The formation and bulk composition of modern juvenile continental crust: The Kohistan arc

Abstract

The intraoceanic Kohistan arc, northern Pakistan, exposes a complete crustal section composed of infracrustal basal cumulates formed at ≤55km depth, a broadly basaltic/gabbroic lower crust, a 26km thick calc-alkaline batholith, and 4km of a volcanoclastic/sedimentary sequence. The bulk composition of the Kohistan arc crust is approximated by estimating the relative volumes of exposed rocks through detailed field observations, in particular along a representative km-wide transect across the arc, through geobarometric constrains to determine the unit thicknesses, and through satellite images to estimate their lateral extent. We separated the arc into three major units: lower, mid-, and mid- to upper crust, which contain a total of 17 subunits whose average compositions were derived from employing a total of 594 whole rock analyses. The volume-integrated compositions of each unit yield the bulk composition of the arc crust. While the details of the resulting bulk composition depend slightly on the method of integration, all models yield an andesitic bulk supra Moho composition, with an average SiO2 of 56.6–59.3wt.% and XMg of 0.51–0.55. The Kohistan arc composition is similar to global continental crust estimates, suggesting that modern style arc activity is the dominant process that formed the (preserved) continental crust. A slight deficit in high field strength and incompatible elements in the Kohistan arc with respect to the global continental crust can be mitigated by adding 6–8wt.% of (basaltic) intraplate type magmas. Our results document that infra arc processes, even in a purely oceanic environment, result in an overall andesitic crust composition in mature arcs, contrary to the widely accepted view that oceanic arcs are basaltic. Bulk crust differentiation from a basaltic parent occurs through foundering of ultramafic cumulates. Our results imply that secondary reworking processes such as continental collision are of secondary importance to explain the major element chemistry of the bulk continental crust composition.