Petrology of chloritoid–ilmenite-rich rocks in the Indus Suture mélange of Pakistan: Implications for the Cretaceous paleolatitude of Kohistan

Abstract

The Indian Plate and Kohistan Island Arc are juxtaposed to the north of Peshawar along the Indus Suture, which is characterized by a tectonic mélange consisting of ultramafic, mafic, and a variety of sedimentary rocks. Within metavolcanic greenstones, there are small, lensoid bodies comprising quartz, white mica (up to 4.5 wt% FeO), chlorite (pseudothuringite to ripidolite), pyrite, and abundant chloritoid (up to 60%) and ilmenite. These lenses consist essentially of SiO 2, TiO 2, Al 2O 3 and iron oxide. There is a clear petrographic and geochemical (major, trace and RE element) transition from these rocks to their host greenstones, suggesting a common parentage. The greenstones contain high TiO 2, Fe 2O 3 and P 2O 5. Their chondrite-normalized patterns show enrichment in light REE, depletion in heavy REE, negative Eu and positive Yb anomalies. Because the greenstones have probably been altered, the present set of data is not capable of deciphering the tectonic setting of these rocks, but previous geochemical studies have suggested an island arc setting for volcanic rocks in the mélange. By chemical analogy with lateritized basalts, the chloritoid-rich rocks are considered to be the product of weathering of basalts in tropical, probably equatorial, conditions before the northward drift of Kohistan. Mélange formation and greenschist facies metamorphism occurred during the collision of the Indian Plate and the Kohistan Magmatic Arc, to its north, in the Late Cretaceous-Early Tertiary.