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1.
Three Far East diatomites, Puzanov (Kunashir Island, Kuril Islands), Sergeevskii, and Terekhovka (Southern Primorye) are characterized in detail. Rock-forming taxa are identified (Puzanov—Aulacoseira subarctica (O. Mull.) Hawort and Stephanodiscus niagarae var. pusanovae Genkal et Cherepanova; Sergeevskii—Staurosira construens var. venter (Ehr.) Gran., Aulacoseira italica (Ehr.) Sim., and representatives of the genus Cymbella with large valves (up to 175 Jim in length); Terekhovka—Aulacoseira praegranulata var. praeislandica f. praeislandica (Sim.) Moiss.). Morphometric analysis of valves of the dominant taxa showed a low variability of valve parameters for the Puzanov diatomite and a high one for the Sergeevskii and Terekhovka diatomites. The lake environments of the diatoms forming the diatomites were reconstructed based on the elemental composition of diatom valves and the ecological structure of diatom paleocommunities. The high oxygen and low silicon concentrations in valves of the Puzanov diatomite, most likely, indicate that the diatoms were part of plankton communities formed in a large deep freshwater lake. Significant concentrations of silicon in valves of the Terekhovka diatomite, in contrast, suggest that the diatoms occurred in benthic ecotopes in a shallow lake. The age of the deposits was refined by detailed studies of Aulacoseira valves, using light and scanning electron microscopes. For example, the presence of Aulacoseira taxa of the “prae” group in the Terekhovka diatomite confirms its Pliocene age, and the presence of valves of the present species A. italica in the Sergeevskii diatomite points to its Late Pliocene age. The identified features of the diatomites permit their use in practice.  相似文献   

2.
This paper reports the first fossil (Tertiary) occurrence of freshwater sponges of the genus Ephydatia in the southern hemisphere. The sponges appear in diatomite lacustrine sediments of Late Miocene Quillagua Formation (Chile, Atacama region). The investigated specimens represent a new species, Ephydatia chileana sp. nov., which is close to the Recent cosmopolitan E. fluviatilis. On the basis of sedimentological and diatom assemblage data, sponge-bearing diatomites have been interpreted as deposited in open offshore shallow lacustrine conditions with slightly alkaline waters. The sponges show malformations, similar to some diatoms and probably caused by high heavy metal concentrations in a lake water. These concentrations are related to hydrothermal activity, which favored the leaching of volcanic rocks that outcrop extensively in the catchment.  相似文献   

3.
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.  相似文献   

4.
We report results of an integrated study of volcanosedimentary rocks of the Middle-Late Miocene Dzhilinda Formation consisting of the prevolcanic sedimentary Lower Dzhilinda and volcanosedimentary Upper Dzhilinda subformations. The section was studied in three wells drilled near Lake Mukhal (Khoigot paleovalley, Vitim upland). The Dzhilinda sediments contain ubiquitous Alveolophora jouseana (Moiss.) Moiss. typical of Miocene environments. Deposition occurred in two main stages represented by different water (diatoms) and land (spores and pollen) plant communities. The upsection successive changes in diatom species are attendant with changes in pollen and spore assemblages. The K-Ar ages of lavas indicate that the mostly sedimentary lower section of the Dzhilinda Formation deposited between 12 and 14 Ma and the more volcanic upper section, with a lacustrine lens at base, formed at about 10.8–9.5 Ma. The isotope dating of volcanic rocks agrees with the ages inferred from the diatom and spore-pollen analyses. The prevolcanic Lower Dzhilinda subformation deposited during the Middle Miocene climate optimum. The stratigraphy, lithology and facies of sediments suggest that the Dzhilinda deposition was associated with the development of a deep freshwater lake in the conditions of active tectonism and volcanism.  相似文献   

5.
The freshwater diatom flora from the tuffogenous sedimentary rocks of the submarine Ulleung Plateau of the southern Sea of Japan is studied and its complete taxonomic composition (96 species and varieties belonging to 33 genera) is revealed. It is characterized by species diversity and abundance of representatives of the freshwater genus Aulacoseira. The predominance of Aulacoseira is characteristic of the Miocene lacustrine deposits of the Sea of Japan and other regions, but the presence of ellipsoid species of narrow stratigraphic distribution allows the age of the recognized diatom flora to be limited to the Early Miocene. The species diversity and abundance of diatoms in the deposits of the Ulleung Plateau indicate the presence in the Early Miocene of a large freshwater lake in this region. Such lakes also existed on other large submarine rises of the Sea of Japan and its periphery before the onset of the tectonic activation which led to the descent of vast fragments of the continent and expansion of the Sea of Japan.  相似文献   

6.
A diatomite, about 60 m thick, of late Palaeocene-early Eocene age crops out in northern Jutland, Denmark. The diatomite is locally termed ‘Moler’. Frustules of marine diatoms constitute c. 65% (by weight) of the diatomite, and clay minerals, chiefly montmorillonite, make up the remainder. Slight variations in the relative supply of diatom frustules and clay minerals are preserved undisturbed in laminated diatomite, while lamination is partly destroyed by burrowing organisms in weakly laminated diatomite and obliterated by total bioturbation in structureless diatomite; these three facies alternate throughout the sequence. The presence or absence of infaunal burrowing organisms is interpreted as a record of the content of dissolved oxygen in the water above the sediment-water interface and hence of the position of the redox potential discontinuity. Interspaced in the diatomite are 179 identifiable layers of volcanic ash. These ash layers provide a means of precise lateral correlation. They show that levels of laminated diatomite may be followed throughout the basin and therefore that changes between anoxic and oxic conditions occurred simultaneously across the area. The laminated diatomites may consequently be interpreted as representing short-term anoxic events, of which twelve have been recognized.  相似文献   

7.
沱沱河盆地是冻土天然气水合物潜在分布区之一,其内发育下—中二叠统开心岭群九十道班组、上二叠统乌丽群那益雄组、上三叠统结扎群巴贡组、中—渐新统雅西措组4套烃源岩以及不同类型的火山岩。研究表明,火山岩岩石类型主要为玄武岩、玄武安山岩、安山岩、玄武质粗面安山岩和粗面安山岩。火山岩主量元素低TiO_2,Al_2O_3含量较高,K_2O含量较低,K_2ONa_2O;火山岩的稀土元素配分模式为轻稀土富集型;微量元素配分模式呈锯齿状,Ta、Nb、P_2O_5、TiO_2、Y、Yb以及铁族元素Sc、Cr、Ni亏损,综合判断青海南部沱沱河地区火山岩形成于岛弧环境。结合火山岩的地球化学特征,推断火山活动可以加快烃源岩的热成熟,进而促使有机质裂解,产生热解气,为水合物的生成提供气源条件。同时,火山岩的气孔构造发育,连通性较好,有可能成为水合物的储层。火山热液具有的高压使周围的烃源岩产生裂隙,这些裂隙不仅是水合物气源的运移通道,还可以为水合物提供形成场所和储存空间。  相似文献   

8.
This study is the first detailed account of freshwater to restricted marine molluscs from the Upper Cretaceous sedimentary rocks in the northern sector of the Neuquén Basin. The fossils are from the Campanian–Maastrichtian Loncoche Formation in southern Mendoza, west-central Argentina, which records the initial connection of the Neuquén Basin to the Atlantic Ocean. Six species of bivalves (Diplodon bodenbenderi, Pleiodon? sp., Isognomon? mechanquilensis, Mactridae? indet., Panopea? sp., and Laternula sp.) and three of gastropods (Paleoanculosa macrochilinoides, Paleoanculosa ameghiniana, and a possible cerithioidean) are described. Specimens were collected from fine to coarse sandstones, which may be massive or with planar stratification, planar-cross stratification or trough-cross stratification, and a few from bioclastic limestones and mudstones. Although the sections are from 50 to 300 m thick, the specimens are found only in the lower 120 m. Molluscs represent autochthonous/parautochthonous assemblages composed of mostly non-broken gastropods and articulated bivalve specimens, some of which show signs of postmortem transport; however, they were not removed far from their original habitat. Review of the habitats of living genera supports the inference of dominantly freshwater palaeoenvironments in the lower and middle part of the Loncoche Formation, with restricted marine influence in the southernmost localities studied where there are a few samples that contain specimens belonging to predominantly marine groups (e.g., Laternula, Panopea).  相似文献   

9.
The highly deformed c. 3800 Ma Isua supracrustal belt is a fragment of a more extensive Early Archaean sedimentary and volcanic succession intruded by and tectonically intercalated with tonalitic and granitic Amftsoq gneisses in the period 3800-3600 Ma. The supracrustal rocks recrystallised under amphibolite facies conditions between 3800 and 3600 Ma, in the Late Archaean and locally at c. 1800 Ma. Layered sequences of rock of sedimentary and probable volcanic origin form over 50% of the belt. Bodies of high MgAl basic rocks and ultramafic rocks were intruded into the layered sequences prior to isoclinal folding and intrusion of Amitsoq gneisses. The layered rocks which are < 1 km thick are divided into two sequences, that are in faulted contact with each other. The way-up of these sequences has been determined from facing-directions of locally-preserved graded layering in felsic metasediments at several localities. The overall upwards change in sedimentary succession is interpreted as showing change from dominantly basic to dominantly felsic volcanism which provided the major clastic component of the sediments. Clastic sedimentation took place against a background of chemical sedimentation, shown by interlayers of banded iron formation, metachert and calc-silicate rocks throughout the sequences. The felsic rocks locally preserve graded bedding and possible conglomerate structures, indicating deposition from turbidite flows and possibly as debris flows. Nodules in the felsic rocks contain structures interpreted as fiammé. There is an irregular enrichment in K2O/Na2O in many of the felsic rocks at constant SiO2 and Al2O3 content, interpreted as owing to alteration of original andesitic to dacitic volcanic rocks. Banded iron formations locally contain conglomeratic structures suggesting sedimentary reworking, possibly under shallow water conditions. Lithological and geochemical characters of the clastic components of the supracrustal sequences are consistent with derivation from felsic and basic volcanic rocks and do not require a continental source.  相似文献   

10.
松辽盆地东缘六台地区营城组露头地质调查和实验室分析显示,貌似正常碎屑岩的营二段是一套复杂的火山熔岩和沉积岩之间的过渡岩性。这套岩性包括了碎屑熔岩、凝灰岩、沉凝灰岩、凝灰质碎屑岩和正常碎屑岩,显示了它们形成于火山—沉积盆地的过渡环境,即火山斜坡过渡到冲积平原和湖泊环境。与火山作用有关岩相有4种端元的火山碎屑岩相:弹射坠落爆发空落相、喷射降落爆发空落相、热碎屑流相和热基浪相。火山物质堆积在火山斜坡以及前方的冲积平原上,往往以火山碎屑岩相或者冲积扇相、辫状河相保存;它们堆积在湖泊中则为三角洲相、扇三角洲相、较深水相和沼泽相。这些相的空间组合反映了火山喷发—碎屑堆积的过程。根据旋回发育营二段可以区分出3种类型的充填序列:沉积—火山充填序列、沉积充填序列和火山充填序列。较为完整的序列为粗碎屑岩—火山岩和火山碎屑岩—细碎屑岩,对应着裂谷类盆地断裂作用—火山作用—沉积充填的过程,记录在特定的地层结构中。  相似文献   

11.
The exact biological source of the C20 highly branched isoprenoid (HBI) present in sediments from aquatic systems is unclear. We therefore examined the relationship between the distribution of fossil diatoms and the concentration of the C20 HBI in a Late Glacial sedimentary record from the Hässeldala Port paleolake in southern Sweden. Using Bayesian multiple linear regression analysis, we show that its concentration is linked primarily to the production of the diatom taxon Gomphonema acuminatum, which accounts for the largest proportion of the temporal variability in the biomarker. By analogy with modern observations, we argue that an increasing amount of G. acuminatum biomass in our sedimentary record reflects increasing oligotrophy in the paleolake during the summer growing season, especially at times defined by subdued hydrologic flow. Our conclusions are corroborated by the δ13C composition of the C20 HBI biomarker, which points to a negative photosynthetic fractionation between atmospheric CO2 and the pool of dissolved inorganic carbon during diatom bloom, a distinct phenomenon at times of inhibited hydrological flow. Accordingly, we suggest that the C20 HBI biomarker can be effectively used to reconstruct the trophic state of the paleolake at Hässeldala Port, while its stable isotope composition can provide physicochemical information about the lake conditions during the dry summer season.Moreover, we note that the major hydrological shifts recorded in the G. acuminatum-C20 HBI stratigraphy do not coincide with the pollen zone boundaries. We thus infer that aquatic and terrestrial environmental responses to climate change are substantially decoupled through the hydrological system, which highlights the necessity for multi-proxy investigations to decipher past climate events.  相似文献   

12.
Fossil skeletal remains of the Afro–Asian labeonine genus Garra are described on the basis of more than 300 specimens from the Pliocene diatomites of Jradzor, Central Armenia. Extant species of the genus Garra exhibit a wide distribution ranging from China, South Asia, and Borneo, through Middle East, the Arabian Peninsula, and Africa, where these fishes inhabit a variety of freshwater biotopes. The Pliocene fossils from the Jradzor site provide the first evidence of Garra in the fossil record. The inadequate preservation of the available material did not allow a detailed taxonomic attribution at the species level. Remains of Garra sp. were found associated to a moderately diverse assemblage, which includes also remains of additional freshwater fishes (Capoeta sp., Leuciscus cf. souffia, Leuciscus sp.), amphibians (Pelophylax cf. ridibundus), and large mammals (Rhinocerotidae indet.), as well as of monocotyledon plants. According to the diatom flora, the diatomites of Jradzor can be assigned to the Pliocene and were deposited in a freshwater lacustrine setting that was characterized by high productivity, eutrophic conditions, and standing macrophyte vegetation along the littoral zone. The presence of abundant resorptive pharyngeal teeth suggests that the Pliocene palaeolake of Jradzor was characterized by resident populations of Garra. Extant species of this genus are currently absent in the freshwaters of Armenia. The record of the genus Garra from Jradzor suggests that the Araks–Kura River drainage was connected, at least in part, with the Euphrates and Tigris River drainage during the Pliocene. The extinction of Garra (and other thermophilous taxa) from Araks and Kura River drainage was probably due to Plio-Pleistocene tectonic uplift of the Armenian Highland and consequent progressive climate cooling.  相似文献   

13.
Despite abundant data on volcaniclastic sand(stone), the compositional, spatial and temporal distribution of volcanic detritus within the sedimentary record is poorly documented. One of the most intricate tasks in optical analysis of sand(stone) containing volcanic particles is to distinguish grains derived by erosion of ancient volcanic rocks (i.e. palaeovolcanic, noncoeval grains) from grains generated by active volcanism (subaqueous and/or subaerial) during sedimentation (neovolcanic, coeval grains). Deep-marine volcaniclastic sandstones of the Middle Topanga Group of southern California are interstratified with 3000-m-thick volcanic deposits (both subaqueous and subaerial lava and pyroclastic rocks, ranging from basalt, andesite to dacite). These rocks overlie quartzofeldspathic sandstones (petrofacies 1) of the Lower Topanga Group, derived from deep erosion of a Mesozoic magmatic arc. Changes in sandstone composition in the Middle Topanga Group provide an example of the influence of coeval volcanism on deep-marine sedimentation. Volcaniclastic strata were deposited in deep-marine portions of a turbidite complex (volcaniclastic apron) built onto a succession of intrabasinal lava flows and on the steep flanks of subaerially emplaced lava flows and pyroclastic rocks. The Middle Topanga Group sandstones are vertically organized into four distinctive petrofacies (2–5). Directly overlying basalt and basaltic-andesite lava flows, petrofacies 2 is a pure volcanolithic sandstone, including vitric, microlitic and lathwork volcanic grains, and neovolcanic crystals (plagioclase, pyroxene and olivine). The abundance of quenched glass (palagonite) fragments suggests a subaqueous neovolcanic provenance, whereas sandstones including andesite and minor basalt grains suggest subaerial neovolcanic provenance. This petrofacies probably was deposited during syneruptive Periods, testifying to provenance from both intrabasinal and extrabasinal volcanic events. Deposited during intereruptive periods, impure volcanolithic petrofacies 3 includes both neovolcanic (85%) and older detritus derived from plutonic, metamorphic and palaeovolcanic rocks. During post-eruptive periods, the overlying quartzofeldspathic petrofacies 4 and 5 testify to progressive decrease of neovolcanic detritus (48–14%) and increase of plutonic-metamorphic and palaeovolcanic detritus. The Upper Topanga Group (Calabasas Formation), conformably overlying the Middle unit, has dominantly plutoniclastic sandstone (petrofacies 6). Neovolcanic detritus is drastically reduced (4%) whereas palaeovolcanic detritus is similar to percentages of the Lower Topanga Group (petrofacies 1). In general, the volcaniclastic contribution represents a well-defined marker in the sedimentary record. Detailed compositional study of volcaniclastic strata and volcanic particles (including both compositional and textural attributes) provides important constraints on deciphering spatial (extrabasinal vs. intrabasinal) and temporal relationships between neovolcanic events (pre-, syn-, inter- and post-eruptive periods) and older detritus.  相似文献   

14.
The long-term histories of the neighboring Nakuru–Elmenteita and Naivasha lake basins in the Central Kenya Rift illustrate the relative importance of tectonic versus climatic effects on rift-lake evolution and the formation of disparate sedimentary environments. Although modern climate conditions in the Central Kenya Rift are very similar for these basins, hydrology and hydrochemistry of present-day lakes Nakuru, Elmenteita and Naivasha contrast dramatically due to tectonically controlled differences in basin geometries, catchment size, and fluvial processes. In this study, we use eighteen 14C and 40Ar/39Ar dated fluvio-lacustrine sedimentary sections to unravel the spatiotemporal evolution of the lake basins in response to tectonic and climatic influences. We reconstruct paleoclimatic and ecological trends recorded in these basins based on fossil diatom assemblages and geologic field mapping. Our study shows a tendency towards increasing alkalinity and shrinkage of water bodies in both lake basins during the last million years. Ongoing volcano-tectonic segmentation of the lake basins, as well as reorganization of upstream drainage networks have led to contrasting hydrologic regimes with adjacent alkaline and freshwater conditions. During extreme wet periods in the past, such as during the early Holocene climate optimum, lake levels were high and all basins evolved toward freshwater systems. During drier periods some of these lakes revert back to alkaline conditions, while others maintain freshwater characteristics. Our results have important implications for the use and interpretation of lake sediment as climate archives in tectonically active regions and emphasize the need to deconvolve lacustrine records with respect to tectonics versus climatic forcing mechanisms.  相似文献   

15.
云南省兰坪县石登—兔峨地区火山岩岩石学特征   总被引：1，自引：0，他引：1       下载免费PDF全文
李玫 《中国地质》2004,31(1):64-69
石登—兔峨地区上三叠统小定西组火山岩属云南西部澜沧江沿岸三叠系火山岩带的一部分，本文从岩相学、岩石化学和地球化学等多方面对该火山岩特征进行了系统、全面分析，并从理论角度对其形成的大地构造环境进行了判别。结果表明：该火山岩以安山岩为主，玄武岩为次，属钙碱性系列，形成于岛弧大地构造环境。  相似文献   

16.
The Cayconi Formation of the Crucero Basin, Puno Department, southeastern Peru, has been described as a 800–1000 m sequence of Oligocene and Miocene fanglomerate and lacustrine sedimentary rocks, interlayered with mafic and silicic volcanic rocks, and unconformably overlying Paleozoic and Cretaceous successions. On the basis of new field and petrological investigations, key aspects of the stratigraphic relationships of the rocks comprising this formation, and hence the viability of this lithostratigraphic name, are questioned. Thus, several sedimentary units previously assigned to the Cayconi Formation are reinterpreted as Cretaceous or older. We further argue that the formational terminology fails to accomodate the great variety of volcanic rocks, which are, moreover, disposed in isolated eruptive fields. We therefore propose establishment of the Crucero Supergroup as a broad, yet flexible framework for lithostratigraphic subdivision of the Tertiary sequences of the Cordillera Oriental of southeastern Peru. This new nomenclature accomodates the voluminous two-mica ash flow tuffs (Macusani Volcanics) and associated rocks of the Quenamari Meseta, a succession excluded from the existing lithostratigraphic classification scheme. It also permits distinction between the petrogenetically-contrasted upper Oligocene — Lower Miocene and Middle Miocene — Upper Miocene volcanic suites, which dominate, respectively, the Picotani and Quenamari Groups comprising the proposed Crucero Supergroup. Finally, the economically important granitic/rhyolitic intrusive centers cogenetic with the volcanic rocks are readily assignable to intrusive lithodemes in each group.  相似文献   

17.
New data on the composition, age, structure, and geodynamic settings of the Upper Silurian–lower Middle Carboniferous rocks in the Baikal–Vitim Fold System (BVFS) are reported. These rocks make up the Early Hercynian structural stage of the BVFS, within which the Uda–Vitim, Vitimkan–Tsipa, and Turka–Kurba lithotectonic zones are recognized. The Early Hercynian stage combines rocks of two stratigraphic levels: Upper Silurian–Upper Devonian (S2–D3) and Upper–Devonian–Middle Carboniferous (D3–C21). On the basis of lithostratigraphic and spatiotemporal relationships between sedimentary and volcanic–sedimentary complexes of the Early Hercynian stage three formations are identified, which characterize the main tectonic regimes of the early half of the Late Paleozoic. It has been established that a common paleobasin that evolved through consecutively changing geodynamic settings at the passive and active continental margins existed in the Silurian (?)–early Middle Carboniferous in the northeastern part (in present-day coordinates) of the western Transbaikal region.  相似文献   

18.
The Colider Group ignimbrites and rhyolites, located between Guarantã do Norte and Serra do Cachimbo, Brazil derives from a felsic magmatism with a wide spatial distribution at the Alta Floresta Gold Province (AFGP). The geological mapping and petrographic studies allow distinguishing three volcanic successions of the Colider Group at the study area, named Lower (LS), Intermediate (IS) and Upper (US). The LS characterizes by its abundance in pyroclastic density current deposits, such as ignimbrites and surge deposits, with subordinated effusive rocks. The US consists of effusive rhyolites and trachytes and, subordinately, rheomorphic ignimbrites. The magmas that generated the volcanic rocks of the LS is of a subsolvus character. The US rocks present, dominantly, a mineralogy indicating a hypersolvus character. This characteristic can be indicating a variation of the fluid pressure conditions within the magma chambers that generated this magmatism. Regarding the chemical composition of the volcanic rocks from the two successions, we can point out that both have a metaluminous to slightly peraluminous character and affinity with A2-type magmas. The zircon U-Pb LA-ICPMS isotopic data of a LS rhyolite indicate a crystallization age of 1810 ± 9 Ma, interpreted as the Lower Succession age. One of the hypotheses that may be suggested to the generation of the volcanic successions is that the petrographic, lithochemical, and geochronological contrasts indicate the occurrence of two distinct magmatic events, one of ∼1.80 Ga and another of ∼1.76 Ga. The second hypothesis is that both successions belong to the same magmatic event, which occurred from 1.81 Ga to 1.76 Ga.  相似文献   

19.
在东准噶尔卡拉麦里地区的五彩湾一带出露一套具磨拉石特征的火山沉积建造, 下部为具磨拉石特征的砾岩、砂岩, 上部为一套中基性的火山熔岩夹中酸性的火山凝灰岩.1:20万卡拉麦里山幅将其归入下石炭统松喀尔苏组.通过1:5万地质调查研究发现, 该火山沉积建造底部以一套粗砾岩高角度不整合于下-中泥盆统卡拉麦里组之上, 含晚泥盆世植物化石Prelepidodendron sp.(先鳞木), 中上部火山岩LA-ICP-MS锆石U-Pb年龄为346.8±3.3 Ma, 且被年龄为341.1±4.0 Ma~340.9±5.1 Ma后碰撞花岗岩侵入, 表明其形成时代为晚泥盆世-早石炭世, 时代上对应于北准噶尔地层分区的上泥盆统克安库都克组.该套地层中上部的火山岩的岩石组合为玄武岩、玄武安山岩、夹少量的流纹质凝灰岩, 岩石化学特征上属钙碱性-高钾钙碱性系列, (La/Yb)N=2.97~6.66, Nb、Ta亏损, 部分样品Zr、Ti弱显亏损, Nb/U、Ce/Pb比值分别为7.43~20.88、3.17~12.45.地球化学特征表明其兼具板内火山岩和弧火山岩的某些特点, 形成于后碰撞伸展环境, 是卡拉麦里洋盆于晚泥盆世之前闭合后后碰撞岩浆活动的产物.这一研究成果对广为关注的卡拉麦里洋盆的闭合时间进行了很好的限定.   相似文献   

20.
扬子西缘广泛出露的新元古代澄江组是一套与晋宁运动密切相关的沉积岩夹火山岩系,其形成时限对华南新元古代区域地层格架、地层划分对比及古大陆再造具有重要意义。出露于滇中易门地区的新元古代澄江组底部发育有厚数米的凝灰岩夹层,本次采集凝灰岩进行LA-ICP-MS锆石U-Pb定年研究,结果表明,锆石206Pb/238U年龄加权平均值为812±5. 5Ma(n=17,MSWD=0.46),可以代表滇中澄江组的底界年龄,从而进一步确定澄江组底界年龄为812Ma左右。此外,凝灰岩中捕获的锆石年龄信息指示扬子西缘可能存在2. 5Ga、1. 8～1. 6Ga、1. 5Ga、1. 3Ga及1.0Ga等数期重要的区域构造-热事件。  相似文献   

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