistan: Implications for post-collisional magmatism

Abstract

The genesis and tectonic controls on the emplacement of post-collisional granites in the Karakoram Block (KB) in northern Pakistan are unclear due to lack of detailed studies. Zircon U-Pb dating indicates that the Sumayar pluton was emplaced at ∼ 9.6 Ma and represented one post-collisional magmatism. The Sumayar granite samples belong to strong peraluminum granites with aluminum saturation indexes of 1.1–1.2. Zircons from the granite exhibit strong negative εHf(t) values mostly of between −30 and −25, indicating a supracrustal source. Whole-rock geochemical data display enrichment in light rare earth elements and large ion lithophile elements, depletion in high field strength elements, and strong negative Eu anomalies. The low CaO/Na2O ratios and high Rb/Sr ratios together with the presence of tourmaline, suggest that the primary magma was formed from dehydrating melting of B-rich metapelites at moderate temperature conditions. In comparison with other post-collisional S-type granites in the KB, two groups were recognized. The first group represented by the Baltoro granite and other granites of ca. 36–15 Ma, is volatile-depleted and B-deficient, and formed from partial melting of greywacke source. The second group represented by the Sumayar granite and other small plutons of 9.6–3.0 Ma, formed from dehydration melting of B-rich metapelitic source. The emplacements of these S-type granitoids are attributed to the episodic partial melting of accretionary wedge rocks and associated crustal anatexes, due to the heat generation during the post-collisional stage at the hanging wall along the collisional boundary between the Kohistan Island Arc and the Asian Plate.