Journal articles: 'P–T pseudosection'

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N.Jowhar, T. "Computer Programs for P-T History of Metamorphic Rocks using Pseudosection Approach." International Journal of Computer Applications 41, no. 8 (March 31, 2012): 18–25. http://dx.doi.org/10.5120/5561-7639.

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Cheng, H., and D. Cao. "Protracted garnet growth in high-P eclogite: constraints from multiple geochronology and P-T pseudosection." Journal of Metamorphic Geology 33, no. 6 (July 6, 2015): 613–32. http://dx.doi.org/10.1111/jmg.12136.

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Zuluaga, C. A. "The effect of zoned garnet on metapelite pseudosection topology and calculated metamorphic P-T paths." American Mineralogist 90, no. 10 (October 1, 2005): 1619–28. http://dx.doi.org/10.2138/am.2005.1741.

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Volkova, N. I., E. I. Mikheev, A. V. Travin, A. G. Vladimirov, A. S. Mekhonoshin, and V. V. Khlestov. "P–T CONDITIONS, U/Pb AND 40Ar/39Ar ISOTOPIC AGES OF UHT GRANULITES FROM CAPE KALTYGEI, WESTERN BAIKAL REGION." Geodynamics & Tectonophysics 12, no. 2 (June 23, 2021): 310–31. http://dx.doi.org/10.5800/gt-2021-12-2-0526.

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The study is focused on metapelitic granulites of Cape Kaltygei (Western Baikal region) that contain a diagnostic mineral assemblage of ultrahigh temperature (UHT) metamorphic rocks (orthopyroxene+sillimanite+quartz). The pseudosection-based thermobarometry yields peak metamorphic temperature and pressure values (T=950 °C, P=~9 kbar) and suggests near-isobaric cooling (IBC) conditions during the retrograde evolution of the granulites. The U/Pb zircon age estimates for metamorphism (~1.87 Ga) support the data published by other researchers. The SHRIMP-II U-Pb dating of zircon cores yields a minimum protolith age of 1.94–1.91 Ga. Biotites and amphiboles from granulites of Cape Kaltygei show the 40Ar/39Ar isotopic ages that are close to the Early Paleozoic accretion-collision system of the Western Baikal region.

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Cruciani, G., M. Franceschelli, and H. J. Massonne. "Low-temperature metamorphic evolution of a pre-Variscan gabbro: a case study from the Palaeozoic basement of northwest Sardinia, Italy." Mineralogical Magazine 75, no. 6 (December 2011): 2793–812. http://dx.doi.org/10.1180/minmag.2011.075.6.2793.

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AbstractA lenticular metagabbro crops out in an early Palaeozoic metasedimentary sequence at Nurra, northwest Sardinia. The metagabbro consists of variable proportions of early-formed coarse-grained albite, chlorite, epidote and apatite, later biotite and amphibole, and late stilpnomelane. Clinopyroxene and ilmenite are rare relict igneous minerals; albite has completely replaced primary plagioclase.The metamorphic evolution of the Nurra metagabbro has been investigated by pseudosection modelling for a fixed bulk-rock composition in the Na2O—CaO—K2O—FeO—MgO—A12O3—SiO2—H2O (NCKFMASH) model system with added Ti and Mn in the P-T range 1-11 kbar and 150-450°C. The P—T path of the metagabbro is a loop with a prograde segment overprinted by later metamorphic re-equilibration. The pressure peak was at ⩽7 kbar and ∼400°C. The subsequent temperature peak, at ∼440°C, was accompanied by a decrease in pressure to ∼3 kbar. The final P—T evolution of the metagabbro is characterized by near-isobaric cooling to 250—300°C, with the formation of stilpnomelane. The P—T path of the Nurra gabbro is typical of continental orogenic belts that have undergone crustal thickening.

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de Hoÿm de Marien, Luc, Pavel Pitra, Florence Cagnard, and Benjamin Le Bayon. "Prograde and retrograde P–T evolution of a Variscan high-temperature eclogite, French Massif Central, Haut-Allier." BSGF - Earth Sciences Bulletin 191 (2020): 14. http://dx.doi.org/10.1051/bsgf/2020016.

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The P–T evolution of a mafic eclogite sample from the Haut-Allier was studied in order to constrain the dynamic of the Variscan subduction in the eastern French Massif Central. Three successive metamorphic stages M1, M2 and M3, are characterized by assemblages comprising garnet1-omphacite-kyanite, garnet2-plagioclase, and amphibole-plagioclase, respectively, and define a clockwise P–T path. These events occurred at the conditions of eclogite (M1; ∼ 20 kbar, 650 °C to ∼ 22.5 kbar, 850 °C), high-pressure granulite (M2; 19.5 kbar and 875 °C) and high-temperature amphibolite facies (M3; < 9 kbar, 750–850 °C), respectively. Pseudosection modelling of garnet growth zoning and mineralogy of the inclusions reveal a prograde M1 stage, first dominated by burial and then by near isobaric heating. Subsequent garnet1 resorption, prior to a renewed growth of garnet2 is interpreted in terms of a decompression during M2. High-pressure partial melting is predicted for both the M1 temperature peak and M2. M3 testifies to further strong decompression associated with limited cooling. The preservation of garnet growth zoning indicates the short-lived character of the temperature increase, decompression and cooling cycle. We argue that such P–T evolution is compatible with the juxtaposition of the asthenosphere against the subducted crust prior to exhumation driven by slab rollback.

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Nasipuri, P., A. Bhattacharya, and S. Das. "Metamorphic reactions in dry and aluminous granulites: a Perple_X P–T pseudosection analysis of the influence of effective reaction volume." Contributions to Mineralogy and Petrology 157, no. 3 (September 4, 2008): 301–11. http://dx.doi.org/10.1007/s00410-008-0335-8.

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Tiwari, Ashish Kumar, and Tapabrato Sarkar. "P-T-t evolution of sapphirine-bearing semipelitic granulites from Vadkampatti in Eastern Madurai Domain, southern India: Insights from petrography, pseudosection modelling and in-situ monazite geochronology." Precambrian Research 348 (September 2020): 105866. http://dx.doi.org/10.1016/j.precamres.2020.105866.

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Holmberg, Johanna, Michał Bukała, Pauline Jeanneret, Iwona Klonowska, and Jarosław Majka. "Decompressional equilibration of the Midsund granulite from Otrøy, Western Gneiss Region, Norway." Geologica Carpathica 70, no. 6 (December 1, 2019): 471–82. http://dx.doi.org/10.2478/geoca-2019-0027.

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Abstract The Western Gneiss Region (WGR) of the Scandinavian Caledonides is an archetypal terrain for high-pressure (HP) and ultrahigh-pressure (UHP) metamorphism. However, the vast majority of lithologies occurring there bear no, or only limited, evidence for HP or UHP metamorphism. The studied Midsund HP granulite occurs on the island of Otrøy, a locality known for the occurrence of the UHP eclogites and mantle-derived, garnet-bearing ultramafics. The Midsund granulite consists of plagioclase, garnet, clinopyroxene, relict phengitic mica, biotite, rutile, quartz, amphibole, ilmenite and titanite, among the most prominent phases. Applied thermodynamic modelling in the NCKFMMnASHT system resulted in a pressure–temperature (P–T) pseudosection that provides an intersection of compositional isopleths of XMg (Mg/Mg+Fe) in garnet, albite in plagioclase and XNa (Na/Na+Ca) in clinopyroxene in the stability field of melt + plagioclase + garnet + clinopyroxene + amphibole + ilmenite. The obtained thermodynamic model yields P–T conditions of 1.32–1.45 GPa and 875–970 °C. The relatively high P–T recorded by the Midsund granulite may be explained as an effect of equilibration due to exhumation from HP (presumably UHP) conditions followed by a period of stagnation under HT at lower-to-medium crustal level. The latter seems to be a more widespread phenomenon in the WGR than previously thought and may well explain commonly calculated pressure contrasts between neighboring lithologies in the WGR and other HP–UHP terranes worldwide.

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Tropper, Peter, and Christoph Hauzenberger. "How well do pseudosection calculations reproduce simple experiments using natural rocks: an example from high-P high-T granulites of the Bohemian Massif." Austrian Journal of Earth Sciences 108, no. 1 (2015): 123–38. http://dx.doi.org/10.17738/ajes.2015.0008.

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Tsujimori, Tatsuki, and Keisaku Matsumoto. "P-T pseudosection of a glaucophane-epidote eclogite from Omi serpentinite mélange, SW Japan: a preliminary report." Journal of the Geological Society of Japan 112, no. 6 (2006): 407–14. http://dx.doi.org/10.5575/geosoc.112.407.

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Cruciani, Gabriele, Marcello Franceschelli, and Norma Brogioni. "Early stage evolution of the mafic-ultramafic belt at La Melada, Sierra de San Luis, Argentina: P–T constraints from metapyroxenite pseudosection modelling." Journal of South American Earth Sciences 37 (August 2012): 1–12. http://dx.doi.org/10.1016/j.jsames.2012.01.001.

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Clark, Chris, and Martin Hand. "Decoding Mesoproterozoic and Cambrian metamorphic events in Willyama Complex metapelites through the application of Sm–Nd garnet geochronology and P–T pseudosection analysis." Gondwana Research 17, no. 1 (January 2010): 59–74. http://dx.doi.org/10.1016/j.gr.2009.09.002.

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Kullerud, Kåre, Pritam Nasipuri, Erling J. K. Ravna, and Rune S. Selbekk. "Formation of corundum megacrysts during H2O-saturated incongruent melting of feldspar: P–T pseudosection-based modelling from the Skattøra migmatite complex, North Norwegian Caledonides." Contributions to Mineralogy and Petrology 164, no. 4 (May 17, 2012): 627–41. http://dx.doi.org/10.1007/s00410-012-0765-1.

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Bijnaar, Ginny, Manfred J. van Bergen, and Theo E. Wong. "The kyanite quartzite of Bosland (Suriname): evidence for a Precambrian metamorphosed alteration system." Netherlands Journal of Geosciences - Geologie en Mijnbouw 95, no. 4 (November 3, 2016): 447–65. http://dx.doi.org/10.1017/njg.2016.38.

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AbstractThis article investigates the origin of a rare occurrence of kyanite quartzites in the Palaeoproterozoic greenstone belt of Suriname. The rocks form elongated hills in the Bosland area, Brokopondo district, where they are associated with meta-sedimentary, meta-volcanic and granitic lithologies. Their mineral content and unusual Si- and Al-rich chemical composition are inferred to be the result of advanced argillic alteration of felsic volcanic tuffs and a later overprint by regional metamorphism up to lower amphibolite facies during the Trans-Amazonian orogeny. Structurally, the Bosland area seems centred within a contractional strike-slip duplex of a major dextral fault system. The alteration was probably associated with a high-sulphidation environment and involved significant to almost complete removal of alkali and alkaline earth elements. Pseudosection modelling and textures suggest that the precipitation–temperature (P–T) history of the kyanite quartzites started with shallow (<2kbar) hydrothermal alteration of the acidic tuffaceous volcanics, possibly in the andalusite stability field (T>350°C), and ended in peak metamorphic conditions in the kyanite–staurolite stability field (P>4kbar andT=500–650°C). Alteration events that preceded the peak of Trans-Amazonian metamorphism may be more common in the rock record of Suriname's greenstone belt, which lends support to the hypothesis that gold mineralisations in the region can be pre-orogenic.

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PRAKASH, DIVYA, DEEPAK, PRAVEEN CHANDRA SINGH, CHANDRA KANT SINGH, SUPARNA TEWARI, MAKOTO ARIMA, and HARTWIG E. FRIMMEL. "Reaction textures and metamorphic evolution of sapphirine–spinel-bearing and associated granulites from Diguva Sonaba, Eastern Ghats Mobile Belt, India." Geological Magazine 152, no. 2 (August 14, 2014): 316–40. http://dx.doi.org/10.1017/s0016756814000399.

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AbstractThe Diguva Sonaba area (Vishakhapatnam district, Andhra Pradesh, South India) represents part of the granulite-facies terrain of the Eastern Ghats Mobile Belt. The Precambrian metamorphic rocks of the area predominantly consist of mafic granulite (±garnet), khondalite, leptynite (±garnet, biotite), charnockite, enderbite, calc-granulite, migmatic gneisses and sapphirine–spinel-bearing granulite. The latter rock type occurs as lenticular bodies in khondalite, leptynite and calc-granulite. Textural relations, such as corroded inclusions of biotite within garnet and orthopyroxene, resorbed hornblende within pyroxenes, and coarse-grained laths of sillimanite, presumably pseudomorphs after kyanite, provide evidence of either an earlier episode of upper-amphibolite-facies metamorphism or they represent relics of the prograde path that led to granulite-facies metamorphism. In the sapphirine–spinel-bearing granulite, osumilite was stable in addition to sapphirine, spinel and quartz during the thermal peak of granulite-facies metamorphism but the assemblage was later replaced by Crd–Opx–Qtz–Kfs-symplectite and a variety of reaction coronas during retrograde overprint. Variable amounts of biotite or biotite+quartz symplectite replaced orthopyroxene, cordierite and Opx–Crd–Kfs–Qtz-symplectite at an even later retrograde stage. Peak metamorphic conditions of c. 1000°C and c. 12 kbar were computed by isopleths of XMg in garnet and XAl in orthopyroxene. The sequence of reactions as deduced from the corona and symplectite assemblages, together with petrogenetic grid and pseudosection modelling, records a clockwise P–T evolution. The P–T path is characteristically T-convex suggesting an isothermal decompression path and reflects rapid uplift followed by cooling of a tectonically thickened crust.

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Jung, Stefan, Soenke Brandt, Rebecca Bast, Erik E. Scherer, and Jasper Berndt. "Metamorphic petrology of a high-T /low-P granulite terrane (Damara belt, Namibia) - Constraints from pseudosection modelling and high-precision Lu-Hf garnet-whole rock dating." Journal of Metamorphic Geology 37, no. 1 (October 2, 2018): 41–69. http://dx.doi.org/10.1111/jmg.12448.

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Prakash, D., DK Patel, MK Yadav, B. Vishal, S. Tewari, R. Yadav, SK Rai, and CK Singh. "Prograde polyphase regional metamorphism of pelitic rocks, NW of Jamshedpur, eastern India: constraints from textural relationship, pseudosection modelling and geothermobarometry." Geological Magazine 157, no. 7 (November 11, 2019): 1045–67. http://dx.doi.org/10.1017/s0016756819001171.

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AbstractThe study area belongs to the Singhbhum metamorphic belt of Jharkhand, situated in the eastern part of India. The spatial distribution of the index minerals in the pelitic schists of the area shows Barrovian type of metamorphism. Three isograds, viz. garnet, staurolite and sillimanite, have been delineated and the textural study of the schists has revealed a time relation between crystallization and deformation. Series of folds with shifting values of plunges in the supracrustal rocks having axial-planar schistosity to the folds have been widely cited. Development of these folds could be attributed to the second phase of deformation. In total, two phases of deformation, D1 and D2, in association with two phases of metamorphism, M1 and M2, have been lined up in the study area. Chemographic plots of reactant and product assemblages corresponding to various metamorphic reactions suggest that the pattern of metamorphic zones mapped in space is in coherence with the temporal-sequential change during prograde metamorphism. The prograde P–T evolution of the study area has been obtained using conventional geothermobarometry, internally consistent winTWQ program and Perple_X software in the MnNCKFMASHTO model system. Our observations suggest that the progressive metamorphism in the area is not related to granitic intrusion or migmatization but that it was possibly the ascending plume that resulted in the M1 phase of metamorphism followed by D1 deformation. The second and prime metamorphic phase, M2, with its possible heat source generated by crustal overloading, was preceded by D1 and it lasted until late- to post-D2 deformation.

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BASUSARBADHIKARI, A., and S. BHOWMIK. "Constraining the metamorphic evolution of a cryptic hot Mesoproterozoic orogen in the Central Indian Tectonic Zone, using P–T pseudosection modelling of mafic intrusions and host reworked granulites." Precambrian Research 162, no. 1-2 (April 5, 2008): 128–49. http://dx.doi.org/10.1016/j.precamres.2007.07.014.

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van Hinsberg, Vincent, Catherine Crotty, Stan Roozen, Kristoffer Szilas, and Alexander Kisters. "Pressure–Temperature History of the >3 Ga Tartoq Greenstone Belt in Southwest Greenland and Its Implications for Archaean Tectonics." Geosciences 8, no. 10 (September 30, 2018): 367. http://dx.doi.org/10.3390/geosciences8100367.

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The Tartoq greenstone belt of southwest Greenland represents a well-preserved section through >3 Ga old oceanic crust and has the potential to provide important constraints on the composition and geodynamics of the Archaean crust. Based on a detailed structural examination, it has been proposed that the belt records an early style of horizontal convergent plate tectonics where elevated temperatures, compared to the modern-day, led to repeated aborted subduction and tonalite–trondhjemite–granodiorite (TTG) type melt formation. This interpretation hinges on pressure–temperature (P–T) constraints for the belt, for which only preliminary estimates are currently available. Here, we present a detailed study of the pressure–temperature conditions and metamorphic histories for rocks from all fragments of the Tartoq belt using pseudosection modelling and geothermobarometry. We show that peak conditions are predominantly amphibolite facies, but range from 450 to 800 °C at up to 7.5 kbar; reaching anatexis with formation of TTG-type partial melts in the Bikuben segment. Emplacement of the Tartoq segments into the host TTG gneisses took place at approximately 3 Ga at 450–500 °C and 4 kbar as constrained from actinolite–chlorite–epidote–titanite–quartz parageneses, and was followed by extensive hydrothermal retrogression related to formation of shear zone-hosted gold mineralisation. Tourmaline thermometry and retrograde assemblages in mafic and ultramafic lithologies constrain this event to 380 ± 50 °C at a pressure below 1 kbar. Our results show that the convergent tectonics recorded by the Tartoq belt took place at a P–T gradient markedly shallower than that of modern-day subduction, resulting in a hot, weak and buoyant slab unable to generate and transfer ‘slab pull’, nor sustain a single continuous downgoing slab. The Tartoq belt suggests that convergence was instead accomplished by under-stacking of slabs from repeated aborted subduction. The shallow P–T path combined with thermal relaxation following subduction stalling subsequently resulted in partial melting and formation of TTG melts.

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JASTRZĘBSKI, MIROSŁAW. "New insights into the polyphase evolution of the Variscan suture zone: evidence from the Staré Město Belt, NE Bohemian Massif." Geological Magazine 149, no. 6 (February 28, 2012): 945–63. http://dx.doi.org/10.1017/s0016756812000040.

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AbstractForming a northern continuation of the Moldanubian Thrust Zone, the Staré Město Belt comprises an E-verging thrust stack of three narrow lithotectonic units that exhibit variations in their respective P–T records. The upper and lower units form the respective margins of the hanging wall and footwall of the suture zone and are dominated by amphibolite grade metasedimentary successions. The middle unit is defined by an elongated body of MORB-like amphibolites that contains inserts of migmatized mica schists. Integrating both structural studies and pseudosection modelling in the MnNCKFMASH system shows that the present-day tectonic architecture of the Staré Město Belt is the result of a polyphase Variscan evolution. During a frontal, WNW–ESE-directed (in present-day coordinates) collision between the Bohemian Massif terranes and the Brunovistulian terrane, the metasedimentary rocks of the Staré Město Belt experienced tectonic burial to depths corresponding to 7–9 kbar. The continuous indentation and underthrusting of the Brunovistulian terrane led to top-to-the-ESE folding and uplift of these rocks to depths corresponding to 5.5–6.0 kbar at peak temperature. At depths corresponding to 5.5 kbar, the Staré Město Belt underwent subsequent dextral (top-to-the-NNE) shearing that was locally associated with nearly isobaric heating, possibly related to the emplacement of a Carboniferous tonalite body in the axial part of the Staré Město Belt. Subsequent tectonic compression resulted in the Variscan WNW-dipping metamorphic foliations becoming locally (N)NE- or ESE-dipping.

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Li, Zilong, Xiaoqiang Yang, Yinqi Li, M. Santosh, Hanlin Chen, and Wenjiao Xiao. "Late Paleozoic tectono–metamorphic evolution of the Altai segment of the Central Asian Orogenic Belt: Constraints from metamorphic P–T pseudosection and zircon U–Pb dating of ultra-high-temperature granulite." Lithos 204 (September 2014): 83–96. http://dx.doi.org/10.1016/j.lithos.2014.05.022.

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Nemec, Ondrej, Marián Putiš, Peter Bačík, Peter Ružička, and Zoltán Németh. "Metamorphic Conditions of Neotethyan Meliatic Accretionary Wedge Estimated by Thermodynamic Modelling and Geothermobarometry (Inner Western Carpathians)." Minerals 10, no. 12 (December 6, 2020): 1094. http://dx.doi.org/10.3390/min10121094.

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Metamorphic evolution of an accretionary wedge can be constrained by a reconstructed P–T conditions of the oceanic and continental margin fragments. This paper deals with the metamorphic overprinting of the Inner Western Carpathians (IWC) Meliatic Triassic–Jurassic paleotectonic units after the closure of the Neotethyan Meliata Basin. Medium to high-pressure and lower temperature conditions were estimated by Perple_X pseudosection modelling, combined with garnet–phengite, calcite–dolomite and chlorite thermometers and chlorite–phengite and phengite barometers. The Late Jurassic subductional burial to a maximum 50 km depth was estimated from the Bôrka Unit continental margin fragments at 520 °C and 1.55 GPa. This is compatible with the metamorphic peak garnet–glaucophane–phengite assemblage of blueschist facies in metabasites. The Jaklovce Unit oceanic fragments were subducted to maximum 35–40 km at 390–420 °C and 1.1–1.3 GPa. Metabasalts and metadolerites contain winchite, riebeckite, actinolite, chlorite, albite, epidote and phengite. A glaucophane-bearing metabasalt recorded an intra-oceanic subduction in blueschist-facies conditions. Rare amphibolite-facies metabasalts of this unit indicate the base of an inferred oceanic crust sliver obducted onto the continental margin wedge. The Meliata Unit oceanic/continental margin flysch calciclastic and siliciclastic metasediments suggest the burial to approximately 15–20 km at 250–350 °C and 0.4–0.6 GPa. This is indicated by a newly formed albite, K-feldspar, illite–phengite and chlorite associated with quartz and/or calcite and dolomite in these rocks. Magnesio-hastingsite to magnesio-hornblende bearing metagabbro with newly formed metamorphic magnesio-riebeckite and actinolite is an inferred detached Meliatic block tectonically emplaced in a Permian salinar mélange in the Silica Nappe hanging wall. Reconstructed P–T paths indicate variable metamorphic conditions from the medium-pressure to high-pressure subduction of the Bôrka and Jaklovce units to the Meliata Unit shallow burial in an accretionary wedge during Late Jurassic to Early Cretaceous Meliaticum evolution. Mélange blocks of Meliaticum incorporate different juxtaposed Meliatic paleotectonic units exposed in nappe outliers overlying the IWC Gemeric and Veporic superunits.

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Dutta, Upama, Ayan Kumar Sarkar, Sadhana M. Chatterjee, Anirban Manna, Alip Roy, and Subhrajyoti Das. "Petrological implications of element redistribution during metamorphism: insights from meta-granite of the South Delhi Fold Belt, Rajasthan, India." Geological Magazine 159, no. 5 (February 23, 2022): 735–60. http://dx.doi.org/10.1017/s0016756821001345.

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AbstractMeta-granites of the South Delhi Fold Belt, northwestern India, contain spectacular reaction textures formed during the metamorphic replacement of primary minerals. Textural relationships imply that amphibole was replaced sequentially in two stages. Epidote + titanite + quartz symplectite formed syn-tectonically on amphibole grain boundaries/fractures, followed by post-deformational growth of euhedral garnet overprinting amphibole grains. Besides occurring as symplectite grown during deformation, titanite in this rock also developed as a post-tectonic corona around magnetite. Parent magnetite contains exsolutions of ilmenite and/or ultrafine lamellae of Ti-rich oxide (Ti-oxd). Textures involving coronal titanite suggest their formation through a magnetite + ilmenite(/Ti-oxd) + plagioclase → titanite reaction. Compositional attributes and the calculation of the gain versus loss of components during the reaction suggest that the Mn2+ for garnet (XSpss = 0.23–0.29) that grew replacing amphibole was supplied by ilmenite (Mn2+ is 0.118–0.128 apfu) as it disintegrated to form coronal titanite. The redistribution of components between the metamorphic reaction sites connects the texturally unrelated domains and suggests that these zones were in chemical equilibrium during metamorphism. We estimated the P–T conditions of metamorphism for these post-tectonic assemblages as ∼650–700 °C from pseudosection modelling and conventional thermometry. Zircon data from this study suggest that the granitic rock crystallized at 988.8 ± 8.8 Ma. We propose that the metamorphic phases replaced the primary minerals during the mid Neoproterozoic tectonic activity reported from this terrane. The syn-tectonic symplectitic assemblage formed as the temperature increased during prograde metamorphism, and the post-tectonic minerals developed at peak conditions following the cessation of deformation.

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Diener, J. F. A., and A. Dziggel. "Can mineral equilibrium modelling provide additional details on metamorphism of the Barberton garnet amphibolites?" South African Journal of Geology 124, no. 1 (March 1, 2021): 211–24. http://dx.doi.org/10.25131/sajg.124.0003.

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Abstract The Stolzburg domain to the south of the Barberton Greenstone Belt preserves evidence for a 3.23 Ga subduction–collision tectonic event. Garnet amphibolite greenstone remnants have previously yielded conventional thermobarometric P-T estimates of 12 to 15 kbar at 600 to 650°C, 8 to 11 kbar at 650 to 700°C and 7.5 to 8.5 kbar at 560 to 640°C from, respectively, the Inyoni shear zone along the western margin of the Stolzburg domain, the central part of the domain and from the Tjakastad schist belt on the boundary with the main body of the Barberton Greenstone Belt. Pseudosection calculations constrain the stability conditions of the peak metamorphic assemblages at the three localities to be 10 kbar at 675 to 690°C, ~10 kbar at 700°C and ~7 and 10 kbar at 660°C respectively. Although it is possible that the peak metamorphic assemblages may be displaced to somewhat lower conditions if Mn is considered in the calculations, these estimates are generally in good agreement with existing estimates, and confirm that the Stolzburg domain exposes an intact mid- to lower-crustal section that was metamorphosed in a relatively cool environment at 3.23 Ga. Our results do not support previously documented higher-pressure conditions, and we contend that the mineral assemblages used to derive these estimates can equally reflect the conditions determined here. The presence of albite-epidote inclusion assemblages in garnet indicates that the likely prograde path involved a component of heating at depth, which is typical of subduction–collision environments and markedly different from the heating–burial paths expected for sinking greenstones in a vertical tectonic model.

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Li, Zilong, Xiaoqiang Yang, Yinqi Li, M. Santosh, Hanlin Chen, and Wenjiao Xiao. "Corrigendum to “Late Paleozoic tectono-metamorphic evolution of the Altai segment of the Central Asian Orogenic Belt: Constraints from metamorphic P–T pseudosection and zircon U–Pb dating of ultra-high-temperature granulite” [Lithos (2014)]." Lithos 204 (September 2014): 268–70. http://dx.doi.org/10.1016/j.lithos.2014.07.003.

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Hoschek, Gert. "Comparison of calculated P-T pseudosections for a kyanite eclogite from the Tauern Window, Eastern Alps, Austria." European Journal of Mineralogy 16, no. 1 (February 23, 2004): 59–72. http://dx.doi.org/10.1127/0935-1221/2004/0016-0059.

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Zhang, Lifei, Qianjie Wang, and Shuguang Song. "Lawsonite blueschist in Northern Qilian, NW China: P–T pseudosections and petrologic implications." Journal of Asian Earth Sciences 35, no. 3-4 (July 2009): 354–66. http://dx.doi.org/10.1016/j.jseaes.2008.11.007.

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29

Du, Jin-Xue, Li-Fei Zhang, Xiao-Jie Shen, and Thomas Bader. "A new P-T-t path of eclogites from Chinese southwestern Tianshan: constraints from P-T pseudosections and Sm-Nd isochron dating." Lithos 200-201 (July 2014): 258–72. http://dx.doi.org/10.1016/j.lithos.2014.04.009.

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30

Mposkos, E., and I. Baziotis. "STUDY OF THE METAMORPHIC EVOLUTION OF A CARBONATE-BEARING METAPERIDOTITE FROM THE SIDIRONERO COMPLEX (CENTRAL RHODOPE, GREECE) USING P-T AND P(T)-XCO2 PSEUDOSECTIONS." Bulletin of the Geological Society of Greece 43, no. 5 (July 31, 2017): 2667. http://dx.doi.org/10.12681/bgsg.11674.

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The carbonate-bearing metaperidotite from Sidironero Complex, north of the Xanthi town is composed primarily of olivine and orthopyroxene megacrysts and of Ti-clinohumite, tremolite, chlorite, dolomite, magnesite, talc, antigorite and spinel group minerals. The metaperidotite underwent a prograde HP metamorphism probably isofacial with the neighboring amphibolitized eclogites. Calculated P-T and P(T)-XCO2 phase diagram sections (pseudosections) for the bulk rock composition showed that XCO2 in the fluid phase was extremely low (≤0.008) at the first stages of the metamorphism and increased up to 0.022 at the peak P-T conditions ~1.5 GPa and 690 0C. The prograde metamorphism probably started from a hydrated and carbonated assemblage including talc+chlorite+magnesite+dolomite and proceeded with tremolite and antigorite formation before olivine growth, and orthopyroxene formation after olivine growth (Ol-1). Matrix dolomite, breakdown of chlorite (Chl-1) to Cr spinel+olivine and of Ti-clinohumite to olivine+Mg-ilmenite occurred during decompression. The P-T path is constrained by the absence of clinopyroxene in the metaperidotite.

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Colás, Vanessa, Ignacio Subías, José María González-Jiménez, Joaquín A. Proenza, Isabel Fanlo, Antoni Camprubí, William L. Griffin, Fernando Gervilla, Suzanne Y. O’Reilly, and Monica F. Escayola. "Metamorphic fingerprints of Fe-rich chromitites from the Eastern Pampean Ranges, Argentina." Boletín de la Sociedad Geológica Mexicana 72, no. 3 (November 28, 2020): A080420. http://dx.doi.org/10.18268/bsgm2020v72n3a080420.

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Chromitites hosted in the serpentinized harzburgite bodies from Los Congos and Los Guanacos (Eastern Pampean Ranges, north Argentina) record a complex metamorphic evolution. The hydration of chromitites during the retrograde metamorphism, their subsequent dehydration during the prograde metamorphism and the later-stage cooling, have resulted in a threefold alteration of chromite: i) Type I is characterized by homogeneous Fe3+- and Cr-rich chromite; ii) Type II chromite contains exsolved textures that consist in blebs and fine lamellae of a magnetite-rich phase hosted in a spinel-rich phase; iii) Type III chromite is formed by variable proportions of magnetite-rich and spinel-rich phases with symplectitic texture. Type I chromite shows lower Ga and higher Co, Zn and Mn than magmatic chromites from chromitites in suprasubduction zone ophiolites as a consequence of the redistribution of these elements between Fe3+-rich non-porous chromite and silicates during the prograde metamorphism. Whereas, the spinel-rich phase in Type III chromite is enriched in Co, Zn, Sc, and Ga, but depleted in Mn, Ni, V and Ti with respect to the magnetite-rich phase, due to the metamorphic cooling from high-temperature conditions. The pseudosection calculated in the fluid-saturated FCrMACaSH system, and contoured for Cr# and Mg#, allows us to constrain the temperature of formation of Fe3+-rich non-porous chromite by the diffusion of magnetite in Fe2+-rich porous chromite at <500 ºC and 20 kbar. The subsequent dehydration of Fe3+-rich non-porous chromite by reaction with antigorite and chlorite formed Type I chromite and Mg-rich olivine and pyroxene at >800 ºC and 10 kbar. The ultimate hydration of silicates in Type I chromite and the exsolution of Type II and Type III chromites would have started at ~600 ºC. These temperatures are in the range of those estimated for ocean floor serpentinization (<300 ºC and <4 kbar), the regional prograde metamorphism in the granulite facies (800 ºC and <10 kbar), and subsequent retrogression to the amphibolite facies (600 ºC and 4-6.2 kbar) in the host ultramafic rocks at Los Congos and Los Guanacos. A continuous and slow cooling from granulite to amphibolite facies produced the exsolution of spinel-rich and magnetite-rich phases, developing symplectitic textures in Type III chromite. However, the discontinuous and relatively fast cooling produced the exsolution of magnetite-rich phase blebs and lamellae within Type II chromite. The P-T conditions calculated in FCrMACaSH system and the complex textural and geochemical fingerprints showed by Type I, Type II and Type III chromites leads us to suggest that continent-continent collisional orogeny better records the fingerprints of prograde metamorphism in ophiolitic chromitites.

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Zeh, A. "Inference of a detailed P -T path from P -T pseudosections using metapelitic rocks of variable composition from a single outcrop, Shackleton Range, Antarctica." Journal of Metamorphic Geology 19, no. 4 (July 2001): 329–50. http://dx.doi.org/10.1046/j.0263-4929.2000.00314.x.

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Braga, R., H. J. Massonne, and L. Morten. "An early metamorphic stage for the Variscan Ulten Zone gneiss (NE Italy): evidence from mineral inclusions in kyanite." Mineralogical Magazine 71, no. 06 (December 2007): 691–702. http://dx.doi.org/10.1180/minmag.2007.071.6.691.

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Abstract The early P-T evolution of a garnet-kyanite gneiss from the Variscan Ulten Zone has been defined by detailed petrographic observations leading to the detection of chlorite-epidote- and staurolite-bearing assemblages enclosed in kyanite porphyroblasts. Calculations of P-T pseudosections in the system NaCaKFeMgAlSiHO allowed us to constrain the evolution of these relics to the earliest metamorphic stages. The overall path shows a P-T increase to a peak of 11–12 kbar and 600–650°C followed by decompressional heating to 720°C and 9–10 kbar and final cooling at 7 kbar, 550–600°C. This clockwise P-T path reflects crustal thickening and subsequent thermal decay related to the continental collision of the Variscan orogeny 330–340 Ma ago. Our study demonstrates that large kyanite porphyroblasts may preserve assemblages related to prograde metamorphic stages. As a result, the detection of mineral inclusions in kyanite can complement many similar studies on mineral suites hosted in garnet and zircon.

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Vance, Derek, and Emma Mahar. "Pressure-temperature paths from P - T pseudosections and zoned garnets: potential, limitations and examples from the Zanskar Himalaya, NW India." Contributions to Mineralogy and Petrology 132, no. 3 (August 28, 1998): 225–45. http://dx.doi.org/10.1007/s004100050419.

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35

Cirrincione, Rosolino, Gaetano Ortolano, Antonino Pezzino, and Rosalda Punturo. "Poly-orogenic multi-stage metamorphic evolution inferred via P–T pseudosections: An example from Aspromonte Massif basement rocks (Southern Calabria, Italy)." Lithos 103, no. 3-4 (July 2008): 466–502. http://dx.doi.org/10.1016/j.lithos.2007.11.001.

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36

Liu, Fenglin, Lifei Zhang, Xiaoli Li, Alexander I. Slabunov, Chunjing Wei, and Thomas Bader. "The metamorphic evolution of Paleoproterozoic eclogites in Kuru-Vaara, northern Belomorian Province, Russia: Constraints from P-T pseudosections and zircon dating." Precambrian Research 289 (February 2017): 31–47. http://dx.doi.org/10.1016/j.precamres.2016.11.011.

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37

Castellan, Paulo, Gustavo Viegas, and Frederico M. Faleiros. "Brittle–ductile fabrics and P–T conditions of deformation in the East Pernambuco shear zone (Borborema Province, NE Brazil)." Journal of the Geological Society 178, no. 1 (September 22, 2020): jgs2020–109. http://dx.doi.org/10.1144/jgs2020-109.

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Fabrics of the East Pernambuco shear zone (EPSZ) were studied via microstructural analysis, mineral chemistry and isochemical phase diagram modelling to constrain the pressure and temperature conditions of deformation during shearing. Granitic mylonites show fractured feldspar porphyroclasts embedded in a fine-grained, recrystallized quartzo-feldspathic matrix. These mylonites grade laterally into banded ultramylonites characterized by stretched feldspar clasts alternated with recrystallized quartz bands. Fractures in these ultramylonites are filled by phyllosilicates. The mineral chemistry of the feldspars points to systematic changes between porphyroclasts, grains within fractures and fine-grained mixtures. Quartz crystallographic fabrics in the mylonites suggest activation of prism slip, while the ultramylonites show the activation of both rhomb and basal slip systems. Thermodynamic modelling suggests that the mylonites were formed at 4.75 ± 0.25 kbar and 526 ± 9°C, while the ultramylonites yield conditions of 5.9 ± 1 kbar and 437 ± 17°C. These observations suggest that the EPSZ records a heterogeneous path of strain accommodation, marked by decreasing temperature from its western sector to its eastern termination. The differences in metamorphic conditions are consistent with a transitional, brittle–ductile strain regime. Such characteristics indicate that the EPSZ is a Neoproterozoic shear belt nucleated and heterogeneously exhumed at the brittle–ductile transition, possibly in an intracontinental setting.Supplementary Material: EPMA analysis of feldspars in Caruaru and Gravatá domains and T-X(O2) pseudosections are available at https://doi.org/10.6084/m9.figshare.c.5125957

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Ramacciotti, Carlos Dino, César Casquet, Edgardo Gaspar Baldo, Sebastián Osvaldo Verdecchia, Matías Martín Morales, and Priscila Soledad Zandomeni. "Metamorfismo de alto gradiente P/T en la Sierra de Pie de Palo (Sierras Pampeanas, Argentina): modelado de equilibrio de fases minerales e implicancias geodinámicas en el antearco famatiniano." Andean Geology 46, no. 3 (September 30, 2019): 526. http://dx.doi.org/10.5027/andgeov46n3-3198.

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The Sierra de Pie de Palo (SPP, Western Sierras Pampeanas) shows evidence of two regional metamorphisms: one Mesoproterozoic attributed to the Grenvillian orogeny and other of Ordovician age related to the Famatinian orogeny. The Neoproterozoic-to-Cambrian sedimentary successions that cover the Grenvillian basement only record the Ordovician event. One staurolite-schist from the Ediacaran Difunta Correa Metasedimentary Sequence collected in the southeastern side of the SPP allows to constrain, by means of pseudosections, a prograde evolution from ca. 3 kbar and 515 ºC up to ca. 9 kbar and 640 ºC corresponding to a high P/T gradient. The SPP and the immediately east Loma de Las Chacras outcrop were part of the famatinian forearc which shows a progressive decrease of P (from ca. 13 kbar to 6 kbar), T (from ca. 900 ºC to 450 ºC), and P/T gradient (from ca. 85 ºC/kbar to 35 ºC/kbar) towards the active continental margin on the west. The Caucete Group, in the western side of the SPP, represents the westernmost part of the forearc, near to the active continental margin. Metamorphism was apparently coeval with the Famatinian magmatism and with ductile underthrusting at ca. 470-465 Ma, which led to burial of the forearc beneath the magmatic arc.

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Kryza, R., A. P. Willner, H. J. Massonne, A. Muszyński, and H. P. Schertl. "Blueschist-facies metamorphism in the Kaczawa Mountains (Sudetes, SW Poland) of the Central-European Variscides: P-T constraints from a jadeite-bearing metatrachyte." Mineralogical Magazine 75, no. 1 (February 2011): 241–63. http://dx.doi.org/10.1180/minmag.2011.075.1.241.

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AbstractSodic pyroxene is reported from an Ordovician metatrachyte of the Kaczawa Mountains, SW Poland. Its composition ranges from Jd0.98Ae0.02 to Jd0.15Ae0.85. Relict jadeite and phengite (up to 3.75 Si atoms per fomula unit) belong to the peak-pressure assemblage of an early HP-LT event. Later greenschist-facies stages are represented by riebeckite, biotite, chlorite, low-Si potassic white mica and actinolite. P-T pseudosections calculated for the range 200–450°C, 3–13 kbar allow evaluation of the conditions of formation of jadeite in the metatrachyte and derivaton of a P-T path. Considering the position of prograde, peak and retrograde metamorphic assemblages and respective mineral compositions, we can derive the following equilibration stages: 8.5±0.5 kbar, 270±20°C for the pressure maximum, 6.0±1.0 kbar, 310±20°C for the temperature maximum and 3.5±0.5 kbar, 280±20°C as well as <3.5 kbar, <280°C for the retrograde stages.The metamorphic gradient for the peak-pressure is estimated at ∼10°C/km, which is typical of a subduction setting involving subducted continental crust, in particular of an exhumation channel within a collision zone of a microplate. Based on earlier structural observations, the ESE-oriented subduction in the NE Bohemian Massif was confined with WNW thrusting and followed by extension and ESE backward normal faulting during Devonian–Early Carboniferous times.

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Yuan, Haiqi, Jian Wang, and Keiko Hattori. "Ultrahigh-Pressure Metamorphism and P-T Path of Xiaoxinzhuang Eclogites from the Southern Sulu Orogenic Belt, Eastern China, Based on Phase Equilibria Modelling." Minerals 12, no. 2 (February 8, 2022): 216. http://dx.doi.org/10.3390/min12020216.

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Three types of eclogites were identified in the Xiaoxinzhuang area in the northern Sulu ultrahigh pressure (UHP) terrene based on their petrographic, compositional characteristics and locations. They are composed of garnet, omphacite, amphibole, epidote, phengite, quartz/coesite, rutile, apatite, ilmenite and kyanite. Garnet in eclogite exhibits weak compositional zoning, which shows an increase in Xgr and a decrease in Xpy from core to mantle, and a decrease in Xgr and a slight increase in Xpy from mantle to rim. Phengite inclusions in garnet show higher Si, up to 3.424 p.f.u., than those in the matrix. Pseudosections calculated using THERMOCALC in the NCKFMASHTO system for three representative samples record three stages of metamorphism: (I) prograde stage, (II) post- Pmax decompression and heating to the Tmax stage and (III) retrograde stage. Stage-I was recorded in garnet cores with mineral assemblage of garnet + omphacite ± amphibole ± lawsonite + phengite + quartz + rutile, and the P-T condition is constrained at 23.5–26.4 kbar and 623–655 °C. The Pmax, 41.5 kbar at 801 °C, is revealed from garnet enclosed by coarse-grained garnet with the mineral assemblage of garnet + omphacite + phengite + coesite + rutile. Stage-II produced garnet rim with mineral assemblage of garnet + omphacite + amphibole + quartz + rutile + metabasite melt, which constrained the P-T conditions of 21.4–23.0 kbar and 869–924 °C. Stage-III, recorded by unzoned garnet grain with the mineral assemblage of garnet + omphacite + amphibole + ilmenite + rutile + metabasite melt, constrained P-T conditions of 13.5–16.4 kbar and 813–852 °C. The data suggest that the rocks in the Xiaoxinzhuang area were subducted to a depth of over 135 km and underwent an UHP metamorphism. The P-T-t path revealed by the Xiaoxinzhuang eclogites is different from those in other areas of the Sulu UHP terrane, suggesting that they represent different rock slices during the subduction and exhumations.

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41

STIPSKA, P., and R. POWELL. "Constraining the P-T path of a MORB-type eclogite using pseudosections, garnet zoning and garnet-clinopyroxene thermometry: an example from the Bohemian Massif." Journal of Metamorphic Geology 23, no. 8 (October 2005): 725–43. http://dx.doi.org/10.1111/j.1525-1314.2005.00607.x.

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42

Li, Yang, Yang Yang, Yi-Can Liu, Chiara Groppo, and Franco Rolfo. "Muscovite Dehydration Melting in Silica-Undersaturated Systems: A Case Study from Corundum-Bearing Anatectic Rocks in the Dabie Orogen." Minerals 10, no. 3 (February 27, 2020): 213. http://dx.doi.org/10.3390/min10030213.

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Corundum-bearing anatectic aluminous rocks are exposed in the deeply subducted North Dabie complex zone (NDZ), of Central China. The rocks consist of corundum, biotite, K-feldspar and plagioclase, and show clear macro- and micro-structural evidence of anatexis by dehydration melting of muscovite in the absence of quartz. Mineral textures and chemical data integrated with phase equilibria modeling, indicate that coarse-grained corundum in leucosome domains is a peritectic phase, reflecting dehydration melting of muscovite through the reaction: Muscovite = Corundum + K-feldspar + Melt. Aggregates of fine-grained, oriented, corundum grains intergrown with alkali feldspar in the mesosome domains are, instead, formed by the dehydration melting of muscovite with aluminosilicate, through the reaction: Muscovite + Al-silicate = Corundum + K-feldspar + Melt. P-T pseudosections modeling in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2 system constrains peak pressure-temperature (P-T) conditions at 900–950 °C, 9–14 kbar. The formation of peritectic corundum in the studied rocks is a robust petrographic evidence of white mica decompression melting that has occurred during the near-isothermal exhumation of the NDZ. Combined with P-T estimates for the other metamorphic rocks in the area, these new results further confirm that the NDZ experienced a long-lived high-T evolution with a near-isothermal decompression path from mantle depths to lower-crustal levels. Furthermore, our new data suggest that white mica decompression melting during exhumation of the NDZ was a long-lasting process occurring on a depth interval of more than 30 km.

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Safonov, Oleg, Valentina Butvina, and Evgenii Limanov. "Phlogopite-Forming Reactions as Indicators of Metasomatism in the Lithospheric Mantle." Minerals 9, no. 11 (November 6, 2019): 685. http://dx.doi.org/10.3390/min9110685.

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Phlogopite is widely accepted as a major mineral indicator of the modal metasomatism in the upper mantle within a very wide P–T range. The paper reviews data on various phlogopite-forming reactions in upper-mantle peridotites. The review includes both descriptions of naturally occurring reactions and results of experiments that model some of these reactions. Relations of phlogopite with other potassic phases, such as K-richterite, sanidine and K-titanates, are discussed. These data are taken as a basis for thermodynamic modeling of the phlogopite-forming reactions for specific mantle rocks in terms of log(aH2O) − log(aK2O) diagrams (pseudosections) using the Gibbs free energy minimization. These diagrams allow estimation of potassium-water activity relations during metasomatic transformations of mantle rocks, prediction sequences of mineral assemblages with respect to these parameters and comparison of metasomatic processes in the rocks of different composition. This approach is illustrated by examples from peridotite xenoliths from kimberlites.

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ALESSIO, BRANDON L., MORGAN L. BLADES, GEORGE MURRAY, BENJAMIN THORPE, ALAN S. COLLINS, DAVID E. KELSEY, JOHN FODEN, JUSTIN PAYNE, SALAH AL-KHIRBASH, and FRED JOURDAN. "Origin and tectonic evolution of the NE basement of Oman: a window into the Neoproterozoic accretionary growth of India?" Geological Magazine 155, no. 5 (March 7, 2017): 1150–74. http://dx.doi.org/10.1017/s0016756817000061.

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AbstractThe Omani basement is located spatially distant from the dominantly juvenile Arabian–Nubian Shield (ANS) to its west, and its relationship to the amalgamation of those arc terranes has yet to be properly constrained. The Jebel Ja'alan (NE Oman) basement inlier provides an excellent opportunity to better understand the Neoproterozoic tectonic geography of Oman and its relationship to the ANS. To understand the origin of this basement inlier, we present new radiogenic isotopic data from igneous bodies in Jebel Ja'alan. U–Pb and 40Ar/39Ar geochronological data are used to constrain the timing of magmatism and metamorphism in the jebel. Positive εHf and εNd values indicate a juvenile origin for the igneous lithologies. Phase equilibria modelling is used to constrain the metamorphic conditions recorded by basement. Pressure–temperature (P–T) pseudosections show that basement schists followed a clockwise P–T path, reaching peak metamorphic conditions of c. 650–700°C at 4–7.5 kbar, corresponding to a thermal gradient of c. 90–160°C/kbar. From the calculated thermal gradient, in conjunction with collected trace-element data, we interpret that the Jebel Ja'alan basement formed in an arc environment. Geochronological data indicate that this juvenile arc formed during Tonian time and is older than basement further west in Oman. We argue that the difference in timing is related to westwards arc accretion and migration, which implies that the Omani basement represents its own tectonic domain separate to the ANS and may be the leading edge of the Neoproterozoic accretionary margin of India.

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45

Diener, Johann F. A., Åke Fagereng, and Sukey A. J. Thomas. "Mid-crustal shear zone development under retrograde conditions: pressure–temperature–fluid constraints from the Kuckaus Mylonite Zone, Namibia." Solid Earth 7, no. 5 (September 16, 2016): 1331–47. http://dx.doi.org/10.5194/se-7-1331-2016.

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Abstract. The Kuckaus Mylonite Zone (KMZ) forms part of the larger Marshall Rocks–Pofadder shear zone system, a 550 km-long, crustal-scale strike-slip shear zone system that is localized in high-grade granitoid gneisses and migmatites of the Namaqua Metamorphic Complex. Shearing along the KMZ occurred ca. 40 Ma after peak granulite-facies metamorphism during a discrete tectonic event and affected the granulites that had remained at depth since peak metamorphism. Isolated lenses of metamafic rocks within the shear zone allow the P–T–fluid conditions under which shearing occurred to be quantified. These lenses consist of an unsheared core that preserves relict granulite-facies textures and is mantled by a schistose collar and mylonitic envelope that formed during shearing. All three metamafic textural varieties contain the same amphibolite-facies mineral assemblage, from which calculated pseudosections constrain the P–T conditions of deformation at 2.7–4.2 kbar and 450–480 °C, indicating that deformation occurred at mid-crustal depths through predominantly viscous flow. Calculated T–MH2O diagrams show that the mineral assemblages were fluid saturated and that lithologies within the KMZ must have been rehydrated from an external source and retrogressed during shearing. Given that the KMZ is localized in strongly dehydrated granulites, the fluid must have been derived from an external source, with fluid flow allowed by local dilation and increased permeability within the shear zone. The absence of pervasive hydrothermal fractures or precipitates indicates that, even though the KMZ was fluid bearing, the fluid/rock ratio and fluid pressure remained low. In addition, the fluid could not have contributed to shear zone initiation, as an existing zone of enhanced permeability is required for fluid infiltration. We propose that, following initiation, fluid infiltration caused a positive feedback that allowed weakening and continued strain localization. Therefore, the main contribution of the fluid was to produce retrograde mineral phases and facilitate grain-size reduction. Features such as tectonic tremor, which are observed on active faults under similar conditions as described here, may not require high fluid pressure, but could be explained by reaction weakening under hydrostatic fluid pressure conditions.

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RÉGNIER, JEAN-LUC, JOCHEN E. MEZGER, and CEES W. PASSCHIER. "Metamorphism of Precambrian–Palaeozoic schists of the Menderes core series and contact relationships with Proterozoic orthogneisses of the western Çine Massif, Anatolide belt, western Turkey." Geological Magazine 144, no. 1 (October 19, 2006): 67–104. http://dx.doi.org/10.1017/s0016756806002640.

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The tectonic setting of the southern Menderes Massif, part of the western Anatolide belt in western Turkey, is characterized by the exhumation of deeper crustal levels onto the upper crust during the Eocene. The lowermost tectonic units of the Menderes Massif are exposed in the Çine Massif, where Proterozoic basement orthogneisses of the Çine nappe are in tectonic contact with Palaeozoic metasedimentary rocks of the Selimiye nappe. In the southern Çine Massif, orthogneiss and metasedimentary rocks are separated by the southerly dipping Selimiye shear zone, preserving top-to-the-S shearing under greenschist facies conditions. In contrast, in the western Çine Massif, the orthogneiss is deformed and mylonitic near the contact with the metasedimentary rocks. The geometry of the mylonite zone and the observed shear directions change from north to southwest. In the north, the mylonite zone dips shallowly to the north, with top-to-the-N shear sense indicators showing northward thrusting of the orthogneiss over the metasedimentary rocks. In the southwest, the mylonite zone resembles a steep N–S striking strike-slip shear zone associated with top-to-the-SSW sense of shear. Overall, the geometry of the mylonite shear zone is consistent with northward movement of the orthogneiss relative to the metasedimentary rocks. Different shear senses are attributed to strain partitioning.AFM diagrams and P–T pseudosections with mineral parageneses of metasedimentary rocks of the Selimiye nappe and metasedimentary enclaves within the orthogneiss of the Çine nappe indicate a single Barrovian-type metamorphism. An earlier higher pressure phase is evident from staurolite–chloritoid inclusions in garnets of the Çine nappe, suggesting a clockwise P–T path. A similar path is inferred for the Selimiye nappe. Index minerals and the sequence of mineral parageneses point to a single amphibolite facies metamorphic event affecting metasedimentary rocks of both nappes, which predates Eocene emplacement of the high pressure–low temperature Lycian and Cycladic blueschist nappes. Northward thrusting of the orthogneiss onto the metasedimentary rocks of the Selimiye nappe is coeval with amphibolite facies metamorphism. Recently postulated polymetamorphism cannot be supported by this study. Petrological data provide no evidence for burial of the lower units of the Menderes Massif to depth greater than 30 km during closure of the Neo-Tethys. A major pre-Eocene tectonic event associated with top-to-the-N thrusting and Barrovian-type metamorphism could lend support to the idea of a Neo-Tethys (sensu stricto) suture south of the Menderes Massif and below the Lycian nappes.

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Greenfield, A. M. R., E. D. Ghent, and J. K. Russell. "Geothermobarometry of spinel peridotites from southern British Columbia: implications for the thermal conditions in the upper mantle." Canadian Journal of Earth Sciences 50, no. 10 (October 2013): 1019–32. http://dx.doi.org/10.1139/cjes-2013-0037.

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Spinel lherzolite xenoliths within alkali basalts exposed at Rayfield River and Big Timothy Mountain, south-central British Columbia, represent samples of the underlying lithospheric mantle. Electron microprobe analysis shows that most xenoliths comprise compositionally homogeneous grains of olivine, orthopyroxene, clinopyroxene, and spinel. We applied the following mineral-pair geothermometers to these rocks: orthopyroxene–clinopyroxene, spinel–orthopyroxene, and spinel–olivine. Temperatures calculated using the Brey and Köhler calibration of two-pyroxene thermometry were constrained in pressure by being required to lie on a model geotherm we develop for this region of B.C. The model geotherm is constrained to produce a temperature at the Moho (33 km) of 825 ± 25 °C to match the lowest temperature peridotite xenoliths recovered in this study. Although the overall effect of pressure on the temperature calculations is negligible (∼2 °C for 0.1 GPa), the simultaneous solution of the model geotherm and the pressure-dependent Brey–Köhler two-pyroxene thermometry removes the need for adopting an arbitrary pressure. We take these temperatures to represent peak mantle lithosphere temperatures. Fourteen Rayfield River xenoliths return two-pyroxene temperatures between 841 and 962 °C corresponding to depths of 34–42 km. Orthopyroxene–spinel and olivine–spinel results are 889 ± 60 and 825 ± 88 °C, respectively. Five Big Timothy xenoliths have two-pyroxene temperatures spanning 840–1058 °C and corresponding to depths of 34–48 km. Mean orthopyroxene–spinel and olivine–spinel temperatures are 844 ± 63 and 896 ± 232 °C, respectively. We argue that the differences in ranges of temperature do not represent closure temperatures imposed during cooling either in the mantle or during transport by the magma. Rather, these differences reflect differences in the original calibrations of the geothermometers or different degrees of equilibration in exchange reactions in dry rocks. Isochemical phase diagrams (pseudosections) constrain the pressure–temperature (P–T) field in which spinel is stable. These diagrams suggest that the spinel-bearing peridotites equilibrated at pressures ranging from ∼9.6 to 14 kbar (10 kbar = 1 GPa).

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48

Belic, Maximilian, Christoph A. Hauzenberger, and Yunpeng Dong. "Multistage Metamorphic Evolution of Retrograded Eclogites from the Songshugou Complex, Qinling Orogenic Belt, China." Journal of Petrology 60, no. 11 (November 1, 2019): 2201–26. http://dx.doi.org/10.1093/petrology/egaa007.

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Abstract The Qinling Orogenic Belt is one of the major collisional orogens in eastern Asia and marks the boundary between the North China Craton and South China Craton. The Songshugou complex is the largest basic to ultrabasic body to be found in the North Qinling Belt, and was emplaced as a lens-shaped body at the southern margin of the Qinling Group. A detailed petrological investigation of garnet amphibolite, augen amphibolite and well-foliated amphibolite together with garnet zoning patterns of major and trace elements, inclusions in garnet, and thermodynamic modelling indicate a multistage metamorphic history. Garnets clearly show characteristics of discontinuous growth, as they display optically light-colored snowball-textured cores surrounded by a darker mantle with few inclusions as well as chemically a sudden increase in grossular and decrease in almandine components. A partly resorbed rim is not recognized optically but mineral inclusions and a discontinuous chemical composition of garnet are proof of this third garnet growth stage. Rare earth element distribution patterns of garnet also show clear evidence for discontinuous growth and allow us to identify the reactions responsible for garnet growth. Garnet core compositions as well as amphibole inclusions allow us to constrain a P–T window where this rock equilibrated in a first stage. Calculated pseudosections and the application of the garnet–amphibole thermometer indicate an upper amphibolite- to lower granulite-facies metamorphic episode at 630–740 °C and 0·7–0·9 GPa. The presence of relict omphacite as well as a discontinuously grown garnet mantle with rutile inclusions clearly places the peak metamorphic stage in the eclogite facies. Garnet (XGrs, XAlm, XPrp) and omphacite isopleths (XMg, XNa) constrain this event at 1·7–2·1 GPa and 570–650 °C. Consistent temperatures of 500–650 °C were also determined by clinopyroxene–garnet geothermobarometry for this event. Growth of an outermost rim as well as different stages of garnet breakdown to plagioclase + amphibole coronae and the nearly complete replacement of former omphacite by a variety of symplectites point to an intricate retrograde P–T path. In more strongly retrograded samples plagioclase + amphibole ± quartz pseudomorphs entirely replace former garnet grains. Certain coronae around garnets and symplectites also contain prehnite and pumpellyite, which formed during a late retrograde stage or during a different event at very low P–T conditions (250–350 °C). Based on the detailed petrological study, we favour a multistage metamorphic history of the Songshugou metabasic rocks. The age of the eclogite-facies metamorphic event must be related to the deep subduction of the Songshugou complex during the early Paleozoic, although the age of garnet core growth remains enigmatic. The development of garnet cores indicates an upper amphibolite-facies regional metamorphic overprint succeeded by an eclogite-facies event around 500 Ma and subsequent retrogression seen in replacement of garnet and formation of symplectite. The latest imprint evidenced by prehnite and pumpellyite may be the result of fluid infiltration during the fading orogenic phase or represents a low-temperature overprint by a later process, probably related to the uplift of the North Qinling terrane at around 420 Ma.

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Karmakar, Shreya, Subham Mukherjee, and Upama Dutta. "Origin of corundum within anorthite megacrysts from anorthositic amphibolites, Granulite Terrane, Southern India." American Mineralogist 105, no. 8 (August 1, 2020): 1161–74. http://dx.doi.org/10.2138/am-2020-7108.

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Abstract Growth of corundum in metamorphosed anorthosites and related basic-ultra-basic rocks is an exceptional feature, and its origin remains elusive. We describe the occurrence of and offer an explanation for the genesis of corundum in anorthositic amphibolites from ~2.5 Ga old basement of the Granulite Terrane of Southern India (GTSI). The studied amphibolites from two localities, Manavadi (MvAm) and Ayyarmalai (AyAm), contain anorthite lenses (An90–99) with euhedral to elliptical outline set in a finer-grained matrix of calcic plagioclase (An85–90) and aluminous amphibole (pargasite-magnesiohastingsite). The lenses, interpreted as primary magmatic megacrysts, and the matrix are both recrystallized under static condition presumably during the regional high pressure (HP) metamorphism (~800 °C, 8–11 kbar) at ~2.45 Ga. Corundum occurs in the core of some of the recrystallized anorthite lenses (An95–99) in two modes: (1) Dominantly, it forms aggregates with magnetite (with rare inclusion of hercynite; in MvAm) or spinel (and occasionally hematite-ilmenite; in AyAm). The aggregates cut across the polygonal grain boundaries of the anorthite and contain inclusions of anorthite. (2) Corundum also occurs along the grain boundaries or at the triple junctions of the polygonal anorthite grains, where it forms euhedral tabular grains, sieved with inclusions of anorthite or forms skeletal rims around the recrystallized anorthite, such that it seems to be intergrown with anorthite. Combined petrological data and computed phase relations are consistent with growth of corundum in an open system during regional metamorphism in the presence of intergranular fluids. Two mechanisms are proposed to explain the formation of the corundum in the amphibolites: (1) corundum + magnetite/spinel aggregates formed dominantly by oxy-exsolution of pre-existing Al-Fe-Mg-(Ti)-spinel. This pre-existing spinel may be primary magmatic inclusions within the anorthite phenocrysts or could have formed due to reaction of primary magmatic inclusions of olivine with the host anorthite. Pseudosections of fO2-nH2O-T-P in the CaO–FeO–MgO–Al2O3–SiO2–H2O (CFMASH) system indicate that fO2 and H2O strongly influence the formation of corundum + amphibole from the initial magmatic assemblage of anorthite (phenocrysts) + spinel ± olivine (inclusions). (2) The corundum with anorthite presumably formed through desilification and decalcification of anorthite, as is indicated by computed phase relations in isobaric-isothermal chemical potential diagrams (µSiO2-µCaO) in parts of the CASH system. Growth of corundum in this mode is augmented by high activity of anorthite in plagioclase, high pressure, and low-to-medium temperature of metamorphism. This study thus presents a new viable mechanism for the origin of corundum in anorthositic amphibolites, and basic-ultra-basic rocks in general, which should provide new insight into lower crustal processes like high-pressure metamorphism.

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Dachs, Edgar, Stan Roozen, and Artur Benisek. "A new activity model for Fe–Mg–Al biotites: II—Applications in the K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH) system." Contributions to Mineralogy and Petrology 176, no. 3 (March 2021). http://dx.doi.org/10.1007/s00410-020-01771-4.

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AbstractThe new biotite activity model and standard-state thermodynamic properties of Ann, Phl, and Eas presented in part-I were used to make pseudosections of bulk compositions representing experimental Fe–Mg exchange equilibria and (model) pelitic bulk rock compositions in the system K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH), using mainly the software Perple_X. These pseudosection calculations (termed ‘our calculation(s)’ in the following) were compared to analogous ones performed with the solution model of biotite and thermodynamic data cited in White et al. (J Metamorph Geol 32:261–286, 2014, 10.1111/jmg.12071), termed ‘W14 calculation’. Our calculations with the experimental bulk composition used by Zhou (Ti–Mg–Fe biotites: formation, substitution, and thermodynamic properties at 650 to 900 °C and 1.1 Kb with fO2 defined by the CH4–graphite buffer. PhD thesis, State University of New York, 1994) in his experimental study of the Fe–Mg exchange between biotite (Bt) and olivine (Ol) confirm that biotite had no or only minimal octahedral Al (AlVI) in these experiments. The experimental data of Ferry and Spear (—FS78, Contrib Mineral Petrol 66:113–117, 1978, 10.1007/BF00372150) on the Fe–Mg distribution between biotite and garnet (Grt) are well reproduced by our calculations. The computed composition of biotite (XFe) in equilibrium with garnet of Alm90Py10 composition and the resulting lnKD values as a function of temperature are in good agreement with the experimental brackets. An analogous W14 calculation on the same Fe-rich bulk composition predicts too high XFeBt in order of 0.1 mol fraction. The AlVI contents of biotite of about 0.3–0.45 apfu, as measured by Gessman et al. (Am Mineral 82:1225–1240, 1997, 10.2138/am-1997-11-1218) in similar biotite–garnet exchange experiments performed with Alm80Py20 and Alm70Py30 garnets, are well reproduced by our, as well as by W14 calculations. The extent of Tschermak substitution in biotite in the FS78 experiments, which had Fe-richer bulk compositions, has not been measured. Comparing the FS78 biotites with the ones from Gessman et al. (Am Mineral 82:1225–1240, 1997, 10.2138/am-1997-11-1218), it is very likely that the biotites reported in FS78 contained AlVI in the same order of ca. 0.3–0.4 apfu. A T–XFe (= molar FeO/(FeO + MgO) pseudosection demonstrates the bulk composition dependence of lnKD of the Mg/FeGrt/Bt exchange reaction in high-variance fields. Further comparisons, demonstrating the application of the new biotite solution model in the KFMASH system, are presented in pseudosections constructed for an average model pelite, as well as for a natural high-T/low-P and a natural high-P metapelite. The pseudosections show that biotite according to our biotite model breaks down at lower temperatures and pressures than predicted from the W14 biotite model in the KFMASH system. This means that KFMASH biotite can break down before the wet solidus is reached, which can explain the existence of dry high-T/low-P metapelites. At higher pressures, biotite according to our calculations breaks down at lower pressures than computed with the W14 biotite model. Before biotite breaks down, however, its AlVI content based on our calculations could potentially be used for pseudosection barometry, similarly as the Si-in-phengite barometer. These trends need to be confirmed by a future extension of our model which incorporates Ti, Fe3+ and a di–tri-octahedral substitution.

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Journal articles on the topic 'P–T pseudosection'

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