Petrogenetic, geochemical, and geochronological constraints on magmatic evolution of the Chilas Complex gabbros, Kohistan arc, NW Himalaya

Abstract

The Kohistan Arc preserves an exquisite and ideal petrogenetic record for studying mafic magmatism in an arc environment. Here we report the geochronological, mineralogical, and geochemical data of the gabbros from the Chilas Complex of Kohistan in order to understand their genesis, timing, role in crustal thickening, and tectonic evolution in Neo‐Tethys. The mineral composition of orthopyroxene (Opx) in gabbros displays transitional to metamorphic behaviour with high amount of Al2O3 (0.59–2.66 wt.%), suggesting the influence of subsolidus re‐equilibration during cooling or magmatic crystallization. Thermobarometric constraints propose the crystallization of Chilas gabbros around 892–985°C and ≤1.5 GPa pressure coupled with high H2O content (5.6%–7.7%). Geochemical modelling and isotopic peculiarities such as (87Sr/86Sr)i (0.000018–0.704140) and 206Pb/204Pb (18.526–18.554) suggest that the source of gabbros was depleted, likely controlled by peridotite melts and experienced insignificant crustal contamination. Relatively lower ratios of Nb/Yb (0.89–3.32) and TiO2/Yb (0.14–0.79) imply that the garnet was not a residual phase during the partial melting of a mantle source. The Chilas gabbros with depletion in Nb and Ta, enrichment in light rare earth element (LREE)s, the lower Ce/Pb ratios, plot of Th/La versus Th, and negative anomalies of Eu provide a consistent clue regarding the negligible participation of recycled components. Zircon U–Pb concordant ages of 83–86 Ma indicate the timing of emplacement and a Cretaceous magmatic event in the Chilas Complex. We infer that the Cretaceous magmatic flare‐up is a significant crustal growth event of Kohistan and consider the underplating of mantle‐derived magmas as one of the primary reasons for the crustal thickening based on zircon Hf isotopic signatures. The average chemical composition of Kohistan Complex provides support for the generation of andesitic magmas in arcs and the “andesite model” of continental crustal growth. We propose the similar petrogenesis for the Nidar, Kargil, and Chilas gabbros linking the formation of these variants within Neo‐Tethys based on identical magmatic records.