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1.
The relative timing of two discrete pulses of metamorphic fluid flow is constrained based on chemical zoning in several garnet crystals from Kvaløya, Troms, northern Norway. The garnet crystals measured 1–2 cm in diameter and were contained within c. 1.6 Ga, staurolite grade metasediments. Major element zoning indicates that garnet grew under normal prograde conditions in the garnet and/or staurolite zones. Timing constraints are based on comparisons between major and trace element chemical zoning, oxygen isotope (δ18O) zoning and deformational (inclusion trail) zoning in one of the garnet. We interpret at least two pulses of metamorphic fluid flow. The first pulse occurred during the syn‐tectonic growth interval. The δ18O zoning was reversed relative to ‘normal’ prograde zoning and the δ18O maximum was located within the syn‐tectonic growth zone, displaced 3–4 mm from the garnet core. The fluid might have been sourced in neighbouring calcareous pelites and may also have caused formation of an Y ring. The second (and subsequent) pulse(s) occurred during/after the post‐tectonic growth interval. δ18O was locally increased at the garnet rim, particularly where the rim was sheared. The incomplete rim was also enriched in calcium. Transport of oxygen and calcium by metamorphic fluids is well documented. Transport of Y is both problematic and poorly understood, but might have been facilitated by complexing with F and/or CO2.  相似文献   

2.
Metamorphic equilibration requires chemical communication between minerals and may be inhibited through sluggish volume diffusion and or slow rates of dissolution in a fluid phase. Relatively slow diffusion and the perceived robust nature of chemical growth zoning may preclude garnet porphyroblasts from readily participating in low‐temperature amphibolite facies metamorphic reactions. Garnet is widely assumed to be a reactant in staurolite‐isograd reactions, and the evidence for this has been assessed in the Late Proterozoic Dalradian pelitic schists of the Scottish Highlands. The 3D imaging of garnet porphyroblasts in staurolite‐bearing schists reveals a good crystal shape and little evidence of marginal dissolution; however, there is also lack of evidence for the involvement of either chlorite or chloritoid in the reaction. Staurolite forms directly adjacent to the garnet, and its nucleation is strongly associated with deformation of the muscovite‐rich fabrics around the porphyroblasts. “Cloudy” fluid inclusion‐rich garnet forms in both marginal and internal parts of the garnet porphyroblast and is linked both to the production of staurolite and to the introduction of abundant quartz inclusions within the garnet. Such cloudy garnet typically has a Mg‐rich, Mn‐poor composition and is interpreted to have formed during a coupled dissolution–reprecipitation process, triggered by a local influx of fluid. All garnet in the muscovite‐bearing schists present in this area is potentially reactive, irrespective of the garnet composition, but very few of the schists contain staurolite. The staurolite‐producing reaction appears to be substantially overstepped during the relatively high‐pressure Barrovian regional metamorphism reflecting the limited permeability of the schists in peak metamorphic conditions. Fluid influx and hence reaction progress appear to be strongly controlled by subtle differences in deformation history. The remaining garnet fails to achieve chemical equilibrium during the reaction creating distinctive patchy compositional zoning. Such zoning in metamorphic garnet created during coupled dissolution–reprecipitation reactions may be difficult to recognize in higher grade pelites due to subsequent diffusive re‐equilibration. Fundamental assumptions about metamorphic processes are questioned by the lack of chemical equilibrium during this reaction and the restricted permeability of the regional metamorphic pelitic schists. In addition, the partial loss of prograde chemical and textural information from the garnet porphyroblasts cautions against their routine use as a reliable monitor of metamorphic history. However, the partial re‐equilibration of the porphyroblasts during coupled dissolution–reprecipitation opens possibilities of mapping reaction progress in garnet as a means of assessing fluid access during peak metamorphic conditions.  相似文献   

3.
Chemical zoning in the outer few 10s of microns of garnet porphyroblasts has been investigated to assess the scale of chemical equilibrium with matrix minerals in a pelitic schist. Garnet porphyroblasts from the Late Proterozoic amphibolite facies regional metamorphic mica schists from Glen Roy in the Scottish Highlands contain typical prograde growth zoning patterns. Edge compositions have been measured via a combination of analysis of traverses across the planar edges of porphyroblast surfaces coupled to X-ray mapping of small areas within polished thin sections at the immediate edge of the porphyroblasts. These approaches reveal local variation in garnet composition, especially of grossular (Ca) and almandine (Fe) components, with a range at the edge from <7 mol.% grs to >16 mol.% grs, across distances of less than 50 µm. This small-scale patchy compositional zoning is as much variation as the core–rim compositional zoning across the whole of a 3 mm porphyroblast. Ca and Fe heterogeneity occurs on a scale suggesting a combination of inefficient diffusive exchange across grain boundaries during prograde growth and the evolving microtopography of the porphyroblast surface control garnet composition. The latter creates haloes of compositional zoning adjacent to some inclusions, which typically extend from the inclusion towards the porphyroblast edge during further growth. The lack of a consistent equilibrium composition at the garnet edge is also apparent in the internal zoning of the porphyroblast and so processes occurring during entrapment of some mineral inclusions have a profound influence on the overall chemical zoning. Garnet compositions and associated zoning patterns are widely used by petrologists to reconstruct P–T–t paths for crustal rocks. The evidence of extremely localized (10–50 µm scale) equilibrium during growth further undermines these approaches.  相似文献   

4.
In the classical view of metamorphic microstructures, fast viscous relaxation (and so constant pressure) is assumed, with diffusion being the limiting factor in equilibration. This contribution is focused on the only other possible scenario – fast diffusion and slow viscous relaxation – and brings an alternative interpretation of microstructures typical of high‐grade metamorphic rocks. In contrast to the pressure vessel mechanical model applied to pressure variation associated with coesite inclusions in various host minerals, a multi‐anvil mechanical model is proposed in which strong single crystals and weak grain boundaries can maintain pressure variation at geological time‐scales in a polycrystalline material. In such a mechanical context, exsolution lamellae in feldspar are used to show that feldspar can sustain large differential stresses (>10 kbar) at geological time‐scales. Furthermore, it is argued that the existence of grain‐scale pressure gradients combined with diffusional equilibrium may explain chemical zoning preserved in reaction rims. Assuming zero net flux across the microstructure, an equilibrium thermodynamic method is introduced for inferring pressure variation corresponding to the chemical zoning. This new barometric method is applied to plagioclase rims around kyanite in felsic granulite (Bohemian Massif, Czech Republic), yielding a grain‐scale pressure variation of 8 kbar. In this approach, kinetic factors are not invoked to account for mineral composition zoning preserved in rocks metamorphosed at high grade.  相似文献   

5.
This study explores garnet coronas around hedenbergite, which were formed by the reaction plagioclase + hedenbergite→garnet + quartz, to derive information about diffusion paths that allowed for material redistribution during reaction progress. Whereas quartz forms disconnected single grains along the garnet/hedenbergite boundaries, garnet forms ～20‐μm‐wide continuous polycrystalline rims along former plagioclase/hedenbergite phase boundaries. Individual garnet crystals are separated by low‐angle grain boundaries, which commonly form a direct link between the reaction interfaces of the plagioclase|garnet|hedenbergite succession. Compositional variations in garnet involve: (i) an overall asymmetric compositional zoning in Ca, Fe2+, Fe3+ and Al across the garnet layer; and (ii) micron‐scale compositional variations in the near‐grain boundary regions and along plagioclase/garnet phase boundaries. These compositional variations formed during garnet rim growth. Thereby, transfer of the chemical components occurred by a combination of fast‐path diffusion along grain boundaries within the garnet rim, slow diffusion through the interior of the garnet grains, and by fast diffusion along the garnet/plagioclase and the garnet/hedenbergite phase boundaries. Numerical simulation indicates that diffusion of Ca, Al and Fe2+ occurred about three to four, four and six to seven orders of magnitude faster along the grain boundaries than through the interior of the garnet grains. Fast‐path diffusion along grain boundaries contributed substantially to the bulk material transfer across the growing garnet rim. Despite the contribution of fast‐path diffusion, bulk diffusion through the garnet rim was too slow to allow for chemical equilibration of the phases involved in garnet rim formation even on a micrometre scale. Based on published garnet volume diffusion data the growth interval of a 20‐μm‐wide garnet rim is estimated at ～103–104 years at the inferred reaction conditions of 760 ± 50 °C at 7.6 kbar. Using the same parameterization of the growth law, 100‐μm‐ and 1‐mm‐thick garnet rims would grow within 105–106 and 106–107 years respectively.  相似文献   

6.
Garnet porphyroblasts in sillimanite‐bearing pelitic schists contain complex textural and compositional zoning, with considerable variation both within and between adjacent samples. The sillimanite‐bearing schists locally occur in regional Barrovian garnet zone assemblages and are indicative of a persistent lack of equilibrium during prograde metamorphism. Garnet in these Dalradian rocks from the Scottish Highlands preserves evidence of a range of metamorphic responses including initial growth and patchy coupled dissolution–reprecipitation followed by partial dissolution. Individual porphyroblasts each have a unique and variable response to prograde metamorphism and garnet with mainly flat compositional profiles co‐exists with those containing largely unmodified characteristic bell‐shaped Mn profiles. This highlights the need for caution in applying traditional interpretations of effective volume diffusion eliminating compositional variation. Cloudy garnet with abundant fluid inclusions is produced during incomplete modification of the initial porphyroblasts and these porous garnet are then particularly prone to partial replacement in sillimanite‐producing reactions. The modification of garnet via a dissolution–reprecipitation process releases Ca into the effective whole‐rock composition, displacing the pressure–temperature positions of subsequent isograd reactions. This represents the first report of internal metasomatism controlling reaction pathways. The behaviour of garnet highlights the importance of kinetic factors, especially deformation and fluids, in controlling reaction progress and how the resulting variability influences subsequent prograde history. The lack of a consistent metamorphic response, within and between adjacent schists, suggests that on both local and regional scales these rocks have largely not equilibrated at peak metamorphic conditions.  相似文献   

7.
Garnet crystallization in metapelites from the Barrovian garnet and staurolite zones of the Lesser Himalayan Belt in Sikkim is modelled utilizing Gibbs free energy minimization, multi‐component diffusion theory and a simple nucleation and growth algorithm. The predicted mineral assemblages and garnet‐growth zoning match observations remarkably well for relatively tight, clockwise metamorphic P–T paths that are characterized by prograde gradients of ～30 °C kbar?1 for garnet‐zone rocks and ～20 °C kbar?1 for rocks from the staurolite zone. Estimates for peak metamorphic temperature increase up‐structure toward the Main Central Thrust. According to our calculations, garnet stopped growing at peak pressures, and protracted heating after peak pressure was absent or insignificant. Almost identical P–T paths for the samples studied and the metamorphic continuity of the Lesser Himalayan Belt support thermo‐mechanical models that favour tectonic inversion of a coherent package of Barrovian metamorphic rocks. Time‐scales associated with the metamorphism were too short for chemical diffusion to substantially modify garnet‐growth zoning in rocks from the garnet and staurolite zones. In general, the pressure of initial garnet growth decreases, and the temperature required for initial garnet growth was reached earlier, for rocks buried closer toward the MCT. Deviations from this overall trend can be explained by variations in bulk‐rock chemistry.  相似文献   

8.
Large garnet poikiloblasts hosted by leucosome in metapelitic gneiss from Broken Hill reflect complex mineral–melt relationships. The spatial relationship between the leucosomes and the garnet poikiloblasts implies that the growth of garnet was strongly linked to the production of melt. The apparent difficulty of garnet to nucleate a large number of grains during the prograde breakdown of coexisting biotite and sillimanite led to the spatial focussing of melting reactions around the few garnet nuclei that formed. Continued reaction of biotite and sillimanite required diffusion of elements from where minerals were reacting to sites of garnet growth. This diffusion was driven by chemical potential gradients between garnet‐bearing and garnet‐absent parts of the rock. As a consequence, melt and peritectic K‐feldspar also preferentially formed around the garnet. The diffusion of elements led to the chemical partitioning of the rock within an overall context in which equilibrium may have been approached. Thus, the garnet‐bearing leucosomes record in situ melt formation around garnet porphyroblasts rather than centimetre‐scale physical melt migration and segregation. The near complete preservation of the high‐grade assemblages in the mesosome and leucosome is consistent with substantial melt loss. Interconnected networks between garnet‐rich leucosomes provide the most likely pathway for melt migration. Decimetre‐scale, coarse‐grained, garnet‐poor leucosomes may represent areas of melt flux through a large‐scale melt transfer network.  相似文献   

9.
Devolatilization reactions during prograde metamorphism are a key control on the fluid distribution within subduction zones. Garnets in Mn-rich quartz schist within the Sanbagawa metamorphic belt of Japan are characterized by skeletal structures containing abundant quartz inclusions. Each quartz inclusion was angular-shaped, and showed random crystallographic orientations, suggesting that these quartz inclusions were trapped via grain boundary cracking during garnet growth. Such skeletal garnet within the quartz schist formed related to decarbonation reactions with a positive total volume change (?V t > 0), whereas the euhedral garnet within the pelitic schists formed as a result of dehydration reaction with negative ?V t values. Coupled hydrological–chemical–mechanical processes during metamorphic devolatilization reactions were investigated by a distinct element method (DEM) numerical simulation on a foliated rock that contained reactive minerals and non-reactive matrix minerals. Negative ?V t reactions cause a decrease in fluid pressure and do not produce fractures within the matrix. In contrast, a fluid pressure increase by positive ?V t reactions results in hydrofracturing of the matrix. This fracturing preferentially occurs along grain boundaries and causes episodic fluid pulses associated with the development of the fracture network. The precipitation of garnet within grain boundary fractures could explain the formation of the skeletal garnet. Our DEM model also suggests a strong influence of reaction-induced fracturing on anisotropic fluid flow, meaning that dominant fluid flow directions could easily change in response to changes in stress configuration and the magnitude of differential stress during prograde metamorphism within a subduction zone.  相似文献   

10.
A dramatic demonstration of the role of intergranular solubility in promoting chemical equilibration during metamorphism is found in the unusual zoning of garnet in pelitic schist exposed at Harpswell Neck, Maine, USA. Many garnet crystals have irregular, patchy distributions of Mn, Cr, Fe and Mg in their inclusion‐rich interiors, transitioning to smooth, concentric zoning in their inclusion‐poor outer rims; in contrast, zoning of Ca and Y is comparatively smooth and concentric throughout. We re‐assess the disputed origin of these zoning features by examining garnet growth in the context of the thermal and structural history of the rocks, and by evaluating the record of fluid–rock interaction revealed in outcrop‐scale veining and fluid‐inclusion assemblages. The transition in the character of garnet zoning correlates with the onset of a synkinematic, simple‐shear‐dominated phase of garnet growth and with a shift in the composition of the intergranular fluid from CO2‐rich to H2O‐rich. Compositional variations in garnet are therefore best explained by a two‐stage growth history in which intergranular diffusive fluxes reflect differences in the concentration of dissolved species in these two contrasting fluids. Interiors of garnet crystals grew in the presence of a CO2‐rich fluid, in which limited solubility for Mn and Cr (and perhaps Fe and Mg) produced patchy disequilibrium overprint zoning, while appreciable solubility for Ca and Y permitted their rock‐wide equilibration. Rims grew in the presence of an H2O‐rich fluid, in which high intergranular concentrations for all elements except Cr enabled diffusion over length scales sufficient for rock‐wide equilibration. This striking example of partial chemical equilibrium during reaction and porphyroblast growth implies that thermal effects may commonly be subsidiary in importance to solubilities in the intergranular medium as determinants of length scales for metamorphic equilibration.  相似文献   

11.
冀磊  刘福来  王舫  田忠华 《岩石学报》2021,37(2):513-529
石榴夕线片麻岩是中、下地壳主要组成岩石之一,岩石内石榴石和夕线石的结晶学优选方位会显著影响地壳深部流变性质,因此探讨特征变质矿物的变形机制和主要受控因素对构造带深部演化过程有深远意义。本文选取红河-哀牢山韧性剪切带内石榴夕线片麻岩为研究对象,通过定向切片内显微构造、电子探针、X-ray成分扫描、电子背散射衍射(EBSD)和相平衡模拟综合研究,揭示出石榴石在溶解沉淀反应过程中具有明显的粒度敏感性,不同粒径石榴石表现出截然不同的长宽比、成分环带、包裹体排列方式和压力影发育情况。石榴石表面流体活动明显截切早期生长环带。EBSD分析揭示石榴石破碎颗粒以绕〈112〉轴机械旋转为主,溶解过程主要集中于颗粒表面和裂隙内高曲率位置。夕线石的EBSD结果表明基质内夕线石以绕〈010〉轴旋转为主,而流体作用明显区域夕线石则以(100)[001]滑移为主。岩石相平衡模拟限定岩石变质峰期P-T条件达高压麻粒岩相,退变过程中同剪切变形导致大量流体渗入而形成降温降压退变轨迹,由~9.5kbar、760℃演化至~6.0kbar、500~600℃,并在粗粒石榴石内保存早期进变质环带,剪切抬升过程中石榴石内普遍发育垂直剪切方向的裂隙,并在流体作用下进一步改造其形态。此研究揭示红河-哀牢山剪切带内除前人报道的石榴石高温韧性变形外,还存在大量中-上地壳层次同变质反应下的溶解-沉淀蠕变作用。因此,石榴石变质-变形的综合研究有助于揭示变质杂岩带挤压-剪切-伸展多阶段构造演化过程。  相似文献   

12.
Migmatites with sub‐horizontal fabrics at the eastern margin of the Variscan orogenic root in the Bohemian Massif host lenses of eclogite, kyanite‐K‐feldspar granulite and marble within a matrix of migmatitic paragneiss and amphibolite. Petrological study and pseudosection modelling have been used to establish whether the whole area experienced terrane‐wide exhumation of lower orogenic crust, or whether smaller portions of higher‐pressure lower crust were combined with a lower‐pressure matrix. Kyanite‐K‐feldspar granulite shows peak conditions of 16.5 kbar and 850 °C with no clear indications of prograde path, whereas in the eclogite the prograde path indicates burial from 10 kbar and 700 °C to a peak of 18 kbar and 800 °C. Two contrasting prograde paths are identified within the host migmatitic paragneiss. The first path is inferred from the presence of staurolite and kyanite inclusions in garnet that contains preserved prograde zoning that indicates burial with simultaneous heating to 11 kbar and 800 °C. The second path is inferred from garnet overgrowths of a flat foliation defined by sillimanite and biotite. Garnet growth in such an assemblage is possible only if the sample is heated at 7–8 kbar to around 700–840 °C. Decompression is associated with strong structural reworking in the flat fabric that involves growth of sillimanite in paragneiss and kyanite‐K‐feldspar granulite at 7–10 kbar and 750–850 °C. The contrasting prograde metamorphic histories indicate that kilometre‐scale portions of high‐pressure lower orogenic crust were exhumed to middle crustal levels, dismembered and mixed with a middle crustal migmatite matrix, with the simultaneous development of a flat foliation. The contrasting P–T paths with different pressure peaks show that tectonic models explaining high‐pressure boudins in such a fabric cannot be the result of heterogeneous retrogression during ductile rebound of the whole orogenic root. The P–T paths are compatible with a model of heterogeneous vertical extrusion of lower crust into middle crust, followed by sub‐horizontal flow.  相似文献   

13.
A detailed analysis of chemical zoning in two garnet crystals from Harpswell Neck, Maine, forms the basis of an interpretation of garnet nucleation and growth mechanisms. Garnet apparently nucleates initially on crenulations of mica and chlorite and quickly overgrows the entire crenulation, giving rise to complex two‐dimensional zoning patterns depending on the orientation of the thin section cut. Contours of Ca zoning cross those of Mn, Fe and Mg, indicating a lack of equilibrium among these major garnet constituents. Zoning of Fe, Mg and Mn is interpreted to reflect equilibrium with the rock matrix, whereas Ca zoning is interpreted to be controlled by diffusive transport between the matrix and the growing crystal. Image analysis reveals that the growth of garnet is more rapid along triple‐grain intersections than along double‐grain boundaries. Moreover, different minerals are replaced by garnet at different rates. The relative rate of replacement by garnet along double‐grain boundaries is ordered as muscovite > chlorite > plagioclase > quartz. Flux calculations reveal that replacement is limited by diffusion of Si along double‐grain boundaries to or from the local reaction site. It is concluded that multiple diffusive pathways control the bulk replacement of the rock matrix by garnet, with Si and Al transport being rate limiting in these samples.  相似文献   

14.
Prograde P–T paths recorded by the chemistry of minerals of subduction‐related metamorphic rocks allow inference of tectonic processes at convergent margins. This paper elucidates the changing P–T conditions during garnet growth in pelitic schists of the Sambagawa metamorphic belt, which is a subduction related metamorphic belt in the south‐western part of Japan. Three types of chemical zoning patterns were observed in garnet: Ca‐rich normal zoning, Ca‐poor normal zoning and intrasectoral zoning. Petrological studies indicate that normally‐zoned garnet grains grew keeping surface chemical equilibrium with the matrix, in the stable mineral assemblage of garnet + muscovite + chlorite + plagioclase + paragonite + epidote + quartz ± biotite. Pressure and temperature histories were inversely calculated from the normally‐zoned garnet in this assemblage, applying the differential thermodynamic method (Gibbs' method) with the latest available thermodynamic data set for minerals. The deduced P–T paths indicate slight increase of temperature with increasing pressure throughout garnet growth, having an average dP/dT of 0.4–0.5 GPa/100 °C. Garnet started growing at around 470 °C and 0.6 GPa to achieve the thermal and baric peak condition near the rim (520 °C, 0.9 GPa). The high‐temperature condition at relatively low pressure (for subduction related metamorphism) suggests that heating occurred before or simultaneously with subduction.  相似文献   

15.
Four amphibolite facies pelitic gneisses from the western Mongolian Altai Range exhibit multistage aluminosilicate formation and various chemical‐zoning patterns in garnet. Two of them contain kyanite in the matrix and sillimanite inclusions in garnet, and the others have kyanite inclusions in garnet with sillimanite or kyanite in the matrix. The Ca‐zoning patterns of the garnet are different in each rock type. U–Th–Pb monazite geochronology revealed that all rock units experienced a c. 360 Ma event, and three of them were also affected by a c. 260 Ma event. The variations in the microstructures and garnet‐zoning profiles are caused by the differences in the (i) whole‐rock chemistry, (ii) pressure conditions during garnet growth at c. 360 Ma and (iii) equilibrium temperatures at c. 260 Ma. The garnet with sillimanite inclusions records an increase in pressure at low‐P (~5.2–7.2 kbar) and moderate temperature conditions (~620–660 °C) at c. 360 Ma. The garnet with kyanite inclusions in the other rock types was also formed during an increase in pressure but at higher pressure conditions (~7.0–8.9 kbar at ~600–640 °C). The detrital zircon provenance of all the rock types is similar and is consistent with that from the sedimentary rocks in the Altai Range, suggesting that the provenance of all the rock types was a surrounding accretionary wedge. One possible scenario for the different thermal gradient is Devonian ridge subduction beneath the Altai Range, as proposed by several researchers. The subducting ridge could have supplied heat to the accretionary wedge and elevated the geotherm at c. 360 Ma. The differences in the thermal gradients that resulted in varying prograde P–T paths might be due to variations in the thermal regimes in the upper plate that were generated by the subducting ridge. The c. 260 Ma event is characterized by a relatively high‐T/P gradient (~25 °C km?1) and may be due to collision‐related granitic activity and re‐equilibrium at middle crustal depths, which caused the variations in the aluminosilicates in the matrix between the rock units.  相似文献   

16.
The analysis of texture, major element and oxygen isotope compositions of cloudy garnet crystals from a metapelite sampled on Ikaria Island (Greece) is used to assess the model of growth and re‐equilibration of these garnet crystals and to reconstruct the pressure–temperature–fluid history of the sample. Garnet crystals show complex textural and chemical zoning. Garnet cores (100–200 μm) are devoid of fluid inclusions. They are characterized by growth zoning demonstrated by a bell‐shaped profile of spessartine component (7–3 mol.%), an increase in grossular from 14 to 22 mol.% and δ18O values between 9.5 ± 0.3‰ and 10.4 ± 0.2‰. Garnet inner rims (90–130 μm) are fluid inclusion‐rich and show a decreasing grossular component from 22 to 5 mol.%. The trend of the spessartine component observed in the inner rim allows two domains to be distinguished. In contrast to domain I, where the spessartine content shows the same trend as in the core, the spessartine content of domain II increases outwards from 2 to 14 mol.%. The δ18O values decrease towards the margins of the crystals to a lowest value of 7.4 ± 0.2‰. The outer rims (<10 μm) are devoid of fluid inclusions and have the same chemical composition as the outermost part of domain II of the inner rim. Garnet crystals underwent a four‐stage history. Stage 1: garnet growth during the prograde path in a closed system for oxygen. Garnet cores are remnants of this growth stage. Stage 2: garnet re‐equilibration by coupled dissolution–reprecipitation at the temperature peak (630 < T < 650 °C). This causes the creation of porosity as the coupled dissolution–reprecipitation process allows chemical (Ca) and isotopic (O) exchange between garnet inner rims and the matrix. The formation of the outer rim is related to the closure of porosity. Stage 3: garnet mode decreases during the early retrograde path, but garnet is still a stable phase. The resulting garnet composition is characterized by an increasing Mn content in the inner rim’s domain II caused by intracrystalline diffusion. Stage 4: dissolution of garnet during the late retrograde path as garnet is not a stable phase anymore. This last stage forms corroded garnet. This study shows that coupled dissolution–reprecipitation is a possible re‐equilibration process for garnet in metamorphic rocks and that intra‐mineral porosity is an efficient pathway for chemical and isotopic exchange between garnet and the matrix, even for otherwise slow diffusing elements.  相似文献   

17.
Major and trace‐element zoning in garnet, in combination with Rb–Sr, Sm–Nd and Lu–Hf geochronology, provide evidence for a protracted garnet growth history for the Zermatt‐Saas Fee (ZSF) ophiolite, western Alps. Four new Lu–Hf ages from Pfulwe (c. 52–46 Ma) and one from Chamois (c. 52 Ma) are very similar to a previously published Lu–Hf age from Lago di Cignana. Overall, the similarity of geochronological and garnet zoning patterns suggests that these three localities had a similar prograde tectonic history, commensurate with their similar structural position near the top of the ZSF. Samples from the lower part of the ZSF at Saas Fee and St. Jacques, however, produced much younger Lu–Hf ages (c. 41–38 Ma). Neither differences in whole‐rock geochemistry, which might produce distinct garnet growth histories, nor rare‐earth‐element zoning in garnet, can account for the age differences in the two suites. This suggests a much later prograde history for the lower part of the ZSF, supporting the idea that it was subducted diachronously. Such a model is consistent with changes in subduction vectors based on plate tectonic reconstructions, where early oblique subduction, which produced long prograde garnet growth, changed to more orthogonal subduction, which corresponds to shorter prograde garnet growth. Six new Rb–Sr phengite ages range from c. 42 to 39 Ma and, in combination with previously published Rb–Sr ages, constrain the timing of the transition from eclogite to upper greenschist facies P–T conditions. The proximity of the ZSF in the Saas Fee region to the underlying continental Monte Rosa unit and the similarity of peak‐metamorphic ages suggest these two units were linked for part of their tectonic history. This in turn indicates that the Monte Rosa may have been partly responsible for rapid exhumation of the ZSF unit.  相似文献   

18.
High-pressure schists (2–2.5 GPa) from the Eclogite Zone in the Tauern Window contain honeycomb garnet in which fine webs of garnet surround strain-free quartz ± carbonate grains. High-resolution X-ray computed tomography shows that the garnet webs form a cellular structure that coats all surfaces of the inclusions. Electron backscatter diffraction analysis shows that the garnet cells are crystallographically continuous with more massive garnet regions, and that the quartz ± carbonate inclusions have random orientations; in contrast, matrix quartz exhibits a prominent crystallographic preferred orientation (CPO). High-resolution transmission electron microscopy shows few dislocations in either the garnet or the inclusion quartz. Most honeycomb garnet is chemically homogeneous, but some displays asymmetric core–rim zoning. Taken together, these observations are most consistent with formation of the garnet sheets via precipitation from a wetting fluid along quartz–quartz grain boundaries, or possibly via wholesale precipitation of garnet + quartz ± carbonate from a fluid. In either case, a silicate-rich aqueous fluid must have been present. The likelihood that a fully wetting fluid existed at high pressure has important implications for rheology during subduction of metasedimentary rocks: strain may be accommodated by grain rotation and sliding in an aqueous silicate slurry, rather than via dislocation creep mechanisms at high pressures. The absence of a CPO in early quartz may thus point to involvement of a pervasive grain-boundary fluid rather than requiring low differential stresses during subduction.  相似文献   

19.
An automated method for the calculation of P–T paths based on garnet zoning is presented and used to interpret zoning in metapelitic schist from the southern Canadian Cordillera. The approach adopted to reconstruct the P–T path is to match garnet compositions along a radial transect with predictions from thermodynamic forward models, while iteratively modifying the composition to account for fractional crystallization. The method is applied to a representative sample of garnet‐ and staurolite‐bearing schist from an amphibolite facies Barrovian belt in the southern Canadian Omineca belt. Garnet zoning in these schists is concentric and largely continuous from core to rim. Three zones are present, the first two of which coincide with sector‐zoned cores of garnet crystals. Similar zoning is developed in rocks that contain or lack staurolite, respectively, suggesting garnet growth was restricted to the initial part of the prograde P–T path prior to the development of staurolite. Growth zoning in large garnet crystals has not been significantly modified by diffusion. This interpretation is based on zoning characteristics of garnet crystals and is further supported by results of a forward model incorporating the effects of simultaneous fractional crystallization and intracrystalline diffusion. The P–T path calculated for this rock includes an initial, linear stage with a high dP/dT, and a later stage dominated by heating. The approach adopted in this study may have application to other garnet‐bearing rocks in which growth zoning is preserved.  相似文献   

20.
During prograde metamorphism garnet and, in some higher grade samples, staurolite were produced in a chlorite-chloritoid schist, part of the Precambrian Z to Cambrian Hoosac Formation near Jamaica, VT. Garnet grew during two prograde events separated by a retrogression. This sequence resulted in distinctive inclusion textures and zoning anomalies in garnet produced by diffusive alteration. Textures, reaction space analysis, and mineral compositional variations constrain the possible sequence of reactions in these rocks. Below the staurolite isograd, and to some unknown extent above it, garnet grew by the reaction chloritoid+chlorite+quartz→garnet+H2O. With increasing grade the mineral compositions are displaced towards lower Mn/Fe and higher Mg/Fe ratios. The data are compatible with equilibrium with respect to exchange reactions for the matrix assemblages on a thin section scale and with minerals having closely followed equilibrium paths during reaction. The staurolite isograd coincides with the reaction chloritoid+quartz→garnet+staurolite+chlorite+H2O. This reaction is continuous and trivariant with ZnO becoming an additional component concentrated in staurolite. During this reaction both the Mn/Fe and Mg/Fe ratios of the phases appear to have decreased. This new chemical trend is recorded by garnet zoning profiles and is compatible with trends predicted from phase diagrams. Thus there are two distinct types of garnet zoning reversals in these samples. One is near the textural unconformity and is best explained by diffusive alteration during partial resorption of first stage garnet. The other occurs near the outer rim of garnet in staurolite zone samples and marks the onset of a new prograde garnet producing reaction.  相似文献   

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