82-1 Detrital Garnet Compositional Constraints on the Development of the Himalaya

Abstract:

In sedimentary provenance studies, detrital garnet chemistry has traditionally been applied to distinguish broadly between mafic and felsic source rocks or basic metamorphic and igneous lithologies. However, matching detrital garnet compositions to those in hinterland bedrock sources provides access to associated metadata (tectonic and unit affiliations, metamorphic or magmatic conditions, structural and geographic locations, relationships to drainage pathways and geomorphological features), enabling the use of sedimentary archives to test models of exhumation and sediment routing. The Himalayan Siwalik Group foreland basin deposits contain a wealth of information regarding Himalayan tectonics, erosion, paleoclimate, and paleontological changes. A core problem is whether its provenance records a simple, regionally progressive unroofing sequence or a more complex system in which sediment supply varied with local catchments, drainage reorganizations, thrust activation, and differential exhumation/erosion of specific units. Here, detrital garnet compositions from the Siwalik Group (n = 2,515; Surai, Tinau, Muksar Khola, and Karnali River, Nepal) and the Pre-Siwalik (n = 14; Kasauli Fm., NW India) were compared with those in the Himalayan Garnet Analysis Database (n = 12,434; HGAD v1.0, tinyurl.com/HimalayanGarnetDB). The maximum depositional ages range from 27.7 to 2.8 Ma. Almandine, Grossular, Pyrope, and Spessartine end-members were compared using Aitchison distance and dominance thresholds were applied if non-unique unit matches occurred. Siwalik Group garnets are similar to those from the Himalayan core [Greater Himalayan Crystallines (GHC), Lesser Himalaya Sequence (LHS), Tethyan Himalaya Sequence (THS), and Main Central Thrust]. Suture-zone and magmatic garnets first appear at ~15.1 Ma, with Gangdese Batholith-affiliated garnets increasing in the Middle–Upper Siwalik Group. Siwalik Group matches ≤ 300 km apart occur at faults, klippen, tectonic windows, gneiss domes, or contact-metamorphic settings, clustering at amphibolite facies. Siwalik Group garnets match those from the Himalayan core, but Muksar Khola samples are similar to those from the Indian Shield (interpreted as upper LHS or skarn sources), gneiss domes, and High Himalayan leucogranites. Pre-Siwalik garnets compositionally match those from GHC and LHS pelites, notably from a Sutlej Basin klippe and tectonic window. The data suggest that sediment was derived from multiple Himalayan structural levels, rather than from the progressive unroofing of deeper crust. The results demonstrate that detrital garnet can serve as a quantitative provenance tool rather than merely a broad metamorphic or lithology indicator.

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