小兴安岭构造带早侏罗世两期俯冲事件:镁铁质侵入岩的锆石年代学、元素地球化学及Sr-Nd-Li同位素制约

Two stages of Early-Jurassic subduction in the Lesser Xing’an Range:Constraints from the zircon chronology,elemental geochemistry and Sr-Nd-Li isotopes of the mafic intrusions

位于小兴安岭构造带北缘的二龙山橄榄辉长岩及星火角闪辉长岩,锆石LA-ICP-MS定年结果分别为201±1Ma和186±1Ma。两者的矿物组成不同,星火角闪辉长岩角闪石含量较高(可达15%)。主微量元素组成显示在岩浆演化过程中二龙山橄榄辉长岩经历了橄榄石和单斜辉石的分离结晶,而星火角闪辉长岩主要经历了橄榄石的分离结晶,此外,两者显著的Sr正异常及一定程度的Eu正异常(1.07~1.38),表明两者在岩浆演化过程中都存在明显的斜长石堆晶作用。原始岩浆计算表明两者都具有弧形微量元素组成,富集大离子亲石元素和轻稀土元素,而亏损高场强元素(Nb和Ta),二龙山橄榄辉长岩属于钙碱性到高钾钙碱性岩浆岩,而星火角闪辉长岩属于钾玄岩系列。两者地幔源区都经历了俯冲沉积物熔流体交代作用的影响。此外,两者具有较为一致的Li同位素组成,同MORB以及岛弧玄武岩的锂同位素组成较为一致,或者略低于MORB,进一步限定俯冲交代物质来源为陆源沉积物。两者具有明显不同的Sr-Nd同位素组成,表明小兴安岭构造带北缘东西侧具有明显的地幔不均一性。位于小兴安岭构造带北缘西侧的186Ma星火角闪辉长岩,同古太平洋板块俯冲后撤导致的弧后伸展拉张有关;而小兴安岭构造带北缘东侧的201Ma二龙山橄榄辉长岩,与位于南部张广才岭构造带东缘的209~202Ma镁铁质岩浆岩带同期,共同揭示牡丹江洋俯冲事件发生的时限为209~201Ma,整体而言牡丹江洋俯冲事件开始的时间南部略早于北部。

The Erlongshan olivine gabbro and the Xinghuo hornblende gabbro are located in the northern margin of the Lesser Xing’an Range,they yielded crystallization ages of 201±1Ma and 186±1Ma,respectively.They have different petrographic compositions,the Xinghuo hornblende gabbro contains more hornblendes.The composition of major and trace elements shows that the Erlongshan olivine gabbro experienced olivine and clinopyroxene fractionation during the magmatic evolution,while only the fractionation of olivine plays an important role during the magmatic evolution of the Xinghuo hornblende gabbro.Moreover,both of them are characterized with strong Sr positive anomalies and certain degrees of Eu positive anomalies(1.07~1.38),suggesting obviously plagioclase cumulate.The results of parent magma recalculation reveal that,both of them exhibit arc-like trace-element patters,enriched in LILE and LREE but depleted in HFSE(Nb and Ta).The Erlongshan olivine gabbro belongs to calc-alkaline to high-potassium calc-alkaline magmatic rocks,while the Xinghuo hornblende gabbro belongs to shoshonite series,both of them derived from mantle wedge metasomatized by sedimentary melt and fluid.The Erlongshan olivine gabbro and the Xinghuo hornblende gabbro have relatively similar Li isotopic compositions,δ7Li is consistent with or slightly lower than the MORB and island-arc basalts,further define the subduction metasomatic material as terrigenous sediment.The difference of Sr-Nd isotopic compositions between the two intrusions indicates that mantle heterogeneity exists in the east-west margin of the northern Lesser Xing’an Range.The occurrence of Xinghuo hornblende gabbro in the western margin of the northern Lesser Xing’an Range was related to the back-arc stretching caused by the palaeo-Pacific slab subducting W-NW beneath East Asia.While the Erlongshan olivine gabbro in the eastern margin of the northern Lesser Xing’an Range,together with the 209~202Ma mafic magmatic belt located in the southern Zhangguangcai Range,indicating the subduction of the Mudanjiang ocean occurred during 209~201Ma,and the subduction started slightly earlier in the south than in the north.