Metamorphic Pressure-temperature Conditions Research Articles

Minerálne zloženie laminovaných vápnitých fylitov z Črmeľskej doliny pri Košiciach (severné gemerikum, Slovenská republika)

Studied calcareous phyllites represent a part of Carboniferous Črmeľ Formation of the Northern Gemericum Unit (Western Carpathians, Slovak Republic). The mineral association of calcareous phyllites is composed of quartz, calcite, muscovite, Mg-Fe chlorites, albite and accessory minerals (fluorapatite, titanite, zircon and tourmalines). Chemical composition of analyzed muscovites has higher contents of Si (3.38 - 3.40 apfu) with K (up to 0.94 apfu), Fe2+ (up to 0.18 apfu) and Mg (up to 0.33 apfu). Chlorites contain Mg in range 2.12 - 2.36 apfu and Fe2+ in range 2.12 - 2.36 apfu with Fe/(Fe+Mg) ratio between 0.49 and 0.54. Chemical composition of albites is Ab97.9-99.7 An1.9Or0.5. Content of F- (up to 0.07 apfu) and OH- (up to 0.07 apfu) in titanite is slightly increased with lower amount Ti (0.89 - 0.92 apfu). In titanites not very significant (Al, Fe3+) + (OH, F) ↔ Ti + O substitution was also identified. Zircon consists of Si (up to 1.04 apfu), Zr (up to 0.96 apfu) and very low content of Hf (up to 0.02 apfu). Two types of tourmalines in calcareous phyllites are also present. Zonal tourmalines with central parts composed of schorl (1.95 - 2.47 apfu Fe; 0.93 - 0.97 apfu Mg; 0.45 apfu Na) and peripheral parts composed of dravite (1.09 - 1.19 apfu Fe; 1.66 - 1.93 apfu Mg; Na up to 0.79 apfu) and indistinctly zoned to non-zoned tourmalines were identified as magnesio-foitite (1.01 - 1.34 apfu Fe; 1.23 - 1.70 apfu Mg; 0.52 - 0.67 pfu vacancy). Studies of calcareous phyllites indicated metamorphic pressure-temperature conditions of 8 - 9 kbar at 330 - 340 °C using chlorite geothermometer and phengite geobarometer.

Clay mineralogy and geochemistry of pelitic rocks in the middle Permian Fanjiatun formation, East Changchun, China: implications for metamorphism and provenance

Metamorphic pressure–temperature conditions and sedimentary provenance were determined for the middle Permian Fanjiatun Formation in east Changchun, China, based on clay mineralogy, vitrinite reflectance and geochemical data. The pelitic rocks in the Fanjiatun Formation contain illite + kaolinite + chlorite ± mixed-layer chlorite/smectite and detrital quartz + plagioclase. Illite crystallinity in the formation varies from 0.38 to 0.49 (average = 0.44), and consists dominantly of the 2M1 and mixed 2M1 and 1Md polytypes. The illite b0 dimension is 8.9860–9.0130 A (average = 9.0010 A), indicating the Fanjiatun Formation experienced very-low-grade metamorphism at <1.5 kbar. The metamorphic temperatures obtained based on vitrinite reflectance are 191–308 °C, indicating the formation experienced anchizone-zone facies metamorphism. Al2O3/TiO2 ratios vary from 23.93 to 24.22 (average = 24.11), and the chemical index of alteration is 64–69 (average = 67). Rare earth element contents are high and exhibit enrichment in light rare earth elements and negative Eu anomalies. Trace element characteristics indicate that the source of the pelitic rocks was moderately weathered silicic rocks. The sediments were deposited in a reducing continental setting.

Reprint of Very low-grade metamorphism of the upper Permian Linxi Formation from the southern Da Xing'an Mountains, NE China: Constraints from clay mineral genesis

The metamorphic pressure–temperature conditions and the timing of metamorphism were determined for the upper Permian Linxi Formation, southern Da Xing'an Mountains, NE China, based on the genetic characteristics of clay minerals and K–Ar dating of illite from pelitic rocks. Detrital minerals are dominated by quartz and plagioclase, and clay minerals consist mainly of illite, chlorite, and mixed-layer chlorite/smectite. The XRD analytical results indicate that illite crystallinities are in the range of 0.25–0.57 (average value, 0.35), that illite polytypism is dominated by 2M1 and minor 2M1+1Md and 1Md types, and the illite b dimension is in the range of 8.9982–9.0312Å (average, 9.0174Å). These characteristics indicate that the strata containing the illite experienced very low-grade metamorphism. The temperatures estimated by vitrinite reflectance thermometry are in the range of 202°C–293°C (average, 245°C), indicating that the pelitic rocks of the upper Permian Linxi Formation experienced anchizone-grade metamorphism. The illite grains used for dating are <0.15μm in size, which is similar to that of autogenic illite grains. The K–Ar age of the illite is 127.64±2.12Ma, which is interpreted to represent the timing of the very low-grade metamorphism.

Constraints on anthophyllite formation in thermally metamorphosed peridotites from southwestern Japan

Anthophyllite or another species of Mg-amphibole commonly occurs in an intervening zone between the higher grade orthopyroxene zone and lower grade talc zone in progressively metamorphosed peridotites. However, the anthophyllite zone is absent in some of the thermally metamorphosed peridotite complexes in SW Japan despite the existence of the other zones. A comparative study presented here reveals similarities in rock composition and metamorphic pressure–temperature conditions at high-grade zones between the metaperidotite complexes, and differences in the following respects. The metaperidotite complex that contains an anthophyllite zone has less abundant magnetite and olivine that is more homogeneous than the complex where the anthophyllite zone is absent. It is likely that the degree of cation diffusion in olivine crystals depends on duration of heat retention in metaperidotites during thermal metamorphism, which is supported by the variation in mineralogy of intrusive rocks and pelitic hornfelses surrounding the metaperidotites, and by calculations based on a simplified model of thermal conduction. The long duration of heat retention looks to be a necessary condition for the formation of anthophyllite crystals, which have a sluggish nucleation rate. In addition, the circulation of reducing fluids during prolonged metamorphism likely promoted the decomposition of magnetite and the growth of anthophyllite, into which iron is preferentially distributed. This study cautions about kinetic controls and redox conditions for anthophyllite formation in metaperidotites.

Pressure-temperature conditions of the Ryoke metamorphic rocks in Yanai district, SW Japan

Pressure-temperature conditions of metamorphism in the Yanai district, Ryoke belt, SW Japan, have been determined using garnet-biotite thermometry in combination with an empirically calibrated barometer in the assemblage common in pelitic and siliceous rocks, garnet + biotite + plagioclase + quartz. The barometer estimates pressure difference between a well-established sample and unknown samples based on the reaction, % MathType!Translator!2!1!AMS LaTeX.tdl!TeX -- AMS-LaTeX! <![CDATA[% MathType!MTEF!2!1!+- % feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeiCaiaabM % hacaqGYbGaae4BaiaabchacaqGLbGaaGjbVlabgUcaRiaaysW7caqG % NbGaaeOCaiaab+gacaqGZbGaae4CaiaabwhacaqGSbGaaeyyaiaabk % hacaaMe8Uaey4kaSIaaGjbVlaabwgacaqGHbGaae4CaiaabshacaqG % VbGaaeOBaiaabMgacaqG0bGaaeyzaiaaysW7cqGHRaWkcaaMe8Uaae % yCaiaabwhacaqGHbGaaeOCaiaabshacaqG6bGaaGjbVlabg2da9iaa % ysW7caqGWbGaaeiAaiaabYgacaqGVbGaae4zaiaab+gacaqGWbGaae % yAaiaabshacaqGLbGaaGjbVlabgUcaRiaaysW7caqGHbGaaeOBaiaa % b+gacaqGYbGaaeiDaiaabIgacaqGPbGaaeiDaiaabwgaaaa!780C! $$ {\text{pyrope}}\; + \;{\text{grossular}}\; + \;{\text{eastonite}}\; + \;{\text{quartz}}\; = \;{\text{phlogopite}}\; + \;{\text{anorthite}} $$ Pressure and pressure gradient increased with increasing temperature such that pressures of high-grade areas exceeded that of the triple point of aluminosilicates. The thermobaric structure of the study area shows that pressure increased up to 5 kbar with southward increase in metamorphic temperature up to the highest-grade area, the garnet-cordierite zone. Further south, pressure was almost the same as that of the garnet-cordierite zone and temperature decreased. This asymmetric distribution of metamorphic conditions on both sides of the garnet-cordierite zone can explain the asymmetric distribution of metamorphic zones; the K-feldspar-cordierite zone and sillimanite-K-feldspar zone on the north and south sides of the garnet-cordierite zone, respectively. The breakdown reaction of muscovite and quartz defines the beginning of both the K-feldspar-cordierite zone and sillimanite-K-feldspar zone, which took place under low and high pressures, respectively. These thermobaric structures suggest that temperature varied laterally at mid-crustal level during the peak of metamorphism.

Zircon formation during fluid circulation in eclogites (Monviso, Western Alps): implications for Zr and Hf budget in subduction zones

The zircons from an eclogite and an enclosed eclogite-facies vein from the Monviso ophiolite (Western Alps) display contrasting chemical and morphologic features and document different stages of the evolution of the ophiolite. The zircons from the eclogite show a typical magmatic zoning and are enriched in heavy rare earth elements (HREEs) over middle rare earth elements (MREEs) and have an accentuated negative Eu anomaly, which indicates that the grains co-crystallised with plagioclase. These magmatic zircons document the formation of oceanic crust at 163 ± 2 Ma. In contrast, zircons from the vein contain inclusions of garnet, omphacite, and rutile, which indicate that they crystallised under eclogite-facies conditions. The vein zircons have Th/U ratios < 0.09, lack Eu anomalies, and are only weakly enriched in HREE with respect to MREE. These features are consistent with a garnet-bearing, plagioclase-free, i.e., eclogite-facies paragenesis. Vein zircons yield an age of 45 ± 1 Ma, which is evidence for Eocene subduction-zone metamorphism of the Monviso ophiolite. In the vein, the apparent coexistence of zircon, omphacite, and garnet permits the determination of a set of trace element distribution coefficients among these minerals at high pressure. This set of partitioning can demonstrate chemical equilibrium among these phases in rocks that show less clear evidence of textural equilibrium. In addition, zircon age can now be linked to sensors of metamorphic pressure-temperature conditions. The presence of zircon and rutile in the vein is another example of high field strength element (HFSE) mobility over short distances in aqueous fluids at eclogite-facies conditions. However, the concentrations of Zr and Hf in the aqueous fluid are estimated to be at least a factor of 10 less than primitive mantle values. Mass balance calculations demonstrate that zircon hosts > 95% of the bulk Zr, 90% of Hf, and ∼25% of U in the vein. Zircon is a residual phase in subducted basalts and sediments up to temperatures of at least 800 to 900 °C. Therefore, residual zircon in subducted crust, together with rutile, control the HFSE in liberated subduction zone fluids/melts and might be partly responsible for negative Zr and Hf anomalies in subduction zone magmas.

Amphibolite facies metamorphism in the Schapenburg schist belt: A record of the mid-crustal response to 3.23 Ga terrane accretion in the Barberton greenstone belt

The Schapenburg Schist Belt is one of several large greenstone remnants exposed in the granitoid-dominated terrane to the south of the Barberton greenstone belt and is unique in that it contains a well-developed metasedimetary sequence in addition to the typical mafic-ultramafic volcanic rocks. Detrital zircons within the metasediments have ages as young as ~3.24 Ga and consequently, these rocks correlate with Fig Tree Group sediments exposed in the central portions of the Barberton greenstone belt some 60km to the north, where they are metamorphosed to greenschist facies conditions. The Schapenburg metasediments are relatively K2O-poor, and are commonly characterised by the peak metamorphic assemblage garnet + cordierite + gedrite + biotite + quartz ± plagioclase. Other assemblages recorded are garnet + cummingtonite + biotite + quartz, cordierite + biotite + sillimanite + quartz and cordierite + biotite + anthophyllite. In all cases the peak assemblages are texturally very well equilibrated and the predominantly almandine garnets from all rock types show almost flat zonation patterns for Fe, Mg, Mn and Ca. Analysis using FMASH reaction relations, as well as a variety of geothermometers and barometers, has constrained the peak metamorphic pressure-temperature conditions to 640 ± 40°C and 4.8 ± 1.0 kbar. The maximum age of metamorphism is defined by the ~3.23 Ga age of a syntectonic tonalite intrusion into the central portion of the schist belt. Thus, sedimentation, burial to mid-crustal depths, and amphibolite facies equilibration were achieved in a time span similar to ~15 Ma. The strong bedding-parallel foliation in the metasediments dips to the east at an angle of 75 to 85° In this foliation plane elongated cordierite rods produced during prograde metamorphism define a close to vertical mineral lineation. The metasedimentary succession youngs to the east and in this direction is overlain by older Onverwacht Group rocks. Thus, the cordierite lineation might represent a transposed lineation developed during thrust stacking. Post-kinematic static recrystallization has largely obscured the prograde history of the rocks. However, the pressure-temperature conditions of peak metamorphism, the age of metamorphism, the rapidity of metamorphism following sedimentation, the presence of cordierite on the prograde path, as well as the timing of metamorphism relative to deformation strongly suggest that metamorphism occurred in an arc-style collisional setting. Thus, the rocks of this study most likely represent an exhumed mid-crustal terrane equilibrated during the proposed ~3230 Ma terrane accretion event in the Barberton Greenstone Belt, and record an apparent geothermal gradient typical for portions of modern orogenic belts of this type.

Petrology of the Garnet Amphibolites from the Tejici Village - Povlen Mt., Western Serbia

Different metamorphic rocks discovered near the village of Tejici (Povlen Mt., Western Serbia) represent members of the olistostrome m?lange metamorphosed during the obduction/emplacement of some still hot ultramafic body. They occupy the area of about 2 km2. The garnet amphibolites are of highest metamorphic grade in the area of Tejici and were chosen as the most convenient rocks for determination the pressure-temperature conditions of metamorphism and of ultramafics during their emplacement.

Beryllium and boron in subduction zone minerals: An ion microprobe study

Secondary ion mass spectrometry was used to measure the distribution of Be, B, Rb, Sr, Y, Nb, Ba, and Ce among minerals in rocks from the Franciscan Complex and Catalina Schist paleosubduction zone terranes. The rocks examined represent a wide range of metamorphic pressure-temperature conditions (6–15 kb, 300–750°C) and include metasomatized and migmatitic samples. White mica is the primary host of Be (2.5 ppm) and B (107 ppm) in the rocks examined (average concentrations). Amphibole (Be 0.7, B 15 ppm), lawsonite (Be 0.9, B 37 ppm), and biotite (Be 0.9, B 25 ppm) may also contain significant amounts of both Be and B. Omphacite may host Be (4.1 ppm in veins, 0.7 ppm groundmass) and chlorite is a minor host of B (22 ppm). Minerals in these subduction zone metamorphic rocks that lack measurable Be or B include albite, apatite, calcite, clinozoisite, epidote, garnet, ilmenite, sphene, stilpnomelane, plagioclase, quartz, rutile, and zoisite. White mica, biotite, and stilpnomelane are the major mineral hosts of Rb (90, 300, and 30 ppm, respectively) and Ba (2100, 1000, and 4700 ppm). Epidote and lawsonite are the major host minerals of Sr (600 and 300 ppm) and Ce (12 and 15 ppm). Garnet, epidote, lawsonite, and sphene host Y (100, 30, 40, and 60 ppm, respectively). Sphene (77 ppm, 1 analysis), ilmenite, and rutile host Nb. Evidence of veining and metasomatism is strongly correlated with Be and/or B enrichment of white mica, omphacite, amphibole, and lawsonite at the P-T conditions examined. Both Be and B appear to be transported in aqueous fluids at depths of 20–50 km in subduction zones. The transfer of Be, B, and other elements from the subducted slab into the mantle wedge can be accomplished by aqueous fluids. Transfer of the elements hosted by white mica could result directly from the breakdown of white mica through pressure-dependent dehydration reactions or, alternatively, the Be and B in white mica could be partitioned into infiltrating fluids produced at deeper levels in the slab and subsequently transported into the region of are magma genesis.

A molecular water pump in quartz dislocations

RECENTLY Bakker and Janssen1 have reported experiments suggesting that water leaks out of bubbles in quartz preferentially along dislocations against thermodynamic gradients in fugacity, chemical potential and pressure. This effect indicates that the density of gas-rich fluid inclusions cannot be calculated from metamorphic pressure-temperature conditions and, conversely, that the density of the fluid cannot be used to infer metamorphic pressure-temperature conditions. Using computer modelling techniques, I show here that water not only diffuses easily along dislocations in quartz, but can also be 'pumped' along them. The simulations indicate that in dislocation cores, water molecules dissociate and the protons and hydroxyl groups become strongly bound to kinks. A shear stress can do work on kinks to move them along a dislocation, dragging with them these water-bearing species and sweeping other, undissociated water molecules before them. © 1992 Nature Publishing Group.

Metamorphic pressure‐temperature conditions of Indian Plate rocks south of the main mantle thrust, lower swat, Pakistan

The Lower Swat rock sequence, in northern Pakistan, is composed of greenschist and amphibolite facies Indian plate rocks that crop out in a dome directly south of the Main Mantle thrust zone and Kohistan arc terrane. The metamorphism is Eocene to Oligocene in age and records the collision of the Indian plate with the Kohistan arc. This paper presents mineral assemblage, mineral composition and garnet zoning data on the Lower Swat rock sequence in order to estimate the pressure‐temperature conditions of metamorphism and to infer a possible pressure‐temperature‐time path. The stratigraphic sequence consists of Precambrian to Cambrian(?) Manglaur formation unconformably overlain by late Paleozoic to early Mesozoic Alpurai group. The Alpurai group is further subdivided into the Marghazar, Kashala, Saidu and Nikanai Ghar formations. The Manglaur formation, in the core of the dome, is at kyanite grade. Metamorphic grade decreases in the overlying Alpurai group where a garnet “isograd” is mapped in calcareous schist of the Kashala formation. Mineral composition geothermometers and geobarometers indicate final metamorphic equilibrium conditions of about 600°–700°C and 9–11 kbar in the Manglaur and Marghazar formations. These conditions are associated with a phase of static recrystallization which postdates the main phases of deformation. Correlation of the structural‐tectonic history with the pressure‐temperature estimates, and with the garnet zoning analysis, suggests that in the Eocene the Lower Swat rock sequence was subducted to a depth of 35 to 45 km beneath the MMT suture melange. Subduction was followed immediately by exhumation. The initiation of exhumation is believed to have been triggered by a change from subduction to strike slip motion in the MMT, thereby allowing the relatively buoyant Lower Swat crust to rise. The entire metamorphic‐deformational cycle lasted from 7 to 16 m.y., ending in the late Eocene about 38 Ma when the rocks cooled through the argon blocking temperature in hornblende (≈ 550°C).

Regional ultrahigh-pressure coesite-bearing eclogitic terrane in central China: Evidence from country rocks, gneiss, marble, and metapelite

The Dabie Mountains represent a fault-bounded, coesite-bearing eclogite gneissic terrane within the collisional belt between the Sino-Korean and Yangtze cratons. The following geologic evidence indicates that this terrane has been subjected to regional ultrahigh-pressure metamorphism. (1) The country rocks have a metamorphic pressure-temperature evolution compatible with that of the eclogites, on the basis of the occurrence of quartz pseudomorphs after coesite, calcite pseudomorphs after aragonite, relict high-pressure assemblages in different rock types of the country rocks, and consistent, comparable compositional zonations of garnets in both country rocks and eclogites. (2) Peak metamorphic pressure-temperature conditions consistently decrease both from east to west in the collisional belt and from north to south in the Dabie Mountains, and clockwise pressure-temperature paths are common for eclogites over the region. (3) Layered eclogites interbedded with gneisses and amphibolites contain omphacite, kyanite, and garnet, indicating a minimum pressure of 20 kbar at 600 °C for formation of these rocks.

Extremal bounds on geotherms in eroding mountain belts from metamorphic pressure-temperature conditions

Summary. We present a technique based on linear programming for inverting metamorphic pressure-temperature (PT) information from one or more stratigraphic horizons in an eroded mountain belt to determine maximum and minimum bounds on geotherms as a function of time in the orogen. These extremal bounds characterize the set of all possible geotherms at a particular time in the history of the mountain belt consistent with the metamorphic data. Using synthetic PT data generated from several assumed initial thermal profiles, we demonstrate the sensitivity of the extremal bounds, determined by this inverse technique, to the number, distribution, and accuracy of the PT measurements. We conclude that 20 Myr after orogeny, the set of all possible geotherms is reasonably well-constrained. Reconstruction of tight bounds on possible geotherms as early as 10 Myr after the orogeny requires accurate PT measurements from more than one structural horizon. Finally, we provide an example of extremal bounds consistent with real metamorphic PT data from the Lower Schieferhulle in the Tauern Window of the Austrian Alps. Combined with estimates of the mechanical strength of the continental lithosphere supporting the Alps derived from gravity observations, our results suggest that the temperature at the elastic/ductile transition in continental lithosphere exceeds 540°C.

Three-stage metamorphic history of a whiteschist from Sar e Sang, Afghanistan, as part of a former evaporite deposit

Small volumes (in the cm3 range) of a talc-kyanite schist exhibit mosaic equilibria characterized by mineral assemblages conventionally attributed to vastly different pressure temperature conditions of metamorphism. On the basis of petrographic and microprobe studies these assemblages are attributed to three consecutive stages of metamorphism of a chemically exceptional rock composition falling largely into the model system MgO-Al2O3-SiO2-H2O. Stage 1 typified by Mg chlorite-quartz-talc and some paragonite was followed during stage 2 by talc-kyanite, Mg gedrite-quartz, and the growth of large dravites. In stage 3 pure Mg cordierite formed with or without corundum and/or talc, and kyanite was partly converted into sillimanite. Pressures and temperature during this final stage of metamorphism were probably near 5–6 kb, 640 ° C. The preservation of this succession of mineral assemblages related to each other through isochemical reactions suggests that the main factors governing the metamorphic history of this whitheschist were compositional changes of the coexisting fluids with time, whereas pressure temperature variations may be subordinate. In the Sar e Sang area such chemical variations of the metamorphic fluids are probably caused by progressive metamorphism and mobilization of a former evaporite deposit. Microprobe analyses of the phases gedrite and talc indicate variable degrees of sodium incorporation into these phases according to the substitution NaAl→Si.

Pan-African Orogeny in Northern Nigeria

An area of approximately 12,000 sq km of Precambrian to lower Paleozoic rocks around Zaria in northern Nigeria has recently been mapped for the first time. Results from this survey have been combined with limited data from adjacent areas to determine the nature of the Pan-African event in this part of Nigeria. A crystalline basement complex of gneisses, migmatites (Dahomeyan), and relicts of an ancient metasedimentary sequence (Birrimian) underlies most of the region. Infolded into this basement, as north-south-trending synclinoria, is a younger metasedimentary sequence (Ka-tangan) forming the remnants of an earlier, more extensive supracrustal cover. A suite of syntectonic to late tectonic granites and granodiorites (late Precambrian to lower Paleozoic) has intruded both the basement and its cover. The structure of the area is dominated by north-south trends typical over much of Nigeria. There is evidence to suggest that the whole area was affected by two successive phases of intense deformation resulting in tight isoclinal folding about (1) east-northeast-west-southwest, and (2) north-south axes. The crystalline basement was reactivated and the supracrustal cover regionally metamorphosed. Two phases of progressive metamorphism accompanied the deformations, which were separated and followed by static metamorphic phases. Pressure-temperature conditions of metamorphism appear to have remained fairly constant throughout, giving rise to a low pressure facies series similar to the Buchan type. Extensive migmadzation of the crystalline basement is believed to have accompanied the first deformation, followed by further differentiation during the second phase to produce homogeneous gneisses and intrusive granites. Jointing, fracturing, and faulting followed the orogeny, and a major northeast-southwest-trending transcurrent fault system crosses the area. It is obvious from the structural and metamorphic evidence that the Pan-African event reached the status of an orogeny in northwestern Nigeria.