PETROLOGY OF MANSEHRA GRANITIC COMPLEX, HAZARA AREA, NORTHWESTERN HIMALAYA, PAKISTAN

Abstract

The Mansehra Granitic Complex (MGC) is mainly comprised of Mansehra Granite (MG), Hakale Granite (HG), microgranitic (MIG) and leucogranitic (LG) bodies along with pegmatites and aplites. Geochemical classification diagrams place these granites in the high calc-alkaline, quartz-rich, peraluminous granitoid field. The Mansehra Granite is a porphyritic and massive body that is locally foliated, whereas the Hakale Granite is sub-porphyritic to non-porphyritic pluton. The Susalgali Granite Gneiss is sheared Mansehra Granite. Harker’s variation diagrams show that MG and HG are derived from magmas of the common non- homogeneous source rock Tanawal Formation through fractional crystallization process in a closed system without considerable contamination. Field relationships, geochemical and mineralogical characteristics of the MGC reveal the peraluminous S-type nature of this Complex. The zircon saturation temperature of MG (749-852 oC), HG (709-779 oC), LG (749-754 oC) and MIG (692-696 oC) is comparable with crystallization temperatures of the peraluminous S-type Lesser Himalayan Indian granites (~670-817 oC). The geochemical characteristics of the MG revealed that the magma was probably generated through biotite dehydration melting of the metasediments of Tanawal Formation at pressure > 5 kbr and temperature > 700 oC, while HG melt was most likely originated at relatively shallower crustal level and lower temperature by muscovite fluid-absent melting of pelites. The occurrence of andalusite in the contact aureole of Mansehra Granite, association of perthitic microcline along negative Nb, Sr and Ti anomalies in spidergrams and higher Rb/Sr ratios in granitic rocks of the MGC may reveal the upper crustal signatures and low pressure shallow emplacement (< 15 km) of these bodies. The leucogranitic bodies associated with the MGC are most likely the products of Na 2 O-rich residual melt of the MG, whereas microgranites may have been derived from boron-rich residual magma of the HG by insurgent boiling and subsequent quenching. In the light of U-Pb zircon systematics of the MGC, a middle Mesoproterozoic to early Neoproterozoic age (ca. 1300-985 Ma) has been proposed for the granite protolith in Hazara area. Whereas, the inherited age components of ca. 985-920, vi880-800 and 690-500 Ma may be interpreted as the ages of post-depositional metamorphic fabric development in the source Tanawal Formation. U-Pb zircon dating of Lesser Himalayan granites also revealed inherited age components at ca. 980 ca. 800 Ma and ca. 700-500 Ma. The age segments of ca. 490 Ma, ca. 475 Ma and ca. 466 Ma (middle to upper Ordovician) represent the intrusive ages of the MG, LG and HG, respectively. The mean age of Mansehra Granite (ca. 480 Ma) is younger than the reported Rb/Sr age of 516±16 Ma (Le Fort et al., 1980). The U-Pb zircon systematics of Mansehra Granite is comparable with the reported Rb/Sr and U-Pb zircon ages of the Himalayan granites and gneisses. Moreover, the depletion of Ba, Sr, Nb and Ti in spidergrams of the MGC allows correlation with the early Paleozoic (500±25 Ma) Lesser Himalayan S-type granites. According to the similarity of mineralogical, geochemical, structural features and U-Pb zircon dating of the MGC (ca. 466-490 Ma) with the peraluminous S-type Himalayan granites, it may be assumed that Mansehra Complex is associated with the Pan African orogeny. However, convincing evidence is lacking. Hence, the genesis of MGC can be better explained by emplacement of Cambro- Ordovician granites along the northern margin of Gondwana.