Geochemistry and petrogenesis of Oligocene volcaniclastic rocks from the Chagai arc: implications for the emplacement of porphyry copper deposits

The Chagai–Raskoh arc is an east–west trending subduction-related magmatic belt in the western part of Pakistan. The initial Cretaceous to Paleocene volcanism in the arc is tholeiitic, and from the Eocene onwards, it becomes calc-alkaline in nature. The Oligocene volcaniclastic rocks are found in the western part of the Chagai–Raskoh arc. These rocks and are mainly andesitic tuffs and belong to the calc-alkaline series. Normalized trace element patterns with negative Nb anomalies and moderate enrichments in K, Sr, Rb and light rare earth elements (LREE) confirm a composition consistent with a continental margin (Andean)-type arc environment. The LREE enriched chondrite-normalized REE patterns with negative Eu anomalies are consistent with their calc-alkaline signature and suggest plagioclase fractionation during differentiation. The Oligocene volcaniclastic rocks from the Chagai–Raskoh arc are more enriched in large ion lithophile elements (LILE) and LREE than the older rocks of the arc. This enrichment in LILEs and LREEs suggests that during the Oligocene–Miocene, more subduction-related fluids were added to the sub-arc mantle source from the subducting Arabian slab and may have been responsible for the porphyry copper deposits in the arc.