摘要
本文首次报告了班公湖-怒江结合带内蓬错地区接奴群火山岩夹层中具有富硅(70. 5%~75. 2%)、高镁(2. 4%~3. 5%)和高Mg~#(69. 4~75. 7)等特殊地球化学特征的流纹质岩石(流纹英安岩、流纹岩),并对该岩石进行镜下鉴定、造岩矿物电子探针分析、锆石U-Pb和Lu-Hf同位素分析及全岩地球化学测试。岩相学观察发现,在斑晶和基质中均可见一定量的黑云母以及少量白云母。电子探针分析结果揭示该套高镁质岩石镁质成分主要由黑云母(MgO=9. 2%~20. 0%)贡献;白云母也具有富镁(MgO=1. 4%~4. 0%)的特征,因含量较少故贡献较弱。岩石具典型的弧火山岩特征,富集Rb、Th、U等大离子亲石元素(LILE),相对亏损Nb、Ta、Ti等高场强元素(HSFE),并具明显的Sr、Eu负异常。2件锆石U-Pb年龄结果(161. 5±0. 5Ma、163. 3±1. 7Ma)指示其为中-晚侏罗世火山活动的产物,与前人报道该地区高镁安山岩年龄一致,且具一致的ε_(Hf)(t)值和地球化学特征,指示与高镁安山岩具有成因联系。研究结果表明蓬错高镁质流纹岩是由俯冲洋壳沉积物熔体与地幔组分发生交代作用经由角闪石等矿物相的结晶分异形成的酸性岩浆,并水下喷发的产物;结合地球化学特征,指示其形成于与洋壳俯冲相关的活动大陆边缘弧环境。因研究区地处班公湖-怒江结合带内部,夹持于南羌塘和北拉萨地块之间;且研究区出露老地层(二叠纪之前),南北两侧均有蛇绿岩展布,故提出蓬错高镁流纹质火山岩可能形成于班公湖怒江洋内部小洋盆洋壳俯冲至微陆块之下的构造地质背景。蓬错流纹质岩石为首次报道的具有高镁、富硅特征的流纹质岩石,这种特殊岩石的发现不仅完善高镁岩石家族体系(高镁玄武岩、高镁安山岩、高镁英安岩和高镁流纹岩),同时也为研究班怒洋演化提供详实的证据。
In this study, we report the results of zircon U-Pb ages, Hf isotopic analysis and systematic whole-rock geochemical data for the Late Jurassic rhyolitic rocks (rhyolitic dacite, rhyolite), which occur as the volcanic interlayers of the Jienu Group in the Peng Tso area of the Bangong-Nujiang suture zone. Furthermore, we carried out detail microscopic identification and electron probe analysis. Electron probe analysis reveals that the magnesium component of the High-Mg (HM) rocks is mainly contributed by biotite (MgO=9.2%~20.0%). The zircon U-Pb dating of two samples indicate that they crystallized at ca.163~161Ma. Geochemically, the rhyolitic rocks are characteristic by high SiO2 (70.5%~75.2%), MgO (2.4%~3.5%) and Mg#(69.4~75.7). All samples are enriched in large ion lithophilic elements (LILE) and relatively depleted high field strength elements (HSFE), with obvious negative Sr, Eu anomalies, which are typical characteristics of arc volcanic rocks. Their εHf(t) values exhibit a narrow range of variation and mainly concentrated on -9^-7. Our new data, combined with previously published date about HM andesites in the same area, indicate they have genetically related. The results show that in an arc environment of active continental margin related to oceanic crust subduction, the melts formed by subducted oceanic crust sediment metasomatismed by mantle component and then crystallized of mineral phases such as hornblende formed the HM rhyolitic rocks. Since the study area is located inside the Bangong-Nujiang suture zone, it is far away from the South Qiangtang and North Lhasa terranes, and there are many Pre-Permian stratum inside and ophiolites on both sides, so we deem that the high-magnesium rhyolitic volcanic rocks may be the result of the subduction of the oceanic crust between the microcontinent in the Bangong-Nujiang Ocean. The discovery of HM rhyolitic rocks not only complement the high Mg rocks family system (HM basalt, HM andesite, HM dacite and HM rhyolite), but also provide detailed evidence for the study of the evolution of the Bangong-Nujiang Ocean.
作者
李海峰
刘治博
陈伟
王楠
王嘉星
张开江
李发桥
王超
LI HaiFeng;LIU ZhiBo;CHEN Wei;WANG Nan;WANG JiaXing;ZHANG Kaijiang;LI FaQiao;WANG Chao(School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083 , China;Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 1 (X)037 , China;College of Earth Sciences, Chengdu University of Technology, Chengdu 610059 , China)
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2019年第3期799-815,共17页
Acta Petrologica Sinica
基金
中国地质调查局地质调查项目(DD2016026)资助