摘要
大洋斜长花岗岩是指分布在蛇绿岩或现今洋壳中的少量长英质侵入体,可形成在洋中脊及俯冲带等多种构造环境,虽然其在洋壳和蛇绿岩中所在体积很小,但对探究大洋岩石圈演化、俯冲起始及洋-陆转化机制具有重要意义。本文对北秦岭造山带西段商丹缝合带内新识别出的斜长花岗岩开展了岩石学、地球化学和锆石U-Pb年代学的综合研究。全岩地球化学结果指示花岗闪长岩具有高的SiO2、Al2O3含量以及较低的K2O、TiO2含量,属于低钾亚碱性、准铝质到弱过铝质的岩石。微量元素相对富Sr、贫Nb、Ta和Ti,具有平坦的稀土元素配分模式,是弧前环境中由俯冲板片在浅部层次部分熔融成因的大洋斜长花岗岩。3个斜长花岗岩样品分别记录了526±4 Ma、515±4 Ma和517±6 Ma的锆石U-Pb年龄,结合前人同时代的玻安岩、高镁安山岩的报道,西秦岭商丹缝合带保留有与伊豆-小笠原-马里亚纳(IBM)俯冲带相似的弧前岩石组合,这些弧前的玻安岩、高镁安山岩和斜长花岗岩表明商丹洋在早寒武世发生了洋壳的初始俯冲。结合区域地质资料,认为中央造山带可能在寒武纪发生了与IBM俯冲带规模相当的俯冲起始。
Oceanic plagioganites refers to felsic intrusions which can be generated in different tectonic setting including midocean ridge and subduction zone.Plagiogranite is volumetrically minor component of oceanic crust and ophiolites,but it is crucial to probe the evolution of oceanic lithosphere,subduction initiation and mechanism of oceanic-continental transformation.In this paper,we carried out an integrated study of petrology,geochemistry and zircon U-Pb chronology for the newly identified plagiogranite in the Tianshui area of the western part of the Shangdan suture zone.The whole rock geochemistry indicate that the plagiogranite have high content of SiO2 and Al2O3 and low content of K2O and TiO2,and show characteristics of low potassium subalkaline,metaluminous to weak peraluminous.In terms of trace element composition,they are relatively enriched in Sr and depleted in Nb,Ta and Ti,and show a characteristic of REE trends is relatively flat.Generally,it has geochemical affinities to oceanic plagiogranite which formed via partial melting of subducted oceanic crust slabs in shallow level and developed in the forearc area.Zircon U-Pb datings of plagiogranite samples yield age of 526±4 Ma,515±4 Ma and 517±6 Ma,respectively.Combined with previous studies data,we suggest that the Shangdan suture zone of the West Qinling records similar forearc magmatic rock associations as that of the Izu-Bonin-Mariana(IBM)subduction zone.The boninites,High-Mg andesites and plagiogranites generated almost simultaneously in forearc setting,reflecting the initial subduction of the Shangdan ocean at Early Cambrian.Combining the present study with regional geological data reveals that the northern Central China Orogenic system experienced a simultaneous subduction initiation of Proto-Tethys at Cambrian,similar to the scale in the IBM subduction zone.
作者
郭祺
张建新
毛小红
武亚威
GUO Qi;ZHANG Jan-xin;MAO Xiao-hong;WU Ya-wei(Institute of Geology Chinese Academy of Geological Sciences,Beijing 100037,China;School of Earth and Space Sciences,Peking University,Beijing 100871,China)
出处
《岩石矿物学杂志》
CAS
CSCD
北大核心
2024年第2期304-329,共26页
Acta Petrologica et Mineralogica
基金
国家自然科学基金项目(42072237,U2344214)
中国地质调查局地质调查项目(DD20221649)
中国地质科学院基本科研业务费项目(JKYZD202310)。
关键词
西秦岭
商丹缝合带
斜长花岗岩
部分熔融
俯冲起始
West Qinling
Shangdan suture zone
plagiogranite
partial melting
subduction initiation
