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1.
The basement of the North China craton (NCC) can be divided into eastern and western blocks separating the Trans-North China orogen on the basis of petrologic associations, structures, metamorphic processes, and isotopic ages. Aluminous gneiss khondalites occur in the western block, and record a clockwise metamorphic P–T history characterized by nearly isothermal decompression following peak metamorphism at ca. 1.3 GPa and 825°C. Four metamorphic stages are recognized based on mineral assemblages. The early prograde metamorphic assemblage contains Ky+Bt+Ms+Grt+Pl+Qtz. The peak metamorphic mineral assemblage is characterized by Grt+Sil+Bt+Kfs+Pl+Qtz and the formation of cordierite after garnet, leading to a retrograde assemblage of Grt+Sil+Crd+Pl+Kfs+Qtz. Garnet retrogrades to biotite and the formation of pervasive matrix muscovite define a final metamorphic stage, inferred at ca. < 0.6 GPa and 700°C. Quantified metamorphic stages and a related clockwise P–T path derived from pseudosection analysis in the KMASH system suggest collision of the north Yinshan block with the South Ordos block at 1.92 Ga, before final suturing of the entire NCC basement.  相似文献   

2.
The Legs Lake shear zone is a crustal‐scale thrust fault system in the western Canadian Shield that juxtaposes high‐pressure (1.0+ GPa) granulite facies rocks against shallow crustal (< 0.5 GPa) amphibolite facies rocks. Hangingwall decompression is characterized by breakdown of the peak assemblage Grt + Sil + Kfs + Pl + Qtz into the assemblage Grt + Crd + Bt ± Sil + Pl + Qtz. Similar felsic granulite occurs throughout the region, but retrograde cordierite is restricted to the immediate hangingwall of the shear zone. Textural observations, petrological analysis using P–T/P–MH2O phase diagram sections, and in situ electron microprobe monazite geochronology suggest that decompression from peak conditions of 1.1 GPa, c. 800 °C involved several distinct stages under first dry and then hydrated conditions. Retrograde re‐equilibration occurred at 0.5–0.4 GPa, 550–650 °C. Morphology, X‐ray maps, and microprobe dates indicate several distinct monazite generations. Populations 1 and 2 are relatively high yttrium (Y) monazite that grew at 2.55–2.50 Ga and correspond to an early granulite facies event. Population 3 represents episodic growth of low Y monazite between 2.50 and 2.15 Ga whose general significance is still unclear. Population 4 reflects low Y monazite growth at 1.9 Ga, which corresponds to the youngest period of high‐pressure metamorphism. Finally, population 5 is restricted to the hydrous retrograded granulite and represents high Y monazite growth at 1.85 Ga that is linked directly to the synkinematic garnet‐consuming hydration reaction (KFMASH): Grt + Kfs + H2O = Bt + Sil + Qtz. Two samples yield weighted mean microprobe dates for this population of 1853 ± 15 and 1851 ± 9 Ma, respectively. Subsequent xenotime growth correlates with the reaction: Grt + Sil + Qtz + H2O = Crd. We suggest that the shear zone acted as a channel for fluid produced by dehydration of metasediments in the underthrust domain.  相似文献   

3.
The Seiland Igneous Province of the North Norwegian Caledonides consists of a suite of deep-seated rift-related magmatic rocks emplaced into paragneisses during late Precambrian to Ordovician time. In the south-eastern part of the province, contact metamorphism of the paragneisses and later reworking of intrusives and associated contact aureoles have resulted in the development of three successive metamorphic stages. The contact metamorphic assemblage (M1) Opx + Grt + Qtz + Pl + Kfs + Hc + Ilm ± Crd is preserved in xenolithic rafts of paragneiss within metagabbro. Geothermobarometric calculations yield 930-960d? C and 5-6.5 kbar for the contact metamorphism. M1 was followed by cooling, accompanied by strong shearing, formation of the gneiss foliation and recrystallization at intermediate-P granulite facies conditions (M2). Stable M2 phases are Cpx + Opx + Pl +Ilm ± Hbl in metagabbro and Grt ± Sil ± Opx + Kfs + Qtz + Pl ± Bt + Ilm in host paragneiss. The M2 conditions are estimated to 700-750d? C and 5-7 kbar. A subsequent pressure increase is recorded in the M3 episode, which is associated with recrystallization in narrow ductile shear zones and secondary growth on M2 minerals. M3 is defined by the assemblages Grt + Cpx ± Opx + Pl + Ru + Qtz in metagabbro, and Grt ± Ky + Qtz + Pl ± Kfs + Bt + Ru in host paragneiss. M3 conditions are estimated to 650-700d? C and 8-10 kbar. The substantial pressure increase related to the M2 → M3 transition is interpreted to be a result of (early?) Caledonian overthrusting. Chemical zoning in cordierite and biotite suggest rapid cooling following the M3 event. The proposed P-T-t evolution implies that the tectonic evolution of the Seiland Igneous Province was long (at least 330 Ma) and complex and involved initial rifting and extension followed by crustal thickening and compression.  相似文献   

4.
Eclogites in the Tromsø area, northern Norway, are intimately associated with meta-supracrustals within the Uppermost Allochthon of the Scandinavian Caledonides (the Tromsø Nappe Complex). The whole sequence, which includes pelitic to semipelitic schists and gneisses, marbles and calc-silicate rocks, quartzofeldspathic gneisses, metabasites and ultramafites, has undergone three main deformational/metamorphic events (D1/M1, D2/M2 and D3/M3). Detailed structural, microtextural and mineral chemical studies have made it possible to construct separate P–T paths for these three events. Chemically zoned late syn- to post-D1 garnets with inclusions of Bt, Pl and Qtz in Ky-bearing metapelites indicate a prograde evolution from 636°C, 12.48 kbar to c. 720°C, 14–15 kbar. This latter result is in agreement with Grt–Cpx geothermometry and Grt–Cpx–Pl–Qtz geobarometry on eclogites and trondhjemitic to dioritic gneisses. Maximum pressures at c. 675°C probably reached 17–18 kbar based on Cpx–Pl–Qtz inclusions in eclogitic garnets, and Grt–Ky–Pl–Qtz and Jd–Ab–Qtz in trondhjemitic gneisses. Post-D1/pre-D2 decompressional breakdown of the high-P assemblages indicates a substantial drop in pressure at this stage. Inclusions and chemical zoning in syn- to post-D2 garnets from metapelites record a second episode of prograde metamorphism, from 552°C, 7.95 kbar, passing through a maximum pressure of 10.64 kbar at 644°C, with final equilibration at c. 665°C, 9–10 kbar. The corresponding apparently co-facial paragenesis Grt + Cpx + Pl + Qtz in metabasites yields c. 635°C, 8–10 kbar. In the metapelites post-D3, Grt in apparent equilibrium with Bt, Phe and Pl yield c. 630°C, 9 kbar. The D1/M1 and D2/M2 episodes are exclusively recorded in the Tromsø Nappe Complex and must thus pre-date the emplacement of this allochthonous unit on top of the underlying Lyngen Nappe, while the D3/M3 episode is common for the two units. A previously published Sm–Nd mineral isochron (Grt–Cpx–Am) on a partly retrograded and recrystallized ecologite of 598 ± 107 Ma represents either the timing of formation of the eclogites or the post-eclogite/pre-D2 decompression stage, while a Rb–Sr whole rock isochron of an apparently post-D1/pre-D2 granite of 433 ± 11 Ma is consistent with a K–Ar age of post-D1/pre-D2 amphiboles from a retrograded eclogite of 437 ± 16 Ma which most likely record cooling below the 475–500°C isotherm after the M3 metamorphism.  相似文献   

5.
The Meatiq basement, which is exposed beneath late Proterozoic nappes of supracrustal rocks in the Central Eastern Desert of Egypt, was affected by three metamorphic events. The ophiolite cover nappes show only the last metamorphic overprint. The M1 metamorphic event (T ≥750 °C) is restricted to migmatized amphibolite xenoliths within the Um Ba′anib orthogneiss in the structurally lowest parts of the basement. Typical upper amphibolite facies M2 mineral assemblages include Grt–Zn-rich Spl–Qtz±Bt, Grt–Zn-rich Spl–Ms–Kfs–Bt–Sil–Qtz and locally kyanite in metasedimentary rocks. The mineral assemblages Ms–Qtz–Kfs–Sil in the matrix and Sil–Grt in garnet cores indicate that peak M2 P–T conditions exceeded muscovite and staurolite stabilities. Diffusional equilibration at M2 peak temperature conditions caused homogeneous chemical profiles across M2 garnets. Abundant staurolite in garnet rims and the matrix indicates a thorough equilibration during M2 at decreasing temperature conditions. M2 P–T conditions ranged from 610 to 690 °C at 6–8 kbar for the metamorphic peak and 530–600 °C at about 5.8 kbar for the retrograde stage. However, relic kyanite indicates pressures above 8 kbar, preceeding the temperature peak. A clockwise P–T path is indicated by abundant M2 sillimanite after relic kyanite and by andalusite after sillimanite. M2 fluid inclusions, trapped in quartz within garnet and in the quartz matrix show an array of isochores. Steepest isochores (water-rich H2O-CO2±CH4/N2 inclusions) pass through peak M2 P–T conditions and flatter isochores (CO2-rich H2O-CO2±CH4/N2 inclusions) are interpreted to represent retrograde fluids which is consistent with a clockwise P–T path for M2. The M3 assemblage Grt–Chl in the uppermost metasedimentary sequence of the basement limits temperature to 460 to 550 °C. M3 temperature conditions within the ophiolite cover nappes are limited by the assemblage Atg–Trem–Tlc to<540 °C and the absence of crysotile to >350 °C. The polymetamorphic evolution in the basement contrasts with the monometamorphic ophiolite nappes. The M1 metamorphic event in the basement occurred prior to the intrusion of the Um Ba′anib granitoid at about 780 Ma. The prograde phase of the M2 metamorphic event took place during the collision of an island arc with a continent. The break-off of the subducting slab increased the temperature and resulted in the peak M2 mineral assemblages. During the rise of the basement domain retrograde M2 mineral assemblages were formed. The final M3 metamorphic event is associated with the updoming of the basement domain at about 580 Ma along low-angle normal faults.  相似文献   

6.
We report a new occurrence of incipient charnockite from Mavadi in the Trivandrum Granulite Block (TGB), southern India, and discuss the petrogenesis of granulite formation in an arrested stage on the basis of petrography, geothermobarometry, and mineral equilibrium modeling. In Mavadi, patches and lenses of charnockite (Kfs?+?Qtz?+?Pl?+?Bt?+?Grt?+?Opx?+?Ilm?+?Mag) of about 30 to 220 cm in length occur within Opx-free Grt-Bt gneiss (Kfs?+?Qtz?+?Pl?+?Bt?+?Grt?+?Ilm). The application of mineral equilibrium modeling on the charnockite assemblage in the NCKFMASHTO system to constrain the conditions of charnockitization defines a P–T range of 800 °C at 4.5 kbar to 850 °C at 8.5 kbar, which is broadly consistent with the results from the conventional geothermobarometry (810–880 °C at 7.7–8.0 kbar) on these rocks. The P–T conditions are lower than the peak metamorphic conditions reported for the ultrahigh-temperature granulites from this area (T?>?900 °C). The heterogeneity in peak P–T conditions within the same crustal block might be related to local buffering of metamorphic temperatures by the Opx-Bt-Kfs-Qtz assemblage. The result of T versus mole H2O (M(H2O)) modeling demonstrated that the Opx-free assemblage in the Grt-Bt gneiss is stable at M(H2O)?=?0.3 to 1.5 mol%, and orthopyroxene occurs as a stable mineral at M(H2O) <0.3 mol%, which is consistent with the petrogenetic model of incipient charnockite related to the lowering of the water activity and stabilization of orthopyroxene through the breakdown of biotite by dehydration caused by the infiltration of CO2-rich fluid from external sources. We also propose a possible alternative mechanism to form charnockite from Grt-Bt gneiss through slight variations in bulk-rock chemistry (particularly for the K- and Fe-rich portion of Grt-Bt gneiss) that can enhance the stability of orthopyroxene rather than that of biotite, with K-metasomatism playing a possible role.  相似文献   

7.
Low‐pressure crystal‐liquid equilibria in pelitic compositions are important in the formation of low‐pressure, high‐temperature migmatites and in the crystallization of peraluminous leucogranites and S‐type granites and their volcanic equivalents. This paper provides data from vapour‐present melting of cordierite‐bearing pelitic assemblages and augments published data from vapour‐present and vapour‐absent melting of peraluminous compositions, much of which is at higher pressures. Starting material for the experiments was a pelitic rock from Morton Pass, Wyoming, with the major assemblage quartz‐K feldspar‐biotite‐cordierite, approximately in the system KFMASH. A greater range in starting materials was obtained by addition of quartz and sillimanite to aliquots of this rock. Sixty‐one experiments were carried out in cold‐seal apparatus at pressures of 1–3.5 kbar (particularly 2 kbar) and temperatures from 700 to 840 °C, with and without the addition of water. In the vapour‐present liquidus relations at 2 kbar near the beginning of melting, the sequence of reactions with increasing temperature is: Qtz + Kfs + Crd + Sil + Spl + V = L; Qtz + Kfs + Crd + Spl + Ilm + V = Bt + L; and Qtz + Bt + V = Crd + Opx + Ilm + L. Vapour‐absent melting starts at about 800 °C with a reaction of the form Qtz + Bt = Kfs + Crd + Opx + Ilm + L. Between approximately 1–3 kbar the congruent melting reaction is biotite‐absent, and biotite is produced by incongruent melting, in contrast to higher‐pressure equilibria. Low pressure melts from pelitic compositions are dominated by Qtz‐Kfs‐Crd. Glasses at 820–840 °C have calculated modes of approximately Qtz42Kfs46Crd12. Granites or granitic leucosomes with more than 10–15% cordierite should be suspected of containing residual cordierite. The low‐pressure glasses are quite similar to the higher‐pressure glasses from the literature. However, XMg increases from about 0.1–0.3 with increasing pressure from 1 to 10 kbar, and the low‐temperature low‐pressure glasses are the most Fe‐rich of all the experimental glasses from pelitic compositions.  相似文献   

8.
The Mollendo–Camana Block (MCB) is a 50 × 150 km Precambrian inlier of the Andean belt that outcrops along the Pacific coast of southern Peru. It consists of stromatic migmatites of Paleoproterozoic heritage intensely metamorphosed during the Grenville event (c. 1 Ga; U‐Pb and U‐Th‐Pb ages on zircon and monazite). In the migmatites, aluminous mesosomes (FMAS) and quartzofeldspathic leucosomes (KFMASH), contain various amounts of K‐feldspar (Kfs), orthopyroxene (XMg Opx = 0.86), plagioclase (Pl), sillimanite (Sil; exceptionally kyanite, Ky) ilmenite (Ilm), magnetite (Mag), quartz (Qtz), and minor amounts of garnet (XMg Grt = 0.60), sapphirine (XMg Spr = 0.87), cordierite (XMg Crd = 0.92) and biotite (XMg Bt = 0.83). The ubiquitous peak mineral assemblage is Opx‐Sil‐Kfs‐Qtz‐(± Grt) in most of the MCB, which, together with the high Al content of orthopyroxene (10% Al2O3) and the local coexistence of sapphirine‐quartz, attest to regional UHT metamorphism (> 900 °C) at pressures in excess of 1.0 GPa. Fluid‐absent melting of biotite is responsible for the massive production of orthopyroxene that proceeded until exhaustion of biotite (and most of the garnet) in the southern part of the MCB (Mollendo‐Cocachacra areas). In this area, a first stage of decompression from 1.1–1.2 to 0.8–0.9 GPa at temperatures in excess of 950 °C, is marked by the breakdown of Sil‐Opx to Spr‐Opx‐Crd assemblages according to several bivariant FMAS reactions. High‐T decompression is also shown by Mg‐rich garnet being replaced by Crd‐Spr‐ and Crd‐Opx‐bearing symplectites, and reacting with quartz to produce low‐Al‐Opx‐Sil symplectites in quartz‐rich migmatites. Neither osumilite nor spinel‐quartz assemblages being formed, isobaric cooling at about 0.9 GPa probably followed the initial decompression and proceeded with massive precipitation of melts towards the (Os) invariant point, as demonstrated by Bt‐Qtz‐(± pl) symplectites in quartz‐rich migmatites (melt + Opx + Sil = Bt + Grt + Kfs + Qtz). Finally, Opx rims around secondary biotite attest to late fluid‐absent melting, compatible with a second stage of decompression below 900 °C. The two stages of decompression are interpreted as due to rapid tectonic denudation whereas the regional extent of UHT metamorphism in the area, probably results from large‐scale penetration of hot asthenospheric mantle at the base of an over‐thickened crust.  相似文献   

9.
Garnet–biotite–(sillimanite) gneiss (～700 °C, 7 kbar) of the Otter Lake area in the Western Grenville Province (Canadian Shield) occurs as granitic gneiss (group 4) that forms a large part of the Otter Complex, and as widely distributed, more heterogenous metasedimentary gneiss (group 2). In one sample of group 4 gneiss (Qtz25 Pl34 Kfs28 Bt10 Grt2.5 Sil1) the true diameter (determined by serial grinding) of subhedral garnet crystals ranges from 0.2 to 3.0 mm, with a mode at 1.0 mm. Nearest‐neighbour measurements in this sample, and in surfaces of nine additional samples (all <5% garnet) confirm that garnet crystals are distributed mainly at random; slight clustering was detected in two samples. In one sample of group 4 gneiss, microprobe analyses on sections through crystal centres (obtained by serial slicing), reveal that small crystals and margins to large crystals contain more Fe and Mn and less Mg than the broad central regions of large crystals. Based on these and previous results, together with theoretical considerations, a crystallization model is proposed, in which, (i) garnet was produced by the continuous reaction, Ms + Bt + Qtz → Grt + Kfs + H2O, (ii) nucleation occurred by the random selection of randomly distributed Ms–Bt–Qtz triple junctions, (iii) the rate of linear growth remained constant, and (iv) as temperature increased, the rate of nucleation first increased slowly, then remained nearly constant, and finally declined. Within‐population compositional homogenization was followed, on cooling, by local Fe–Mg–Mn exchange with biotite.  相似文献   

10.
ABSTRACT The metasedimentary sequence of the Deep Freeze Range (northern Victoria Land, Antarctica) experienced high-T/low-F metamorphism during the Cambro-Ordovician Ross orogeny. The reaction Bt + Sil + Qtz = Grt + Crd + Kfs + melt was responsible for the formation of migmatites. Peak conditions were c. 700–750° C, c. 3.5–5 kbar and xH2Oc. 0.5). Distribution of fluid inclusions is controlled by host rock type: (1) CO2-H2O fluid inclusions occur only in graphite-free leucosomes; (2) CO2–CH4± H2O fluid inclusions are the most common type in leucosomes, and in graphite-bearing mesosomes and gneiss; and (3) CO2–N2–CH4 fluid inclusions are observed only in the gneiss, and subordinately in mesosomes. CO2–H2O mixtures (41% CO2, 58% H2O, 1% Nad mol.%) are interpreted as remnants of a synmig-matization fluid; their composition and density are compatible P–T–aH2O conditions of migmatization (c. 750° C, c. 4 kbar, xH2Oc. 0.5). CO2-H2O fluid in graphite-free leucosomes cannot originate via partial melting of graphite-bearing mesosomes in a closed system; this would have produced a mixed CO2–CH4 fluid in the leucosomes by a reaction such as Bt + Sil + Qtz + C ± H2O = Grt + Crd + Kfs + L + CO2+ CH4. We conclude that an externally derived oxidizing CO2-H2O fluid was present in the middle crust and initiated anatexis. High-density CO2-rich fluid with traces of CH4 characterizes the retrograde evolution of these rocks at high temperatures and support isobaric cooling (P–T anticlockwise path). In unmigmatized gneiss, mixed CO2–N2–CH4 fluid yields isochores compatible with peak metamorphic conditions (c. 700–750° C, c. 4–4.5 kbar); they may represent a peak metamorphic fluid that pre-dated the migmatization.  相似文献   

11.
Abstract Dehydration-melting reactions, in which water from a hydrous phase enters the melt, leaving an anhydrous solid assemblage, are the dominant mechanism of partial melting of high-grade rocks in the absence of externally derived vapour. Equilibria involving melt and solid phases are effective buffers of aH2,o. The element-partitioning observed in natural rocks suggests that dehydration melting occurs over a temperature interval during which, for most cases, aH2o is driven to lower values. The mass balance of dehydration melting in typical biotite gneiss and metapelite shows that the proportion of melt in the product assemblage at T± 850°C is relatively small (10–20%), and probably insufficient to mobilize a partially melted rock body. Granulite facies metapelite, biotite gneiss and metabasic gneiss in Namaqualand contain coarse-grained, discordant, unfoliated, anhydrous segregations, surrounded by a finer grained, foliated matrix that commonly includes hydrous minerals. The segregations have modes consistent with the hypothesis that they are the solid and liquid products of the dehydration-melting reactions: Bt + Sil + Qtz + PI = Grt ° Crd + Kfs + L (metapelite), Bt + Qtz + Pl = Opx + Kfs + L (biotite gneiss), and Hbl + Qtz = Opx + Cpx + Pl + L (metabasic gneiss). The size, shape, distribution and modes of segregations suggest only limited migration and extraction of melt. Growth of anhydrous poikiloblasts in matrix regions, development of anhydrous haloes around segregations and formation of dehydrated margins on metabasic layers enclosed in migmatitic metapelites all imply local gradients in water activity. Also, they suggest that individual segregations and bodies of partially melted rock acted as sinks for soluble volatiles. The preservation of anhydrous assemblages and the restricted distribution of late hydrous minerals suggest that retrograde reaction between hydrous melt and solids did not occur and that H2O in the melt was released as vapour on crystallization. This model, combined with the natural observations, suggests that it is possible to form granulite facies assemblages without participation of external fluid and without major extraction of silicate melt.  相似文献   

12.
A sequence of at least three Al2SiO5-bearing mineral assemblages are preserved in successively overprinted ductile shear zones in the Willimantic window, Connecticut. The ductile deformation, localized at and near the boundary between the Putnam-Nashoba terrane and underlying Avalon terrane is characterized by a network of anastomozing shear zones that outline metre-scale tectonic blocks of migmatitic Kfs + Sil + Gt + Bi + Pg + Qtz + Ilm + Ru gneiss. These assemblages record Acadian or older metamorphic conditions of 6 kbar, 700d? C. Mylonitic gneisses in shear zones that define block margins were formed by reconstitution and recrystallization of the migmatitic gneiss. The reconstituted rocks exhibit relict Ky + St + Grt (+Pl + Bt + Qtz + Rt + Ilm) assemblages and require a minimum pressure for the Ky-Str grade metamorphism of 8.5 kbar. Kyanite in block margins is widely replaced by sillimanite, and locally by andalusite, during a period of post-Alleghanian ductile deformation. The interiors of blocks do not record this sequence of polymorphs. The pattern of reconstitution is accounted for by localization of strain along block margins within a regionally extensive terrane-bounding fault zone. Strain provided the activation energy for recrystallization and retrograde mineral reactions. The P-T conditions of post-Alleghanian ductile deformation evolved from 600d? C and 6 kbar to 550d? C and 3 kbar. The occurrence of Ky + Str-bearing assemblages, overprinting Acadian Kfs + Sil-bearing assemblages and subsequently overprinted by Alleghanian sillimanite- and andalusite-bearing assemblages, along with reset hornblende 40Ar/39 Ar mineral ages from Mississippian to Permian, requires a prograde Alleghanian metamorphism of rocks previously metamorphosed during the Acadian. Thus, mineral assemblages from gneisses in the Willimantic fault zone retain evidence of a protracted tectonothermal evolution that included high-grade Acadian orogenesis, tectonic loading resulting from Alleghanian collision of Avalon with North America, and tectonic exhumation in Permo-Triassic time. The c.3-kbar pressure decrease between prograde and retrograde Alleghanian metamorphic conditions corresponds to 10 km of crust that must have been tectonically excised from the base of the Putnam-Nashoba terrane cover sequence following Alleghanian orogenesis in south-eastern New England.  相似文献   

13.
In the contact aureole of the Oligocene granodiorite of Cima di Vila, granitic pegmatites of Variscan age were strongly deformed during eo‐Alpine regional metamorphism, with local development of ultramylonites. In the ultramylonite matrix, consisting of quartz, plagioclase, muscovite and biotite, microstructures show grain growth of quartz within quartz ribbons, and development of decussate arrangements of mica. These features indicate that dynamic recrystallization related to mylonite development was followed by extensive static growth during contact metamorphism. K‐feldspar porphyroclasts up to 1.5 cm are mantled by myrmekite that forms a continuous corona with thickness of about 1 mm. In both XZ and YZ sections, myrmekite tubules are undeformed, and symmetrically distributed in the corona, and oligoclase‐andesine hosts have random crystallographic orientation. Myrmekite development has been modelled from the P–T–t evolution of the ultramylonites, assuming that the development of the ultramylonites occurred during eo‐Alpine metamorphism at c. 450 °C, 7.5 kbar, followed by contact metamorphism at c. 530 °C, 2.75 kbar. Phase diagram pseudosections calculated from the measured bulk composition of granitic pegmatite protolith indicate that the equilibrium assemblage changes from Qtz–Phe–Ab ± Zo ± Cpx ± Kfs during the ultramylonite stage to Qtz–Pl(An30–40)–Ms–Kfs–Bt(Ann55) during the contact metamorphic stage. The thermodynamic prediction of increasing plagioclase mode and anorthite content, change of white mica composition and growth of biotite, occurring during the end of the heating path, are in agreement with the observed microstructures and analysed phase compositions of ultramylonites. Along with microstructural evidence, this supports the model that K‐feldspar replacement by myrmekite took place under static conditions, and was coeval with the static growth accompanying contact metamorphism. Myrmekite associated with muscovite can develop under prograde (up‐temperature) conditions in granites involved in polymetamorphism.  相似文献   

14.
The early Precambrian khondalite series is widely distributed in the Jining-Zhuozi-Fengzhen-Liangcheng area, southeastern Inner Mongolia. The khondalite series mainly consists of sillimanite garnet potash feldspar (or two-feldspar) gneiss and garnet biotite plagioclase gneiss. These gneissic rocks have commonly experienced granulite-facies metamorphism. In zircons separated from sillimanite garnet potash feldspar gneisses, many mineral inclusions, including Sil, Grt, Ky, Kfs, Qtz and Ap, have been identified by the Laser Raman spectroscopy. Generally, prograde metamorphic mineral inclusion assemblages such as Ky + Kfs + Qtz + Ap and Ky + Grt + Kfs + Qtz are preserved in the core of zircon, while peak granulite-facies metamorphic minerals including Sil + Grt + Kfs + Qtz and Sil + Grt + Kfs + Qtz + Ap are identified in the mantle and rim of the same zircon. However, in some zircons are only preserved the peak metamorphic minerals such as Sil + Grt + Kfs + Qtz and Sil + Grt + Kfs + Qtz + Ap from core to ri  相似文献   

15.
We discuss upper-amphibolite to granulite facies, early Palaeozoic metamorphism and partial melting of aluminous greywackes from the Sierra de Comechingones, SE Sierras Pampeanas of Central Argentina. Consistent P–T estimates, obtained from equilibria involving Al and Ti exchange components in biotite and from more traditional thermobarometric equilibria, suggest that peak metamorphism of the exposed section took place at an essentially constant pressure of 7–8 kbar, and at temperatures ranging from 650 to 950 °C. Mineral compositions record an initial decompression, after peak metamorphism, of c. 1.5 kbar, which was accompanied by a cooling of c. 100 °C. Upper-amphibolite facies gneisses consist of the assemblage Qtz+Pl+Bt+Grt+Rt/Ilm. The transition to the granulite facies is marked by the simultaneous appearance of the assemblage Kfs+Sil and of migmatitic structures, suggesting that the amphibolite to granulite transition in the Sierra de Comechingones corresponds to the beginning of melting. Rocks with structural and/or chemical manifestations of partial melting range from metatexites, to diatexites, to melt-depleted granulites, consisting of the assemblage Grt+Crd+Pl+Qtz+Ilm±Ath. The melting stage overlapped at least partially with decompression, as suggested by the occurrence of cordierite, in both the migmatites and the residual granulites, of two distinct textural types: idiomorphic porphyroblasts (probably representing peritectic cordierite) and garnet-rimming coronas. Metapelitic rocks are unknown in the Sierra de Comechingones. Therefore, it appears most likely that the Al-rich residual assemblages found in the migmatites and residual granulites were formed by partial melting of muscovite- and sillimanite-undersaturated metagreywackes. We propose a mechanism for this that relies on the sub-solidus stabilization of garnet and the ensuing changes in the octahedral Al content of biotite with pressure and temperature.  相似文献   

16.
Four assemblages from calcic pelitic schists from South Strafford,Vermont, have been studied in detail to determine the relationshipbetween reaction history and compositional zoning of minerals.The lowest-grade assemblage is garnet + biotite + chlorite +plagioclase + epidote + quartz + muscovite + graphite + fluid.Along a path of isobaric heating, the net reaction is Chl +Ms + Ep + Gr = Grt + Bt + Pl + fluid. Garnet grows with decreasingFe/(Fe + Mg) and XSpa, (from 0•2 to 0•05), XGra staysnearly constant between 0•20 and 0•25, and plagioclasegrows with XAn increasing from peristerite to 0•2–0•5. The subsequent evolution depends on whether chlorite or epidotereacts out first. If chlorite is removed from the assemblagefirst, the net reaction along an isobaric heating path becomesGrt + Ms + Ep + Qtz + Gr = Bt + Pl + fluid. XAn of plagioclaseincreases to 0•20–0•70, depending on the bulk-rockcomposition and changes in pressure and temperature. If epidoteis removed first, the assemblage becomes a simple pelite andthe net reaction becomes Chl + Pl + Ms + Qtz = Grt + Bt + H2O.Plagioclase is consumed to provide Ca for growing garnet, andXAn, Fe/(Fe + Mg) of garnet, XGra, and XSpa all decrease. Afterboth chlorite and epidote are removed, continued heating upto the metamorphic peak of {small tilde}600C produces littleprogress of the reaction Grt + Ms = Bt + Pl; and XAn increases. The four assemblages have been numerically modeled using theGibbs method starting with measured compositions. The modelssuccessfully predict the observed compositional zoning and trendsof mineral growth and consumption along the computed P–Tpaths. The models also predict the compositional mineral zoningthat would have resulted from other P–T paths. * Present address: Department of Geology, University of Alabama, Tuscaloosa, Alabama 35487  相似文献   

17.
Pressure-temperature conditions of pelites in the Ballachulishaureole, Scotland, have been determined from a calibrated petrogeneticgrid and from published geothermometers and geobarometers. Tocalibrate the mineral reactions in the grid, thermodynamic datafor appropriate end members of Ms, Chi, Qtz, And, Sil, Ky, Crn,Crd, Kfs, and Bt were derived from experimental data. This approachwas hampered by the unknown compositions of many of the mineralsused in the experiments, and by apparent inconsistency betweenthe experiments. A best compromise grid that satisfies mostof the data was obtained, which is applicable to the Ballachulishand other contact aureoles. In this grid, the first developmentof sillimanite is constrained to lie between the Richardsonet al. (1969) and Holdaway (1971) andalusite-sillimanite boundaries. A pressure estimate of 3.0 + 0.5 kb is obtained from the calibratedgrid, within 0.3 kb of estimates from geobarometry and fromtwo other independent petrological studies. Temperatures rangedfrom 560?20?C at the first development of cordierite in theassemblage Ms+Qtz+Chl+Crd+Bt to 750–800?C in Grt+Crd+Hyassemblages in pelitic screens within the igneous complex. In graphitic slates, in contrast to non-graphitic pelites, anentire andalusite-bearing subzone is developed, and initialcordierite development occurs further from the igneous contacts.The presence of graphite lowered aH2o in the slaters, expandingthe stability field of the andalusite-bearing assemblage And+Qtz+Bt+Ms+Crdrelative to the assemblage Kfs+Qtz+Bt+Ms+Crd developed in non-graphiticunits. Initial development of cordierite in the assemblage Ms+Qtz+Chl+Crd+Btwas also promoted by reduced aH2o in graphitic slates. The regular sequence and spacing of mineral zones in the aureolesuggests that gross equilibrium was attained during contactmetamorphism, even though the thermal metamorphic pulse is estimatedto have been less than 0.2 Ma (Buntebarth, in press). Thereis no evidence for reaction overstepping in cordierite-producingreactions.  相似文献   

18.
An ultra-high-pressure (UHP) metamorphic slab at Yangkou Beach near Qingdao in the Sulu region of China consists of blocks of eclogite facies metagabbro, metagranitoid, ultramafic rock and mylonitic orthogneisses enclosed in granitic gneiss. A gradational sequence from incipiently metamorphosed gabbro to completely recrystallized coesite eclogite formed at ultra-high-pressures was identified in a single 30 m block; metagabbro is preserved in the core whereas coesite eclogite occurs along the block margins. The metagabbro contains an igneous assemblage of Pl+Aug+Opx+Qtz+Bt+Ilm/Ti-Mag; it shows relict magmatic textures and reaction coronas. Fine-grained garnet developed along boundaries between plagioclase and other phases; primary plagioclase broke down to Ab+Ky+Ms+Zo±Grt±Amp. Augite is rimmed by sodic augite or omphacite, whereas orthopyroxene is rimmed by a corona of Cum±Act and Omp+Qtz layers or only Omp+Qtz. In transitional rocks, augite and orthopyroxene are totally replaced by omphacite, and the lower-pressure assemblage Ab+Ky+Phn+Zo+Grt coexists with domains of Omp (Jd70–73)+Ky±Phn in pseudomorphs after plagioclase. Both massive and weakly deformed coesite-bearing eclogites contain Omp+Ky+Grt+Phn+Coe/Qtz+Rt, and preserve a faint gabbroic texture. Coesite inclusions in garnet and omphacite exhibit limited conversion to palisade quartz; some intergranular coesite and quartz pseudomorphs after coesite also occur. Assemblages of the coronal stage, transitional and UHP peak occurred at about 540±50 °C at c. 13 kbar, 600–800 °C at ≥15–25 kbar and 800–850 °C at >30 kbar, respectively. Garnet from the coronal- through the transitional- to the eclogite-stage rocks show a decrease in almandine and an increase in grossular±pyrope components; garnet in low-grade rocks contains higher MnO and lower pyrope components. The growth textures of garnet within pseudomorphs after plagioclase or along grain boundaries between plagioclase and other phases are complex; the application of garnet zoning to estimate P–T should be carried out with caution. Some garnet enclosing quartz aggregates as inclusions shows radial growth boundaries; these quartz aggregates, as well as other primary and low-P phases, persisted metastably at UHP conditions due to sluggish reactions resulting from the lack of fluid during prograde and retrograde P–T evolution.  相似文献   

19.
ABSTRACT The high-grade rocks (metapelite, quartzite, metagabbro) of the Hisøy-Torungen area represent the south-westernmost exposures of granulites in the Proterozoic Bamble sector, south Norway. The area is isoclinally folded and a metamorphic P–T–t path through four successive stages (M1-M4) is recognized. Petrological evidence for a prograde metamorphic event (M1) is obtained from relict staurolite + chlorite + albite, staurolite + hercynite + ilmenite, cordierite + sillimanite, fine-grained felsic material + quartz and hercynite + biotite ± sillimanite within metapelitic garnet. The phase relations are consistent with a pressure of 3.6 ± 0.5 kbar and temperatures up to 750–850°C. M1 is connected to the thermal effect of the gabbroic intrusions prior to the main (M2) Sveconorwegian granulite facies metamorphism. The main M2 granulite facies mineral assemblages (quartz+ plagioclase + K-feldspar + garnet + biotite ± sillimanite) are best preserved in the several-metre-wide Al-rich metapelites, which represent conditions of 5.9–9.1 kbar and 790–884°C. These P–T conditions are consistent with a temperature increase of 80–100°C relative to the adjacent amphibolite facies terranes. No accompanying pressure variations are recorded. Up to 1-mm-wide fine-grained felsic veinlets appear in several units and represent remnants of a former melt formed by the reaction: Bt + Sil + Qtz→Grt + lq. This dehydration reaction, together with the absence of large-scale migmatites in the area, suggests a very reduced water activity in the rocks and XH2O = 0.25 in the C–O–H fluid system was calculated for a metapelitic unit. A low but variable water activity can best explain the presence or absence of fine-grained felsic material representing a former melt in the different granulitic metapelites. The strongly peraluminous composition of the felsic veinlets is due to the reaction: Grt +former melt ± Sil→Crd + Bt ± Qtz + H2O, which has given poorly crystalline cordierite aggregates intergrown with well-crystalline biotite. The cordierite- and biotite-producing reaction constrains a steep first-stage retrograde (relative to M2) uplift path. Decimetre- to metre-wide, strongly banded metapelites (quartz + plagioclase + biotite + garnet ± sillimanite) inter-layered with quartzites are retrograded to (M3) amphibolite facies assemblages. A P–T estimate of 1.7–5.6 kbar, 516–581°C is obtained from geothermobarometry based on rim-rim analyses of garnet–biotite–plagioclase–sillimanite–quartz assemblages, and can be related to the isoclinal folding of the rocks. M4 greenschist facies conditions are most extensively developed in millimetre-wide chlorite-rich, calcite-bearing veins cutting the foliation.  相似文献   

20.
Stromatic metatexites occurring structurally below the contact with the Ronda peridotite (Ojén nappe, Betic Cordillera, S Spain) are characterized by the mineral assemblage Qtz+Pl+Kfs+Bt+Sil+Grt+Ap+Gr+Ilm. Garnet occurs in low modal amount (2–5 vol.%). Very rare muscovite is present as armoured inclusions, indicating prograde exhaustion. Microstructural evidence of melting in the migmatites includes pseudomorphs after melt films and nanogranite and glassy inclusions hosted in garnet cores. The latter microstructure demonstrates that garnet crystallized in the presence of melt. Re‐melted nanogranites and preserved glassy inclusions show leucogranitic compositions. Phase equilibria modelling of the stromatic migmatite in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2–O2–C (MnNCaKFMASHOC) system with graphite‐saturated fluid shows P–T conditions of equilibration of 4.5–5 kbar, 660–700 °C. These results are consistent with the complete experimental re‐melting of nanogranites at 700 °C and indicate that nanogranites represent the anatectic melt generated immediately after entering supersolidus conditions. The P–T estimate for garnet and melt development does not, however, overlap with the low‐temperature tip of the pure melt field in the phase diagram calculated for the composition of preserved glassy inclusions in garnet in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) system. A comparison of measured melt compositions formed immediately beyond the solidus with results of phase equilibria modelling points to the systematic underestimation of FeO, MgO and CaO in the calculated melt. These discrepancies are present also when calculated melts are compared with low‐T natural and experimental melts from the literature. Under such conditions, the available melt model does not perform well. Given the presence of melt inclusions in garnet cores and the P–T estimates for their formation, we argue that small amounts (<5 vol.%) of peritectic garnet may grow at low temperatures (≤700 °C), as a result of continuous melting reactions consuming biotite.  相似文献   

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