小兴安岭西北部早——中侏罗世TTG花岗岩年代学、地球化学特征及构造意义

Early-Middle Jurassic TTG Granites in Northwest of Lesser Xing'an Range:Its Geochronology,Geochemical Characteristics and Tectonic Significance

通过对出露于小兴安岭西北部白石砬子地区中生代花岗岩的研究,查明该地区存在一套由英云闪长岩-花岗闪长岩-石英闪长岩和少量闪长岩为组合的花岗岩,锆石U-Pb LA-ICP-MS测年分别获得(171.1±1.3)、(173.9±3.9)、(178.3±1.5)和(169.1±6.2)Ma的岩体侵位年龄,证明其形成于早—中侏罗世。通过岩石地球化学方法判别该花岗岩组合具有高Al_2O_3型T_1T_2G_1花岗岩特征,整体具有富硅、富铝,钠质,贫铁、镁、钛,属于偏铝质-弱过铝质钙碱性系列,富集轻稀土,相对亏损重稀土,富集Rb、Ba、Th、U、Sr等大离子亲石元素,相对亏损Nb、Ta、Zr、Hf等高场强元素,Eu具有轻微负异常的特点,与安第斯I型花岗岩具有相似性。其低Sr/Y值、(La/Yb)N值,弱或无负δEu异常等特征反映源区残留相矿物以角闪石、辉石为主,有少量斜长石,石榴石可能有少量残留。岩体露头有闪长质岩浆与英云(花岗)闪长质岩浆两种不完全机械混合的现象,反映可能存在两个以上的不同源区。另外,岩体不规则侵入元古宙地层(兴华渡口群和落马湖群)和岩浆中残留有大量早期捕获锆石,说明在岩浆上升侵位过程中与围岩发生同化混染作用。综合研究认为,该早—中侏罗世TTG岩浆形成与蒙古—鄂霍茨克洋的闭合作用密切相关,反映侏罗纪蒙古—鄂霍茨克洋存在向南侧微陆块的俯冲作用。

This paper reports the Mesozoic TTGs outcropped in the Northwestern Lesser Xing＇an Range,which was caused by the subduction of the Mongol-Okhotsk Ocean.The TTGs are composed mainly of tonalite,granodiorite,quartz-diorite,and diorite.The zircon U-Pb LA-ICP-MS dating provided the age of（171.1±1.3）Ma,（173.9±3.9）Ma,（178.3±1.5）Ma and（169.1±6.2）Ma corresponding to the four samples,indicating that the magma formed during the Early-Middle Jurassic.They are the High-Al2O3 T1T2G1 based on the O＇Connor＇s An-Ab-Or normative classification,and petrochemically belong to metaluminous or weakly peraluminous calc-alkaline series with enrichment in Si and Al,while poor in Fe,Mg,Ti,and Na.The LILE（e.g.Rb,Ba,Th,U,Sr）and LREE are enriched,while HFSE（e.g.Nb,Ta,Zr,Hf）and HREE are relatively depleted with slightly negative Eu anomalies.Such characteristics show that the TTGs have a similarity with type I granite in the Andes type orogenic belt.The low ratios of Sr/Y and（La/Yb）Nand the weakly negativeδEu indicate that the residual minerals in the source are hornblende,pyroxene and a little of plagioclase,and little garnet.The co-existence of diorite and tonalite in outcrops reflect that an incomplete and mechanical mixing of two or more magmas might occur,and the intrusion of the TTGs to the Proterozoic strata（such as Xinghuadukou Group and Luomahu Group）and the fact that the TTGs contains lots of captured zircons show that the TTGs magma experienced the assimilation and contamination of the wall rocks during their ascending up through the lithosphere.The authors conclude that the Jurassic TTGs were closely related to the closure of the Mongol-Okhotsk Ocean,and the subduction towards the southern massifs in that period.