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1.
40Ar/39Ar geochronological data on hornblende, biotite and K-feldspar provide constraints on the cooling path experienced by a high-grade metamorphic complex from the Mühlig–Hofmannfjella and Filchnerfjella (6–8°E), central Dronning Maud Land, Antarctica, during the late Neoproterozoic-early Palaeozoic Pan–African orogeny. Hornblende ages yield c. 481 Ma, biotite ages range from c. 466 Ma to c. 435 Ma, whereas K-feldspar ages of the gneisses are c. 437 Ma. The 40Ar/39Ar data suggest initial cooling at a rate of ~10 °C/Myr between 481 and 465 Ma, followed by a lower cooling rate of ~6 °C/Myr during the subsequent c. 30 million years. The K-feldspar 40Ar/39Ar ages place a lower time limit on the duration of the exhumation, by the time of thermal relaxation to a stable continental geotherm. The 40Ar/39Ar data reflecting cooling indicate tectonic exhumation related to orogenic collapse during a later phase of the Pan–African orogeny.  相似文献   

2.
We have studied the oxygen isotopic composition of rocks from a 100 km transect through the central Superior province of Ontario, representing progressively the shallower terrains of the Kapuskasing structural zone (KSZ), the Wawa gneiss terrane (WGT), and the Michipicoten greenstone belt (MGB). These rocks range in age from 2.76 to 2.60 Ga, and correspond to a section through approximately 20 km of crustal thickness. Equivalent lithologic types have similar range of 18O values at each crustal level: tonalitic to granodioritic rocks: 6.4 to 9.5; dioritic and anorthositic rocks: 5.5 to 7.6; mafic gneisses: group 1 (majority): 5.7 to 7.1; group 2: 8.1 to 9.5. 18O values exhibit a remarkable correlation with SiO2 values, similar to that observed in unaltered plutonic rocks of equivalent composition. Paragneisses have significantly higher 18O values: 9.3 to 12.2. Low-grade metavolcanic and metasedimentary rocks of the MGB are 18O-enriched compared to their high-grade equivalents in the KSZ and WGT: 7.4 to 13.3 for mafic to felsic metavolcanic rocks; 11.4 to 14.7 for clastic metasediments. Coexisting minerals exhibit 18O-fractionation consistent with equilibrium, but corresponding to uniform isotopic temperatures about 553 to 584°C across the entire transect, lower than the inferred metamorphic temperatures in the highest-grade (KSZ) terrane. The lack of distinctive isotopic differences between equivalent rock types in the KSZ, WGT and MGB suggests that there is no significant gradient in 18O with depth in the crust. The majority of mafic gneisses (group 1) appear to have been emplaced either as subaerial extrusives, intrusive sills, or, less likely, as submarine extrusives that were hydrothermally altered at high temperatures. The less abundant group 2 mafic rocks have the 18O values typical of greenstones that were altered at low temperature by seawater, and isotopically resemble low-grade rocks in the Michipicoten and Abitibi belts. In general, no major changes in whole-rock isotopic composition appear to have occurred during granulite facies metamorphism, implying limited flux of water or CO2. The continuous linear gradient in 18O versus SiO2 in the high-grade rocks cannot be due to differentiation of a mafic source magma. A model involving an association between mantle-derived mafic magma and 18O-enriched crustal materials is more consistent with the oxygen isotopic and the REE data.McMaster Isotopic, Nuclear and Geochemical Studies Group Publication 163; LITHOPROBE Publication 168.  相似文献   

3.
High-pressure (HP) rocks at Tehuitzingo, on the western margin of the HP belt within the Paleozoic Acatlán Complex (southern México), occur in a klippe that was thrust over low-grade clastic rocks. The youngest detrital zircon cluster in the low-grade rocks yielded U-Pb ages of 481 ± 16 Ma, which provide an older limit for deposition. The HP rocks are composed of metabasites, serpentinite, granite (482 ± 3 Ma) and mica schist (youngest concordant detrital zircon: 433 ± 3 Ma). The schist and granite are inferred to be high-grade equivalents of lower Paleozoic, low-grade rocks exposed elsewhere in the Acatlán Complex, from which they are inferred to have been removed by subduction erosion. Mineral analyses indicate that the subducted rocks underwent HP metamorphism and polyphase deformation at depths of ~ 50 km (~ 16 kbar and 750 °C: eclogite facies). Subsequent retrogression passed through epidote-amphibolite to greenschist facies, which was synchronous with W-vergent thrusting over the low-grade clastic rocks. Deposition of the low-grade rocks and thrusting are bracketed between either 481–329 Ma (Ordovician-Mississippian), and was followed by F3 synformal folding. Cooling through ca. 385 °C is indicated by 329 ± 1 and 316–317 ± 2 Ma, 40Ar/39Ar muscovite plateau ages in HP rocks, which are 5–17 my younger than those of the adjacent Piaxtla eclogites suggesting younger exhumation. The petrology, P-T conditions and ages of the Piaxtla Suite is consistent with an extrusion channel within the Acatlán Complex along the active western margin of Pangea during the Carboniferous. Detrital zircon populations in the low-grade psammite (ca. 481, 520–650, 720, 750, 815, 890, 1050 and 2750 Ma) and the HP schist (ca. 457–480, 534, 908, 954–1150, 1265, 1845 and 2035 Ma) indicate derivation from the Ordovician Acatlán granitoids, Neoproterozoic Brasiliano orogens, 900–750 Ma Goiás arc (Amazonia), 1–1.3 Ma Oaxaquia, and more ancient sources in Oaxaquia/Amazonia.  相似文献   

4.
The compositional variations in epidotes, Ca2Al2(Fe3+, Al)-Si3O12(OH), from a prograde Mesozoic rock series in the Eastern Alps, Austria, are systematically related to metamorphic grade and the oxidation state of the rock. With increasing metamorphic grade the average composition of the zoned epidotes shifts to Fe3+-poorer compositions reflecting not only the effect of temperature and total pressure but also the concomitant decrease of the oxidation state of the rocks. Oxidized hematite-bearing assemblages are: 90 mole % Al2FeEp (greenschists) 70 mole % Al2FeEp (garnet amphibolites) 58 mole % Al2FeEp (eclogites); reduced sulfidebearing assemblages are: 42 mole % Al2FeEp (greenschists) 24 mole % Al2FeEp (garnet amphibolites) 23 mole % Al2FeEp (eclogites).A similar compositional evolution of the epidotes as in the spatial sequence of the samples can be observed within the single zoned crystals, reflecting the temporal changes of temperature, total pressure and oxygen fugacity during the prograde crystallization. The Fe3+-contents of core and rim decrease with increasing metamorphic grade and decreasing oxidation state. Generally the zoned epidotes consist of a Fe3+-rich core (90 to 63 mole % Al2FeEp) and a Fe3+-poorer rim (55 to 23 mole % Al2FeEp). Core and rim of the epidote crystals are separated by a compositional gap the extension of which is independent of the bulk rock composition, the oxidation state, and the mineralogical composition of the assemblages but becomes smaller with increasing metamorphic grade: 7253 mole % Al2FeEp (low grade greenschists, 400° C) 6355 mole % Al2FeEp (higher grade greenschists, 500° C), and 6055 mole % Al2FeEp (garnet amphibolites, 500–550° C). At the temperature conditions of the highest grade garnet amphibolites and eclogites (550° C) the compositional gap closes at a composition of 58 mole % Al2FeEp.The data presented thus confirm clearly the existence of an asymmetric miscibility gap in the monoclinic Al-Fe(III)-epidote solid solution series, which, for the first time, has been assumed by Strens (1964, 1965).A model is proposed that describes the prograde compositional evolution of the epidotes studied through the competing mechanisms of growth and diffusional Al-Fe3+ exchange and their dependence on metamorphic grade and oxidation state.  相似文献   

5.
This paper presents data on the geological position, geochemistry, age, and isotopic characteristics of the granitoids of the southern part of the Voznesenka terrane, Southern Primorye (Muraviev–Amursky Peninsula and its vicinities). All of the studied granitoids were formed in three stages: the Ordovician, Silurian, and Permian. The Silurian and Permian ages of the granitoid intrusions have been previously determined (Ostrovorussky Massif, 432–422 Ma, and 250 ± 4 Ma, early and late associations, respectively; Sedanka massif, 261 ± 3 Ma). The granites of the Artem and Nadezhdinsky massifs define an U–Pb zircon age of 481 ± 6 and 452 ± 4 Ma, respectively. The geochemical and isotope data show mainly the crustal nature of the granitoids. Their formation was related to melting of relatively immature rocks of the continental crust (mafic–intermediate volcanic rocks). The Nd isotope composition of the granitods (TNd(DM–2) = 1.3 Ga) indicates the absence of the mature ancient crust at the basement of the southern Voznesenka terrane. The maximum contribution of mantle sources to the granite formation is recorded in the Permian associations. A comparison of the peaks of intrusive magmatism in the southern part of the Voznesenka terrane and adjacent territories suggests that the formation of the granitoids of the Muraviev–Amursky Peninsula and its vicinities was caused by the interaction of continental blocks with two oceanic basins: the Paleoasian (and its fragments) and Paleopacific ones.  相似文献   

6.
A suite of the fossil-rich marine-land interbedded strata(Nanshuangyashan Formation) is distributed at the eastern margin of the Jiamusi massif in the eastern Heilongjiang Province, NE China. The authors had recently discovered a suite of arkose beneath the marine-land interbedded strata, which overlays unconformably on the Permain granite in the eastern margin of the Jiamusi massif. The LA-ICP-MS zircon U-Pb dating indicate that all detrital zircons from the analysed four arkose samples show the four population ages of 800 Ma, 538–481 Ma, 269–250 Ma and 223–215 Ma. The former three population ages are widely recorded in the Jiamusi-Khanka massif and the Songnen massif. The later group is the minimal age population in the analyzed samples, limiting the sedimentation time of the arkoses occurred after the Late Triassic. At present, the minimal age population is not recorded in the Jiamusi massif, but the granites with the ages of 228–210 Ma are widely distributed in the Songnen-Zhangguangcai Range massif and the Khanka massif. The predominantly Permian zircons are characterized by oscillatory zoning and euhedral shapes, with variable zircon ε_(Hf)(t) values(-5.5 to +11.2), indicating that they were derived from mixture sources, possibly mixed with components of the Songnen-Zhangguangcai Range massif and the Jiamusi-Khanka massif. These results, combined with regional analyses, indicate that the closing of Mudanjiang ocean and Panthalassa ocean possibly existed from Early Permian to Late Triassic.  相似文献   

7.
Despite its small area (5.6 km2), the Penguins Island brings magmatic information concerning mantle geochemical heterogeneities in southwestern Indian Ocean. The volcanism that built the island was firstly associated with marine deposits, and secondly with aerial, giving then abundant volcanic breccias and lava flows. The rocks are weakly differentiated and result of magmatic fractionation from picritic to tephritic types. KAr ages are near 1.1 Ma. The magmatic source may be related to a HIMU reservoir or to EMI ± EMII ones, depending on the geochemical evidences that are taken into account. However, comparisons with the Marion Island, on the same oceanic plateau and far to the west, as well as with Kerguelen Islands, far to the east, suggest a very heterogeneous mantle source. To cite this article: A. Giret et al., C. R. Geoscience 334 (2002) 481–488.  相似文献   

8.
Geological, geochemical, and geochronological studies of rocks have been performed at the Okunyovo rare-metal ore magmatic complex (Western Sayan Mountains) composed of alkali granites and related F–Be mineralization. The geological data obtained and the features of the geochemical similarity of granite and ore mineralization identified are indicative of their genetic relation. Riebeckite from two granite samples has been subjected to 40Ar/39Ar analyses. The estimated age values of 481.5 ± 2.7 Ma and 486.5 ± 5.8 Ma are consistent within the limits of error and determine the age of Okunyovo alkali granite within 481–486 Ma ago. These data have made it possible to relate the studied rock and ore formation to the development of the Early Paleozoic ASFR controlled by the mantle plume. Along with the Okunyovo rare-metal granite massif, within the Sayan Region of the magmatic province, there are a number of Early Paleozoic alkali massifs related to the Aryskan, Raduga, and Kazyr rare-metal deposits. They are located in the conjugation area of the Western and Eastern Sayan ridges, in the Early Paleozoic Eastern Sayan rare-metal magmatism zone specialized in Be, W, Mo, Zr, Nb, and REEs.  相似文献   

9.
This paper examines the effect of cluster organisations on the international competitive advantages of clusters in the European life sciences industry. Data on coordinated cluster organisations are used to assess whether cluster development policies enhance the attractiveness of clusters to multinational corporations (MNCs). We estimate mixed logit models using a sample of 481 greenfield investments by life sciences MNCs in West-European regions during the period 2010–2013. Contrary to the beliefs of policymakers, our results indicate that both the presence of cluster organisations and higher-order activities of cluster organisations have only a small effect in attracting greenfield investments. In addition, cluster organisations help attract economic activities with less specific location requirements, such as production plants and sales and marketing offices. This finding provides valuable insights for policymakers, as investing public funds in cluster organisations is not found to attract high value-added business activities, such as those that occur in headquarters and R&D facilities, which are often the target of such investments.  相似文献   

10.
The effects of intrusive thermal stress have been studied on a number of Pleistocene sediment samples obtained from Leg 64 of the DSDP-IPOD program in the Gulf of California. Samples were selected from Sites 477, 478 and 481 where the organic matter was subjected to thermal stress from sill intrusions. For comparison purposes, samples from Sites 474 and 479 were selected as representative of unaltered material.The GC and GC-MS data show that lipids of the thermally unaltered samples were derived from microbial and terrestrial higher-plant detritus. Samples from sill proximities were found to contain thermally-derived distillates and those adjacent to sills contained essentially no lipids. Curie point pyrolysis combined with GC and GC-MS was used to show that kerogens from the unaltered samples reflected their predominantly autochthonous microbial origin. Pyrograms of the altered kerogens were much less complex than the unaltered samples, reflecting the thermal effects. The kerogens adjacent to the sills produce little or no pyrolysis products since these intrusions into unconsolidated, wet sediments resulted in in situ pyrolysis of the organic matter.Examination of the kerogens by ESR showed that spin density and line width pass through a maximum during the course of alteration but ESR g-values show no correlation with maturity. Stable carbon isotope (δ13C) values of kerogens decrease by 1–1.5‰ near the sills at Sites 477 and 481 and the atomic N/C decreases slightly with proximity to a smaller sill at Site 478. Differences in maturation behavior between Site 477 and 481 and Site 478 are attributed to dissimilarities in thermal stress and to chemical and isotopic heterogeneity of Guaymas Basin protokerogen.  相似文献   

11.
This study presents new zircon U–Pb geochronology, geochemistry, and zircon Hf isotopic data of volcanic and subvolcanic rocks that crop out in the Bayanhushuo area of the southern Great Xing’an Range (GXR) of NE China. These data provide insights into the tectonic evolution of this area during the late Mesozoic and constrain the evolution of the Mongol–Okhotsk Ocean. Combining these new ages with previously published data suggests that the late Mesozoic volcanism occurred in two distinct episodes: Early–Middle Jurassic (176–173 Ma) and Late Jurassic–Early Cretaceous (151–138 Ma). The Early–Middle Jurassic dacite porphyry belongs to high-K calc-alkaline series, showing the features of I-type igneous rock. This unit has zircon εHf(t) values from +4.06 to +11.62 that yield two-stage model ages (TDM2) from 959 to 481 Ma. The geochemistry of the dacite porphyry is indicative of formation in a volcanic arc tectonic setting, and it is derived from a primary magma generated by the partial melting of juvenile mafic crustal material. The Late Jurassic–Early Cretaceous volcanic rocks belong to high-K calc-alkaline or shoshonite series and have A2-type affinities. These volcanics have εHf(t) and TDM2 values from +5.00 to +8.93 and from 879 to 627 Ma, respectively. The geochemistry of these Late Jurassic–Early Cretaceous volcanic rocks is indicative of formation in a post-collisional extensional environment, and they formed from primary magmas generated by the partial melting of juvenile mafic lower crust. The discovery of late Mesozoic volcanic and subvolcanic rocks within the southern GXR indicates that this region was in volcanic arc and extensional tectonic settings during the Early–Middle Jurassic and the Late Jurassic–Early Cretaceous, respectively. This indicates that the Mongol–Okhotsk oceanic plate was undergoing subduction during the Early–Middle Jurassic, and this ocean adjacent to the GXR may have closed by the Late Middle Jurassic–Early Late Jurassic.  相似文献   

12.
We present new U–Pb isotopic age data for detrital zircons from 16 deformed sandstones of the Ross Supergroup in north Victoria Land, Antarctica. Zircon U/Th ratios primarily point to dominantly igneous parent rocks with subordinate contributions from metamorphic sources. Comparative analysis of detrital zircon age populations indicates that inboard stratigraphic successions (Wilson Terrane) and those located outboard of the East Antarctic craton (the Bowers and Robertson Bay terranes) have similar ~ 1200–950 Ma (Mesoproterozoic–Neoproterozoic) and ~ 700–490 Ma (late Neoproterozoic–Cambrian, Furongian) age populations. The affinity of the age populations of the sandstones to each other, as well as Gondwana sources and Pacific-Gondwana marginal stratigraphic belts, challenges the notion that the outboard successions form exotic terranes that docked with Gondwana during the Ross orogeny and instead places the terranes in proximity to each other and within the peri-Gondwana realm during the late Neoproterozoic to Cambrian. The cumulative zircon age suite from north Victoria Land yields a polymodal age spectra with a younger, primary 700–480 Ma age population that peaks at ~ 580 Ma. Cumulative analysis of zircons with elevated U/Th ratios (> 20) indicating metamorphic heritage yield ~ 657–532 Ma age probability peaks, which overlap with the younger dominantly igneous zircon population. The data are interpreted to give important new evidence that is consistent with ongoing convergent arc magmatism by ~ 626 Ma, which provided the dominant zircon-rich igneous rocks and subordinate metamorphic rocks. Maximum depositional ages as young as ~ 493–481 Ma yielded by deformed sequences in the outboard Bowers and Robertson Bay terrane samples provide new support for late Cambrian to Ordovician deformation in outboard sectors of the orogen, consistent with tectonic models that call for cyclic phases of contraction along the north Victoria Land sector of the Ross–Delamerian orogen.  相似文献   

13.
Characteristic latest Neoproterozoic and Early Paleozoic acritarchs and associated organic-walled microfossils are recorded from the sediments of Marwar Supergroup encountered in BGW-A well (Bikaner-Nagaur Basin) from 1123–481 m depth. Six distinct acritarch assemblages, broadly comparable with globally known Ediacaran (Vendian) and Cambrian assemblages are recognised. The recovered microfossils provide precise age for different units of the Marwar Supergroup whose ages, till now, were poorly understood due to absence or paucity of invertebrate and other mega and microfossils.  相似文献   

14.
《International Geology Review》2012,54(14):1706-1727
ABSTRACT

The Tehuitzingo ultramafic body (Acatlán Complex, southern Mexico) is a strongly serpentinized harburgitic body that encloses chromitite bodies and blocks of eclogitic rocks. Hydrous retrograde metamorphism on chromitite bodies resulted in the formation of: i) partly altered chromite characterized by unaltered cores surrounded by Fe2+-rich and Al-depleted porous chromite containing chlorite; ii) porous chromite corresponding to a chromite that was entirely transformed to Fe2+-rich and Al-depleted porous chromite; and iii) zoned chromite formed by modified cores surrounded by Fe3+-rich non-porous chromite and magnetite rims. The content of minor and trace elements (Ga, Ti, Ni, Zn, Co, Mn, V, Sc) in the cores of partly altered chromite preserves the magmatic fingerprint of back-arc-bearing chromitites, while the cores of zoned chromite are enriched in Zn, Co and Mn but depleted in Ga, Ti, Ni and Sc, and display a metamorphic signature. P-T pseudosections performed in the system CrMFASH allow us to constrain the temperature of formation of Fe2+-rich chromite by reaction of magmatic chromite with olivine between 584 and 449 °C; while the new thermodynamic calculations performed in the FMASH system shows that the Fe3+-rich non-porous chromite and magnetite rims in zoned chromites were formed at lower temperatures (i.e. 270–340 ºC and < 20 kbar). Temperatures of alteration obtained using our new thermodynamic model are in agreement with the qualitative estimation that use minor and trace elements in Fe2+-rich chromite (i.e. 560–700 ºC). These temperatures are more restricted than those estimated for chlorite hosted in partly altered (193–481 ºC) and porous chromite (158–255 ºC), and those reported in host serpentinite and associated eclogitic rocks (210–399 ºC). A comparison of P-T conditions estimated by our new thermodynamic models and the distribution of minor and trace elements in the different zones of chromites allow us to infer that the alteration took place during the hydrous retrograde metamorphism, and that the high-pressure metamorphism did not modify the composition of chromites. Thus, the microstructural zoning displayed by chromite grains was formed during the exhumation of a segment of a back-arc-related oceanic lithosphere, at similar temperature conditions than the Tehuitzingo serpentinite.  相似文献   

15.
Lyså, A., Jensen, M. A., Larsen, E., Fredin, O. & Demidov, I. N.* 2010: Ice‐distal landscape and sediment signatures evidencing damming and drainage of large pro‐glacial lakes, northwest Russia. Boreas, Vol. 40, pp. 481–497. 10.1111/j.1502‐3885.2010.00197.x. ISSN 0300‐9483. Sediments from river sections and the morphology of the upper reaches of Severnaya Dvina and Vychegda in northwest Russia show evidence of the existence of large ice‐dammed lakes in the area twice during the Weichselian. During the Late Weichselian, three separate ice‐dammed lakes (LGM lake(s)) existed, the largest one at about 135 m a.s.l. having a volume of about 1510 km3. Stepwise and rapid lake drainage is suggested to have taken place within less than 1000 years. The locations of various passpoints controlled the drainage, and when the lake was at its maximum level water spilled southeastwards into the Volga basin. Later, but before the lake water finally drained into the White Sea, water was routed northeastwards into the southeastern part of the Barents Sea. The oldest lake, the White Sea lake, existed around 67–57 ka ago, slightly in conflict with earlier palaeogeographic reconstructions regarding the chronology. The extent of the lake was constrained by, in addition to the Barents Sea ice‐sheet margin in the north, thresholds in the drainage basin. Later, one threshold was eroded and lowered during the LGM lake drainage. Given a lake level of about 115 m a.s.l., a lake area of about 2.5 × 104 km3 and a water volume of about 4800 km3, the lake drainage northwards and into the ocean probably impacted the ocean circulation.  相似文献   

16.
New U–Pb zircon data from metagranites and metavolcanic rocks of the Schist-Graywacke Complex Domain and the Schistose Domain of Galicia Tras-os-Montes Zone from central and NW Iberia contribute to constrain the timing of the Cambro-Ordovician magmatism from Central Iberian and Galicia Tras-os-Montes Zones which occurred between 498 and 462 Ma. The crystallization ages of the metagranites and metavolcanic rocks from the northern Schist-Graywacke Complex Domain are as follows: (a) in west Salamanca, 489 ± 5 Ma for Vitigudino, 486 ± 6 Ma for Fermoselle and 471 ± 7 Ma for Ledesma; (b) in northern Gredos, 498 ± 4 Ma for Castellanos, 492 ± 4 Ma for San Pelayo and 488 ± 3 Ma for Bercimuelle; (c) in Guadarrama, 490 ± 5 Ma for La Estación I, 489 ± 9 Ma for La Cañada, 484 ± 6 Ma for Vegas de Matute (leucocratic), 483 ± 6 Ma for El Cardoso, 482 ± 8 Ma for La Morcuera, 481 ± 9 Ma for Buitrago de Lozoya, 478 ± 7 Ma for La Hoya, 476 ± 5 Ma for Vegas de Matute (melanocratic), 475 ± 5 Ma for Riaza, 473 ± 8 Ma for La Estación II and 462 ± 11 Ma for La Berzosa; and (d) in Toledo, 489 ± 7 Ma for Mohares and 480 ± 8 Ma for Polán. The crystallization ages of the metagranites from the Schistose Domain of Galicia Tras-os-Montes Zone are 497 ± 6 Ma for Laxe, 486 ± 8 Ma for San Mamede, 482 ± 7 Ma for Bangueses, 481 ± 5 Ma for Noia, 480 ± 10 for Rial de Sabucedo, 476 ± 9 Ma for Vilanova, 475 ± 6 Ma for Pontevedra, 470 ± 6 Ma for Cherpa and 462 ± 8 Ma for Bande. This magmatism is characterized by an average isotopic composition of (87Sr/86Sr)485Ma ≈ 0.712, (εNd)485Ma ≈ ?4.1 and (TDM) ≈ 1.62 Ga, and a high zircon inheritance, composed of Ediacaran–Early Cambrian (65 %) and, to a lesser extent, Cryogenian, Tonian, Mesoproterozoic, Orosirian and Archean pre-magmatic cores. Combining our geochronological and isotopic data with others of similar rocks from the European Variscan Belt, it may be deduced that Cambro-Ordovician magmas from this belt were mainly generated by partial melting of Ediacaran–Early Cambrian igneous rocks.  相似文献   

17.
Metagranitic orthogneisses are abundant in the Central Iberian Zone (CIZ). This felsic magmatism has a highly peraluminous composition (A/CNK = 1.07–1.62) defining a typical S-type granite character, common in crustal thickening environments. The studied Spanish Central System (SCS) orthogneisses yield Late Cambrian to Early Ordovician U–Pb zircon ages (496 to 481 Ma), overlapping with the available literature data (mostly from 477 to 500 Ma). These orthogneisses are intrusive into metasedimentary sequences from the northern CIZ that have been recently dated at about 536 Ma. Late Ediacaran inherited zircons are common in the SCS orthogneisses (10% to 75% of the total zircon population). Most inheritance ranges from Neoproterozoic to Late Mesoproterozoic in age (0.52 to 1.25 Ga) and shows marked positive ԐHft values (>+5). This long period of Proterozoic juvenile input is only recognized in the metasedimentary rocks of the Schist–Greywacke Complex, outcropping in the southern CIZ. The proposed linkage between the southern CIZ metasediments (as sources) and the studied orthogneisses is reinforced by their similar Nd isotopic signatures (from − 2.81 to − 4.95) and the highly peraluminous character of the orthogneisses. The intrusion of this felsic magmatism within the northern CIZ, having been generated by melting of the more distal southern CIZ metasediments, together with their recycled crustal origin, suggests crustal thickening of the northern Gondwana margin during a period of flat subduction. The orthogneisses define a large linear S-type magmatic belt cropping out for over 650 km from central Spain to Galicia. The ~ 35 Ma delay between sedimentation and granite intrusion is a typical time interval for crustal thickening models. This thickening stage evolved toward a passive margin setting, allowing the deposition of the siliciclastic Ordovician series which covered the previous terranes. Minor Floian-aged tholeiitic magmatism, giving rise to scarce metabasite outcrops in the SCS, probably postdates thickening and marks this tectonic change in central Iberia.  相似文献   

18.
The Pattern Informatics (PI) technique [Tiampo, K.F., Rundle, J.B., McGinnis, S., Gross, S., Klein, W., 2002. Mean-field threshold systems and phase dynamics: An application to earthquake fault systems, Europhys. Lett., 60, 481–487] is founded on the premise that changes in the seismicity rate are a proxy for changes in the underlying stress. This new approach to the study of seismicity quantifies its local and regional space–time patterns and identifies regions of local quiescence or activation. Here we use a modification of the PI method to quantify localized changes surrounding the epicenters of large earthquakes in California in an attempt to objectively quantify the rupture zones of these upcoming events. We show that this method can be used to forecast the size and magnitude of future earthquakes.  相似文献   

19.
ABSTRACT

The Suyunhe porphyry Mo deposit, located in the West Junggar terrane, is the largest molybdenum deposit found in Xinjiang to date, with a proven reserve of 0.57 Mt. The Suyunhe deposit is associated with Early Permian granitic rocks, which emplaced into the volcano-sedimentary sequences of the Middle Devonian Barluk Formation. Four metallogenic stages are identified in this study. Stage I is marked by the quartz-magnetite-K-feldspar±biotite±pyrite±molybdenite veins, which mainly occurred in the intensively potassic alternation zone and were formed at high temperature (>481°C), high salinity (58.6?65.18 wt.%), and relatively high oxygen fugacity conditions with a fluid system of NaCl-H2O-CO2. Stage II is the main metallogenic stage and develops numerous quartz-molybdenite±pyrite veins associated with muscovite–chlorite alteration, which were formed by immiscible fluids at medium-high temperature (210?427°C), medium-high salinity (43.36?49.90 wt.%), and relatively low oxygen fugacity conditions with the fluid system of NaCl-H2O-CO2-CH4-C2H6. After the main Mo-mineralization, quartz-polymetallic sulphides veins associated with quartz–sericite alteration were formed by fluids at medium-low temperature, low-salinity conditions with the fluid system of NaCl-H2O-CO2 in stage III. The following quartz-polymetallic sulphide veins are quartz-calcite±pyrite veins associated with calcite alteration, which were formed by fluids at low temperature and low-salinity conditions with a fluid system of NaCl-H2O in stage IV.

The δ18O‰ values indicate that the ore fluids of stages I and II are dominated by magmatic water, whereas stages III and IV are dominated by meteoric water. A wide range of δ34S‰ values (?7.1 to 3.4‰) of sulphides between stages I and II indicates that increasing the reducibility plays an important role in molybdenum mineralization. The δ13CCH4 values suggest that CH4 of the ore fluids mainly results from the assimilation–contamination of carbonaceous country rocks, and partly derives from magma. However, the δ13CCO2 values suggest that CO2 of the ore fluids mainly originates from magma, and minor derives from wall-rocks as well as meteoric water.  相似文献   

20.
The Beypazar? granitoid has been studied with respect to multi-radiometric dating and oxygen isotopic geothermometry. Radiometric dating of the granitoid yields zircon U-Pb isochron ages ranging from 72.5 ± 12.6 to 78.6 ± 4.7, and K-Ar ages of 71.4 ± 2.8 to 74.9 ± 2.9 and 59.5 ± 2.2 to 75.4 ± 2.9 Ma for hornblende and biotite, respectively. Oxygen isotope thermometry for the granitoid gives temperatures of 550 ± 25°C to 605 ± 30, 390 ± 15 to 540 ± 25°C, and 481 ± 5 to 675 ± 10°C, for hornblende, biotite, and K-feldspar, respectively, when paired with quartz. The systematic differences among ages according to different techniques used on different minerals are used to reconstruct the cooling history of the granite. The results yield rapid cooling rates of 33.3°C/Ma from 800°C to 550°C, and slow cooling rates of about 15 ± 0.5°C/Ma from 550 to 300°C. Rapid subsolidus cooling between 600°C and 550°C is documented by 40Ar/39Ar ages on amphibole and biotite between 71.4 ± 2.8 and 75.4 ± 2.9 Ma. Younger ages on biotites from two samples (59.5 ± 2.2 and 64.4 ± 2.5) are probably caused by loss of Ar. The reason for this possible Ar loss can be interpreted as slower subsolidus cooling (~375°C) ages. There is an apparent spatial and temporal relationship between the intrusion-cooling of the Beypazar? granitoid and the evolution of the ?zmir–Ankara–Erzincan ocean belonging to the northern Neo-Tethyan ocean domain.  相似文献   

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