Himalayan architecture constrained by isotopic tracers from clastic sediments

Abstract

We report the first geochemical traverse to integrate U-Pb ages and Hf data on single detrital zircons with bulk-rock Sm-Nd-Rb-Sr isotopic measurements across the breadth of the Himalayan orogen, in northwest India. Our dataset, from key tectonostratigraphic units in the Sutlej valley, provides detailed insights into both the provenance of the sedimentary formations, and their tectonic history. Whole-rock Nd data provide information on the mean age of source areas contributing detritus to the sedimentary units in the mountain belt, regardless of their current metamorphic grade. U-Pb ages of detrital zircons from the same units identify the age of melt formation in the source rocks, while Lu-Hf isotope data from zircons resolve discrete crust formation ages in the multiple eroding terrains. Complex whole-rock Sr systematics hint at the extent of pre-Himalayan thermal events. Our data confirm the geochemical distinction between the older Lesser Himalayan Formations and those of the depositionally younger units (including the High Himalayan Crystalline Series) that has been recorded along the strike of the Himalaya from the western syntaxis to Nepal. Palaeoproterozoic metasediments of the Jutogh Group and Rampur Formation yield εNd(500) values below - 17, whereas Neoproterozoic and younger metasediments yield εNd(500) above - 13. The latter group encompasses the Vaikrita Group gneisses of the High Himalayan crystallines and the overlying Haimanta Group, but also includes a number of units in thrust sheets north of the Main Boundary Thrust in the Lesser Himalayan Zone (designated the 'Outer Lesser Himalaya' to distinguish them from the Palaeoproterozoic 'Inner Lesser Himalaya'). Zircon populations from the Inner Lesser Himalaya are characterised by Palaeoproterozoic-Late Archaean ages (2.6-1.8 Ga), whereas the depositionally younger units contain populations both of this age, and a younger period (Meso- to Neo-Proterozoic; 1.1-0.8 Ga). Detrital zircon ages are younger than their respective Hf-isotope derived crustal formation ages by 0.7-2.1 Ga, indicating that the source regions of the detrital zircons consisted of older terranes with considerable amounts of reworking and renewed magmatism. The distinct feature of the High Himalayan crystallines, which is also shared with the Outer Lesser Himalaya of the Sutlej traverse, is the presence of Meso- to Neo-Proterozoic detritus derived from a complex source area. In addition, while 500 Ma granitoids are apparently confined to Neoproterozoic units such as the High Himalayan crystallines and Haimanta Group, 1.8-1.9 Ga granitoid bodies (e.g., the Wangtu gneiss in the Sutlej valley) are exclusively associated with Inner Lesser Himalayan units. Our data suggest that, while the Inner Lesser Himalaya sediments derived their detritus from partially reworked Archaean crust similar to the Aravalli craton, depositionally younger units such as the HHCS represent a mixture of detritus from this ancient crust and a more juvenile source region.