Late Jurassic granitoids in the Xilamulun Mo belt, Northeastern China: geochronology, geochemistry, and tectonic implications

The Xilamulun Mo belt of Northeastern China, located in the southeastern segment of the Central Asia Orogenic Belt (CAOB), is composed of large deposits of porphyry Mo and quartz-vein-type Mo, which are related to Mesozoic granitoids. Previous studies led to the conclusion that all granitoids in the region formed during the Cretaceous and Triassic, but our new laser ablation inductively coupled plasma mass spectrometry U–Pb zircon dating of magmatic zircons from five samples of four mineralized plutons (Nailingou, Longtoushan, and Hashitu granites and Erbadi and Hashitu granite porphyries) reveals that these range in age from 143.8 ± 1.2 to 149.5 ± 1.0 Ma. These granites show post-collisional (A-type) geochemical characteristics (e.g. enrichment in total alkali, LILE, and LREE and depletion in Eu, Ba, P, and Nb). The Erbadi, Longtoushan, Hashitu, and Longtoushan granitoids exhibit moderately positive Hf isotopic compositions (εHf(t) = −0.3 to 10.2), indicating that granitic magmas may reflect mixtures of mantle melts and continental crust. These mineralized granites were all emplaced along a major fault over a time span of ~6 million years during the Late Jurassic. We conclude that igneous activity and mineralization resulted from the rollback of the subducted Palaeo-Pacific plate beneath Eurasia. Confirming that the Late Jurassic granitic intrusives are related to the Mo mineralization is useful for understanding the Mesozoic tectonic evolution of the Xilamulun Mo belt and also has significant implications for the regional exploration of ores.

中国东北地区西拉木伦钼矿带（Xilamulun Mo belt）位于中亚造山带（Central Asia Orogenic Belt, CAOB）的东南段，由与中生代花岗岩类相关的大型斑岩型钼矿和石英脉型钼矿矿床组成。以往研究认为该区所有花岗岩类均形成于三叠纪与白垩纪，但我们对4个矿化岩体（奈林沟、龙头山、哈什吐花岗岩体以及额巴迪、哈什吐花岗斑岩体）的5件样品中的岩浆锆石开展的激光剥蚀电感耦合等离子体质谱锆石U-Pb定年结果显示，这些岩体的形成年龄介于143.8 ± 1.2 Ma至149.5 ± 1.0 Ma之间。这些花岗岩具备碰撞后（A型）地球化学特征，具体表现为富总碱、大离子亲石元素（LILE）与轻稀土元素（LREE），且亏损Eu、Ba、P及Nb元素。额巴迪、龙头山、哈什吐及龙头山花岗岩类呈现中等程度的正Hf同位素组成（εHf(t) = -0.3 至 10.2），指示花岗岩岩浆可能为幔源熔体与大陆地壳的混合产物。上述矿化花岗岩均沿大型断裂带侵位，形成时间跨度约600万年，处于晚侏罗世时期。我们认为，该区域的岩浆活动与成矿作用源于欧亚大陆下方俯冲的古太平洋板块板片回转。确认晚侏罗世花岗岩侵入体与钼矿化的成因关联，有助于理解西拉木伦钼矿带的中生代构造演化，同时对区域矿产勘查具有重要的指导意义。