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1.
Tectonothermal evolution of the gneiss complex at Salur in the Eastern Ghats Grnaulite Belt of India
Textural relations, thermobarometry and petrogenetic grid considerations in the syn-tectonic granitoid massif and the enveloping metasedimentary gneisses at Salur are consistent with a counter-clockwise P–T –t path for the rocks. The low-P/high-T prograde sector is documented by the pre- to syn-D1 cordierite±orthopyroxene±garnet±spinel–bearing metatexite leucosomes in metapelites. Heating and loading of the rocks (syn- to post-D1) resulted in the formation of garnet+orthopyroxene± cordierite-bearing diatexites, and decomposition of cordierite in metatexite leucosomes to orthopyroxene+sillimanite+biotite+quartz symplectites. Near-peak temperature, 850 °C at 8.0 kbar, was reached syn- to post-D2. Post-peak cooling resulted in the stabilization of coronal grossular and anorthite+calcite symplectites at the expense of scapolite+wollastonite+calcite assemblages in calc-silicate gneisses, and the resetting of cation exchange temperatures at 700–750 °C. Near-isothermal decompression at c. 700–750 °C is manifested by the decomposition of garnet porphyroblasts in the granitoid gneisses to plagioclase+orthopyroxene/ilmenite/biotite two-phase coronas and restabilization of cordierite at garnet margins in metapelites. Subsequent low-P, near-isobaric cooling led to the overprinting of granulite facies assemblages by muscovite+calcite assemblages, and further resetting of cation exchange thermometers to lower temperatures c. 600 °C. The tectonothermal evolution of the Salur gneiss complex vis-a-vis the Eastern Ghats Belt is therefore consistent with high degrees of lower crustal melting, followed by prograde heating of the cover rocks due to magma invasion synchronous with crustal compression, and finally thermal relaxation over a protracted period punctuated by tectonic/erosional denudation of the thickened crust. 相似文献
2.
A sequence of psammitic and pelitic metasedimentary rocks from the Mopunga Range region of the Arunta Inlier, central Australia, preserves evidence for unusually low pressure (c. 3 kbar), regional‐scale, upper amphibolite and granulite facies metamorphism and partial melting. Upper amphibolite facies metapelites of the Cackleberry Metamorphics are characterised by cordierite‐andalusite‐K‐feldspar assemblages and cordierite‐bearing leucosomes with biotite‐andalusite selvages, reflecting P–T conditions of c. 3 kbar and c. 650–680 °C. Late development of a sillimanite fabric is interpreted to reflect either an anticlockwise P–T evolution, or a later independent higher‐P thermal event. Coexistence of andalusite with sillimanite in these rocks appears to reflect the sluggish kinematics of the Al2SiO5 polymorphic inversion. In the Deep Bore Metamorphics, 20 km to the east, dehydration melting reactions in granulite facies metapelites have produced migmatites with quartz‐absent sillimanite‐spinel‐cordierite melanosomes, whilst in semipelitic migmatites, discontinuous leucosomes enclose cordierite‐spinel intergrowths. Metapsammitic rocks are not migmatised, and contain garnet–orthopyroxene–cordierite–biotite–quartz assemblages. Reaction textures in the Deep Bore Metamorphics are consistent with a near‐isobaric heating‐cooling path, with peak metamorphism occurring at 2.6–4.0 kbar and c. 750–800 °C. SHRIMP U–Pb dating of metamorphic zircon rims in a cordierite‐orthopyroxene migmatite from the Deep Bore Metamorphics yielded an age of 1730 ± 7 Ma, whilst detrital zircon cores define a homogeneous population at 1805 ± 7 Ma. The 1730 Ma age is interpreted to reflect the timing of high‐T, low‐P metamorphism, synchronous with the regional Late Strangways Event, whereas the 1805 Ma age provides a maximum age of deposition for the sedimentary precursor. The Mopunga Range region forms part of a more extensive low‐pressure metamorphic terrane in which lateral temperature gradients are likely to have been induced by localised advection of heat by granitic and mafic intrusions. The near‐isobaric Palaeoproterozoic P–T–t evolution of the Mopunga Range region is consistent with a relatively transient thermal event, due to advective processes that occurred synchronous with the regional Late Strangways tectonothermal event. 相似文献
3.
The East Hebei terrane of North China Craton is characterized by the dome-and-keel structure, a common feature in most Archean cratons, where supracrustal rocks of granulite facies commonly occur as enclaves or rafts in tonalite–trondhjemite–granodiorite (TTG) gneisses. The metamorphic P–T paths of the granulites are significant for addressing the Archean tectonic regimes. Two types of granulite facies paragneiss with pelitic and greywacke compositions from the western margin of Qian'an gneiss dome are documented for their petrography, mineral chemistry, phase equilibria modelling using thermocalc, and zircon dating. Anticlockwise P–T paths involving the pre-peak pressure increase to the ultra-high temperature peak conditions and post-peak cooling and decompression processes were recognized. The pre-peak pressure increase process was constrained for a pelitic granulite mainly based on the spinel and cordierite inclusions in garnet and rutile corona around ilmenite, where the transition from spinel to garnet is modelled at 6–7 kbar at a fixed T = 1,000°C. For greywacke granulite, the pre-peak pressure increase evolution can be ascertained from the textural relation that orthopyroxene is surrounded by garnet, and the outwards increasing grossular (from 0.03 to 0.05) in the core of the atoll-like garnet (Grt-A), to occur from ~7 kbar at ~1,000°C. The peak P–T conditions for pelitic granulite are roughly limited to 7–11 kbar/890–1,050°C on the basis of the stability of the inferred peak assemblage involving garnet, perthite, sillimanite, rutile/ilmenite, and quartz. The peak P–T condition for greywacke granulite can be well constrained as 9–10 kbar/>1,000°C on the basis of the maximum grossular content (XGrs = 0.045–0.050) in the core of subhedral garnet (Grt-B) and the mantle of Grt-A together with an average re-integrated anorthite content (XAn = 0.07) in K-feldspar. The peak temperature condition is consistent with the ternary feldspar thermometer results mostly of 950–1,020°C for antiperthite and perthite in greywacke granulite, and in accordance with the development of oriented needle-like exsolution of Ti±Fe oxides in garnet from pelitic granulite. The post-peak cooling and decompression process was consistent with the decreasing XGrs in the mantle of Grt-A and core of Grt-B in greywacke sample, and the final-stage cooling conditions can be well constrained from the stability of final assemblages marked by the later growth of biotite, as 8–9 kbar/820–880°C for pelitic granulite and 6–9 kbar/840–890°C for greywacke granulite. Zircon dating yields provenance ages from 3.34 to 2.57 Ga and metamorphic ages of c. 2.50 Ga for the two types of granulite. The metamorphic ages overlap the final pulse of the Neoarchean magmatic activity of TTGs that ranges from c. 2.56 to c. 2.48 Ga with a peak at c. 2.52 Ga. Combining the development of dome-and-keel structures, the penecontemporaneity between the metamorphism of supracrustal rocks and TTG magmatic activity, and also the unique anticlockwise P–T paths, we prefer a vertical sagduction regime to interpret the tectonic evolution of the East Hebei terrane, which may be also significant for other Archean cratons. 相似文献
4.
The investigated area around Sarvapuram represents a part of the Karimnagar granulite terrane of the Eastern Dharwar Craton, India. Garnet–bearing gneiss is hosted as enclaves, pods within granite gneiss and charnockite. It is largely made up of garnet, orthopyroxene, cordierite, biotite, plagioclase, K–feldspar, sillimanite and quartz. The peak metamorphic stage is represented by the equilibrium mineral assemblage i.e. garnet, orthopyroxene, cordierite, biotite, plagioclase, sillimanite and quartz. Breakdown of the garnet as well as preservation of the orthopyroxene–cordierite symplectite, formation of cordierite with the consumption of the garnet + sillimanite + quartz represents the decompressional event. The thermobarometric calculations suggest a retrograde P–T path with a substantial decompression of c. 3.0 kbar. The water activity(XH2 O) conditions obtained with the win TWQ program for core and symplectite compositions from garnet–bearing gneiss are 0.07–0.14 and 0.11–0.16 respectively. The quantitative estimation of oxygen fugacity in garnet–bearing gneiss reveal log f O2 values ranging from-11.38 to-14.05. This high oxidation state could be one of the reasons that account for the absence of graphite in these rocks. 相似文献
5.
Mineral textures in metapelitic granulites from the northern Prince Charles Mountains, coupled with thermodynamic modelling in the K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (KFMASHTO) model system, point to pressure increasing with increasing temperature on the prograde metamorphic path, followed by retrograde cooling (i.e. an anticlockwise P–T path). Textural evidence for the increasing temperature part of the path is given by the breakdown of garnet and biotite to form orthopyroxene and cordierite in sillimanite‐absent rocks, and through the break‐down of biotite and sillimanite to form spinel, cordierite and garnet in more aluminous assemblages. This is equated to the advective addition of heat from the regional emplacement of granitic and charnockitic magmas dated at c. 980 Ma. A subsequent increase in pressure, inferred from the break‐down of spinel and quartz to sillimanite, cordierite and garnet in aluminous rocks, is attributed to crustal thickening related to upright folding dated at 940–910 Ma. The terrane attained peak metamorphic temperatures of c. 880 °C at pressures of c. 6.0–6.5 kbar during this event. Subsequent cooling is inferred from the localised breakdown of cordierite and garnet to form biotite and sillimanite that developed in the latter stages of the same event. The textural observations described are interpreted via the application of P–T and P–T–X pseudosections. The latter show that most rock compositions preserve only fragments of the overall P–T path; a result of different rock compositions undergoing mineral assemblage changes, or changes in mineral modal abundance, on different sections of the P–T path. The results also suggest that partial melting during granulite facies metamorphism, coupled with melt loss and dehydration, initiated a switch from pervasive ductile, to discrete ductile/brittle deformation, during retrograde cooling. 相似文献
6.
A. ALI 《Journal of Metamorphic Geology》2010,28(4):397-422
The sequential growth of biotite, garnet, staurolite, kyanite, andalusite, cordierite and fibrolitic sillimanite, their microstructural relationships, foliation intersection axes preserved in porphyroblasts (FIAs), geochronology, P–T pseudosection (MnNCKFMASH system) modelling and geothermobarometry provide evidence for a P–T–t–D path that changes from clockwise to anticlockwise with time for the Balcooma Metamorphic Group. Growth of garnet at ～530 °C and 4.6 kbar during the N–S‐shortening event that formed FIA 1 was followed by staurolite, plagioclase and kyanite growth. The inclusions of garnet in staurolite porphyroblasts that formed during the development of FIAs 2 and 3 plus kyanite growth during FIA 3 reflect continuous crustal thickening from c. 443 to 425 Ma during an Early Silurian Benambran Orogenic event. The temperature and pressure increased during this time from ～530 °C and 4.6 kbar to ～630 °C and 6.2 kbar. The overprinting of garnet‐, staurolite‐ and kyanite‐bearing mineral assemblages by low‐pressure andalusite and cordierite assemblages implies ～4‐kbar decompression during Early Devonian exhumation of the Greenvale Province. 相似文献
7.
P–T–t–d evolution of orogenic middle crust of the Roc de Frausa Massif (Eastern Pyrenees): a result of horizontal crustal flow and Carboniferous doming? 
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下载免费PDF全文 C. Aguilar M. Liesa P. Štípská K. Schulmann J. A. Muñoz J. M. Casas 《Journal of Metamorphic Geology》2015,33(3):273-294
Structural, petrological and textural studies are combined with phase equilibria modelling of metapelites from different structural levels of the Roc de Frausa Massif in the Eastern Pyrenees. The pre‐Variscan lithological succession is divided into the Upper, Intermediate and Lower series by two orthogneiss sheets and intruded by Variscan igneous rocks. Structural analysis reveals two phases of Variscan deformation. D1 is marked by tight to isoclinal small‐scale folds and an associated flat‐lying foliation (S1) that affects the whole crustal section. D2 structures are characterized by tight upright folds facing to the NW with steep NE–SW axial planes. D2 heterogeneously reworks the D1 fabrics, leading to an almost complete transposition into a sub‐vertical foliation (S2) in the high‐grade metamorphic domain. All structures are affected by late open to tight, steeply inclined south‐verging NW–SE folds (F3) compatible with steep greenschist facies dextral shear zones of probable Alpine age. In the micaschists of the Upper series, andalusite and sillimanite grew during the formation of the S1 foliation indicating heating from 580 to 640 °C associated with an increase in pressure. Subsequent static growth of cordierite points to post‐D1 decompression. In the Intermediate series, a sillimanite–biotite–muscovite‐bearing assemblage that is parallel to the S1 fabric is statically overgrown by cordierite and K‐feldspar. This sequence points to ~1 kbar of post‐D1 decompression at 630–650 °C. The Intermediate series is intruded by a gabbro–diorite stock that has an aureole marked by widespread migmatization. In the aureole, the migmatitic S1 foliation is defined by the assemblage biotite–sillimanite–K‐feldspar–garnet. The microstructural relationships and garnet zoning are compatible with the D1 pressure peak at ~7.5 kbar and ~750 °C. Late‐ to post‐S2 cordierite growth implies that F2 folds and the associated S2 axial planar leucosomes developed during nearly isothermal decompression to <5 kbar. The Lower series migmatites form a composite S1–S2 fabric; the garnet‐bearing assemblage suggests peak P–T conditions of >5 kbar at suprasolidus conditions. Almost complete consumption of garnet and late cordierite growth points to post‐D2 equilibration at <4 kbar and <750 °C. The early metamorphic history associated with the S1 fabric is interpreted as a result of horizontal middle crustal flow associated with progressive heating and possible burial. The upright F2 folding and S2 foliation are associated with a pressure decrease coeval with the intrusion of mafic magma at mid‐crustal levels. The D2 tectono‐metamorphic evolution may be explained by a crustal‐scale doming associated with emplacement of mafic magmas into the core of the dome. 相似文献
8.
Interpretations based on quantitative phase diagrams in the system CaO–Na2O–K2O–TiO2–MnO–FeO–MgO–Al2O3–SiO2–H2O indicate that mineral assemblages, zonations and microstructures observed in migmatitic rocks from the Beit Bridge Complex (Messina area, Limpopo Belt) formed along a clockwise P–T path. That path displays a prograde P–T increase from 600 °C/7.0 kbar to 780 °C/9–10 kbar (pressure peak) and 820 °C/8 kbar (thermal peak), followed by a P–T decrease to 600 °C/4 kbar. The data used to construct the P–T path were derived from three samples of migmatitic gneiss from a restricted area, each of which has a distinct bulk composition: (1) a K, Al‐rich garnet–biotite–cordierite–sillimanite–K‐feldspar–plagioclase–quartz–graphite gneiss (2) a K‐poor, Al‐rich garnet–biotite–staurolite–cordierite–kyanite–sillimanite–plagioclase–quartz–rutile gneiss, and (3) a K, Al‐poor, Fe‐rich garnet–orthopyroxene–biotite–chlorite–plagioclase–quartz–rutile–ilmenite gneiss. Preservation of continuous prograde garnet growth zonation demonstrates that the pro‐ and retrograde P–T evolution of the gneisses must have been rapid, occurring during a single orogenic cycle. These petrological findings in combination with existing geochronological and structural data show that granulite facies metamorphism of the Beit Bridge metasedimentary rocks resulted from an orogenic event during the Palaeoproterozoic (c. 2.0 Ga), caused by oblique collision between the Kaapvaal and Zimbabwe Cratons. Abbreviations follow Kretz (1983 ). 相似文献
9.
C. A. Smit D. D. Van Reenen T. V. Gerya L. L. Perchuk 《Journal of Metamorphic Geology》2001,19(3):249-268
The Southern Marginal Zone of the late Archean Limpopo Belt of southern Africa is an example of a high‐grade gneiss terrane in which both upper and lower crustal deformational processes can be studied. This marginal zone consists of large thrust sheets of complexly folded low‐strain gneisses, bound by an imbricate system of kilometre‐wide deep crustal shear zones characterized by the presence of high‐strain gneisses (‘primary straight gneisses’). These shear zones developed during the decompression stage of this high‐grade terrane. Low‐ and high‐strain gneisses both contain similar reaction textures that formed under different kinematic conditions during decompression. Evidence for the early M1/D1 metamorphic phase (> 2690 Ma) is rarely preserved in low‐strain gneisses as a uniform orientation of relict Al‐rich orthopyroxene in the matrix and quartz and plagioclase inclusions in the cores of early (M1) Mg‐rich garnet porphyroblasts. This rare fabric formed at > 820 °C and > 7.5 kbar. The retrograde M2/D2 metamorphic fabric (2630–2670 Ma) is well developed in high‐strain gneisses from deep crustal shear zones and is microscopically recognized by the presence of reaction textures that formed synkinematically during shear deformation: M2 sigmoid‐shaped reaction textures with oriented cordierite–orthopyroxene symplectites formed after the early M1 Mg‐rich garnet porphyroblasts, and syn‐decompression M2 pencil‐shaped garnet with oriented inclusions of sillimanite and quartz formed after cordierite under conditions of near‐isobaric cooling at 750–630 °C and 6–5 kbar. The symplectites and pencil‐shaped garnet are oriented parallel to the shear fabric and in the stretching direction. Low‐strain gneisses from thrust sheets show similar M2 decompression cooling and near‐isobaric cooling reaction textures that formed within the same P–T range, but under low‐strain conditions, as shown by their pseudo‐idioblastic shapes that reflect the contours of completely replaced M1 garnet and randomly oriented cordierite–orthopyroxene symplectites. The presence of similar reaction textures reflecting low‐strain conditions in gneisses from thrust sheets and high‐strain conditions in primary straight gneisses suggests that most of the strain during decompression was partitioned into the bounding shear zones. A younger M3/D3 mylonitic fabric (< 2637 Ma) in unhydrated mylonites is characterized by brittle deformation of garnet porphyroclasts and ductile deformation of the quartz–plagioclase–biotite matrix developed at < 600 °C, as the result of post‐decompression shearing under epidote–amphibolite facies conditions. 相似文献
10.
S. L. HARLEY 《Journal of Metamorphic Geology》1998,16(4):541-562
Highly magnesian and aluminous migmatitic gneisses from Mather Peninsula in the Rauer Group, Eastern Antarctica, preserve ultrahigh temperature (UHT) metamorphic assemblages that include orthopyroxene+sillimanite±quartz, garnet+sillimanite±quartz and garnet+orthopyroxene±sillimanite. Garnet that ranges up to XMg of 71.5 coexists with aluminous orthopyroxene that shows zoning from cores with 7.5–8.5 wt% Al2O3 to rims with up to 10.6 wt% Al2O3 adjacent to garnet. Peak P–T conditions of 1050 °C and 12 kbar are retrieved from Fe–Mg–Al thermobarometry involving garnet and orthopyroxene, in very good agreement with independent constraints from petrogenetic grids in FeO–MgO–Al2O3–SiO2 and related chemical systems. Sapphirine, orthopyroxene and cordierite form extensive symplectites and coronas on the early phases. The specific reaction textures and assemblages involving these secondary phases correlate with initial garnet XMg , with apparent higher-pressure reaction products occurring on the more magnesian garnet, and are interpreted to result from an initial phase of ultrahigh temperature near-isothermal decompression (UHT-ITD) from 12 to 8 kbar at temperatures in excess of 950 °C. Later textures that involved biotite formation and then partial breakdown, along with garnet relics, to symplectites of orthopyroxene+cordierite or cordierite+spinel may reflect hydration through back-reaction with crystallizing melts on cooling below 900–850 °C, followed by ITD from 7 to 8 kbar to c. 5 kbar at temperatures of 750–850 °C. The tectonic significance of this P–T history is ambiguous as the Rauer Group records the effects of Archean tectonothermal events as well as high-grade events at 1000 and 530 Ma. Late-stage biotite formation and subsequent ITD can be correlated with the P–T history preserved in the Proterozoic components of the Rauer Group and hence with either 1000 or 530 Ma collisional orogenesis. However, whether the preceding UHT-ITD history reflects a temporally unrelated event (e.g. Archean) or is simply an early stage of either the late-Proterozoic or Pan-African tectonism, as recently deduced for similar UHT rocks from other areas of the East Antarctica, remains uncertain. 相似文献
11.
Dehydration melting and devolatilization during exhumation of high-grade metapelites: the Tatra Mountains, Western Carpathians 总被引：1，自引：0，他引：1
Partial melting and retrogression related to Variscan tectonic exhumation have been recognized in the high-grade metapelites of the Tatra Mountains, Western Carpathians. Staurolite and kyanite relics document an early stage of the prograde metamorphism at c. 600 °C and 9–10 kbar. An increase in temperature to >730 °C at 11–12 kbar resulted in partial melting and incipient migmatization in the stability field of kyanite. Further heating at decreasing pressure during the earliest stage of exhumation led to the dehydration-melting of muscovite and biotite at >750–800 °C and 6–10 kbar, producing garnet-bearing granite as leucosomes in migmatite. Subsequent cooling is documented by garnet resorption by biotite and sillimanite (a reversal of the prograde biotite dehydration-melting reaction). This was followed by nearly isothermal decompression to c. 4–5 kbar producing cordierite and some melt due to biotite decomposition. Later nearly isobaric cooling led to cordierite pinitization and formation of orthoamphibole, chlorite and carbonates. Densities of primary, monophase CO2–N2 inclusions (0.69–1.06 g cm?3) from the migmatite leucosome are consistent with the near-peak and retrograde conditions. Highly varying N2 contents (5–30 mol%) are thought to result from the nitrogen uptake in retrograde K-bearing minerals, or dilution by CO2 liberated during interaction of melt-derived water with metapelite graphite. The relatively high nitrogen content, not observed until now in migmatites, could have been inherited from the high-pressure metamorphism stage. It is assumed that the water-absent composition of fluid inclusions is not representative of the bulk water content (XH2O≤0.7), which was masked by mechanical separation of the CO2- and H2O-dominated immiscible phases, and/or by post-entrapment modifications of the fluid inclusions. Decompression and the final stage of exhumation were accomplished by top-to-the-south thrusting as well as west–east (orogen-parallel) extension. They were most probably related to regional uplift and gravitational collapse of thermally weakened Variscan crust. 相似文献
12.
Partial melting in the Inzie Head gneisses: the role of water and a petrogenetic grid in KFMASH applicable to anatectic pelitic migmatites 总被引：5，自引：0，他引：5
A sequence of prograde isograds is recognized within the Dalradian Inzie Head gneisses where pelitic compositions have undergone variable degrees of partial melting via incongruent melting reactions consuming biotite. Three leucosome types are identified. At the lowest grades, granitic leucosomes containing porphyroblasts of cordierite (CRD‐melt) are abundant. At intermediate grades, CRD‐melt mingles with garnetiferous leucosomes (GT‐melt). At the highest grades, CRD‐melt coexists with orthopyroxene‐bearing leucosomes (OPX‐melt), while garnet is conspicuously absent. The prograde metamorphic field gradient is constrained to pressures of 2–3 kbar below the CRD‐melt isograd, and no greater than 4.5 kbar at the highest grade around Inzie Head. A petrogenetic grid, calculated using thermocalc , is presented for the K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH) system for the phases orthopyroxene, garnet, cordierite, biotite, sillimanite, H2O and melt with quartz and K‐feldspar in excess. For the implied field gradient, the reaction sequence predicted by the grid is consistent with the successive prograde development of each leucosome type. Compatibility diagrams suggest that, as anatexis proceeded, bulk compositions may have been displaced towards higher MgO content by the removal of (relatively) ferroan granitic leucosome. An isobaric (P = 4 kbar) T–aH2O diagram shows that premigmatization fluids must have been water‐rich (aH2O > 0.85) and suggests that, following the formation of small volumes of CRD‐melt, the system became fluid‐absent and melting reactions buffered aH2O to lower values as temperatures rose. GT‐ and OPX‐melt formed by fluid‐absent melting reactions, but a maximum of 7–11% CRD‐melt fraction can be generated under fluid‐absent conditions, much less than the large volumes observed in the field. There is strong evidence that the CRD‐melt leucosomes could not have been derived by buoyantly aided upwards migration from levels beneath the migmatites. Their formation therefore required a significant influx of H2O‐rich fluid, but in a quantity insufficient to have exhausted the buffering capacity of the solid assemblage plus melt. Fluid : rock ratios cannot have exceeded 1 : 30. The fluid was channelled through a regionally extensive shear zone network following melt‐induced failure. Such an influx of fluid at such depths has obvious consequences for localized crustal magma production and possibly for cordierite‐bearing granitoids in general. 相似文献
13.
Metamorphism of high‐P metagreywacke from the Eastern Himalayan syntaxis: phase equilibria and P–T path 下载免费PDF全文
下载免费PDF全文 High‐pressure (HP) metagreywacke from the Namche Barwa Complex, Eastern Himalayan Syntaxis (EHS), consists of garnet, biotite, plagioclase, quartz, rutile and ilmenite with or without K‐feldspar, sillimanite, cordierite, spinel and orthopyroxene. Two types of metagreywacke are recognized: medium‐temperature (MT) and high‐temperature (HT) types. Garnet in the MT metagreywacke shows significant growth zoning and contains lower MgO than the weakly zoned garnet in the HT metagreywacke. Petrographic observations and phase equilibria modelling for four representative samples indicate that both types of metagreywacke experienced clockwise P–T paths subdivided into three stages: stage I is the pre‐peak prograde to pressure peak (Pmax) stage characterized by progressive increase in P–T conditions. The Pmax conditions are estimated using the garnet composition with maximum CaO, being 12.5–13.5 kbar and 685–725 °C for the MT metagreywacke, and 15–16 kbar and 825–835 °C for the HT one. Stage II is the post‐Pmax decompression with heating or near‐isothermal to Tmax stage and the Tmax conditions, constrained using the garnet compositions with maximum MgO, are 11 kbar and 760 °C for the MT metagreywacke, and ~12 kbar and 830–845 °C for the HT one. The modelled mineral assemblages at Tmax are garnet + biotite + K‐feldspar + rutile + plagioclase ± ilmenite in the presence of melt for both types of metagreywacke, consistent with the petrographic observations. Stage III is the post‐Tmax retrograde metamorphism, characterized by decompression and cooling. The modelling suggests that the melts with high Na/K ratios (1.7–5.2) have been produced during stages I and II, which could be responsible for the formation of sodium‐rich leucogranites. This study and previous results indicate that the Higher Himalayan Crystallines in the EHS consist of MT–HP and HT–HP metamorphic units separated by a speculated tectonic contact. Petrological and structural discontinuities within the EHS cannot be easily interpreted with ‘tectonic aneurysm’ model. 相似文献
14.
Aluminous reaction textures in orthoamphibole-bearing rocks from the Froland area, Bamble, south Norway, record the prograde pressure–temperature path of the high-grade Kongsbergian Orogeny (c. 1600–1500 Ma) and the low–mid amphibolite facies overprint during the Sveconorwegian Orogeny (c. 1100–1000 Ma). The rocks contain anthophyllite/gedrite, garnet, cordierite, biotite, quartz, andalusite, kyanite, Cr-rich staurolite, tourmaline, ilmenite, rutile and corundum in a variety of parageneses. The P–T path is deduced from petrographic observations, mineral chemistry and zoning, geothermometry and (N)FMASH equilibria. The results indicate the sequence of metamorphic stages outlined below. (a) An M1 phase characterized by the presence of strongly deformed andalusite, gedrite and tourmaline. (b) An M2 phase with the development of kyanite after andalusite and the growth of staurolite associated with strong Na–Al–Mg zoning in orthoamphibole, indicating an increase in pressure (4 8 kbar) and temperature (500° 650°C). (c) Pressure decrease at high P (6–7 kbar) and high T (600–700 °C) during M3a with the production of cordierite ° Corundum between kyanite, staurolite and orthoamphibole and cordierite growth between corundum and orthoamphibole. (d) Temperature increase to 740 ± 60 °C and 7 kbar; static growth of garnet (M3b) at the metamorphic climax (peak T). The heat supply necessary to explain the temperature increase between the M3a and M3b phases is correlated with synkinematic enderbitic–charnockitic and basic intrusions in the Arendal granulite facies terrain. (e) M3b metamorphic conditions were followed by an initial isobaric cooling path (early M4) and late-stage pressure decrease (late M4). Early M4 conditions of 6–7 kbar and 550–600 °C, assuming PH2O < Ptotal are indicated by a retrograde talc–kyanite–quartz assemblage in late quartz–cordierite veins. Late M4 conditions of 3–4 kbar and 420–530 °C are inferred from a kyanite–andalusite–chlorite–quartz assemblage in vein-cordierite. The M1–M3 stages are interpreted as being the result of the same metamorphic P–T path, which was caused by both tectonic and magmatic thickening. A prolonged crustal residence time is proposed for the Bamble sector before uplift during the later stages of M4 occurred. 相似文献
15.
F. J. KORHONEN M. BROWN M. GROVE C. S. SIDDOWAY E. F. BAXTER J. D. INGLIS 《Journal of Metamorphic Geology》2012,30(2):165-192
The Fosdick migmatite–granite complex in West Antarctica records evidence for two high‐temperature metamorphic events, the first during the Devonian–Carboniferous and the second during the Cretaceous. The conditions of each high‐temperature metamorphic event, both of which involved melting and multiple melt‐loss events, are investigated using phase equilibria modelling during successive melt‐loss events, microstructural observations and mineral chemistry. In situ SHRIMP monazite and TIMS Sm–Nd garnet ages are integrated with these results to constrain the timing of the two events. In areas that preferentially preserve the Devonian–Carboniferous (M1) event, monazite grains in leucosomes and core domains of monazite inclusions in Cretaceous cordierite yield an age of c. 346 Ma, which is interpreted to record the timing of monazite growth during peak M1 metamorphism (～820–870 °C, 7.5–11.5 kbar) and the formation of garnet–sillimanite–biotite–melt‐bearing assemblages. Slightly younger monazite spot ages between c. 331 and 314 Ma are identified from grains located in fractured garnet porphyroblasts, and from inclusions in plagioclase that surround relict garnet and in matrix biotite. These ages record the growth of monazite during garnet breakdown associated with cooling from peak M1 conditions. The Cretaceous (M2) overprint is recorded in compositionally homogeneous monazite grains and rim domains in zoned monazite grains. This monazite yields a protracted range of spot ages with a dominant population between c. 111 and 96 Ma. Rim domains of monazite inclusions in cordierite surrounding garnet and in coarse‐grained poikiloblasts of cordierite yield a weighted mean age of c. 102 Ma, interpreted to constrain the age of cordierite growth. TIMS Sm–Nd ages for garnet are similar at 102–99 Ma. Mineral equilibria modelling of the residual protolith composition after Carboniferous melt loss and removal of inert M1 garnet constrains M2 conditions to ～830–870 °C and ～6–7.5 kbar. The modelling results suggest that there was growth and resorption of garnet during the M2 event, which would facilitate overprinting of M1 compositions during the M2 prograde metamorphism. Measured garnet compositions and Sm–Nd diffusion modelling of garnet in the migmatitic gneisses suggest resetting of major elements and the Sm–Nd system during the Cretaceous M1 overprint. The c. 102–99 Ma garnet Sm–Nd ‘closure’ ages correspond to cooling below 700 °C during the rapid exhumation of the Fosdick migmatite–granite complex. 相似文献
16.
Cordierite gneisses of southern Kerala,India: petrology,fluid inclusions and implications for crustal uplift history 总被引：2，自引：0，他引：2
M. Santosh 《Contributions to Mineralogy and Petrology》1987,96(3):343-356
The cordierite-bearing gneisses occurring as elongate patches in an 8- to 10-km-wide zone along the Achankovil fault-lineament at the northern margin of the southern Kerala crustal segment represent an important lithological unit in the Archaean granulite terrane of south India. The textural relationships in these rocks are consistent with the following main reactions: (1) garnet+quartz=cordierite+hypersthene; (2) garnet+sillimanite+quartz=cordierite; (3) hypersthene+sillimanite+quartz=cordierite; (4) sillimanite+spinel=cordierite+corundum; and (5) biotite+quartz+sillimanite=cordierite+K-feldspar. Many of the mineral associations and reaction textures, including the remarkable preservation of symplectites, are indicative of partial replacement of high-pressure assemblages by cordierite-bearing lower-pressure ones during an event of rapid decompression. Temperature estimates from coexisting mineral phases show 710° (garnet-biotite), 791° (garnet-cordierite) and 788° C (garnet-orthopyroxene). Pressure estimates from mineral assemblages range from 5.4 to 7 kb. Detailed fluid inclusion studies in quartz associated with cordierite show high-density CO2 (0.80–0.95 g/cm3) as the dominant fluid phase, with traces of probable CH4 (?) in the sillimanite-bearing rocks. The isochore for the higher-density fluid inclusions defines a pressure of 5.5 kb. The fracture-bound CO2 and CO2-H2O (±CH4?) inclusions indicate simultaneous entrapment at 400° C and 1.7 kb in the cordierite-hypersthene assemblage and 340° C and 1.2 kb in the cordierite-sillimanite assemblage. The P-T path delineated from combined solid and fluid data corresponds to the piezothermic array of the gneisses and is characterized by T-convex nature, indicative of rapid and virtually isothermal crustal uplift, probably aided by extensional tectonics. 相似文献
17.
Metapelitic granulites from Jetty Peninsula,east Antarctica: formation during a single event or by polymetamorphism? 总被引：5，自引：0，他引：5
M. HAND I. SCRIMGEOUR R. POWELL K. STÜWE C. J. L. WILSON 《Journal of Metamorphic Geology》1994,12(4):557-573
Granulite facies metasedimentary gneiss exposed on Jetty Peninsula, east Antarctica, contains assemblages involving garnet-sillimanite-biotite-cordierite-spinel-ilmenite-rutile and garnet-orthopyroxene-cordierite-biotite, as well as quartz and K-feldspar. Peak assemblages involve garnet + sillimanite + ilmenite (±rutile) and garnet + orthopyroxene. P-T calculations suggest formation conditions of approximately 800d? C at 7-7.5 kbar. Cooling from peak conditions is suggested by biotite + garnet (±sillimanite) overprinting some peak assemblages. A subsequent increase in temperature is inferred from the formation of cordierite + garnet + biotite + ilmenite, garnet + sillimanite + cordierite + ilmenite and cordierite + orthopyroxene assemblages during D2. In slightly zincian bulk compositions, hercynitic spinel + cordierite + sillimanite constitutes the peak D2 assemblage. Average pressure calculations indicate peak pressures of 5.9 ±0.4 kbar at 700d? C for the cordierite-bearing D2 assemblages. Available radiometric data suggest that peak metamorphism occurred at c. 1000 Ma and D2 occurred after 940 ± 20 Ma. The following two possibilities exist for the metamorphic evolution. (1) The formation of the lower pressure cordierite-bearing assemblages is associated with a separate metamorphic event (M2), unrelated to the peak assemblage (M1), and the lower pressure assemblages have no relevance in terms of a single tectonothermal event. (2) The cordierite-bearing assemblages formed during a progression from peak conditions. In this case, the lower pressure assemblages reflect a broadly decompressional metamorphic evolution, during which temperatures fluctuated. Comparison with P-T paths from granulites of similar age in adjacent areas suggests that the second possibility should be preferred. The cooling interval between peak conditions and the development of cordierite-bearing coronas and symplectites suggests affinities with isobarically cooled granulites of similar age immediately to the west, and the low-P/high-T post-peak conditions are similar to the later stages of decompressional paths recognized in much of east Antarctica. 相似文献
18.
R. D. Staples H. D. Gibson R. G. Berman J. J. Ryan M. Colpron 《Journal of Metamorphic Geology》2013,31(7):729-753
Amphibolite facies metasedimentary schists within the Yukon‐Tanana terrane in the northern Canadian Cordillera reveal a two‐stage, polymetamorphic garnet growth history. In situ U‐Th‐Pb Sensitive High Resolution Ion Microprobe dating of monazite provide timing constraints for the late stages of garnet growth, deformation and subsequent decompression. Distinct textural and chemical growth zoning domains, separated by a large chemical discontinuity, reveal two stages of garnet growth characterized in part by: (i) a syn‐kinematic, inclusion‐rich stage‐1 garnet core; and (ii) an inclusion‐poor, stage‐2 garnet rim that crystallized with syn‐ to post‐kinematic staurolite and kyanite. Phase equilibria modelling of garnet molar and compositional isopleths suggest stage‐1 garnet growth initiated at ~600 °C, 8 kbar along a clockwise P–T path. Growth of the compositionally distinct, grossular‐rich, pyrope‐poor inner portion of the stage‐2 overgrowth is interpreted to have initiated at higher pressure and/or lower temperature than the stage‐1 core along a separate P–T loop, culminating at peak P–T conditions of ~650–680 °C and 9 kbar. Stage‐2 metamorphism and the waning development of a composite transposition foliation (ST) are dated at c. 118 Ma from monazite aligned parallel to ST, and inclusions in syn‐ to post‐ST staurolite and kyanite. Slightly younger ages (c. 112 Ma) are obtained from Y‐rich monazite that occurs within resorbed areas of both stage‐1 and stage‐2 garnet, together with retrograde staurolite and plagioclase. The younger ages obtained from these texturally and chemically distinct grains are interpreted, with the aid of phase equilibria calculations, to date the growth of monazite from the breakdown of garnet during decompression at c. 112 Ma. Evidence for continued near‐isothermal decompression is provided by the presence of retrograde sillimanite, and cordierite after staurolite, which indicates decompression below ~4–5 kbar prior to cooling below ~550 °C. As most other parts of the Yukon‐Tanana terrane were exhumed to upper crustal levels in the Early Jurassic, these data suggest this domain represents a tectonic window revealing a much younger, high‐grade tectono‐metamorphic core (infrastructure) within the northern Cordilleran orogen. This window may be akin to extensional core complexes identified in east‐central Alaska and in the southeastern Canadian Cordillera. 相似文献
19.
Formation of eclogite, and reaction during exhumation to mid-crustal levels, Snowbird tectonic zone, western Canadian Shield 总被引：1，自引：1，他引：1
J. A. BALDWIN R. POWELL M. L. WILLIAMS P. GONCALVES 《Journal of Metamorphic Geology》2007,25(9):953-974
A re‐evaluation of the P–T history of eclogite within the East Athabasca granulite terrane of the Snowbird tectonic zone, northern Saskatchewan, Canada was undertaken. Using calculated pseudosections in combination with new garnet–clinopyroxene and zircon and rutile trace element thermometry, peak metamorphic conditions are constrained to ～16 kbar and 750 °C, followed by near‐isothermal decompression to ～10 kbar. Associated with the eclogite are two types of occurrences of sapphirine‐bearing rocks preserving a rich variety of reaction textures that allow examination of the retrograde history below 10 kbar. The first occurs as a 1–2 m zone adjacent to the eclogite body with a peak assemblage of garnet–kyanite–quartz interpreted to have formed during the eclogite facies metamorphism. Rims of orthopyroxene and plagioclase developed around garnet, and sapphirine–plagioclase and spinel–plagioclase symplectites developed around kyanite. The second variety of sapphirine‐bearing rocks occurs in kyanite veins within the eclogite. The veins involve orthopyroxene, garnet and plagioclase layers spatially organized around a central kyanite layer that are interpreted to have formed following the eclogite facies metamorphism. The layering has itself been modified, with, in particular, kyanite being replaced by sapphirine–plagioclase, spinel–plagioclase and corundum–plagioclase symplectites, as well as the kyanite being replaced by sillimanite. Petrological modelling in the CFMAS system examining chemical potential gradients between kyanite and surrounding quartz indicates that these vein textures probably formed during further essentially isothermal decompression, ultimately reaching ～7 kbar and 750 °C. These results indicate that the final reaction in these rocks occurred at mid‐crustal levels at upper amphibolite facies conditions. Previous geochronological and thermochronological constraints bracket the time interval of decompression to <5–10 Myr, indicating that ～25 km of exhumation took place during this interval. This corresponds to minimum unroofing rates of ～2–5 mm year?1 following eclogite facies metamorphism, after which the rocks resided at mid‐crustal levels for 80–100 Myr. 相似文献
20.
Spinel–cordierite symplectites partially replacing andalusite occur in metapelitic rocks within the cores of several country rock diapirs that have ascended into the upper levels of layered mafic/ultramafic rocks in the Bushveld Complex. We investigate the petrogenesis of these symplectites in one of these diapirs, the Phepane dome. Petrographic evidence indicates that at conditions immediately below the solidus the rocks were characterized by a cordierite‐, biotite‐ and K‐feldspar‐rich matrix and 5–10 mm long andalusite porphyroblasts surrounded by biotite‐rich fringes. Phase relations in the MnNCKFMASHT model system constrain the near‐solidus prograde path to around 3 kbar and imply that andalusite persisted metastably into the sillimanite + melt field, where the fringing relationship between biotite and andalusite provided spatially restricted equilibrium domains with silica‐deficient effective bulk compositions that focused suprasolidus reaction. MnNCKFMASHT pseudosections that model these compositional domains suggest that volatile phase‐absent melting reactions consuming andalusite and biotite initially produced a moat of cordierite surrounding andalusite; reaction progressed until all quartz was consumed. Spinel is predicted to grow with cordierite at around 720 °C. Formation of the aluminous solid products was strongly controlled by the receding edge of andalusite grains, with symplectites forming at the andalusite‐cordierite moat interface. Decompression due to melt‐assisted diapiric rise of the floor rocks into the overlying mafic/ultramafic rocks occurred close to the thermal peak. Re‐crossing of the solidus at P = 1.5–2 kbar, T > 700 °C resulted in preservation of the symplectites. Two features of the silica‐deficient domains inhibited resorption of spinel. First, the cordierite moat armoured the symplectites from reaction with crystallizing melt in the outer part of the pseudomorphs. Second, an up‐T step in the solidus at low‐P, which may be in excess of 100 °C higher than the quartz‐saturated solidus, resulted in high‐T crystallization of melt on decompression. Even in metapelitic rocks where melt is retained, preservation of spinel is favoured by decompression. 相似文献