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11.
W. G. Ernst 《Contributions to Mineralogy and Petrology》1997,128(1):30-44
Thin mafic dikes, possibly correlative with the Independence dike swarm of SE California, transect uppermost Proterozoic–Cambrian
metasedimentary strata in the White-Inyo Range. Textures and bulk-rock chemistry indicate that the protoliths were diabases
and microdiorites, accompanied by Ca + Mg + Fe +Ni + Cr-rich hornblende (± minor augite) cumulates. Analytical data suggest
crystal settling and fractionation at shallow depths. Most of the dikes lie in the mapped aureoles of – and were metamorphosed
by – voluminous Late Jurassic granitoid plutons; however, a few metadikes cut these plutons and must have been recrystallized
during the emplacement of Cretaceous granitic stocks. The mafic metadikes thus include members of two or more temporally distinct
suites, pre-Late Jurassic, and latest Jurassic–Cretaceous. Neoblastic mineral assemblages and element partitioning within
these nonfoliated mafic metadikes reflect lower-to-upper greenschist facies overprints; metamorphic parageneses, coincident
with those developed in the metasedimentary wallrocks, are defined by the production of chlorite, biotite, white mica, epidote,
and actinolite, and by albitization of the igneous plagioclase. Based on analytical and mineralogic data obtained in this
study, the following conclusions regarding subsolidus recrystallization of the mafic metadikes are advanced: (1) Newly grown
minerals and phase assemblages are systematic in their areal distributions. (2) Metamorphic grade increases chiefly toward
the north and east, toward the Late Jurassic granitoids. (3) Element fractionation among coexisting neoblastic phases is regular,
and compatible with a close approach to chemical equilibrium. (4) Assemblages 3–5 km from the granitic intrusive contacts
reflect lowermost greenschist facies physical conditions. (5) Investigated mafic dikes exhibit mineral parageneses isofacial
with the regional/contact metamorphic assemblages previously documented for the enclosing pre-Mesozoic clastic country rocks.
Clearly, mafic dikes of several ages of injection and recrystallization are present in the central White-Inyo Range, making
correlation with the Independence dike swarm problematic. In any case, the dikes record localized contact metamorphism that
took place sporadically over portions of an approximately 100 million year interval.
Received: 13 March 1996 / Accepted: 24 December 1996 相似文献
12.
近期博斯腾湖水位变化及其原因分析 总被引:27,自引:5,他引:27
新疆博斯腾湖1987年以来湖泊水位的变化(上升)与主要补给河流开都河径流量的变化有直接关系, 而这与发源于天山中段降水和高山冰雪融水的河流, 受到气候变化影响很大有关. 全球变化研究结果显示, 中亚干旱区是全球温度上升幅度较大的地区. 当地的气象资料表明, 过去20 a年平均温度明显升高的趋势, 对水资源储量和补给来源影响深远. 相似文献
13.
We discuss the potential variations of the biological pump that can be expected from a change in the oceanic circulation
in the ongoing global warming. The biogeochemical model is based on the assumption of a perfect stoichiometric composition
(Redfield ratios) of organic material. Upwelling nutrients are transformed into organic particles that sink to the deep ocean
according to observed profiles. The physical circulation model is driven by the warming pattern as derived from scenario computations
of a fully coupled ocean-atmosphere model. The amplitude of the warming is determined from the varying concentration of atmospheric
CO2. The model predicts a pronounced weakening of the thermohaline overturning. This is connected with a reduction of the transient
uptake capacity of the ocean. It yields also a more effective removal of organic material from the surface which partly compensates
the physical effects of solubility. Both effects are rather marginal for the evolution of atmospheric pCO2. Running climate models and carbon cycle models separately seems to be justified.
Received: 9 August 1995 / Accepted: 22 April 1996 相似文献
14.
New P–T constraints on the Tamayen glaucophane‐bearing rocks,eastern Taiwan: Perple_X modelling results and geodynamic implications
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I. Baziotis C.‐H. Tsai W. G. Ernst B.‐M. Jahn Y. Iizuka 《Journal of Metamorphic Geology》2017,35(1):35-54
New pseudosection modelling was applied to better constrain the P–T conditions and evolution of glaucophane‐bearing rocks in the Tamayen block of the Yuli belt, recognized as the world's youngest known blueschist complex. Based on the predominant clinoamphibole, textural relationships, and mineral compositions, these glaucophane‐bearing high‐P rocks can be divided into four types. We focused on the three containing garnet. The chief phase assemblages are (in decreasing mode): amphibole + quartz + epidote + garnet + chlorite + rutile/titanite (Type‐I), phengite + amphibole + quartz + garnet + chlorite + epidote + titanite + biotite + magnetite (Type‐II), and amphibole + quartz + albite + epidote + garnet + rutile + hematite + titanite (Type‐III). Amphibole exhibits compositional zoning from core to rim as follows: glaucophane → pargasitic amphibole → actinolite (Type‐I), barroisite → Mg‐katophorite/taramite → Fe‐glaucophane (Type‐II), glaucophane → winchite (Type‐III). Using petrographic data, mineral compositions and Perple_X modelling (pseudosections and superimposed isopleths), peak P–T conditions were determined as 13 ± 1 kbar and 550 ± 40 °C for Type‐I, 10.5 ± 0.5 kbar and 560 ± 30 °C for Type‐II (thermal peak) and 11 ± 1 kbar and 530 ± 30 °C for Type‐III. The calculations yield higher pressures and temperatures than previously thought; the difference is ~1–6 kbar and 50–200 °C. The three rock types record similar P–T retrograde paths with clockwise trajectories; all rocks followed trajectories with substantial pressure decrease under near‐isothermal conditions (Type‐I and Type‐III), with the probable exception of Type‐II where decompression followed colder geotherms. The P–T paths suggest a tectonic environment in which the rocks were exhumed from maximum depths of ~45 km within a subduction channel along a relative cold geothermal gradient of ~11–14 °C km?1. 相似文献
15.
16.
Johannes Bouman Sietse Rispens Thomas Gruber Radboud Koop Ernst Schrama Pieter Visser Carl Christian Tscherning Martin Veicherts 《Journal of Geodesy》2009,83(7):659-678
One of the products derived from the gravity field and steady-state ocean circulation explorer (GOCE) observations are the
gravity gradients. These gravity gradients are provided in the gradiometer reference frame (GRF) and are calibrated in-flight
using satellite shaking and star sensor data. To use these gravity gradients for application in Earth scienes and gravity
field analysis, additional preprocessing needs to be done, including corrections for temporal gravity field signals to isolate
the static gravity field part, screening for outliers, calibration by comparison with existing external gravity field information
and error assessment. The temporal gravity gradient corrections consist of tidal and nontidal corrections. These are all generally
below the gravity gradient error level, which is predicted to show a 1/f behaviour for low frequencies. In the outlier detection, the 1/f error is compensated for by subtracting a local median from the data, while the data error is assessed using the median absolute
deviation. The local median acts as a high-pass filter and it is robust as is the median absolute deviation. Three different
methods have been implemented for the calibration of the gravity gradients. All three methods use a high-pass filter to compensate
for the 1/f gravity gradient error. The baseline method uses state-of-the-art global gravity field models and the most accurate results
are obtained if star sensor misalignments are estimated along with the calibration parameters. A second calibration method
uses GOCE GPS data to estimate a low-degree gravity field model as well as gravity gradient scale factors. Both methods allow
to estimate gravity gradient scale factors down to the 10−3 level. The third calibration method uses high accurate terrestrial gravity data in selected regions to validate the gravity
gradient scale factors, focussing on the measurement band. Gravity gradient scale factors may be estimated down to the 10−2 level with this method. 相似文献
17.
An introduction to ultrahigh-pressure metamorphism 总被引:6,自引:0,他引:6
Abstract Ultrahigh-pressure (UHP) metamorphism refers to mineralogical and structural readjustment of supracrustal protoliths and associated mafic-ultramafic rocks at mantle pressures greater than ∼ 25 kbar (80-90 km). Typical products include metapelite, quartzite, marble, granulite, eclogite, paragneiss and orthogneiss; minor mafic and ultramafic rocks occur as eclogitic-ultramafic layers or blocks of various dimensions within the supracrustal rocks. For appropriate bulk compositions, metamorphism at great depths produces coesite, microdiamond and other characteristic UHP minerals with unusual compositions. Thus far, at least seven coesite-bearing eclogitic terranes and three diamond-bearing UHP regions have been documented. All lie within major continental collision belts in Eurasia, have similar supracrustal protoliths and metamorphic assemblages, occur in long, discontinuous belts that may extend several hundred kilometers or more, and typically are associated with contemporaneous high-P blueschist belts. This paper defines the P-T regimes of UHP metamorphism and describes mineralogical, petrological and tectonic characteristics for a few representative UHP terranes including the western gneiss region of Norway, the Dora Maira massif of the western Alps, the Dabie Mountains and the Su-Lu region of east-central China, and the Kokchetav massif of the former USSR. Prograde P-T paths for coesite-bearing eclogites require abnormally low geothermal gradients (approximately 7°C/km) that can be accomplished only by subduction of cold, oceanic crust-capped lithosphere ± pelagic sediments or an old, cold continent. The preservation of coesite inclusions in garnet, zircon, omphacite, kyanite and epidote, and microdiamond inclusions in garnet and zircon during exhumation of an UHP terrane requires either an extraordinarily fast rate of denudation (up to 10 cm/year) or continuous refrigeration in an extensional regime (retreating subduction zone). 相似文献
18.
Bent-over buoyant jets distorted by a crosscurrent develop a vortex pair structure and can bifurcate to produce two distinct lobes which diverge from one another downwind. The region downwind of the source between the lobes has relatively low proportions of discharged fluid. Factors invoked by previous workers to cause or enhance bifurcation include buoyancy, release of latent heat at the plume edge by evaporating water droplets, geometry and orientation of the source, and the encounter with a density interface on the rising path of the plume. We suggest that the pressure distribution around the vortex pair of a rising plume may initially trigger bifurcation. We also report new experimental observations confirming that bifurcation becomes stronger for stronger bent-over plumes, identifying that bifurcation can also occur for straight-edged plumes but gradually disappears for stronger plumes which form a gravity current at their final level and spread for a significant distance against the current. Observations from satellites and the ground are reviewed and confirm that volcanic plumes can show bifurcation and a large range of bifurcation angles. Many of the bifurcating plumes spread out at the tropopause level and suggest the tropopause may act on the plumes as a density interface enhancing bifurcation. Even for quite moderate bifurcation angles, the two plume lobes become rapidly separated downwind by distances of tens of kilometers. Such bifurcating plumes drifting apart can only result in bilobate tephra fall deposits. The tephra fall deposit from the 16 km elevation, SE spreading, bifurcating volcanic plume erupted on 15 May 1981 from Mt Pagan was sampled by previous workers and clearly displayed bilobate characteristics. Examples of bilobate tephra fall deposits are reviewed and their origin briefly discussed. Bilobate deposits are common and may result from many causes. Plume bifurcation should be considered one of the possible mechanisms which can account for come examples of bilobate tephra fall deposits. 相似文献
19.
20.
Ernst A. Brugger 《Geoforum》1982,13(2):177-192
In most Third World countries regional planning and regional policy is formally well-developed, but in practice is not implemented. In spite of increasing regional disparities, narrow sectoral and centrally decided programmes and projects dominate over territorial and decentralized ones. This article examines theoretically and empirically why, in the case of Costa Rica, the implementation problem is so difficult and which strategic approaches would best suit an effective regional policy. 相似文献