Documenting basin scale, geometry and provenance through detrital geochemical data: Lessons from the Neoproterozoic to Ordovician Lesser, Greater, and Tethyan Himalayan strata of Bhutan

Abstract

Detrital zircon (DZ) ages, augmented with εNd(0) and δ13C isotopic values from 18 new and 22 published samples collected from Lesser Himalayan (LH), Greater Himalayan (GH) and Tethyan Himalayan (TH) rocks in Bhutan, support deposition of >7km of sedimentary rock in late Cambrian–Ordovician time and provide a stratigraphic framework for the pre-collisional Indian margin. Youngest GH DZ grains become younger upsection from 900Ma to 477Ma. Youngest DZ grains in TH samples are ~490–460Ma. Both the LH Jaishidanda Formation (Fm), and the LH Baxa Group overlie Paleoproterozoic LH rocks. The Jaishidanda Fm exhibits distinct populations of youngest DZ peaks, 475–550Ma, and 800–1000Ma. The Baxa Group (Manas, Pangsari, and Phuntsholing formations) contains youngest DZ peaks at both 500–525Ma and 0.9–1.0Ga. However, most samples from the Baxa Group in western Bhutan contain no grains younger than 1.8Ga. Samples from the LH Paro Fm, which sits directly under the MCT in western Bhutan, have youngest DZ peaks at 0.5, 0.8, 1.0, 1.7, 1.8Ga. εNd values generally match DZ spectra, with samples that contain old, youngest grain populations corresponding to more negative εNd signatures. The Paro Fm is an exception where εNd (0) values from quartzite samples are quite negative (−19 to −24) whereas the εNd (0) values from interbedded schist contain younger detritus (−12 to −17). δ13C values from the Jaishidanda, Paro and Manas formations have δ13C values (−1.8 to +6) suggestive of deposition over late Neoproterozoic to Ordovician time. δ13C values from the Pangsari Fm vary from −2.8 to +1.8, compatible with deposition in the early- to middle Neoproterozoic. The young, latest Cambrian–Ordovician grains preserved in TH, GH and LH rocks suggest that the late Cambrian–Ordovician orogeny, documented in GH rocks throughout the orogen, served as a significant sediment source in Bhutan.