大兴安岭南段小乌兰沟正长花岗岩成因:锆石LA-ICP-MS U-Pb年代学、地球化学及Hf同位素的制约

Petrogenesis of the Syenogranite in the Xiaowulangou Area of Southern Great Xing’an Range: Constraints from Zircon LA-ICP-MS U-Pb Geochronology,Geochemistry and Hf Isotopes

大兴安岭南段西坡发育有大量晚侏罗世−早白垩世花岗岩,深入讨论岩石成因对该地区中生代地球动力学背景及其构造演化的研究具有重要地质意义.报道了小乌兰沟正长花岗岩的锆石LA‐ICP‐MS U‐Pb年龄,全岩地球化学及锆石Hf同位素数据.小乌兰沟正长花岗岩的锆石^(206)Pb/^(238)U加权平均年龄为139.4±0.7 Ma,属早白垩世.样品主量元素表现为富硅、富钾的高钾钙碱性系列岩石,A/CNK值介于1.00~1.11之间,属弱过铝质.微量元素富集Rb、Th、U等大离子亲石元素(LILE)和轻稀土元素(LREE),强烈亏损Ba、Sr、P、Ti等高场强元素(HFSE),Eu负异常明显(δEu=0.007~0.009),Zr+Nb+Ce+Y值基本小于350×10^(−6),含磁铁矿,未见原生白云母和碱性暗色矿物,属高分异I型花岗岩.小乌兰沟正长花岗岩ε_(Hf)(t)值介于+5.5~+8.9之间,Hf同位素模式年龄t_(DM2)变化于700~947 Ma之间.结合区域研究,小乌兰沟正长花岗岩为上元古界变基性岩与中奥陶统变中性岩在低压、高温条件下发生部分熔融作用形成,是蒙古−鄂霍茨克洋闭合后伸展作用的产物.

TheLate Jurassic-Early Cretaceous granites are widely developed in the west slop of Southern Great Xing’an Range,and an in-depth study of the petrogenesis is of great geological significance to reveal the Mesozoic geodynamic background and tectonic evolution. In this paper, we report for the zircon LA-ICP-MS U-Pb age and Hf isotopic data, whole-rock major and trace element composition data from Xiaowulangousyeno granite. LA-ICP-MS U-Pb dating yields a weighted mean of 139.4±0.7 Ma,indicating that Xiaowulangousyeno granite formed in the Early Cretaceous. Geochemically, the syenogranite belongs to Si-K-rich calc-alkaline series, A/CNK=1.00~1.11 and are weak peraluminous. The syenogranite enriched in Rb, Th, U(LILE) and light rare earth elements(LREE), relatively depleted in Ba, Sr, P, Ti(HFSE), withobvious Eu negative anomaly(δEu=0.007~0.009).In addition, the(Zr+Nb+Ce+Y) values of syenograniteis less than 350×10^(-6). The syenogranite contains magnetite, and no primary muscovite and alkaline mafic minerals. All these features above suggest that Xiaowulangousyenogranite belongs to highly fractionated I-type granite. The zircons ε_(Hf)(t) values of syenogranite are +5.5~ +8.9, and the two stage model ages(t_(DM2)) range700 Ma to 947 Ma. Combined with the reginoal geological background, we conclude that the Xiaowulangousyenograniteoriginated from the partial melting of upper Proterozoic meta-mafic rocks and middle Ordovician meta-andesite rocks under low pressure and high temperature, and the syenogranitewas the product of extension after the closure of Mongol-Okhotsk Ocean.