A Cretaceous dike swarm provides evidence of a spreading axis in the back-arc basin of the Kohistan paleo-island arc, northwestern Himalaya, Pakistan

A 100-4000 m wide and 15 km long dike swarm, consisting of basalt and dolerite, occurs at the base of the Thelichi Formation in the Kohistan paleo-island arc terrane, north Pakistan. The dikes contain hornblende (altered from diopsidic-augite), diopsidic-augite (relics; ophitic to subophitic texture), chlorite, epidote, sphene, apatite, zircon, ilmenite, titanomagnetite and magnetite. The geochemistry reveals two groups of dikes: (1) Higher TiO2 (2.74-3.50 wt%), Na2O, Fe2O3 and lower Al2O3 (12.65-14.16 wt%) and MgO (3.73-5.04 wt%); (2) Lower TiO2 (1.24-2.05 wt%), Na2O, Fe2O3 and higher Al2O3 (14.02-16.52 wt%) and MgO (3.98-7.52 wt%). The MgO contents (3.73-7.52-wt%) show a variation in the dikes from relatively primitive to more evolved compositions. The dikes contain high amounts of both LILE and HFSE. The major, trace and rare-earth elements data confirm the MORB affinity and the back-arc basin origin of the dike swarm. The NW-SE orientation of the dike swarm and its 134 ± 3 Ma K-Ar age suggest the spreading axis of the back-arc basin in the Early Cretaceous.