Study of the tectonic uplift history of the Sillai Patti granitic gneiss, Pakistan: constraints from zircon fission-track dating

Abstract

A group of alkaline igneous complexes called the Peshawar Plain Alkaline Igneous Province (PAIP) is exposed in an arcuate fashion north of the Peshawar Plain in northwestern Pakistan. The PAIP, which stretches from Tarbela in the east to Loe-Shilman near the Pakistan–Afghanistan border in the west, consists mainly of granites, syenites, gabbros, ijolites and carbonatites. Field relationships suggest that the Malakand granite and Sillai Patti carbonatite are younger than the Malakand and Sillai Patti granitic gneisses. However, a zircon fission-track age from the Sillai Patti granitic gneiss is less than the absolute ages of the Malakand granite (zircon U–Pb age=270 Ma ) and Sillai Patti carbonatite (calcite U–Pb age=30 Ma, biotite K–Ar age=31±2 Ma, zircon fission-track age=32.3±1.4 Ma ). Therefore, the zircon fission-track age of 24.4±2.9 Ma from the Sillai Patti granitic gneiss represents a time of post metamorphic denudation history of the area, when these rocks passed through the 210°C isotherm. This isotherm corresponds to a depth of about 6.7 km within the earth's crust from the present-day surface.