Geochemical evidence for a widespread Paleoproterozoic continental arc-back-arc magmatism in the Lesser Himalaya during the Columbia supercontinent assembly

Abstract

The geodynamic evolution of the proto-northern Indian continental margin during Paleoproterozoic is widely debated. The proposed models include (i) a mantle plume triggered rift-related tectonic setting at a passive continental margin and (ii) an Andean type active continental margin. A widespread quasi-continuous belt of Paleoproterozoic (∼1.9–1.8 Ga) granitoids and orthogneisses, as well as mafic volcanic and intrusive rocks intercalated with metasedimentary rocks, occur all along the strike of Lesser Himalaya. A geochemical re-investigation is presented here using the existing geochemical data on mafic volcanic and intrusive rocks from the Lesser Himalayan Sequence of Himachal, Garhwal, and Nepal sectors to address the tectonic affiliation of these rocks and to infer the geodynamic setting of the proto-northern continental margin of the Indian Shield. These mafic magmatic rocks are classified as subalkaline basalt and basaltic andesite with calc-alkaline magmatic affinity to transitional character between calc-alkaline and tholeiites. Broad initial ɛNd range reveals a heterogeneous mantle source developed by mixing between a depleted mantle and a recycled component. Mantle source enrichment of these rocks is attributed to the interaction between the siliceous partial melt from the subducted sediment-laden oceanic crust and the mantle wedge, which is further supported by compositional similarity with experimental hybrid partial melts of eclogite-peridotite reaction. Estimated mantle potential temperature from the composition of these mafic rocks is comparable to that of mid-oceanic ridge basalts and does not invoke a mantle plume in their genesis. Geochemical proxies based on the major oxide abundances reveal the generation of these rocks from olivine-poor lithologies. It is suggested that these rocks were generated in a back-arc setting during the assembly of the Columbia supercontinent at the proto-northern continental margin of the Indian Shield. Distinct Nd isotopic systematics in these rocks compared to the Paleoproterozoic metamafic rocks of Cathaysia Block of South China Craton questions their purported linkage in the Columbia configuration.