Source and mode of the Permian Panjal Trap magmatism: Evidence from zircon U–Pb and Hf isotopes and trace element data from the Himalayan ultrahigh-pressure rocks

Abstract

We present an integrated study of LA-ICP-MS U–Pb age, Hf isotopes, and trace element geochemistry of zircons from the Himalayan eclogites (mafic rocks) and their host gneisses (felsic rocks) from the Kaghan Valley in Pakistan in order to understand the source and mode of their magmatic protoliths and the effect of metamorphism. Zircons from the so-called Group I (high-pressure) eclogites yielded U–Pb mean ages of 259±10Ma (MSWD=0.74), whereas those of Group II (ultrahigh-pressure) eclogites yielded 48±3Ma (MSWD=0.71). In felsic gneisses the central or core domains of zircons yielded ages similar to those from Group I eclogites but zircon overgrowth domains yielded 47±1Ma (MSWD=1.9). Trace element data suggest a magmatic origin for Group I-derived (having Th/U ratios: >0.5) and metamorphic origin for Group II-derived (Th/U<0.07) zircons, respectively. Zircon Hf isotope data, obtained from the same dated spots, show positive initial 176Hf/177Hf isotopic ratios referred to as “ƐHf(t)” of around +10 in Group I eclogites; +7 in Group II eclogites; and +8 in felsic gneisses zircons, respectively, thus indicate a juvenile mantle source for the protolith rocks (Panjal Traps) with almost no contribution from the ancient crustal material. The similar ƐHf(t) values, identical protolith ages and trace element compositions of zircons in felsic (granites or rhyolites) and mafic (basalt and dolerite) rocks attest to a bimodal magmatism accounting for the Panjal Traps during the Permian. Later, during India–Asia collision in Eocene times, both the felsic and mafic lithologies were subducted to mantle-depths (>90km: coesite-stable) and experienced ultrahigh-pressure metamorphism before their final exhumation.