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Thermal evolution of the orogenic lower crust during exhumation within a thickened Moldanubian root of the Variscan belt of Central Europe 总被引：4，自引：5，他引：4
At the eastern margin of the Bohemian Massif (Variscan belt of Central Europe), large bodies of felsic granulite preserve mineral assemblages and structures developed during the early stages of exhumation of the orogenic lower continental crust within the Moldanubian orogenic root. The development of an early steep fabric is associated with east–west-oriented compression and vertical extrusion of the high-grade rocks into higher crustal levels. The high-pressure mineral assemblage Grt-Ky-Kfs-Pl-Qtz-Liq corresponds to metamorphic pressures of ∼18 kbar at ∼850 °C, which are minimum estimates, whereas crystallization of biotite occurred at 13 kbar and ∼790 °C during decompression with slight cooling. The late stages of the granulite exhumation were associated with lateral spreading of associated high-grade rocks over a middle crustal unit at ∼4 kbar and ∼700 °C, as estimated from accompanying cordierite-bearing gneisses. The internal structure of a contemporaneously intruded syenite is coherent with late structures developed in felsic granulites and surrounding gneisses, and the magma only locally explored the early subvertical fabric of the felsic granulite during emplacement. Consequently, the emplacement age of the syenite provides an independent constraint on the timing of the final stages of exhumation and allows calculation of exhumation and cooling rates, which for this part of the Variscan orogenic root are 2.9–3.5 mm yr−1 and 7–9.4 °C Myr−1 , respectively. The final part of the temperature evolution shows very rapid cooling, which is interpreted as the result of juxtaposition of hot high-grade rocks with a cold upper-crustal lid. 相似文献
2.
In high-Mg, Al metapelites, monophase sapphirine corona occur around spinel–corundum aggregates in monomineralic cordierite
layers, and bi-phase orthopyroxene–sillimanite aggregates replace locally warped sapphirine in polygonized cordierite aggregates.
P–T phase topologies computed (Perple_X software) using compositions of cordierite-rich layers that host the reaction textures
did not match the assemblages for the discontinuous reactions spinel + corundum + cordierite → sapphirine and sapphirine + cordierite → orthopyroxene + sillimanite.
Instead, the reaction assemblages were reproduced using P–T pseudosection analysis for micro-domain reaction volumes estimated from compositions of product phases in the volume proportion
they occur. The results are consistent with known phase relations deduced using Schreinmakers P–T grids. Apparently, the compositions of cordierite-rich layers that hosted the reaction textures were inadequate chemical
proxies for determining P–T–X relations of phase-boundary controlled reactions influenced by compositions of the nearest-neighbor minerals in the proportion
they react (effective composition), and not in the proportion they existed in the layer/bulk rock. In other words, P–T–X phase topologies and reconstructed P–T paths in dry and aluminous rocks may be best understood by thermodynamic modeling of reactions using effective reaction volume
compositions rather than the bulk composition of the rock or the mineralogical layer that host the reaction textures. 相似文献
3.
Shale and greywacke compositions from the Archean to Phanerozoic record a secular change in the siliciclastic material that comprises much of Earth's continental margins, past and present. This study explores the metamorphic consequence of these compositional changes, by comparing phase equilibrium models constructed for average Archean, Proterozoic, and Phanerozoic shale and greywacke compositions equilibrated along two Barrovian-type geotherms: 1330℃/GPa(A) and 800 ℃/GPa(B). Our models show that Archean siliciclastic rocks can retain up to 4 vol.% water at middle to lower crustal conditions, nearly twice that of Proterozoic and Phanerozoic compositions. The increased ferromagnesium content of Archean siliciclastic rocks stabilizes chlorite to higher temperatures and results in a biotite-rich assemblage at solidus temperatures. Accordingly, water-absent biotite dehydration melting is predicted to play a greater role in the generation of melt in the metamorphism of Archean aged units,and water-absent muscovite dehydration melting is of increasing importance through the Proterozoic and Phanerozoic. This secular variation in predicted mineral assemblages demonstrates the care with which metamorphic facies diagrams should be applied to Archean compositions. Moreover, secular changes in the composition of shale and greywacke is reflected in the evolution of anatectic melt towards an increasingly less viscous, Ca-rich, and Mg-poor monzogranite. 相似文献
4.
Marcos García-Arias 《Journal of Metamorphic Geology》2020,38(9):993-1010
The recent publication of new activity–composition models by Holland, Green and Powell (2018; Journal of Petrology 59 : 881–900), with a melt model calibrated for source compositions ranging from peridotitic to granitic, opens the door to the modelling of multiple petrogenetic processes at supersolidus conditions in which the composition of the melt phase changes considerably, without having to change the melt model. This melt model is also the first one using the internally consistent thermodynamic databases published by T. Holland and R. Powell that contains TiO2 and Fe2O3, further expanding the application of this model to more realistic geological scenarios. The accompanying mineral models are also the first in containing some minor elements, like TiO2 in garnet and K2O in clinopyroxene. Consequently, it is relevant to test the applicability of these new models to a large P–T–X range of conditions before they can be used in full. Thermodynamic calculations made with the software Perple_X using these models were compared to experimental results, namely the modal proportions and the composition of the melt and several mineral solution phases. The experiments chosen for the comparative study covered a wide range of source compositions (from mafic to felsic), pressure (from 0.3 to 2.1 GPa), temperature (from 700 to 1,150°C) and total and added water content (structural water: 0.15–1.48 wt%; added water: 0–8 wt%; total water: 0.15%–8.15%). The results indicate that the extended melt model reproduces well the composition of the experimental melts, with an inverse correlation between component amount and fit: the best match is found for SiO2 (−0.8% on average) and the worst match is found for those elements with the lowest amounts, TiO2 and MgO (+241% and +235% on average, respectively; values indicate calculated minus experimental, times 100 and divided by experimental). The TiO2 content in the melt model increases dramatically with increasing pressure, from +90% for P < 1.5 GPa to +593% for P > 1.5 GPa. No comparison was made on the Fe2O3 content, as the published iron contents of the experimental melts were always reported as FeOt. In some cases, there is a substantial mismatch in the modal proportions between experiments and calculations, with the reactant phases less abundant and product phases more abundant in the calculations, an effect that is attributed to kinetic effects in the experiments and to the selected clinoamphibole model. Finally, the extended melt model was compared to the tonalitic melt model of Green et al. (2016; Journal of Metamorphic Geology, 34 : 845–869). Both melt models produce very similar results for SiO2, Al2O3, Na2O and K2O, with slightly better results for the tonalitic melt model in FeOt and MgO and for the extended melt model in CaO. No comparison is made on TiO2 because the tonalitic melt model does not include this component. In summary, the new activity–composition models represent a significant contribution to thermodynamic calculations on the evolution of siliceous magmas where their composition, temperature and pressure changes substantially. 相似文献
5.
Diffusion-controlled development of silica-undersaturated domains in felsic granulites of the Bohemian Massif (Variscan belt of Central Europe) 总被引：2，自引：0，他引：2
Lucie Tajčmanová Jiří Konopásek James A. D. Connolly 《Contributions to Mineralogy and Petrology》2007,153(2):237-250
Plagioclase rims around metastable kyanite crystals appear during decompression of high-pressure felsic granulites from the
high-grade internal zone of the Bohemian Massif (Variscan belt of Central Europe). The development of the plagioclase corona
is a manifestation of diffusion-driven transfer of CaO and Na2O from the surrounding matrix and results in isolation of kyanite grains from the quartz- and K-feldspar-bearing matrix. This
process establishes Si-undersaturated conditions along the plagioclase–kyanite interface, which allow crystallization of spinel
during low-pressure metamorphism. The process of the plagioclase rim development is modeled thermodynamically assuming local
equilibrium. The results combined with textural observations enable estimation of equilibration volume and diffusion length
for Na and Ca that extends ∼400–450 and ∼450–550 μm, respectively, around each kyanite crystal. Low estimated bulk diffusion
coefficients suggest that the diffusion rate of Ca and Na is controlled by low diffusivity of Al across the plagioclase rim. 相似文献
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