Documenting basin scale, geometry and provenance through detrital geochemical data: lessons from Neoproterozoic to Ordovician strata of Bhutan

We present 3517 detrital zircon (DZ) ages augmented with both whole-rock Nd and δ13C isotopic values from 22 new and 21 published samples collected from Greater (GH), Tethyan (TH), and Lesser Himalayan (LH) rocks in the Bhutan Himalaya. TH samples (n=7) are dominated by young, Cambro-Ordovician DZ grains with only 2 samples with ~0.9 Ga youngest DZ peaks. GH samples (n=10) also contained a significant component of Cambro-Ordovician youngest DZ peaks, and youngest DZ peaks become younger upsection from 900 Ma at the base to 455 Ma at the top. The LH Jaishidanda Formation (Fm) (distal LH) is exposed directly under the Main Central thrust (MCT) and overlies Paleoproterozoic LH rocks. The Jaishidanda Fm (n=8) has 2 populations of youngest DZ peaks, one between 475-550 Ma, and one between 800-1000 Ma, with no observed trend between stratigraphic level and age of youngest DZ peak. The Baxa Group (proximal LH) displays a wider range of youngest zircons. Samples from the Manas Fm in eastern Bhutan have youngest DZ peaks at both 500-525 Ma and 0.9-1.0 Ga. 2 samples from the Manas Fm in western Bhutan also yield ~500 Ma DZs, however most Baxa Group samples from western Bhutan (Manas, Pangsari and Phuntsoling Fms) contain ≥1.8 Ga youngest DZ peaks. The LH Paro Fm (n=5), which sits directly under the MCT in western Bhutan, displays a wide range of youngest DZ peaks (0.5, 0.8, 1.0, 1.7, and 1.8 Ga). ɛNd values generally match DZ spectra with older DZ corresponding to more negative ɛNd signatures, except for the Paro Fm where ɛNd (0) values from quartzite samples are quite negative (-19 to -24) while ɛNd (0) values from interbedded schist require younger detritus (-12 to -17). δ13C values for LH samples are also quite variable. δ13C values combined with youngest DZ peak ages can be used to help constrain deposition age. The Jaishidanda and Manas Fms have very positive δ13C values (+3 to +6) suggesting deposition in late Neoproterozoic to early Cambrian time, with deposition continuing into the Ordovician for the Jaishidanda Fm. δ13C values from the Pangsari Fm vary between -2.8 to +1.8, compatible with deposition in the Neoproteorzoic. δ13C values from marbles within the Paro Fm vary between -1.8 and +4, compatible with deposition during Cambro-Ordovician time. These combined samples highlight a significant depositional event during the Ordovician. The most distal units to the Indian craton (TH, GH and the LH Paro and Jaishidanda Fms) contain the highest percentage of young detritus, supporting a northern source for Ordovician zircon grains within quartzite units as well as the clays preserved as schist in the Paro Fm. More proximal LH units (Baxa Group) did not receive the young DZ from the north, even though Baxa deposition most likely overlapped in time with the Jaishidanda and Paro Fms. The source of young (Cambrian) DZ for the Baxa Fm was most likely orogenic belts associated with the assembly of the Gondwana supercontinent to the south and east.