Remagnetization of igneous rocks in Gupis area of Kohistan arc, northern Pakistan

Abstract

The Kohistan arc was formed due to subduction of neo-Tethyan oceanic crust beneath Asia. The arc is comprised of volcanic, plutonic and sedimentary rocks of Mesozoic to Tertiary age, formed prior and after the suturing of the Indian and Asian continents. Paleomagnetic investigations have been carried out on Paleocene volcanic and plutonic rocks exposed in the northern part of the arc. A total of 110 samples from 16 sites were drilled. According to rock-magnetic studies the main magnetic carrier is magnetite. Optical microscopy study reveals that low-grade metamorphism have effected all rocks. Magnetite is found as both a primary magmatic mineral and secondary alteration product in all samples. Samples of volcanics yield post tilting characteristic remanent magnetizations (ChRM). The in situ mean direction of the ChRMs of the intrusives is similar to the in situ mean direction of volcanics. The presence of secondary magnetite in plutons, the similarity of in situ mean ChRM of plutons with that of post-tilting ChRMs of similar age volcanics and dissimilarity of the mean ChRM of plutons from expected directions at the time of formation of plutons support a secondary origin for the ChRM of plutons. As the ChRM directions of the volcanics and intrusives are the same, it implies that both ChRMs were acquired during the same remagnetization event. Comparing the mean paleolatitude (25 ± 6°N) from Gupis area with those from Indian APWP and considering the fact that there was prevailing heating event in Lower Tertiary in the area, the acquisition age of this secondary remanent magnetization can be bracketed between 50 and 35 Ma.