额尔古纳地块韩家园子‒富林地区中生代火成岩的成因及其对蒙古‒鄂霍茨克洋演化的启示

Geochronology and Geochemistry of Mesozoic Igneous Rocks in the Hanjiayuanzi⁃Fulin Area of the Erguna Massif:Constraints on the Tectonic Evolution of the Mongol⁃Okhotsk Ocean

额尔古纳地块东缘韩家园子‒富林地区紧邻蒙古‒鄂霍茨克缝合带,其广泛出露的早侏罗世‒早白垩世火成岩对于完善蒙古‒鄂霍茨克洋俯冲‒闭合历史具有重要意义.本文对韩家园子‒富林地区的中生代火成岩进行岩相学、锆石U-Pb年代学、全岩主‒微量元素地球化学研究.LA-ICP-MS锆石U-Pb结果显示韩家园子钾长花岗岩锆石U-Pb年龄为196±2 Ma,代表其侵位时代为早侏罗世;富林地区光华组粗安岩锆石U-Pb年龄为122±2 Ma,暗示其结晶时代为早白垩世.早侏罗世钾长花岗岩为准铝质的I型花岗岩,Mg^(#)值较低(36),Nb/Ta比值(16.55~17.05)接近于原始地幔,暗示岩浆应来源于新生下地壳的部分熔融.同时,钾长花岗岩富集大离子亲石元素Rb、Ba、K,亏损高场强元素Nb、Ta、Ti,与典型弧型火成岩的地球化学特征相一致,结合区域上发育同时代准铝质或弱过铝质I型花岗岩的事实,表明其可能与蒙古‒鄂霍茨克洋南向俯冲至额尔古纳地块有关.相比较下,早白垩世粗安岩具有较低的SiO_(2)含量(59.67%~59.93%)和较高的Mg^(#)值(42~43),同时富集大离子亲石元素Rb、Ba、K,亏损高场强元素Nb、Ta、Ti,富集Sr,亏损Th,暗示其可能是富集岩石圈地幔重熔的产物.鉴于区域上其他来源于富集岩石圈地幔的早白垩世钙碱性火山岩呈面状分布的特征,以及早白垩世A型花岗岩和变质核杂岩的存在,暗示粗安岩为蒙古‒鄂霍茨克洋闭合后伸展环境下的产物.结合区域最新火成岩和沉积岩资料,认为蒙古‒鄂霍茨克洋在早侏罗世‒早白垩世发生俯冲‒碰撞‒后碰撞作用,其在大兴安岭北部闭合时间应介于晚侏罗世末期和早白垩世早期(约150~140 Ma).

The Hanjiayuanzi-Fulin area in the eastern Erguna Massif,located adjacent to the Mongol-Okhotsk suture zone,is characterized by numerous Early Jurassic to Early Cretaceous igneous rocks,which is important to reconstruct the subduction and closure history of the Mongol-Okhotsk Ocean.In this study,we present new petrology,LA-ICP-MS zircon U-Pb ages and wholerock geochemical data for the Mesozoic igneous rocks from the Hanjiayuanzi-Fulin area.Zircon U-Pb ages of Hanjiayuanzi Kfeldspar granite and Fulin trachyandesite are 196±2 Ma and 122±2 Ma,corresponding to the Early Jurassic and Early Cretaceous,respectively.The K-feldspar granite belongs to metaluminous I-type granite,with low Mg^(#) value(36)and Nb/Ta ratios(16.55-17.05)close to the primitive mantle,which could be derived from partial melting of the juvenile lower crust.It is enriched in large ion lithophile elements(LILEs,e.g.,Rb,Ba,K),and depleted in high field strength elements(HFSEs,e.g.,Nb,Ta,Ti),showing the geochemical characteristics of typical arc igneous rocks.Combined with the spatial distribution of contemporary metaluminous-weakly peraluminous I-type granites,we propose that the Early Jurassic Hanjiayuanzi K-feldspar granite may be related to the southward subduction of the Mongol-Okhotsk Ocean beneath the Erguna massif.By contrast,the trachyandesite has lower SiO_(2) contents(59.67%-59.93%),higher Mg^(#) values(42-43),and is also enriched in LILEs and depleted in HFSEs.Besides,it has an enrichment of Sr and depletion of Th,suggesting an origin from the enriched lithospheric mantle.Based on the regional distribution of Early Cretaceous calc-alkaline volcanic rocks,A-type granites and metamorphic core complexes,we suggest that the Early Cretaceous trachyandesite was formed in an extensional environment after the closure of the Mongol-Okhotsk Ocean.Combined with published data from igneous and sedimentary rocks,the closure time of the Mongol-Okhotsk Ocean in the northern Great Xing’an Range is suggested to be between latest Late Jurassic and earliest Early Cretaceous(~150-140 Ma).