Zircon U-Pb ages, geochemical and Sr-Nd-Hf-O isotopic data of newly discovered ultrapotassic and adakitic rocks in the Kohistan Island Arc, North Pakistan: constraints on petrogenesis and post-collisional alkaline magmatism

Abstract

The succession from adakitic granite to peralkaline ultrapotassic rocks within collisional orogens provides valuable insights into the regional tectonic evolution. This study presents detailed petrography, zircon U-Pb ages, whole-rock geochemical data, and isotopic data from newly discovered adakitic granite and peralkaline ultrapotassic syenite rocks from the Kohistan Batholith in the Kohistan Island Arc (KIA), northern Pakistan. Zircon U-Pb dating indicates that the adakitic granite was emplaced at ∼33.6 Ma, slightly earlier than the syenite at ∼31.2 Ma. The adakitic granite is characterized by high contents of SiO _2, K ₂O, Na ₂O, Al ₂O _3, Sr, and Sr/Y ratios, as well as low MgO, Ni, and Cr, which likely reflects partial melting of water-fluxed mafic rocks in the thickened lower crust. The positive ε _Hf(t) (avg. +3.6) and mantle-like δ ¹⁸O _zir (avg. +5.3 ‰) values of the adakitic granite suggest that the magma was derived from a thickened lower crustal mafic source, with no significant contribution from subducted sediments or upper crustal materials. The syenite is potassic to ultrapotassic/shoshonitic, characterized by high K ₂O contents (8.7–10.3 wt%) and K ₂O/Na ₂O ratios (2.2–3.2), and exhibits enrichment in LILE and LREE while being depleted in HFSE. Geochemical and Sr–Nd–Hf–O isotopic data indicate that the syenite likely derived from partial melting of the metasomatized lithospheric mantle beneath the thickened lower crust. The low Ba/La (avg. 7.4) and Hf/Sm (avg. 0.06) ratios, combined with isotopic modeling, reveal a 10–20 % contribution of subducted pelagic marine sediments to the mantle source of ultrapotassic peralkaline rocks in the KIA, which is linked to subduction of the earlier Tethyan Ocean crust. These geochemical results provide evidence that slab break-off occurred in the western Himalaya ∼10 Ma after the India–KIA collision at ∼45 Ma, and resulted in the emplacement of the adakitic and ultrapotassic rocks, similar to post-collisional alkaline magmatism observed elsewhere in the Tethys Orogenic Belt. These coeval but petrogenetically distinct magmas highlight the interplay of crustal and mantle heterogeneity during post-collisional evolution across the Tethyan Orogen. In addition, this study refines our understanding of the post-collisional alkaline magmatism in the KIA and provides a broader geodynamic model applicable to other similar convergent orogenic belts worldwide.

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