Revisiting the origin and significance of the Paleoproterozoic complex in the Himalaya: new insights from apatite geochemistry

This study examines the tectono-thermal evolution of the Paleoproterozoic complex in the Arun area, eastern Nepal, highlighting the role of apatite geochemistry, along with zircon U–Pb dating and whole-rock analyses. New zircon dating from orthogneisses in the Main Central Thrust (MCT) zone and High Himalayan Crystalline Sequence (HHCS) yield crystallisation ages of ca. 1.79–1.78 Ga and 1.85–1.81 Ga, respectively. Combined with published data, coeval Paleoproterozoic magmatism in the HHCS and Lesser Himalayan Sequences support a once-unified continental crust. Detrital zircon data from HHCS metasediments indicate deposition during the Paleoproterozoic, contemporaneous with magmatism. Whole-rock-apatite compositions exclude an I-type origin, while high Y and low Ce-As contents in apatite support a crustal-derived S-type affinity. Negative Eu anomalies in apatite from MCT orthogneisses imply plagioclase-rich protoliths; in contrast, apatite from HHCS orthogneisses lacks Eu anomalies and shows low Mn-Y contents, suggesting garnet-bearing, plagioclase-poor protoliths. The Paleoproterozoic magmatism was likely driven by rift- or plume-related processes, ultimately leading to the development of a passive margin. Multiple pulses at ca. 1.87, 1.82, and 1.79 Ga, with mafic underplating, triggered crustal melting. Protolith differences in the MCT hanging wall reflect lateral variations in HHCS lithostratigraphy, more pronounced than previously recognised.