西昆仑山库地蛇绿岩的特征及其构造意义

库地蛇绿岩套由下部超镁铁质岩、辉长岩、幔源型花岗岩和上部基性火山岩、复理石等成分构成,形成于震旦纪-寒武纪。通过基性的辉长岩、玄武岩的矿物学、岩石学和地球化学研究得出,辉长岩表现出与典型蛇绿岩套中的堆晶辉长岩的特征相一致;而玄武岩则与大洋中脊玄武岩的特征一致,认为该蛇绿岩套是形成于洋中脊的蛇绿岩套。结合库地岛弧花岗岩的特征,提出塔里木盆地南缘在震旦纪-早古生代时期存在一个广阔的大洋,这一大洋在志留纪末期关闭。     

北祁连西段中元古代早期蛇绿岩的确定

北祁连西段朱龙关河—托来山北坡一带,希罕地保存着中元古代早期蛇绿岩带。该蛇绿岩带形成于下元古界北大河群古陆壳结晶基底上,由裂陷到裂谷,尔后演化为小洋盆所残留下来的洋壳残片。根据采自蛇绿岩中辉绿岩岩墙的３ 颗单颗粒锆石年龄测定值（１．７～１．８Ｇａ）信息,表明洋盆发育于中元古代早期。同时它还经历了沟、弧、盆体系发育过程。     

Probe and SIMS investigation of clinopyroxene inclusions in chromites from the Troodos chromitites (Cyprus): Implications for dunite–chromitite genesis

Chromite ores formation is still a debated topic and, in this study, detailed analyses of major, minor elements by EMPA and Rare Earth Elements (REE) by SIMS, were performed on silicate inclusions detected in chromite grains of chromitite pods, enveloping dunites and on clinopyroxene of lherzolitic host rock of well known Trodoos chromite ores. Results show the complexity of relationships among lithologies that reflects the subtlety of genetic events and of chromite ore occurrence.Analyses of textural and chemical features have led to a better understanding of the chromite ore genetic process, that is related to a supra-subduction geodynamic setting where partial melting processes were overlapped by metasomatic events.Metasomatism, that marks the general genetic context, is characterized by the presence of a hydrated boninitic melt that favors the precipitation of chromite ores. Chromite ores, typical of ophiolite complexes and usually enveloped by dunite are, in this case, characterized by the presence of two different types of dunites whose geochemical differences are reflected by olivine mineral chemistry and by different REE patterns of analyzed clinopyroxenes. Such geochemical marks, related to different magmatic and metasomatic events, could be a main key for location of fertile or barren dunites in terms of chromite ore occurrence in the field.     

蛇绿岩就位机制及时限

蛇绿岩就位机制可以划分出4种:1)碰撞仰冲型:被动大陆边缘或岛弧与洋壳碰撞时,俯冲到一定深度的硅铝质物质在浮力作用下折返,并上驮相对完整的大洋岩石圈残片到达地表;2)增生底垫型:洋底、海沟沉积物及海底较高地形的上层物质从俯冲板块上刮削下来,持续底垫到上覆板块之下,使大洋岩石圈残片逐渐被动抬高;3)俯冲剥离型:断裂发育相...     

Sedimentary record of the northward flight of India and its collision with Eurasia (Ladakh Himalaya,India)

Abstract

— Stratigraphic and petrographic analysis of the Cretaceous to Eocene Tibetan sedimentary succession has allowed us to reinterpret in detail the sequence of events which led to closure of Neotethys and continental collision in the NW Himalaya.

During the Early Cretaceous, the Indian passive margin recorded basaltic magmaüc activity. Albian volcanic arenites, probably related to a major extensional tectonic event, are unconformably overlain by an Upper Cretaceous to Paleocene carbonate sequence, with a major quartzarenite episode triggered by the global eustatic sea-level fall at the Cretaceous/Tertiary boundary. At the same time, Neotethyan oceanic crust was being subducted beneath Asia, as testified by calc-alkalic volcanism and forearc basin sedimentation in the Transhimalayan belt.

Onset of collision and obduction of the Asian accretionary wedge onto the Indian continental rise was recorded by shoaling of the outer shelf at the Paleocene/Eocene boundary, related to flexural uplift of the passive margin. A few My later, foreland basin volcanic arenites derived from the uplifted Asian subduction complex onlapped onto the Indian continental terrace. All along the Himalaya, marine facies were rapidly replaced by continental redbeds in collisional basins on both sides of the ophiolitic suture. Next, foreland basin sedimentation was interrupted by fold-thrust deformation and final ophiolite emplacement.

The observed sequence of events compares favourably with theoretical models of rifted margin to overthrust belt transition and shows that initial phases of continental collision and obduction were completed within 10 to 15 My, with formation of a proto-Himalayan chain by the end of the middle Eocene.     

新疆科克森套蛇绿岩套玄武岩的地质地球化学

新疆北部科克森套地区玄武岩出露于超镁铁质岩体的北侧,与玄武质凝灰岩共生,厚度达50～80 m;玄武岩普遍具有片理化、斑状结构;岩石地球化学特征表明,该玄武岩属低碱(Na2O+K2O均<3%,σ值在0.60～1.19)、低钛(TiO2在0.73%～1.22%)、富镁(MgO在7.43%～9.72%)的拉斑玄武岩系列,具有原始岩浆特征(其Mg#值在0.65～0.71),稀土总量较低(ΣREE在29.68×10-6～44.91×10-6),稀土及微量元素配分曲线平缓,可能形成于洋中脊构造环境。该区玄武岩的发现和岩石地球化学研究,佐证了科克森套蛇绿岩套的存在。     

内蒙朝克山蛇绿岩地球化学: 洋内弧后盆地的产物？

朝克山蛇绿岩是内蒙贺根山地区出露最好的蛇绿岩之一,可能形成于中晚石炭世。朝克山蛇绿岩中的基性岩具有LREE亏损、类似N-MORB的稀土配分模式,而相对N-MORB富集大离子亲石元素,亏损Nb、Ta等高场强元素又类似岛弧火山岩的成分特征,因此,我们认为朝克山蛇绿岩应形成于弧后盆地。将朝克山蛇绿岩的基性岩与现代Mariana洋内弧后盆地和Okinawa陆缘弧后盆地的玄武岩及同属中亚造山带的、形成于洋内弧后盆地的新疆库尔提蛇绿岩对比,朝克山蛇绿岩更类似于Mariana玄武岩和库尔提蛇绿岩,因此其很可能形成于洋内弧后盆地而不是大陆边缘弧后盆地环境。     

Mantle petrology and mineralogy of the Thetford Mines Ophiolite Complex

The Ordovician Thetford Mines ophiolite complex (TMOC) formed by boninite-fed seafloor-spreading, probably in a fore-arc environment. The mantle section is dominated by foliated harzburgite (≤ 5–6% clinopyroxene), cut by dunitic (± chromitite cores) and orthopyroxenitic veins and dykes. Contrasting structures, textures and mineral compositions allow us to subdivide the mantle. The granular-textured rocks of the Duck Lake Block (DLB) have two steeply-dipping foliations. The older foliation strikes NW, is sub-perpendicular to the Moho, and is interpreted to have resulted from upflow of the asthenosphere beneath the spreading ridge. This fabric is overprinted by a 2nd ductile foliation striking ENE, oriented sub-parallel to the Moho, which we interpreted as having formed by crust–mantle shear as the lithosphere migrated away from the spreading ridge. The DLB mantle has a limited range of spinel Cr# (100Cr / (Cr + Al) = 51–71). Comparison with experimentally determined residual spinel compositions (equilibrium melting) implies a maximum loss of 27–38% melt if the protolith had a fertile MORB mantle composition. However, interstitial-textured clinopyroxene may have high TiO₂ (< 0.04wt.%) and Na₂O (< 0.27wt.%), and some interstitial spinel has higher TiO₂ (< 0.09wt.%), suggesting interaction with (or crystallization from) an “impregnating” melt. Interstitial tremolitic amphibole also indicates the passage of late hydrous fluids. The harzburgite in the Caribou Mountain Block (CMB) has a porphyroclastic texture, with a strong, locally mylonitic foliation striking roughly N–S, parallel to the orientation of seafloor-spreading related paleo-normal faults in the crust. These fabrics and textures imply a colder, lithospheric deformation, possibly related to tectonic denudation (oceanic core complex). This would explain problematic lava/mantle contacts, favour infiltration of seawater, serpentinization, and reduced fO₂ conditions. The CMB mantle shows a wider range of mineral compositions than the DLB, with spinel Cr# (28–86) implying ≤ 15–45% of equilibrium melting. Locally higher TiO₂ in spinel (< 0.05wt.%) and clinopyroxene (< 0.11wt.%), a local rimward decrease in spinel Cr#, clinopyroxene Cr#, and olivine Fo-content, and traces of interstitial amphibole, are attributed to the circulation of an evolved hydrous melt during peridotite deformation. This suggests that the lower limit to the extent of melting inferred for the CMB (15%), established on the basis of Al-rich spinel rims and neoblasts, is probably too low. On the other hand, the higher inferred degree of depletion of the CMB is probably unaffected by the metasomatic overprint and is a more robust conclusion.