勉略构造带新元古代洋盆俯冲作用:来自略阳地区纸房沟岩片火山岩锆石U-Pb年龄和Lu-Hf同位素的证据  

The subduction of Neoproterozoic Ocean in the Mianlue tectonic zone:evidence from zircon U-Pb age and Lu-Hf isotopes from volcanic rocks in the Zhifanggou tectonic slice, Lueyang area

作　　者：徐通 张晓团 焦建刚[2] 张红强 贾力 XU Tong;ZHANG Xiaotuan;JIAO Jiangang;ZHANG Hongqiang;JIA Li(Shaanxi Institute of Geological Survey,Academician Expert Workstation of Shaanxi Province,Xi’an,710054;School of Earth Science and Resources,Chang’an University,Xi’an,710054)

机构地区：[1]陕西省地质调查院,陕西省院士专家工作站,西安710054 [2]长安大学地球科学与资源学院,西安710054

出　　处：《地质学报》2020年第12期3607-3624,共18页Acta Geologica Sinica

基　　金：陕西省自然科学基础研究计划项目(编号2018JQ4030);陕西省公益性地质调查科研类项目(编号201906)联合资助的成果。

摘　　要：纸房沟岩片位于勉略构造带略阳地区。通过细致的岩石学和地球化学研究,将纸房沟岩片内的火山岩分为拉斑玄武岩系列(Ⅰ类)和钙碱性火山岩系列(Ⅱ类)。Ⅰ类火山岩稀土配分曲线左倾,富Na贫P,整体具有N-MORB特点,形成于洋脊构造环境。相比Ⅰ类火山岩,Ⅱ类火山岩具有更高的SiO2(49.02%~61.86%)和K2O(0.32%~1.55%)含量,相对亏损Nb、Ta、P和Ti,形成于陆缘弧构造环境。Ⅱ类火山岩锆石Lu-Hf同位素测试结果显示,εHf(t)值主体为负值,介于-8.01^+0.77,表明其主要为古老地壳物质熔融成因。Ⅱ类火山岩LA-ICP-MS锆石U-Pb测年结果为854±3Ma(MSWD=0.12,n=30)、844±4Ma(MSWD=0.03,n=15),表明纸房沟岩片火山岩结晶时代为新元古代早中期。综合前人成果认为,纸房沟岩片火山岩为新元古代勉略洋盆俯冲的产物,俯冲作用一直持续到800Ma左右,该俯冲过程很可能是对全球性Rodinia超大陆聚合事件的响应。The Zhifanggou tectonic slice is located in the Mianlue tectonic zone, Lueyang area. Based on detailed petrological and geochemical studies, we identifiedthe main rock types in the Zhifanggou tectonic slice asa combination of tholeiite(type Ⅰ) and calc-alkaline volcanic rocks(type Ⅱ). Type Ⅰ volcanic rocks possess the features of normal mid-ocean ridge basalt such as left-leaning REE patterns, are Na-rich, and P-poor, indicating that they were generated in a mid-ocean ridge setting. Type Ⅱ volcanic rocks have higher SiO2(49.02%~61.86%) and K2O(0.32%~1.55%)concentrations than type Ⅰ andnegative anomaliesof Nb, Ta, P, Ti, suggesting generation in a continental marginal arc setting. The zircon Lu-Hf isotopic composition show that the zircon εHf(t) of type Ⅱ volcanic rocks ranges from-8.01 to +0.77, suggesting that these melts were derived from old continental crust material. The LA-ICP-MS zircon U-Pb age determined using laser ablation-inductively coupled plasma-mass spectrometry from Type Ⅱ volcanic rocks is 854±3 Ma(MSWD=0.12, n=30) and 844±4 Ma(MSWD=0.03, n=15), indicating the crystallizationage of the volcanic rocks from Zhifanggou tectonic slice asMiddle Neoproterozoic. The volcanic rocks from Zhifanggou tectonic slice are considered to bethe product of subduction of the Neoproterozoic Mianlue Ocean that did not enduntil about ~800 Ma. This process islikely to have been a response to the amalgamation of Rodinia supercontinent.

关 键 词：勉略构造带 纸房沟岩片 火山岩 新元古代 南秦岭南缘 RODINIA超大陆 

分 类 号：P597[天文地球—地球化学] P542[天文地球—地质学]