Growth and Chemical Evolution of the Kohistan Arc Crust, Northern Pakistan, Western Himalayas

Abstract Fossil arcs which expose the middle‐lower crust have the potential to shed light on active arc processes because magmatic rocks from the entire history of the arc can be examined. We present new geochemical data from the middle crust of the Kohistan Arc and use existing geochemical and geochronological literature data from this arc to constrain the growth and chemical evolution of the arc crust by reconstructing the chemical evolution of intrusive and extrusive magmatism. The ultramafic‐mafic lower crust of the Kohistan Arc is older than much of the intermediate‐felsic middle‐upper crust. The middle‐upper crust of the Kohistan Arc has higher (Ce/Yb) N and lower 143 Nd/ 144 Nd ratios than the older lower crust. The later intermediate to felsic plutons of this arc were formed by fractional crystallization of basaltic magmas. The Kohistan Arc formed in two main stages: (a) subduction initiation and early intra‐oceanic arc (e.g., Izu‐Bonin‐Mariana) magmatism (155–100 Ma) that formed the lower crust; and (b) mature intra‐oceanic arc (85–60 Ma) magmatism (e.g., Cascade Arc) followed by collisional (60–40 Ma) magmatism that generated the upper‐middle crust. Stage 1 volcanic and plutonic rocks have low (Ce/Yb) N ratios, whereas their Stage 2 equivalents have high (Ce/Yb) N ratios. The (Ce/Yb) N ratios and fractionation depths of the volcanic and plutonic rocks of the Kohistan Arc increased with decreasing age, reflecting the progressive maturation and crustal thickening of the arc. Lavas erupted at mature (∼100 Ma old) entirely intra‐oceanic arcs are geochemically similar to those found at “continental” arcs (e.g., Cascade Arc).