CONSTRAINING THE BOUNDARY OF THE AFGHAN CONTINENTAL BLOCK WITHIN PAKISTAN: EVIDENCE FROM THE CHAGAI-RASKOH ARC

The Chagai-Raskoh arc is located in western Pakistan and extends into Iran and Afghanistan. The arc forms an elongate body trending EW and is roughly 500km long by 150km wide. Activity along the arc began in the Late Cretaceous and continued through into the Quaternary.

Current theories to explain the multiple phases of volcanism in the Chagai-Raskoh arc propose that these lavas are the result of intra-oceanic convergence in the Neo-Tethys. Our geochemical data supports this model in that the initial phases of volcanism are entirely oceanic arc. The increasing proximity of the Afghan Block is evidenced by increasing continental signatures in the lavas, followed by much younger continental arc volcanism. However, in the literature there is debate about the influence of the Afghan Block on the volcanism and also over the location of the Afghan Block-Makran Accretionary complex boundary.

The oldest volcanic rocks in the arc are primarily basalts and basaltic-andesites which are form both pillow sequences and massive flows. They are tholeiitic lavas with typical island arc characteristics and an N-MORB source. Sometime after the Senonian the basalt-dacite lavas, although initially tholeittic, become calc-alkaline with weak continental signatures. The youngest units are Quaternary through to Miocene, are more siliceous, have calc-alkaline fractionation trends, and strong continental arc signatures.

Dating old, up to greenschist metamorphosed, lavas can be very difficult. Hence in order to attempt to determine when the Chagai-Raskoh arc collided with the Afghan Block we are targeting zircons in a sequence of intrusions found cutting the arc.

Intruded into the oldest units are diorite and granodiorite intrusions which are most likely Oligocene to Miocene. They range between having strong continental arc signatures to having no continental signatures. Our preliminary results suggest that intrusions as young as 53Ma show strong continental arc signatures. This strongly supports our hypothesis that the Chagai-Raskoh arc collided with the Afghan Block during the Senonian.