Geochronology, trace elements and hf isotopic geochemistry of zircons from swat orthogneisses, northern pakistan

Abstract

In this study, zircon grains are applied for U-Pb dating, Hf isotopes and trace elements to reveal the origin of magmatism and tectonic evolution of Late Paleozoic rocks of the Indian plate, Northern Pakistan. Most of the zircons are characterized by oscillatory zoning, depletion of light rare earth elements (LREE) and enrichment of heavy rare earth elements (HREE) with Ce and Eu anomalies. The yielded ages for these rocks are 256 ± 1.9 Ma and are plotted in the zones defined for the continental setting with few deviated toward the mid-oceanic ridge and the oceanic arc setting. Deviated zircons are recognized as inherited zircons by displaying a high concentration of normalized primitive La and Pr values, while others are plotted in the continental zones. Rare earth elements (REE) and trace elements including Th, Hf, U, Nb, Sc and Ti discriminate Swat orthogneisses into the within plate setting and the inherited zircons are plotted in the orogenic or the arc-related setting. The LREE discriminated these zircons into a magmatic zone with inherited zircons deviated toward the hydrothermal zone. The temperature calculated for these rocks based on the Ti content in zircon ranges from 679 to 942°C. The ϵ Hf(t) ranging from -11.1 to +1.4 reveals that the origin is the continental crust with the minute input of the juvenile mantle.