The petrology and geochemistry of a newly discovered suite of high-pressure garnet + clinopyroxene-bearing rocks from the Monotonous Series of the Moldanubian Zone of the Bohemian Massif, southwest Czech Republic have been investigated. Three types [common eclogites, quartz ± kyanite ± (clino)zoisite eclogites and garnet-hornblende-clinopyroxenites] are distinguished by petrography and geochemistry. All underwent a significant degree of partial breakdown under granulite and amphibolite facies conditions during exhumation. Important features include the growth of orthopyroxene in breakdown domains after garnet and omphacite and anorthite + spinel ± corundum ± exceedingly peraluminous sapphirine replacing kyanite. Garnet zoning and inclusion patterns support a prograde evolution from low pressures for at least some of the samples. The post-eclogite stage granulite facies overprint indicates that high temperatures prevailed during exhumation, but preservation of zoning in some garnets and the results of diffusion modelling suggest that this overprint took place over a very short time-scale. The geochemical and petrological results allow characteristic differences to be recognized between these eclogites and metabasites found in other tectonic units of South Bohemia and consequently the assigning of all high-pressure rocks to a single, now disrupted, tectonic unit is a gross simplification that seriously misrepresents the tectono-metamorphic history of the region. 相似文献
Fifteen samples of (Mg,Fe)SiO3 majorite with varying Fe/Mg composition and one sample of (Mg,Fe)(Si,Al)O3 majorite were synthesized at high pressure and temperature under different conditions of oxygen fugacity using a multianvil press, and examined ex situ using X-ray diffraction and Mössbauer and optical absorption spectroscopy. The relative concentration of Fe3+ increases both with total iron content and increasing oxygen fugacity, but not with Al concentration. Optical absorption spectra indicate the presence of Fe2+–Fe3+ charge transfer, where band intensity increases with increasing Fe3+ concentration. Mössbauer data were used in conjunction with electron microprobe analyses to determine the site distribution of all cations. Both Al and Fe3+ substitute on the octahedral site, and charge balance occurs through the removal of Si. The degree of Mg/Si ordering on the octahedral sites in (Mg,Fe)SiO3 majorite, which affects both the c/a ratio and the unit cell volume, is influenced by the thermal history of the sample. The Fe3+ concentration of (Mg,Fe)(Si,Al)O3 majorite in the mantle will reflect prevailing redox conditions, which are believed to be relatively reducing in the transition zone. Exchange of material across the transition boundary to (Mg,Fe) (Si,Al)O3 perovskite would then require a mechanism to oxidize sufficient iron to satisfy crystal-chemical requirements of the lower-mantle perovskite phase. 相似文献
Grain-size reduction may be a possible mechanism for the origin of localized deformation in the ductile regime. I investigated the effects of grain-size reduction due to dynamic recrystallization, cataclasis, and syntectonic metamorphic reaction on the stress envelope in the lithospheric mantle during extension by using a simple one-dimensional model. In this model, the lithosphere extends uniformly with a constant strain rate, and a fall in rock strength appears as a decrease in stress. Because grain-size distribution at the onset of extension is unknown, I regarded the steady state grain-size due to dynamic recrystallization as the initial size. Then, I evaluated the maximum effects of grain-size reduction by dynamic recrystallization during extension, and consequently examined the effects of grain-size reduction by cataclasis and metamorphic reaction under conditions when dynamic recrystallization occurs significantly. I find that it is difficult to bring about localized rheological weakening by grain-size reduction owing to dynamic recrystallization. In contrast, grain-size reduction by cataclasis can cause localized weakening during extension. There is a wide-ranging rate of grain-size reduction by means of cataclasis that causes localized weakening just below the Moho. I specified the reaction from spinel-lherzolite to plagioclase-lherzolite that plays a role in grain-size reduction by syntectonic metamorphism. The reaction occurs at depths less than 35 km, which is independent of the initial thermal state of the lithosphere. Localized rheological weakening can occur if the following conditions are satisfied: (1) grain-size before the reaction is greater than 0.7 mm under dry conditions and greater than 0.5 mm under wet conditions, and it decreases down to those values by the reaction; (2) grain-size decreases down to less than initial grain-size, when the dominant deformation mechanism is GSS creep at the onset of extension. It is also noted that dry conditions are more favourable for localized weakening. 相似文献
Large instrumental mass fractionation (IMF) may occur during measurements of oxygen isotope ratios by SIMS. Part of this fractionation depends on crystal structure and mineral composition. In order to improve the accuracy of SIMS measurements, we gathered 6 commonly used garnet standards and prepared 6 others to adequately cover the composition range Alm0-73, Prp0-99, Grs0-20. Electron microprobe analyses were performed at UBP-Clermont to check the chemical homogeneity of these standards. Oxygen isotope compositions were determined by laser fluorination and mass spectrometry at UW-Madison. Ten SIMS sessions and 336 δ18O measurements at CRPG-Nancy, on a Cameca IMS1270 instrument, demonstrate that the standards are homogeneous with external reproducibility of 0.3‰ (1σ). In terms of δ18O, SIMS measurements indicate that, during a single session, IMF can vary up to 6.3‰ from one garnet standard to another. In most of the sessions, IMF can be correlated with the grossular content. However, for a satisfactory correction scheme, we suggest the combination of the 3 main components (Ca, Fe, Mg). This is done using a simple least square calculation routine. The correction coefficients determined for each session can be used to calculate the IMF and correct the measured isotopic ratio of a garnet of known chemical composition. This way, we were able to reproduce the δ18O values of most of the Fe-Mg-Ca garnet standards within ± 0.6‰. Interestingly, the use of only 3 end-member standards (AlmCMG, PrpMM, GrsSE) plus a standard of intermediate composition (e.g. UWG-2) is sufficient to reproduce δ18O within the same precision. Thus, linear interpolation among end-member standards is satisfactory in the case of the garnet solid-solutions. Two studies carried out on zoned garnets from the Alps and the Pyrenees indicate that matrix effects become significant when variations in grossular contents are important (> 10%). In order to obtain reliable isotope ratio measurements on Fe-Mg-Ca garnets using a SIMS, we suggest a correction scheme using at least 3 reliable end-member standards plus a standard of intermediate composition (a garnet standard closest to the average composition of the analysed garnet). This allows cross-checking and incorporates a correction based on the variations in composition of zoned crystals. 相似文献
Garnets in UHP eclogites from Bixiling in Dabieshan were investigated by Fourier transform infrared spectroscopy (FTIR). The results indicate that all garnets contain structural water that occurs as hydroxyl (OH−) and non-structural molecular water (H2O) possibly in the form of sub-microscopic fluid inclusions. The structural hydroxyl contents range from 92 to 1735 ppm (H2O wt.) and most are between 200 and 1000 ppm. Therefore, garnet in eclogite can recycle surface water into the mantle. Various water contents were observed among different samples of the same outcrop (∼150 m) and in different domains of the same sample (∼1 cm). This variability in structural H2O contents suggests that the mobility of fluids during UHP metamorphism was very limited, and that both subduction and exhumation processes of UHP rocks occurred in a short time interval. 相似文献
Rare earth element (REE) and other trace element (Y, Sr, Ti, Cr, V, Na) abundances in garnet from a garnet-bearing metapelite, a pelitic migmatite, a syn-tectonic granite and a post-tectonic leucogranite were measured by secondary ion mass spectrometry (SIMS) in order to identify the effective variables on the trace element distribution between garnet and the host rock. Garnet from the garnet-bearing metapelite, the pelitic migmatite and the syn-tectonic granite is zoned with respect to REE. The cores are enriched by a factor of 2–3 relative to the rims. For the garnets from the garnet-bearing metapelite equilibrium distribution following a simple Rayleigh fractionation is responsible for the decreasing concentrations in REE from core to rim. Garnet from the pelitic migmatite shows a more complex trace element pattern following distinct enrichment and depletion patterns for Ti, V, Cr and REE from core to rim. These features suggest disequilibrium between garnet and the associated melt in which the enrichment of trace elements probably correspond to a period of open-system behaviour in these rocks at a time when the garnet, originally nucleated in the metamorphic environment was incorporated into the melt. The garnet from the syn-tectonic granite shows stepwise decreasing concentrations in REE from core to rim: a REE-rich core can be distinguished from a broad REE-depleted rim. Notably, from core to rim an inflection of the Yb / Er and Yb / Dy ratios is visible. Whereas the decrease of HREE abundance in the core region of the garnet from the syn-tectonic granite may arise from equilibrium partitioning during garnet growth, the inflection can be interpreted as a result of partial melting. Garnet cores with high Yb / Er and Yb / Dy > 1 nucleated in the metamorphic environment without the presence of a melt whereas the rims with lower Yb / Er and Yb / Dy < 1 crystallized in the presence of a melt. Garnet from the leucogranite has lower REE abundances and is considered to be of igneous origin. In contrast to garnet from the other samples, its core has low trace element abundances, whereas its rim is significantly enriched in REE but depleted in Ti. These features suggest that only the outermost rim was in equilibrium with the melt. For this garnet, liquid diffusion controlled partitioning is more likely to explain the extreme trace element variation. An evaluation of Sm and Nd concentrations in garnet and a comparison of Sm–Nd and U–Pb garnet ages and U–Pb monazite ages form the terrane indicate that the observed LREE systematics in the different garnet species are a primary feature and are not homogenized by volume diffusion during high grade amphibolite facies conditions. 相似文献
Within the metamorphic basement of the Coastal Cordillera of central Chile, the Western Series constitutes the high-pressure (HP)/low-temperature (LT) part (accretionary prism) of a fossil-paired metamorphic belt dominated by metagreywackes. In its eastern part, blocks derived from small lenses of garnet amphibolite with a blueschist facies overprint are locally intercalated and associated with serpentinite and garnet mica-schist. Continuously developed local equilibria were evaluated applying various independent geothermobarometric approaches. An overall anticlockwise PT path results. The prograde path evolved along a geothermal gradient of 15 °C/km, passing the high-pressure end of greenschist facies until a transient assemblage developed within albite-epidote amphibolite facies transitional to eclogite facies at peak metamorphic conditions (600–760 °C, 11–16.5 kbar; stage I). This peak assemblage was overprinted during an external fluid infiltration by an epidote blueschist facies assemblage at 350–500 °C, 10–14 kbar (stage II) indicating nearly isobaric cooling. The retrograde equilibration stage was dated with a Rb–Sr mineral isochron at 305.3±3.2 Ma, somewhat younger (296.6±4.7 Ma) in an adjacent garnet mica-schist. Localized retrograde equilibration continued during decompression down to 300 °C, 5 kbar. The retrograde evolution is identical in the garnet amphibolite and the garnet mica-schist.
The counterclockwise PT path contrasts the usual clockwise PT paths derived from rocks of the Western Series. In addition, their ages related to stage II are the oldest recorded within the fossil wedge at the given latitude. Its “exotic” occurrence is interpreted by the path of the earliest and deepest subducted material that was heated in contact with a still hot mantle. Later accreted and dehydrated material caused hydration and cooling of the earliest accreted material and the neighbouring mantle. After this change also related to rheological conditions, effective exhumation of the early subducted material followed at the base of the hydrated mantle wedge within a cooler environment (geothermal gradient around 10–15 °C/km) than during its burial. The exotic blocks thus provide important time markers for the onset of subduction mass circulation in the Coastal Cordillera accretionary prism during the Late Carboniferous. Continuous subduction mass flow lasted for nearly 100 Ma until the Late Triassic. 相似文献