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1.
We report the field, petrographic and mineral chemical characteristics of relict super‐silicic (=majoritic) garnet microstructures from the Otrøy peridotites in the Western Gneiss Region, Norway. The evidence for the former existence of super‐silicic garnet consists of two‐pyroxene exsolution microstructures from garnet. Estimates of the initial composition of the super‐silicic garnet imply pressures of 6–6.5 GPa, indicating that the Otrøy garnet peridotites were derived from depths >185 km. The garnet peridotites consist of inter‐banded variable compositions with c. 50% garnet peridotite and 50% garnet‐free peridotite. Two distinct garnet types were identified: (a) normal matrix garnet, grain‐size ≤4 mm, and (b) large isolated single garnet crystals and/or (polycrystalline) garnet nodules up to 10 cm in size. Large garnet nodules occur only within limited bands within the garnet peridotites. The relicts of super‐silicic garnet were exclusively found in some (not all) of the larger garnet nodules. Petrographic observations revealed that the microstructure of nodular garnet consists of the following four characteristic elements. (1) Individual garnet nodules are polycrystalline, with grain sizes of 2–8 mm. Garnet grain boundaries are straight with well‐defined triple junctions. (2) Some garnet triple junctions and garnet grain boundaries are decorated by interstitial orthopyroxene. (3) Cores of larger polycrystalline garnet contain two‐pyroxene exsolution microstructures. (4) Precipitation‐free rims (2 mm thick) surround garnet cores containing the exsolved pyroxene microstructure. Pyroxene exsolution from super‐silicic garnet was subsequently followed by brittle–ductile deformation of garnet. Both exsolved pyroxene needles and laths become undulous or truncated by fractures. Simultaneous garnet plasticity is indicated by the occurrence of high densities of naturally decorated dislocations. Transmission electron microscopy observations indicate that decoration is due to Ti‐oxide precipitation. Estimates of the P–T conditions for mineral chemical equilibration were obtained from geothermobarometry. The mineral compositions equilibrated at mantle conditions around 805±40 °C and 3.2±0.2 GPa. These P–T estimates correspond to cold continental lithosphere conditions at depths of around 105 km. From a combination of both depth estimates it can be concluded that the microstructural memory of the rock extends backwards to twice as great a depth range as obtained by thermobarometric methods. Available geochronological and geochemical data of Norwegian garnet peridotites suggest a multi‐stage, multi‐orogenic exhumation history.  相似文献   

2.
Our experimental simulations of the exhumation path of mantle peridotites show that high‐temperature (1400 °C) decompression of lherzolite from 14 to 13 and 12 GPa results in exsolution of interstitial blebs of diopside and Mg2SiO4 (wadsleyite) lamellae from majoritic garnet. At lower pressures (from 8 to 5 GPa, at T = 1400 °C) only enstatite exsolves as blebs at garnet boundaries. Continuous high‐temperature decompression from 14 to 7 GPa produces zoned majoritic garnet containing blebs of exsolved pyroxenes inside garnet rims. No intracrystalline precipitation of pyroxene was observed in garnet, although such lamellae are found in some natural garnet peridotites. The explanation appears to be the three orders of magnitude difference in grain size between experimental and natural specimens. Our data suggest that Mg2SiO4 and diopside exsolutions reflect the deepest point of the exhumation path of garnet peridotites, whereas enstatite precipitation may be restricted to garnets with less majoritic component at shallower depths.  相似文献   

3.
P–T  paths based on parageneses in the immediate vicinity of former high-temperature contact zones between mantle peridotites and granulitic country rocks of the Central Vosges (NE France) were derived by applying several conventional thermometers and thermobarometric calculations with an internally consistent dataset. The results indicate that former garnet peridotites and garnet–spinel peridotites were welded together with crustal rocks at depths corresponding to 1–1.2 GPa. The temperature of the crustal rocks was about 650–700 °C at this stage, whereas values of 1100 °C (garnet peridotites) and 800–900 °C (garnet–spinel peridotites) were calculated for the ultramafic rocks. After emplacement of the mantle rocks, exhumation of the lower crust took place to a depth corresponding to 0.2–0.3 GPa. The temperatures of the incorporated peridotite slices were still high (900–1000 °C) at this stage. This is indicated by the presence of high- T  /low- P parageneses ( c . 800 °C, 0.2–0.3 GPa) in a small (1–10 m) contact aureole around a former garnet peridotite. Crustal rocks distant to the peridotites equilibrated in the same pressure range at lower temperature (650–700 °C). High cooling rates (102–103 °C Ma−1) were calculated for a garnet–biotite rock inclusion in the peridotites and for the crustal rocks at the contact by applying garnet–biotite diffusion modelling. Minimum rates of 0.75–7.5 cm a−1 are required for vertical ascent of rock units (30 km vertical distance) derived from the crust–mantle boundary, resulting in a late Variscan (340 Ma) high- T  /low- P event.  相似文献   

4.
Equilibrium pressure–temperature (P–T) conditions were estimated for kyanite‐bearing eclogite from Nové Dvory, Czech Republic, by using garnet–clinopyroxene thermometry and garnet–clinopyroxene–kyanite–coesite (or quartz) barometry. The estimated P–T conditions are 1050–1150 °C, 4.5–4.9 GPa, which are mostly the same as previously estimated values for garnet peridotite from Nové Dvory (～1100–1250 °C, 5–6 GPa). Such very high‐P conditions, which correspond to about 150‐km depth, have been obtained for some garnet peridotites in the Gföhl Unit of the Bohemian Massif, but pressure conditions of eclogites associated with the garnet peridotites have not been so well constrained. This is the first substantial finding of eclogite that gives such very high‐P conditions in the Gföhl Unit of the Bohemian Massif. The Gföhl Unit mainly consists of felsic granulite or migmatitic gneiss, but these rock types do not display high‐P (>2.5 GPa) evidence. It is unclear whether both the peridotite body and surrounding felsic rocks in the Gföhl Unit were buried to very deep levels, but at least some garnet peridotites and associated eclogites in the Gföhl Unit have ascended from about 150‐km depth.  相似文献   

5.
We report here the detailed microstructure and chemistry of pyroxene exsolution from a polycrystalline garnet porphyroblast of the Western Gneiss Region (WGR) garnet peridotite, Otrøy, Norway. For both clinopyroxene (Cpx) and orthopyroxene (Opx), the same basic crystallographic relationship is found with the host garnet: (100)py//{112}grt, (010)px//{110}grt and (001)px//{111}grt for the majority (>90%) of its intracrystalline pyroxene rods. In addition, this pattern is also exhibited by some interstitial Opx and a subpopulation of both pyroxenes shows a different pattern or no discernible pattern. The results provide quantitative microstructural evidence demonstrating an exsolution (precipitation) origin of both the intracrystalline Cpx and Opx and the small interstitial Opx crystals. The reconstructed precursor majoritic garnet, taking into account both the intracrystalline pyroxenes and interstitial Opx, was characterized by Si = ～3.07 cation per formula unit that corresponds to a minimum pressure of 7.7 GPa (～250 km depth). We also deduce from the observation of Opx being the majority of intracrystalline precipitates and 100% of the interstitial ones that the precursor majoritic garnet probably originated from a pressure less than ～10 GPa (～300 km depth). A multistage decomposition hypothesis is proposed for this WGR majoritic garnet during exhumation of the peridotite from 250 to 300 km depth to explain the topotaxy and chemistry of the exsolved pyroxenes.  相似文献   

6.
Rocks containing breakdown products of majoritic garnet, derivedfrom the deep upper mantle, occur in kimberlite xenoliths andin orogenic peridotites from Otrøy in Norway. The Otrøyperidotites are banded harzburgites and dunites with similarcompositions to mantle xenoliths from Precambrian cratons andPhanerozoic supra-subduction-zone peridotites. Pressure–temperature(P–T) paths deduced for the Otrøy peridotites andkimberlite xenoliths from South Africa are consistent with emplacementof deep mantle peridotites into cratonic lithosphere by asthenospherediapirism. Numerical thermo-convection models provide insightinto the possible P–T histories of deep upper-mantle rocks.In the models, material from the base of the convecting systemis transported to depths of 60–100 km by convection andsmall (50–100 km) diapirs. Diapir intrusion induces small-scaleconvection in the low-viscosity deeper part of the thermochemicallydefined lithosphere. Small-scale convection in the craton rootcan produce complex P–T paths, complex recurrent meltinghistories and complex compositional structure in the craton.P–T paths derived from the numerical models for asthenospherediapirism in a hot upper mantle are consistent with the sequenceof sub-solidus P–T conditions deduced for the cratonicperidotites. KEY WORDS: asthenosphere diapirs; cratonic lithosphere; deep upper mantle; majoritic garnet  相似文献   

7.
Garnet peridotites occur as lenses, blocks or layers within granulite–amphibolite facies gneiss in the Dabie-Sulu ultra-high-pressure (UHP) terrane and contain coesite-bearing eclogite. Two distinct types of garnet peridotite were identified based on mode of occurrence and petrochemical characteristics. Type A mantle-derived peridotites originated from either: (1) the mantle wedge above a subduction zone, (2) the footwall mantle of the subducted slab, or (3) were ancient mantle fragments emplaced at crustal depths prior to UHP metamorphism, whereas type B crustal peridotite and pyroxenite are a portion of mafic–ultramafic complexes that were intruded into the continental crust as magmas prior to subduction. Most type A peridotites were derived from a depleted mantle and exhibit petrochemical characteristics of mantle rocks; however, Sr and Nd isotope compositions of some peridotites have been modified by crustal contamination during subduction and/or exhumation. Type B peridotite and pyroxenite show cumulate structure, and some have experienced crustal metasomatism and contamination documented by high 87Sr/86Sr ratios (0.707–0.708), low εNd( t ) values (−6 to −9) and low δ18O values of minerals (+2.92 to +4.52). Garnet peridotites of both types experienced multi-stage recrystallization; some of them record prograde histories. High- P–T  estimates (760–970 °C and 4.0–6.5±0.2 GPa) of peak metamorphism indicate that both mantle-derived and crustal ultramafic rocks were subducted to profound depths >100 km (the deepest may be ≥180–200 km) and experienced UHP metamorphism in a subduction zone with an extremely low geothermal gradient of <5 °C km−1.  相似文献   

8.
A suite of exceptional mineral inclusions in diamonds from the São Luiz river, Juina province, Brazil, shows a wide range of garnet/majorite mineral compositions co-existing with clinopyroxene; the overall bulk compositions are eclogitic. The inclusions have a wide variety of textural arrangements, but crystallographic data obtained by EBSD shows that each inclusion consists of a single garnet with constant crystallographic orientation whilst clinopyroxene grains have preferred orientation with relation to garnet {110} and <111>. This suggests that the inclusions were originally single phase majoritic garnets, and that they preserve various states of progressive unmixing (exsolution) into lower pressure garnet and clinopyroxene compositions during transport of the host diamonds towards the Earth’s surface. On the basis of high pressure–temperature experimental data some of the original majoritic garnets must have come from depths of 450 km or more, and therefore resided in the transition zone and asthenospheric upper mantle. Particularly extensive re-equilibration of many inclusions took place at depths of ca 180–200 km (probably close to the base of the continental lithosphere). The partially unmixed state of the inclusions provides a unique opportunity for using mineral diffusion data to roughly estimate the rate of transport through the asthenospheric upper mantle, and within error this rate is found to be broadly compatible with expected transport rates by upper mantle convection or plume flow.  相似文献   

9.
The Western Alps are a classic subduction-related collisional orogen with well-preserved, deeply subducted ophiolitic remnants of oceanic lithosphere. Some (e.g. Monviso, Voltri) were recognized as a palaeo-subduction channel, with tectonic blocks showing a wide range of pressure–temperature conditions. We herein evaluate for the first time the metamorphic homogeneity of the extensive Zermatt-Saas ophiolite. Zermatt-Saas peak eclogitic assemblages are represented by omphacite–garnet ± phengite ± epidote ± lawsonite ± glaucophane in MORB-derived metabasalts and garnet–chloritoid–talc ± lawsonite ± phengite in hydrothermalized metabasalts. Thermobarometric estimates with thermocalc and Raman Spectroscopy of carbonaceous material reveal homogeneous peak burial conditions at around 540 ± 20 °C and 23 ± 1 kbar. P – T paths indicate that the whole of the ophiolite, at least 60 km across, strikingly underwent similar burial and exhumation patterns and detached from the slab at depths around 80 km. The Zermatt-Saas ophiolite thus appears to be the world's largest oceanic lithosphere fragment exhumed from such depths, which provides important constraints on interplate coupling mechanisms.  相似文献   

10.
A means for estimating pressures in natural samples based on both the coupled substitution (Na+)[1+] (Ti + [VI]Si)[4+] = (M)[2+] (Al + Cr)[3+], and the classic pyroxene-stoichiometry majorite-substitution into garnet at high-pressure, is derived for garnets with majoritic chemistry. The technique is based on a compilation of experimental data for different bulk compositions. It is compositionally and thermally robust and can be used to estimate pressures experienced by natural materials during formation of majoritic garnet. In addition, it can be used either retrospectively, or in new experimental studies to establish the pressures of crystallization of reaction products, and determine if disequilibrium is recorded by the chemistries of majoritic garnets. Pressures are calculated based on majoritic chemistries in chondritic meteorites and diamond inclusions. Majoritic garnets associated with Mg perovskite in shocked L chondrites (n = 4) yield uniform pressures of 23.8 ± 0.2 GPa that are slightly higher than pressures recorded by majoritic garnet in shock-derived melt veins in L chondrites (22.4 ± 0.6 GPa; n = 5). Similar pressures are also exhibited by shock-derived majoritic garnets in H chondrites (22.2 ± 1.1 GPa; n = 3). Diamond inclusions with eclogitic and peridotitic majoritic garnet chemistries exhibit mean pressures of 10.7 ± 2.7 GPa (n = 30) and 8.3 ± 1.6 GPa (n = 15) respectively, consistent with a sub-lithospheric origin. However, pressures defined by majoritic diamond inclusions from Jagersfontein (22.3 ± 0.8 GPa and 16.9 ± 1 GPa), Monastery (15.7 ± 7 GPa) and Kankan (15.5 ± 0.2 GPa) show that these inclusions originated from the mantle transition zone. Thus, this new single-phase method for pressure estimation has unmatched potential to map the depth of formation of garnets with majoritic chemistries that occur as diamond inclusions in all parageneses except those that include Ca silicate perovskite. The derived pressures confirm the sub-lithospheric origin of eclogitic majoritic diamond inclusions, and thus provide a more comprehensive picture of the important role of storage of oceanic lithosphere in the transition zone.  相似文献   

11.
 Multianvil experiments were carried out at 10–15 GPa and 1600–1700 °C to match the compositions of majoritic garnet inclusions from diamonds, and to determine the compositions of other phases potentially coexisting with these inclusions in the source. Most experiments produced coexisting majoritic garnet, diopsidic clinopyroxene, one or more (Mg,Fe)2SiO4 polymorphs, and quenched carbonatic melt. The experimental garnets had relatively high Ca and Fe contents similar to the observed Ca and Fe contents of the inclusions. The resulting Si contents confirmed that the depth of origin of the inclusion with the highest Si content did not exceed 410 km, thus none of the majoritic garnet inclusions found so far originated in the transition zone (410–660 km). The evidence from inclusions and experiments is consistent with the presence of an eclogite layer occurring globally between 200 and 410 km. Compositional variations observed among more than 100 majoritic garnet inclusions with their Si content, which is a measure of pressure and depth, are consistent with the origin of the eclogite layer by crystal fractionation in a magma ocean. The compositions of olivine coexisting with majoritic garnet in the experimental products had the average Fe/(Fe + Mg) ratios between 0.16 and 0.28. Inclusions with such high Fe contents have not been found; the Fe/(Fe + Mg) ratio of the olivine inclusions in diamonds usually varies between 0.05 and 0.09. Hence, the mantle between 200 and 410 km may not contain olivine. In the absence of olivine, the discontinuity at 410 km is most likely a chemical boundary between the 200-km-thick eclogite layer and a more mafic transition zone. Received: 15 March 2001 / Accepted: 14 September 2001  相似文献   

12.
ABSTRACT An Early Palaeozoic UHP metamorphic belt was recently discovered in the North Qaidam Mountains in the north-eastern part of the Tibet Plateau. The belt contains abundant eclogite with lesser amounts of garnet peridotite and gneiss. The paragneiss contains abundant zircon with coesite inclusions whereas the eclogite has garnet with coesite pseudomorphs and omphacite with exsolved quartz. The calculated peak metamorphic conditions for the eclogite are T  = 730 °C and P  = 2.8 GPa. The garnet peridotites are magnesium-rich with Mg#s [100 × Mg/(Mg + Fe)] up to 92. Olivine in the peridotites is highly magnesian (Fo > 91.5) and TiO2-poor and is typical of low- T garnet peridotites. Calculated peak metamorphic conditions for the peridotites are T  = 837 °C and P  = 2.5 GPa. U–Pb and Sm–Nd isotope dating indicate that continental subduction started during the early Ordovician (∼ 495 Ma). The North Qaidam UHP, together with the North Qilian HP belt about 300 km to the north, constitute a pair of genetically related early Palaeozoic subduction zones along the northern margin of the Tibet Plateau.  相似文献   

13.
中国东部上地幔岩石相转变及其意义   总被引：2，自引：0，他引：2  
樊祺诚  隋建立 《地球科学》2009,34(3):387-391
中国东部新生代玄武岩和大别-苏鲁超高压变质带中的含石榴石相橄榄岩, 带来了发生在上地幔的尖晶石→石榴石相转变和铝辉石→贫铝辉石+石榴石的重要信息, 为中国东部上地幔岩石结构分层奠定了重要基础.通过岩石学与实验岩石学的研究, 推导出发生相转变的P-T条件, 为建立中国东部大陆上地幔岩石分层结构提供了重要约束.尖晶石二辉橄榄岩向石榴石二辉橄榄岩相转变发生在55~70km, 随着深度增加, 石榴石二辉橄榄岩从富铝石榴石二辉橄榄岩(70~120km) 转变为贫铝石榴石二辉橄榄岩(> 120~150km).   相似文献   

14.
Garnet-bearing mantle peridotites, occurring as either xenoliths in volcanic rocks or lenses/massifs in high-pressure and ultrahigh-pressure terrenes within orogens, preserve a record of deep lithospheric mantle processes. The garnet peridotite xenoliths record chemical equilibrium conditions of garnet-bearing mineral assemblage at temperatures (T) ranging from ~700 to 1,400°C and pressures (P) > 1.6–8.9 GPa, corresponding to depths of ~52–270 km. A characteristic mineral paragenesis includes Cr-bearing pyropic garnet (64–86 mol% pyrope; 0–10 wt% Cr2O3), Cr-rich diopside (0.5–3.5 wt% Cr2O3), Al-poor orthopyroxene (0–5 wt% Al2O3), high-Cr spinel (Cr/(Cr + Al) × 100 atomic ratio = 2–86) and olivine (88–94 mol% forsterite). In some cases, partial melting, re-equilibration involving garnet-breakdown, deformation, and mantle metasomatism by kimberlitic and/or carbonatitic melt percolations are documented. Isotope model ages of Archean and Proterozoic are ubiquitous, but Phanerozoic model ages are less common. In contrast, the orogenic peridotites were subjected to ultrahigh-pressure (UHP) metamorphism at temperature ranging from ~700 to 950°C and pressure >3.5–5.0 GPa, corresponding to depths of >110–150 km. The petrologic comparisons between 231 garnet peridotite xenoliths and 198 orogenic garnet peridotites revealed that (1) bulk-rock REE (rare earth element) concentrations in xenoliths are relatively high, (2) clinopyroxene and garnet in orogenic garnet peridotites show a highly fractionated REE pattern and Ce-negative anomaly, respectively, (3) Fo contents of olivines for off-cratonic xenolith are in turn lower than those of orogenic garnet and cratonic xenolith but mg-number of garnet for orogenic is less than that of off-cratonic and on-cratonic xenolith, (4) Al2O3, Cr2O3, CaO and Cr# of pyroxenes and chemical compositions of whole rocks are very different between these garnet peridotites, (5) orogenic garnet peridotites are characterized by low T and high P, off-cratonic by high T and low P, and cratonic by medium T and high P and (6) garnet peridotite xenoliths are of Archean or Proterozoic origin, whereas most of orogenic garnet peridotites are of Phanerozoic origin. Taking account of tectonic settings, a new orogenic garnet peridotite exhumation model, crust-mantle material mixing process, is proposed. The composition of lithospheric mantle is additionally constrained by comparisons and compiling of the off-cratonic, on-cratonic and orogenic garnet peridotite.  相似文献   

15.
ABSTRACT Ion probe traverses across garnets from peridotites of the Caledonides of Norway and the Variscides of Poland show zoning patterns for Y, V, Zr, Cr, Ti and the REE. The complexly zoned patterns of garnets from the Bystrzyca Górna peridotite, Poland, are interpreted in terms of a changing P–T history (isobaric cooling followed by decompression and cooling). Weak rimward gradients in REE concentrations in garnets from the Almklovdalen and Sandvika peridotites, Norway, may be relicts of the original growth history of the garnets, but the nearly flat Y, V, Zr, Cr and Ti profiles from the same garnets imply a later period of near-homogenization at uniform P–T. Crushed garnet separates from each body were separated into three or more fractions on the assumption that density and magnetic susceptibility vary with Fe/Mg ratio, and Fe/Mg ratios change from garnet core to rim. Sm-Nd garnet–clinopyroxene ‘ages’ were determined for each fraction to determine whether they are also zoned. Four garnet fractions from the Góry Sowie peridotite give nearly the same ages (397–412 Ma) that are believed to span the interval of garnet growth. Garnet fractions from the Norwegian peridotites define scattered ages (816–1350 Ma) that are suspect, but hint at a Sveconorwegian equilibration event. The data indicate the Variscan and Norwegian peridotites had different histories, despite superficial mineralogical and tectonic similarities. Norwegian garnet peridotites had a long pre-Caledonian history and were extracted from a relatively cold mantle whereas the Variscan garnet peridotites had a comparatively short pre- or Eo-Variscan history and were extracted from a hot mantle.  相似文献   

16.
Garnets from recrystallized, staurolite- and kyanite-bearing mica schists from the central Saualpe basement, representing the host rocks of the type-locality eclogites, give concordant Sm–Nd garnet–whole-rock isochron ages between 88.5±1.7 and 90.9±0.7 Ma. The millimetre-sized, mostly inclusion-free grains show fairly homogeneous element profiles with pyrope contents of 25–27%. Narrow rims with an increase in Fe and Mn and a decrease in Mg document minor local re-equilibration during cooling. According to phengite geothermobarometry, peak metamorphic conditions at 90 Ma were close to 20  kbar and 680  °C and similar to those recorded by the eclogites. The garnet rims record about 575  °C/7  kbar for the final stages of metamorphism. A phengitic garnet–mica schist, sampled at the immediate contact with the Gertrusk eclogite, gave a garnet–whole-rock Sm–Nd age of 94.0±2.7 Ma.
Garnet porphyroclasts separated from a pegmatite–mylonite of the Koralpe plattengneiss near Stainz are unzoned and show spessartine contents of 15%. Composition and Sm–Nd ages of close to 260 Ma point to a magmatic origin for these garnets.
The garnet data from the Saualpe document an intense Alpine metamorphism for this part of the Austroalpine basement. The mica schists recrystallized during decompression and rapid exhumation, at the final stages of and immediately following a high- P event. The Koralpe data show that high Alpine temperatures did not reopen the Sm–Nd isotope system, implying a closure temperature in excess of c . 600  °C for this isotopic system in garnet.  相似文献   

17.
The Makbal Complex in the northern Tianshan of Kazakhstan and Kyrgyzstan consists of metasedimentary rocks, which host high‐P (HP) mafic blocks and ultra‐HP Grt‐Cld‐Tlc schists (UHP as indicated by coesite relicts in garnet). Whole rock major and trace element signatures of the Grt‐Cld‐Tlc schist suggest a metasomatized protolith from either hydrothermally altered oceanic crust in a back‐arc basin or arc‐related volcaniclastics. Peak metamorphic conditions of the Grt‐Cld‐Tlc schist reached ~580 °C and 2.85 GPa corresponding to a maximum burial depth of ~95 km. A Sm‐Nd garnet age of 475 ± 4 Ma is interpreted as an average growth age of garnet during prograde‐to‐peak metamorphism; the low initial εΝd value of ?11 indicates a protolith with an ancient crustal component. The petrological evidence for deep subduction of oceanic crust poses questions with respect to an effective exhumation mechanism. Field relationships and the metamorphic evolution of other HP mafic oceanic rocks embedded in continentally derived metasedimentary rocks at the central Makbal Complex suggest that fragments of oceanic crust and clastic sedimentary rocks were exhumed from different depths in a subduction channel during ongoing subduction and are now exposed as a tectonic mélange. Furthermore, channel flow cannot only explain a tectonic mélange consisting of various rock types with different subduction histories as present at the central Makbal Complex, but also the presence of a structural ‘dome’ with UHP rocks in the core (central Makbal) surrounded by lower pressure nappes (including mafic dykes in continental crust) and voluminous metasedimentary rocks, mainly derived from the accretionary wedge.  相似文献   

18.
This paper presents monomineral and multiphase inclusions in garnet from eclogites and clinopyroxenites, which form layers and boudins in garnet peridotites from two areas in the Moldanubian zone of the Bohemian Massif. The garnet peridotites occur in felsic granulites and reached UHP conditions prior to their granulite facies overprint. In addition to complex compositional zoning, garnets from hosting eclogites and clinopyroxenites preserve inclusions of hydrous phases and alkali silicate minerals including: amphiboles, chlorites, micas and feldspars. Amphibole, biotite and apatite inclusions in garnet have a high concentration of halogens; CO2 and sulfur are involved in carbonates and sulfide inclusions, respectively. The inclusion patterns and compositional zoning in garnet in combination with textural relations among minerals, suggest that the ultramafic and mafic bodies are derived from lithospheric mantle above the subduction zone and were transformed into garnet pyroxenites and eclogites in the subduction zone. Based on compositional, mineral and textural relations, all of these rocks along with the surrounding crustal material were overprinted by granulite facies metamorphism during their exhumation.  相似文献   

19.
The garnet-olivine Fe-Mg exchange geothermometer and the garnet-olivine-plagioclase geobarometer have been simultaneously calibrated using reversed experimental data based on the model reactions and between 900 and 1500 °C at 9.1–95.0 kbar and between 4.7 and 7.0 kbar at 750–1050 °C, respectively. The resulting garnet-olivine thermometer reproduces experimental temperatures mostly within ±75 °C and the garnet-olivine-plagioclase barometer reproduces experimental pressures well within ±0.19 kbar. These new thermobarometers use the same garnet and olivine activity models and are thermodynamically consistent. Application of these thermobarometers to garnet peridotites from mantle xenoliths, orogenic garnet peridotites over the world and the Adirondack olivine-bearing metagabbros yielded reasonable P–T results. The present garnet-olivine thermometer can be used to measure medium-high-grade to ultrahigh-grade, low-pressure to ultrahigh–high-pressure garnet peridotites and metagabbros, whereas the garnet-olivine-plagioclase barometer has limited application to garnet-olivine-plagioclase-bearing granulites.  相似文献   

20.
This paper reports an occurrence of medium-pressure granulite facies calc-silicate rocks intercalated with pelitic gneisses in the Higo metamorphic terrane, central Kyushu, Japan, which is classified as a low- P /high- T (andalusite-sillimanite type) metamorphic belt. Three equilibrium stages are recognized in the calc-silicate rock based on reaction textures: M1 stage characterized by an assemblage of porphyroblastic garnet + coarse-grained clinopyroxene + plagioclase included in the clinopyroxene; M2 stage by two kinds of breakdown products of garnet, one is plagioclase + coronitic clinopyroxene within garnet and the other is plagioclase + vermicular clinopyroxene surrounding garnet; and M3 stage by amphibole replacing clinopyroxene. The key assemblage in the calc-silicate rock common to M1 and M2 stages is Grt + Cpx + Pl ± Qtz, which constrains the pressure and temperature ( P – T ) conditions for these stages by Fe–Mg exchange reaction and the two univariant net-transfer reactions: 2Grs + Alm + 3Qtz = 3Hd + 3An or 2Grs + Prp + 3Qtz = 3Di + 3An. The P – T conditions for M1 and M2 stages were estimated to be about 8.4 ± 1.9 kbar and 680 ± 122 °C, and 6.7 ± 1.9 to 8.9 ± 2.2 kbar and 700 ± 130 to 820 ± 160 °C, respectively. Estimates are consistent with an isobaric heating P – T path. The high peak temperature conditions at normal crustal depths and the prograde isobaric heating path probably require heat advection due to melt migration during the high- T metamorphism.  相似文献   

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