西藏冈底斯带中段南木林县仁堆侵入体LA-ICP-MS锆石U-Pb年龄及岩石成因
LA-ICP-MS zircon U-Pb ages and petrogenesis of Rindu intrusion near Namling, middle Gangdise magmatic belt
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摘要: 对冈底斯带中段仁堆地区二云母花岗岩进行LA-ICP-MS锆石U-Pb定年，主量和微量元素地球化学分析表明，二云母花岗岩的结晶年龄为226.3±0.98Ma。二云母花岗岩为高钾钙碱性强过铝质花岗岩，SiO2含量为72.83%~74.99%，K2O/Na2O为1.58~2.37，Alk为7.37%~8.72%，A/CNK为1.21~1.39（>1.1），刚玉标准分子ω(C-norm)大于1%。稀土元素总量为61.71×10-6~175.16×10-6，其中，轻稀土元素总量为53.93×10-6~165.49×10-6，重稀土元素总量为7.588×10-6~13.880×10-6，轻、重稀土元素分馏明显，（La/Yb）N介于10.49~42.170之间。在稀土元素配分模式图上，表现为轻稀土元素富集的右倾型，有明显的负Eu异常，δEu介于0.264~0.57之间；在微量元素蛛网图上，表现出Ba、Nb、Sr、Ti、P亏损和Rb、K、U富集的特征。二云母花岗岩是增厚的冈底斯地壳在印支晚期应力松弛阶段，由于构造减压诱发部分熔融形成的。Abstract: This paper reports LA-ICP-MS zircon U-Pb ages and whole rock major and trace element analyses of two-mica granite in the middle part of the Gangdise terrane, Tibet. Magma zircons from the two-mica granite yielded a weighted 206Pb/238U mean age of 226.3±0.98Ma. The two-mica granite is of strongly peraluminous high-K calc-alkaline series, with SiO2 values from 72.83% to 74.99%, K2O/Na2O values from 1.58 to 2.37, total alkali content from 7.37% to 8.72%, A/CNK ratios from 1.21 to 1.39(>1.1), and ω(C-norm) higher than 1%. The ∑REE values vary from 61.71×10-6 to 175.16×10-6, LREE from 53.93×10-6 to 165.49×10-6, HREE from 7.588×10-6 to 13.880×10-6, REE patterns are inclined toward the HREE side with intensive enrichment of LREE, and (La/Yb)N values are from 10.49 to 42.17. Eu anomalies are strong, with Eu/Eu? values varying from 0.264 to 0.57. In the primitive mantel-normalized spidergrams of trace elements, the depletion of Ba, Nb, Sr, Ti and enrichment of Rb, K, U are obvious. It is suggested that the magma for the two-mica granite was dominantly derived from partial melting of argillaceous rocks in the crust, induced by dehydration of mica minerals.
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Keywords:
- LA-ICP-MS zircon U-Pb dating /
- geochemistry /
- strongly peraluminous granite /
- Indosinian /
- Gangdise /
- Tibet
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