Tectono–Magmatic Evolution of the Central Kohistan Magmatic Arc, Pakistan—New Constraints from Zircon U–Pb Dating, Hf Isotopes and Geochemistry

Abstract New zircon U–Pb dating and Hf isotope analyses, along with bulk-rock geochemical data of plutonic rocks, indicate that the tectono–magmatic evolution of the Kohistan Arc (KA) in northern Pakistan was more complex than previously suggested. The new results obtained from (quartz) diorites, granodiorites, quartz monzonites, and granites from the central part of the KA (District Swat) indicate two major stages of tectono–magmatic evolution. Stage I comprises the pre-collisional phase of KA formation related to two magmatic pulses at 77–72 Ma and 63–53 Ma, and stage II the subsequent collision of the KA with the Indian plate at 47–44 Ma, which resulted in significant arc crust thickening, as indicated by the (La/Yb)N, Sr/Y, and (Gd/Yb)N ratios. In contrast to former studies, the new data indicate the involvement of significant amounts of ancient crust already during stage I, highlighted by variations in εHft values from 0 to +10, and for two distinct magma sources during stage II. Magmas parental to the stage IIa Shahibagh granodiorite resulted from the remelting of the previously formed KA crust, as indicated by superchondritic εHft values up to +10 and geochemical characteristics, whereas typical stage II granitoids (Jut Banda, Diwanger) were formed by melting of the sub-arc mantle, enriched by sediment-derived fluids during subduction of the Indian plate beneath the KA, resulting in subchondritic εHft values between 0 and −5.