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引用本文:彭银彪,于胜尧,张建新,李三忠,孙德有. 北阿尔金地区早古生代洋壳俯冲时限:来自斜长花岗岩和花岗闪长岩的证据[J]. 中国地质, 2018, 45(2): 334-350.
PENG Yinbiao,YU Shengyao,ZHANG Jianxin,LI Sanzhong,SUN Deyou. Timing of Early Paleozoic oceanic crust subduction in North Altun: Evidence from plagiogranite and granodiorite[J]. Geology in China, 2018, 45(2): 334-350(in Chinese with English abstract).
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北阿尔金地区早古生代洋壳俯冲时限:来自斜长花岗岩和花岗闪长岩的证据
彭银彪,于胜尧,张建新,李三忠,孙德有
1.中国地质科学院地质研究所, 北京 100037;2.中国海洋大学海洋地球科学学院, 海底科学与探测技术教育部重点实验室, 山东 青岛 266100;3.青岛海洋科学与技术国家实验室海洋地质功能实验室, 山东 青岛 266061;4.吉林大学, 吉林 长春 130061
摘要:
蛇绿岩在不同演化阶段自身形成的花岗质岩石和侵入到蛇绿岩中的花岗质岩石对于蛇绿岩的精确定年具有重要意义,是揭示洋壳俯冲时限的有力证据。对北阿尔金红柳沟-拉配泉蛇绿岩中斜长花岗岩和花岗闪长岩的锆石U-Pb及Lu-Hf同位素分析表明,红柳沟斜长花岗岩和花岗闪长岩的锆石LA-ICP-MS U-Pb定年结果分别为(501±3)Ma和(496±2)Ma,表明北阿尔金洋的俯冲时限可能开始于中寒武世或更早。斜长花岗岩和花岗闪长岩的锆石εHft)值均为正值,结果分别为1.6~5.6和3.3~6.9,反映其源区均为亏损型地幔。全岩地球化学分析结果表明,斜长花岗岩具有高SiO2、高Sr、低Y和相应的高Sr/Y等类似于埃达克质岩石的特征,可能来自热的洋壳俯冲到石榴角闪岩相条件下变基性岩发生小比例部分熔融形成,且其形成深度应该在40~50 km;花岗闪长岩属于高钾钙碱性系列岩石,可能代表了岛弧环境下下地壳基性岩石部分熔融的产物。年代学分析表明,北阿尔金洋可能存在南北双向俯冲,并且北阿尔金洋向北俯冲可能略早于向南俯冲。北阿尔金和北祁连的俯冲时限对比研究表明,北阿尔金早古生代缝合带是北祁连早古生代缝合带的西延部分。
关键词:  北阿尔金地区  俯冲时限  斜长花岗岩  花岗闪长岩  早古生代
DOI:10.12029/gc20180209
分类号:P588.12;P597
基金项目:国家重点研发计划项目(2017YFC0601401),国家自然科学基金项目(41572053、41630207、41572180),中国地质调查局项目(1212011502700)和同位素地球化学国家重点实验室(中国科学院广州地球化学研究所)(SKLabIG-KF-16-02)联合资助。
Timing of Early Paleozoic oceanic crust subduction in North Altun: Evidence from plagiogranite and granodiorite
PENG Yinbiao1, YU Shengyao2,3,4, ZHANG Jianxin1, LI Sanzhong2,3,4, SUN Deyou5
1.Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;2.Key Laboratory of Submarine Geosciences and Prospecting Technique, Ministry of Education;3.College of Marine Geoscience, Ocean University of China, Qingdao 266100, Shangdong, China;4.Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266061, Shandong, China;5.Jilin University, Changchun 130061, Jilin, China
Abstract:
The granitic rocks that were formed from ophiolite or intruded into the ophiolites are important for the precise dating of ophiolites, and some of them can be the powerful evidence for constraining the timing of subduction. Based on detailed studies of field geological background and petrographical features, the authors conducted analysis of whole-rock major, trace and rare earth elements and zircon U-Pb ages as well as Lu-Hf isotopes for the plagiogranite and granodiorite. The weighted mean ages of plagiogranite and granodiorite from Hongliugou-Lapeiquan ophiolite in North Altun Mountains determined by LA-ICP-MS UPb method are 501±3 Ma and 496±2 Ma, respectively, suggesting that the subduction timing of the northern Altun Ocean might have started in the mid-Cambrian or earlier. The zircons from plagiogranite and granodiorite exhibit positive εHf (t) values of 1.6-5.6 and 2.7-6.9, separately, implying that the plagiogranites and granodiorites were all derived from the depleted mantle. Geochemical analysis indicates that the plagiogranites are characterized by high SiO2, high Sr, low Y and corresponding high Sr/Y, which are similar to features of adakitic rocks, suggesting that they resulted from the small proportion of partial melting of metabasites during the hot oceanic subduction to the depth of 40-50 km, belonging to the amphibolite facies. The granodiorite belongs to the series of high-Na calc-alkaline rocks, and may represent the product of mafic rocks remelting from the lower crustal in an island arc setting. The geochronological analysis indicates that the north Altun Ocean might have experienced both northward and southward subductions, and the northward subduction of the northern Altun Ocean may be slightly earlier than the southward subduction. The comparative study of the subduction time between the northern Altun and northern Qilian shows that the northern Altun Early Paleozoic suture zone is the extension of the North Qilian Early Paleozoic suture zone.
Key words:  North Altun area  subduction timing  plagiogranite  granodiorite  early Paleozoic