Geochronology and petrogenesis of alkaline rocks from the Peshawar Plain Alkaline Igneous Province, NW Himalaya, Pakistan: Implications for rare metal mineralization and geodynamic settings

Alkaline rocks of the Peshawar Plain Alkaline Igneous Province (PPAIP) constitute a major magmatic unit within the Indian plate in northwestern Pakistan. However, their tectonic settings, petrogenesis, and potential for associated rare metal mineralization remain poorly constrained. In this study, we present UPb zircon ages, mineral and whole-rock geochemical data, and NdHf isotope compositions for the Baru granite and its associated pegmatite, providing new insights into their origin. Zircon UPb dating of the Baru granite and the pegmatite yielded crystallization ages of 270.6 ± 1.7 Ma and 270.3 ± 1.5 Ma, respectively. Zircon grains exhibit positive ɛ_Hf(t) values, ranging from +4.1 to +7.3 for the Baru granite and from +5.2 to +10.0 for the pegmatite. The nearly identical ages and highly similar Hf isotope compositions indicate a co-magmatic origin for the granite and the pegmatite. Geochemically, the Baru granite is characterized by high SiO₂, Rb, Nb, U, and Ta contents, low Yb/Ta, Y/Nb ratios, and positive bulk-rock ɛ_Nd(t) values (+2.8 to +4.5). These features are consistent with fractionated anorogenic A-type granites derived from an OIB-like mantle source. Trace element compositions of zircons from the granite and the pegmatite exhibit systematic evolutionary trends, including increasing concentrations of HREEs, Hf, U, Y, Nb, and Ta, coupled with decreasing Eu anomalies and Ti concentrations. These trends strongly suggest that prolonged fractional crystallization of an alkaline magma, progressing from the granite to the pegmatite, was responsible for the enrichment of rare metals in the residual pegmatitic melt. The Baru granite shares similar emplacement ages and HfNd isotope compositions with other Permian alkaline rocks in the NW Himalaya, southern Qiangtang, and Tibet. This spatial and temporal correlation implies that these alkaline rocks formed within a large-scale extension regime across the Tethys realm. This early Permian magmatic event was likely triggered by a mantle plume, which played an active role during rifting along the northern margin of Gondwana.