Report of CA. 760 Ma Mafic Rocks in the Eastern Himalayan Orogen: Petrogenesis and Geodynamic Implications

Constraints on the Neoproterozoic evolution of the Himalayan terrane remain poorly understood due to the scarcity of Neoproterozoic magmatic rocks. In this study, we report for the first time Middle Neoproterozoic mafic rocks from the eastern Himalayan orogen. Zircon U–Pb dating indicates that these rocks crystallized at approximately 760 Ma and can be divided into two distinct groups. Group 1 mafic rocks have E-MORB-like compositions and are enriched in incompatible elements and exhibit relatively higher initial (87Sr/86Sr)i ratios (0.7053–0.7063), lower positive whole-rock εNd(t) values (3.0 to 3.4), and zircon εHf(t) values ranging from 4.9 to 10.4. They also show low Nb/Th ratios and high Th/Yb, Nb/Yb, and (La/Sm)N ratios, suggesting a lithospheric mantle source. In contrast, Group 2 mafic rocks have N-MORB-like compositions and are characterized by light rare earth element (LREE)-depleted patterns, lower initial (87Sr/86Sr)i ratios (0.7033–0.7040), and higher positive whole-rock εNd(t) (4.8 to 6.0) and zircon εHf(t) values (4.6 to 10.9). Their high Nb/Th ratios and low Th/Yb, Nb/Yb, and (La/Sm)N ratios indicate an origin involving interaction between the lithospheric mantle and depleted asthenospheric mantle. The absence of coeval volcanic and sedimentary records, combined with high La/Y and Ti/V ratios, suggests that these mafic rocks differ from typical arc or back-arc basin suites but are consistent with an intraplate setting. Integrating previous studies on multistage Neoproterozoic magmatism in India and the Himalayas, we propose that the ca. 760 Ma mafic rocks in the eastern Himalaya were likely formed within an intraplate continental rift system.