Journal of Petrology最新文献

{"title":"The effects of partial melting and metasomatism on peridotite water contents: insights from Shuangliao Volcano Group, Northeast China","authors":"Chang-Yu Zhu, Huan Chen, Yan-Tao Hao","doi":"10.1093/petrology/egae003","DOIUrl":"https://doi.org/10.1093/petrology/egae003","url":null,"abstract":"Many minerals within the subcontinental lithospheric mantle (SCLM) contain structurally bonded hydrogen (commonly referred to as “water”), which significantly impacts their physical properties and associated geodynamic processes. Observations from different localities worldwide make understanding the behavior of hydrogen during partial melting and mantle metasomatism a contentious issue, as different localities reveal either melting or metasomatism as the controlling factor. To provide new insights, major elements, trace elements and water contents of peridotite xenoliths from three volcanoes of the Shuangliao Volcano Group in Northeast China were analyzed. Minerals display variations in major and trace elements, particularly in clinopyroxene. Most olivine contain no observable water, while orthopyroxene and clinopyroxene retained 14-157 μg/g and 46-351 μg/g of water, respectively. Samples were divided into 3 types according to trace element patterns, namely, type 1: Light Rare Earth Elements (LREE)-depleted samples; type 2: “spoon-shaped” samples featuring slight enrichment of the most incompatible elements (La and Ce) and relative depletion of Mid-REE; and type 3: LREE-enriched samples. Combined with major element trends, the Shuangliao SCLM experienced varying degrees of partial melting and cryptic metasomatism. Different water contents in the Shuangliao SCLM are a combined result of melting and metasomatism: less metasomatized samples (LREE-depleted and “spoon-shaped”) preserved the control of water contents by partial melting, while strongly metasomatized samples (LREE-enriched), equilibrated at higher oxygen fugacity and temperature, display considerable post-melting modifications of water contents, possibly associated with Fe redox. These characteristics suggest that hydrous and oxidized melts/fluids likely released by the stagnant Pacific slab in the Big Mantle Wedge (BMW) have metasomatized the shallow SCLM beneath Shuangliao, which indicates the circulation of materials released by the stagnant slab throughout the upper mantle.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"99 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-01-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139482666","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The elusive Congo craton margin during Gondwana breakup: Insights from lithospheric mantle structure and heat-flow beneath the Xaudum kimberlite province, NW Botswana","authors":"Azhar M Shaikh, Sebastian Tappe, Fanus Viljoen, Mike C J de Wit","doi":"10.1093/petrology/egae002","DOIUrl":"https://doi.org/10.1093/petrology/egae002","url":null,"abstract":"The continental lithospheric mantle (CLM) beneath the southern margin of the Congo craton has remained elusive mainly because of thick Phanerozoic sedimentary cover concealing possible kimberlite and lamproite diatremes. In this study, we explore this lithospheric mantle section by using major and trace element compositions of mantle-derived clinopyroxene and garnet xenocrysts from kimberlites of the ca. 84 Ma Nxau Nxau cluster in NW Botswana, which is part of the poorly known Xaudum kimberlite province extending into northern Namibia. We utilize these data to better understand the thermal and compositional evolution of the lithospheric mantle at the southern margin of the Congo craton. The clinopyroxene population (83 individual grains) comprises Cr-rich and Cr-poor diopsides with variable major (Al2O3, Na2O, Mg#) and incompatible trace element (U, Th, Zr, Hf, Nb, Ta, REEs) compositions. The large garnet population studied (496 individual grains) is dominated by lherzolitic G9 (38%) and `megacrystic´ G1 (41%) compositions, with minor contributions from Ti-metasomatized G11 (7%) and eclogitic G3 (6%) cratonic mantle sources. Harzburgitic G10 garnet is very rare (two grains only), consistent with a lherzolite-dominated CLM section in a craton margin position. The eclogitic garnet population has compositions akin to garnet from high-Mg cratonic mantle eclogite xenoliths, and such compositions have recently been interpreted as metasomatic in origin within the mantle xenoliths literature. Pressure–temperature calculations using the single-grain clinopyroxene technique reveal a relatively cold cratonic geotherm of 37-38 mW/m2 for the study region during the Late Mesozoic. For peridotitic garnets, projections of calculated Ni-in-garnet temperatures onto the independently constrained regional conductive geotherm suggest that lherzolite dominates at <145 km depths, whereas high-Ti lherzolitic G11 garnets and `megacrystic´ G1 garnets originate mostly from greater depths, down to the lithosphere base at 150 to 210 km depth. The apparent confinement of ´megacrystic´ G1 garnet to the bottom of the lithosphere suggests formation from infiltrating asthenosphere-derived proto-kimberlite liquids during melt–rock interactions. In general, the data suggest that the CLM beneath NW Botswana is depleted to about 145 km depth, and between 145-210 km depths a thick metasomatized layer is identified, representing the transition into the underlying asthenosphere. A relatively thin lithosphere beneath NW Botswana is consistent with the proposed craton margin setting, especially when compared to the thicker cratonic roots beneath the central regions of the Congo and Kalahari cratons in Angola and South Africa, respectively, reaching down to 250 km depth and possibly even deeper. The compositional dissimilarity between the deepest-derived garnets from kimberlites in NW Botswana (i.e., from the diamond stability field) and garnets that occur as inclusions in diamond f","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"25 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-01-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139482534","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Are microstructures in plutonic rocks primary or secondary?: a re-examination of the metasomatism hypothesis for the roof-sourced autoliths in the Skaergaard intrusion","authors":"Marian B Holness, Jens C Ø Andersen, Olivier Namur, Troels F D Nielsen","doi":"10.1093/petrology/egae001","DOIUrl":"https://doi.org/10.1093/petrology/egae001","url":null,"abstract":"The roof-derived autoliths in the floor cumulates of the Skaergaard Intrusion have been argued to have been extensively metasomatized and recrystallised, forming the foundation of the hypothesis that microstructures in plutonic rocks are essentially metamorphic. However, the augite-plagioclase-plagioclase dihedral angles and plagioclase core composition of the autoliths match with those of the roof rocks, demonstrating that they were generally solid on arrival at the floor, with no subsequent microstructural or compositional modification. Many autoliths have mafic rinds, which were used as evidence of metasomatism: these rinds fall into two groups. The rarely developed rind rock of Irvine et al. (1998) is most likely chilled magma infiltrating along fractures in the roof rocks, either associated directly with detachment of roof material, or occurring before final detachment. Thin mafic rims are widespread in LZc and MZ, present at the tops of the more elongate autoliths, with a corresponding felsic rim at the base of the most elongate. The close correspondence of thin rim development with autolith shape, rather than composition, is argued to be evidence that they formed as a result of differential migration of immiscible conjugate interstitial liquids: the dense Fe-rich liquid flowed downwards and ponded on the tops of impermeable autoliths, whereas its buoyant Si-rich conjugate flowed upwards and was trapped underneath. Any differences in microstructure and bulk composition of the autoliths compared to the remaining exposures of the roof sequence reflect the wider range of lithologies in the now-eroded regions of the roof.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"21 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-01-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139482669","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"An amphibole perspective on the recent magmatic evolution of Mount St. Helens","authors":"Franziska Keller, Maren Wanke, Nico Kueter, Marcel Guillong, Olivier Bachmann","doi":"10.1093/petrology/egad093","DOIUrl":"https://doi.org/10.1093/petrology/egad093","url":null,"abstract":"Compositional variations of amphibole stratigraphically recovered from multiple eruptions at a given volcano have a great potential to archive long-term magmatic processes in its crustal plumbing system. Calcic amphibole is a ubiquitous yet chemically and texturally diverse mineral at Mount St. Helens (MSH), where it occurs in dacites and in co-magmatic enclaves throughout the Spirit Lake stage (last ~4000 years of eruptive history). It forms three populations with distinct geochemical trends in key major and trace elements, which are subdivided into a high-Al (11–14.5 wt% Al2O3), a medium-Al (10–12.5 wt% Al2O3), and a low-Al (7.5–10 wt% Al2O3) amphibole population. The oldest investigated tephra record (Smith Creek period, 3900–3300 years B.P.) yields a bimodal amphibole distribution in which lower-crustal, high-Al amphibole cores (crystallized dominantly from basaltic andesite to andesite melts) and upper-crustal, low-Al amphibole rims (crystallized from rhyolitic melt) document occasional recharge of a shallow silicic mush by a more mafic melt from a lower-crustal reservoir. The sudden appearance of medium-Al amphiboles enriched in incompatible trace elements in eruptive periods younger than 2900 years B.P. is associated with a change in reservoir conditions towards hotter and drier magmas, which indicates recharge of the shallow silicic reservoir by basaltic melt enriched in incompatible elements. Deep-crystallizing, high-Al amphibole, however, appears mostly unaffected by such incompatible-element-enriched basaltic recharge, suggesting that these basalts bypass the lower crustal reservoir. This could be the result of the eastward offset position of the lower crustal reservoir relative to the upper crustal storage zone underneath the MSH edifice. Amphibole has proven to be a sensitive geochemical archive for uncovering storage conditions of magmas at Mount St. Helens. In agreement with geophysical observations, storage and differentiation have occurred in two main zones: an upper crustal and lower crustal reservoir (the lower one being chemically less evolved). The upper crustal silicic reservoir, offset to the west of the lower crustal reservoir, has captured compositionally unusual mafic recharge (drier, hotter, and enriched in incompatible trace elements in comparison to the typical parental magmas in the region), resulting in an increased chemical diversity of amphiboles and their carrier intermediate magmas, in the last ~3000 years of Mount St. Helens’s volcanic record.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"48 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-01-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139412699","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Oligocene high-MgO alkali basalts in central Tibet: implications for magma–mush mixing and mantle processes","authors":"Yue Qi, Qiang Wang, Gang-Jian Wei, Xiu-Zheng Zhang, Wei Dan, Zong-Yong Yang, Lu-Lu Hao, Wan-Long Hu","doi":"10.1093/petrology/egad091","DOIUrl":"https://doi.org/10.1093/petrology/egad091","url":null,"abstract":"High-MgO (> 9 wt.%) basaltic rocks can be primary magmas and used to constrain the geochemistry and temperature of the mantle. However, high MgO contents can also result from mixing between evolved melts and antecrysts or xenocrysts, and thus the whole-rock composition might not represent the solidified equivalents of primary magma. Whether such mixing with crystals can result in erroneous interpretations of mantle processes remains unclear. This study presents a petrological and geochemical investigation of the post-collision high-MgO (> 9 wt.%) Lugu volcanic rocks in the southern Qiangtang terrane, central Tibet. The Lugu volcanic rocks comprise porphyritic and intersertal alkali basalts. Zircon U–Pb ages and 40Ar/39Ar dating suggest that the two types of alkali basalts were erupted at ca. 29 Ma. Based on detailed petrographic observations and geochemical analysis, the porphyritic alkali basalts may represent near-primary melts, which are characterised by low SiO2 contents (40.9–45.1 wt.%), high CaO/Al2O3 ratios (1.1–1.5), and arc-like trace element patterns. We suggest these basalts were derived by partial melting of enriched garnet peridotite (> 3GPa) in the presence of H2O and CO2. These geochemical features are different from those of the ca. 30 Ma (ultra)-potassic rocks in the Qiangtang terrane, indicating that heterogeneous lithospheric mantle existed beneath the Qiangtang terrane during the Oligocene. In contrast, although the intersertal alkali basalts have high MgO contents (> 9 wt.%), evidence from mineral chemistry indicates that whole-rock compositions of the intersertal alkali basalts represent mixtures of evolved residual melts and cumulate crystals. They were the product of polybaric fractional crystallisation and subsequent mixing of crystals and residual melts in a magmatic plumbing system. Furthermore, when intersertal alkali basalts are assumed to be primary melts, they would have been derived by partial melting of shallow (~2.5 GPa) CO2-poor pyroxenite or peridotite. These conditions are different from interpretations of the nature of mantle source and melting conditions for porphyritic alkali basalts. Our results highlight that the interpretation of petrogenetic processes should be preceded by detailed mineralogical investigations.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"10 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2023-12-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138692375","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Connecting the dots: the lava domes’ perspective of magmatism related to an ignimbrite flare-up","authors":"L Smithies Sarah, M Gravley Darren, A R Gualda Guilherme","doi":"10.1093/petrology/egad090","DOIUrl":"https://doi.org/10.1093/petrology/egad090","url":null,"abstract":"Dome eruptions associated with rhyolitic calderas offer an important insight into how extremely large (>>10 km3), rhyolitic magma systems are constructed through time. We focus on rhyolitic calderas in the central Taupō Volcanic Zone leading to, during, and immediately following the 350 – 240 ka ignimbrite flare-up. We identified 103 dome eruptions that are dated between ca. 650 and 150 ka and collated 239 literature whole-rock compositions from these domes. For each composition, we modelled the pressure of magma extraction from the magma mush and the mineral assemblage of the mush using the rhyolite-MELTS geobarometer. We calculated extraction temperatures using zircon saturation geothermometry. We show that magmas are extracted from typically quartz-bearing magma mush at a wide range of depths (~50 – 425 MPa, ~2 – 16 km) and temperatures (~750 – ~850 °C). Throughout the central TVZ, there are two dominant extraction pressure modes at 1) 150 – 175 MPa and 2) 250 – 325 MPa, consistent with 1) the depth of the brittle-ductile transition (~6 km) and just below typical pre-eruptive storage depths of other TVZ magmas (100 – 150 MPa, ~4 – 6 km); and 2) partial melt regions imaged below ~8 km by previous geophysical studies. In some regions, there is a clear correlation between crustal structures, the depth of magma extraction, and the composition of the magmas. In the Whakamaru caldera, the domes erupted inside the caldera following caldera collapse are extracted from ~225 to ~350 MPa at ~810 °C and have orthopyroxene-bearing compositions dissimilar to the caldera-forming eruption. These domes are aligned along normal faults, suggesting that rifting creates pathways for magma extraction from a deeper mush rejuvenated by recharge. The domes erupted along the structural margins of the Whakamaru caldera have very evolved, hornblende-bearing compositions, similar to the caldera-forming eruption and shallow, colder extraction from ~100 – ~200 MPa at ~770 °C, suggesting the mush feeding these domes is a remnant of the older caldera-forming magma system mobilised along the caldera-bounding faults. Two structural levels of magma extraction at ~6 km and 9 – 12 km are persistent throughout the flare-up period and across the central TVZ region, demonstrating the need for further investigation into the factors controlling the depth of mush development.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"28 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2023-12-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138692193","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Plutonic nature of a transcrustal magmatic system: evidence from ultrahigh resolution Sr-disequilibria in plagioclase microantecrysts from the southern Taupo Volcanic Zone, New Zealand","authors":"Daniel A Coulthard Jr, Raimundo Brahm, Charline Lormand, Georg F Zellmer, Naoya Sakamoto, Yoshiyuki Iizuka, Hisayoshi Yurimoto","doi":"10.1093/petrology/egad087","DOIUrl":"https://doi.org/10.1093/petrology/egad087","url":null,"abstract":"The residence timescales of antecrystic minerals contribute a key piece of information regarding the petrologic evolution of transcrustal magmatic systems and may be inferred using a combination of observations derived from microanalytical chemistry and diffusion modelling. Here, we present state-of-the-art stacked CMOS-type active pixel sensor (SCAPS) isotopographic images of tephra-hosted plagioclase microantecrysts from Tongariro Volcanic Centre in the southern Taupo Volcanic Zone, New Zealand. These crystals exhibit high-frequency Sr and anorthite zonation at sub-micron spatial resolution. We also find that all crystals display high-frequency intracrystalline Sr chemical potential variations, indicating that they have not resided at magmatic temperature for diffusive relaxation to advance significantly. To quantify crystal residence times at the well-constrained magmatic temperatures of these tephras, we first forward-modeled intracrystalline Sr diffusion over time using numerical methods. Results were then analyzed using novel spatial Fourier-transform techniques developed to understand the systematics the diffusive decay of Sr disequilibria in the spatial frequency domain. This ultimately permitted the estimation of Sr concentration profiles at crystal formation, prior to uptake into the carrier melt at the onset of eruption. Our data imply residence times of days to weeks for the studied microantecrysts. This is inconsistent with long antecryst residence times in magmatic mushes at elevated temperatures, pointing instead to a cool plutonic nature of the magmatic plumbing system beneath the southern Taupo Volcanic Zone.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"27 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2023-12-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138692374","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Plumbing system architecture and differentiation processes of the Nyiragongo volcano, DR Congo","authors":"Sander M Molendijk, Olivier Namur, Ephrem Kamate Kaleghetso, Paul R D Mason, Benoît Smets, Jacqueline Vander Auwera, David A Neave","doi":"10.1093/petrology/egad088","DOIUrl":"https://doi.org/10.1093/petrology/egad088","url":null,"abstract":"The Nyiragongo volcano is one of the most alkali-rich volcanic centres on the planet (Na2O + K2O generally >10 wt.%, agpaitic index up to 1.34), characterized by a semi-permanently active lava lake which hosts silica-undersaturated (SiO2 <40 wt.%), low viscosity lavas. To improve our understanding of this unique magmatic system, we present a set of 291 samples, acquired during new field excursions between 2017 and 2021. The major and trace element composition of all samples was measured, revealing a lithological range extending from primitive picrites (Mg# 82) erupted from parasitic cones to a variety of highly evolved nephelinites, leucitites, and melilitites erupted from the main edifice as recently as 2002, 2016, and 2021. We measured major and trace element compositions from the full spectrum of minerals present in all sampled lithologies. From these we calculated that the main magma reservoirs feeding Nyiragongo are at approximately 9 – 15 and 21 – 33 km depth, in agreement with recent seismic observations. Fractional crystallization modelling using observed mineral compositions and proportions was performed to quantitatively link the lithologies to specific residual liquid fractions assuming evolution from an olivine-melilite parental melt. Our modelling indicates that fractionation cumulate formation in deep chambers reduces the melt fraction remaining to ~60%, after which melts are injected into upper, liquid dominated magma chambers where fractionation and accumulation of clinopyroxene, melilite, and feldspathoids dominate. Characterisation of mineral textures and geochemistry reveals high crystal mobility in a repeatedly recharging plumbing system split between liquid-dominated, evolved magma chambers and more solid-dominated, primitive mushes, decreasing in liquid fraction with depth.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"165 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2023-12-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138555136","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Correction to: A Comparison of Oxygen Fugacities of Strongly Peraluminous Granites across the Archean-Proterozoic Boundary and Strongly Peraluminous Granites across the Archean-Proterozoic Transition","authors":"","doi":"10.1093/petrology/egad089","DOIUrl":"https://doi.org/10.1093/petrology/egad089","url":null,"abstract":"","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"87 ","pages":""},"PeriodicalIF":3.9,"publicationDate":"2023-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139016638","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Competing roles of evolving P-T conditions, equilibration volume and chemical potential landscape in the formation of corona texture: a case study from the Southern Granulite Terrane, India","authors":"Anindita Dey, Sirina Roy Choudhury, Pulak Sengupta","doi":"10.1093/petrology/egad083","DOIUrl":"https://doi.org/10.1093/petrology/egad083","url":null,"abstract":"Corona texture is defined by the development of partial or complete rim of one or more minerals around a central reactant mineral depicting limited mass transport (local equilibration) in the length scale of the coronitic layers. The mineral assemblages forming corona texture in a metamorphic rock are commonly used to trace the P-T-X conditions through which the rock evolved during various tectonic processes. However, without a proper assessment of the changes in the equilibration volume (EV) and its effect on the mineralogy, any petrological interpretation deduced from the coronitic texture may be incorrect. In this study, we demonstrate that the double corona texture, observed in a suite of Mg-Al rich ortho-amphibole cordierite-bearing rock from the Cauvery Shear Systemin (Southern Granulite Terrane, India), developed in response to the continuously evolving EV. The studied rock contains aluminosilicate porphyroblasts that are set in a matrix of ortho-amphibole ± quartz. The aluminosilicate porphyroblasts are rimmed successively by an inner symplectic corona of sapphirine + cordierite, and an outer mono-mineralic corona of cordierite (near ortho-amphibole). Locally, patches of corundum with a rind of cordierite grow preferentially along the interface of aluminosilicate and the inner symplectic corona. Based on detailed petrography and mineral composition analyses, the corona textures are interpreted to have formed through a sequence of different chemical reactions that occurred in local micro-domains. We calculated quantitative P-T pseudosection in a NCFMASHT (Na2O-CaO-FeO-MgO-Al2O3-SiO2- H2O-TiO2) system and activity-adjusted P-T petrogenetic grid in a MASH (MgO-Al2O3-SiO2- H2O) system which, together, suggest that the coronitic assemblages were formed in response to a steeply decompressive retrograde P-T path from >8.8 kbar to <6 kbar, at a nearly constant temperature of ~700°C. Changes in EV in response to the limited transport of chemical components during the formation of corona texture were investigated through isothermal P-μMgO, P-μSiO2, and P-μMgO-μSiO2 MASH diagrams. Our results quantitatively model the continuously changing chemical potential landscape (P-μMgO- μSiO2 evolution path) around the central aluminosilicate porphyroblast within the corona-bearing micro-domain. The path demonstrates that a gradually shrinking EV around the central aluminosilicate during retrogression led to the sequential change of mineral reactions and equilibrium mineral assemblages, and resulted in the formation of multiple coronae. Unavailability of fluids and/or rapid exhumation is considered as the most dominant factors responsible for the decreasing elemental mobility and the consequent shrinking in EV in the studied rock.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"25 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2023-11-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138516843","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}