Petrogenesis and Tectono-Magmatic Evolution of Neoproterozoic Kainchwa Granitoids in the NW Himalaya: Insights from Petrography and Whole-Rock Geochemistry

ABSTRACT The pre-collisional granitoids of the Lesser Himalaya preserve records of Proterozoic magmatic activity along the northern active margin of the Indian Plate. This study provides a comprehensive geological mapping, petrological investigation, and whole-rock geochemical analysis of the Kainchwa Granitoid Complex (KGC) in the NW Himalaya, aiming to elucidate its geological characteristics, tectono-magmatic evolution, emplacement mechanisms, and contact relationships with the surrounding country rocks. Geological mapping reveals that the Neoproterozoic KGC intrudes the Naura Formation along the S1 foliation plane, forming tongues and apophyses with sharp contact boundaries. The granitoids are classified into five variants: porphyroblastic gneiss, medium-grained augen gneiss, foliated porphyritic granite, fine-grained leucocratic granite, and garnet-bearing mesocratic granite. Geochemical data reveal high concentrations of SiO2 (50.48–73.97 wt.%), Al2O3 (13.08– 22.37 wt.%), K2O (2.03–5.59 wt.%), and Na2O (0.55–4.42 wt.%), with highly variable Na2O + K2O (4.28–9.14 wt.%), ΣREE (176.60–847.71 ppm), Rb (67–453 ppm), and Nb (6–23 ppm), indicating a possible heterogeneous source for these granitoids. The CIPW normative approach, utilising parameters Q’=100×Q/ (Q+Or+Ab+An) and ANOR = 100×An/(Or+An), places the KGC within the alkali granite field. The granitic rocks of KGC exhibit elevated SiO2 and are strongly peraluminous, with an A/CNK ratio of 1.65–3.38, similar to S-type granitoids. Total Alkali-Silica (TAS) and Harker diagrams confirm its alkaline to calc-alkaline nature, demonstrating systematic removal of ferromagnesian phases during crystallisation. The REE diagram indicates light rare earth element (LREE) enrichment ([La/Yb] N = 5.08–26.08) and moderate to slightly negative Eu anomalies (Eu/Eu* = 0.18–0.82), suggesting plagioclase fractionation under reducing conditions. Well-fractionated LREE patterns, reflected in consistent (La/Yb)N values, suggest a uniform fractionation process across KGC samples. Discrimination diagrams, such as Rb/Sr versus Rb/Ba and La versus La/Sm, further indicate that the KGC originated from sedimentary sources with low clay content and high maturity, characteristic of psammitic or pelitic compositions, underscoring the granitoid complex’s uniform tectonic history and sedimentary derivation.