青藏高原东缘休瓦促钨钼矿区复式岩体时空分布及演化意义

Temporal-spatial distribution and evolution implication of the composite intrusion in the Xiuwacu W-Mo deposit,SE Tibetan Plateau

关键词:  休瓦促  复式岩体  结晶分异  地质年代学  地球化学

Key words:  Xiuwacu  composite intrusion  fractionation crystallization  geochronology  geochemistry

摘要:

休瓦促钨钼矿区位于义敦地体南段，是该区域唯一存在晚三叠世、晚白垩世和新生代酸性侵入体直接接触形成复式岩体的矿区。目前尚未对该复式岩体的岩浆演化过程开展过系统深入研究，限制了对该矿区及区域构造事件和岩浆演化的认识。本文通过系统梳理前人研究成果，开展东矿段黑云母花岗岩，西矿段二长花岗岩、花岗斑岩地质年代学和地球化学分析，探讨休瓦促复式岩体岩浆演化过程及其对区域构造的指示意义。研究表明，东矿段黑云母花岗岩结晶年龄为205.4±4.3 Ma，属高钾的钾玄质系列，偏铝质，富集轻稀土和大离子亲石元素Rb、Nd、Sm等，亏损重稀土和高场强元素Nb、Ta、Zr、Hf等，具有较明显的负Eu异常，氧逸度较高(f_O2=-22.8~-6.8，平均-12.2)，ε_Hf(t)为正值(0.0~3.9)，ε_Nd(t)=-2.11~-1.18，⁸⁷Sr/⁸⁶Sr=0.707248~0.708070，锆石Hf二阶段模式年龄T_DM2=1.32~1.68 Ga；西矿段二长花岗岩、花岗斑岩和细晶岩具有相同的物质来源和极为相似的地球化学特征，花岗斑岩年龄为80.2±1.5 Ma，与黑云母花岗岩相比，具有更强烈的负Eu异常，更低的氧逸度(f_O2=-33.2~-3.2，平均-23.0)，更低的ε_Hf(t)值(-8.4~-2.1)、ε_Nd(t)值(-8.59~-5.14)；更高的⁸⁷Sr/⁸⁶Sr值(0.709987~0.713559)，更老的锆石Hf二阶段模式年龄(1.62~2.36 Ga)。研究认为：晚三叠世黑云母花岗岩在甘孜-理塘洋西向俯冲的开放系统中通过岩浆结晶分异作用形成，岩浆主要来源于亏损地幔和少量俯冲洋壳物质；晚白垩世岩体物质来源于区域拉张环境下的古老下地壳和少量幔源物质，在相对封闭系统下通过岩浆结晶分异作用形成，同时在岩浆演化过程中受围岩同化混染作用影响。与成矿相关花岗岩的成因研究对揭示区域构造和成矿事件具有重要意义。

Abstract:

Located in the south section of the Yidun terrane (SYT), the Xiuwacu W-Mo deposit occurs in a composite intrusion composed by the Late Triassic, late Cretaceous and Cenozoic intrusions. The study on the magmatic evolution process of the composite intrusion is poor, limiting the understanding of the deposit and the relation ship between regional tectonic events and magmatic activities. Basing on previous research results, this paper carries out geochronological and geochemical analysis of the biotite granite in the east ore section, and monzogranite, granite porphyry, and aplite in the west ore section, so as to explore the magmatic evolution process of the Xiuwacu composite intrusion and its indicative significance to regional tectonic background. With a crystallization age of biotite granite of 205.4 ±4.3 Ma, the Xiuwacu composite intrasion belongs to high potassium shoshonitic aluminous series, enriched in light rare earth (LREE) and large ion lithophile elements(LILE) Rb, Nd and Sm, depleted in heavy rare earth (HREE) and high field strength elements (HFSE) Nb, Ta, Zr and HF. It is with an obvious negative Eu anomaly and a high oxygen fugacity (fo₂ =-22.8~-6.8, average-12.2). The positive ε_Hf(t) values are from 0.0 to 3.9, the ε_Nd (t) are -2.11 to -1.18, the ⁸⁷Sr / ⁸⁶Sr are from 0.707248 to 0.708070, and the zircon Hf two-stage model age is 1.32~1.68 Ga. The monzogranite, granite porphyry and aplite in the west ore section have the same material source and very similar geochemical characteristics. The age of granite porphyry is 80.2 ± 1.5 Ma. Compared with the biotite granite, it is of a stronger negative Eu anomaly, a lower oxygen fugacity (fo₂ = -33.2 ~ -3.2, average -23.0), a lower temperature, a negative ε_Hf(t) value (-8.4 ~ -2.1) and ε_Nd(t) value (-8.59 ~ -5.14) a higher ⁸⁷Sr / ⁸⁶Sr value (0.709987 ~ 0.713559) and an older zircon Hf two-stage model age (1.62 ~ 2.36 Ga). The results show that the Late Triassic biotite granite was formed by fractionation crystallization due to the Gantze-Litang oceanic westward subduction in an open-system. The parental magma mainly comes from depleted mantle and a small amount of subduction oceanic crust material. The late Cretaceous intrusion mainly comes from the ancient lower crust and a small amount of mantle derived material under the regional tensile environment. The magmatic fractionation crystallization under the relatively closed-system is affected by the assimilation and contamination of surrounding rock in the process of magmatic evolution. It is of great significance for revealing the relationship between regional tectonic and metallogenic events.