Pre-Himalayan crustal growth in the Indian plate: Implications from U-Pb dating, geochemical and Sr-Nd isotopic systematics

Abstract

This study presents detailed petrography, bulk rock geochemistry, U-Pb dating, and Sr-Nd isotopes of pre-Himalayan granite gneisses and granites exposed in the Neelum Valley, NW Pakistan. To understand pre-Himalayan crustal growth history in the Indian plate, basement granitoids provide useful constraints. Two granites (the Jura and Neelum granites) and two granitic gneisses (the Nauseri and Jura gneisses) were investigated with the goal of their petrogenesis and constraining their geotectonic settings. Petrographically both granites and gneisses contain plagioclase, K-feldspar, quartz with S-type granite paragenesis of muscovite, garnet and tourmaline. Geochemically granites and gneisses are characterized by strongly peraluminous (A/NK > 1.1) character with a distinct mafic metapelitic sources. Notable negative Ta, Nb, and Ti compared to La and K, and a pronounced positive Pb anomalies in the studied rocks indicate convergence related source with a similar rare earth element pattern of upper continental crust. Two U-Pb zircon age populations, ca. 2429 ± 17 Ma and 1844 ± 11 Ma were obtained from the Nauseri gneiss whereas Jura gneiss yielded 865 ± 5.6 Ma. In contrast, Neelum and Jura granites yielded ages of 736 ± 11 Ma and 459.1 ± 3.5 Ma, respectively. Sr-Nd isotopic composition ((87Sr/86Sr)i = 0.7099 to 0.7303; εNd(t) = -8.1 to −3.6) together with the major and trace element geochemistry indicate that parental magmas of granites and gneisses were derived from the partial melting of upper continental crust during syn-collision tectonic settings. The Sr-Nd isotopes coupled with two-stage model ages further suggest incorporation of Archean and Paleoproterozoic reworked continental material that contained some mafic metapelitic component as well. Tectonically the parental magma of gneisses and granites could be linked with the global-scale orogenic events that operated during the assembly and rifting of the supercontinents of Columbia to Pangaea. Protoliths of Nauseri gneiss was possibly formed during the assemblage of Columbia supercontinent (1.8 Ga). Later, the Pan-African Orogeny provided additional pulse of magmatism that resulted Jura gneiss and Neelum granite. Moreover, the Bhimphedian orogeny in the Indian plate during Ordovician times generated the Jura granite of ca. 460 Ma.