Geochemical Architecture of the Lower- to Middle-crustal Section of a Paleo-island Arc (Kohistan Complex, Jijal–Kamila Area, Northern Pakistan): Implications for the Evolution of an Oceanic Subduction Zone

Abstract

The processes active in the deep crust above an oceanic subduction zone during its evolution have been constrained through a detailed geochemical study (major and trace elements and Sr, Nd and Pb isotopes) of representative samples through an ∼30 km thick exhumed crustal section of the Cretaceous Kohistan oceanic island arc (Northern Pakistan). The use of both trace elements and radiogenic isotopes reveals two distinct geochemical suites (suites A and B) within the Jijal–Patan–Kiru–Kamila (JPKK) complex. Suite A is characterized by a progressive enrichment in 207Pb and a decrease in 143Nd/144Nd with increasing LaN/SmN. Suite B has higher 207Pb/204Pb and lower 143Nd/144Nd ratios with approximately constant LaN/SmN. By combining trace elements with different partitioning behaviour it is demonstrated that there is an increasing contribution of the subduction component in the magmas with time. It is also possible to distinguish a slab component imprint carried by aqueous fluids from one corresponding to sediment melts. Intrusive granites are abundant in the upper levels of the JPKK section. All were generated at the arc root level (Jijal crustal section) during dehydration-melting of hornblende-rich plutonic rocks. A three-stage geodynamic model is proposed for the evolution of the arc over a period of ∼30 Myr. The first stage (∼117–105 Ma) starts with the onset of subduction, which was followed by the building of the volcanic arc. The second stage (∼105–99 Ma to ∼96–91 Ma) corresponds to a major igneous event, which was characterized by abundant magma underplating and granulite-facies metamorphism at the arc base. Recycling of the residual–cumulative lower crust into the convective asthenospheric mantle was efficient during this stage, and was probably related to thermo-mechanical erosion of the base of the crust. The last stage (∼95–85 Ma) corresponds to a period of low magmatic activity, which marked the end of the intra-oceanic subduction. This is related to the formation of a ‘cold blanket’ above the slab surface as a result of thermo-mechanical erosion of the cold walls of the subduction zone (i.e. the upper part of the slab and the base of the overriding plate), and corner flow dragging the cold material into the zone of melt generation. Ultimately, a voluminous magmatic pulse occurred around 85 Ma (forming the Chilas complex), before arc–continent collision.