Credit: © Jean-Louis Bodinier

During the past ∼500 million years, the accretion of island arcs has been the main process of the growth of continental crust, but only two examples worldwide are known to have been preserved on continents, complete with the arcs' roots. New geochemical analyses for one of them — the Kohistan Arc in Pakistan — reveal a multi-stage process for arc evolution and more than one source of magma, suggesting that island arc formation is more complicated than previously thought.

Bruno Dhuime from the University of Montpellier and colleagues1 obtained detailed data on the mineralogy and elemental and isotopic composition of rocks from the Jijal complex at the base of the Kohistan Arc, which formed when the ancient Neo-Tethyan ocean plate slid beneath the oceanic part of the Karakorum plate. The isotopic compositions of the ultramafic (very magnesium-rich) and mafic (moderately magnesium-rich) rock units — which would have been relatively similar had the complex formed from a single parent magma — were actually found to be distinct. The data suggest that the two units were formed by different processes and at different times.

The root of the Kohistan Arc thus records a complex history of arc formation, with compositionally and mineralogically distinct rocks forming at specific periods through the arc's evolution.