Geology of the Chalt–Babusar transect, Kohistan terrane,N. Pakistan: implications for the constitution and thickening of island-arc crust

Abstract

The Kohistan island arc terrane is sandwiched between the collided Indian and Karakoram plates in the Himalaya of North Pakistan. Structures related to collision, during which the arc was thrust onto the leading edge of continental India along the Main Mantle Thrust, have resulted in exposure of an almost complete section of arc crust. Mapping along a transect across the east end of the arc terrane provides new data concerning the magmatic emplacement of several of the principal units. The base of the arc here is occupied by a major stratiform ultramafic–gabbroic complex, the Sapat complex. This was intruded into the base of a thick pile of meta-volcanic rocks which make up the Kamila amphibolite belt, and which comprise a varied sequence of basalts some with MORB-type tholeiitic affinities and some with island-arc tholeiitic affinities as well as calc-alkaline andesites. Ultramafic and gabbronorite rocks of the Chilas complex are intrusive into the top of the Kamila amphibolite belt. The upper part of the crust comprises meta-sediments and meta-volcanic rocks of the Jaglot and Yasin-Chalt Groups. These were formed in one or more arc-related basins, and host much of the Kohistan batholith. A three-stage history of crustal thickening can be documented for the Kohistan arc. From its initiation at ca. 125–120 Ma until ca. 90 Ma, the arc grew downward through magmatic emplacement, into its base, of stratiform ultramafic–gabbroic plutonic complexes, and upward through extrusion of volcanic sequences. In Stage 2, the focus of crustal growth shifted upwards from the base of the arc with emplacement of the Chilas complex along the interface between the Kamila amphibolite belt and the overlying volcano-sedimentary cover. This stage of crustal thickening was accompanied by shortening associated with the 90–80 Ma Kohistan–Karakoram collision. Finally, in Stage 3 (80–45 Ma), the Kohistan batholith was emplaced into deformed cover rocks of the uppermost part of the arc crust.