Abstract
Although the Himalayan orogenic belt is dominant by Oligocene to Miocene leucogranites, it initiated magmatic activity from middle Eocene. However, the exact distribution and genesis of early magmatism is yet to be resolved. This study identified new outcrops of middle Eocene magmatism, Haweng granodiorite porphyries, from northwest Langkazi in the northern Tethyan Himalaya, southern Tibet. Identical zircon U–Pb ages were obtained by secondary ion mass spectrometry (SIMS) (17JT13: 45.3±0.4 Ma; 17JT16: 44.5±0.8 Ma) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) (17JT15: 44.3±0.8 Ma; 12FW75: 44.4±0.6 Ma) method for different outcrops there. Titanite LA-ICP-MS U-Pb analyses also gave consistent lower intercept ages for samples 17JT13 (45.3 ± 0.5 Ma) and 17JT15 (44.5 ± 0.6 Ma). Zircon metamorphic rims of sample 17JT15 recorded a younger thermal event of 29.9±0.4 Ma. The analyzed samples possess high SiO2 (69.98–73.53 wt.%), Al2O3 (15.07–16.15 wt.%), variable Na2O (3.94–5.81 wt.%) with Na2O/K2O ratios of 1.57–7.88, and A/CNK values of 1.08–1.27 indicative of sodic peraluminous series. They show variable Sr (342–481 ppm), Rb (37.9–133 ppm) concentrations, and low Rb/Sr ratios (0.10–0.39), radiogenic Sr isotopes (87Sr/86Sr(t): 0.7190 to 0.7251) and unradiogenic Nd–Hf isotopes with eNd(t) values of -13.54 to -11.55 and eHf(t) values of -11.97 to -9.37, respectively. The variation of major and trace elements, such increase of Na2O and Sr, and decreases of K2O and Rb, resulted from cumulation of plagioclase and crystal fractionation of K-feldspar during magma evolution. The Haweng granodiorite porphyries were derived from partial melting of dominant amphibolites and variable metasedimentary rocks. The newly identified outcrops help conform the EW trending middle Eocene magmatic belt along the Yarlung Tsangpo suture zone, resulting from breakoff of Neo-Tethyan slab at ca. 45 Ma.
Highlights
The Cenozoic Himalayan orogeny was accompanied by longlasting magmatism
Evidence of early anatexis during prograde metamorphism was reported from frontal Himalaya, the ubiquitous existences of monazites of this stage preclude the possibility of widely partial melting, as the melting would consume monazites and erase their records of earlier metamorphism (Gibson et al, 2004; Kohn et al, 2005)
The younger age group from the black zircon rims reflects a later event of zircon overgrowth or recrystallization, possibly under low temperature radiometric ages document the existence of early Oligocene thermal event in the study area, which has been reported along the Yarlung Tsangpo suture zone (YTSZ) (e.g., Chen et al, 2015)
Summary
The Cenozoic Himalayan orogeny was accompanied by longlasting magmatism. Deciphering the petrogenesis of these magmatic rocks is a key issue to understand the formation and evolution of the orogenic belt. Melt generation and granite emplacement are mainly related to the exhumation stage of the Greater Himalayan Sequence (GHS) during the late Oligocene to early Miocene (Harris and Massey, 1994; Hodges, 2000; Searle, 2013; Zeng and Gao, 2017). With respect to this dominant magmatic stage, less attention has been paid to the magmatism during prograde metamorphism, especially initial stage of partial melting. Only some coeval granitic dikes were found in the Ramba dome (Liu et al, 2014)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
