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New U/Pb and Fission track ages and their implication for the tectonic history of the Lower Kohistan Arc Complex, Northern Pakistan

 G. ZEILINGER, J.-P. BURG, U. SCHALTEGGER & D. SEWARD

Department of Earth Science, ETH Zurich, Switzerland

The Kohistan Arc Complex separates the Indian and Asian plates in NW Pakistan. It was formed as an Island Arc during Mesozoic times. Accretion to Asia and subsequent thrusting onto the northern margin of the Indian plate along the northward dipping Indus Suture Zone (ISZ) constitute the collisional history. The early tectonic evolution of the arc can be subdivided into 2 stages: (1) a juvenile stage (estimated at ca. 110-95 Ma): lithospheric growth through partial melting of a fertile mantle in an intraoceanic subduction environment and SW-thrusting expressed by anastomosing shear zones in the southern, i.e. lower part of the arc imposed by subduction of the Tethys oceanic crust; and (2) an intra-arc rifting stage (around 80 Ma) with the emplacement of large bodies of gabbronorite and of felsic dykes.

We will present two groups of new ages obtained in the Lower Kohistan Arc Complex: 1) crystallization ages of gabbroic to granodioritic intrusions reflecting the early arc related history and 2) zircon and apatite fission track (FT) cooling ages related to the younger continent/arc – continent collision and post-collisional uplift.

Results for U/Pb dating

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 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 diorite body was emplaced into the same environment at 92 Ma. The whole complex was again penetrated by mantle melts (gabbronorites) during rifting. A gabbronorite from the so-called Chilas Complex yielded an age of 85 Ma. A granitoid kyanite-bearing dyke yielded an age of 83 Ma.

Results for FT dating

Zircon and apatite fission-track ages were obtained from a weakly deformed, 1860 Ma old diorite ca. 100 m beneath the ISZ close to Jijal (the uppermost Indian plate) and from sheared gabbro-diorites and Kiru amphibolites representing the hanging wall, ca. 20 km north of the ISZ. To the south of the ISZ, zircon and apatite FT ages are 23±2 (2s) Ma and 3.7±1 Ma, while to the north they are 28±6 Ma and 11±2 Ma, respectively.

Tectonic Interpretation

Age determinations suggest a two-stage model for the Kohistan Complex: (A) an initial arc-building stage starting pre-100 Ma and lasting to at least 90 Ma, characterized by the emplacement of gabbroic to granodioritic melts; and (B) the extensional intra-arc rifting stage at ca. 85 Ma, characterized by the emplacement of gabbronorites. Subsequently felsic dykes intruded into the overlying pile under high-pressure conditions, most likely created by the injection of small volumes of granitic melts, generated by anatexis at the base of the arc. Despite the continuous nature of subduction and melt generation we anticipate magmatism to happen in multiple, short-lived differentiation events. The results demonstrate that the rates of magmatic differentiation in an arc environment are very high and can produce juvenile melts of granitic composition with depleted mantle isotopic characteristics within 2 million years. The studied granite and felsic dyke are part of the arc magmatism and are not derived by post-obduction melting of crustal sources within the Indian plate.

The FT ages yield tectonothermal information postdating the suturing which occurred at approximately 55 Ma ago. With this initial data set, there is clearly a younging from north to south (upper plate à lower plate). This could represent a tilting effect, or a more discrete event across the ISZ. If the latter is the case (to be tested with more data) then there is evidence for back sliding of the hanging wall (Kohistan Arc Complex) along the ISZ since at least 23 Ma. This trend is also recorded with apatite FT data which point to a maximum exhumation rate of ca. 0.8 mm/a in the footwall over the last 3.7 Ma and of ca. 0.3 mm/a for the last 11 Ma in the hanging wall of the ISZ. As a preliminary interpretation this indicates a normal sense of movement along the ISZ since 23 Ma in support of the structural evidence presented by Burg et al. (1996).

References

Burg, J.P., Chaudhry, M.N., Ghanzanfar, M., Anczkiewicz, R., and Spencer, D., 1996. Structural evidence for back sliding of the Kohistan arc in the collisional system of northwest Pakistan. Geology, 24, 739 - 742.