Mineralogy and geochemistry of calc-alkaline magmatic rocks from the Mansehra Granitic Complex, NW Himalaya, Pakistan: insights into petrogenesis and tectonic setting

Abstract

The Mansehra Granitic Complex (MGC) is mainly comprised of Mansehra Granite, Hakale Granite, pegmatitic, microgranitic, and leucogranitic bodies in the northwest Lesser Himalaya of Pakistan. The Mansehra Granite is generally massive but occasionally gneissic due to local shearing manifested by solid-state deformation. Geochemical classification diagrams placed these plutonic rocks in calc-alkaline, peraluminous granite fields. The zircon saturation temperatures of the Mansehra Granite, Hakale Granite, and Leucogranite are 749–852, 709–779, and 749–754 °C, respectively. The temperature range (709–852 °C) of the MGC rocks is comparable with crystallization temperatures (670–817 °C) of the peraluminous S-type Lesser Himalayan Indian granites. Rb/Sr and Sr/Ba ratios suggest that the granitic melt of Mansehra Granite was derived through biotite dehydration-melting of the protolith Tanawal Formation at > 5 kbr pressure and > 700 °C temperature, whereas the Hakale Granite magma was generated at a relatively shallower depth and lower temperatures by muscovite fluid-absent anatexis of the source rock. The occurrence of andalusite in the contact aureole of Mansehra Granite, presence of perthitic microcline along with negative Nb, Sr, and Ti anomalies in spider diagram and higher Rb/Sr ratios reveal upper crustal signatures and low-pressure shallow emplacement (< 15 km) of the MGC magmatic bodies. The occurrence of psammitic restites, migmatites, and geochemical characteristics suggests the S-type trait of these plutonic rocks. The leucogranitic bodies most likely are the products of Na2O-rich residual melt and boron-rich fluids of the MGC magma. Harker’s variation diagrams indicate that the MG, HG, and LG display significant to poor relationships which suggest imperfect mixing of the non-homogenous protolith during partial crustal melting. However, scattering in some of the plots reflects the heterogeneity of the protolith and irregular distribution of minerals in the granitic melt of the MGC. Owing to affiliation with the continental-continental, syn-collisional geotectonic field, petrogenesis of the MGC rocks can be attributed to an Andean-type Cambro-Ordovician episode along the northern margin of east Gondwana.