Tectono-magmatic evolution of the central Kohistan Arc, NW Pakistan: Granitoid emplacement revealed by mineral chemistry

The central Kohistan Arc consists of Late Cretaceous to Early Tertiary, subduction-related, calc-alkaline to high-K granitoids with varied textures and compositions, reflecting several stages of a complex tectono-magmatic evolution. In this study, we present major elements data of amphibole, feldspar, and biotite, along with trace elements data of zircon from seven plutons, previously grouped into two categories: (1) pre-50 Ma intra-arc subduction-related and (2) post-50 Ma arc-continent collision-related granitoids. Amphibole compositions indicate that the intra-arc-related Late Cretaceous to Paleocene granitoids (Matiltan granite and Gabral quartz diorite) were less hydrous (∼4.6–5.8 wt% H$_2$O) and crystallized at lower temperatures (740–745 °C) and pressures (3.0–3.6 kbar) corresponding to shallow mid-crustal depth, with oxygen fugacity of (logfO2: −17 to −15). The latter, Early Eocene (53 Ma) Deshai quartz diorites, which represents transitional magmatism between the waning intra-arc subduction phase and the onset of arc-continental collision, and the collision related Mid-Eocene (46–44 Ma) Diwanger, Shahibagh, and Jut Banda granitoids crystallized at relatively higher temperatures (774–785 °C) and pressures (3.7–4.5 kbar), indicating emplacement level in the deep mid crust as well as record oxygen fugacity of logfO2 = −14 to −13, and H$_2$O contents of 5.7–7.2 wt%. ... mehrThe biotite and zircon compositions consistently reflect a calc-alkaline subduction-related origin for all plutons. Zircon Eu-anomalies additionally suggest a progressive deepening of the magma sources from ca. 40 to 65 km depth, consistent with a change of the tectonic regime from arc-related to collision-related magmatism accompanied by crustal thickening and assimilation.