用于新生代定年的Ar-Ar法标准样品候选样品初测结果

Ar Ar法定年的特点是必须要有一套年龄从小到大的标准样。迄今用于新生代定年的国内标准样极少。为满足新生代矿物Ar Ar法定年的需要,初选了一个标准样候选者BT 1透长石。样品总重366g,粒级6080目,纯度100%,缩分为100瓶,每瓶3 66g。Ar Ar阶段加热法初测结果为:全部12个阶段给出的总气体年龄为30 8±0 9Ma,412阶段视年龄十分接近,年龄谱平坦,对应的39Ar析出量达96%,坪年龄为29 6±0 4Ma,等时年龄为29 6±0 6Ma,MSWD=1 01。40Ar/36Ar初始值为293 6±3 9,与尼尔值295 5相当。重复测定结果为:全部气体年龄为31 0±0 9Ma(全部9个阶段),坪年龄为29 5±0 4Ma,等时年龄为29 4±0 6Ma,MSWD=1 94,40Ar/36Ar初始值为282 2±6 3。这些结果表明,BT 1透长石不含过剩氩,作为新生代定年的Ar Ar法标准样品候选者是较为理想的。     

Paleozoic serpentinite-enclosed chromitites from Tehuitzingo (Acatlán Complex, southern Mexico): a petrological and mineralogical study

The serpentinites and associated chromitite bodies in Tehuitzingo (Acatlán Complex, southern Mexico) are in close relationship with eclogitic rocks enclosed within a metasedimentary sequence, suggesting that the serpentinites, chromitites and eclogitic rocks underwent a common metamorphic history.Primary chromites from the chromitite bodies at Tehuitzingo are of refractory-grade (Al-rich) and have a chemical composition similar to that expected to be found in an ophiolitic environment. The chromite grains in chromitites and serpentinites are systematically altered to ‘ferritchromite’. The alteration trend is usually characterized by a decrease in the Al, Mg and Cr contents coupled by an increase in Fe³⁺ and Fe²⁺.The Tehutizingo chromitites have low Platinum Group Elements (PGE) contents, ranging from 102 to 303 ppb. The chondrite-normalized PGE patterns are characterized by an enrichment in the Ir-subgroup elements (IPGE=Os, Ir, Ru) relative to the Pd-subgroup elements (PPGE=Rh, Pt, Pd). In addition, all chromitite samples display a negative slope from Ru to Pd [(Os+Ir+Ru)/(Pt+Pd)=4.78−14.13]. These patterns, coupled with absolute PGE abundances, are typical of ophiolitic chromitites elsewhere. Moreover, all the analyzed samples exhibit chondrite-normalized PGE patterns similar to those found for non-metamorphosed ophiolitic chromitites. Thus, the PGE distribution patterns found in the Tehuitzingo chromitites have not been significantly affected by any subsequent Paleozoic high-pressure (eclogite facies) metamorphic event.The chondrite-normalized PGE patterns of the enclosing serpentinites also indicate that the PGE distribution in the residual mantle peridotites exposed in Tehuitzingo was unaffected by high-pressure metamorphism, or subsequent hydrothermal alteration since the serpentinites show a similar pattern to that of partially serpentinized peridotites present in mantle sequences of non-metamorphosed ophiolites.Our main conclusion is that the chromitites and serpentinites from Tehuizingo experienced no significant redistribution (or concentration) of PGE during the serpentinization process or the high-pressure metamorphic path, or during subsequent alteration processes. If any PGE mobilization occurred, it was restricted to individual chromitite bodies without changing the bulk-rock PGE composition.Our data suggest that the Tehuitzingo serpentinites and associated chromitites are a fragment of oceanic lithosphere formed in an arc/back-arc environment, and represent an ophiolitic mantle sequence from a supra-subduction zone, the chemical composition of which remained essentially unchanged during the alteration and metamorphic events that affected the Acatlán Complex.     

Estimating coefficients of volume compressibility from compression of strata and piezometric changes in a multiaquifer system in west Taiwan

This paper estimates the coefficients of volume compressibility from variation in compressible layer thickness and changes in piezometric heads by using detail ground surface surveys and a multilayer monitoring well at a selected site (Shigang) within the Choshui River alluvial fan in west Taiwan. The paper integrates various types of in situ monitoring tools, including leveling surveys, continuous global position system (GPS) stations, multilevel layer compression and groundwater pressure head-monitoring wells, to investigate the situation and progress of the subsidence problem in the region. The results from the cross-analyses of the measured data show that surface settlement caused by the compression of strata is between the depths of 60 and 210 m where the clayey stratum within 120-180 m was most pronounced and contributes up to 53% of the total compression. The results indicate that the clayey stratum is under normal consolidation. The results also reflect the fact that 20% of settlement contribution comes from the sandy stratum within 90-120 m; the elasto-plastic behavior of this sandy stratum is clear. The coefficients of volume compressibility of the clayey and sandy stratum analysed from the stratum's compression records; they were 6.38×10⁻⁸ and 5.71×10⁻⁹ m²/N, respectively. Ultimately, this parameter estimation would permit to control and predict land subsidence based on change in pressure head which are related to groundwater extraction.     

Petrology and Geochemistry of Early Cretaceous Bimodal Continental Flood Volcanism of the NW Etendeka, Namibia. Part 1: Introduction, Mafic Lavas and Re-evaluation of Mantle Source Components

The bimodal NW Etendeka province is located at the continentalend of the Tristan plume trace in coastal Namibia. It comprisesa high-Ti (Khumib type) and three low-Ti basalt (Tafelberg,Kuidas and Esmeralda types) suites, with, at stratigraphicallyhigher level, interstratified high-Ti latites (three units)and quartz latites (five units), and one low-Ti quartz latite.Khumib basalts are enriched in high field strength elementsand light rare earth elements relative to low-Ti types and exhibittrace element affinities with Tristan da Cunha lavas. The unradiogenic²⁰⁶Pb/²⁰⁴Pb ratios of Khumib basalts are distinctive, most plottingto the left of the 132 Ma Geochron, together with elevated ²⁰⁷Pb/²⁰⁴Pbratios, and Sr–Nd isotopic compositions plotting in thelower ¹⁴³Nd/¹⁴⁴Nd part of mantle array (EM1-like). The low-Tibasalts have less coherent trace element patterns and variable,radiogenic initial Sr (     

U-Pb Zircon Calendar for Namaquan (Grenville) Crustal Events in the Granulite-facies Terrane of the O'okiep Copper District of South Africa

The O'okiep Copper District is underlain by voluminous 1035–1210Ma granite gneiss and granite with remnants of metamorphosedsupracrustal rocks. This assemblage was intruded by the 1030Ma copper-bearing Koperberg Suite that includes jotunite, anorthosite,biotite diorite and hypersthene-bearing rocks ranging from leuconoriteto hypersthenite. New sensitive high-resolution ion microprobeage data demonstrate the presence of 1700–2000 Ma zirconas xenocrysts in all of the intrusive rocks, and as detritalzircon in the metasediments of the Khurisberg Subgroup. Thesedata are consistent with published Sm–Nd model ages ofc. 1700 Ma (T_CHUR) and c. 2000 Ma (T_DM) of many of the intrusivesthat support a major crust-forming event in Eburnian (Hudsonian)times. In addition, U–Th–Pb analyses of zirconsfrom all major rock units define two tectono-magmatic episodesof the Namaquan Orogeny: (1) the O'okiepian Episode (1180–1210Ma), represented by regional granite plutonism, notably theNababeep and Modderfontein Granite Gneisses and the Concordiaand Kweekfontein Granites that accompanied and outlasted (e.g.Kweekfontein Granite) regional tectonism [F₂(D₂)] and granulite-faciesmetamorphism (M₂); (2) the Klondikean Episode (1020–1040Ma), which includes the intrusion of the porphyritic RietbergGranite and of the Koperberg Suite that are devoid of regionalplanar or linear fabrics. Klondikean tectonism (D₃) is reflectedby major east–west-trending open folds [F₃(D_3a)], andby localized east–west-trending near-vertical ductilefolds [‘steep structures’; F₄(D_3b)] whose formationwas broadly coeval with the intrusion of the Koperberg Suite.A regional, largely thermal, amphibolite- to granulite-faciesmetamorphism (M₃) accompanied D₃. This study demonstrates, interalia, that the complete spectrum of rock-types of the KoperbergSuite, together with the Rietberg Granite, was intruded in ashort time-interval (<10 Myr) at c. 1030 Ma, and that therewere lengthy periods of about 150 Myr of tectonic quiescencewithin the Namaquan Orogeny: (1) between the O'okiepian andKlondikean Episodes; (2) from the end of the latter to the formalend of Namaquan Orogenesis 800–850 Ma ago. KEY WORDS: U–Pb, zircon; O'okiep, Namaqualand; granite plutonism; granulite facies; Koperberg Suite; Namaquan (Grenville) Orogeny     

A Finite Volume Scheme for the Transport of Radionucleides in Porous Media

The COUPLEX1 Test case (Bourgeat et al., 2003) is devoted to the comparison of numerical schemes on a convection–diffusion–reaction problem. We first show that the results of the simulation can be mainly predicted by a simple analysis of the data. A finite volume scheme, with three different treatments of the convective term, is then shown to deliver accurate and stable results under a low computational cost.     

Gravity anomalies and associated tectonic features over the Indian Peninsular Shield and adjoining ocean basins

A combined gravity map over the Indian Peninsular Shield (IPS) and adjoining oceans brings out well the inter-relationships between the older tectonic features of the continent and the adjoining younger oceanic features. The NW–SE, NE–SW and N–S Precambrian trends of the IPS are reflected in the structural trends of the Arabian Sea and the Bay of Bengal suggesting their probable reactivation. The Simple Bouguer anomaly map shows consistent increase in gravity value from the continent to the deep ocean basins, which is attributed to isostatic compensation due to variations in the crustal thickness. A crustal density model computed along a profile across this region suggests a thick crust of 35–40 km under the continent, which reduces to 22/20–24 km under the Bay of Bengal with thick sediments of 8–10 km underlain by crustal layers of density 2720 and 2900/2840 kg/m³. Large crustal thickness and trends of the gravity anomalies may suggest a transitional crust in the Bay of Bengal up to 150–200 km from the east coast. The crustal thickness under the Laxmi ridge and east of it in the Arabian Sea is 20 and 14 km, respectively, with 5–6 km thick Tertiary and Mesozoic sediments separated by a thin layer of Deccan Trap. Crustal layers of densities 2750 and 2950 kg/m³ underlie sediments. The crustal density model in this part of the Arabian Sea (east of Laxmi ridge) and the structural trends similar to the Indian Peninsular Shield suggest a continent–ocean transitional crust (COTC). The COTC may represent down dropped and submerged parts of the Indian crust evolved at the time of break-up along the west coast of India and passage of Reunion hotspot over India during late Cretaceous. The crustal model under this part also shows an underplated lower crust and a low density upper mantle, extending over the continent across the west coast of India, which appears to be related to the Deccan volcanism. The crustal thickness under the western Arabian Sea (west of the Laxmi ridge) reduces to 8–9 km with crustal layers of densities 2650 and 2870 kg/m³ representing an oceanic crust.     

Regional moment tensor uncertainty due to mismodeling of the crust

The retrieval of earthquake moment tensor (MT) requires the response of the medium, in which seismic waves travel from the hypocenter to the stations, to be known. In inverting long-period (LP) seismic data (teleseismic and LP regional records), a gross earth model is sufficient; with decreasing periods, a more detailed model is needed. This is the case when waveforms of weak earthquakes at regional distances are to be inverted. Regional moment tensors (RMTs) of mostly Mediterranean earthquakes are determined on a routine basis by the Swiss Seismological Survey (SED) by using averaged models of the earth's crust. By inverting broad-band records of the M_w=4.8 earthquake near Udine, N Italy, on Feb. 14, 2002, we tested the sensitivity of the MT solution with respect to possible errors in the earth model used and in the location of the hypocenter depth. We perturbed the P and S velocities and the thickness in the 1-D earth model in the range from 3% to 30% of the parameter values and constructed estimates of confidence regions of the MT and error bars of the source time function (STF) and scalar moment in three frequency bands. Similarly, these error characteristics were determined assuming a mislocation in the hypocenter depth. We found that, in the band of periods from 25 to 50 s, the mechanism is resolved well (at the confidence level 95% at least) up to an earth model uncertainty of 30%, in the passband 10–25 s up to about 10%, but it is undetermined completely at periods of 5–10 s. An error in hypocenter depth of as much as double the value reported by the location procedure does not destroy the resolution of the mechanism at periods above 10 s. In the RMT catalog of the SED, earthquakes of M_w greater than about 3.5 are processed at periods above 30 s; thus, the solutions for these events are robust with respect to a possible uncertainty in the earth model used. Mechanisms of weaker earthquakes, retrieved from short periods, should be interpreted with caution.     

Late-Stage Ductile Deformation in Xiongdian-Suhe HP Metamorphic Unit, North-Western Dabie Shan, Central China

New structural and petrological data unveil a very complicated ductile deformation history of the Xiongdian-Suhe HP metamorphic unit, north-western Dabie Shun, central China. The finegrained symplectic amphibolite-facies assemblage and coronal structure enveloping eclogite-facies garnet,omphacite and phengite etc., representing strain-free decompression and retrogressive metamorphism,are considered as the main criteria to distinguish between the early-stage deformation under HP metamorphic conditions related to the continental deep subduction and collision, and the late-stage deformation under amphibolite to greenschist-facies conditions occurred in the post-eclogite exhumation processes.Two late-stages of widely developed, sequential ductile deformations D3 and D4, are recognized on the basis of penetrative fabrics and mineral aggregates in the Xiongdian-Suhe HP metamorphic unit, which shows clear, regionally, consistent overprinting relationships. D3 fabrics are best preserved in the Suhe tract of low post-D3 deformation intensity and characterized by steeply dipping layered mylonitic amphibolites associated with doubly vergent folds. They are attributed to a phase of tectonism linked to the initial exhumation of the HP rocks and involved crustal shortening with the development of upright structures and the widespread emplacement of garnet-bearing granites and felsic dikes. D4 structures are attributed to the main episode of ductile extension (D^24) with a gently dipping foliation to the north and common intrafolial, recumbent folds in the Xiongdian tract, followed by normal sense top-to-the northductile shearing (D^24) along an important tectonic boundary, the so-called Majiawa-Hexiwan fault (MHF), the westward continuation of the Balifan-Mozitan-Xiaotian fault (BMXF) of the northern Dabie Shan. It is indicated that the two stages of ductile deformation observed in the Xiongdian-Suhe HP metamorphic unit, reflecting the post-eclogite compressional or extrusion wedge formation, the subhorizontal ductile extension and crustal thinning as well as the top-to-the north shearing along the high-angle ductile shear zones responsible for exhumation of the HP unit as a coherent slab, are consistent with those recognized in the Dabie-Sulu UHP and HP metamorphic belts, suggesting that they were closely associated in time and space. The Xiongdian-Suhe HP metamorphic unit thus forms part of the Triassic(250-230 Ma) collision orogenic belt, and can not connect with the South Altun-North Qaidam-North Qinline UHP metamorphic belt formed durin~ the Early Paleozoic (500-400Ma).     

正交法在注浆材料优化设计中的应用

应用正交试验设计法，对粘土固化浆液的配比进行了优化，得到了最佳配比，并将其应用于洛湛铁路益娄段岩溶路基的注浆加固，收到了很好的整治效果。