The detrital zircon U-Pb-Hf isotopes of the Triassic sediments in northern Pakistan: Implications for crustal evolution of the NW Indian continent

Abstract

Detrital zircon analysis of the Triassic Kashala Formation of the Alpurai Group in Swat, Pakistan, west of Indus syntaxis, record Archean to Paleozoic ages. The U-Pb age spectra show distinctive age clusters at 3510–3100 Ma, 2700–2400 Ma, 1750–1500 Ma, 1350–750 Ma, 700–600 Ma and 580–470 Ma, with one youngest age of 229 Ma, suggesting secular and episodic continental growth. The prominent negative εHf(t) values indicate dominantly crustal reworking together with a minor contribution from a juvenile crust. The age pattern is similar to the detrital zircon from Tethyan Himalaya and South India, showing two major peaks at 926 Ma and 532 Ma, and corresponding to Greenville and Pan-African orogenies, respectively. The U-Pb age peaks between ca. 1750–1600 Ma and 900–750 Ma are correlated well with the peaks of detrital zircon from Precambrian and Ordovician rocks in the Hazara region. Detrital zircon analysis suggests proximal source areas. Based on combined U-Pb and Hf isotope analysis and grain morphology, Paleoproterozoic detrital zircon grains were derived from basement rocks of the Besham Complex, which is presently exposed in the Indus syntaxis. The zircon trace element geochemistry reveals mainly felsic and mafic igneous rocks of the continental crust, suggesting that proximal Proterozoic-Paleozoic intrusions in the Swat and Hazara regions as a possible source.