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31.
32.
The pyrope-coesite rocks and their country rocks at Parigi,Dora Maira Massif,Western Alps: detailed petrography,mineral chemistry and PT-path 总被引:1,自引:0,他引:1
Both the coarse- and fine-grained varieties of the partly coesite-bearing pyrope-quartzites, their interlayered jadeite-kyanite rocks, and the biotite-phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents, which were also studied analytically by microprobe. Prior to pyrope and coesite growth, the Mg-rich metapelites were talc-kyanite-chlorite-rutile-ellenbergerite schists which — upon continued prograde metamorphism — developed first pyrope megacrysts in silica-deficient local environments at the expense of chlorite + talc + kyanite, and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc + kyanite. Based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations, a prograde PT-path — simplified for water activity = 1 — is constructed which passes through the approximate PT-conditions 16 kbar and 560° C, 29 kbar and 720° C, and finally up to 37 kbar at about 800° C, where the Mg-rich metapelite was a pyrope-coesite rock with phengite, kyanite, and talc still present. During the retrograde path, pyrope was altered metasomatically either into phlogopite + kyanite + quartz or, at a later stage, to chlorite + muscovite + quartz. Both assemblages yield PT-constraints, the latter about 7–9 kbar, 500–600° C. The country rock gneisses have also endured high-pressures of at least 15 kbar, but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies (about 5 kbar, 450° C). Microprobe data are presented for the following minerals: pyrope, ellenbergerite, dumortierite (unusually MgTi-rich), jadeite, vermiculite (formed after Na-phlogopite?), paragonite, and for several generations of phengite, chlorite, talc, phlogopite, dravite, and glaucophane in the high-pressure rocks, as well as for biotite, chlorite, phengites, epidote, garnet, albite, and K-feldspar in the country rock gneisses. An outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their PT-stability limits. 相似文献
33.
Saxicolous species of lichens are able to induce and accelerate weathering of their rock substrate, and effects of lichens on substrate can be attributed to both physical and chemical causes. This paper is focused on biotic weathering actions of epilithic and endolithic species on the different rock types (sandstones and volcanogenic rocks) in Antarctica. The patterns, mechanisms, processes and neoformations of rock-weathering resulting from lichen colonization are expounded in detail. Furthermore, it is pointed out that, for a better understanding of the impacts of lichens on environments, the studies on the rate of biotic weathering and the comprehensive involvement of the lichen effects on weathering of natural rocks remain to be carried out in Antarctica. 相似文献
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Marco Bohnhoff Martina Rische Thomas Meier Dirk Becker George Stavrakakis Hans-Peter Harjes 《Tectonophysics》2006,423(1-4):17
The volcanic arc of the Hellenic subduction zone with its four volcanic centers is of major relevance when evaluating the seismovolcanic hazard for the Aegean region. We present results from a 22-station temporary seismic network (CYCNET) in the central Hellenic Volcanic Arc (HVA). CYCNET recordings allow to analyze the level and spatio-temporal evolution of microseismic activity in this region for the first time. A total of 2175 events recorded between September 2002 and July 2004 are analyzed using statistical methods, cluster analysis and relative relocation techniques. We identify distinct regions with significantly varying spatio-temporal behavior of microseismicity. A large portion of the seismic activity within the upper crust is associated with the presence of islands representing horst structures that were generated during the major Oligocene extensional phase. In contrast, the central part of the Cyclades metamorphic core complex remains aseismic considering our magnitude threshold of 1.8 except one spot where events occur swarm-like and with highly similar waveforms.The highest activity in the study area was identified along the SW–NE striking Santorini–Amorgos zone. Within this zone the submarine Columbo volcano exhibits strong temporal variations of seismic activity on a high background level. This activity is interpreted to be directly linked to the magma reservoir and therein the migration of magma and fluids towards the surface. NE of Columbo where no volcanic activity has yet been reported we observe a similar seismicity pattern with small-scaled activity spots that might represent local pathways of upward migrating fluids or even developing volcanic activity within this zone of crustal weakness. In contrast, the Santorini and Milos volcanic complexes do not show significant temporal variations and low to moderate background activity, respectively. Relating our results to the distribution of historical earthquakes and the GPS-derived horizontal velocity field we conclude that the Santorini–Amorgos zone is presently in the state of right-lateral transtension reflecting a major structural boundary of the volcanic arc subdividing it into a seismically and volcanically quiet western and an active eastern part. 相似文献
37.
Gültekin Topuz Rainer Altherr Winfried H. Schwarz Abdurrahman Dokuz Hans-Peter Meyer 《International Journal of Earth Sciences》2007,96(5):861-873
The Kurtoğlu metamorphic complex, that forms part of the pre-Liassic basement of the Sakarya zone in northern Turkey, consists
of at least two tectonic units. Blueschist-facies rocks of unknown metamorphic age in the southern part of the complex are
tectonically overlain by Variscan low-pressure high-temperature metamorphic rocks. The latter comprise mica schists and fine-grained
gneisses, cut by metaleucogranitic dikes, as well as migmatitic biotite gneisses and subordinate amphibolite intercalations.
Structural data indicate that metamorphism and penetrative deformation occurred after dyke intrusion. Peak metamorphic conditions
of the mica schists, fine-grained gneisses and metaleucogranites are estimated to ∼650°C and ∼0.4 GPa, based on phase relationships
in the system NCKFMASH, Fe–Mg partitioning between garnet and biotite as well as garnet-aluminosilicate-quartz-plagioclase
(GASP) and garnet-plagioclase-biotite-quartz (GBPQ) barometry. Peak temperatures of the migmatitic biotite gneisses and amphibolite
intercalations are not well constrained but might have been significantly higher (690–740°C), as suggested from hornblende-plagioclase
thermometry. 40Ar–39Ar incremental dating on muscovite and biotite fractions from the mica schists and fine-grained gneisses yielded plateau ages
of ∼323 Ma. Significantly older model ages of ∼329 and ∼337 Ma were obtained on muscovite fractions from two metaleucogranite
samples. These fractions contain both relict igneous and newly formed metamorphic muscovite. 相似文献
38.
Giampiero Iaffaldano Hans-Peter Bunge Martin Bücker 《Earth and Planetary Science Letters》2007,260(3-4):516-523
Past plate motions display a range of variability, including speedups and slowdowns that cannot easily be attributed to changes in mantle related driving forces. One key controlling factor for these variations is the surface topography at convergent margins, as previous modeling shows that the topographic load of large mountain belts consumes a significant amount of the driving forces available for plate tectonics by increasing frictional forces between downgoing and overriding plates. Here we use this insight to pose a new tectonic inverse problem and to infer the growth of mountain belts from a record of past plate convergence. We introduce the automatic differentiation method, which is a technique to produce derivative code free of truncation error by source transformation of the forward model. We apply the method to a publicly available global tectonic thin-shell model and generate a simple derivative code to relate Nazca/South America plate convergence to gross topography of the Andes mountain belt. We test the code in a search algorithm to infer an optimal paleotopography of the Andes 3.2 m.y. ago from the well-known history of Nazca/South America plate convergence. Our modeling results are in excellent agreement with published estimates of Andean paleotopography and support the notion of strong feedback between mountain belt growth and plate convergence. 相似文献
39.
Previous studies have highlighted the crucial role of land degradation in tropical African climate. This effect urgently has
to be taken into account when predicting future African climate under enhanced greenhouse conditions. Here, we present time
slice experiments of African climate until 2025, using a high-resolution regional climate model. A supposable scenario of
future land use changes, involving vegetation loss and soil degradation, is prescribed simultaneously with increasing greenhouse-gas
concentrations in order to detect, where the different forcings counterbalance or reinforce each other. This proceeding allows
us to define the regions of highest vulnerability with respect to future freshwater availability and food security in tropical
and subtropical Africa and may provide a decision basis for political measures. The model simulates a considerable reduction
in precipitation amount until 2025 over most of tropical Africa, amounting to partly more than 500 mm (20–40% of the annual
sum), particularly in the Congo Basin and the Sahel Zone. The change is strongest in boreal summer and basically reflects
the pattern of maximum vegetation cover during the seasonal cycle. The related change in the surface energy fluxes induces
a substantial near-surface warming by up to 7°C. According to the modified temperature gradients over tropical Africa, the
summer monsoon circulation intensifies and transports more humid air masses into the southern part of West Africa. This humidifying
effect is overcompensated by a remarkable decrease in surface evaporation, leading to the overall drying tendency over most
of Africa. Extreme daily rainfall events become stronger in autumn but less intense in spring. Summer and autumn appear to
be characterized by more severe heat waves over Subsaharan West Africa. In addition, the Tropical Easterly Jet is weakening,
leading to enhanced drought conditions in the Sahel Zone. All these results suggest that the local impact of land degradation
and reduction of vegetation cover may be more important in tropical Africa than the global radiative heating, at least until
2025. This implies that vegetation protection measures at a national scale may directly lead to a mitigation of the expected
negative implications of future climate change in tropical Africa. 相似文献
40.
<正>New occurrences of jadeitite,jadeite quartzites,and jadeite-lawsonitequartzites have recently been discovered in the Rio San Juan Complex(RSJC)of the northern Dominican Republic.These rocks are found in serpentinite mélanges associated with a former intra-oceanic 相似文献