Petrogenesis of Jurassic granitoids in the west central Lhasa subterrane, Tibet, China: the Geji example

<p>The Jurassic magmatic and volcanic rocks are widespread along the west central Lhasa subterrane. However, the petrogenesis of these rocks is poorly understood because of lacking high-quality geochronology and geochemical data. Here, we present new zircon U–Pb age and Hf isotopic data, whole-rock geochemical and Sr–Nd–Pb isotopic data for the Songduole and Qiangnong plutons in Geji area. LA-ICP-MS dating of zircon yield crystallization ages of 172.1 ± 1.9 and 155.9 ± 1.2 Ma for the Songduole and Qiangnong plutons, respectively. Geochemically, Songduole and Qiangnong granodiorite are characterized by high MgO (2.63–3.49 wt%), high Mg# (49–50), and low TiO<sub>2</sub> (0.48–0.57 wt%). Besides, all rocks show metaluminous, calc-alkaline signatures, with strong depletion of Nb, Ta, and Ti, enrichment of large-ion lithophile (e.g. Rb, Th, K), and a negative correlation between SiO<sub>2</sub> and P<sub>2</sub>O<sub>5</sub>. All these features are indicative of arc-related I-type magmatism. Five samples from the Songduole granodiorite have whole rock (<sup>87</sup>Sr/<sup>86</sup>Sr)<sub>i</sub> of 0.71207–0.71257, <i>ε</i><sub>Nd</sub>(<i>t</i>) values of −15.1 to −13.9, zircon <i>ε</i><sub>Hf</sub>(<i>t</i>) values of −17.4 to −10.5, (<sup>206</sup>Pb/<sup>204</sup>Pb)<sub>t</sub> ratios of 18.402–18.854, (<sup>207</sup>Pb/<sup>204</sup>Pb)<sub>t</sub> ratios of 15.660–15.736, and (<sup>208</sup>Pb/<sup>204</sup>Pb)<sub>t</sub> ratios of 38.436–39.208. Samples from the Qiangnong granodiorite have (<sup>87</sup>Sr/<sup>86</sup>Sr)<sub>i</sub> of 0.71230–0.71252, <i>ε</i><sub>Nd</sub>(<i>t</i>) values of −15.1 to −14.2, zircon <i>ε</i><sub>Hf</sub>(<i>t</i>) values of −12.6 to −6.4, (<sup>206</sup>Pb/<sup>204</sup>Pb)<sub>t</sub> ratios of 18.688–18.766, (<sup>207</sup>Pb/<sup>204</sup>Pb)<sub>t</sub> ratios of 15.696–15.717, and (<sup>208</sup>Pb/<sup>204</sup>Pb)<sub>t</sub> ratios of 38.546–39.083. These geochemical signatures indicate that the two plutons most likely originated from partial melting of the ancient Lhasa lower crust with obvious inputs of mantle-derived melts. Combined with regional geology, our results indicate that the Jurassic magmatism in the west central Lhasa subterrane most likely resulted from the southward subduction of the Bangong Ocean lithosphere beneath the central Lhasa terrane.</p>