Late Riphean Sedimentation in the Central Russian Aulacogen

Result of the analysis of combined structural, facies, mineralogical, and micropaleontological data suggest that the formation of the Central Russian Aulacogen was related to the evolution of a major strike-slip system in the Late Riphean on the Russian Plate. The strike-slip system led to the formation of parageneticaly interrelated structures: central anticlinal uplift framed by two troughs (structurally isolated echelon of lake-type sedimentary basins). The structural pattern of Late Riphean riftogenesis was responsible for the isolation of the sedimentation system. The variable intensity of extension at opposite flanks of the aulacogen is reflected in the fanlike form of the present-day structure and composition of sedimentary complexes, particularly the presence of red-colored molasse in the widest (southwestern) part of the study region. The growth of the central uplift is inferred from the enrichment of heavy fraction in epidote from blastomylonites derived from the lithostructural framing (rocks of the tectonic melange).     

Geothermal regime and Jurassic source rock maturity of the Junggar basin, northwest China

We analyze the thermal gradient distribution of the Junggar basin based on oil-test and well-logging temperature data. The basin-wide average thermal gradient in the depth interval of 0–4000 m is 22.6 °C/km, which is lower than other sedimentary basins in China. We report 21 measured terrestrial heat flow values based on detailed thermal conductivity data and systematical steady-state temperature data. These values vary from 27.0 to 54.1 mW/m² with a mean of 41.8 ± 7.8 mW/m². The Junggar basin appears to be a cool basin in terms of its thermal regime. The heat flow distribution within the basin shows the following characteristics. (1) The heat flow decreases from the Luliang Uplift to the Southern Depression; (2) relatively high heat flow values over 50 mW/m² are confined to the northern part of the Eastern Uplift and the adjacent parts of the Eastern Luliang Uplift and Central Depression; (3) The lowest heat flow of smaller than 35 mW/m² occurs in the southern parts of the basin. This low thermal regime of the Junggar basin is consistent with the geodynamic setting, the extrusion of plates around the basin, the considerably thick crust, the dense lithospheric mantle, the relatively stable continental basement of the basin, low heat generation and underground water flow of the basin. The heat flow of this basin is of great significance to oil exploration and hydrocarbon resource assessment, because it bears directly on issues of petroleum source-rock maturation. Almost all oil fields are limited to the areas of higher heat flows. The relatively low heat flow values in the Junggar basin will deepen the maturity threshold, making the deep-seated widespread Permian and Jurassic source rocks in the Junggar basin favorable for oil and gas generation. In addition, the maturity evolution of the Lower Jurassic Badaowan Group (J_1b) and Middle Jurassic Xishanyao Group (J_2x) were calculated based on the thermal data and burial depth. The maturity of the Jurassic source rocks of the Central Depression and Southern Depression increases with depth. The source rocks only reached an early maturity with a R₀ of 0.5–0.7% in the Wulungu Depression, the Luliang Uplift and the Western Uplift, whereas they did not enter the maturity window (R₀ < 0.5%) in the Eastern Uplift of the basin. This maturity evolution will provide information of source kitchen for the Jurassic exploration.     

新疆库鲁克塔格南华系砂岩碎屑组成对其物源及盆地演化的指示

库鲁克塔格南华系记录了塔里木北缘同期的火山-沉积事件和蚀源区物质组成及演化的信息。该区不同剖面内南华系各组砂岩碎屑组分的统计分析显示,贝义西组砂岩在不同剖面内组成差异显著:西山口剖面以岩屑砂岩为主,且岩屑为火山岩岩屑与沉积岩岩屑,物源为再旋回地层,而依格孜塔格剖面以长石砂岩和岩屑长石砂岩为主,岩屑主要为变质岩岩屑,物源为下伏古元古代高级变质岩。砂岩碎屑组成在剖面上垂向的变化表明贝义西组沉积期与照壁山组沉积期间（725 Ma±）存在一个沉积转型事件,导致贝义西组之上的照壁山组、阿勒通沟组及特瑞艾肯组砂岩组成在不同区域趋于一致,转变为代表基底隆起-过渡大陆区物源的典型“长石砂岩”。南华系砂岩碎屑组成与大陆裂谷盆地沉积砂岩相似,且物源区存在由前裂谷地层-过渡裂谷肩部-切割裂谷肩部-克拉通内部的连续演化过程,是库满凹陷早期裂解的岩相学记录。     

Structure and stages in development of the cataplatform cover in the Central Russian‒Belomorian province

The results of correlation between seismic sequences (based on the CMP data) and lithostratigraphic units (based on drilling data) of the preplate sedimentary section are discussed. By the structure and interrelations of seismic sequences, as well composition and facies features of lithological varieties, three successive stages in the formation of the cataplatform cover of the province are recognizable: (1) main stage of the graben formation in the Central Russian and White Sea?Pinega regions; (2) terminal stage of the graben formation?initial stage of postrift subsidence in all regions; (3) the formation of a ‘protosyneclise” (non-riftogenic depression) in the Orsha region. The model for explaining the formation of the Orsha depression is proposed.     

燕山中段常州沟组砂岩岩石学、地球化学特征及地质意义

中元古代常州沟组是华北克拉通北缘燕山地区变质结晶基底上的第一套沉积盖层,不整合覆盖于新太古代各类片麻岩之上。为了解常州沟组时期的古环境、古气候及其物质来源、构造环境和盆地性质,在野外地质调查的基础上,对常州沟组砂岩进行了粒度分析和地球化学分析测试。岩石组合、沉积构造和粒度分析结果指示区内常州沟组沉积环境演化由下至上依次为冲积扇、辫状河和碎屑潮坪。常州沟组砂岩地球化学特征具有高SiO₂、Al₂O₃和K₂O,低TiO₂、Fe₂O₃、FeO、MgO含量的特点,成熟度较高,化学蚀变指数(CIA)介于52.00～73.23之间,化学风化作用指数(CIW)范围为93.04～98.68。由于岩石遭受钾交代作用影响,结合A-CN-K图解、Th/U-Th图解和古气候判别图综合分析,认为源区可能经历了较强的风化作用,古气候温暖、潮湿。Cr/Zr、Th/Sc、Ba/Sr、Rb/Sr值和La/Th-Hf图解指示常州沟组源区属性主要为上地壳环境,原岩成分以长英...     

Mineralogy and petrography of Riphean rocks in the Moscow graben

Based on the study of Upper Precambrian rocks penetrated by the Pavlovskii Posad parametric borehole in the 1770–4780 m interval, the Riphean succession is underlain by quartzose sandstones. The aulacogen within the Moscow Syneclise was covered by sediments accumulated during the intense chemical weathering of a peneplain. Above the 3550 m level, Upper Precambrian rocks comprise arkoses with abundant garnet. The immature (in terms of lithology and mineralogy) arkosic sequence was probably accumulated in grabenshaped structures (aulacogens).     

Structure of the North Kara Shelf from results of seismostratigraphic analysis

The sedimentary cover section of the North Kara Shelf had been subdivided based on extensive seismic data, and seismic correlation was carried out. The predominant role of Upper Riphean-Middle Paleozoic rocks has been corroborated. A series of relatively deepwater basins filled with primarily terrigenous fly-schoid rocks up to 7–9 km in thickness existed in the Late Riphean-Vendian at the place of the shelf. In the Cambrian, isolated basins merged into a wide and shallow-water basin as a result of the Baikalian reactivation in southeast Severnaya Zemlya and probably in Taimyr. After the pre-Ordovician hiatus, a vast sedimentation basin with a regressive section was formed on the shelf from Ordovician to Late Devonian. Shallow-water marine and near-shore carbonate and carbonate-terrigenous sequences accumulated in this basin and gave way to continental and less frequent near-shore, marine, variegated, and red beds in the Devonian. The thickness of the Ordovician-Devonian sequence reaches 6 km. Since the Mid-Carboniferous, block emergence and deep erosion of Ordovician-Devonian complexes have occurred in the north Kara shelf as a result of Hercynian events in northern Taimyr, Severnaya Zemlya, and in the southern Kara Sea. No Permian-Triassic rifts existed on the North Kara Shelf. At that time, the shelf was an area of erosion. The thickness of the Middle Carboniferous-Cretaceous sequence was insignificant and gradually increased toward Barents Sea troughs. The newly formed Svyataya Anna and Voronin troughs arose due to opening of the Eurasia Basin of the Arctic Ocean. The terrane concept has been subjected to criticism. The available data show that a large epi-Grenvillian continental block existed, and the North Kara region was part of it. Collision of the northern continent with the Paleosiberian Platform in the Late Paleozoic resulted in the formation of the Hercynian fold arc to the south of the North Kara Shelf, and an inverted orogenic arch arose at the place of shelf basin. The individual geological features that distinguish the North Kara Shelf from the Barents Sea troughs and the South Kara Syneclise are emphasized. The ancient pre-Riphean basement, a system of Late Riphean-Vendian relatively deepwater troughs and basins, Hercynian tectonic inversion, deep erosion of the most uplifted part of the arch, and significant block motions are the distinguishing features of the North Kara Shelf.     

Meso- and Neoproterozoic complexes of the cover in the southeastern Siberian Platform: Formation conditions and main tectonic features

The large (more than 200000 km²) Uchur-Maya Meso- and Neoproterozoic intracontinental basin is delineated for the first time in the southeastern Siberian Platform, including the territory overlapped by the Ediacaran to Cambrian plate complex. The tectonic elements of higher orders—uplifts, domes, and deeps formed during the Neoproterozoic and Late Mesozoic tectonomagmatic activity—are outlined. The tectonic nappes of Paleoproterozoic rocks from 200 to 400 m thick were established between the Calymmian and Ectasian to Stenian rocks in the southeastern portion of the basin. The thickness of sedimentary rocks increases toward the Yudoma-Maya Aulacogen. The considerable thickness of Meso- and Neoproterozoic rocks and bitumen contents in sedimentary rocks are favorable factors testifying to the petroleum resource potential of the basin. The southern part of the basin is promising for ore deposits (primarily, uranium and gold) in zones of structural and stratigraphic unconformities and within the sedimentary cover itself. This study may serve as a background for further, more detailed tectonic investigations and substantiation of strategic lines of geological exploration.     

Geochemistry of Proterozoic Radioactive Arkoses in Khammam District,Andhra Pradesh,India: Evidence for K-rich Evolved Granitoid Upper Crust in the Late Archaean

Geochemical studies on radioactive arkoses (43–153 ppm U and 387–862 ppm Th) of the Proterozoic Pakhal Supergroup from Bangaruchilka, Khammam district, Andhra Pradesh, India, indicate that their gross major and trace element chemistry reflect their mineral composition. Chemically, arkoses are rich in silica (83% to 88% SiO₂) and potassium (3% to 5% K₂O), with consistently high Al₂O₃/Na₂O (36 to 50) and K₂O/Na₂O (18 to 25) ratios, which indicate that they are chemically mature sediments. The arkoses also show higher concentrations of Ti, V, Cr, Ni, Cu, Y, Zr, Nb, La and Pb.The values (60% to 68%) of chemical alteration index (CIA) of studied arkoses are moderate, and indicate that the source rocks have undergone lesser degree of chemical weathering. Tectonic setting discriminate plots of Fe₂O₃ (total)/MgO vs. TiO₂ and Al₂O₃/SiO₂, and K₂O/Na₂O vs. SiO₂ and SiO₂/Al₂O₃ indicate that the Bangaruchilka arkoses represent the sediments that were deposited in passive continental margin (PM), which is further supported by association of platformal type of sediments (quartzites and phyllites) with them. Unlike middle Archaean sedimentary rocks, the studied arkoses are depleted in Na₂O, MgO and CaO, and distinctly enriched in SiO₂ and K₂O. These geochemical features match with post-Archaean clastic sediments, which argues for the involvement of late Archaean granitic crust in supply of detritus of studied arkoses. Enrichment of potassium alongwith abundant microcline and elevated concentrations of Y, Zr, Nb, U, Th, La, etc. in arkoses indicate K-rich evolved granitoid upper crust in the provenance during the late Archaean-early Proterozoic period.Dominance of mechanical weathering over chemical weathering favoured arkose formation, and also transfer of radio-elements with clastic sediments into the Pakhal basin. After sedimentation, uranium seems to have been remobilised from the rocks of Archaean-Pakhal system, consequent to post-sedimentation structural disruptions, and concentrated along suitable structural loci. Therefore, it is likely to encounter significant concentration of uranium in close proximity of unconformable Archaean and Pakhal contacts and tectonic zones, thereby making Pakhal basin and its environs a suitable terrain to search for concealed uranium mineralisation.     

早古生代阿拉善地块与华北地块之间的关系:来自阿拉善东缘中奥陶统碎屑锆石的信息

阿拉善东缘奥陶纪地层位于鄂尔多斯(华北地块)与北祁连早古生代造山带之间的过渡地区,该区的构造背景一直是长期争论的问题,它涉及到阿拉善地块是否与华北地块相连、奥陶系的物源以及"贺兰拗拉槽"是否存在等问题。分布于阿拉善地块东缘的中奥陶统米钵山组的碎屑锆石LA-ICP-MSU-Pb年龄测试表明,样品中数量最多的锆石年龄为900～950Ma,Alxa-1的峰值年龄为916Ma,Alxa-2的峰值年龄为953Ma,次者在494～623Ma之间,这个区间内存在多个峰值,如Alxa-1存在505Ma和588Ma两个主要峰值,Alxa-2则存在494Ma、517Ma、623Ma等几个峰值。在2.5Ga左右两个样品都存在一个弱的峰值,Alxa-1峰值为2517Ma,而Alxa-2峰值为2552Ma和2670Ma。除此之外,两个样品都有个别大于3.0Ga的成分,Alxa-1样品中最年轻的锆石为451±8Ma,Alxa-2样品则为483±4Ma。这些年龄以及沉积特征表明:(1)传统认为的奥陶纪"贺兰拗拉槽"并不存在,鄂尔多斯西南缘地区以及阿拉善东部地区当时属于北祁连早古生代周缘前陆盆地系统;(2)早古生代主要物源来自北祁连造山带,新元古代物源来自阿拉善地块;(3)鄂尔多斯西缘整个米钵山组的锆石年龄分布及其变化,指示出北祁连造山带(岛弧)逐渐靠近阿拉善地块,其间洋盆逐渐消失的过程;(4)阿拉善地块基底与华北有明显差别,阿拉善地块明显受到新元古代和古生代构造热事件的影响,两者可能是在中奥陶世或之后才拼贴在一起。     

Diagenesis of diatomite from the Kolubara Coal Basin,Baroševac,Serbia

Diatomite associated with the Kolubara Coal Basin was studied to better understand early stage silica diagenesis of shallow water deposits. The Kolubara Basin consists of Neogene siliciclastic rocks, diatomite, marlstone and rare carbonates. Palaeozoic metamorphic and Mesozoic sedimentary and igneous basement rocks are transgressively overlain by Upper Miocene sandstone, siltstone, shale and mudstone. This Upper Miocene section is transgressively overlain by the Pontian section, which contains diatomite and coal beds. White and grey diatomite forms beds 0.7-2.2 m thick that are continuous over an area of about 2 km². Siliceous rocks vary in composition from diatomite (81-89 per cent SiO₂) to diatom-bearing shale (58-60 per cent SiO₂). Siliceous deposits are laminated in places, with the laminae defined by variations in clay minerals, organic matter and diatoms. Diatomite shows only incipient diagenesis characterized by the fragmentation of diatom frustules, the minor to moderate corrosion of frustules and the formation of minor amounts of opal-A' (X-ray amorphous inorganic opal) cement. The low degree of diagenesis results from the young age of the deposits, low burial temperatures and possibly also from the presence of abundant organic matter and the dissolution of kaolinite. The presence of only weak diagenesis is also reflected by the characteristically poor consolidation of the rocks and low rank of the associated coal.     

Geochemistry of source rocks in the Es<Subscript>3</Subscript> and Es<Subscript>4</Subscript> members in the Shahejie Formation of the Western Depression,Liaohe Oilfield,China

The Western Depression of the Liaohe Basin is the major exploration area of the Liaohe Oilfield, and its main source rocks consist of the third and fourth members of the Shahejie Formation (Es3 and Es4). These source rocks are widely distributed in the depression, with semi-deep lake and fan delta as the main sedimentary facies, brown oil shale and black gray-dark gray mudstone as the main rocks, and a total thickness of 270-1450 m. The kerogens are mainly of the types I and IIA, and partly of the type IIB and least of the type III. The Ro values range from 0.4%-0.8%, indicating an evolution stage from immature to mature. The maturity of Es4 source rocks is rela-tively high, reaching the early mature stage, but their distribution and thickness are lower than those of Es3. Besides, according to biomarker analysis, it is thought that the source rocks of Es3 and Es4 are characterized by mixed input, and most of the source rocks were formed in the brackish water-saline and strongly oxygen-free environment. Fur-thermore, the Qingshui, Niuxintuo and Chenjia sags are believed to possess greater potential for hydrocarbon gen-eration and expulsion, for they are source rocks with a larger thickness, have higher organic carbon contents, belong to better organic matter types and possess higher maturities.     

Mid-Cretaceous uplift and erosion on the northern margin of the Ligurian Tethys deduced from thermal history reconstruction

The paleogeography during Early Cretaceous of the northern margin of the Ligurian Tethys is poorly constrained because of deformation and erosion during Pyrenean and Alpine orogenic phases. The present-day limit between Lower Cretaceous sediments in the South–East basin, located at the northwestern margin of the Ligurian Tethys, and basement rocks is the consequence of a protracted erosion history. Lower Cretaceous sediments observed today in the basin, even close to the present-day outcropping border, are characteristic of pelagic environments. A larger extent of a Lower Cretaceous cover on the basement must then be considered. This study focuses on the western part of this margin (the Causses basin), in the South of the Massif Central (France), using several thermochronometers and geothermometers to decipher the former extent of the sedimentary cover. Apatite fission track thermochronology on basement rocks surrounding the Causses basin suggests that these rocks cooled from temperatures higher than 110°C during the mid-Cretaceous. Average fluid inclusion homogenisation temperatures between 94°C and 108°C are recorded in calcite veins from outcropping Toarcian and Aalenian shales. In the shales, Tmax values, temperature obtained by Rock–Eval pyrolysis of organic matter, are in agreement with these elevated temperatures. Different explanations for these relatively high temperatures, which cannot be explained by the present-day sedimentary serie in the basin, have been tested using a 1D thermal modelling procedure (Genex). For a 95±10-mW/m² paleoflux, thick sedimentary deposits (2.5±0.3 km) including 1.3±0.3 km of Lower Cretaceous sediments cover the South of the Massif Central; these formations have been subsequently eroded from mid-Cretaceous time onwards. This study confirms that the South of the Massif Central was a site of marine sedimentation during the Early Cretaceous where a thick sedimentary sequence was once deposited.     

Geological framework of the Archean and Paleoproterozoic Tanami Region, Northern Territory

The Tanami Region, a poorly exposed, mostly Paleoproterozoic province within the North Australian Craton, hosts a number of significant gold deposits in diverse settings. Rare exposures of 2,520–2,500 Ma amphibolite facies Archean gneiss and metasedimentary rocks form basement to the thick overlying metasedimentary succession of the 1,880–1,830 Ma Tanami Group. The basal unit of the Tanami Group is the Dead Bullock Formation, a fining-upward deep-water succession dominated by siltstone, carbonaceous siltstone, iron-rich siltstone and mafic sills. Carbonaceous- and iron-rich lithologies in the upper Dead Bullock Formation represent important hosts for gold mineralization. The conformably overlying Killi Killi Formation represents turbiditic sedimentary rocks that are correlated with the widespread Lander Rock beds of the Arunta Region. Sedimentation of the Tanami Group was terminated by regional deformation and greenschist to amphibolite facies metamorphism during the Tanami Event (D₁/M₁), at around 1,830 Ma. The Tanami Group is unconformably overlain by rhyolite, siliciclastic sedimentary rocks, and felsic ignimbrite of the Ware Group that were deposited at about 1,825–1,810 Ma. Subsequent ESE–WNW to SE–NW directed shortening (D₂), followed by NE–SW to E–W directed shortening (D₃), has resulted in open NE F₂- and NW F₃-trending folds in both the Tanami and Ware Groups. Voluminous granitoids, dominated by I-type, biotite granodiorite, and monzogranite were intruded in the interval 1,825–1,790 Ma and have been subdivided using geochemical criteria into the Birthday, Frederick, and Grimwade Suites. Basalt and immature sedimentary rocks of the Mount Charles Formation are restricted in extent to the Tanami mine corridor, and are interpreted to reflect a continental rift succession that was deposited around 1,800 Ma, with an early Archean sedimentary provenance. Steep S to SE dipping F₄-fold structures of Tanami and Ware Group metasedimentary rocks, many spatially associated with 1,825–1,790 Ma granitoid intrusions, indicate a period of SSE-directed regional shortening (D₄) syn-to-post the regional granitoid intrusive phase. A network of N to NW striking faults, several of which are interpreted as oblique thrusts with a component of left lateral movement, indicates a period of D₅ convergence during WSW–ENE to E–W directed shortening. The Tanami mine corridor fault system comprises a network of N, NE to ENE striking D₅ faults that merge with N to NW striking faults and probably accommodated movement between granite core domains. D₅ faulting is associated with the main phase of gold mineralization in suitable structural–lithological traps. The Paleoproterozoic basement of the Tanami Region is unconformably overlain by quartz sandstone, lithic arenite, and conglomerate of the Pargee Sandstone. Pargee Sandstone may represent syn-tectonic sedimentation related to the 1,730 Ma Strangways Orogeny, and is unconformably overlain by the late Paleoproterozoic platform cover succession of the Birrindudu Group. The Paleoproterozoic basement and cover sequences have subsequently undergone several episodes of faulting, collectively termed D₆₊. The Paleoproterozoic evolution of the Tanami Region is interpreted to have occurred in an intracratonic setting, but was fundamentally influenced by tectonic events in the adjacent Halls Creek Orogen (1,835–1,805 Ma Halls Creek Orogeny) and Arunta Region (1,815–1,800 Ma Stafford Event). The boundaries between the Tanami Region and Kimberley Region to the northwest and the Arunta Region to the southeast are transitional, and are largely defined by the presence or absence of identifiable Dead Bullock Formation.     

内蒙古阿鲁科尔沁旗陶海营子剖面林西组地球化学特征及其对物源-构造背景的制约

大兴安岭中南段上二叠统林西组发育厚层的暗色泥页岩,是区域上重要的上古生界生烃层系之一。阿鲁科尔沁旗陶海营子剖面是林西组的典型剖面之一,本文以该剖面为研究对象,对取自该剖面林西组的10件砂、泥岩样品进行主量元素、微量元素和稀土元素测试分析。剖面样品元素分析和物源判别函数（F₁-F₂）、Ni-TiO₂、La/Th-Hf图解判别结果表明,陶海营子剖面林西组沉积物来源多样,主要来源于上地壳长英质火成物源区和石英岩沉积物源区,还有少量来自中、基性岩火成物源区。剖面样品微量元素PAAS（后太古庙澳大利亚页岩）标准化蛛网图、稀土元素球粒陨石标准化配分模式图、K₂O/Na₂O-SiO₂、Zr/Th、TiO₂-（TFe₂O₃+MgO）图解、物源构造背景判别函数（F₁-F₂）''的分析判别和剖面样品与不同构造环境砂岩地球化学参数对比结果表明,陶海营子剖面林西组物源构造背景具有被动大陆边缘、活动大陆边缘和大陆岛弧特征,构造背景较复杂。综合分析可知,陶海营子剖面林西组物源主要为被动大陆边缘背景下的火成岩、石英质沉积岩,以及活动大陆边缘和大陆岛弧背景下的火成岩。结合前人相关研究成果,推测兴蒙造山带内伸展作用背景下的晚石炭世-二叠纪岩浆型被动陆缘沉积建造和与俯冲背景有关的古生代弧岩浆岩应该是陶海营子剖面林西组的主要物源。     

http://www.sciencedirect.com/science/article/pii/S1674987113001114

The Deccan Syneclise is considered to have significant hydrocarbon potential.However,significant hydrocarbon discoveries,particularly for Mesozoic sequences,have not been established through conventional exploration due to the thick basalt cover over Mesozoic sedimentary rocks.In this study,near-surface geochemical data are used to understand the petroleum system and also investigate type of source for hydrocarbons generation of the study area.Soil samples were collected from favorable areas identified by integrated geophysical studies.The compositional and isotopic signatures of adsorbed gaseous hydrocarbons(methane through butane) were used as surface indicators of petroleum micro-seepages.An analysis of 75 near-surface soil-gas samples was carried out for light hydrocarbons(C1-C4) and their carbon isotopes from the western part of Tapti graben,Deccan Syneclise,India.The geochemical results reveal sites or clusters of sites containing anomalously high concentrations of light hydrocarbon gases.High concentrations of adsorbed thermogenic methane(C_1 = 518 ppb) and ethane plus higher hydrocarbons(ΣC_(2+) = 977 ppb) were observed.Statistical analysis shows that samples from 13% of the samples contain anomalously high concentrations of light hydrocarbons in the soil-gas constituents.This seepage suggests largest magnitude of soil gas anomalies might be generated/source from Mesozoic sedimentary rocks,beneath Deccan Traps.The carbon isotopic composition of methane,ethane and propane ranges are from-22.5‰ to-30.2‰ PDB,-18.0‰to 27.1‰ PDB and 16.9‰-32.1‰ PDB respectively,which are in thermogenic source.Surface soil sample represents the intersection of a migration conduit from the deep subsurface to the surface connected to sub-trappean Mesozoic sedimentary rocks.Prominent hydrocarbon concentrations were associated with dykes,lineaments and presented on thinner basaltic cover in the study area,which probably acts as channel for the micro-seepage of hydrocarbons.     

安徽巢湖地区石炭纪-早二叠世碳酸盐岩微相与沉积环境

安徽巢湖凤凰山剖面出露石炭系金陵组(C1j)、和州组(C1h)、黄龙组(C2h)、船山组(C2c)以及下二叠统栖霞组下部臭灰岩段(P1q)等5个含碳酸盐岩地层。本文在野外剖面实测的基础上,对这些碳酸盐岩进行了岩相与微相分析。通过对碳酸盐岩显微薄片的观察分析,共识别出14个微相,分属于干旱近海岸蒸发台地(潮坪)、局限海台地、开阔台地内部浅滩、开阔海台地等4种沉积环境。本文研究结果表明,石炭纪-早二叠世期间,巢湖地区主要为浅海碳酸盐台地环境,期间发生了3次比较大规模的海侵-海退旋回,并间隔有若干次小规模的海平面升降。     

Origin and age of ultramafic rocks and gabbros in the southern Puna of Argentina: an alleged Ordovician suture revisited

Ultramafic rocks and gabbros are exposed in the southern Puna (NW Argentina) in tectonic association with continental arc-related Ordovician (volcano) sedimentary successions and granitoids. The origin of this mafic rock suite has been debated for three decades as either representing an Ordovician terrane suture, primitive Ordovician arc-related rocks or relics of the pre-Ordovician basement in tectonic contact with the Ordovician retro-arc basin successions. We present the first U–Pb ages of primary and inherited zircon from gabbros of this mafic–ultramafic assemblage. LA-ICP-MS analyses on cores and rims of these zircon grains yielded a concordia age of 543.4 ± 7.2 Ma for the gabbroic rocks. Other analysed zircons have Mesoproterozoic, and Early Ediacaran core and rim ages indicating that the magmas also assimilated Meso- and Neoproterozoic crustal material prior to final crystallization. The mafic rocks witnessed higher metamorphic grade than associated Ordovician rocks, which are unmetamorphosed or only affected by anchimetamorphism. The gabbros are mostly tholeiitic and enriched in Zr, Th, as well as other incompatible elements and have εNd _t=540Ma ranging from 1.3 to 7.4 with most of the values between 5 and 7. ¹⁴⁷Sm/¹⁴⁴Nd ratios show evidence of weak crustal contamination. The mafic rocks do not reveal any affinity to mid-ocean ridge basalts in their geochemistry but point instead to an emplacement in an active plate margin arc environment. Chromites from ultramafic rocks show typical Ti, Al, Cr#, Fe³⁺ abundances found in magmatic arc rocks. The formation of the gabbros and the associated ultramafic rocks in the southern Argentine Puna is related to the evolution of the margin of the Pampia terrane, including the Puncoviscana basin, during the Late Neoproterozoic and earliest Cambrian. In contrast to previous interpretations, the rocks predate the Ordovician evolution of the Central proto-Andean active margin. Consequently, interpretations assuming these rocks to represent an oceanic terrane suture of Ordovician age have to be dismissed as much as all palaeotectonic models that define Ordovician terranes in the Central Andes based on assumption that the ultramafic rocks and gabbros exposed in the southern Puna mark plate boundaries.     

从西南天山超高压变质带多期构造变形看天山古生代构造演化

通过对西南天山阿克雅孜和木扎尔特地区高压-超高压变质带构造几何学和岩石变形相关运动学的详细剖析,厘定出高压-超高压变质岩石及其相关围岩的构造单元。详细研究表明,研究区可划分为三个构造单元:北部单元、中部单元和南部单元。确定了每个构造单元的构造几何学特征及各个构造单元之间的相互关系。通过分析岩石变形特征和叠加关系,确定了岩石所记录多期变形的运动学特征。根据研究区的多期构造变形特点,建立了阿克雅孜和木扎尔特河地区构造演化序列。共划分出四期构造可识别的事件(E1-E4),分别代表了E1:高压-超高压岩石折返过程;E2:高压-超高压岩石造山带的早期改造过程;E3:北部构造事件对高压-超高压造山带影响;E4:走滑构造对高压-超高压造山带的叠加。沿造山带系列构造分析表明,西南天山高压-超高压带中发育的四期构造事件沿中天山北缘具有很好的一致性,各期构造事件也有一定的横向可对比性。在此基础上通过对多期变形事件的构造背景的探讨,建立了整个天山在古生代的构造拼合过程,揭示我国西部洋壳相关的深俯冲造山带形成过程和参与深俯冲作用(超)高压变质岩的变形变质历史。