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Geotectonics - The dunite–wehrlite–clinopyroxenite–gabbro massif in Eastern Chukotka, a key object for geodynamic reconstructions of the Vel’may terrane, which represents... 相似文献
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Based on the Intensive Field Campaign(IFC-1)data of Boreal Ecosystem-Atmosphere Study(BOREAS).a three-dimensional meso-β scale model is used to simulate the effect of boreal forests onthe lower atmosphere.A fine horizontal resolution of 2 km×2 km is used in order to distinguish thevegetative heterogeneity in the boreal region.A total of 20×25 grid points cover the entire sub-modeling area in BOREAS' South Study Area(SSA).The ecosystem types and their coverage ineach grid square are extracted from the North American Land Cover Characteristics Data Base(NALCCD)generated by the U.S.Geographical Survey(USGS)and the University of Nebraska-Lincoln(UNL).The topography of the study area is taken from the Digital Elevation Map(DEM)of USGS.The model outputs include the components of the energy balance budget within the canopyand at the ground.the turbulence parameters in the atmospheric boundary layer and the wind.temperature and humidity profiles extending up to a height of 1500 m.In addition to the fine timeand spatial step,the unique feature of the present model is the incorporation of both dynamic andbiological effects of the Boreal forest into the model parameterization scheme.The model resultscompare favorably with BOREAS' IFC-1 data in 1994 when the forest was in the luxuriant growingperiod. 相似文献
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S. Siegesmund H. Jelsma J. Becker G. Davies P. Layer E. van Dijk L. Kater M. Vinyu 《International Journal of Earth Sciences》2002,91(1):20-34
In order to constrain the temporal relationship between granite (sensu lato) emplacement and metamorphism, isotope work was carried out on the minerals zircon and apatite (U-Pb), garnet (Pb-Pb) and hornblende (Ar-Ar) from wall rock samples in the Shamva area in Zimbabwe. The area, encompassing parts of the Chinamora and Murehwa batholiths and a wedge-shaped greenstone belt segment in between, is commonly quoted in the literature as an example illustrating pluton emplacement processes and deformational models for the Archean. New U-Pb dating of apatite from a boudinaged pegmatite within mafic schists in the batholith-greenstone contact zone has yielded an age of 2619 +28/-24 Ma. This age is interpreted as the best estimation of the intrusion age of this unit, depending on the assumed closure temperature, and provides an upper age limit for the syntectonic emplacement of the now gneissic granites. Pb-Pb dating of late kinematic garnets in cordierite-bearing rocks within the greenstone belt wall rocks gives an age of 2623NJ Ma. Together, this timing of relatively late, syntectonic plutonism and metamorphic mineral growth at ca. 2.62 Ga compares well with existing zircon crystallization ages for felsic volcanics (2645dž Ma, 2643NJ Ma) and post-tectonic porphyritic monzogranites (2601ᆢ Ma). Ar-Ar hornblende ages for mafic schists from different areas within the greenstone belt wall rocks range between 2621 and 2498 Ma and have been interpreted to indicate mixing between metamorphic ages and cooling ages. The data support a geological model whereby volcanism and sedimentation are associated with an early phase of regional deformation at ca. 2.64 Ga, which may have started earlier and lasted longer, and evolves into the voluminous emplacement of granites (now gneissic granites) in the batholiths at approximately 2.62 Ga. Emplacement of post-tectonic tabular monzogranites takes place at ca. 2.60 Ga. 相似文献
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A global two-dimensional chemistry model is developed to study long-term trends of CH_4 sinceindustrial revolution.The sources of CH_4,CO and NO_x are parameterized as functions of latitudeand time.With two long-term emission scenarios,long-term trends of CH_4 are simulated.The resultshave a good agreement with observation from ice cores.The modeled CH_4 increased from 760 ppbvin 1840 to 1611.9 ppbv in 1991, while the modeled number concentration of tropospheric OHdecreased from 7.17×10~5 cm~(-3)in 1840 to 5.79×10~5 cm~(-3) in 1991.The increase of atmosphericCH_4 can be explained by the increase of emission of CH_4 and build-up because of decrease of OHradicals that remove CH_4 from the atmosphere.The model is also used to simulate the distribution of CH_4.Comparisons between the modelresults and observations show that the model can simulate both latitudinal distribution and seasonalvariation of CH_4 well. 相似文献
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Pedro Oyhantabal Siegfried Siegesmund Klaus Wemmer Robert Frei Paul Layer 《Lithos》2007,98(1-4):141-159
In the southernmost Dom Feliciano Belt of Uruguay, highly fractionated calc-alkaline granites, mildly alkaline granites, shoshonitic volcanics, and peralkaline intrusions and volcanics are spatially and temporal associated with the evolution of shear zones. Four representative magmatic unites of this diverse association were petrographic and geochemically investigated: the Solís de Mataojo Complex, a medium to high K2O calc-alkaline granite with signature typical of mature continental arcs and post-collisional settings; the Maldonado granite, highly fractionated calc-alkaline to alkaline, with characteristics that are transitional between both types of series; the Pan de Azúcar Pluton, with characteristics typical of post-collisional alkaline granites and the Las Flores shoshonitic basalts.
Geochemistry and geotectonic setting point out that slab breakoff was most likely the mechanism associated with the generation of high-K calc-alkaline magmas (Solís de Mataojo and Maldonado) shortly after collision. Extension associated to the formation of molassic basins and emplacement of dolerites and basalt flows with shoshonitic affinity (Las Flores) 15and finally a shift to magmas with alkaline signatures (Pan de Azúcar) simultaneous with a second transpressional phase were probably linked with lithospheric thinning through delamination. This evolution took place between 615 and 575 Ma, according to available data. Contrary to previous proposals, which considered this magmatism to represent the root of a continental magmatic arc, a post-collisional environment, transitional from orogenic to anorogenic, during transcurrent deformation is proposed. 相似文献
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W. U. Reimold G. Q. J. Pybus F. J. Kruger P. W. Layer C. Koeberl 《Journal of African Earth Sciences》2000,31(3-4)
The Anna's Rust Sheet (ARS) and a suite of mineralogically and chemically related intrusions in the core and collar of the Vredefort Dome (in particular, the Vredefort Mafic Complex: VMC) represent a newly recognised type of high Ti gabbro in this central part of the Kaapvaal Craton. This lithology, referred to as the Vredefort Type IV mafic intrusion, is distinguished from chemically similar Type V intrusions (the Karoo dolerites) by the presence of glomeroporphyritic plagioclase and higher Th content and from Type III intrusions (≈ 1600 Ma gabbro) by the lack of cross-cutting pseudotachylitic breccia veinlets. Petrographic features and both major and trace element compositions of all Type IV intrusions are very similar. Based on its Rb-Sr isotope age and character, a gabbroic intrusion from Majuba Colliery (Mpumalanga Province) is also thought to belong to the ARS (Type IV) suite of tholeiitic intrusions. Rb-Sr isotopic analysis resulted in a preferred age of 1052±11 Ma (2ω) for biotite and plagioclase data for ARS, VMC and Majuba samples. The Rb-Sr age for the ARS is further supported by 40Ar-39Ar stepheating ages for plagioclase and pyroxene separates from two ARS and VMC samples, which favour formation of this gabbroic intrusion at ca 1000 Ma ago. These results suggest that an ≈ 120 m thick sheet intrusion may be present throughout a major part of the Vredefort Dome. While Kibaran-age (ca 1–1.2 Ga) alkaline, both mafic and felsic, magmatism, as well as tectonic and hydrothermal activity at that time, have been known in the central Kaapvaal Craton, a widespread tholeiitic magmatic component has now been added to this record. There is a strong likelihood that this magmatic event occurred throughout the southern African subcontinent and perhaps into Antarctica. 相似文献
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Andreas Dietrich Ronald Gutierrez Eric P. Nelson Paul W. Layer 《Mineralium Deposita》2012,47(3):233-249
The San José district is located in the northwest part of the Deseado massif and hosts a number of epithermal Ag–Au quartz
veins of intermediate sulfidation style, including the Huevos Verdes vein system. Veins are hosted by andesitic rocks of the
Bajo Pobre Formation and locally by rhyodacitic pyroclastic rocks of the Chon Aike Formation. New 40Ar/39Ar constraints on the age of host rocks and mineralization define Late Jurassic ages of 151.3 ± 0.7 Ma to 144.7 ± 0.1 Ma for
volcanic rocks of the Bajo Pobre Formation and of 147.6 ± 1.1 Ma for the Chon Aike Formation. Illite ages of the Huevos Verdes
vein system of 140.8 ± 0.2 and 140.5 ± 0.3 Ma are 4 m.y. younger than the volcanic host rock unit. These age dates are among
the youngest reported for Jurassic volcanism in the Deseado massif and correlate well with the regional context of magmatic
and hydrothermal activity. The Huevos Verdes vein system has a strike length of 2,000 m, with several ore shoots along strike.
The vein consists of a pre-ore stage and three main ore stages. Early barren quartz and chalcedony are followed by a mottled
quartz stage of coarse saccharoidal quartz with irregular streaks and discontinuous bands of sulfide-rich material. The banded
quartz–sulfide stage consists of sulfide-rich bands alternating with bands of quartz and bands of chlorite ± illite. Late-stage
sulfide-rich veinlets are associated with kaolinite gangue. Ore minerals are argentite and electrum, together with pyrite,
sphalerite, galena, chalcopyrite, minor bornite, covellite, and ruby silver. Wall rock alteration is characterized by narrow
(< 3 m) halos of illite and illite/smectite next to veins, grading outward into propylitic alteration. Gangue minerals are
dominantly massive quartz intergrown with minor to accessory adularia. Epidote, illite, illite/smectite, and, preferentially
at deeper levels, Fe-chlorite gangue indicate near-neutral pH hydrothermal fluids at temperatures of >220°C. Kaolinite occurring
with the late sulfide-rich veinlet stage indicates pH < 4 and a temperature of <200°C. The Huevos Verdes system has an overall
strike of 325°, dipping on average 65° NE. The orientations of individual ore shoots are controlled by vein strike and intersecting
north-northwest-striking faults. We propose a structural model for the time of mineralization of the San José district, consisting
of a conjugate shear pair of sinistral north-northwest- and dextral west-northwest-striking faults that correspond to R and R′ in the Riedel shear model and that are related to master faults (M) of north-northeast-strike. Veins of 315° strike can
be interpreted as nearly pure extensional fractures (T). Variations in vein strike predict an induced sinistral shear component
for strike directions of >315°, whereas strike directions of <315° are predicted with an induced dextral strike–slip movement.
The components of the structural model appear to be present on a regional scale and are not restricted to the San José district. 相似文献
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Matthew E. Brueseke Jeffrey A. Benowitz Jeffrey M. Trop Kailyn N. Davis Samuel E. Berkelhammer Paul W. Layer Bethany K. Morter 《地学学报》2019,31(1):59-66
The Oligocene to Present Wrangell Volcanic Belt (WVB) extends for ~500 km across south‐central Alaska (USA) into Canada at a volcanic arc‐transform junction. Previously, geochemistry documented mantle wedge and slab‐edge melting in <12 Ma WVB volcanic rocks; new geochemistry shows that the same processes characterized ~18–30 Ma WVB magmatism in Alaska. New 40Ar/39Ar ages demonstrate that WVB magmatism in Alaska initiated at ~30 Ma due to flat‐slab subduction of the Yakutat microplate and that the dextral Totschunda fault was active at this time. Our results, together with prior studies, show that Alaskan WVB magmatism occurred chiefly due to subduction and should be considered a volcanic arc (e.g. the Wrangell Arc). The WVB provides a long‐term geological record of subduction, strike‐slip and magmatism. Slab‐edge upwelling, flat‐slab defocused fluid‐flux and faults acting as magma conduits are likely responsible for the exceptionally large volcanoes and high eruption rates of the Wrangell Arc. 相似文献
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Stefan Löbens Frithjof A. Bense Klaus Wemmer István Dunkl Carlos H. Costa Paul Layer Siegfried Siegesmund 《International Journal of Earth Sciences》2011,100(2-3):671-694
The Sierras Pampeanas in central and north-western Argentina constitute a distinct morphotectonic feature between 27°S and 33°S. The last stage of uplift and deformation in this area are interpreted to be closely related to the Andean flat-slab subduction of the Nazca plate beneath the South American plate. K–Ar fault gouge dating and low-temperature thermochronology along two transects within the Sierra de Comechingones reveal a minimum age for the onset of brittle deformation about 340 Ma, very low exhumation rates since Late Paleozoic time, as well as a total exhumation of about 2.3 km since the Late Cretaceous. New Ar–Ar ages (7.54–1.91 Ma) of volcanic rocks from the San Luis volcanic belt support the eastward propagation of the flat-slab magmatic front, confirming the onset of flat-slab related deformation in this region at 11.2 Ma. Although low-temperature thermochronology does not clearly constrain the signal of the Andean uplift, it is understood that the current structural relief related to the Comechingones range has been achieved after the exhumation of both fault walls (circa 80–70 Ma). 相似文献