古亚洲洋晚石炭世俯冲作用:梅劳特乌拉蛇绿岩中扎嘎音高镁安山岩证据  

Late Carboniferous intraoceanic subduction of the Paleo-Asian Ocean: New evidences from the Zagayin high-Mg andesite in the Meilaotewula SSZ ophiolite

作　　者：王金芳 李英杰 李红阳 董培培 WANG Jinfang;LI Yingjie;LI Hongyang;DONG Peipei(School of Nature Resources,Hebei GeoUniversity,Shijiazhuang,050031)

机构地区：[1]河北地质大学资源学院,石家庄050031

出　　处：《地质论评》2020年第2期289-306,共18页Geological Review

基　　金：国家自然科学基金资助项目“内蒙古西乌旗迪彦庙蛇绿岩年代学、地球化学及大地构造意义”(编号:41502211);中国地质调查局项目“内蒙古1:5万高力罕牧场三连等四幅区调”(编号:1212011120711);“内蒙古1:5万沙日勒昭等四幅区调”(编号:1212011120701)的成果。

摘　　要：内蒙古贺根山缝合带梅劳特乌拉SSZ(Supra Subduction Zone)型蛇绿岩中新发现扎嘎音晚石炭世高镁安山岩,岩性为玄武安山岩和安山岩。扎嘎音玄武安山岩-安山岩MgO(4.33%~9.82%)、Mg#值(57~72)和SiO2含量(55.34%~64.12%)较高;而TiO2(0.30%~0.60%)、K2O(0.06%~0.73%)、Al2O3(12.18%~17.32%)含量和FeOT/MgO值(0.70~1.34)较低,Na2O/K2O值(2.23~40.88)较高。稀土元素总量较低(14.29×10-6~60.20×10-6),稀土配分曲线总体为略显右倾的平坦型,(La/Yb)N为0.57~2.58,无明显Eu异常。岩石富集大离子亲石元素Rb、Ba、Th、U和Sr等,亏损高场强元素Nb、Ta、Ti和P等,具有较低的Ti/V值。该玄武安山岩-安山岩具有高镁安山岩的地球化学特征,与日本西南Setouchi岛弧火山岩带的sanukite(有人音译为"赞岐岩")相类似,可能为俯冲洋壳+俯冲深积物的脱水流体与俯冲沉积物部分熔融形成的硅质熔体与上覆地幔楔橄榄岩平衡反应成因。锆石LA-ICP-MS U-Pb测年表明,扎嘎音玄武安山岩的成岩年龄为315.0±2.3 Ma,反映了晚石炭世高镁安山岩浆作用事件。扎嘎音玄武安山岩-安山岩为晚石炭世形成于洋壳俯冲弧前环境的高镁安山岩组合,为古亚洲洋晚石炭世洋内俯冲作用提供了证据。结合贺根山缝合带的壳幔电性结构特征和晚古生代蛇绿岩-岛弧岩浆岩时空分布与演化关系,提出了扎嘎音高镁安山岩的古亚洲洋晚石炭世洋内俯冲模式。Objectives: The Zagayin high-Mg andesite is newly recognized in the Meilaotewula SSZ( Supra Subduction Zone)-type ophiolite of the Hegenshan suture,Inner Mongolia. High-Mg andesites are usually believed to be resulted from the equilibrium interaction between overlying mantle peridotite and siliceous melt derived from dehydrated fluid of subducted oceanic crust + sediments,representing intraoceanic arc and initial subduction.However,whether the Late Carboniferous Paleo-Asian Ocean was still in subduction stage is lacks of further definitely petrological and chronological evidences and constraints. Therefore,we carried out zircon LA-ICP-MS UPb geochronological and geochemical study on the Zagayin high-Mg andesite to discuss its origin,and to provide new evidences for the tectonic evolution of the eastern Central Asian Orogenic Belt.Methods: Based on field geological surveying,petrological,geochemical and LA-ICP-MS zircon U-Pb geochronological study,this paper discusses the petrogenesis and tectonic setting of the Zagayin high-Mg andesite,and the subduction of the Paleo-Asian Ocean.Results: The Zagayin high-Mg andesite is mainly composed of basaltic andesite and andesite,which has relatively high contents of MgO( 4. 33% ~ 9. 82%,Mg#57 ~ 72) and Si O2( 55. 34% ~64. 12%),low those of Ti O2( 0. 30% ~0. 60%),K2 O( 0. 06% ~ 0. 73%),Al2 O3( 12. 18% ~17. 32%),low FeOT/MgO values( 0. 70 ~ 1. 34),and high Na2 O/K2 O values( 2. 23% ~ 40. 88%). The total REE is low( 14. 29 × 10-6~ 60. 20 × 10-6). Rare earth curve is of flat slightly right pattern( LaN/Yb N= 0. 57 ~ 2. 58),without pronounced Eu anomaly. The rocks are enriched in Rb,Ba,Th,U and Sr large ion lithophile elements,depleted in Nb,Ta,Ti and P high field strength elements,and with low Ti/V. It has typical characteristics of high-Mg andesite( HMA) and is similar to sanukite of the Setouchi arc volcanic belt in the SW Japan. The zircon U-Pb LA-ICP-MS dating shows that the age of the Zagayin basaltic andesite is 315. 0 ±2. 3 Ma,reflecting the Late Carboniferous high-Mg

关 键 词：高镁安山岩 SSZ型蛇绿岩 洋内俯冲作用 晚石炭世 古亚洲洋 

分 类 号：P534.45[天文地球—古生物学与地层学] P542[天文地球—地质学] P588.144