摘要
形成于弧前背景的岩浆岩可能记录了俯冲起始的重要证据。本文以北祁连走廊南山蛇绿混杂岩带新识别出的富铌辉长岩和斜长花岗岩为研究对象,在野外观察的基础上,结合岩石学、地球化学、同位素地球化学和年代学的综合研究,对原特提斯洋初始俯冲的时间进行约束,并探讨其形成过程及构造意义。锆石U-Pb定年结果显示富铌辉长岩样品锆石加权平均^(206)Pb/^(238)U年龄为512±4 Ma;两个斜长花岗岩样品的锆石加权平均^(206)Pb/^(238)U年龄分别为522±3 Ma和519±1 Ma。全岩地球化学结果显示富铌辉长岩的Nb含量为7.49×10^(-6)~10.80×10^(-6),TiO_(2)含量为1.50%~2.08%,Nb/U值为11.9~13.4,(Nb/La)N>0.5,明显高于岛弧玄武岩,其εNd(t)值介于+4.38~+5.78之间。斜长花岗岩样品K_(2)O含量(0.31%~1.66%)和K_(2)O/Na2O值(0.05~0.43)较低,稀土元素分布模式较为平缓,具有轻微的Eu正或负异常,相对富集Ba、Sr、U等大离子亲石元素,亏损Nb、Ta等高场强元素,结合其正的εNd(t)值(+3.35)和高、正的锆石εHf(t)值(主要在6.2~12.9之间),类似于弧前大洋斜长花岗岩。综合这些研究资料和区域地质,我们认为百经寺斜长花岗岩可能形成于俯冲初始阶段高温环境下俯冲洋壳部分熔融,随着俯冲继续,俯冲板片在更深部的部分熔融形成埃达克质熔体,并交代地幔楔形成富铌辉长岩。这些研究资料表明北祁连所代表的原特提斯洋的初始俯冲发生在早寒武世。
The magmatic rocks formed in the forearc setting are considered to possibly record key evidences for subduction initiation.In this paper,the niobium-rich gabbro and plagiogranite are newly reported in the Corridor Nan-shan ophiolitic melange zone of the North Qilian Orogen.Based on field observation,combined with comprehensive studies of petrology,geochemistry,isotope geochemistry and chronology,the time of initial subduction of proto-Tethys Ocean is constrained and its evolution model and tectonic implication are discussed.Zircon U-Pb dating results reveal a weighted average^(206)Pb/^(238)U zircon age at 512±4 Ma for a niobium-enriched gabbro sample,and 522±3 Ma and 519±1 Ma for two plagiogranite samples.The niobium-rich gabbro samples have Nb contents of 7.49×10^(-6)~10.80×10^(-6),TiO_(2)of 1.50%~2.08%,Nb/U values of 11.9~13.4 and(Nb/La)N>0.5,which are obviously higher than those of island-arc basalts.εNd(t)values of Nb-enriched grabbro are between+4.38 and+5.78.Plagiogranite samples have low K_(2)O content(0.31%~1.66%)and low ratios of K_(2)O/Na2O(0.05~0.43).The chondrite-normalized rare earth element pattern is relatively flat,with weakly positive Eu or negative anomalies.The plagioclase samples are relatively enriched in large ion lithophile elements such as Ba,Sr and U,and depleted in high field strength elements such as Nb and Ta.Combing their positiveεNd(t)value(+3.35)and highly positiveεHf(t)ranging(mostly between 6.2 to 12.9),we suggest an oceanic plagiogranite.The new data,along with the regional geology,indicate the Baijingsi plagiogranites are resulted from partial melting of the young and hot subducting oceanic slab at shallow depths in relatively high temperature during subduction initiation stage.Subsequently,the partial melting of subducting slab led to product of adakitic melts,which metasomatized or interacted with the upper mantle wedge peridotites,then the partial melting of metasomatized mantle peridotites gave rise to Nb-enriched basaltic magma.These data indicate that initial stage of subduction of the Prototethyan ocean(Paleo-Qilian ocean)occurred in the Early Cambrian.
作者
张同悦
张建新
于胜尧
祁钰
李传志
ZHANG Tong-yue;ZHANG Jan-xin;YU Sheng-yao;QI Yu;LI Chuan-zhi(MOE Key Lab of Submarine Geosciences and Prospecting Techniques,Ocean University of China,Qingdao 266100,China;The Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;Laboratory for Marine Mineral Resources,Laoshan Laboratory,Qingdao 266237,China)
出处
《岩石矿物学杂志》
CAS
CSCD
北大核心
2024年第2期250-273,共24页
Acta Petrologica et Mineralogica
基金
中国地质调查局地质调查项目(DD20221649)
国家自然科学基金项目(42372247)
山东省自然科学基金项目(ZR2020JQ16)。
关键词
初始俯冲
富铌辉长岩
斜长花岗岩
北祁连
原特提斯洋
subduction initiation
Nb-enriched gabbro
plagiogranite
North Qilian
Proto-Tethys Ocean
