新疆阿尔泰南缘加尔巴斯套铁矿床成矿时代及成矿作用研究

加尔巴斯套铁矿床位于阿尔泰南缘克兰火山沉积盆地中。该矿床与产于额尔齐斯构造带北侧的可斯尔鬼花岗岩有直接的成因关系,属于典型的矽卡岩型富铁矿。岩体SHRIMP U-Pb年龄表明,该矿床略晚于287 Ma形成;流体包裹体和稳定同位素研究显示,成矿阶段流体除岩浆水外,还有较多大气降水的参与,成矿流体主要表现为中低温、低盐度、中低密度特征;成矿物质主要来源于走滑-伸展背景下的可斯尔鬼花岗岩。结合前人研究成果,笔者认为在晚石炭世—早二叠世时期,阿尔泰南缘由走滑-伸展引发的大规模岩浆作用造就了区内有关铜、锌、镍、金、铁等广泛的成矿作用。     

辽西河坎子碱性侵入杂岩体及钼多金属矿床同位素年代学研究

对辽西河坎子地区与碱性杂岩体相关的钼多金属矿床进行了同位素年代学研究。所获黑云母正长花岗岩锆石LA-ICP-MS U-Pb同位素加权平均年龄为(235.3±1.0) Ma,MSWD值为0.68;河坎子钼多金属矿床内辉钼矿的Re-Os同位素等时线年龄为(224.0±1.3) Ma,MSWD值为0.72。碱性杂岩体与相关的钼多金属矿床具有密切的空间关系,两者的形成时间亦比较接近,据此可推测,河坎子碱性杂岩体与相关的钼多金属矿床均为印支期构造-岩浆作用及流体活动的产物。印支期内频繁的岩浆-热液活动为该地区内钼、铜、金多金属元素的活化、迁移、富集提供了充足的热源条件。古大陆内部伸展构造环境中的碱性岩浆作用及流体活动为钼多金属矿床的形成提供了动力、物质和流体来源。     

青海省祁连县东沟铜矿床成矿特征及成因研究

通过对东沟铜矿床的成矿特征、流体包裹体地球化学以及矿床成因研究,获得了如下认识:① 成矿环境为晚寒武世的大洋扩张脊环境,容矿岩石为蛇绿岩套上部的基性火山岩,矿床成矿金属组合为Cu-Zn(少量),围岩蚀变主要有硅化、绿泥石化、碳酸盐化及绿帘石化等,其中,硅化、绿泥石化与矿体的关系最为密切;② 对不同类型矿石中黄铁矿、黄铜矿的电子探针分析均表明,具有低温或中低温热液成矿的特征;③ 流体包裹体地球化学研究表明,该区成矿期石英中包裹体类型简单,仅有纯液体包裹体和液体包裹体,气相分数低,液体包裹体气相成分以CO₂为主,液相成分主要为H₂O;矿床成矿流体温度为150～200℃,盐度w(NaCl_eq)为13%～16%,密度为0.87～0.95 g/cm³;④ 矿床属于"塞浦路斯"型块状硫化物矿床。关键字 地球化学;成矿特征;流体包裹体;塞浦路斯型矿床;东沟铜矿;北祁连     

华北地块北缘西段巴音诺尔公—狼山地区牙马图岩体的岩浆混合成因

华北地块北缘西段巴音诺尔公—狼山地区的牙马图岩体以二长花岗岩为主,岩体中广泛发育岩浆暗色包体,二者界线明显。包体为岩浆结构,大多数具有塑性外形,发育淬冷边、反向脉,存在多种不平衡结构和矿物组合,如斜长石环带、石英眼斑、针状磷灰石等,显示岩浆混合特征;包体的SiO2含量为48.40%～55.40%,寄主花岗岩SiO2含量为65.03%～72.85%,具有明显的SiO2含量间隔;与寄主花岗岩相比,包体的Fe、Mg、Ca、Ti含量较高;包体和寄主花岗岩的主要氧化物之间具有很好的线性关系,微量元素和稀土元素特征相似。包体和寄主花岗岩的这些地球化学特征显示出明显的岩浆混合趋势。岩相学和元素地球化学特征表明暗色包体是基性岩浆侵入到酸性岩浆淬冷的产物,牙马图岩体存在两种岩浆的混合作用。     

西藏冈底斯带东段石炭纪构造环境讨论

冈底斯带东段石炭纪构造环境存有争议。本文通过岩石地球化学分析,发现石炭纪诺错组和来姑组火山岩具有碱性系列向拉斑系列过渡的特征,具有双峰火山岩特点;石炭纪玄武岩样品的平均化学成分与大陆拉斑玄武岩的平均值较为接近;微量元素除Ti、Yb、Y之外,其他元素都富集,其中Rb、Th、Ce富集程度相对较高;稀土元素特征表现为轻稀土富集,铕、铈异常不明显。显示出石炭纪玄武岩的地球化学特征均与大陆拉斑玄武岩相似。构造环境判别图解显示,石炭纪玄武岩样品主要落入板内环境的大陆拉张带(或初始裂谷)玄武岩区。因而认为冈底斯东段在石炭纪属于陆内裂谷或被动陆缘裂谷环境。     

Petrogenesis of early Silurian intrusions in the Sanchakou area of Eastern Tianshan,Northwest China,and tectonic implications: geochronological,geochemical, and Hf isotopic evidence

ABSTRACT

Palaeozoic intrusions in Eastern Tianshan are important for understanding the evolution of the Central Asian Orogenic Belt (CAOB). The Sanchakou intrusions situated in Eastern Tianshan (southern CAOB), are mainly quartz diorite and granodiorite. A comprehensive study of zircon U–Pb ages, zircon trace elements, whole-rock geochemistry, and Lu–Hf isotopes were carried out for the Sanchakou intrusive rocks. LA-ICP-MS zircon U–Pb dating yielded crystallization ages of 439.7 ± 2.5 Ma (MSWD = 0.63, n = 21) for the quartz diorite, and 430.9 ± 2.5 Ma (MSWD = 0.21, n = 21) and 425.5 ± 2.7 Ma (MSWD = 0.04; n = 20) for the granodiorites. These data, in combination with other Silurian ages reported for the intrusive suites from Eastern Tianshan, indicate an early Palaeozoic magmatic event in the orogen. In situ zircon Hf isotope data for the Sanchakou quartz diorite shows ε_Hf(t) values of +11.2 to +19.6, and the two granodioritic samples exhibit similar ε_Hf(t) values from +13.0 to +19.5. The Sanchakou plutons show metaluminous to weakly peraluminous, arc-type geochemical and low-K tholeiite affinities, and display trace element patterns characterized by enrichment in K, Ba, Sr, and Sm, and depletion in Nb, Ta, Pb, and Ti. The geochemical and isotopic signatures indicate that the Sanchakou dioritic and granodioritic magmas were sourced from a subducted oceanic slab, and subsequently underwent some interaction with peridotite in the mantle wedge. Combined with the regional geological history, we suggest the Sanchakou intrusions formed due to the northward subduction of the Palaeo-Tianshan Ocean beneath the Dananhu–Tousuquan arc during early Silurian time.     

Origin and geodynamic environments of the metamorphic sole rocks from the İzmir–Ankara–Erzincan suture zone (Tokat,northern Turkey)

ABSTRACT

The Late Cretaceous accretionary complex of the ?zmir–Ankara–Erzincan suture zone, near Artova, is composed mainly of peridotites (variably serpentinized), amphibolite, garnet-micaschist, calc-schist, marble, basalt, sandstones, neritic limestones. The metamorphic rocks were interpreted as the metamorphic sole rocks occurring at the base of mantle tectonites, because: (i) amphibolites were observed together with the serpentinized peridotites suggesting their occurrences in the oceanic environment; (ii) foliation in amphibolites and serpentinized peridotites run subparallel to each other; (iii) all these metamorphic rocks and serpentinized peridotites are cross-cut by the unmetamorphosed dolerite dikes with island arc tholeiite-like chemistry. Geochemical characteristics of the amphibolites display enriched mid-ocean ridge basalt (E-MORB)- and ocean island basalt (OIB)-like signatures. The dolerite dikes, on the other hand, yield an island arc tholeiite-like composition. Geothermobarometric investigations of the metamorphic sole rocks suggest that the metamorphic temperature was ~650 ± 30°C and the pressure condition was less than 0.5 GPa. Dating of hornblende grains from amphibolite yielded age values ranging from 139 ± 11 to 157 ± 3.6 Ma (2σ). The oldest weighted average age value is regarded as approximating the timing of the intra-oceanic subduction. These cooling ages were interpreted to be the intra-oceanic subduction/thrusting time of the ?zmir–Ankara–Erzincan oceanic domain.     

Review Section

ABSTRACT

The petrology, geochronology, and geochemistry of the early Permian volcanic rocks from Houtoumiao area, south Xiwuqi County in central Inner Mongolia of China, are studied to elucidate the early Permian tectonic setting of the region. The volcanic rocks, which are interbedded with sandstone, feature both mafic and felsic compositions and show a bimodal nature. Zircon U–Pb dating reveals that the volcanic rocks formed at 274–278 Ma, similar to the ages of bimodal magmatism in neighbouring areas. The mafic rocks are composed of tholeiitic basalt, basaltic andesite, basaltic trachyandesite, and trachyandesite. They are rich in Th, U, and LILEs, depleted in HFSEs Nb, Ta, and Ti, and have positive ε_Nd(t) values (+3.6 to +7.9). Geochemical analyses indicate that the mafic rocks originated from metasomatized lithospheric mantle. The felsic volcanic rocks are mainly rhyolite, with minor trachyte and dacite. They have different evolutionary tendencies of major elements, chondrite-normalized REE patterns, and isotopic compositions from the mafic volcanic rocks, which preclude formation by fractional crystallization of mafic melts. The ε_Nd(t) values of the felsic rocks are similar to those of the Carboniferous Baolidao arc rocks in the region. It is suggested that Permian felsic melts originated from the partial melting of Carboniferous juvenile arc-related rocks. By comparison with typical Cenozoic bimodal volcanism associated with several tectonic settings, including rift, post-collisional setting, back-arc basin, and the Basin and Range, USA, the bimodal volcanic rocks in central Inner Mongolia display similar petrological and geochemical characteristics to the rocks from back-arc basin and the Basin and Range, USA. Based on the analysis of regional geological data, it is inferred that the early Permian bimodal volcanic rocks in the study area formed on an extensional continental margin of the Siberian palaeoplate after late Carboniferous subduction–accretion.     

Geochemistry and detrital modes of Proterozoic sedimentary rocks,Bayana Basin,north Delhi fold belt: implications for provenance and source-area weathering

Bayana Basin, sited along the eastern margin of the north Delhi fold belt of the Aravalli Craton, contains an ～3000?m-thick sequence comprising one volcanic and seven sedimentary formations of the Delhi Supergroup. The sedimentary units are the Nithar, Jogipura, Badalgarh, Bayana, Damdama, Kushalgarh, and Weir formations in order of decreasing age. Petrographic study of the sandstones as well as major and trace elements (including rare earth elements) and bulk-rock analyses of the shales and sandstones allow the determination of their provenance, source-rock weathering, and basinal tectonic setting. The sandstones are quartz rich and were derived mainly from exhumed granitoids typical of a craton interior. Geochemical patterns of the sandstones and shales are similar. However, trace element abundances are low in sandstones, probably due to quartz dilution. The coarser clastic Damdama and Weir sandstones, which occur at higher stratigraphic levels, have strikingly low trace element concentrations compared with the underlying Bayana and Badalgarh sandstones. All samples show uniform LREE-enriched patterns with negative Eu-anomalies (Eu/Eu*?=?0.16–0.23) and are similar to those of post-Archaean Australian shales (PAAS). However, the (La/Yb) _n ratios (averages 11–18) of all the sedimentary units are higher than those of PAAS, except for the Bayana Sandstone, which has low values (average 6.77). The chemical index of alteration (70–78) and the plagioclase index of alteration (87–97) values and the A–CN–K diagram suggest moderate to intense weathering of the source area.

The provenance analyses indicate that basin sedimentation was discontinuous. It received input from a terrain comprising granitoids, mafic rocks, sedimentary sequences, and tonalite-trondhjemite-granodiorite (TTG) suites. The Nithar and Badalgarh sandstones received input from a source consisting predominantly of granitoids. The succeeding Damdama and Weir sandstones received debris from granitoids and TTG in different proportions. The Kushalgarh shale was possibly derived from a source consisting granites and mafic rocks with a TTG component. The pre-existing sedimentary formations also contributed intermittently during the different phases of sedimentation.

Bulk-rock geochemical data suggest Mesoarchaean gneisses and late Archaean granites of BGC/BGGC (Banded Gneissic Complex/Bundelkhand Granitic Gneiss Complex) basement as possible source terrains. These data indicate deposition in a continental rift setting. The coeval formation of many rift-related Proterozoic sedimentary basins in the BGC/BGGC terrain suggests that the North Indian Craton underwent major intracratonic extension during Proterozoic time, probably triggering the break up of Earth's first supercontinent.     

Petrochemistry of Mesozoic mafic intrusions related to the Paraná Magmatic Province,Uruguay

The Cuaró Formation is part of the sequence of Mesozoic mafic intrusions related to the Early Cretaceous break-up of Gondwana and represents the southernmost occurrences within the Paraná Magmatic Province in Uruguay. We present field data, petrography and lithogeochemical results regarding these dike swarms and sills that crop out in the southern extreme of the Paraná Basin. Dolerites and sills mainly exhibit glomeroporphyritic textures; the phenocrysts consist of plagioclase, clinopyroxene, relicts of olivine and titaniferous magnetite. Bulk-rock geochemical analyses allowed their classification as low-Ti subalkaline tholeiitic basalts and andesitic basalts. Trace element data indicated that the protoliths of these intrusions include the subcontinental lithospheric mantle, as is generally recognized for other Gondwana-related continental flood basalt provinces.