班公湖-怒江缝合带西段拉果错蛇绿岩中斜长岩成因及其对中特提斯洋演化的制约  

Petrogenesis of anorthosite in the Laguoco ophiolite,western part of the BangongNujiang suture zone and its constraint to the evolution of the Meso-Tethys Ocean

作　　者：唐跃[1] 翟庆国[1] 胡培远[1] 王伟[1] TANG Yue;ZHAI Qingguo;HU Peiyuan;WANG Wei(Key Laboratory of Deep-Earth Dynamics of the Ministry of Natural Resources/Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China)

机构地区：[1]自然资源部深部动力学重点实验室/中国地质科学院地质研究所,北京100037

出　　处：《地质通报》2021年第8期1265-1278,共14页Geological Bulletin of China

基　　金：国家自然科学基金项目《蛇绿岩中异剥钙榴岩成因研究——以藏北班戈地区为例》(批准号:42002069);《藏北湖区蛇绿岩对班公湖-怒江洋盆演化的制约》(批准号:42072268);第二次青藏高原综合科学考察项目《任务7-3之特提斯域大陆增生与第三极形成》(编号:2019QZKK0703);中国博士后科学基金资助项目《异剥钙榴岩成岩过程中交代流体的来源:来自Ca同位素的约束》(编号:2020M670401);中国地质调查局项目《拉萨-腾冲构造岩浆带区域地质调查》(编号:DD20190060);《中国蛇绿岩综合研究》(编号:DD20190370)。

摘　　要：拉果错蛇绿岩位于青藏高原北部的改则地区,是狮泉河-永珠带内保存最好、岩石组合最完整的蛇绿岩之一。为进一步约束拉果错蛇绿岩成因及其构造属性,对其中斜长岩开展了详细的岩石学、锆石U-Pb年代学和地球化学研究。结果显示,斜长岩主要呈不规则脉体或透镜体侵入到辉长岩中,锆石U-Pb定年结果表明斜长岩形成于162 Ma,略晚于蛇绿岩中基性岩形成时代。这些锆石均具有明显正的εHf(t)值(+15.8~+19.7),暗示其岩浆源自亏损的地幔源区。斜长岩具有明显低SiO 2、TiO 2和高CaO的特征,具有与正常型大洋中脊玄武岩(N-MORB)类似的稀土元素配分形式和低的微量元素含量,富集Rb、Ba、Th和Sr,亏损Nb、Ta、Zr等微量元素。此外,斜长岩显示出高的Nb/La值和低的Th/Nb、Ba/Nb值,暗示其源自富集地幔源区高程度的部分熔融,形成过程还受到俯冲组分的影响。综合上述特征,推测拉果错斜长岩是地幔柱影响下再富集的亏损地幔源区高程度部分熔融的产物。俯冲过程中,再富集地幔源区(地幔楔)产生的原始岩浆快速上涌过程,并伴随着富Mg-Fe矿物的结晶分异,残余富Ca、Al岩浆。残余岩浆侵入到俯冲带上盘洋壳中(辉长岩),由于快速减压而形成斜长岩。结合区域研究成果,表明拉果错蛇绿岩形成于与早侏罗世—晚侏罗世早期俯冲相关的构造背景。The Laguoco ophiolite is distributed in the Gaize area,north Tibetan plateau,and is one of the most complete and well-preserved ophiolites in the Shiquanhe-Yongzhu belt.The petrological,zircon U-Pb geochronology and geochemistry of anorthosite intruding gabbros are studied to manifest the genesis and tectonic affinities of the Laguoco ophiolite.The anorthosite was emplaced within gabbros mainly as irregular dikes or lenses,with zircon U-Pb ages of 162±1 Ma,slightly later than mafic rocks of the ophiolite.All zircons yield remarkable positiveεHf(t)values(+15.8~+19.7),indicating that the magma was derived from a long-term depleted mantle source.All anorthosites are characterized by low SiO 2,TiO 2 and high CaO contents,MORB-like REE patterns with low trace element concentrations,enrichment of Rb,Ba,Th and Sr,and depletion of Nb,Ta and Zr.Moreover,these samples show high Nb/La but low Th/Nb and Ba/Nb ratios,which suggests their magma was affected by enriched mantle with variable inputting of subduction materials.The comprehensive analysis suggests that the Laguoco anorthosite was derived from high degree partial melting of depleted mantle source with re-enrichment of plume.The primary magma deriving from(enriched)mantle wedge rapidly ascended into oceanic crust in company with the crystallization differentiation of Mg-Fe oxides.Then,residual Ca,Al-bearing magma injected into the gabbros and formed anorthosite with the decrease of pressure.Combining with regional geology,it is suggested that the Laguoco ophiolite was formed in a subduction-related setting during the Jurassic.

关 键 词：班公湖-怒江缝合带 狮泉河-永珠带 蛇绿岩 斜长岩 锆石U-PB年龄 

分 类 号：P534.52[天文地球—古生物学与地层学] P588.12[天文地球—地质学]