Metamorphic Evolution
13
Citation
25
Reference
10
Related Paper
Citation Trend
Keywords:
Isograd
Paleozoic rocks in the Catalonian Coastal Ranges are in their largest part affectedby alow- tovery-low grade Hercynian metamorphism. Amphibolite facies conditions are only found in restricted areas such as the southwestern part of the Guilleries massif where upper amphibolite facies conditions are reached. Metamorphic grade increases from top to bottom of the Paleozoic stratigraphic sequence and the metamorphic peak is diachronous, being progressively older in the lower grade metamorphic zones. The isograd pattern, mineral assemblages, mineral chemistry and preserved reaction textures are consistent with a low pressure metamorphism possibly evolving from a previous Barrovian type event. The metamorphic climax in the high grade zone was reached after the seconddeformational phase. Calculatedpeak P-Tconditions are 620-640 OC and around 3.5 Kb . A latter episode of decompression from the maximum conditions to 1-2 Kb, with an associated temperature decrease to 530-550 OC, is recognized. The intrusion of late Hercynian granitoids produced contact metamorphic aureoles where the pyroxene-hornfels facies is locally reached.
Diachronous
Isograd
Massif
Hornfels
Pyroxene
Cite
Citations (9)
The Flin Flon – Athapapuskow Lake area, situated in the Flin Flon Greenstone Belt, Manitoba, consists of ocean-floor and island-arc assemblages, deformed and metamorphosed during the Trans-Hudson Orogeny (∼1.86–1.69 Ga). A new map of metamorphic mineral assemblages and isograds has been compiled that reveals a largely coherent regional metamorphic sequence increasing in metamorphic grade from prehnite–pumpellyite to amphibolite facies. Regional metamorphism postdates most of the deformation within the area, with the exception of the reactivation of major block-bounding faults. The regional prograde sequence has been subdivided into 10 metamorphic zones, separated by 9 isograds, that describe the transition from prehnite–pumpellyite to greenschist to amphibolite facies. The formation of contact metamorphic aureoles, pre-dating regional metamorphism, record conditions up to amphibolite facies. Equilibrium phase diagrams for the island-arc (low-Mg) and ocean-floor (high-Mg) assemblages were calculated and allow for the evaluation of the modelling techniques and determination of pressure–temperature conditions. Discrepancies between the modelling predictions and natural observations occur due to (1) limitations in the thermodynamic models for some of the complex minerals (e.g., amphibole); and (2) metastable persistence of some minerals to higher grade due to sluggish reaction kinetics. Notwithstanding these discrepancies, the modelling suggests that metamorphosed mafic rocks in the Flin Flon – Athapapuskow Lake area reached about 430–480 °C and 3.0–4.5 kbar. Peak metamorphic conditions within contact aureoles that preceded regional metamorphism did not exceed 500 °C (at a pressure between 2.7 and 4.4 kbar). The metamorphic field gradient records a transition from 250–300 °C/1.5–2.3 kbar to 430–480 °C/3–4.5 kbar (100–150 °C/kbar), defining a geothermal gradient of approximately 25–31 °C/km.
Greenschist
Isograd
Orogeny
Cite
Citations (3)
On the basis of lithology and metamorphic grade,the Precambrian basement of the Trans-North China Orogen( TNCO) is divided into the high-grade metamorphic terrains and granite-greenstone belts. The former consist of the Taihua,Luliang,Fuping,Hengshan,Huai 'an and Xuanhua complexes,which underwent high-amphibolite to granulite faces metamorphism. The granitegreenstone belts include the Dengfeng,Zanhuang and Wutai domains,metamorphosed in greenschist to amphibolite facies. Their P-T estimates show that the granulites and / or retrograded eclogites in the Hengshan,Huai'an and Xuanhua domains record the maximum pressures of the peak metamorphism,and the granulites in the Hengshan,Fuping and Xuanhua complexes yield the maximum tempretures of the peak metamorphism. It was found that the high-pressure amphibolites and metapelites were exposed in the Zanhuang and Wutai metamorphic complexes. The basement rocks from the TNCO,regardless of their protolith and metamorphic grade,record a similar metamorphic history characterized by the prograde, peak, nearly isothermal decompression and retrogressive cooling metamorphic stages,defining near-isothermal decompressional clockwise P-T paths,which related to continental collision in the central section of the North China Craton. Geochronological data of the complexes in the TNCO indicate three group metamorphic ages,~ 1. 85 Ga,~ 1. 95 Ga and ~ 2. 5Ga. The ubiquitous metamorphic ages of ~ 1. 85 Ga retrieved by miscellaneous methods throughout the metamorphic domains of the TNCO, representing the peak metamorphism occurred at the Palaeoproterozoic, and ~ 1. 95 Ga metamorphic ages suggest a prograde segement of this metamorphic event. Those of ~ 2. 5Ga represent a regional granulite-facies metamorphic event related to the intrusion and underplating of large amounts of mantle-derived magma in some old block,occurred at the Late Neoarchean. However,the relationship between the metamorphic stages and the metamorphic ages is still unidentified.
Protolith
Greenschist
Isograd
Cite
Citations (16)
Isograd
Sequence (biology)
Orogeny
Cite
Citations (40)
Protolith
Isograd
Diachronous
Foliation (geology)
Cite
Citations (1)
Abstract Garnet porphyroblasts in sillimanite‐bearing pelitic schists contain complex textural and compositional zoning, with considerable variation both within and between adjacent samples. The sillimanite‐bearing schists locally occur in regional Barrovian garnet zone assemblages and are indicative of a persistent lack of equilibrium during prograde metamorphism. Garnet in these Dalradian rocks from the Scottish Highlands preserves evidence of a range of metamorphic responses including initial growth and patchy coupled dissolution–reprecipitation followed by partial dissolution. Individual porphyroblasts each have a unique and variable response to prograde metamorphism and garnet with mainly flat compositional profiles co‐exists with those containing largely unmodified characteristic bell‐shaped Mn profiles. This highlights the need for caution in applying traditional interpretations of effective volume diffusion eliminating compositional variation. Cloudy garnet with abundant fluid inclusions is produced during incomplete modification of the initial porphyroblasts and these porous garnet are then particularly prone to partial replacement in sillimanite‐producing reactions. The modification of garnet via a dissolution–reprecipitation process releases Ca into the effective whole‐rock composition, displacing the pressure–temperature positions of subsequent isograd reactions. This represents the first report of internal metasomatism controlling reaction pathways. The behaviour of garnet highlights the importance of kinetic factors, especially deformation and fluids, in controlling reaction progress and how the resulting variability influences subsequent prograde history. The lack of a consistent metamorphic response, within and between adjacent schists, suggests that on both local and regional scales these rocks have largely not equilibrated at peak metamorphic conditions.
Sillimanite
Pelite
Isograd
Dalradian
Metasomatism
Cite
Citations (12)