【作者】 刘敏；

【导师】 赵志丹； 朱弟成；

【作者基本信息】 中国地质大学(北京) ， 矿物学、岩石学、矿床学， 2012， 博士

【摘要】 聂荣微陆块在地理上位于现今的羌塘地块和拉萨地块之间，呈眼球状、近东西向分布于班公湖-怒江缝合带中段，南北两侧均有班公湖-怒江洋的蛇绿混杂岩残留，是青藏高原内部确定具有前寒武纪结晶基底的地区之一，对于研究班公湖-怒江洋在早中生代时期的构造岩浆演化具有重要价值。聂荣微陆块上大规模出露的侏罗纪早期岩体总体上呈岩基状侵位于新元古界变质岩和古生界碎屑岩地层中，且在露头上呈暗色包体和/或脉体与寄主花岗岩类共生的形式。侵入岩的锆石LA–ICP–MS U–Pb定年结果，在误差范围内基本可以分成~185 Ma和~175 Ma两组，说明研究区岩浆侵位活动主要有两期。早侏罗世中期(~185 Ma)侵入岩属于偏铝质-弱过铝质高钾钙碱性-钾玄质碱性系列的闪长岩、二长闪长岩、花岗闪长岩、二长花岗岩到碱长花岗岩；中侏罗世早期(~175 Ma)侵入岩属于偏铝质-过铝质钾玄质碱性-过碱性系列的基性碱性正长岩、碱性二长闪长岩到中性正长岩、闪长岩再到酸性二长花岗岩和碱长花岗岩。两期侵入岩均具有明显的富碱特征，在成因类型上主要是富碱的I-型花岗岩类；均富集轻稀土和大离子亲石元素，亏损高场强元素，同时，均具有富集的Nd同位素和Sr同位素以及古老的Nd模式年龄；并且均属高温低压岩石系列。加之其锆石Hf同位素所表现出的显著不均一性，指示两期侵入体均形成于一个包含有地壳重熔、岩浆混合、围岩混染、结晶分异在内的复杂的，并且涉及到古老基底(占较大比例)和富集岩石圈地幔甚至是亏损地幔的多组分的、幔源物质参与程度逐渐增强的连续演化岩浆作用过程。结合区域同时期变质沉积作用的研究成果，聂荣微陆块侏罗纪早期富碱侵入岩是在后碰撞伸展背景下，由被动陆缘的复杂壳幔相互作用产生的。早侏罗世中期侵入岩的形成标志着班怒洋壳北向俯冲作用、聂荣微陆块-羌塘地块对接碰撞作用的结束和后碰撞作用的开始；而中侏罗世早期侵入岩，特别是超基性、过碱性岩的形成则标志着板片的彻底断离和后碰撞作用时期的最终结束。更多还原

【Abstract】 Nyainrong Microcontinent located geographically between the present-dayQiangtang and Lhasa Terranes, and sandwiched by ophiolitic fragments ofBangong-Nujiang suture zone both in its north and south, comprises one of the fewexposures of metamorphosed crystalline basement in Tibet Plateau. And bounded bywhich, the north(Qiangtang) and south(Lhasa) sides of the Bangong-Nujiang suturezone show very distinct geologic features, making the Nyainrong microcontinent to bean ideal object for regional research on many geological problems, especially theEarly Mesozoic tectonomagmatic evolution of the Bangong-Nujiang Ocean.The widespread, generally undeformed early Jurassic intrusions intrude in theNeoproterozoic gneiss and the Paleozoic clasolites in the Microcontinent, withmultiple exposed mafic enclaves and host granitoid. Zircon LA-ICP-MS U-Pb datingdata accordant within the range of the sampling errors, yields two main magmaemplacement eras of ~185 Ma and ~175 Ma. Moreover, the dioritic and felsic magmaemplacement are coeval in ~185 Ma, and the mafic and the felsic are coeval in ~175Ma also.The ~185 Ma intrusions of Nyainrong pluton are metaluminous to aluminous,and high-K calc-alkaline to shoshonitic-alkaline, ranging from diorite toalkali-feldspar-granite in composition. While, the ~175 Ma intrusions form part of ashoshonitic-alkaline to peralkaline that ranges in composition from maficalkali-syenite to intermediate monzodiorite and to felsic alkali-granite, with bimodalSiO2contents from metaluminous to peraluminous. They are all significantlycharacteristic of high potassic and high alkalic contents, and are petrogenetic mainlyin I-type series. They are all enriched in LREEs and LILEs but depleted in HFSEs,along with the Eu depletion. Meanwhile, they all have the negative bulk isotopicεNd(t)and zirconεHf(t) values corresponding to the ancient Nd-Hf model ages. And ofparticular note is the variation of the zircon Hf isotopic results are all extremely large,indicating the heterogeneous magma source.Based on the magmatic sequence, petrography, mineralogy, major and traceelement geochemical, bulk rock Sr-Nd isotopic data and zircon Hf isotopiccompositions of Nyainrong pluton, the early Jurassic emplacement magmasource(both the ~185 Ma period and the ~175 Ma period) having both highertemperature and lower pressure turns out to be comagmatic, and subsequent to undergo similar evolutionary process, which is a complex stage process involvingcrustal anatexis, magma mixing, crystal fractionation, and wall rock assimilation ofthe ancient mature crustal(significant input) and enriched lithospheric mantle even asthe depleted mantle sources proposed for the petrogenesis of the early JurassicNyainrong pluton. Besides, during this tectonomagmatic process with multiplemagma sources, the involved mantle derived materials are increased or enhanced withage from ~185 Ma to ~175 Ma.The data reported here, in combination with other diversified literatures aboutcoeval metamorphism-sedimentation of the research region, illustrate that the earlyJurassic shoshonitic and alkaline intrusions in Nyainrong Microcontinent are theproducts of the distinctive post-collisional magmatism along with intensifiedcrustal-mantle interactions in a passive margin due to northward subduction ofBangong-Nujiang oceanic lithosphere and the consequent Nyainrong-Qiangtangcollision.Furthermore, the middle early Jurassic(~185 Ma) magmatism marks the end ofthe Nyainrong-Qiangtang collision and the start of the post-collision and regionalextension; while the early middle Jurassic(~175 Ma) magmatism, especially theemergence of the mafic peralkalic intrusions, is in response to the finalpost-collisional stage which should be compatible with the slab thoroughly break-offand hot asthenospheric upwelling.更多还原