西准噶尔巴音布拉克花岗斑岩成因:岩石地球化学、年代学与Sr-Nd-Hf同位素证据

Petrogenesis of Bayinbulake Granite Porphyry in West Junggar, China: Evidences from Petrogeochemistry, Geochronology and Sr-Nd-Hf Isotopes

西准噶尔雪米斯坦成矿带是中国重要的铀多金属成矿带,带内分布有大量古生代中酸性侵入岩体,这些岩体为探索雪米斯坦成矿带古生代岩浆活动及其构造演化提供了重要依据。巴音布拉克花岗斑岩岩体位于该成矿带东南部。对该花岗斑岩开展了系统的岩相学、锆石U-Pb年代学、岩石地球化学和Sr-Nd-Hf同位素研究。结果表明:该岩体形成于(393.7±1.7)Ma(MSWD值为1.3),属于早泥盆世,侵入于志留系雪米斯坦组地层中;在主量元素组成上,巴音布拉克花岗斑岩具有高Si、贫水、高Al、富碱(特别是富K)以及低Mg和Ca的特征,属准铝质-过铝质A2亚型花岗岩;微量元素组成上,岩石富集Rb、Th、U等,亏损Sr、P、Ti等,球粒陨石标准化稀土元素配分模式整体呈轻稀土元素相对富集的右倾模式,轻、重稀土元素分馏明显,稀土元素总含量较高且具有较为明显的Eu负异常;εNd(t)为4.3~6.7,变化范围很小,说明岩浆相对均一,二阶段Nd模式年龄为591~788 Ma,εHf(t)为3.98~9.35,二阶段Hf模式年龄为794~1130 Ma,成岩物质可能来自于新元古代的亏损地幔。综上所述,巴音布拉克花岗斑岩是在俯冲环境下地幔发生部分熔融,并在沿俯冲带上升过程中混入了少量古老地壳物质形成的。通过与雪米斯坦成矿带西部杨庄岩体对比,巴音布拉克岩体具有形成工业性铀矿的前景。

Xuemisitan metallogenic belt in West Junggar is an important Chinese uranium polymetallic metallogenic belt with the massively distributed Paleozoic intermediate-acid intrusive plutons. These plutons provide an important basis for the exploration of Paleozoic magmatism and tectonic evolution of Xuemisitan metallogenic belt. The petrography, zircon U-Pb geochronology, petrogeochemistry and Sr-Nd-Hf isotopes of Bayinbulake granite porphyry in the southeastern margin of Xuemisitan metallogenic belt were studied systematically. The results show that the granite porphyry is formed in(393.7±1.7)Ma(MSWD is 1.3), belonging to Early Devonian, and invades into Silurian Xuemisitan Formation;the granite porphyry is characterized by high Si, poor H2O, high Al, rich alkali(especially K), low Mg and Fe, belonging to metaluminous-peraluminous series A2 sub-type granite;the granite porphyry is enriched in Rb, Th, U, etc., and depleted in Sr, P, Ti, etc., chondrite-normalized REE pattern is a right-leaning type with relative rich LREE, and the granite porphyry has the characteristics of obvious LREE and HREE fractionation, higher contents of total REE, and apparent Eu negative anomaly;the variation range of Nd isotopic composition is small, εNd(t) is 4.3-6.7 with the two-stage Nd model age of 591-788 Ma, indicating that the magma is relatively homogeneous, and εHf(t) is 3.98-9.35 with the two-stage Hf model age of 794-1 130 Ma, indicating that the diagenetic material maybe come from Neoproterozoic depleted mantle. In general, Bayinbulake pluton is formed in subduction setting, which causes partial melting of the mantle, and is mixed with a small amount of ancient crustal material during the rising along the subduction zone. Compared with Yangzhuang pluton, Bayinbulake pluton has the prospect of forming industrial uranium deposits.