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1.
Cleats and fractures in southwestern Indiana coal seams are often filled with authigenic kaolinite and/or calcite. Carbon- and oxygen-stable isotope ratios of kaolinite, calcite, and coalbed CO2 were evaluated in combination with measured values and published estimates of δ18O of coalbed paleowaters that had been present at the time of mineralization. δ18Omineral and δ18Owater values jointly constrain the paleotemperature of mineralization. The isotopic evidence and the thermal and tectonic history of this part of the Illinois Basin led to the conclusion that maximum burial and heat-sterilization of coal seams approximately 272 Ma ago was followed by advective heat redistribution and concurrent precipitation of kaolinite in cleats at a burial depth of < 1600 m at  78 ± 5 °C. Post-Paleozoic uplift, the development of a second generation of cleats, and subsequent precipitation of calcite occurred at shallower burial depth between  500 to  1300 m at a lower temperature of 43 ± 6 °C. The available paleowater in coalbeds was likely ocean water and/or tropical meteoric water with a δ18Owater  − 1.25‰ versus VSMOW. Inoculation of coalbeds with methanogenic CO2-reducing microbes occurred at an even later time, because modern microbially influenced 13C-enriched coalbed CO2 (i.e., the isotopically fractionated residue of microbial CO2 reduction) is out of isotopic equilibrium with 13C-depleted calcite in cleats.  相似文献   

2.
The Mozambique Belt (MB) of the East Africa Orogen contains large areas of granulite-facies migmatitic gneisses with Archaean and Palaeoproterozoic protolith ages and that were recycled during the Neoproterozoic Pan-African orogeny. The study area is situated along the Great Ruaha River and within the Mikumi National Park in central Tanzania where migmatitic gneisses and mafic to intermediate granulites are interlayered with Neoproterozoic granulite-facies migmatitic metapelites. Mineral textures suggest isothermal decompression, with the peak mineral assemblage comprising Grt–Bt–Ky–Kfs–Pl–Qtz ± Phn ± Ti-Oxide ± melt and amphibolite-facies retrograde assemblage Grt–Bt–Sil–Ms–Kfs–Pl–Qtz ± Fe–Ti-Oxide. The near isothermal retrograde overprint is seen in well-developed formation of pseudomorphs after garnet. The HP granulite-facies assemblages record P–T conditions of 13–14 kbar at 760–800 °C. Retrogression and the release of fluids from crystallizing melts occurred at 7 kbar and 650–700 °C. A fluid inclusion study shows three types of fluid inclusion consisting of nearly pure CO2, as well as H2O–NaCl and H2O–CO2 mixtures. We suggest that a immiscible CO2-bearing brine represents the fluid composition during high-grade peak metamorphism, and that the fluid inclusions containing H2O–NaCl or nearly pure CO2 represent trapped fluids from in situ crystallised melt. The results suggest strong isothermal decompression, which is probably related to a fast exhumation after crustal thickening in the central part of the Mozambique Belt in Tanzania.  相似文献   

3.
Coaly source rocks are sufficiently different from marine and lacustrine source rocks in their organic matter characteristics to warrant separate guidelines for their assessment using Rock-Eval pyrolysis. The rank threshold for oil generation is indicated by the increase in BI (S1/TOC) at Rank(Sr)9–10 (Tmax 420–430 °C, Ro 0.55–0.6%), and the threshold for oil expulsion is indicated by the peak in QI ([S1+S2]/TOC) at Rank(Sr)11–12.5 (Tmax 430–440 °C, Ro 0.65–0.85%). The pronounced rank-related increase in HI (S2/TOC) prior to oil expulsion renders the use of immature samples inappropriate for source rock characterisation. A more realistic indication of the petroleum generative potential and oil expulsion efficiency of coaly source rocks can be gained from samples near the onset of expulsion. Alternatively, effective HI′ values (i.e. HIs near the onset of expulsion) can be estimated by translating the measured HIs of immature samples along the maturation pathway defined by the New Zealand (or other defined) Coal Band. Coaly source rocks comprise a continuum of coaly lithologies, including coals, shaly coals and coaly mudstones. Determination of the total genetic potential of coaly source rock sequences is best made using lithology-based samples near the onset of expulsion.  相似文献   

4.
Carbon biogeochemistry of the Betsiboka estuary (north-western Madagascar)   总被引：1，自引：0，他引：1  
Madagascar’s largest estuary (Betsiboka) was sampled along the salinity gradient during the dry season to document the distribution and sources of particulate and dissolved organic carbon (POC, DOC) as well as dissolved inorganic carbon (DIC). The Betsiboka was characterized by a relatively high suspended matter load, and in line with this, low DOC/POC ratios (0.4–2.5). The partial pressure of CO2 (pCO2) was generally above atmospheric equilibrium (270–1530 ppm), but relatively low in comparison to other tropical and subtropical estuaries, resulting in low average CO2 emission to the atmosphere (9.1 ± 14.2 mmol m−2 d−1). Despite the fact that C4 vegetation is reported to cover >80% of the catchment area, stable isotope data on DOC and POC suggest that C4 derived material comprises only 30% of both pools in the freshwater zone, increasing to 60–70% and 50–60%, respectively, in the oligohaline zone due to additional lateral inputs. Sediments from intertidal mangroves in the estuary showed low organic carbon concentrations (<1%) and δ13C values (average −19.8‰) consistent with important inputs of riverine imported C4 material. This contribution was reflected in δ13C signatures of bacterial phospholipid derived fatty acids (i + a15:0), suggesting the potential importance of terrestrial organic matter sources for mineralization and secondary production in coastal ecosystems.  相似文献   

5.
Ophiolites are key components of the Neoproterozoic Arabian–Nubian Shield (ANS). Understanding when they formed and were emplaced is crucial for understanding the evolution of the ANS because their ages tell when seafloor spreading and terrane accretion occurred. The Yanbu–Onib–Sol Hamed–Gerf–Allaqi–Heiani (YOSHGAH) suture and ophiolite belt can be traced  600 km across the Nubian and Arabian shields. We report five new SHRIMP U–Pb zircon ages from igneous rocks along the Allaqi segment of the YOSHGAH suture in southernmost Egypt and use these data in conjunction with other age constraints to evaluate YOSHGAH suture evolution. Ophiolitic layered gabbro gave a concordia age of 730 ± 6 Ma, and a metadacite from overlying arc-type metavolcanic rocks yielded a weighted mean 206Pb/238U age of 733 ± 7 Ma, indicating ophiolite formation at  730 Ma. Ophiolite emplacement is also constrained by intrusive bodies: a gabbro yielded a concordia age of 697 ± 5 Ma, and a quartz-diorite yielded a concordia age of 709 ± 4 Ma. Cessation of deformation is constrained by syn- to post-tectonic granite with a concordia age of 629 ± 5 Ma. These new data, combined with published zircon ages for ophiolites and stitching plutons from the YOSHGAH suture zone, suggest a 2-stage evolution for the YOSHGAH ophiolite belt ( 810–780 Ma and  730–750 Ma) and indicate that accretion between the Gabgaba–Gebeit–Hijaz terranes to the south and the SE Desert–Midyan terranes to the north occurred as early as 730 Ma and no later than 709 ± 4 Ma.  相似文献   

6.
Application of hornblende thermobarometry and fluid inclusion studies to the Palaeoproterozoic (1.7 Ga) basement rocks from Maddhapara, NW Bangladesh, provide information on the pressure and temperature (P–T) conditions of crystallization, the emplacement depth and composition of magmatic fluid. The basement rocks are predominantly diorite or quartz diorite with a mineral assemblage of plagioclase, hornblende, biotite, quartz, K-feldspar, titanite, and secondary epidote and chlorite. The calculated P–T conditions of the dioritic rocks are 680–725 °C and 4.9–6.4 kbar, which probably correspond to crystallization conditions. Fluid inclusion studies suggest that low- to medium-salinity (0–6.4 wt.% NaCleq) H2O-rich fluids are trapped during the crystallization of quartz and plagioclase. The isochore range calculated for primary aqueous inclusions is consistent with the P–T condition obtained by geothermobarometry. The basement rocks likely crystallized at a depth of 17–22 km, with a minimum average exhumation rate of 12–15 m/Ma during Palaeoproterozoic to Lopingian time. Such slow exhumation indicates low relief continental shield surface during this period.  相似文献   

7.
The Iberian Peninsula and the Maghreb experience moderate earthquake activity and oblique,  NW–SE convergence between Africa and Eurasia at a rate of  5 mm/yr. Coeval extension in the Alboran Basin and a N35°E trending band of active, left-lateral shear deformation in the Alboran–Betic region are not straightforward to understand in the context of regional shortening, and evidence complexity of deformation at the plate contact. We estimate 86 seismic moment tensors (MW 3.3 to 6.9) from time domain inversion of near-regional waveforms in an intermediate period band. Those and previous moment tensors are used to describe regional faulting style and calculate average stress tensors. The solutions associated to the Trans-Alboran shear zone show predominantly strike-slip faulting, and indicate a clockwise rotation of the largest principal stress orientation compared to the regional convergence direction (σ1 at N350°E). At the N-Algerian and SW-Iberian margins, reverse faulting solutions dominate, corresponding to N350°E and N310°E compression, respectively. Over most of the Betic range and intraplate Iberia, we observe predominately normal faulting, and WSW–ENE extension (σ3 at N240°E). From GPS observations we estimate that more than 3 mm/yr of African (Nubian)–Eurasian plate convergence are currently accommodated at the N-Algerian margin,  2 mm/yr in the Moroccan Atlas, and  2 mm/yr at the SW-Iberian margin. 2 mm/yr is a reasonable estimate for convergence within the Alboran region, while Alboran extension can be quantified as  2.5 mm/yr along the stretching direction (N240°E). Superposition of both motions explains the observed left-lateral transtensional regime in the Trans-Alboran shear zone. Two potential driving mechanisms of differential motion of the Alboran–Betic–Gibraltar domain may coexist in the region: a secondary stress source other than plate convergence, related to regional-scale dynamic processes in the upper mantle of the Alboran region, as well as drag from the continental-scale motion of the Nubian plate along the southern limit of the region. In the Atlantic Ocean, the  3.5 mm/yr, westward motion of the Gibraltar Arc relative to intraplate Iberia can be accommodated at the transpressive SW-Iberian margin, while available GPS observations do not support an active subduction process in this area.  相似文献   

8.
O. Nebel  K. Mezger   《Precambrian Research》2008,164(3-4):227-232
Dating low temperature events such as magmatic cooling or (hydro-)thermal surges in Archean and Proterozoic terranes is crucial in defining cratonal thermal stabilization after episodic continental growth during the Archean and Early Proterozoic. Rubidium–Sr chronology is potentially a powerful tool in this regard because of its low closure temperature, i.e., <400 °C in most minerals, but has until now been hampered by its relatively low precision compared to high-temperature chronometers. Consequently, Rb–Sr age investigations have so far failed to provide high-precision age constraints on the cooling of rocks older than 2 Ga. Here, it is demonstrated that internal Rb–Sr microchrons can yield important, high-precision age constraints on the cooling history of Archean intrusions. After careful mineral selection and chemical treatment, a Rb–Sr age of 2543.0 ± 4.4 Ma was obtained from the Archean Great Dyke, Zimbabwe Craton, in contrast to the intrusion age of 2575.8 ± 1 Ma, yielding an ambient average cooling of 5 ± 2 °C/Ma. The non-disturbed magmatic Rb–Sr cooling age of the Great Dyke marks the final stage of Zimbabwe craton stabilization and that the greater craton area did not experience any intensive later reheating event during metamorphic or tectonic events.  相似文献   

9.
The crystalline terrane of the Tongbai–Dabie region, central China, comprising the Earth's largest ultrahigh-pressure (UHP) exposure was formed during Triassic collision between the Sino–Korean and Yangtze cratons. New apatite fission-track (AFT) data presented here from the UHP terrane, extends over a significantly greater area than reported in previous studies, and includes the (eastern) Dabie, the Hong'an (northwestern Dabie) and Tongbai regions. The new data yield ages ranging from 44 ± 3 to 142 ± 36 Ma and mean track lengths between 10 and 14.4 μm. Thermal history models based on the AFT data taken together with published 40Ar/39Ar, K–Ar, apatite and zircon (U–Th)/He and U–Pb data, exhibit a three-stage cooling pattern that is similar across the study region, commencing with an Early Cretaceous rapid cooling event, followed by a period of relative thermal stability during which rocks remained at temperatures within the AFT partial annealing zone (60–110 °C) and ending with a possible renewed phase of accelerated cooling during Pliocene to Recent time. The first cooling phase followed large-scale transtensional deformation between 140 and 110 Ma and is related to Early Cretaceous eastward tectonic escape and Pacific back arc extension. Between this phase and the subsequent slow cooling phase, a transition period from 120 to 80 Ma (to 70 to 45 Ma along the Tan–Lu fault) was characterised by a relatively low cooling rate (3–5 °C/Ma). This transition is likely related to a tectonic response associated with the mid-Cretaceous subduction of the Izanagi–Pacific plate as well as lithospheric extension and thinning in eastern Asia. The present regional AFT age pattern is therefore basically controlled by the Early Cretaceous rapid cooling event, but finally shaped through active Cenozoic faulting. Following the transition phase the subsequent slow cooling phase pattern implies a net reduction in horizontal compressional stress corresponding to increased extension rates along the continental margin due to the decrease in plate convergence. Modelling of the AFT data suggests a possible Pliocene–Recent cooling episode, which may be supported by increased rates of sedimentation observed in adjacent basins. This cooling phase may be interpreted as a response to the far-field effects of the frontal India–Eurasia collision to the west. Approximate estimates suggest that the total amount of post 120 Ma denudation across the UHP orogen ranged from 2.4 to 13.2 km for different tectonic blocks and ranged from 0.8 to 9.7 km during the Cretaceous to between 1.7 and 3.8 km during the Cenozoic.  相似文献   

10.
Low concentrations of Th and Fe in the Yamato (Y)-86032 bulk meteorite support earlier suggestions that Y-86032 comes from a region of the moon far distant from the Procellarum KREEP Terrain (PKT), probably from the lunar farside. 39Ar–40Ar, Rb–Sr, Sm–Nd, and Sm-isotopic studies characterize the chronology of Y-86032 and its precursors in the mega regolith. One of the rock types present in a light gray breccia lithology is an anorthosite characterized by plagioclase with An 93, i.e., more sodic than lunar FANs, but with very low 87Rb/86Sr and 87Sr/86Sr similar to those of FANs. (FAN stands for Ferroan Anorthosite). This “An93 anorthosite” has Nd-isotopic systematics similar to those of nearside norites. A FAN-like “An97 anorthosite” is present in a second light-colored feldspathic breccia clast and has a more negative εNd value consistent with residence in a LREE-enriched environment as would be provided by an early plagioclase flotation crust on the Lunar Magma Ocean (LMO). This result contrasts with generally positive values of εNd for Apollo 16 FANs suggesting the possibility of assymetric development of the LMO. Other possible explanations for the dichotomy in εNd values are advanced in the text. The Y-86032 protolith formed at least 4.43 ± 0.03 Ga ago as determined from a Sm–Nd isochron for mineral fragments from the breccia clast composed predominantly of An93 anorthosite and a second clast of more varied composition. We interpret the mineral fragments as being predominatly from a cogenetic rock suite. An 39Ar–40Ar age of 4.36–4.41 ± 0.035 Ga for a third clast composed predominantly of An97 anorthosite supports an old age for the protolith. Initial 143Nd/144Nd in that clast was −0.64 ± 0.13 ε-units below 143Nd/144Nd in reservoirs having chondritic Sm/Nd ratios, consistent with prior fractionation of mafic cumulates from the LMO. A maximum in the 39Ar–40Ar age spectrum of 4.23 ± 0.03 Ga for a second sample of the same feldspathic breccia clast probably reflects some diffusive 40Ar loss. Lack of solar wind and lunar atmosphere implanted Ar in the light gray breccia clast allows determination of an 39Ar/40Ar age of 4.10 ± 0.02 Ga, which is interpreted as the time of initial brecciation of this litholgy. After correction for implanted lunar atmosphere 40Ar, impact melt and dark regolith clasts give Ar ages of 3.8 ± 0.1 Ga implying melt formation and final breccia assembly 3.8 Ga ago. Some breccia lithologies were exposed to thermal neutron fluences of 2 × 1015 n/cm2, only about 1% of the fluence experienced by some other lunar highlands meteorites. Other lithologies experienced neutron fluences of 1 × 1015 n/cm2. Thus, Y-86032 spent most of the time following final brecciation deeply buried in the megaregolith. The neutron fluence data are consistent with cosmogenic 38Arcos cosmic ray exposure ages of 10 Ma. Variations among differing lithologies in the amount of several regolith exposure indicators, including cosmogenic noble gas abundances, neutron capture induced variations in Sm isotopic abundances, and Ir contents, are consistent with a period of early (>3.8 Ga ago) lunar regolith exposure, subsequent deep burial at >5 m depth, and ejection from the moon 7–10 Ma ago.  相似文献   

11.
Dikes of the eastern Troodos ophiolite of Cyprus intruded at slow ocean-spreading axes with dips ranging up to 15° from vertical and with bimodal strikes (now NE–SW and N–S due to post-88 Ma sinistral microplate rotation). Varied dike orientations may represent local stress fields during dike-crack propagation but do not influence the spatial-distributions or orientation-distributions of dikes' magnetic fabrics, nor of their palaeomagnetic signals. Anisotropy of magnetic susceptibility (AMS) integrates mineral orientation-distributions from each of 1289 specimens sampled from dikes at 356 sites over 400 km2 in the eastern Troodos ophiolite of Cyprus. In 90% of dikes, AMS fabrics define a foliation (kMAX–kINT) parallel to dike walls and a lineation (kMAX) that varies regionally and systematically. Magma-flow alignment of accessory magnetite controls the AMS with a subordinate contribution from the mafic silicate matrix that is reduced in anisotropy by sea-floor metamorphism. Titanomagnetite has less influence on anisotropy. Occasionally, intermediate and minimum susceptibility axes are switched so as to be incompatible with the kinematically reasonable flow plane but maximum susceptibility (kMAX) still defines the magmatic flow axis. Such blended subfabrics of kinematically compatible mafic-silicate and misaligned multidomain magnetite subfabrics; are rare. Areas of steep magma flow (kMAX plunge ≥ 70°) and of shallow magma-flow alternate in a systematic and gradual spatial pattern. Foci of steep flow were spaced 4 km parallel to the spreading axes and 6 km perpendicular to the spreading axes. Ridge-parallel separation of steep flow suggest the spacing of magma-feeders to the dikes whereas ridge-perpendicular spacing of 6 km at a spreading rate of 50 mm/a implies the magma sources may have been active for 240 Ka. The magma feeders feeding dikes may have been ≤ 2 km in diameter. Stable paleomagnetic vectors, in some cases verified by reversal tests, are retained by magnetite and titanomagnetite. In all specimens, the stable components were isolated by three cycles of low-temperature demagnetization (LTD) followed by ≥ 10 steps of incremental thermal demagnetization (TD). 47% of primary A-components [338.2 /+ 57.2 n = 207, α95 = 3.9; mean TUB = 397 ± 8 °C] are overprinted by a B-component [341.4 /+ 63.5, n = 96, α95 = 8.7; mean TUB = 182 ± 11 °C]. A- and B-components are ubiquitous and shared equally by the N–S and NE–SW striking dikes. A-component unblocking temperatures (TUB) are zoned subparallel to the fossil spreading axis. Their spatial pattern is consistent with chemical remagnetization at some certain off-axis distance determined by sea-floor spreading. A-components indicate less microplate rotation and more northerly palaeolatitudes that are consistent with metamorphic remagnetization after some spreading from the ridge-axis. Thus, their magnetizations are younger than those of the overlying volcanic sequence for which ChRMs are commonly reported as 274 /+ 33 (88 Ma).  相似文献   

12.
Jun-Hong Zhao  Mei-Fu Zhou 《Lithos》2008,104(1-4):231-248
Numerous Neoproterozoic felsic and mafic–ultramafic intrusions occur in the Hannan region at the northern margin of the Yangtze Block. Among these, the Wudumen and Erliba plutons consist of granodiorites and have SHRIMP zircon U–Pb ages of  735 Ma. The rocks have high K2O (0.8–3.6 wt.%) and Na2O (4.4–6.4 wt.%) and low MgO (0.4–1.7 wt.%). They also have high Sr/Y (32–209) and (La/Yb)n ratios (4.4–38.6). Their εNd values range from − 0.41 to − 0.92 and zircon initial 176Hf/177Hf ratios from 0.282353 to 0.282581. These geochemical features are similar to those of adakitic rocks produced by partial melting of a thickened lower crust. Our new analytical results, combined with the occurrence of voluminous arc-related mafic–ultramafic intrusions emplaced before 740 Ma, lead us to propose that the crustal evolution in the northern margin of the Yangtze Block during Neoproterozoic involved: (1) rapid crustal growth and thickening by underplating of mafic magmas from the mantle which was modified by materials coming from the subducting oceanic slab from  1.0 to  0.74 Ga, and (2) partial melting of the thickened lower crust due to a thermal anomaly induced by upwelling of asthenosphere through an oceanic slab window, producing the  735 Ma adakitic Wudumen and Erliba plutons. Our model suggests that the crustal thickness was more than 50 km at the northern margin of the Yangtze Block at  735 Ma, and rule out the possibility of a mantle plume impact causing the > 735 Ma magmatism in the region.  相似文献   

13.
Mineralizing fluids at the San Martín skarn show an evolution characterized by prograde and retrograde associations. The prograde mineral associations consist of (1) a massive garnet zone, (2) a tremolite ± garnet zone, and (3) a late association of quartz, sphalerite, calcite and fluorite lining the vugs in the garnet zone. The fluids of the prograde associations exhibit decreasing temperatures of homogenization (Th) and variable salinities. The fluids of the massive garnet zone have salinities of 36 wt.% NaCl equiv. and Th of 645 to 570 °C, corresponding to pressures of 1055 bar. At the tremolite ± garnet zone, Th range from 438 to 354 °C. In the late association at the endoskarn, the following evolution can be drawn: (a) salinities of 50 to 42 wt.% NaCl equiv., and Th of 455 to 346 °C in quartz, (b) salinities of 46 wt.% NaCl equiv., and Th of 415 to 410 °C in sphalerite, (c) salinities of 50 to 37 wt.% NaCl equiv., and Th of 479 to 310 °C in calcite, (d) salinities of 33 to 28 wt.% NaCl equiv. and of 24 to 22 wt.% KCl in fluorite, and (e) two types of fluids with salinities of 2 and 39 wt.% NaCl equiv. and Th 344 and 300 °C, respectively, in later saccharoidal quartz segregations. The retrograde mineral associations comprise pervasive propylitic alteration to carbonization, and mantos with sulfides. Fluids in epidote have salinities of 7.6 wt.% NaCl equiv. and Th of 287 to 252 °C, and in calcite have salinities of 9.2 to 1 wt.% NaCl equiv. and Th of 188 to 112 °C. Fluids in the sulfide assemblages in the mantos have salinities of 8 to 3 wt.% NaCl equiv. and Th 300 °C, with corresponding pressures of 94 bar. Fluids in late epithermal veins close to the intrusive body have salinities of 10 to 5 wt.% NaCl equiv. and Th of 275 to 200 °C, and distal veins show salinities of 2 to 1 wt.% NaCl equiv. and Th of 160 °C.  相似文献   

14.
Despite the common belief that AuI complexes with hydrogen sulfide ligands (H2S/HS−) are the major carriers of gold in natural hydrothermal fluids, their identity, structure and stability are still subjects of debate. Here we present the first in situ measurement, using X-ray absorption fine structure (XAFS) spectroscopy, of the stability and structure of aqueous AuI–S complexes at temperatures and pressures (T–P) typical of natural sulfur-rich ore-forming fluids. The solubility of native gold and the local atomic structure around the dissolved metal in S–NaOH–Na2SO4–H2SO4 aqueous solutions were characterized at temperatures 200–450 °C and pressures 300–600 bar using an X-ray cell that allows simultaneous measurement of the absolute concentration of the absorbing atom (Au) and its local atomic environment in the fluid phase. Structural and solubility data obtained from XAFS spectra, combined with quantum-chemical calculations of species geometries, show that gold bis(hydrogensulfide) Au(HS)2− is the dominant Au species in neutral-to-basic solutions (5.5  pH  8.5; H2O–S–NaOH) over a wide range of sulfur concentrations (0.2 < ΣS < 3.6 mol/kg), in agreement with previous solubility studies. Our results provide the first direct determination of this species structure, in which two sulfur atoms are in a linear geometry around AuI at an average distance of 2.29 ± 0.01 Å. At acidic conditions (1.5  pH  5.0; H2O–S–Na2SO4–H2SO4), the Au atomic environment determined by XAFS is similar to that in neutral solutions. These findings, together with measured high Au solubilities, are inconsistent with the predominance of the gold hydrogensulfide Au(HS)0 complex suggested by recent solubility studies. Our spectroscopic data and quantum-chemical calculations imply the formation of species composed of linear S–Au–S moieties, like the neutral [H2S–Au–SH] complex. This species may account for the elevated Au solubilities in acidic fluids and vapors with H2S concentrations higher than 0.1–0.2 mol/kg. However, because of the complex sulfur speciation in acidic solutions that involves sulfite, thiosulfate and polysulfide species, the formation of AuI complexes with these ligands (e.g., AuHS(SO2)0, Au(HS2O3)2−, Au(HSn)2−) cannot be ruled out. The existence of such species may significantly enhance Au transport by high T–P acidic ore-forming fluids and vapors, responsible for the formation of a major part of the gold resources on Earth.  相似文献   

15.
J.D.A. Piper   《Tectonophysics》2007,432(1-4):133-157
The Southern Uplands terrane is an Ordovician–Silurian back-arc/foreland basin emplaced at the northern margin of the Iapetus Ocean and intruded by granite complexes including Loch Doon (408.3 ± 1.5 Ma) during Early Devonian times. Protracted cooling of this 130 km3 intrusion recorded magnetic remanence comprising a predominant (‘A’) magnetisation linked to initial cooling with dual polarity and mean direction D / I = 237 / 64° (α95 = 4°, palaeopole at 316°E, 21°N). Subsidiary magnetisations include Mesozoic remanence correlating with extensional tectonism in the adjoining Irish Sea Basin (‘B’, D / I = 234/− 59°) and minority populations (‘C’, D / I = 106/− 2° and ‘D’, D / I = 199/1°) recording emplacement of younger ( 395 Ma) granites in adjoining terranes and the Variscan orogenic event. The ‘A’ directions have an arcuate distribution identifying anticlockwise rotation during cooling. A comparable rotation is identified in the Orthotectonic Caledonides to the north and the Paratectonic Caledonides to the south following closure of Iapetus. Continental motion from midsoutherly latitudes ( 40°S) at 408 Ma to equatorial palaeolatitudes by  395 Ma is identified and implies minimum rates of continental movement between 430 and 390 Ma of 30–70 cm/year, more than double maximum rates induced by plate forces and interpreted as a signature of true polar wander. Silurian–Devonian palaeomagnetic data from the British–Scandinavian Caledonides define a 430–385 Ma closed loop comparable to the distributed contemporaneous palaeomagnetic poles from Gondwana. They reconcile pre-430 Ma and post-380 Ma APW from this supercontinent and show that Laurentia–Baltica–Avalonia lay to the west of South America with a relict Rheic Ocean opening to the north which closed to produce Variscan orogeny by a combination of pivotal closure and right lateral transpression.  相似文献   

16.
It has been generally accepted that the South China Block was formed through amalgamation of the Yangtze and Cathaysia Blocks during the Proterozoic Sibaoan orogenesis, but the timing and kinematics of the Sibao orogeny are still not well constrained. We report here SHRIMP U–Pb zircon geochronological and geochemical data for the Taohong and Xiqiu tonalite–granodiorite stocks from northeastern Zhejiang, southeastern margin of the Yangtze Block. Our data demonstrate that these rocks, dated at 913 ± 15 Ma and 905 ± 14 Ma, are typical amphibole-rich calc-alkaline granitoids formed in an active continental margin. Combined with previously reported isotopic dates for the  1.0 Ga ophiolites and  0.97 Ga adakitic rocks from northeastern Jiangxi, the timing of the Sibao orogenesis is thus believed to be between  1.0 and  0.9 Ga in its eastern segment. It is noted that the Sibao orogeny in South China is in general contemporaneous with some other early Neoproterozoic (1.0–0.9 Ga) orogenic belts such as the Eastern Ghats Belt of India and the Rayner Province in East Antarctica, indicating that the assembly of Rodinia was not finally completed until  0.9 Ga.  相似文献   

17.
The release of irradiation-produced noble gas isotopes (38ArCl, 80KrBr, 128XeI and 39ArK) during in vacuo crushing scapolite has been investigated and is compared to quartz. Three thousand crushing strokes released 98% of fluid inclusion-hosted noble gas from quartz. In comparison, 3000 crushing strokes released only 4% of the lattice-hosted 38ArCl from a scapolite gem. In vacuo crushing released lattice Ar preferentially relative to lattice Kr or Xe and prolonged crushing released 88% of the lattice-hosted noble gas in 96,000 crushing strokes. We suggest fast diffusion pathways generated by crushing are an important noble gas release mechanism and we demonstrate two applications of prolonged in vacuo crushing on irradiated scapolite.Firstly, scapolite molar Br/Cl and I/Cl values are shown to vary over a similar range as crustal fluids. The Cl-rich scapolite gem from Hunza, Pakistan has Br/Cl of 0.5–0.6 × 10−3 and I/Cl values of 0.3–2 × 10−6, that are similar to fluids that have dissolved evaporites. In contrast, three out of four skarn-related scapolites from the Canadian Grenville Province have molar Br/Cl values of 1.5–2.4 × 10−3, and I/Cl values of 11–24 × 10−6, that are broadly consistent with skarn formation by magmatic fluids. The fourth Grenvillian scapolite, with only 0.02 wt% Cl, has an exceptionally elevated molar Br/Cl value of up to 54 × 10−3 and I/Cl of 284 × 10−6. It is unclear if these values reflect the composition of fluids formed during metamorphism or preferential incorporation of Br and I in Cl-poor meionitic scapolite.Secondly, the Grenvillian scapolites give plateau ages of between 830 Ma and 400 Ma. The oldest ages post-date regional skarn formation by 200 Myr, but are similar to feldspar cooling ages in the Province. The age variation in these samples is attributed to a combination of factors including variable thermal history and the presence of mineral sub-grains in some of the samples. These sub-grains control the release of 39ArK, 38ArCl and 40Ar* during in vacuo crushing as well as the samples 40Ar* retentivity in nature. Scapolite is suggested as a possible analogue for K-feldspar in thermochronologic studies.  相似文献   

18.
Frictional sliding of gabbro gouge under hydrothermal conditions   总被引：12，自引：0，他引：12  
We investigated the frictional sliding behaviour of gabbro gouge under hydrothermal conditions. Experiments were performed on 1-mm-thick gabbro gouge sandwiched between country rock pieces (with gouge inclined 35° to the sample axis) in a triaxial testing system with argon gas as the confining medium. In the first series, experiments were conducted under effective normal stresses of 200 MPa and 300 MPa respectively, with pore pressure of 10 MPa. For temperature over 400 °C, pore pressure of 30 MPa was also applied to implement supercritical water conditions. At temperatures up to 615 °C, slip rate steps ranging from 0.0488 μm/s to 1.22 μm/s were applied to obtain the rate dependence of friction.At 200 MPa effective normal stress and a pore pressure of 10 MPa, the steady state rate dependence a–b shows velocity-weakening behaviour for temperatures between  200 and  310 °C. The higher temperature limit for velocity-weakening behaviour to occur extends up to  510 °C under supercritical water conditions with a pore pressure of 30 MPa. For the limited sliding distance in our experiments, only velocity-strengthening behaviour occurred at 300 MPa effective normal stress. Considering the limited displacement (< 3.5 mm), velocity-weakening behaviour may not be excluded in the high effective normal stress case for temperature below  510 °C.The coefficient of friction shows an increasing trend with increasing temperature in the low temperature range. The cut-off temperatures for the increasing trend are  250 °C and  440 °C, respectively for the 200 MPa and 300 MPa effective normal stress cases. Above the cut-off temperatures, the coefficient of friction at 1.83 mm permanent displacement varies around an average of 0.73, which is identical to the average for the oven-dried case [He, C., Yao, W., Wang, Z., Zhou, Y., 2006. Strength and stability of frictional sliding of gabbro gouge at elevated temperatures. Tectonophysics 427, 217–229, doi:10.1016/j.tecto.2006.05.023]. Together with the small value of rate dependence (a–b < 0.0073) for the whole temperature range, these results indicate the absence of fluid-assisted creep.With the result of our experiments as a constraint on strength of frictional sliding, comparison between converted strength for strike–slip faults and creep strength of gabbro-like rocks implies fracturing and faulting behaviours in the lower crust of a cool area (Zhangbei) in North China.  相似文献   

19.
Integration of on-land and offshore geomorphological and structural investigations coupled to extensive radiometric dating of co-seismically uplifted Holocene beaches allows characterization of the geometry, kinematics and seismotectonics of the Scilla Fault, which borders the eastern side of the Messina Strait in Calabria, Southern Italy. This region has been struck by destructive historical earthquakes, but knowledge of geologically-based source parameters for active faults is relatively poor, particularly for those running mostly offshore, as the Scilla Fault does. The  30 km-long normal fault may be divided into three segments of  10 km individual length, with the central and southern segments split in at least two strands. The central and northern segments are submerged, and in this area marine geophysical data indicate a youthful morphology and locally evidence for active faulting. The on-land strand of the western segment displaces marine terraces of the last interglacial (124 to 83 ka), but seismic reflection profiles suggest a full Quaternary activity. Structural data collected on bedrock faults exposed along the on-land segment provide evidence for normal slip and  NW-SE extension, which is consistent with focal mechanisms of large earthquakes and GPS velocity fields in the region. Detailed mapping of raised Holocene marine deposits exposed at the coastline straddling of the northern and central segments supplies evidence for two co-seismic displacements at  1.9 and  3.5 ka, and a possible previous event at  5 ka. Co-seismic displacements show a consistent site value and pattern of along-strike variation, suggestive of characteristic-type behaviour for the fault. The  1.5–2.0 m average co-seismic slips during these events document Me  6.9–7.0 earthquakes with  1.6–1.7 ka recurrence time. Because hanging-wall subsidence cannot be included into slip magnitude computation, these slips reflect footwall uplift, and represent minimum average estimates. The palaeoseismological record based on the palaeo-shorelines suggests that the last rupture on the Scilla Fault during the February 6, 1783 Mw = 5.9–6.3 earthquake was at the expected time but it may have not entirely released the loaded stress since the last great event at  1.9 ka. Comparison of the estimated co-seismic extension rate based on the Holocene shoreline record with available GPS velocities indicates that the Scilla Fault accounts for at least  15–20% of the contemporary geodetic extension across the Messina Strait.  相似文献   

20.
Late- to post-magmatic deformation in slightly diachronous contiguous intrusions of the north-western Adamello batholith (Southern Alps, Italy) is recorded as, from oldest to youngest: (i) joints, (ii) solid-state ductile shear zones, (iii) faults associated with epidote-K-feldspar veins and (iv) zeolite veins and faults. Structures (ii) to (iv) are localized on the pervasive precursory network of joints (i), which developed during the earliest stages of pluton cooling. High temperature ( 500 °C), ductile overprinting of joints produced lineations, defined by aligned biotite and hornblende, on the joint surfaces and highly localized mylonites. The main phase of faulting, producing cataclasites and pseudotachylytes, occurred at  250 °C and was associated with extensive fluid infiltration. Cataclasites and pseudotachylytes are clustered along different E–W-striking dextral strike-slip fault zones correlated with the activity of the Tonale fault, a major tectonic structure that bounds the Adamello batholith to the north. Ductile deformation and cataclastic/veining episodes occurred at P = 0.25–0.3 GPa during rapid cooling of the batholith to the ambient temperatures ( 250 °C) that preceded the exhumation of the batholith. Timing of the sequence of deformation can be constrained by 39Ar–40Ar ages of  30 Ma on pseudotachylytes and various existing mineral ages. In the whole composite Adamello batholith, multiple magma pulses were intruded over the time span 42–30 Ma and each intrusive body shows the same ductile-to-brittle structural sequence localized on the early joint sets. This deformation sequence of the Adamello might be typical of intrusions undergoing cooling at depths close to the brittle–ductile transition.  相似文献   

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