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1.
New structural, petrological, chemical, isotope, and paleomagnetic data have provided clues to the Late Riphean–Paleozoic history of the Uda–Vitim island arc system (UVIAS) in the Transbaikalian sector of the Paleoasian ocean, as part of the Transbaikalian zone of Paleozoids. The island arc system consists of three units corresponding to main evolution stages: (i) Upper Riphean (Late Baikalian), (ii) Vendian–Lower Paleozoic (Caledonian), and (iii) Middle–Upper Paleozoic (Hercynian). The earliest stage produced the base of the system composed of Late Riphean ophiolite (971–892 Ma, U-Pb) and volcanic (837–789 Ma, U-Pb) and sedimentary rocks (hemipelagic siliceous sediments and dolerite sills) which represent the Barguzin–Vitim oceanic basin and the Kelyana island arc. The main event of the second stage was the formation of the large UVIAS structure (over 150,000 km2) which comprised the Transbaikalian oceanic basin, the forearc and backarc basins, and the volcanic arc itself, and consisted of many volcanic-tectonic units exceeding 100 km2 in area (Eravna, Oldynda, Abaga, etc.). Lithology, stratigraphy, major–element compositions, and isotope ages of Vendian–Cambrian volcanic rocks and associated sediments indicate strong differentiation of calc-alkaline series and the origin of the island arc system upon oceanic crust, in a setting similar to that of the today’s Kuriles–Kamchatka island arc system. The Middle–Upper Paleozoic stage completed the long UVIAS history and left its imprint in sedimentary and volcanic rocks in superposed trough basins. The rocks were studied in terms of their biostratigraphic and isotope age constraints, as well as major- and trace-element compositions, and were interpreted as products of weathering and tectonic-magmatic rework of the UVIAS units.  相似文献   

2.
《Gondwana Research》2014,25(1):338-357
Four isolated metamorphic complexes located within post-collisional granitoids occupying up to 70% of the total area, were distinguished in Sinai (Egypt) and Elat area (southern Israel), the northernmost part of the Arabian–Nubian Shield. The metamorphic rocks include metasediments, felsic and mafic metavolcanic rocks intruded by granitic, dioritic, and gabbroic plutons, all subjected to penetrative deformation.We present new SIMS U–Pb dating of zircons from 13 rock units comprising metasediments, volcanic rocks, gneisses and plutons from three metamorphic complexes (Sa'al, Feiran–Solaf, and Kid). In addition we present a SIMS U–Pb titanite age of a granitic gneiss previously dated using zircon. On the basis of the new and published U–Pb data, three successive Meso- to Neoproterozoic island arcs formed during a period of ca. 500 My are recognized. The Sa'al arc (represented by the oldest arc rocks in the ANS) evolved from 1.03 to 0.93 Ga (100 My); the Feiran–Elat arc developed from ca. 870 to 740 Ma (130 My), and the Kid arc formed from ca. 640 to 620 Ma (20 My). Evidence for an older, ca. 1.1 Ga, pre-Sa'al island arc was established from the zircon xenocryst population, though no exposures of rocks of this age were found. In the Sa'al and Kid arcs both volcanic and sedimentary rocks are preserved, whereas in the Elat–Feiran arc volcanic rocks are missing. We suggest that at ~ 700 Ma the Elat−Feiran arc was subjected to rifting that resulted in separating of the Qenaia block and its movement to the SE.  相似文献   

3.
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4.
The Precambrian basement rocks exposed along Qift–Quseir asphaltic road, central Eastern Desert of Egypt, exhibit two contrasted tectonic units, each of which has its own lithology structural style and grade of metamorphism. They are intruded by dolerite and diorite dykes. The alkali (Na2O+K2O) and TiO2 contents increase whereas Al2O3, FeO, MgO, CaO and MnO decrease with increasing SiO2 from dolerites to diorites. The trace elements Ti, Zr, Cr, Y and Ni indicate that the dolerites are tholeiitic with slight tendency toward calc-alkaline and formed from basaltic magma in an active continental margin, while diorites are calc-alkaline and were formed by fractional crystallisation of high-alumina basaltic magma in an island arc and active continental margin tectonic environment where they probably represent the forerunner of G1 granites. The molecular ratios Mg values (MgO×100/MgO+FeO) of dolerites range from 47 to 49 while those of diorite range from 51 to 59, indicating that the dolerite and diorite have suffered mild fractionation. Mineral chemistry for the diorites shows that the amphiboles are classified as magnesiohornblende and the plagioclase composition is An39–42 (i.e. the narrow range indicate that the pluton has not suffered extensive fractional crystallisation). The Al content of amphibole displays significant variation with pressure and temperature, also the change of the Ca/(Ca+Na) ratio of plagioclase is dependent on temperature. The amphibole–plagioclase geothermobarometer suggested the P-T formation conditions of studied dykes as 2 kbar and 600 °C.  相似文献   

5.
The island of Korčula, which has an area of 271.47 km2 is located along the north-eastern coast of the Adriatic Sea. Due to the Mediterranean climate, size and karst geology its water resources are very scarce. This paper describes the natural features of the island (air temperature, precipitation, geology, hydrogeology and groundwater) which are important for the water appearance and its distribution in time and space. The water supply of the island has been managed in the following ways: through a pipeline from the mainland, by drawing groundwater and by rain harvesting. Tourism causes high seasonal water needs which are barely met by the existing water supply system. Therefore, present water resource management on the island must be improved. The paper also presents mathematical programming scheme to get optimal costs and benefits of water exploitation on the island. Besides economic aspect, linear programming is applied to social and ecological objectives, as well. This study suggests that island’s water management should be primarily based on wisely using its proper water resources.  相似文献   

6.
We report here for the first time the occurrence of a high-temperature metamorphic/plutonic complex (amphibolites, metagabbros, hornblende pyroxenites and hornblendites) in Calaton Hill, Tablas island, Romblon, Central Philippines. The mineral assemblages and relic magmatic textures in these rocks imply apparent derivation from arc-related protoliths. Major element and trace element data are also comparable to those of gabbroic rocks in arc-related setting. Subsolidus re-equilibration under granulite to amphibolite facies is documented by the triple junctions between mineral phases in the different lithologies, the recrystallization of plagioclase and the presence of coronas around olivine with mineral assemblage of orthopyroxene + amphibole ± green spinel. The formation of hornblendite and the pervasive occurrence of amphiboles in the different lithologies are being attributed to the infiltration of a younger hydrous arc magma which also caused metamorphism and hybridization on the surrounding rocks. The characteristics of the Calaton Hill samples are comparable with those of the well-studied xenoliths from Ichinomegata, NE Honshu arc, Japan. We therefore interpret the Calaton Hill metamorphic/plutonic complex as representative of the lower crust underlying the Philippine island arc.  相似文献   

7.
We studied oceanic mafic igneous rocks of the Mesozoic Banggongco–Nujang suture zone in western Tibet to constrain the tectonic evolution of these rocks and the region as a whole. Two transects were accomplished. Seven basalt samples from the base of the Nadongshan transect (N1 basalts) have flat chondrite-normalized rare earth element (REE) and primitive-mantle-normalized trace element variation diagrams that are similar to MORB. Two basalt samples from the base of the Nadongshan transect (N1 basalts), ten gabbro samples from the middle of Nadongshan transect (N2 gabbros), four basalt samples from the bottom of Tanjiuxiama transect (T1 basalts), and four basalt samples from the top of the Tanjiuxiama transect (T2 basalts) are alkali basalts and have light rare earth element (LREE)-enriched chondrite-normalized REE patterns, and have primitive-mantle-normalized trace element variation diagrams that are enriched in highly incompatible elements, similar to OIB. LREE concentrations increase from N1 basalts to the T1 and T2 basalts, which have (La/Yb)N up to 16 and have even higher (Ce/Sm)N. These data indicate that the Nadong ocean island is an Azores-type ocean island that formed during the mature stage of development of the Banggongco–Nujiang Ocean. The conformable nature of the Nadong ocean island with the Mugagangri Group flysch indicates that the Banggongco–Nujiang Ocean was never a large ocean.  相似文献   

8.
Two island arcs of different ages have been reconstructed in the Neoproterozoic history of southeastern East Sayan: Dunzhugur and Shishkhid. According to earlier concepts, the Dunzhugur arc formed at ～1020 Ma and underwent collision with the Siberian(?) continent at ～810 Ma. The Shishkhid arc formed somewhat earlier than 800 Ma and existed till the end of the Late Baikalian (～600 Ma, from indirect data). This primitive geologic history, when each arc existed for 200 Myr, was suggested because of the deficit of direct data, and its reconstruction cast doubt. In this work we present results of preliminary dating of detrital zircons separated from the volcaniclastic rocks composing the above arcs. We analyzed 12 zircon crystals from the Dunzhugur volcanic clastics, whose 206Pb/238U age varies from 844 ± 8 to 1048 ± 12 Ma (1σ). Five most ancient zircons form a concordant cluster with an age of 1034 ± 9 Ma (2σ). Hence, the arc formed earlier than it was assumed and existed for a long time, most likely, till its collision with the continent. We also studied two zircon samples from the volcaniclastic rocks of the Oka accretionary prism, which probably formed in the Shishkhid arc. All ten crystals of the first sample form a concordant cluster with an age of 813 ± 7 Ma (2σ). The analyzed zircons of the second sample arrange in two clusters, with an age of 775 ± 8 Ma (2σ, nine crystals) and 819 ± 17 Ma (three crystals). Thus, the Shishkhid arc formed earlier than it was assumed, at the end of the Early Baikalian, and underwent active volcanism at least till 775 Ma. Dating of detrital zircons from the volcaniclastics generated at the mature stage of the Shishkhid-arc evolution will help to reconstruct partly or completely its history in the period 775–600 Ma.  相似文献   

9.
Accurate and reliable prediction of shallow groundwater level is a critical component in water resources management. Two nonlinear models, WA–ANN method based on discrete wavelet transform (WA) and artificial neural network (ANN) and integrated time series (ITS) model, were developed to predict groundwater level fluctuations of a shallow coastal aquifer (Fujian Province, China). The two models were testified with the monitored groundwater level from 2000 to 2011. Two representative wells are selected with different locations within the study area. The error criteria were estimated using the coefficient of determination (R 2), Nash–Sutcliffe model efficiency coefficient (E), and root-mean-square error (RMSE). The best model was determined based on the RMSE of prediction using independent test data set. The WA–ANN models were found to provide more accurate monthly average groundwater level forecasts compared to the ITS models. The results of the study indicate the potential of WA–ANN models in forecasting groundwater levels. It is recommended that additional studies explore this proposed method, which can be used in turn to facilitate the development and implementation of more effective and sustainable groundwater management strategies.  相似文献   

10.
The Kuril islands constitute a volcanic island arc-trench system,stretching from eastern Hokkaido(Japan)to Kamchatka(Russia) along the northwestern Pacific subduction system.The current arc consists of several volcanic islands mainly with Neogene basement and capped by several,predominantly andesitic,active subduction stratovolcanoes.Kunashir Island is the southwestern-most island of the arc,just off the Hokkaido coast and represents the study area in this paper.The island is composed of a Lower Complex of mainly late Miocene to Pliocene volcanic rocks,covered by an Upper Complex of younger(basaltic)andesitic lava flows and tuffs on which currently four active volcanic edifices are built.In the Lower Complex sub-volcanic and deeper-seated intrusives of the so-called Prasolov and Dokuchaev magmatic complexes are found.More differentiated,tonalitic-granodioritic rocks were collected from these small intrusive bodies.An early Oligocene zircon LA-ICP-MS U/Pb age of 31 Ma for the Prasolov Complex was obtained,showing that the basement of Kunashir Island is older than previously thought.Thermochronometry(apatite fission-track and U-Th-Sm/He and zircon U-Th/He analyses) further shows that the magmatic basement of the island was rapidly exhumed in the Pleistocene to present levels in a differential pattern,with He-ages ranging from 1.9 to 0.8 Ma.It is shown that the northern section of the island was hereby exhumed more intensely.  相似文献   

11.
An unusual amount of precipitation fell in Panay island during the passage of Typhoon Fengshen (local name: Typhoon Frank) in June 2008. The voluminous amount of rain is attributed to the interaction of the tropical cyclone and southwest monsoon as the latter was enhanced by the former during its passage across central Philippines. Ground and aerial surveys were conducted to document the extensive flooding and landslides that occurred in the island. Artificial damming of rivers followed by breaching resulted to flooding, whereas steep slopes, fractured lithologies and intense precipitation were recognized to have led to the occurrence of landslides. These natural hazards and their causes are presented to contribute to our understanding of how weather systems evolve and what the corresponding effects are on the ground. This, hopefully, can provide significant inputs in improving disaster risk reduction and preparedness programs.  相似文献   

12.
Precision U–Pb (SHRIMP-II) isotope geochronological data, obtained for the first time, make it possible to suggest that sediments of the Neocomian primitive island arc sequence are missed or poorly developed in the South-Western part of the Mainitskii terrane of the Koryak Highlands. However, Late Albian mature island arc tuff and tuffaceous–turbidite formations are common. This enables us to extend the age range of the Mainitskii island arc from the Early Neocomian to the Late Albian and to suggest a two-stage pattern of its development. The isotope-geochronological data obtained for plagiogranite and moderately acid subvolcanics, previously attributed to the Koryak–Western Kamchatka volcanoplutonic belt, indicate that it is possible to combine them into the Middle Miocene postsubduction? polygenic complex. In addition, owing to modern high-precision isotope–geochronological methods, it has become possible to determine the age of gold–sulfide mineralization of the Talyaigin ore field, paragenetically related to the manifestations of the Middle Miocene Vilyuneiveem complex.  相似文献   

13.
Interactions between basaltic melt and orthopyroxenite(Opx)were investigated to gain a better understanding of the consequences of the residence and transport of ocean island basalts(OIBs)within the mantle.The experiments were conducted using a DS-3600 six-anvil apparatus at 3.0–4.5 GPa and 1300–1450℃.The basaltic melt and Opx coexisted at local equilibrium at these pressures and temperatures;the initial melts dissolved Opx,which modified their chemical composition,and clinopyroxene(Cpx)precipitated with or without garnet(Grt).The trace-element contents of Grt,Cpx,and melt were measured and the mineral–melt distribution coefficients(D)of Cpx–melt and Grt–melt were calculated,which can be used to assess the distribution of trace elements between basalt and minerals in the mantle.Two types of reaction rim were found in the experimental products,Cpx,and Cpx+Grt;this result indicates that residual rocks within the mantle should be pyroxenite or garnet pyroxenite.Both rock types are found in mantle xenoliths from Hawaii,and the rare-earth-element(REE)pattern of Cpx in these mantle pyroxenites matches those of Cpx in the experimental reaction rims.Furthermore,residual melts in the experimental products plot in similar positions to Hawaiian high-SiO2OIBs on major-element Harker diagrams,and their trace-element patterns show the signature of residual Grt,particularly in runs at1350℃ and 4.0–4.5 GPa.Trace-element concentrations of the experimental residual melts plot in similar positions to the Hawaiian OIBs on commonly used discrimination diagrams(Ti vs.Zr,Cr vs.Y,Cr vs.V,Zr/Y vs.Zr,and Ti/Y vs.Nb/Y).These results indicate that reaction between basaltic melt and pyroxenite might contribute to the generation of Hawaiian high-SiO2 OIBs and account for their chemical variability.  相似文献   

14.
The Portneuf–Mauricie Domain (PMD), located in the south-central part of the Grenville province, contains Mesoproterozoic Ni–Cu ± platinum-group element (PGE) prospects hosted in a variety of plutonic intrusions (layered, with simple structures, or zoned) and emplaced in a mature island arc setting. A two-stage model is envisaged to explain the formation of magmatic sulfides. An early loss of a small amount of sulfides in the conduits of primitive, hydrous mantle-derived melts under high fO2, resulted in depletion of the magmas in chalcophile and precious metals (Cu/Pd ratios vary from initial mantle values up to 1.6 × 106). Then, nearer the mineralized zones, the magmas interacted with sulfide-bearing country rocks, resulting in felsification of the magmas, assimilation of crustal sulfur (δ 34S values up to +5.5‰), and the formation of an immiscible sulfide liquid. Liquid-sulfide formation was followed by variable interactions between the silicate and sulfide magmas, which were responsible for the enrichment of sulfides in Ni, Cu, and, locally, PGE. Indeed, low R factors are found for prospects hosted in intrusions with a simple internal structure and in layered intrusions whereas high R factors are found for prospects hosted in zoned intrusions. Finally, sulfide melt may have been partly incorporated into later pulses of magma and injected into shallow magma chambers to form the PMD prospects. The PMD prospects share common characteristics with other well-known deposits (Aguablanca, Vammala, Stormyrplunen, and deposits in Alaskan/Ural-type intrusions), attesting to the Ni, Cu, and PGE potential of deposits associated with subduction-zone settings.  相似文献   

15.
The Danish island of Bornholm is located at the southwestern margin of the Fennoscandian Shield, and features exposed Precambrian basement in its northern and central parts. In this paper, we present new U–Pb zircon and titanite ages for granites and orthogneisses from 13 different localities on Bornholm. The crystallization ages of the protolith rocks all fall within the range 1,475–1,445 Ma (weighted average 207Pb/206Pb ages of zircon). Minor age differences, however, may imply a multi-phase emplacement history of the granitoid complex. The presence of occasional inherited zircons (with ages of 1,700–1,800 Ma) indicates that the Bornholm granitoids were influenced by older crustal material. The east–west fabric observed in most of the studied granites and gneisses, presumably originated by deformation in close connection with the magmatism at 1,470–1,450 Ma. Most titanite U–Pb ages fall between 1,450 and 1,430 Ma, reflecting post-magmatic or post-metamorphic cooling. Granitoid magmatism at ca. 1.45 Ga along the southwestern margin of the East European Craton has previously been reported from southern Sweden and Lithuania. The ages obtained in this study indicate that the Bornholm magmatism also was part of this Mesoproterozoic event.  相似文献   

16.
The Pueblo Viejo deposit (production to 1996: 166 t Au, 760 t Ag) is located in the Dominican Republic on the Caribbean island of Hispaniola and ranks as one of the largest high-sulfidation/acid-sulfate epithermal deposits (reserves in 2007: 635 t Au, 3,648 t Ag). One of the advanced argillic ore bodies is cut by an inter-mineral andesite porphyry dike, which is altered to a retrograde chlorite–illite assemblage but overprinted by late-stage quartz–pyrite–sphalerite veins and associated low-grade Au, Ag, Zn, Cd, Hg, In, As, Se, and Te mineralization. The precise TIMS U–Pb age (109.6 ± 0.6 Ma) of the youngest zircon population in this dike confirms that the deposit is part of the Early Cretaceous Los Ranchos intra-oceanic island arc. Intrusion-related gold–sulfide mineralization took place during late andesite–dacite volcanism within a thick pile (>200 m) of carbonaceous sand- and siltstones deposited in a restricted marine basin. The high-level deposit was shielded from erosion after burial under a late Albian (109–100 Ma) ophiolite complex (8 km thick), which was in turn covered by the volcano-sedimentary successions (>4 km) of a Late Cretaceous–Early Tertiary calc-akaline magmatic arc. Estimates of stratigraphic thickness and published alunite, illite, and feldspar K-Ar ages and closure temperatures (alunite 270 ± 20°C, illite 260 ± 30°C, K-feldspar 150°C) indicate a burial depth of about 12 km at 80 Ma. During peak burial metamorphism (300°C and 300 MPa), the alteration assemblage kaolinite + quartz in the deposit dehydrated to pyrophyllite. Temperature–time relations imply that the Los Ranchos terrane then cooled at a rate of 3–4°C/Ma during slow uplift and erosion.  相似文献   

17.
Ischia, one active volcano of the Phlegraean Volcanic District, prone to very high risk, is dominated by a caldera formed 55 ka BP, followed by resurgence of the collapsed area. Over the past 3 ka, the activity extruded evolved potassic magmas; only a few low-energy explosive events were fed by less evolved magmas. A geochemical and Sr–Nd–O isotope investigation has been performed on minerals and glass from products of three of such eruptions, Molara, Vateliero, and Cava Nocelle (<2.6 ka BP). Data document strong mineralogical, geochemical, and isotopic heterogeneities likely resulting from mingling/mixing processes among mafic and felsic magmas that already fed the Ischia volcanism in the past. Detailed study on the most mafic magma has permitted to investigate its origin. The mantle sector below Ischia underwent subduction processes that modified its pristine chemical, isotopic, and redox conditions by addition of ≤1 % of sediment fluids/melts. Similar processes occurred from Southeast to Northwest along the Apennine compressive margin, with addition of up to 2.5 % of sediment-derived material. This is shown by volcanics with poorly variable, typical δ18O mantle values, and 87Sr/86Sr progressively increasing toward typical continental crust values. Multiple partial melting of this modified mantle generated distinct primary magmas that occasionally assimilated continental crust, acquiring more 18O than 87Sr. At Ischia, 7 % of Hercynian granodiorite assimilation produced isotopically distinct, K-basaltic to latitic magmas. A SW–NE regional tectonic structure gave these magmas coming from large depth the opportunity to mingle/mix with felsic magmas stagnating in shallower reservoirs, eventually triggering explosive eruptions.  相似文献   

18.
The urban heat island (UHI), together with summertime heat waves, foster’s biophysical hazards such as heat stress, air pollution, and associated public health problems. Mitigation strategies such as increased vegetative cover and higher albedo surface materials have been proposed. Atlanta, Georgia, is often affected by extreme heat, and has recently been investigated to better understand its heat island and related weather modifications. The objectives of this research were to (1) characterize temporal variations in the magnitude of UHI around Metro Atlanta area, (2) identify climatological attributes of the UHI under extremely high temperature conditions during Atlanta’s summer (June, July, and August) period, and (3) conduct theoretical numerical simulations to quantify the first-order effects of proposed mitigation strategies. Over the period 1984–2007, the climatological mean UHI magnitude for Atlanta-Athens and Athens-Monticello was 1.31 and 1.71°C, respectively. There were statistically significant minimum temperature trends of 0.70°C per decade at Athens and −1.79°C per decade at Monticello while Atlanta’s minimum temperature remained unchanged. The largest (smallest) UHI magnitudes were in spring (summer) and may be coupled to cloud-radiative cycles. Heat waves in Atlanta occurred during 50% of the years spanning 1984–2007 and were exclusively summertime phenomena. The mean number of heat wave events in Atlanta during a given heat wave year was 1.83. On average, Atlanta heat waves lasted 14.18 days, although there was quite a bit of variability (standard deviation of 9.89). The mean maximum temperature during Atlanta’s heat waves was 35.85°C. The Atlanta-Athens UHI was not statistically larger during a heat wave although the Atlanta-Monticello UHI was. Model simulations captured daytime and nocturnal UHIs under heat wave conditions. Sensitivity results suggested that a 100% increase in Atlanta’s surface vegetation or a tripling of its albedo effectively reduced UHI surface temperature. However, from a mitigation and technological standpoint, there is low feasibility of tripling albedo in the foreseeable future. Increased vegetation seems to be a more likely choice for mitigating surface temperature.  相似文献   

19.
《International Geology Review》2012,54(12):1504-1520
This study presents new data relating to the tectonic evolution of the Zhonggang ocean island, within the Mesozoic Banggongco–Nujiang suture zone of northern Tibet, and discusses the implications of these data for the evolution of this region. Thirteen basalt and ten gabbro samples were collected from a sampling transect through this area; these samples have light rare earth element (LREE)-enriched chondrite-normalized REE patterns, and are enriched in highly incompatible elements, yielding primitive-mantle-normalized trace-element variation patterns that are similar to ocean island basalts (OIB). A gabbro dike intruded into basalt of the Zhonggang ocean island and was overlain by basaltic conglomerate, suggesting that this dike was formed after the basalt, but before the basaltic conglomerate. The gabbro dike yields an LA–ICP–MS zircon U–Pb age of 116.2 ± 4.1 Ma, indicating the timing of formation of the Zhonggang ocean island, and suggesting in turn that the Banggongco–Nujiang Neo-Tethys Ocean remained open at this time. These data, combined with the geological history of the region, indicates that the Banggongco–Nujiang Neo-Tethys Ocean opened between the late Permian and the Early Triassic, expanded rapidly between Late Triassic and Middle Jurassic time, and finally closed between the late Early and early Late Cretaceous.  相似文献   

20.
The Sipadan island is the only oceanic island found in Malaysia, and is popularly known for its beautiful corals and diving activities. The aquifer of the island is affected by seawater intrusion associated with groundwater exploitation. Geologically, the…  相似文献   

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