二连—贺根山缝合带晚石炭世埃达克岩的发现及古亚洲洋洋内俯冲作用

Discovery of the Late Carboniferous adakite in the Erenhot-Hegenshan suture zone and intra-oceanic subduction of the Paleo-Asian Ocean

内蒙古二连—贺根山缝合带额很傲包图英云闪长岩体,出露于西乌旗梅劳特乌拉SSZ型蛇绿岩带北侧。为了确定该岩体的岩石成因类型,探讨其构造环境及古亚洲洋俯冲消亡过程,对该岩体进行了岩石学、地球化学和LAICP-MS锆石U-Pb年代学研究。额很傲包图英云闪长岩高硅富铝,富钠贫钾,高锶低钇,富集Rb、Ba、Sr等大离子亲石元素和LREE,亏损Nb、Ta、Ti、P等高场强元素和HREE,无明显Eu异常。岩石学和岩石地球化学特征表明,该英云闪长岩体为高Si埃达克岩(HSA),形成于岛弧环境,具有洋内俯冲洋壳+俯冲深积物部分熔融并与上覆地幔楔橄榄岩反应成因特征。锆石LA-ICP-MS U-Pb测年表明,额很傲包图英云闪长岩体的侵位年龄为(305.6±1.5)Ma,形成时代为晚石炭世。结合二连—贺根山缝合带石炭纪蛇绿岩、石炭纪—二叠纪岛弧型岩浆岩的时空分布与演化特征,认为古亚洲洋二连—贺根山洋盆在石炭纪—早二叠世处于以洋内俯冲为特征的大洋俯冲消亡过程中。

The Ehenaobaotu tonalite is outcropped in the northern part of the Meilaotewula SSZ-type ophiolite of the Erenhot-Hegenshan suture zone in Xi Ujimqin Banner of Inner Mongolia.Based on the study of petrology,geochemistry and zircon LAICP-MS U-Pb geochronology,the genetic type and tectonic setting of the tonalite were determined,and the final closure time and subduction process of the Erenhot-Hegenshan ocean basin of the Paleo-Asian Ocean were discussed.The tonalite is characterized by high SiO2,Al2O3,Na2O,Sr and low K2O,Y contents,enrichment of Rb,Ba,Sr large ion lithophile elements and LREE,and depletion of Nb,Ta,Ti,P high field strength elements and HREE.There is no pronounced Eu anomaly.The lithological and geochemical characteristics show that the Ehenaobaotu pluton is a high Si adakite(HSA)formed in island arc,and was derived from patial melting of the intra-oceanic subductingd oceanic crust and sediment and subsequently reaction with overlying mantle wedge peridotite.Its U-Pb LA-ICP-MS dating yields(305.6±1.5)Ma and suggests that the Ehenaobaotu pluton was formed during Late Carboniferous.Combined with the study of spatio-temporal evloution of the Carboniferous ophiolites and Carboniferous-Permian island arc magmatic rocks in the Erenhot-Hegenshan suture,it is assumed that the Erenhot-Hegenshan oceanic basin of the Paleo-Asian Ocean was in the process of oceanic subduction characterized by intra-oceanic subduction during the Carboniferous-Early Permian.