Northward subduction of the Indian Plate beneath the Kohistan island arc, Pakistan Himalaya; new evidence from isotopic data

The intra-oceanic Kohistan island arc was initiated during the Cretaceous as a result of northward movement of the Indian Plate. The conventional assertion that the arc was erected above a north-dipping subduction zone has been challenged by an alternative hypothesis, which suggests that the arc was built above a south-dipping subduction zone. This hypothesis is based partly on the apparent presence of a "Dupal" signature in mafic rocks of the arc, and partly on the occurrence of boninites exposed in the northern part of the arc. The "Dupal" signature, with enhanced levels of radiogenic lead, is found only in rocks extracted from the mantle in equatorial regions. However, new radiogenic isotope data, presented here, suggest that the isotopic signature of the mafic volcanic rocks of the juvenile arc, rather than being "Dupal"-type, is a function of fluids derived from subduction and dehydration of sea-floor sediments. Boninitic volcanic rocks in the Kohistan arc form a volumetrically minor part of a compositionally diverse sequence in the northern part of the arc. Boninites and high-Mg andesites are found mostly, but not exclusively, in the fore-arc region, and evidence is accumulating that these may also be erupted into back-arc regions. Sedimentological and geochemical data suggest the Kohistan boninites to have been erupted into a short-lived back-arc basin. Both the eruption of boninites into the back-arc and the lack of a "Dupal" signature would be consistent with northward subduction beneath Kohistan, although the latitude at which the arc was initiated remains unresolved.