| 西秦岭凤县地区早古生代岩浆作用特征:来自岩石地球化学、锆石U⁃Pb年龄及Hf同位素的证据 |
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| 引用本文： | 王永,石永红,陈柏林,谭人文,高允,申景辉.西秦岭凤县地区早古生代岩浆作用特征:来自岩石地球化学、锆石U⁃Pb年龄及Hf同位素的证据[J].地球科学,2021,46(6):2016-2036. |
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| 作者姓名： | 王永 石永红 陈柏林 谭人文 高允 申景辉 |
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| 作者单位： | 中国地质科学院地质力学研究所自然资源部古地磁与古构造重建重点实验室,北京 100081;中国地质科学院地质力学研究所自然资源部古地磁与古构造重建重点实验室,北京 100081;中国地质大学地球科学与资源学院,北京 100083;中国地质科学院地质力学研究所自然资源部古地磁与古构造重建重点实验室,北京 100081;中国地质大学地球科学学院,湖北武汉 430074 |
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| 基金项目： | 中央财政地质调查项目DD20190161中央财政地质调查项目DD20160053中国地质科学院中央级公益性科研院所基本科研业务费JYYWF20183702中国地质科学院中央级公益性科研院所基本科研业务费JYYWF20180602 |
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| 摘 要： | 为了研究西秦岭地区早古生代岩浆活动的构造环境及动力学背景，选择西秦岭凤县地区靠近商丹断裂带的龙王沟岩体的花岗岩和闪长岩、唐藏岩体闪长岩进行了地球化学测试和LA-ICP-MS锆石U-Pb年代学测试，并对龙王沟岩体的花岗岩和闪长岩进行了LA-MC-ICP-MS锆石Hf同位素分析.龙王沟岩体花岗岩锆石U-Pb年龄为433±4 Ma和439±5 Ma，闪长岩年龄为453±3 Ma，早于花岗岩侵位.龙王沟岩体的闪长岩和唐藏花岗闪长岩ΣREE分别为(107~181)×10-6和(209~321)×10-6，稀土配分曲线特征相似，都明显右倾，轻、重稀土分馏强烈((La/Yb)N分别为7~13和31~57)，都没有明显的负Eu异常(δEu分别为0.88~1.00和0.79~0.87)，唐藏花岗闪长岩相对于龙王沟闪长岩，具有高的SiO2和Na2O含量，较低的TiO2和CaO含量，较高的ΣREE，Ba和Th相对更富集，轻重稀土分馏更强烈，负Eu异常稍明显.龙王沟闪长岩与唐藏岩体其侵位时代一致，认为他们都来源于下地壳，在上升过程中与幔源基性岩浆发生一定程度的混合，唐藏花岗闪长岩经历的岩浆演化相对较为充分.龙王沟花岗岩的样品Hf同位素显示出了较大的差异，虽然εHf(t)全部为正值，但峰值分别为0~3和7~10，对应的两阶段模式年龄(tDM2)分别为1.1~1.4 Ga和0.8~1.0 Ga，结合其地球化学特征，认为它们形成于活动大陆边缘(岛弧)的构造环境，来源于秦岭群的部分熔融，同时有俯冲作用带来大量的洋壳中的基性岩的加入，在上升侵位过程中经历了较为强烈的结晶分异作用，但没有发生明显的岩浆混合作用.龙王沟岩体是一个由不同性质、不同时代的岩浆侵位形成的复合杂岩体，经历了板块俯冲到碰撞挤压的构造体制转换过程，记录了秦岭洋从俯冲到闭合各阶段的构造演化信息，秦岭洋在古生代的俯冲造山作用持续时间较长，伴随的岩浆活动从早古生代一直持续到中古生代.
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| 关 键 词： | 西秦岭 早古生代 岩浆活动 锆石U-Pb年龄 Hf同位素 地球化学 |
| 收稿时间： | 2020-03-31 |
Characteristics of Early Paleozoic Magmatism in Fengxian Area,West Qinling: Evidence from Petrogeochemistry,Zircon U-Pb Age and Hf Isotope |
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| Wang Yong,Shi Yonghong,Chen Bailin,Tan Renwen,Gao Yun,Shen Jinghui.Characteristics of Early Paleozoic Magmatism in Fengxian Area,West Qinling: Evidence from Petrogeochemistry,Zircon U-Pb Age and Hf Isotope[J].Earth Science-Journal of China University of Geosciences,2021,46(6):2016-2036. |
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| Authors: | Wang Yong Shi Yonghong Chen Bailin Tan Renwen Gao Yun Shen Jinghui |
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| Abstract: | In order to study the tectonic environment and dynamic background of the magmatic activity in the West Qinling orogenic belt in Early Paleozoic, it chooses the granite and diorite in Longwanggou rock mass and granodiorite in Tangzang rock mass, which near the Shang-Dan fault zone, for study. Geochemistry and LA-ICP-MS zircon U-Pb chronology were tested for all type of rocks and Hf isotope analysis for granite and diorite in Longwanggou rock mass. The dating results show that the age of Longwanggou granite is 433±4 Ma and 439±5 Ma, the diorite emplacement time is later than granite with the U-Pb age of 453±3 Ma. ΣREEs of Longwanggou diorite and Tangzang granodiorite are (107-181)×10-6 and (209-321)×10-6, respectively. Rare earth element distribution curve characteristics are similar, both showing clearly to the right-inclined pattern, strong light and heavy rare earth element fractionation strongly ((La/Yb) N at 7-13 and 31-57, respectively), and no obvious negative Eu anomaly (δEu are 0.88-1.00 and 0.79-0.87, respectively). Relative to the Longwanggou diorite, Tangzang granodiorites have high ΣREE, light and heavy rare earth element fractionation is more intense, less negative Eu anomaly. The emplacement times of Longwanggou diorite and Tangzang rock mass are consistent, both of originate and granodiorites are from the lower crust, mixed with mantle-based magma to a certain extent during the rising process. The Tangzang granodiorite experienced relatively sufficient magma evolution. Hf isotopic compositions have distinct differences between the two Longwanggou granite samples. Although the εHf(t) values are all positive, they concentrated on the values of 0-3 and 7-10 respectively, with corresponding two-stage model ages (tDM2) 1.1-1.4 Ga and 0.8-1.0 Ga. Combined with geochemical characteristics, the granite formed in the active continental margin tectonic environment (island arc) and derived from the partial melting of Qinling Group. At the same time, much of basic rocks from oceanic crust were brought into the melt during the plate subduction process, the magma have experienced a relatively strong crystallization differentiation and no obvious mingling process during the emplacement occurred. Longwanggou rock mass is a polyphase complex, experienced the tectonic regime transformation from subduction to collision and compression, and recorded the tectonic evolution information of Qinling ocean from subduction to closure. The subduction and orogenesis of Qinling area in the Paleozoic lasted for a long time and the accompanying magmatic activity also continued from Early Paleozoic to Middle Paleozoic. |
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