Pre-Collisional Permo-Triassic Lamprophyres in Ladakh Reveal a Metasomatized Lithospheric Mantle Predating the Indo–Asian Collision

Potassic to ultrapotassic, volatile-rich alkaline magmas such as lamprophyres originate from deep, metasomatized mantle domains and occur in specialized geodynamic settings. Their enrichment in REE–HFSE and ability to sample the sub-lithospheric mantle make them key indicators of mantle metasomatism, enrichment–depletion cycles, and regional tectonomagmatic evolution. In the early Paleogene, Indo–Asian collision produced widespread post-collisional magmatism across the Trans-Himalaya; however, lamprophyres, lamproites, shoshonites, and alkaline basalts remain largely restricted to regions north of the Indus–Tsangpo Suture Zone (ITSZ). The occurrence of lamprophyres within the Ladakh Batholith is therefore significant in revealing the nature of the sub-Indian lithospheric mantle. These lamprophyres display alkaline to ultrapotassic affinities and yield Permian–Triassic U-Pb apatite ages (212 ± 17 Ma and 329 ± 71 Ma), substantially older than the Cenozoic (80-50 Ma) Ladakh arc magmatism. Whereas the T_DM age calculated from the isotopic Nd systematics imparts a late Proterozoic (1.0 Ga to 0.7 Ga) ages for the origin of source for investigated lamprophyres. The results demonstrate that they are pre-batholith basement intrusions, representing pre-collisional magmatism along the Asian continental margin. Their ages correspond to regional Permo-Triassic alkaline and lamprophyric magmatic events, including those of the Panjal Trap-related province of the NW Himalaya-Karakoram region, indicating a broader mantle metasomatism event predating Himalayan orogenesis. Trace element ratios like Th/La (0.4-0.5) and Nb/Y (1.3-1.9) are closer to the bulk continental crust implementing crustal input during their genesis. Moreover, the lamprophyres also exhibit fractionated REE patterns with pronounced Nb–Hf–Ti anomalies, reflecting crustal involvement or subduction modification during magma generation, while non-radiogenic initial Sr ratios and near-CHUR to negative εNd(t) values (+0.2 to –5.9) point to a subduction-modified, metasomatized mantle source. Carbonate metasomatism-related modification is also evident in these lamprophyres. Collectively, these features imply a major geodynamic episode during the waning stages of Gondwana breakup, involving mantle metasomatism beneath the Indian plate and interaction with remnants of the Tethyan oceanic lithosphere. The Late Paleozoic lamprophyres of the Ladakh region thus record an important pre-Himalayan mantle signature, preserving a magmatic history fundamentally distinct from the Cenozoic Indo-Asian collisional system.