内蒙古索伦山蛇绿岩锆石U-Pb年代学、地球化学特征及其地质意义

为加深对中亚造山带东段晚古生代构造背景的认识,选取内蒙古索伦山蛇绿岩中较新鲜的玄武岩和辉长岩开展岩相学、年代学和地球化学研究。结果表明,玄武岩和辉长岩的LA-ICP-MS锆石U-Pb谐和年龄分别为(276.4±1.4) Ma和(273.9±3.3) Ma,形成于早二叠世。二者均为亚碱性拉斑系列,Mg#分别变化于41~50和62~65之间,均低于幔源原生岩浆值;主量元素与SiO2含量之间具有一定的线性关系,反映岩浆演化过程中分离结晶占主要地位。两种岩石的Th/La、Th/Ce和Lu/Yb值均低于大陆地壳的平均值,而与幔源岩浆的比值接近,推测岩浆演化过程中地壳混染的影响不大。稀土元素组成上,玄武岩表现为轻稀土亏损的N-MORB特征,可能起源于尖晶石二辉橄榄岩亏损地幔的5%~10%部分熔融;辉长岩具有轻稀土略亏损、整体平坦的稀土特征,但稀土元素总量明显比N-MORB更亏损,是源区熔融程度更大所致。此外,二者都富集K、Sr、Ba和Pb等大离子亲石元素而亏损Nb、Ta等高场强元素,具有岛弧岩浆特征,指示它们遭受了俯冲板片含水流体或者俯冲沉积物熔体的影响。上述岩石地球化学特征表明,索伦山蛇绿岩应为俯冲带上盘(SSZ)型,推测其形成于洋内弧后盆地。索伦山SSZ型蛇绿岩的存在说明该区早二叠世仍存在洋内俯冲,综合分析认为古亚洲洋在晚二叠世-早三叠世沿索伦-西拉木伦一线闭合。

Zircon U-Pb Geochronology,Geochemistry Characteristics and Geological Significance of the Solonker Ophiolite,Inner Mongolia

To deepen our understanding on the Late Paleozoic tectonics of the eastern Central Asian Orogenic Belt,a study of the petrology,geochronology and geochemistry of fresh basalts and gabbros in the Solonker ophiolitic mélange belt were reported.Zircon grains from these basalts and gabbros yielded 206Pb/238U ages of(276.7±2.7)Ma and(274.1±3.2)Ma,respectively,indicating an Early Permian emplacement age.Both the basalts and gabbros are tholeiitic,and have low Mg#values of 41-50 and 62-65,respectively,both below the primary magma value.Meanwhile,the basalt and gabbro samples fall along a fractionation trend in the Harker diagrams,suggesting a dominant role of fractional crystallization in the magma evolution.The Th/La,Th/Ce and Lu/Yb ratios of the two rock types are below those of the average continental crust,but similar to those of typical mantle-derived magmas.Therefore,it is inferred that crustal contamination had little influence on the mafic magmas.Rare earth element(REE)compositions show that the basalts are of the N-MORB-type with light REE(LREE)depletions,probably originated from 5%-10%partial melting of the depleted spinel lherzolite mantle.Gabbros are slightly LREE-depleted with a flat REE distribution pattern,and have clearly lower total REEs(ΣREEs)than N-MORB,probably due to a higher degree of partial melting than the basalts in the source region.In addition,both rock types are highly enriched in large ion lithophile elements(LILEs:Rb,Sr,Ba,Pb)and depleted in high-field-strength elements(HFSEs:Nb,Ta),similar to typical island arc magmas.These features imply that their magmas were affected by subduction-related fluids,or were derived from the melting of subducting sediments.The geochemical characteristics indicate that the Solonker ophiolites belong to the supra-subduction zone(SSZ)-type,and may have formed in an intra-oceanic backarc basin.This indicates that intra-oceanic subduction still persisted till at least the Early Permian in the study area.Comprehensive analyses suggest that the Paleo-Asian Ocean(PAO)was closed in the Late Permian to Early Triassic along the Solonker-Xar Moron suture zone.