FIELD RELATIONS, GEOCHEMISTRY AND PETROGENESIS OF THE CRETACEOUS BASALTIC JUTAL DYKES, KOHISTAN, NORTHERN PAKISTAN

A 75 Ma Cretaceous suite of low-medium-K basalt to basaltic andesite dykes intrudes the Kohistan terrane between Jutal and Chalt, Gilgit area, northern Pakistan. Orientational data suggest they intruded in response to NW-SE to N-S tension. The dykes cut the Albian-Aptian Chalt Volcanic Group and the 102 Ma Matum Das trondhjemite pluton. They cut the fold structures and penetrative fabrics associated with the collision between Kohistan and Eurasia and thus constrain the age of formation of the Northern Suture. The dykes define two groups on the basis of trace element geochemistry and geochemical trends. A depleted D-type group has low concentrations of incompatible elements (2-8 x primordial mantle) and (Ce/Yb)N ratios < 1. An enriched E-type group is relatively enriched in both low field strength and light Rare Earth elements and exhibits increasing incompatible element concentrations with increasing Zr. Trace element and field data suggest that the dykes are genetically linked and formed by partial melting of a mantle source which has been metasomatized by subduction-related processes. Models involving (i) different degrees of partial melting (ii) two-stage partial melting and (iii) partial melting of a heterogeneous source are considered.