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    福建长乐—南澳断裂带南务里变质岩碎屑锆石U-Pb年代学及其地质意义

    Detrital zircon U-Pb ages of the Nanwuli metamorphic rocks in the Changle-Nan’ao fault zone, Fujian and their geological significance

    • 摘要: 福建南务里变质岩位于长乐—南澳断裂带北段,主要由矽线石二云母石英片岩和矽线石黑云母石英片岩组成,超覆于片麻状花岗岩之上。岩石学特征表明:南务里变质岩的原岩为陆源碎屑岩。 LA-ICP-MS锆石U-Pb年龄谱出现2个峰值:第一峰值为189.6±5.7 Ma,206Pb/238U年龄集中于199~182 Ma; 第二峰值为1 899±4 Ma,207Pb/206Pb年龄集中于1 922~1 831 Ma。前者锆石自形晶较好,环带明显,Th/U值为0.4~0.96,稀土元素明显分异,属于岩浆锆石; 后者锆石磨圆度较高,环带不明显,Th/U值为0.03~0.3,稀土元素分异不明显,属于变质锆石; 部分为岩浆锆石,Th/U值>0.3,稀土元素分异明显。189.6±5.7 Ma代表该区变质岩原岩的沉积时代,原岩层位相当于早侏罗世梨山组。

       

      Abstract: The Nanwuli metamorphic rocks, located in the northern part of the Changle-Nan’ao fault zone of Fujian Province, consist mainly of sillimanite mica-quartz schist and sillimanite biotite quartz schist and overlay gneissic granite. Petrological characteristics indicate that the Nanwuli metamorphic rocks sourced from terrigenous clastic rocks. LA-ICP-MS zircon U-Pb age spectrum shows two peaks: the first peak is 189.6±5.7 Ma with a 206Pb/238U age range of 199~182 Ma, and the second is 1 899±4 Ma with a 207Pb/206Pb age range of 1 922~1 831 Ma. The former zircons(Early Jurassic zircons)are euhedral crystals with obvious oscillatory zonal texture of magmatic zircons and have Th/U ratios of 0.4~0.96 and distinct REE differentiation. As to the latter(Paleoproterozoic zircons), most of them show better psephicity with no obvious oscillatory zonal texture, have lower Th/U ratios(0.03~0.3)and weak REE differentiation, revealing that they derived from metamorphic rocks. A few Paleoproterozoic magmatic zircons have higher Th/U ratios(>0.3)and obvious REE differentiation, indicating that they might derive from old igneous rocks in the Cathaysia Block. The age of 189.6±5.7 Ma represents the sedimentary time(Early Jurassic)of the Nanwuli metamorphic rocks with its protolith equivalent to the early Jurassic Qingyingshan Formation.

       

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