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    西藏班公湖-怒江成矿带早白垩世吉龙花岗闪长斑岩成因:锆石年代学、Hf同位素及岩石地球化学约束

    Petrogenesis of the early Cretaceous Jilong granodiorite porphyry in the Bangong Co-Nujiang metallogenic belt, Xizang, China: Constraints from zircon U-Pb geochronology, Hf isotopes, and whole-rock geochemistry

    • 摘要: 班公湖-怒江成矿带西段的龙荣地区,发育有吉龙、龙荣、双岔河等众多早白垩世成矿岩体。吉龙岩体位于龙荣地区中部,由黑云母花岗闪长斑岩和黑云母花岗闪长岩组成。LA-ICP-MS锆石U-Pb测年结果显示,花岗闪长斑岩锆石U-Pb年龄为(116.4±0.7)Ma (MSWD = 1.07),表明岩体形成于早白垩世。花岗闪长斑岩全岩地球化学特征为:具有较高的SiO2(63.66%~65.24%)、Al2O3(14.23%~16.21%)、MgO(2.32%~2.53%)含量,较低的P2O5(0.24%~0.26%)、TiO2(0.53%~0.61%)含量,里特曼指数(σ43)为0.86~1.47,铝饱和指数(A/CNK)为0.98~1.12,分异系数(DI)为63~68,富集大离子亲石元素(Rb、K),亏损高场强元素(Nb、Ta、Ti),相对于Rb和Th亏损Ba,Eu负异常(δEu = 0.37~0.58)。总体上,花岗闪长斑岩为钙碱性I型花岗岩,具有典型岛弧型岩浆岩的特征。花岗闪长斑岩具有不均一的锆石Hf同位素组成(εHf(t)= -15.7~-6.8),二阶段Hf模式年龄(tDM2)为2.2~1.6 Ga,具有较低的全岩锆石饱和温度(TZr = 782~790℃)和锆石Ti饱和温度(TTi = 603~772℃),Mg#值为49.38~55.96,表现出壳幔混合的特征。研究揭示,吉龙花岗闪长斑岩是古元古代地壳部分熔融形成的长英质岩浆与少量俯冲流体交代的幔源岩浆混合的产物,早白垩世晚期(120~110 Ma)向北俯冲的班公湖-怒江洋板片的折返可能是其成岩作用的主要动力学机制。

       

      Abstract: Various Early Cretaceous metallogenic intrusions, such as Jilong, Longrong, and Shuangchahe, develop in the Longrong region, western section of the Bangong Co-Nujiang metallogenic belt. The Jilong intrusion lies in the middle of the Longrong region, consisting of biotite granodiorite porphyry and biotite granodiorite. LA-ICP-MS zircon U-Pb dating results show that the zircon U-Pb age of the granodiorite porphyry is (116.4±0.7) Ma (MSWD = 1.07), suggesting that the Jilong intrusion formed during the Early Cretaceous epoch. The whole-rock geochemical characteristics of the granodiorite porphyry exhibit high contents of SiO2 (63.66%~65.24%), Al2O3 (14.23%~16.21%), and MgO (2.32%~2.53%), with low contents of P2O5 (0.24%~0.26%) and TiO2 (0.53%~0.61%). The rocks have a Rittmann index (σ43) ranging from 0.86 to 1.47, an aluminum saturation index (A/CNK) from 0.98 to 1.12, and a coefficient of differentiation (DI) from 63 to 68. They are enriched in large ion lithophile elements (LILEs) such as Rb and K, depleted in high field strength elements (HFSEs) such as Nb, Ta, and Ti. They are also depleted in Ba relative to Rb and Th, with a negative Eu anomaly (δEu = 0.37~0.58). Generally, the granodiorite porphyry is a calc-alkaline arc I-type granitoid with typical island arc magmatite characteristics. It exhibits heterogeneous zircon Hf isotopic compositions (εHf(t)= -15.7~-6.8) with a two-stage Hf model age (tDM2) ranging from 2.2 Ga to 1.6 Ga. Both the whole-rock zircon saturation temperatures (TZr = 782~790℃) and the zircon Ti saturation temperatures (TTi = 603~772℃) are low, with Mg# values ranging from 49.38 to 55.96, presenting characteristics of crust-mantle mixing. It is concluded that Jilong porphyry is the product of mixing felsic magma which forms through the partial melting of the Paleoproterozoic crust, with a small amount of mantle-derived magma under the metasomatism of subduction-related fluids. The inversion of the northward subduction slabs of Bangong Co-Nujiang Tethys in the Early Cretaceous epoch (120~110 Ma) may be the primary dynamic mechanism.

       

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