Early Paleozoic magmatism along the northern margin of East Gondwana

Abstract

Zircon UPb analyses of granitic gneisses from the gneiss domes (Xiaru, Mabja, Lhagoi Kangri, and Kangmar)within the Tethyan Himalaya and from the High Himalaya (Gyirong, Yadong, and Namche Barwa)yield that their protolith ages range from 509 Ma to 429 Ma. Similar to the Himalayan Cenozoic leucogranites, the Paleozoic granites are characterized by (1) high SiO2 (69.9–77.0 wt%), Al2O3 (12.4–16.1 wt%), K2O + Na2O (5.8–10.3 wt%) with K2O/Na2O > 1.0 and A/CNK > 1.0; (2) large variations in Rb (166–851 ppm), Sr (20–172 ppm), Ba (27–1474 ppm) and Rb/Sr ratios (1.0–38.3); (3) low degrees of LREE over HREE fractionation with (La/Yb)N < 9.0 and pronounced negative Eu anomalies with Eu/Eu* = 0.1–0.6; (4) negative Nd isotope ratios ranging from −12.6 to −7.7 and Hf isotope compositions from −15.3 to +1.0. All these geochemical characteristics indicate that the Paleozoic granites are peraluminous granite and derived from partial melting of metasedimentary rocks with various degrees of input of mantle-derived materials. Literature data from Sibumasu, Lhasa, and Qiangtang block suggest that Paleozoic granitoids from Cambrian to early Silurian are ubiquitous and share similar geochemical characteristics to Himalayan Paleozoic granites. Therefore, we infer that Himalaya, Lhasa, Qiangtang and Sibumasu block were present along the northern margin of Gondwana and the Paleozoic granites represented a protracted (~90 myr) Paleozoic magmatism which possibly could be formed by Andean-type subduction followed by asthenospheric upwelling, passive crustal extension, possibly even a rift event.