Petrogenesis of highly fractionated leucogranite in the Himalayas: The Early Miocene Cuonadong example

Abstract

There has been considerable debate concerning the petrogenesis of Cenozoic highly felsic, strongly peraluminous, leucogranites within the Himalayan-Tibetan orogen, with two contrasting viewpoints regarding their origin, either as primary melts from parting melting or magmatic differentiation. This debate hampers our understanding of the tectono-magmatic evolution and associated uplift and exhumation processes during Himalayan orogenesis. This study attempts to resolve this debate by means of an integrated petrographic, geochronological, and geochemical study on the Cenozoic Cuonadong leucogranites (including two-mica granites, garnet-bearing granites, and tourmaline-bearing granites) in the eastern part of the south Himalayan-Tibetan orogen. One SHIRIMP zircon U-Pb dating of two-mica granites and four LA-ICP-MS monazite U-Th-Pb dating of all three types of granites yield ages of ca. 21.9-19.7 Ma. Combined with previous published continuous U-Th-Pb zircon/monazite ages varying from 16.5 to 21.8 Ma, indicating that Cuonadong leucogranites have experienced continuously multiple episodes of intrusion and long-term crystallization from ca.16 to 22 Ma, and lasted for ca. 6 Myrs. Major and trace element geochemistry combined with whole-rock Sr-Nd isotope data suggest that the Cuonadong's three types of leucogranites are highly fractionated and garnet-and tourmaline-bearing granites are more evolved. The Cuonadong leucogranite melts are mainly derived from metapelites within the High Himalayan Crystalline Sequence, and underwent fractional crystallization of biotite, monazite, and plagioclase during melt migration. This study indicates Cuonadong leucogranites are Cenozoic highly fractionated strongly peraluminous leucogranites, which, combined with previous intermittent documentations of fractionated leucogranites, point to an increasing maturation of the continental crust composition during continental orogenesis.

Hydrothermal fluid evolution in the Cuonadong Sn–W–Be polymetallic deposit, southern Tibet: indicated by the in–situ element and boron isotope compositions of tourmaline

“…The B isotope characteristics of the granitoid in the Cuonadong polymetallic deposit indicate that it was mainly derived from partial melting of a metasediment source. This is also consistent with the Sr-Nd isotopic characteristic and high-K calc-alkaline and peraluminous characteristic of the Cuonadong granitoid (Xie et al, 2018;Xie et al, 2020;Chen et al, 2021). Furthermore, the whole Tur-1 from the rim to the core shows constant Mg/(Mg + Fe) and δ 11 B values, Frontiers in Earth Science frontiersin.org which indicates that the fluid composition was stable during Tur-1 formation (Figures 8, 9).…”