青海同仁县隆务峡地区首次发现镁铁质——超镁铁质岩带

在青海省同仁县隆务峡南段的二叠纪地层中首次发现了北西—南东向分布的镁铁质—超镁铁质岩带,命名为隆务峡镁铁质—超镁铁质岩带。该岩带的岩石组合包括纯橄岩、辉石橄榄岩、辉长岩、辉绿岩、枕状玄武岩等。从超镁铁岩—下部堆晶岩—辉绿岩—上部堆晶岩,其稀土元素总量逐渐增加,稀土元素配分型式总体一致,从超镁铁岩的Eu正异常到上部辉长岩的Eu负异常,显示了各岩石间的同源性。根据对其两侧沉积岩中化石的研究,推测该镁铁质—超镁铁质岩带的形成时代为二叠纪。该岩带位于西秦岭造山带与祁连造山带的接舍部,对于揭示秦岭和祁连接合部的大地构造演化有重要意义。     

贵州遵义镍-钼富集层中独居石的发现及成因意义

对贵州遵义天鹅山-黄家湾镍-钼富集层中镍-钼矿石进行了电子探针研究,在镍-钼矿石中发现了稀土独立矿物——独居石,呈不规则的细粒、蠕虫状分布于矿石中,并与镍、钼的独立矿物共生;独居石La和Ce的含量高(La2O3含量变化范围为25.70%～30.52%,Ce2O3含量变化范围为22.96%～27.68%),贫Sm、Th(Sm2O3含量的变化范围为0.49%～0.80%,ThO2含量的变化范围为0%～0.19%),具有热液成因独居石的化学成分特征。镍-钼矿石中稀土矿物独居石的发现为镍-钼矿层的热液成因提供了直接的矿物学证据。     

青藏高原新生代火车头山碱性及钙碱性两套火山岩的地球化学特征及其物源讨论

藏北羌塘火车头山新生代火山岩可区分为钙碱性及碱性两个不同的系列。钙碱性火山岩主要岩石组合为玄武岩-安山岩-英安岩,其SiO_2介于49%～70%之间,Al_2O_3>10%,Na_2O/K_2O>1;其中玄武岩具平坦型稀土配分型式,LREE/HREE为1.3～1.8,(La/Yb)_N为2.87～4.45,无明显铕异常,δEu为0.96～1.09;该套岩石的Mg~#与SiO_2相关关系以及La/sm-La等亲岩浆元素与超亲岩浆元素协变关系表明,它们应为幔源岩浆经分离结晶演化的产物,其岩石组合类型以及低的Sm/Yb值(Sm/Yb=1.53～5.35)表明它们的原始岩浆应来源于岩石圈地幔尖晶石二辉橄榄岩的局部熔融。本区碱性火山岩为一套典型的钾质岩石系列,主要岩石组合类型为碱玄岩-碱玄质响岩-响岩,其SiO_2介于44%～59%之间,Al_2O_3>14%,Na_2O/K_2O介于0.47～1.51之间;岩石轻稀土强烈富集,LREE/HREE为13.20～15.76,(La/Yb)_N=50.44～91.99;其岩石组合类型以及Mg~#与SiO_2相关关系以及La/Sm-La协变关系同样表明它们为共源岩浆分离结晶演化的产物;然而,其较高的Sm/Yb值(Sm/ Yb=2.63～13.98)表明它们并非地幔橄榄岩直接局部熔融的产物,岩石弱的负Eu异常(δEu=0.77～0.85)以及Th、U的强烈富集和Nb、Ta的相对亏损,又反映了原始岩浆中有显著的地壳物质的贡献;该套钾质碱性系列岩石在La/Co-Th/Co同分母协变图上呈直线型分布,而在La/Co-Sc/Th异分母协变图上呈显著的双曲线分布,从而表明其源区为二源混合型,是青藏高原特殊的壳幔混合层局部熔融的产物,这些特征是新生代青藏高原壳幔层圈物质交换的重要岩石学证据。     

山东大尹格庄金矿蚀变岩中矿体分布稳定性的动力学控制参量探讨

元素品位分布的相依性指标——赫斯特指数(Hurst指数)是矿体分布稳定性的动力学控制参量,重标极差(R/S)分析是相依性分析的有效方法。利用R/S分析对胶东大尹格庄金矿黄铁绢英岩内不同勘探线金元素品位系列的Hurst指数计算发现,Hurst指数主要介于0.59至0.89之间,均值为0.75,标准差为0.09,均大于随机行走经验Hurst指数的均值与标准差;计算结果说明元素品位分布存在正相关性,但由于受多种因素影响,其空间分布的相依性具有一定波动。元素品位分布的正相依性显示在特定的地质背景内,矿体连续分布或者间断出现,发育相对稳定,其与大尹格庄金矿的地质事实相吻合。     

阿尔泰额尔齐斯带东段酸性岩墙群地球化学特征及其地质意义

野外地质调查发现在阿尔泰南缘额尔齐斯构造带东段、额尔齐斯活动断裂与富蕴-锡泊渡断裂之间发育了一套未变形的酸性岩墙群。岩墙群侵位于海西期片麻岩化花岗岩和上石炭统深变质的额尔齐斯组岩层中,薄片鉴定和岩石地球化学分析确定为具有细晶结构的流纹斑岩。岩石的SiO_2含量为70.9%～75.38%,K_2O Na_2O含量为7.55%～8.99%;大部分样品Na_2O>K_2O,里特曼指数值为1.8～2.5之间,Al_2O_3=12.80%～14.53%,A/CNK=0.9～1.1,NK/A=0.7～0.9,具有准铝质—弱过铝质、低镁、高钾钠、低钙和锶、高(Fe)_(mol)/(Mg)_(mol)特点,具A型花岗岩类特点,属于亚碱性脉岩。岩石轻稀土富集,大离子亲石元素相对富集,具有明显的中等负铕异常,壳源特征明显。锆石U-Pb二次粒子微探针测年显示岩墙群形成于277～286Ma。推断岩墙是阿尔泰海西期造山运动结束后,在拉张构造环境下的地壳局部熔融产物。     

鄂尔多斯北部直罗组中烃类包裹体地球化学特征及来源分析

对鄂尔多斯盆地东胜地区中侏罗统直罗组砂岩中烃类包裹体进行镜下观察、描述，利用压碎抽提法对烃类包裹体进行色谱—质谱分析，并与白垩系油苗、三叠系油砂及源岩抽提物进行对比，目的是探讨其来源。包裹体油生物标志物成熟度参数，C29ααα甾烷20S/(S+R)、C32αβ藿烷22S/(S+R)比值基本达到平衡值，利用甲基菲指数计算的镜质体反射率参数介于0.64%～0.82％之间，显示包裹体中石油烃已接近成熟—成熟热演化阶段；物质来源及沉积环境参数，ααα20R甾烷百分含量C27>C28相似文献   

Cenozoic post-collisional brittle tectonic history and stress reorientation in the High Zagros Belt (Iran, Fars Province)

The Zagros fold-and-thrust belt of SW-Iran is among the youngest continental collision zones on Earth. Collision is thought to have occurred in the late Oligocene–early Miocene, followed by continental shortening. The High Zagros Belt (HZB) presents a Neogene imbricate structure that has affected the thick sedimentary cover of the former Arabian continental passive margin. The HZB of interior Fars marks the innermost part of SE-Zagros, trending NW–SE, that is characterised by higher elevation, lack of seismicity, and no evident active crustal shortening with respect to the outer (SW) parts. This study examines the brittle structures that developed during the mountain building process to decipher the history of polyphase deformation and variations in compressive tectonic fields since the onset of collision. Analytic inversion techniques enabled us to determine and separate different brittle tectonic regimes in terms of stress tensors. Various strike–slip, compressional, and tensional stress regimes are thus identified with different stress fields. Brittle tectonic analyses were carried out to reconstruct possible geometrical relationships between different structures and to establish relative chronologies of corresponding stress fields, considering the folding process. Results indicate that in the studied area, the main fold and thrust structure developed in a general compressional stress regime with an average N032° direction of σ₁ stress axis during the Miocene. Strike–slip structures were generated under three successive strike–slip stress regimes with different σ₁ directions in the early Miocene (N053°), late Miocene–early Pliocene (N026°), and post-Pliocene (N002°), evolving from pre-fold to post-fold faulting. Tensional structures also developed as a function of the evolving stress regimes. Our reconstruction of stress fields suggests an anticlockwise reorientation of the horizontal σ₁ axis since the onset of collision and a significant change in vertical stress from σ₃ to σ₂ since the late stage of folding and thrusting. A late right-lateral reactivation was also observed on some pre-existing belt-parallel brittle structures, especially along the reverse fault systems, consistent with the recent N–S plate convergence. However, this feature was not reflected by large structures in the HZB of interior Fars. The results should not be extrapolated to the entire Zagros belt, where the deformation front has propagated from inner to outer zones during the younger events.     

U–Pb SHRIMP monazite ages of the giant Morro Velho and Cuiabá gold deposits, Rio das Velhas greenstone belt, Quadrilátero Ferrífero, Minas Gerais, Brazil

U–Pb SHRIMP results of 2672 ± 14 Ma obtained on hydrothermal monazite crystals, from ore samples of the giant Morro Velho and Cuiabá Archean orogenic deposits, represent the first reliable and precise age of gold mineralization associated with the Rio das Velhas greenstone belt evolution, in the Quadrilátero Ferrífero, Brazil. In the basal Nova Lima Group, of the Rio das Velhas greenstone belt, felsic volcanic and volcaniclastic rocks have been dated between 2792 ± 11 and 2751 ± 9 Ma, coeval with the intrusion of syn-tectonic tonalite and granodiorite plutons, and also with the metamorphic overprint of older tonalite–trondhjemite–granodiorite crust. Since cratonization and stable-shelf sedimentation followed intrusion of Neoarchean granites at 2612 + 3/− 2 Ma, it is clear that like other granite–greenstone terranes in the world, gold mineralization is constrained to the latest stages of greenstone evolution.     

On-going orogeny in the outer-arc of the Timor–Tanimbar region, eastern Indonesia

The Timor–Tanimbar islands of eastern Indonesia form a non-volcanic arc in front of a 7 km deep fore-arc basin that separates it from a volcanic inner arc. The Timor–Tanimbar Islands expose one of the youngest high P/T metamorphic belts in the world, providing us with an excellent opportunity to study the inception of orogenic processes, undisturbed by later tectonic events.Structural and petrological studies of the high P/T metamorphic belt show that both deformation and metamorphic grade increase towards the centre of the 1 km thick crystalline belt. Kinematic indicators exhibit top-to-the-north sense of shear along the subhorizontal upper boundaries and top-to-the-south sense in the bottom boundaries of the high P/T metamorphic belt. Overall configuration suggests that the high P/T metamorphic rocks extruded as a thin sheet into a space between overlying ophiolites and underlying continental shelf sediments. Petrological study further illustrates that the central crystalline unit underwent a Barrovian-type overprint of the original high P/T metamorphic assemblages during wedge extrusion, and the metamorphic grade ranged from pumpellyite-actinolite to upper amphibolite facies.Quaternary uplift, marked by elevation of recent reefs, was estimated to be about 1260 m in Timor in the west and decreases toward Tanimbar in the east. In contrast, radiometric ages for the high P/T metamorphic rocks suggest that the exhumation of the high P/T metamorphic belt started in western Timor in Late Miocene time and migrated toward the east. Thus, the tectonic evolution of this region is diachronous and youngs to the east. We conclude that the deep-seated high P/T metamorphic belt extrudes into shallow crustal levels as a first step, followed by doming at a later stage. The so-called ‘mountain building’ process is restricted to the second stage. We attribute this Quaternary rapid uplift to rebound of the subducting Australian continental crust beneath Timor after it achieved positive buoyancy, due to break-off of the oceanic slab fringing the continental crust. In contrast, Tanimbar in the east has not yet been affected by later doming. A wide spectrum of processes, starting from extrusion of the high P/T metamorphic rocks and ending with the later doming due to slab break-off, can be observed in the Timor–Tanimbar region.     

The geology of the Morro Velho gold deposit in the Archean Rio das Velhas greenstone belt, Quadrilátero Ferrífero, Brazil

The Morro Velho gold deposit, Quadrilátero Ferrífero region, Minas Gerais, Brazil, is hosted by rocks at the base of the Archean Rio das Velhas greenstone belt. The deposit occurs within a thick carbonaceous phyllite package, containing intercalations of felsic and intermediate volcaniclastic rocks and dolomites. Considering the temporal and spatial association of the deposit with the Rio das Velhas orogeny, and location in close proximity to a major NNW-trending fault zone, it can be classified as an orogenic gold deposit. Hydrothermal activity was characterized by intense enrichment in alteration zones of carbonates, sulfides, chlorite, white mica±biotite, albite and quartz, as described in other Archean lode-type gold ores. Two types of ore occur in the deposit: dark gray quartz veins and sulfide-rich gold orebodies. The sulfide-rich orebodies range from disseminated concentrations of sulfide minerals to massive sulfide bodies. The sulfide assemblage comprises (by volume), on average, 74% pyrrhotite, 17% arsenopyrite, 8% pyrite and 1% chalcopyrite. The orebodies have a long axis parallel to the local stretching lineation, with continuity down the plunge of fold axis for at least 4.8 km. The group of rocks hosting the Morro Velho gold mineralization is locally referred to as lapa seca. These were isoclinally folded and metamorphosed prior to gold mineralization. The lapa seca and the orebodies it hosts are distributed in five main tight folds related to F1 (the best examples are the X, Main and South orebodies, in level 25), which are disrupted by NE- to E-striking shear zones. Textural features indicate that the sulfide mineralization postdated regional peak metamorphism, and that the massive sulfide ore has subsequently been neither metamorphosed nor deformed. Lead isotope ratios indicate a model age of 2.82 ± 0.05 Ga for both sulfide and gold mineralization. The lapa seca are interpreted as the results of a pre-gold alteration process and may be divided into carbonatic, micaceous and quartzose types. The carbonatic lapa seca is subdivided into gray and brown subtypes. Non-mineralized, gray carbonatic lapa seca forms the hanging wall to the orebodies, and is interpreted as the product of extreme CO₂ metasomatism during hydrothermal alteration. This dolomitic lapa seca ranges in composition from relatively pure limestone and dolomite to silty limestone and dolomite. The brown carbonatic and micaceous lapa secas are the host rocks to gold. These units are interpreted to correspond to the sheared and hydrothermal products of metamorphosed volcaniclastic and/or volcanic rocks of varying composition from dacitic to andesitic, forming various types of schists and phyllites. The high-grade, massive sulfide orebodies occur at the base of the gray carbonatic lapa seca. Both disseminated mineralization and quartz veins are hosted by micaceous lapa seca. The data are consistent with a model of epigenetic mineralization for the lapa seca, from a hydrothermal fluid derived in part from the Archean basement or older crust material.