Geochemistry of the Chagai-Raskoh arc, Pakistan: Complex arc dynamics spanning the Cretaceous to the Quaternary

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 500 km long by 150 km 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 andesites which 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 large ion lithophile element (LILE) enriched, high field strength element (HFSE) depleted, and have negative Nb and Ti anomalies. Also within the Sinjrani Formation are a sequence of Senonian basalt-dacite lavas that were generated from an N-MORB type mantle source, and although they are initially tholeiitic the more evolved lavas are calc-alkaline. Trace element data indicates these lavas have a very minor continental affiliation. The generation of these lavas may mark the increasing proximity of the Afghan continental block. 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 low degrees of partial melting of an enriched source. 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 data supports this model in that the initial phases of volcanism are entirely formed in an oceanic arc. However the increasing proximity of the Afghan Block, ca. 65 Ma, is evidenced by increasing continental signatures in the lavas, followed by much younger continental arc volcanism. We suggest the north-south distribution of oceanic versus continental signatures indicates that the location of the Afghan Block-Makran accretionary complex boundary is beneath the Chagai-Raskoh arc.