Geochemistry of the Chagai Arc, Pakistan: Complex arc Dynamics Spanning the Cretaceous to the Quaternary.

The Chagai arc is located in western Pakistan and extends into Iran and Afghanistan. The arc forms a 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. The oldest volcanic rocks in the arc belong to the Sinjrani and Kuchakki Formations. These rocks are primarily basalts and basaltic-andesites which are form both pillow sequences and massive flows. Geochemically these units are very similar. They are tholeiitic lavas with typical island arc characteristics and an N-MORB source. For example when normalized to N-MORB they are LILE enriched, HFSE depleted, and have negative Nb and Ti anomalies. Intruded into the oldest units are younger diorite and granodiorite intrusions which are most likely Oligocene to Miocene. These rocks are calc-alkaline and but again have island arc affinities with a more depleted N- MORB source than the older lavas. These rocks are more evolved and show a negative europium anomaly on a chondrite normalized spider diagram. The youngest units in this study belong to the Quaternary Koh-e-Sultan and the Miocene Koh-e-Dalil These units have a calc-alkaline fractionation trend and contain more silicic lavas, including dacites, than the older lavas. Chemically these units are very similar; they both contain continental arc signatures and were generated from an E-MORB source. Current theories to explain the multiple phases of volcanism in the Chagai arc propose that these lavas are the result of intra-oceanic convergence in the Neo-Tethys. During the Latest Paleocene the arc collided with the Afghan microplate. Our preliminary data supports this model in that the initial phases of volcanism are entirely oceanic arc, followed by much younger continental arc volcanism.