陕西铜厂闪长岩地球化学、锆石U-Pb定年及Lu-Hf同位素研究
Studies on Geochemistry, Zircon U-Pb Dating and Lu-Hf Isotope Composition of the Tongchang Diorites, Shaanxi Province
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摘要: 对位于陕西汉中略阳县境内与铜厂铜矿关系密切的闪长岩体进行了地球化学、锆石U-Pb年代学及Lu-Hf同位素研究。地球化学研究表明,该岩体具有较高的Al2O3、Na2O和MgO含量,低K2O含量,属低钾钙碱性系列岩石。富集大离子亲石元素(Rb、Ba、Th、K、U),贫高场强元素(Ti、Nb、Ta、HREE),低Sr,显示壳源成因岩浆岩特征。依据锆石特征及锆石U-Pb同位素测年结果确定该岩体形成时代为(843.7±3.8)Ma,属新元古代岩浆作用的产物。锆石Hf同位素测试结果显示:εHf(t)介于-5.96~-30.17,二阶段模式年龄tDMC变化范围较大,介于1 816~3 344 Ma,存在1.81~1.95 Ga、2.13~2.65 Ga、2.72~2.78 Ga、2.97~3.09 Ga和~3.34 Ga 5组年龄峰值,暗示从新太古代到古元古代,扬子陆块曾发生过多期地壳增生事件。铜厂闪长岩是在Rodinia超大陆裂解过程中,由这些古老地壳物质部分熔融的结果。闪长岩体中2.72~2.78 Ga模式年龄峰值的出现,表明鱼洞子群的部分基性物质参与了铜厂闪长岩岩浆的形成。Abstract: The paper presents geochemistry, Zircon U-Pb geochronolgoy and Lu-Hf isotope of Tongchang diorites, which has a close relationship with the Tongchang Copper Deposit in Shaanxi province. Geochemical studies have shown that the diorites belong to low K calc-alkali rock series, which are characterized by higher Al2O3, Na2O, MgO content and lower K2O content. The rock is enriched in large ion lithophile elements (LILE) such as Rb, Ba, Th, U, K, but comparatively depleted in high field strength elements(HFSE) such as Ti, Nb, Ta and HREE. Meanwhile, the content of Sr is low, signifying the characteristics of crustal genesis magmatic rock. According to Zircon features and U-Pb isotopic dating, the rock is formed in a period of 843.7±5.5 Ma, which indicates that it is the product of Neoproterozoic magmatism. Lu-Hf isotope study demonstrates that its εHf(t) is between -5.96 and -30.17, and the value of tDM changes greatly from 1816 Ma to 3344 Ma. Moreover, the existence of five groups of age peak (i.e. 1.81-1.95, 2.13-2.65, 2.72-2.78, 2.97-3.09 and ~3.34 Ga ) indicates that the formation of Tongchang diorites is the result of partial melting of ancient crustal rocks in the breaking of Rodinia supercontinent, and these important crustal accretion events of the Yangtze Block occured from Neoarchean to Paleoproterozoic. The emergence of 2.72-2.78 Ga model age shows that partial basic materials from Yudongzi Group may have joined the formation of Tongchang diorites magmatism.
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Key words:
- Tongchang diorites /
- zircon U-Pb aging /
- zircon Hf isotope /
- Bikou Group /
- basement
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