【作者】 陈晓锋；

【导师】 朱立新； 马生明；

【作者基本信息】 中国地质科学院 ， 地球化学， 2010， 博士

【摘要】 拉萨地块作为青藏高一个非常重要的构造-岩浆带,构造-岩浆作用非常复杂。中生代以来,相对于北部和南部明显由洋壳俯冲所形成的弧型岩浆岩来说,拉萨地块中部的火成岩成因及岩浆作用过程认识程度相对较低。本文对拉萨地块中部念青唐古拉早白垩世—早始新世侵入岩年代学、地球化学及岩石成因等方面进行了详细的研究,并与前人的研究进行了对比,获得了如下几点认识：1.念青唐古拉山北缘中新世前的侵入岩锆石U-Pb定年结果显示,早白垩世花岗闪长岩为133.20±0.92Ma,早古新世石英闪长岩为62.41±0.53Ma,早始新世辉长闪长岩为49.92±0.24Ma。此外,在早白垩世花岗闪长岩中发现了两颗年龄为2543Ma,2573Ma的锆右,表明了拉萨地块可能存在元古代基底。2.早白垩世花岗闪长岩准铝质至弱过铝质,高钾钙碱系列,Mg#平均为40,较高的(87Sr)/(86Sr)i(0.71947-0.72503),较低的εNd(t)(-12.6--12.7),以及较老的亏损地幔模式年龄(约1927Ma),锆石饱和温度最高可达830℃。暗示了岩浆主要来自古老地壳的熔融,与幔源岩浆的底侵作用相关。石英闪长岩低硅、高镁(Mg#平均为50)、富铁、富钙、低碱,准铝质,钙碱系列至钾玄系列,Nb/Ta比值为20,Sr/Y比值为9,暗示了岩浆的成分主要为幔源。均具有活动陆缘钙碱岩系的微量元素分布特征。花岗闪长岩和石英闪长岩具有密切的岩石成因联系,石英闪长岩形成于富集俯冲带组分的地幔熔体在上升过程中混染了一定量壳源物质结晶分异的产物,花岗闪长岩则为底侵作用生产的大量壳源熔体与少量幔源熔体混合并发生一定程度分离结晶后的产物。结合前人的研究成果,推测早白垩世侵入岩与新特提斯洋西段向NE消减有关。3.早古新世侵入岩由石英闪长岩和花岗闪长岩组成,为高钾钙碱性系列,准铝质Ⅰ型花岗岩类微量元素一个显著的特征是具有强烈的的轻重稀土分馏(((La/Yb)N为17-36),暗示了岩浆形成地壳深部,以石榴石作为主要残留物。但与同样发生强烈轻重稀土分馏埃达克岩及TTG与早古新世侵入岩主微量元素特征存在显著差异。结合高压变质玄武岩部分熔融实验及构造环境,推测早古新世侵入岩形成于富集俯冲带组分的幔源岩浆底侵下地壳,导致高压下变质基性岩的部分熔融,且源区富集稀土元素矿物。4.早始新世中基性杂岩由辉长闪长岩、黑云闪长岩及石英闪长岩组成。具有亚碱性拉斑系列的演化趋势,具有幔源岩浆的主微量元素特征,主要为地幔岩部分熔融的产物。Sr-Nd同位素及微量元素地球示踪研究表明,中基性杂岩为壳幔混源的产物,母岩浆最有可能来自于软流圈地幔；二元混合锶模拟表明,幔源岩浆中壳源物质参与的比例可达8%左右。区域对比研究及动力学背景分析表明,中基性杂岩成因可能与新特提斯洋板片的回旋、断离及软流圈物质的上涌及念青唐古拉山先存断裂的发展并切割岩石圈地幔密切相关。5.将念青唐古早白垩世侵入岩与拉萨地块中生代的其他弧型岩浆岩形成年代及时空对比表明,拉萨地块中部在早白垩世存在新特提斯洋向北,班公湖—怒江洋向南的双向俯冲,且俯冲前缘均消减于狮泉河—纳木错-嘉黎蛇绿岩带所代表的洋盆下的软流圈中。6.念青唐古拉北缘早白垩世—早始新世侵入岩的空间产状,侵位构造特征,地球化学特征表明了念青唐古拉巨型花岗岩基是沿着早始新世就已存在的NE向断裂侵位。7.念青唐古拉地区具有良好的Ag-Pb-Zn热液脉型矿床的成矿前景。更多还原

【Abstract】 The Lhasa block is one of the most important tectonic magmatic belts of Tibet Plateau, has a very complex tectonic magmatic evolutionary history. Since Mesozoic, people understand less about the petrogenesis and magmatic processs of igneous rocks exposed in the central of Lhasa block, compared with that of igneous rocks exposed in the north and south of Lhasa block. Through detailed study on chronology, geochemistry, and petrogenesis of Early Cretaceous intrusive rocks exposed in Nyainqentanglha area, the central of Lhasa block. and comparing with the former study, the author put forward some ideas as follows:1. The zircon LA-MC-ICPMS U-Pb dating for Early Cretaceous granodiorite defines a crystallization age of 133.20±0.92Ma, two the inheritance ages of 2543Ma,2573Ma,respectively, indicating there may exist Archeozoic crust for Early Paleocene a crystallization age of 62.41±0.53Ma, for a Early Eocene a age of 49.92±0.24Ma.2. Early Cretaceous granodiorite are metaluminous to weak peraluminous, high-K calc-alkaline series metaluminous to weak peraluminous, high-K calc-alkaline series, have average Mg index of 40, (87Sr/86Sr)i of 0.71947-0.72503,εNd(t)of-12.6--12.7, tDM of about 1927Ma, and the highest magma temperature to reach 830℃calculating by concentration of zirconium in whole-rock and zircon, which indicate the granodioritic magma are mainely originated from partial melting of ancient crust being closely related to underplating of mantle-derived magma. Quartz diorite are poor in Si、alkalinity, rich in Mg、Fe、Ca, metaluminous, calc-alkaline to shsoshonite series, have Nb/Ta ratio of 20, Sr/Y ratio of 9, which indicate quartz -ioritic magma originated from partial melting of mantle. Both granodiorite and quartz diorite show the trace element distribution characteristic as well as calc-alkaline rocks in island arc or active continental margin. Granodiorite and quartz diorite may be derived form the same magmatic processes. Quartz diorite originate from crystallization differentiation of mantle source magma assimilating and contaminating less crust material; while granodiorite originate from crystallization differentiation of massous crust source magma mixing less mantle source magma. Connecting with the predecessors research results, it can be supposed that the petrogenesis may be related to the subducting towards north of Neo-Tethyan.3. Early Paleocene intrusive rocks consist of quartz diorite and granodiorite. Both of them are high-K calc-alkaline series, metaluminous granorite. Rare-earth elements analysis for Early Paleocene intrusive rocks showed that heavy and light rare-earth elements high fractionation((La/Yb)N of 17-36), indicating the magmas originating in deep crust, and leaving garnet as residue. There are remarkble distinct between Adakite and TTG series and Early Paleocene intrusive rocks. According to basalt partial melting experiment, presume that Early Paleocene intrusive rocks originate from metabasalt partial melting under high pressure, relating to mantle magmas metasomatised in subduction zone, and source region rich in REE.4. Early Eocene intermediate-mafic intrusive complexes consist of gabbro-diorite, biotitediorite, and quartz diorite. They show magmas evolving towards subalkaline oleiiticbasalt series. Gabbro-diorite show the major and trace characteristic of mantle source magamas, so they may originate from partial melting of mantle rocks. Sr and Nd isotope and trace element geochemistry study indicate that intermediate-mafic intrusive complexes originate from crust melt mixing mantle melt, and parent magmas most likely is from asthenosphere mantle. Sr isotope mixing simulation show the percent of crust material in magmas is 8%. Regional correlation study and geodynamics analysis indicate the petrogenesis of Early Paleocene intermediate-mafic intrusive complexes is related to the rolling back and then breaking off of Neo-tethys oceans crust, asthenosphere mantle materials acending along fault cutting lithospheric mantle.5. Space-time comparative study on between Nyainqentanglha Early Cretaceous intrusive rocks and Mezoic arch-type igneous rocks exposed in other place in Lhasa block show that at Early Cretaceous Neo-tethys oceans crust subducted towards north of Lhasa block, while Bangonghu-Nujiang oceans crust subducted towards the south,and bothe of their leading edge sinking into asthenosphere mantle under Shiquanhe-Yongzhu-Namucuo-Jiali ocean.6. Study on the trend, emplacement strutrue and geochemistry characteristic of The intrusive rocks older than Early Eocene intrusive rocks show that Nyainqentanglha vast batholith emplaced along the fault of NE trend exsiting ahead of Early Eocene7. It is expected to find some Ag-Pb-Zn hydrothermal vein deposits in Nyainqentanglha area.更多还原