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1.
The phase relations of primitive magnesian andesites and basaltic andesites from the Mt. Shasta region, N California have been determined over a range of pressure and temperature conditions and H2O contents. The experimental results are used to explore the influence of H2O and pressure on fractional crystallization and mantle melting behavior in subduction zone environments. At 200-MPa H2O-saturated conditions the experimentally determined liquid line of descent reproduces the compositional variation found in the Mt. Shasta region lavas. This calc-alkaline differentiation trend begins at the lowest values of FeO*/MgO and the highest SiO2 contents found in any arc magma system and exhibits only a modest increase in FeO*/MgO with increasing SiO2. We propose a two-stage process for the origin of these lavas. (1) Extensive hydrous mantle melting produces H2O-rich (>4.5--6 wt% H2O) melts that are in equilibrium with a refractory harzburgite (olivine + orthopyroxene) residue. Trace elements and H2O are contributed from a slab-derived fluid and/or melt. (2) This mantle melt ascends into the overlying crust and undergoes fractional crystallization. Crustal-level differentiation occurs under near-H2O saturated conditions producing the distinctive high SiO2 and low FeO*/MgO characteristics of these calc-alkaline andesite and dacite lavas. In a subset of Mt. Shasta region lavas, magnesian pargasitic amphibole provides evidence of high pre-eruptive H2O contents (>10 wt% H2O) and lower crustal crystallization pressures (800 MPa). Igneous rocks that possess major and trace element characteristics similar to those of the Mt. Shasta region lavas are found at Adak, Aleutians, Setouchi Belt, Japan, the Mexican Volcanic Belt, Cook Island, Andes and in Archean trondhjemite--tonalite--granodiorite suites (TTG suites). We propose that these magmas also form by hydrous mantle melting.Editorial responsibility: J. Hoefs  相似文献   

2.
Paleogene sediments in fault-bounded basins on Judge Daly Promontory, northeast Ellesmere Island, Canadian High Arctic, are rich in volcanogenic material. Volcanic pebbles within the Cape Back basin near Nares Strait were studied for their petrography, geochemistry, Sr and Nd isotopes, and geochronology to identify and characterize their parent rock. The pebbles are derived from lava flows and ignimbrites of a continental rift-related, strongly differentiated, highly incompatible element enriched, alkaline volcanic suite, the proposed Nares Strait volcanic suite, which is distinct from other alkaline volcanic suites on the northern coasts of Ellesmere Island and Greenland. 40Ar/39Ar amphibole and alkali feldspar ages indicate that volcanism was active around 61–58 Ma and was probably contemporaneous with sedimentation resulting in Middle to Late Paleocene age for deposition within the Cape Back basin and the other Paleogene basins on Judge Daly Promontory.  相似文献   

3.
Major‐ and trace‐element chemistry (including rare‐earth elements), total‐rock Rb‐Sr and U‐Pb and zircon U‐Pb data are used in an attempt to distinguish between two essentially coeval, felsic volcanic suites: the predominantly submarine Woodlawn suite which is associated with massive Cu‐Pb‐Zn sulphide mineralization and the terrestrial Mt Painter suite, with minor vein‐type mineralization. The Woodlawn samples are the unmineralized equivalents of the volcanics in the immediate ore environment.

Alteration perturbs some of the major‐ and trace‐element chemistry, particularly Ca and alkalis, thereby precluding their usefulness. REE patterns exhibit a significant light to heavy rare‐earth enrichment with an average La/Yb of 12 in the Mt Painter volcanics compared with 5.6 in the Woodlawn volcanics. Both suites have a marked negative Eu anomaly, with that of the Woodlawn samples more pronounced (‐45.5) than in the Mt Painter volcanics (‐29.2). A hydrothermally‐altered sample from Woodlawn has apparently lost about 50% of its light rare‐earth elements.

Initial 87Sr/86Sr ratios at about 0.711 are the same for rocks from both suites and differences in initial lead‐isotopic ratios appear negligible.

Zircons from both suites are a mixture of clear euhedral crystals and rounded discrete crystals or rounded cores overgrown by clear zircon. The U‐Pb data substantiate the morphological features in that the zircon suites both contain older inherited Pb but the Mt Painter zircons contain a greater proportion.

Cs concentrations and Cs/Rb and Ti/Zr ratios can be used to distinguish between the Woodlawn suite and the Mt Painter suite.  相似文献   

4.
ABSTRACT

This paper addresses the Jurassic–Cretaceous stratigraphic evolution of fore-arc deposits exposed along the west coast of the northern Antarctic Peninsula. In the South Shetland Islands, Upper Jurassic deep-marine sediments are uncomformably overlain by a Lower Cretaceous volcaniclastic sequence that crops out on Livingston, Snow and Low islands. U-Pb zircon ages are presented for the upper Anchorage Formation (153.1 ± 1.7 Ma) and the Cape Wallace granodiorite of Low Island (137.1 ± 1.7 Ma) as well as 40Ar/39Ar ages of 136–139 Ma for Low Island andesites. Data are also presented for a U-Pb age of 109.0 ± 1.4 Ma for the upper volcanic succession of Snow Island. In combination with published stratigraphy, these data provide a refined chrono- and litho-stratigraphic framework for the deposits herein referred to as the Byers Basin. Tentative correlation is explored with previously described deposits on Adelaide and Alexander islands, which could suggest further continuation of the Byers Basin towards the south. We also discuss possible correlation of the Byers Basin with the Larsen Basin, a sequence that shows the evolution of foreland to back-arc deposits more or less contemporaneously with the fore-arc to intra-arc evolution of the Byers Basin.  相似文献   

5.
Recent volcanic eruptions at Mt. Unzen (Japan) in 1990 and Mt. Pinatubo (Philippines) in 1991 produced voluminous amounts of ash and sediments which inundated widespread areas. In later rehabilitation and reconstruction, it is practical and economical to use these freshly deposited sediments as materials for foundations and embankments. However, the geotechnical properties of young volcanic products have not been fully investigated. Accordingly, we investigated the geotechnical characteristics of volcanic soils associated with three recent eruptions, namely, the Unzen and Izu-Oshima eruptions of 1990 and 1986, respectively, in Japan and the Pinatubo eruption of 1991 in the Philippines. We specifically investigated index properties, permeability and compaction characteristics, and strength and deformation behavior in drained conditions. Additionally, we examined the dynamic properties and liquefaction characteristics of samples taken from Mt. Pinatubo. The results showed that the geotechnical characteristics of the deposits generally varied with the sampling sites. Depending on the location, either upstream or downstream from a volcano, the preferential sizing due to alluvial deposition affects engineering properties of the deposits. For example, volcanic sediments upstream from Mt. Pinatubo have high compressibility and low cyclic strength, whereas those taken downstream show dilative tendencies and high liquefaction strength.  相似文献   

6.
In the Central Atlantic archipelagos – the Canaries, Cape Verde, Madeira and the Azores – tsunami hazard is often regarded as low, when compared with other extreme wave events such as hurricanes and storms. The geological record of many of these islands, however, suggests that tsunami hazard may be underestimated, notwithstanding being lower than in areas adjacent to subduction zones, such as the margins of the Pacific and Indian oceans. Moreover, tsunamis in oceanic islands are generally triggered by local large-scale volcanic flank collapses, for which little is known about their frequency, making it difficult to estimate the probability of a new occurrence. Part of the problem lies in the fact that tsunami deposits are usually difficult to date, and few islands in the world exhibit evidence for repeated tsunami inundation on a protracted timescale. This study reports on the presence of abundant tsunami deposits (conglomerates and sandstones) on Maio Island (Cape Verde) and discusses their stratigraphy, sedimentological characteristics, probable age and tsunamigenic source. Observations indicate that four distinct inundation events of variable magnitude took place during the Pleistocene. One of the tsunami deposits yielded a high-confidence U/Th age of 78·8 ± 0·9 ka, which overlaps within error with the 73 ± 7 ka age proposed for Fogo volcano's flank collapse, an event known to have had a significant tsunami impact on nearby Santiago Island. This shows that the Fogo tsunami also impacted Maio, resulting in runups in excess of 60 m above coeval sea-level at ca 120 km from the source. Two older deposits, possibly linked to recurrent flank collapses of the Tope de Coroa volcano in Santo Antão Island, yielded lower-confidence ages of 479 to 390 ka and 360 to 304 ka. A younger deposit (<78 ka) remains undated. In summary, the geological record of Maio exhibits well-preserved evidence of repeated tsunami inundation, reinforcing the notion that tsunami hazard is not so low at volcanic archipelagos featuring prominent and highly-active volcanoes such as in Cape Verde.  相似文献   

7.
The section of Mesozoic terrigenous deposits over 1200 m thick has been described, and a new geological map of the Stolbovoi Island southern half has been compiled. The inference has been made that the sedimentary sequence represents a single uniform turbidite complex, which is indivisible into lithological units and which does not show transition to shallow-marine facies either in the visible foot section or visible roof section. The complex accumulated in the foreland basin emerged on the margin of the Novosibirsk-Chukchi continental block during the Anyuian orogeny. The presence of upper Volgian deposits (Upper Jurassic) with a visible thickness of 640 m, Berriasian (Ryazanian) deposits about 100-m thick, and lower Valanginian deposits with a visible thickness of ～200 m has been established on the basis of identification of fossils represented by remains of bivalve mollusks (the genus Buchia). Finds of upper Volgian buchias in the southeastern part of the island are inconsistent with field geological observations of the sequence of deposits. Paleontological data acquired for the island southern part suggest the presence of a transverse thrust fault, along which upper Volgian rocks were thrust over lower Neocomian rocks. The possibility of another interpretation of the data has also been considered. The correlation of the Buchia Beds of the Stolbovoi Island, Nordvik Peninsula, the basin of the Anyui River, and northern California has been carried out. The inference about a close relationship between the biota of the Stolbovoi Island and the North Pacific paleobiogeographic realm has been drawn.  相似文献   

8.
Five cores from the southern Tyrrhenian and Ionian seas were studied for their tephra and cryptotephra content in the 4.4–2.0 ka time interval. The chronological framework for each core was obtained by accelerator mass spectrometry 14C dating, the occurrence of distinct marker tephra and stratigraphic correlation with adjacent records. Tephrochronology allowed us to correlate the analyzed deposits with tephra markers associated with Somma-Vesuvius (79 ad ), Ischia Island (Cretaio), Mt Etna (FG, FL and FS) and Campi Flegrei (Astroni-Agnano Monte Spina) events. For the first time in the marine setting, a large single glass data set is provided for the Late Holocene Etnean marker beds including the FS tephra (ca. 4.3 ka). Moreover, unknown deposits from Lipari (ca. 2.2–2.0 ka) and Vulcano (3.6–3.3 ka) are also recognized at more distal sites than previously reported. These results contribute to improve the high-resolution tephrostratigraphic framework of the central Mediterranean Sea. They also provide new insights into the chemical composition and dispersal pattern of tephras that can be used as inter-archive tools for regional and ‘local’ stratigraphic correlations and for addressing paleoclimate research.  相似文献   

9.
Mount Erebus is an active volcano in Antarctica located on Ross Island. A convecting lava lake occupies the summit crater of Mt. Erebus. Since December 1980 the seismic activity of Mt. Erebus has been continuously monitored using a radio-telemetered network of six seismic stations. The seismic activity observed by the Ross Island network during the 1982–1983 field season shows that: (1)Strombolian eruptions occur frequently at the Erebus summit lava lake at rates of 2–5 per day; (2)centrally located earthquakes map out a nearly vertical, narrow conduit system beneath the lava lake; (3)there are other source regions of seismicity on Ross Island, well removed from Mt. Erebus proper. An intense earthquake swarm recorded in October 1982 near Abbott Peak, 10 km northwest of the summit of Mt. Erebus, and volcanic tremor accompanying the swarm, may have been associated with new dike emplacement at depth.  相似文献   

10.
《Chemical Geology》2002,182(2-4):663-695
The record of mafic magmatism from the Proterozoic to the Holocene in southern Australia reflects episodic incompatible element enrichment of the sub-continental lithospheric mantle (SCLM) recording periodic interaction of asthenosphere and lithosphere. The composition of Jurassic and Cainozoic mantle derived magmas is strongly influenced by the geochemical impact on the SCLM of events which took place during the Neoproterozoic and Cambrian. These events include rifting, passive margin development and orogenesis.Neoproterozoic to Cambrian basalts are widespread in western New South Wales, South Australia and Tasmania and reflect mantle decompression during extension and rifting of the Australian–East Antarctic Craton during the development of the proto-Pacific passive margin. These basalts fall into two regionally extensive and very different suites: (i) a voluminous suite of tholeiites and (ii) a highly undersaturated alkaline (nephelinite–basanite) series.Both Jurassic kimberlite magmas from the Adelaide Fold Belt and highly undersaturated Quaternary analcimites and basanites from the Mt. Gambier district of S.E. South Australia, have geochemical characteristics like those of the Precambrian–Cambrian alkaline suites. They have high concentrations of large ion lithophile (LIL), rare earth (RE) and high field strength (HFS) elements, and high HFSE/LILE and LREE/HREE ratios with TDMNd values of 0.5–0.8 Ga. The Jurassic kimberlites appear to sample lithospheric mantle enrichment zones of Late Neoproterozoic to Early Cambrian age. The Quaternary suites result from mixing of contemporary mantle plume components with this old lithospheric enrichment, which is also identified with the occurrence of metasomatic phlogopite, amphibole and apatite in lherzolite mantle xenoliths from a number of Cainozoic volcanoes in Western Victoria.A very different type of lithospheric mantle enrichment took place during the late stages of the Ross–Delamerian Orogeny. This yielded a crustally contaminated mantle zone that mirrors the Cambro-Ordovician position of that orogen. This zone of contaminated lithospheric mantle interacted with a large plume in the Jurassic to yield the highly anomalous Ferrar–Tasmanian–Kangaroo Island basalts and dolerites.  相似文献   

11.
巴里坤盆地夹持于麦钦乌拉岛弧和哈尔里克岛弧之间,其地层展布、断裂构造、油气成藏、盆山关系均不明确.使用大地电磁测深、航磁、地震等方法对其开展综合研究.研究表明:新生界沉积较薄,中生界及上二叠统地层从盆地中心向四周逐渐减薄,下二叠统和石炭系地层山前沉积略厚、向南逐渐变薄;盆地内部存在12条断裂,大多数为近东西走向的逆冲断裂,这些断裂构成研究区的基本构造格架;盆地可划分为北部斜坡、哈尔里克逆冲带、巴里坤坳陷三个二级构造单元;巴里坤坳陷大河构造为油气成藏的有利区域;南边哈尔里克山呈北西向向盆地内逆冲推覆,与盆地以哈尔里克断裂为界,盆地与北边麦钦乌拉山与以麦钦乌拉断裂为界,与西边博格达山以冬屋断裂为界.   相似文献   

12.
侯增谦 《地球化学》1996,25(3):228-241
选择西南太平洋冲绳海槽现代海底烟囱硫化物矿床、日本小坂矿山上向黑矿（第三纪）和中国西南呷村黑矿型矿床（三叠纪）进行了矿石地球化学比较研究。黑矿型矿床矿石吨位－品位模式与其他火山成因块状硫化物（ＶＭＳ）矿床类似，矿田（２０－５０ｋｍ＾２）矿石吨位与单个喷气－沉积型（Ｓｅｄｅｘ）矿床相当，金属总量４－６Ｍｔ，为矿田范围内热液流体搬运的最大金属量。与洋脊环境ＶＭＳ矿床相比，岛弧裂谷环境产出的黑矿型矿床相  相似文献   

13.
In this work, the Upper Eopleistocene coarse deposits of the Fore-Caucasus (containing 30–50% or more of gravel-pebble-boulder material), are discussed. These compose the Belorechenskaya and Novokubansk suites, Neberdzhai and Andree-Dmitrievskaya formations in the western Fore-Caucasus; the Lysogorskaya, Elkhotovskayaovskaya, Rukhs-Dzuar suites, the Kendzhinskaya and Seivenduk formations and their analogs in the eastern areas of the central Fore-Caucasus. The genesis of continental coarse deposits is regarded as mainly alluvial. These deposits are part of the Fore-Caucasus piedmont molasses, formed during two stages. At the first stage (late Miocene) coarse deposits of the Lysogorskaya suite formed in the eastern areas of the central Fore-Caucasus (modern Kabardin, Ossetian and Chechen depressions). At the second stage (late Pliocene-Eopleistocene) these formations deposited throughout the territory of the Fore-Caucasus, but the intensity of processes varied considerably in different areas. In the central Fore-Caucasus, compared with the western part, coeval deposits are coarser in texture; their thickness is several times greater. Study of the Late Miocene coarse molasses of late Eopleistocene of the Fore-Caucasus allows us to clarify the history of the formation of the Great Caucasus in the most intense periods of its development.  相似文献   

14.
http://www.sciencedirect.com/science/article/pii/S1674987114000711   总被引：1，自引：0，他引：1  
The West Junggar region,located in the loci of the Central Asian Orogenic Belt,is a highly endowed metallogenic province with >100 tonnes Au,>0.7 Mt Cu,>0.3 Mt Mo,and >2.3 Mt chromite as well as significant amounts of Be and U.The West Junggar region has three metallogenic belts distributed systematically from north to south:(1) late Paleozoic Saur Au-Cu belt;(2) early Paleozoic XiemisitaiSharburt Be-U-Cu-Zn belt;(3) late Paleozoic Barluk-Kelamay Au-Cu-Mo-Cr belt.These belts host a number of deposits belonging to at least eight economically important styles,including epithermal Au,granite-related Be-U,volcanogenic massive sulfide(VMS) Cu-Zn,podiform chromite,porphyry Cu,hydrothermal quartz vein Au,porphyry-greisen Mo(-W),and orogenic Au.These deposit styles are associated with the tectonics prevalent during their formation.Five tectonic-mineralized epochs can be recognized:(1) Ordovician subduction-related VMS Cu-Zn deposit;(2) Devonian ophiolite-related podiform chromite deposit;(3) early Carboniferous subduction-related epithermal Au and porphyry Cu deposits;(4) late Carboniferous subduction-related granite-related Be-U,porphyry Cu,and hydrothermal quartz vein Au deposits;and(5) late Carboniferous to early Permian subduction-related porphyry-greisen Mo(-W) and orogenic Au deposits.  相似文献   

15.
A Middle Pleistocene widespread tephra, defined here as Hegawa-Kasamori 5 tephra (Hgw-Ks5), has been newly recognized over a broad area of Japan. Large-scale pyroclastic flow deposits associated with co-ignimbrite ash fall deposits (CAFDs) of Hgw-Ks5 have been identified in the proximal southern Kyushu area, south-west Japan. Hgw-Ks5 possibly originated from the Aira caldera in southern Kyushu, and it is widely spread and intercalated with deposits of the Kasamori Formation, Honshu Island, more than 1000 km away from the source. In the north-west area of the Aira Caldera, the tephra is sparsely distributed in the form of non-welded ignimbrites, and is exposed stratigraphically above the well-known Kobayashi-Kasamori tephra. Hgw-Ks5 is characterized through petrographic features, major element geochemistry of glass shards, and refractive indices of orthopyroxene. The results of previous stratigraphic isotope studies indicate that the eruptive age of Hgw-Ks5 is 434–458 ka (Marine Isotope Stage 12). Assuming that the CAFDs originating from the Aira Caldera are distributed concentrically, the apparent volume of Hgw-Ks5, estimated from the area of distribution and CAFD thickness, is ~100 km3. Therefore, a volcanic explosivity index of 7 is assigned to the Hgw-Ks5 eruption.  相似文献   

16.

Four Middle Devonian (381 Ma) granodiorite samples have been recovered from two dredge sites approximately 65 km east of Green Cape, New South Wales. The granodiorite samples are similar in age and composition to members of the Moruya Suite and probably form an along‐strike extension of that suite. The location of granodiorite on the southeastern margin requires that a piece of continental lithosphere was located to the present east of the study area in the Devonian. This piece of lithosphere may now be located somewhere on the western Lord Howe Rise.

A sample of Early Cretaceous leuco‐quartz monzodiorite was also recovered from a dredge site approximately 45 km north‐northeast of Dalmeny, New South Wales. It represents a body that was intruded at essentially the same time as, and is inferred to be of similar origin to, the syenite rocks of the nearby Mt Dromedary and Montague Island complexes.  相似文献   

17.
The Mt. Edgecumbe Volcanic Field (MEVF), located on Kruzof Island near Sitka Sound in southeast Alaska, experienced a large multiple-stage eruption during the last glacial maximum (LGM)-Holocene transition that generated a regionally extensive series of compositionally similar rhyolite tephra horizons and a single well-dated dacite (MEd) tephra. Marine sediment cores collected from adjacent basins to the MEVF contain both tephra-fall and pyroclastic flow deposits that consist primarily of rhyolitic tephra and a minor dacitic tephra unit. The recovered dacite tephra correlates with the MEd tephra, whereas many of the rhyolitic tephras correlate with published MEVF rhyolites. Correlations were based on age constraints and major oxide compositions of glass shards. In addition to LGM-Holocene macroscopic tephra units, four marine cryptotephras were also identified. Three of these units appear to be derived from mid-Holocene MEVF activity, while the youngest cryptotephra corresponds well with the White River Ash eruption at ∼ 1147 cal yr BP. Furthermore, the sedimentology of the Sitka Sound marine core EW0408-40JC and high-resolution SWATH bathymetry both suggest that extensive pyroclastic flow deposits associated with the activity that generated the MEd tephra underlie Sitka Sound, and that any future MEVF activity may pose significant risk to local population centers.  相似文献   

18.
Previous studies of the availability of lithium for use in batteries to power electric vehicles (EVs) have reached the generally encouraging conclusion that resources are sufficient to meet growing demand for the remainder of the 21st century. However, these surveys have not looked past estimates of lithium resource to the geological constraints on deposit size and composition that will allow the resources to be converted to reserves from which lithium can be produced economically. In this survey, we review the relevant geological features of the best characterized pegmatite, brine and other types of lithium deposits and compare their potential for large-scale, long-term production.The comparison shows that the average brine deposit (1.45 Mt Li) is more than an order of magnitude larger than the average pegmatite deposit (0.11 Mt Li) and that brine deposits, especially the large Atacama (Chile) and Uyuni (Bolivia) deposits, have a much larger total lithium resource (21.6 Mt Li). Brine deposits clearly have a much greater capacity for large-scale, long-term production than do pegmatite deposits. However, individual brine deposits differ considerably and successful production from one deposit does not necessarily assure success at others. Conversion of brine resources to reserves will depend on the degree of compartmentalization of brine reservoirs, the extent to which brine can be recovered without dilution from recharge waters and degradation of salt-bearing reservoir rock, and whether other constituents of the brines, including potassium, magnesium, bromine and boron, restrict processing or the capacity of production to respond to changes in demand for lithium.Despite their smaller size and total estimated resource (3.9 Mt), pegmatites will remain of interest because of their wider geographic distribution and consequently lesser susceptibility to supply disruptions and their more lithium-dominant compositions, which might allow more flexible response to market changes. Some deposits will be amenable to large-scale mining, but significant production might also come from selective, underground mining of small deposits in areas with low labor costs. Even the largest pegmatite deposits, such as those at Greenbushes (Australia) and Manono–Kitolo (Democratic Republic of Congo) have estimated resources that are similar to only an average brine deposit. For this reason, continuous, long-term production from pegmatites will require extensive exploration and discovery of new pegmatite deposits and districts.Two other types of lithium deposits, unusual rocks and unusual fluids, have also received attention. Deposits consisting of rocks and minerals that are enriched in lithium by interaction with brines and hydrothermal solutions, including the large Jadar (Serbia) and King Valley (USA) deposits, have estimated resources of about 3.4 Mt Li. Unusual brines in oilfields and the Salton Sea geothermal field (USA) contain an estimated 2 Mt Li. Both of these deposit types will require new processing methods but are likely to become important because of their lithium-dominant compositions and relatively large size.The total lithium resource in these deposits (31.1 Mt Li) far exceeds estimated lithium demand of up to 20 Mt Li (including consideration for recycling) in all markets through 2100. The difference between these two estimates should be vied largely as a challenge rather than a comfort. Evaluation of the economic extractability of these resources will require large investments and result in only partial success. If prices provide sufficient encouragement and exploration is sustained, however, lithium resources should be available for the next century.  相似文献   

19.
The first data on the distribution of Radiolaria and planktonic Foraminifera in the section at Biyuk-Karasu River in central part of the Crimean Mountains, are presented. Based on the study of radiolarian findings, the upper Cretaceous deposits of Biyuk-Karasu section are subdivided into the following biostratigraphic units: Alievium superbum-Phaseliforma turovi (middle Turonian), Dactylodiscus longispinus-Patulibracchium (?) quadroastrum (upper Turonian), and Orbiculiforma quadrata-Patellula sp. B (Turonian-Coniacian boundary deposits). The stratigraphic interval of Alievium superbum-Phaseliforma turovi has been previously characterized by the complex of Alievium superbum-Phaseliforma sp. A (middle Turonian, Mt. Chuku section, SW of the Crimean Mountains, and middle Turonian, Mt. Ak, central part of the Crimean Mountains). Based on the study of Foraminifera findings, the following biostratigraphic subdivisions were identified: Whiteinella paradubia (lower-lower part of the middle Turonian), Marginotruncana pseudolinneiana (uppermost middle Turonian), and Marginotruncana coronata (upper Turonian). The complex of beds with Marginotruncana pseudolinneiana and Marginotruncana coronata are comparable to the deposits of zones of same name in the Crimean-Caucasian region.  相似文献   

20.
Qinling ore belt is the largest known molybdenum belt in the world with a total reserve of >5 Mt of Mo metal. Based on the geochemical behaviour of Mo, the structural settings of the Qinling orogenic belt, and geological events in eastern China, we propose that tectonic settings are of critical importance to the formation of these ore deposits. Molybdenum is very rare in the earth with an abundance of ～0.8 ppm in the continental crust. Both surface- and magmatic-hydrothermal enrichment processes are required for Mo mineralization. It can be easily oxidized to form water-soluble MoO4 – in the surface environment, especially in the Phanaerozoic, and then precipitated under anoxic conditions. Therefore, closed or semi-closed water bodies with large catchment areas and high chemical erosion rates are the most favourable locations for Mo-enriched sediments. The Qinling orogenic belt was located in the tropics during crustal collisions, such that the chemical erosion was presumably intense, whereas the Erlangping back-arc basin was probably a closed or semi-closed water body as a result of plate convergence. More than 90% of the Mo reserves so far discovered in the Qinling molybdenum belt are associated with the Palaeozoic Erlangping back-arc basin. Compiled Re–Os isotopic ages for porphyry deposits (including several carbonate vein deposits) that have been dated show peaks during 220 million years (>0.32 Mt), 145 million years (>?3.5 Mt), and 115 million years (>?0.84 Mt), which correlate well with the three major episodes of granitoid magmatism since the Triassic. The ～220 million year episode of mineralization, represented by the Huanglongpu carbonate vein-type deposit and the Wenquan porphyry deposit, coincided with the formation of the South Qinling syn-orogenic granites as well as the Dabie ultrahigh-pressure metamorphic rocks, suggests a genetic relationship with the collision between South and North China Blocks. The ～145 Ma porphyry Mo deposits, representing the main mineralization, are attributed to reactivation by ridge subduction along the lower Yangtze River belt to the east of the Qinling orogen ～150–140 Ma. The ～115 Ma Mo deposits likely reflect slab rollback of the northwestwards subducting Pacific plate ～125–110 Ma.  相似文献   

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