Precise U-Pb ages from the Kohistan Complex (Northern Pakistan) illustrate rapid formation of arc-type crust

Gerold Zeilinger¹

Urs SCHALTEGGER¹

Jean-Pierre Burg¹

Nawaz Chaudhry²

Hamid Dawood³

Shahid Hussain³

¹ Earth Sciences Dep., ETH-Zentrum, Zurich, Switzerland

² Institute of Geology, Punjab University, Lahore, Pakistan

³ Museum of Natural History, Islamabad, Pakistan

The Kohistan Arc Complex in northern Pakistan was formed in the Tethys ocean in Mesozoic times and subsequently obducted onto the Indian plate along the Indus Suture in the Late Cretaceous - Paleocene times. The tectonic evolution of the arc can be subdivided into (1) a juvenile stage (estimated at ca.110-95 Ma) involving lithospheric growth through partial melting of a fertile mantle in an intraoceanic subduction environment; (2) intra-arc rifting (around 85 Ma) marked by the emplacement of large volumes of volcaniclastic rocks in the intra-arc extensional basin and underplating of the arc crust by gabbronorites; (3) a mature stage with Andean-type granitoid magmatism, which ceased with the India–Asia collision 60 to 40 Ma ago.

The structurally lowest levels of the Kohistan Arc Complex comprise the Jijal complex, which consists of ultramafic rocks overlain by granulite-facies gabbros. The complex is covered by a pile of metamorphic gabbroic to tonalitic dykes and sills overlain by metabasalts and metasediments. This association was intruded by partial melts of mantle origin (gabbros, tonalites, granitoids) representing the first stages of crustal growth (stage 1) in an intraoceanic arc. Precise age determinations yielded for a sub-granulitic gabbro and a granitoid sheet-like intrusion significantly discrete ages of 99 and 97 Ma, respectively; a tonalite body was emplaced into the same environment at 92 Ma. The whole complex was again penetrated by mantle melts (gabbronorites to granitoid dykes) during rifting (stage 2). A gabbronorite from the so-called Chilas Complex and a granitoid kyanite-bearing dyke yielded ages of 85 Ma and 83 Ma, respectively.

The time period for the initial arc buildup with intrusion of gabbroic to granitoid melts is constrained to a short time period between 99 and 92 Ma. Intra-arc rifting characterized by the intrusion of gabbronorites and granitoid dykes is dated between 82 and 85 Ma. The ages represent the first protolith ages from the sub-arc mantle-crust transition. Initial arc magmatism may therefore be constrained to at least three magmatic pulses of <5 m.y. duration with each magmatic cycle most likely comprising gabbroic to granitoid lithologies, pointing to rapid differentiation processes. The granitoid stocks, sheets and dykes were emplaced prior to obduction onto the Indian plate.