Miocene granitic magmatism constrains the early E-W extension in the Himalayan Orogen: A case study of Kung Co leucogranite

Abstract

Himalayan leucogranites record the tectonic evolution of the Himalayan Orogen and provide a window to explore the deep dynamic processes. In this study, we conducted petrological, geochronological, and geochemical investigations of the Kung Co leucogranites that intrude into the shallow Tethys Himalayan Sequence along the north-south trending Nyima-Tingri rift. Whole-rock major and trace elements, and SrNd isotope geochemical investigations of the Kung Co leucogranites revealed that they: (1) had high SiO2 (> 71.5 wt%) and Al2O3 (> 14.8 wt%), relatively high CaO (> 1 wt%) and TiO2 (avg. 0.2 wt%), high Na2O and K2O, and uniform K2O/Na2O ratios (0.98–1.17); (2) were enriched in large-ion lithophilic elements but depleted in high-field-strength elements, and had relatively high Sr (> 114 ppm), low Rb (< 249 ppm), and low Rb/Sr ratios (0.66–1.64); (3) had moderate total REEs, enriched light REEs, and flat heavy REEs patterns, and no or slightly negative Eu anomalies (Eu/Eu* = 0.65–0.86); and (4) had homogeneous initial 87Sr/86Sr ratios (0.738047 to 0.739784) and εNd(t) values (−14.4 to −15.3). Geochemical characteristics and comparisons with Himalayan magmatic sources indicated that the Kung Co leucogranites originated from the partial melting of metagraywacke in the Greater Himalayan Crystalline Complex. Zircon and monazite U-Th-Pb dating confirmed leucogranitic magmatism at ca. 16.2 Ma. Zircon saturation thermometry and monazite saturation thermometry showed that the Kung Co leucogranites had higher melting temperatures than the Miocene Great Himalayan Granites, suggesting an anomalous geothermal gradient. Combined with the previous model of the Miocene evolution of the Himalayan Orogen and geophysical data, we proposed that Himalayan lithospheric delamination and upwelling of the asthenosphere provided additional heat that enabled the partial melting of metagraywacke, while the lithospheric delamination caused east-west extension and associated rifting activity, with magma rapidly intruding upward along the rift to the shallow crust.