| 西藏班公湖—怒江缝合带中段蓬湖蛇绿岩中的洋脊型二辉橄榄岩 |
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| 引用本文： | 卢雨潇,杨经绥,董玉飞,熊发挥,陈晓坚,李观龙,薄容众.西藏班公湖—怒江缝合带中段蓬湖蛇绿岩中的洋脊型二辉橄榄岩[J].地质学报,2019,93(10):2575-2597. |
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| 作者姓名： | 卢雨潇 杨经绥 董玉飞 熊发挥 陈晓坚 李观龙 薄容众 |
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| 作者单位： | 1) 自然资源部深地动力学重点实验室，中国地质科学院地质研究所，北京,100037,1) 自然资源部深地动力学重点实验室，中国地质科学院地质研究所，北京,100037,2) 中国地质大学(北京)地球科学与资源学院，北京，100083,1) 自然资源部深地动力学重点实验室，中国地质科学院地质研究所，北京,100037,2) 中国地质大学(北京)地球科学与资源学院，北京，100083,1) 自然资源部深地动力学重点实验室，中国地质科学院地质研究所，北京,100037,2) 中国地质大学(北京)地球科学与资源学院，北京，100083 |
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| 基金项目： | 本文为国家自然科学基金项目(41720104009)、中国地质调查局项目(DD20160023- 01)、中国地质调查局项目(DD201190060)和地质部行业基金项目(201511022)联合资助。 |
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| 摘 要： | 蓬湖蛇绿岩产于西藏藏北湖区的蓬湖西侧,属班公湖-怒江缝合带中段白拉拉弄-依拉山亚带。该蛇绿岩主要由地幔橄榄岩、堆晶岩和辉绿岩等组成。其中地幔橄榄岩由方辉橄榄岩和二辉橄榄岩组成。蓬湖二辉橄榄岩的橄榄石Fo值介于88.85～90.33之间、斜方辉石的Al2O3含量范围在4.26%～6.60%。与原始地幔相比,蓬湖二辉橄榄岩岩石有较高的MgO含量和较低的Al2O3、CaO和TiO2等易熔组分含量;稀土元素总量介于1.11×10-6～1.53×10-6之间,明显低于原始地幔值,配分模式为轻稀土轻微亏损。在原始地幔微量元素蛛网图中,蓬湖二辉橄榄岩显示Rb、Zr亏损,U、Ta、Sr强烈富集特征。蓬湖二辉橄榄岩的铂族元素总量介于22.9×10-9～27×10-9之间,PGEs球粒陨石标准化图解显示其为接近原始地幔的"平坦型"。以上特征与深海橄榄岩相似,指示它们可能形成于大洋中脊环境。定量模拟估算表明,蓬湖二辉橄榄岩可能来源于地幔中尖晶石相二辉橄榄岩源区,系经历了约5%～10%的部分熔融残余。蓬湖堆晶岩矿物结晶顺序为橄榄石-单斜辉石-斜长石,其中异剥橄榄岩中的单斜辉石Mg#值介于86.92～89.93之间、橄榄石Fo平均值为84.45,明显不同于MOR型蛇绿岩堆晶岩。蓬湖堆晶岩的矿物组成、岩浆结晶顺序和矿物成分均与俯冲带上SSZ型蛇绿岩形成的堆晶岩类似。以上结果表明,蓬湖二辉橄榄岩形成于大洋脊环境,为尖晶石二辉橄榄岩源区经历了不超过10%部分熔融的残余,后期由于洋内俯冲作用经历了岩石-熔体反应,形成了SSZ型堆晶岩和含较高Cr#值尖晶石的方辉橄榄岩。
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| 关 键 词： | 班公湖- 怒江缝合带 蓬湖蛇绿岩 洋脊型二辉橄榄岩 部分熔融 |
| 收稿时间： | 2019/7/30 0:00:00 |
| 修稿时间： | 2019/9/6 0:00:00 |
The MOR- type lherzolites in Peng Co ophiolite in the middle segment of the Bangong- Nujiang suture in Tibet |
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| LU Yuxiao,YANG Jingsui,DONG Yufei,XIONG Fahui,CHEN Xiaojian,LI Guanlong and BO Rongzhong.The MOR- type lherzolites in Peng Co ophiolite in the middle segment of the Bangong- Nujiang suture in Tibet[J].Acta Geologica Sinica,2019,93(10):2575-2597. |
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| Authors: | LU Yuxiao YANG Jingsui DONG Yufei XIONG Fahui CHEN Xiaojian LI Guanlong and BO Rongzhong |
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| Institution: | 1) Key Laboratory of Deep- Earth Dynamics of MNR, Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037,1) Key Laboratory of Deep- Earth Dynamics of MNR, Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037,2) School of Earth Science and Mineral Resources, China University of Geosciences, Beijing, 100083,1) Key Laboratory of Deep- Earth Dynamics of MNR, Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037,2) School of Earth Science and Mineral Resources, China University of Geosciences, Beijing, 100083,1) Key Laboratory of Deep- Earth Dynamics of MNR, Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037 and 2) School of Earth Science and Mineral Resources, China University of Geosciences, Beijing, 100083 |
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| Abstract: | The Peng Co ophiolite is located to the west of Peng lake in the area of lakes in north Tibet, which belongs to the Baila- Yilashan sub- belt of the the middle Bangong- Nujiang ophiolitic belt. The Peng Co ophiolite is mainly composed of mantle peridotites, cumulates, diabase dikes. Peridotites are made up of harzburgites and lherzolites. Mineral chemistry of the Peng Co lherzolites is characterized by Fo contents (88. 85~90. 33) of olivine and Al2O3 content (4. 26%~6. 60%) in orthopyroxenes. Compared to the primitive mantle, the Peng Co lherzolites have relatively higher MgO contents, lower CaO, Al2O3 and TiO2 contents. The total rare- earth element (REE) contents of the lherzolites are 1. 11×10-6~1. 53×10-6, which are lower than those of the primitive mantle. The chondrite- normalized REE patterns of the Peng Co lherzolites display slight loss in LREE. In the primitive mantle- normalized spider diagram, the Peng Co lherzolites exhibit negative Rb and Zr anomalies and intensively positive U, Ta, Sr anomalies. The PGEs contents of Peng Co lherzolites are between 22. 9×10-9~27×10-9. The chondrite- normalized PGEs patterns of the Peng Co lherzolites are consistent with that of the primitive mantle. Mineral and whole- rock geochemistry characteristics of the Peng Co lherzolites show an affinity to abyssal peridotites, indicating that it may have formed in the mid- ocean ridge setting. Through quantitative modeling, we conclude that the Peng Co lherzolites formed after 5%~10% degree of partial melting of the spinel- phase lherzolite mantle source. The crystallization sequence of Peng Co cumulate is olivine- clinopyroxene- plagioclase. The Mg# value of clinopyroxene in cumulate wehrlites ranges from 86. 92 to 89. 93, and the mean value of Fo in olivine is 84. 45, which differ from those of the MOR- type ophiolite cumulates. The mineral composition, sequence of magmatic crystallization and mineral components of Peng Co cumulate are similar to those of the cumulate formed by the SSZ- type ophiolite in the subduction zone. Therefore, we can draw a preliminary conclusion that Peng Co lherzolites were formed in an environment of mid oceanic ridge and were remnants of the spinel lherzolite zone which experienced a partial melting of no more than 10%. In the later period, due to the intra- oceanic subduction, it experienced the rock- melt interaction, and thus formed the SSZ- type cumulates and harzburgite of high Cr value. |
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| Keywords: | Bangong- Nujiang suture zone Peng Co ophiolite MOR- type lherzolites Partial melting |
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