【作者】 刘仁燕；

【导师】 任纪舜； 牛宝贵；

【作者基本信息】 中国地质科学院 ， 构造地质学， 2011， 硕士

【摘要】 秦岭造山带经历了长期而复杂的地质演化,发育有多期、多阶段岩浆侵入体。对各期次岩体的成因及其物源研究,有助于深化对秦岭造山带地质演化机理的认识。小茅岭复式岩体位于南秦岭北缘、小茅岭-陡岭隆起带的西端,本文在对该复式岩体野外观察基础上,开展了岩石学、锆石U-Pb年代学、地球化学及Hf位素研究,并综合区域研究成果,得到以下认识：1.小茅岭复式岩体由早期宋家屋场基性辉绿(辉长)岩和晚期迷魂阵、磨沟峡及叶家湾中性-偏酸性闪长岩组成,闪长岩中含有辉绿(辉长)岩捕掳体。小茅岭复式岩体中见有前震旦系磨沟峡杂岩捕掳体,与复式岩体同期的花岗岩脉侵入磨沟峡杂岩,杂岩之上不整合覆盖有震旦系陡山沱组碎屑岩,故复式岩体应形成于前震旦系磨沟峡杂岩之后,震旦系陡山沱组之前。2.通过LA-ICP-MS锆石U-Pb年代学研究,获得小茅岭复式岩体中宋家屋场岩体侵位年龄为864.4+1.7Ma,迷魂阵岩体为846.7+2.7Ma,磨沟峡岩体为859.4+1.7Ma,叶家湾岩体为861.1±1.8Ma,表明小茅岭复式岩体形成于870～840Ma的新元古代。据此,笔者将小茅岭复式岩体进一步细分为三部分：早期宋家屋场角闪辉绿(辉长)岩体,中期磨沟峡石英闪长岩-英云闪长岩体及叶家湾石英二长岩-二长闪长岩体,晚期迷魂阵闪长岩-石英闪长岩体,为同期不同序次岩浆侵入活动的产物。3.岩石地球化学分析显示,小茅岭复式岩体早期宋家屋场岩体与中晚期磨沟峡、叶家湾、迷魂阵岩体之间,存在成分壁垒,属两个独立的系列。早期宋家屋场岩体FeO、K Fe2O3、MgO、TiO2含量较高,K2O、Na2O含量较低,A／CNK=0.68～0.83；轻稀土元素较重稀土元素略富集,无Eu异常或略具有Eu正异常；Rb、Ba、Th、U等大离子亲石元素含量较低,Nb、Ta、Ti、P等的负异常不明显,(Th/Nb)N比值略大于1, zr含量及Zr／Y比值较高,具有大陆拉张环境玄武岩的特征。中晚期磨沟峡、叶家湾及迷魂阵闪长岩钾、钠含量较高,A/CNK小于1.1；稀土元素、微量元素地球化学特征较为相似,轻稀土较重稀土富集,从迷魂阵闪长岩-石英闪长岩到磨沟峡石英闪长岩-英云闪长岩再到叶家湾石英二长岩-二长闪长岩,随着岩石酸性的增强,轻重稀土分馏逐渐增强,Eu负异常越来越显著；磨沟峡、叶家湾及迷魂阵岩体均富集Rb、Ba、Th、U、K等大离子亲石元素,亏损Nb、Ta、Ti、P等元素,体现了典型岛弧火成岩的地球化学特征。4.Hf同位素研究显示,小茅岭复式岩体从早期到晚期具有不同的物源。早期宋家屋场蚀变角闪辉绿(辉长)岩εHf(t)为7.9～9.9,单阶段模式年龄为1107～1021Ma,可能为来源于受到地壳物质混染的亏损地幔源区。中期磨沟峡及叶家湾岩体Hf同位素特征相似,磨沟峡岩体εHf(t)为4.1-5.1,阶段模式年龄为1476～1427Ma；叶家湾岩体εHf(t)为3.2～5.9,二阶段模式年龄为1548～1371Ma,表明磨沟峡及叶家湾岩体为中元古代中期形成的幔源物质(新生地壳)部分熔融的产物。晚期迷魂阵岩体多数锆石εHf(t)为负值(个别点εHf(t)略大于0),二阶段模式年龄集中在1970～1709Ma之间,源区可能为古元古代晚期古老地壳物质的部分熔融,混有少量幔源增生的地壳物质。5.结合区域地质背景和秦岭造山带构造演化特征,小茅岭复式岩体属秦岭新元古代早期后造山伸展构造环境下形成的岩体。更多还原

【Abstract】 The Qinling orogenic belt has experienced a long time and complex tectonic evolution, with many multi-phase and multi-stage intrusives being developed. The studies on the petrogenesis of the intrusives including source rocks involve the formation of the Qinling orogenic belt, in particular the geodynamic processes. The Xiaomaoling composite intrusives dealt with in the dissertation are exposed at the western end of the Xiaomaoling-Doling uplift belt located in the northern most part of South Qinling. Based on the field work, the petrography, zircon U-Pb geochronology, systematic geochemistry and Hf isotope analysis of the composite intrusives are discussed in this paper.1. The Xiaomaoling composite intrusives are composed of early Songjiawuchang diabase (grabbro) and late Mihunzhen, Mogouxia and Yejiawan diorites. The diorites contain diabase (grabrro) xenoliths. The Xiaomaoling intrusives bear xenoliths of the Pre-Sinian Mogouxia Complex. Synchronous granitic dikes intrude into the Mogouxia Complex, which is covered unconformably by the Sinian Doushantuo group clastic rocks. It follows that the Xiaomaoling composite intrusives were formed after the Pre-Sinian Mogouxia Complex and before the Sinian Doushantuo group.2. The Songjiawuchang intrusive, Mihunzhen intrusive, Mogouxia intrusive and Yejiawan intrusive from the Xiaomaoling intrusives yield mean 206Pb/238U ages of 864.4±1.7 Ma,846.7±2.7 Ma,859.4±1.7 Ma and 861.1±1.8 Ma respectively by LA-ICP-MS zircon U-Pb dating, suggesting that these composite intrusives were formed during the Neoproterozoic (870～840 Ma). Accordingly, the Xiaomaoling composite intrusives are classified into three parts:early Songjiawuchang diabase (grabbro), middle Mogouxia quartz diorite-tonalite and Yejiawan quartz monzonite-monzodiorite and late Mihunzhen diorite-quartz diorite, that is to say, they are products of different evolutionary sequence of synchronous magmas.3. The early Songjiawuchang intrusive and middle to late Mogouxia, Yejiawan and Mihunzhen intrusives belong to two different series on their geochemical analysis. The early Songjiawuchang intrusive has high FeO, Fe2O3, MgO and TiO2 contents, low K2O and Na2O contents, with A/CNK=0.68～0.83; enriched LREE, negligible Eu anomalies or weakly positive Eu anomalies; low Rb, Ba, Th and U contents, slightly negative Nb, Ta, Ti and P anomalies, (Th/Nb)N ratios slightly greater than one, hight Zr content and Zr/Y ratios, showing a feature of the continental tholeiite. The middle to late Mogouxia, Yejiawan and Mihunzhen diorites have high K and Na contents, A/CNK less than 1.1; similar REE and trace elements feature, enriched LREE; LREE and HREE fractionation and negative Eu anomalies becoming stronger with the increasing acidity from Mihunzhen diorite-quartz diorite to Mogouxia quartz diorite-tonalite to Yejiawan quartz monzonite-monzodiorite; enriched Rb, Ba, Th, U and K and depleted Nb, Ta^ Ti and P for the Mogouxia, Yejiawan and Mihunzhen intrusives, indicating a geochemical feature of island arc volcanic rocks.4. The Lu-Hf isotopic analysis shows the Xiaomaoling composite intrusives of different stages have different sources. The early Songjiawuchang amphibolic diabase (gabbro) withεHf(t) value of 7.9～9.9 and TDM1 of 1107～1021 Ma, might be derived from depleted mantle contaminated by crustal material. The middle Mogouxia intrusive and Yejiawan intrusive have similar Hf isotopic feature, the former characterized by eHf(t) value of 4.1～5.1 and TDM2 of 1476～1427 Ma, and the latter byεHf(t) value of 3.2～5.9 and TDM2 of 1548～1371 Ma, indicating that they were derived from partial melting of juvenile crust formed in the middle-late Mesoproterozoic. The late Mihunzhen intrusive has mostly negativeεHf(t) value and TDM2 of 1970～1709 Ma, indicating that it resulted from the partial melting of a late Paleoproterozoic old crust mixed with some mantle material. 5. In combination with regional geological background and tectonic evolution characteristics of the Qinling orogen, it can be seen that the Xiaomaoling composite intrusives were formed in an early Neoproterozoic post-orogenic extensional environment.更多还原