at temperatures between 400 and 700°C. The ultrapotassic fluid appearing at pressures above 15 kbar on the low-temperature sides of the corresponding reaction curves, which show positive dP/dT slopes, is probably supercritical. The P-T positions of the reactions are compatible with KMASH mineral reactions studied previously and with melting investigations in the KMASH system undertaken at temperatures higher than 700°C.
It is possible that natural rocks, chiefly K-rich metasediments subducted as minor portions of the oceanic crust, could give rise to low-temperature ultrapotassic fluids, mainly at temperatures between 300° to 600°C and pressures between 15 and 30 kbar. The ascending K-rich fluids would penetrate the overlying mantle to metasomatize it. After termination of the subduction process, heating of this mantle material, previously cooled by the subducted lithosphere, could lead to the formation of high-temperature K-rich magmas. 相似文献
A 2-mm fragment of an anorthosite was investigated in transmitted light and by high-voltage electron microscopy. A modal composition of 76% plagioclase (anorthite), 23% pyroxene (pigeonite) and about 1% spinel (pleonaste) was deduced. The results indicate a slight olivine-normative chemical composition. The rock is hypidiomorphic to idiomorphic fine-grained with a well-defined parallel-oriented fabric.Under the petrographic microscope, and even in the submicroscopic range, the plagioclase crystals show twinning only along (010) composition planes (predominantly Carlsbad twins). The dislocation density is low and the dominating defect structures are small-angle grain boundaries. The subtexture of the plagioclases is characterized by aI1-P1 phase transformation and by the existence of large antiphase domains.Features indicative of shock were not observed. From the present results it is concluded that the rock fragment is in its original state. Most likely, crystallisation occurred under plutonic conditions in a deep crustal region with the dominant assemblage: plagioclase + pyroxene + spinel. 相似文献
New petrologic and 40Ar/39Ar geochronologic data constrain conditions of Alpine metamorphism along the northwestern border of the Tauern Window. The
P-T estimations based on phengite barometry were determined for samples from units of the Lower Austroalpine nappe complex exposed
above the Southpenninic interior of the Tauern Window, and from upper parts of the Southpenninic “Bündner Schiefer” sequence.
Results suggest that both Mesozoic metasedimentary nappe units (Reckner and Hippold Nappes) and an ophiolitic nappe (Reckner
Complex) of the Lower Austroalpine nappe complex have been metamorphosed at pressures between 8 and 10.5 kbar and temperatures
around 350 °C. The structurally highest Lower Austroalpine unit (Quartzphyllite Nappe) was not affected by high-pressure metamorphism
and records maximum P-T conditions of approximately 4 kbar and 400 °C. Highest parts of the structurally underlying Southpenninic Bündner Schiefer
sequence were metamorphosed at intermediate pressures (6–7 kbar). Temperatures increased in all structural units during decompression.
Whole-rock 40Ar/39Ar plateau ages of silicic phyllites and cherts with abundant high-Si phengites record ages around 50 Ma in the Reckner Nappe,
and 44–37 Ma in the Hippold Nappe and Southpenninic Bündner Schiefer sequence. These ages are interpreted to date closely
the high-pressure metamorphism.
The Lower Austroalpine-Southpenninic border area in the NW Tauern Window appears to have evolved along an indented, fragmented
active continental margin where the Reckner Complex represents one of the oldest sections of the Southpenninic (Piemontais)
Oceanic tract that was originally situated close to, or even within, the Lower Austroalpine continent. During closure of the
Piemontais Ocean, the resultant subduction zone did not entrain components of the Reckner Complex or its cover sequences (Reckner
and Hippold Nappes): therefore “Eoalpine” high-pressure metamorphism did not occur. Sequences exposed within the study area
were subducted to relatively shallow depths during the last stage of consumption of oceanic crust and immediately prior to
final continental collision.
Received: 30 July 1996 / Accepted: 7 April 1997 相似文献
In view of increasing damage due to earthquakes, and the current problems of earthquake prediction, real-time warning of strong ground motion is attracting more interest. In principle, it allows short-term warning of earthquakes while they are occurring. With warning times of up to tens of seconds it is possible to send alerts to potential areas of strong shaking before the arrival of the seismic waves and to mitigate the damage, but only if the seismic source parameters are determined rapidly. The major problem of an early-warning system is the real-time estimation of the earthquake's size. We investigated digitized strong-motion accelerograms from 244 earthquakes that occurred in North and Central America between 1940 and 1986 to find out whether their initial portions reflected the size of the ongoing earthquake. Applying conventional methods of time-series analyses we calculate appropriate signal parameters and describe their uncertainties in relation to the magnitude and epicentral distance. The study reveals that the magnitude of an earthquake can be predicted from the first second of a single accelerogram within ±1.36 magnitude units. The uncertainty can be reduced to about ±0.5 magnitude units if a larger number (≥8) of accelerograms are available, which requires a dense network of seismic stations in areas of high seismic risk. 相似文献
By means of electron-optical methods the structure and orientation of lamella-like garnet exsolutions in an orthopyroxene from the eclogite of Gilsberg, Saxony Granulite Complex, are investigated. It can be shown that besides the macroscopic and microscopic submicroscopic exsolution lamellae also exist; they have widths up to nearly 5 nm. The intergrowth conditions are such shat (100)op is parallel to {100}gr and [001]op parallel to 〈110〉gr. The lamellae are coherent, but sometimes extended dislocation arrangements are found in microscopic lamellae which are caused by a deformation. These investigations point to formation of the rock as a result of a solid state reaction under high pressure conditions with subsequent diffusive-dissociative exsolution of garnet from primary pyroxene. 相似文献
Abstract Four phengite‐bearing eclogites, taken from different depths of the Chinese continental scientific drilling (CCSD) borehole in the Sulu ultrahigh pressure terrane, eastern China, were studied with the electron microprobe. The compositional zonations of garnet and omphacite are moderate, whereas phengite compositions generally vary significantly in a single sample from core to rim by decrease of the Si content. Various geothermobarometric methods were applied to constrain the P‐T conditions of these eclogites on the basis of the compositional variability of the above minerals. The constrained P‐T path for sample B218 is characterized by pressure decrease from ca 3.0 GPa (ca 600°C) to 1.3 GPa (ca 550°C). Eclogite B310 yielded P‐T conditions of 3.0 GPa and 750°C. The path for eclogite B1008 starts at about 650°C and 3.6–3.9 GPa (stage I) followed by a pressure decrease to 2.8–3.0 GPa and a significant temperature rise (stages II and IIIa, 750–810°C). Afterwards, this rock cooled down to 620–660°C at still high pressures (2.5–2.7 GPa, stage IIIb). Retrograde conditions were about 670°C and 1.3 GPa (stage IV). Eclogite B1039 yielded a P‐T path starting at ca 600°C and 3.3–3.9 GPa (stage I). A pressure decrease to about 3.0 GPa (stage II, 590–610°C) and then a moderate isobaric temperature increase to ca 630°C (stage III) followed. Stage IV is characterized by temperatures of 650°C at pressures close to 1.3 GPa. During and after this stage (hydrous) fluids partially rich in potassium penetrated the rocks causing minor changes. Relatively high oxygen fugacities led to andradite and magnetite among the newly formed minerals. We think that the above findings can be best explained by mass flow in a subduction channel. Thus, we conclude that the assembly of UHP rocks of the CCSD site, eclogites, quartzofeldspathic rocks, and peridotites, cannot represent a crustal section that was already coherent at UHP conditions as it is the common belief currently. The coherency was attained after significant exhumation of these UHP rocks. 相似文献
Blueschist was recently recognized within the Lhasa terrane, which is one of the NE Gondwana blocks. In this rock, the Mn and Mg contents of garnet enclosing aegirine-rich clinopyroxene, rutile and quartz decrease and increase, respectively, from core to rim. Amphibole changes from glaucophane through Na–Ca amphibole to Ca amphibole. The Si contents of phengite are high in the centre but low along the rim. The P – T path, starting above 2.5 GPa–450 °C and showing subsequently first a temperature increase to 500 °C and then a pressure release via blueschist conditions to 0.6 GPa, was reconstructed using a P – T pseudosection calculated for the P – T range 0.4–2.8 GPa and 250–650 °C. This path points to deep subduction of a cold oceanic crust probably beneath the NE Gondwana margin during Permo-Triassic times. This finding contributes to a better understanding of the pre-Cenozoic history of major terranes of NE Gondwana. 相似文献