Zircon U–Pb ages, geochemistry, and Sr–Nd–Pb–Hf isotopes of the Mugagangri monzogranite in the southern Qiangtang of Tibet, western China: Implications for the evolution of the Bangong Co‐Nujiang Meso‐Tethyan Ocean

Abstract

We present in‐situ zircon laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) U–Pb ages, whole‐rock geochemistry, and Sr–Nd–Pb–Hf isotopes of the Mugagangri monzogranite in the southern margin of the Qiangtang Block, Tibet, western China. The zircons yield a U–Pb age of ca. 123 Ma. The hornblende‐bearing monzogranite shows metaluminous to weak peraluminous and high‐K calc‐alkaline characteristics exemplified by high silica (SiO2 = 67.57–70.57 wt%), high aluminium (Al2O3 = 14.68–15.78 wt%), high potassium (K2O = 4.00–5.14 wt%), high alkali (K2O + Na2O = 7.88–8.62 wt%), and low calcium contents (CaO = 1.72–2.17 wt%), with the aluminium saturation index (A/CNK) ranging from 0.98 to 1.09, suggesting that the Mugagangri monzogranite is a metaluminous to weak peraluminous I‐type high‐K calc‐alkaline granite. Geochemically, similar to the arc magmas, the monzogranite is enriched in large‐ion lithophile elements, and relatively depleted in high‐field‐strength elements. The monzogranite displays relatively high (87Sr/86Sr)i values (0.70972–0.71240), uniform εNd(t) values (−2.24 to −3.40), variable zircon εHf(t) values (−14.1 to +8.0), and high radiogenic Pb isotopic values (206Pb/204Pb = 18.588–18.790, 207Pb/204Pb = 15.616–15.642, and 208Pb/204Pb = 38.838–39.053). These geochemical characteristics indicate that the monzogranite was derived from a mixed source comprising ancient crustal and mantle materials, and experienced fractional crystallization during emplacement. We propose that the parental magma of the Mugagangri monzogranite was most likely generated during northward subduction of the Bangong Co‐Nujiang Meso‐Tethys Ocean.

Petrogenesis and tectonic environment of the Middle Permian Luerma igneous complex in the southern Lhasa subterrane, Tibet: Evidence from geochemistry; Rb–Sr, Sm–Nd, and Lu–Hf isotopes; and zircon U–Pb geochronology

“…The LS, also known as the Gangdese terrane or Gangdese–Nyainqentanglha terrane, extends over 2000 km from east to west and approximately 300 km from north to south, is located between the BNS in the north and the YTS in the south, and is a popular study area for researching TP dynamics (Figure 1a) (Cao et al, 2023; Ge et al, 2006; Liu & Liu, 2009). The LS roughly comprises 3 parallel subterranes: the southern Lhasa subterrane (SL), Central Lhasa subterrane (CL), and northern Lhasa subterrane (NL) (Figure 1b) (Huang et al, 2020; Huang, Dai, et al, 2021; Zhu et al, 2011). Large‐scale igneous rocks that developed since the Late Jurassic in the LS are generally considered to be the product of the evolution of the Bangong–Nujiang Tethyan Ocean (BNO), Sumdo Tethyan Ocean (SDO), and Yarlung–Tsangpo Neo‐Tethyan Ocean (YTO) during the Mesozoic to Cenozoic (Huang et al, 2018; Huang, Zhang, et al, 2019; Kong et al, 2017; Wei et al, 1997).…”

“…Simplified tectonic and geological maps. (a) Simplified tectonic map of the Qinghai–Tibet Plateau, modified from Liu, Zhang, Huang, Li, Lü, et al (2019), Liu, Xiao, Huang, Zhang, Lan, et al (2019), (b) simplified tectonic map of the Lhasa terrane, modified from Liu, Zhang, et al (2020), Huang, Dai, et al (2021), (c) simplified geological map of the Luerma area, modified from Liu et al (2021); 1— Neogene igneous rocks; 2— Palaeogene igneous rocks; 3— Cretaceous igneous rocks; 4— Jurassic igneous rocks; 5— Triassic igneous rocks; 6— Permian igneous rocks; 7— Linzizong volcanic rocks; 8— Quaternary river valley; 9— Volcanics in the Palaeocene Dianzhong Fm. ; 10— Limestone in the Upper Triassic Jiangrang Fm.…”