Metamorphic Temperature Conditions Research Articles

Antiperthite and Mesoperthite Exsolution Textures in the Zhengjiapo BIF, Changyi Metallogenic Belt, North China Craton: Evidence of UHT Metamorphic Overprint

Paleoproterozoic banded iron formation (BIF) iron ore of the Zhengjiapo region of the Changyi metallogenic belt, Eastern Block of North China Craton contains abundant coexisting antiperthite and mesoperthite textures. The antiperthite and mesoperthite occur in felsic domains of the Zhengjiapo BIF ore and enable derivation of peak temperature metamorphic conditions. Thermodynamic phase modeling shows that equilibrium conditions of corresponding textures, considering the related mineral assemblage of Pl + Qz + Kfs + Mag + Opx + L, are in the range of 870–940 °C and 5.0–6.8 kbar. Ternary feldspar thermometry using reintegrated compositions of antiperthite and mesoperthite in the felsic domain of the studied BIF iron ore reveals even higher peak metamorphic temperatures of 1045–1080 °C. The ultra-high temperature–low pressure conditions of Precambrian BIF have not yet been reported from the North China Craton. The documented ultra-high temperature metamorphism of the Zhengjiapo BIF iron ore in the Changyi metallogenic belt indicates that the BIF was involved in the collision-related tectonic process during Paleoproterozoic to have occurred in the Jiao-Liao-Ji orogenic belt.

Metamorphic P–T conditions, in‐situ LA–ICP–MS biotite–muscovite Rb–Sr geochronology and tectonic implications of two‐mica–quartz schist from the Kuanping Group in the North Qinling Orogenic Belt

The Kuanping Group is an important lithotectonic unit in the North Qinling Tectonic Belt, which has experienced metamorphism from greenschist to high amphibolite facies and is of great significance in defining the orogenic process of the Qinling Orogenic Belt. This study presents the results of petrology, geothermobarometry, phase equilibrium modelling, in‐situ laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) biotite–muscovite Rb–Sr analysis, for the two‐mica–quartz schist of the Kuanping Group in the Hongtuling and Heihe areas. These results are employed to better understand their metamorphic P–T conditions and ages, as well as geological implications. Garnet‐bearing two‐mica–quartz schist in the Hongtuling area records a clockwise P–T trajectory on the basis of phase equilibrium modelling, with peak P–T conditions of ~600°C and ~0.85 GPa. In addition, the Ti‐in‐biotite thermometry yielded metamorphic temperature conditions of ~578°C–580°C and ~590°C–596°C for the garnet‐bearing two‐mica–quartz schist from the Hongtuling area and the two‐mica–quartz schist from the Heihe area, respectively. These results indicate that metamorphic temperature conditions in both areas are higher than the closure temperature of the Rb–Sr isotopic system of muscovite and biotite. In‐situ LA–ICP–MS biotite and muscovite Rb–Sr analyses yielded three groups of isochron age, i.e., ca. 410 Ma, ca. 360 Ma and ca. 320–300 Ma. On the basis of previously published data and the data in this study, the Kuanping Group underwent ca. 440 Ma regional metamorphism in the NQB, retrograde cooling at ca. 410–360 Ma due to exhumation and ca. 320–300 Ma tectonothermal overprinting.

The Moglio-Testico Unit (Ligurian Alps, Italy) as Subducted Metamorphic Oceanic Fragment: Stratigraphic, Structural and Metamorphic Constraints

Along the Western Alps, the oceanic units showing blueschists to eclogite facies metamorphic imprint are classically regarded as fragments of the Ligurian-Piedmont Ocean. These units recorded a strongly deformation related to their subduction, accretion and subsequent exhumation into the Alpine wedge, developed during the convergence between the Europa and Adria Plates. However, some of these units, for example the Moglio-Testico Unit, are less pervasively deformed, providing evidence of their sedimentary evolution as well as the tectono-metamorphic history. Therefore, we present original stratigraphic, structural and thermo-barometric data to characterize the tectono-metamorphic history and the sedimentary evolution of the Moglio-Testico Unit, performing different techniques including fieldwork, structural analysis and chlorite-phengite multiequilibrium thermobarometry. Our dataset indicates that the Moglio-Testico Unit can be considered as a fragment of oceanic cover whose sedimentary evolution reflects that of a portion of oceanic lithosphere approaching to the subduction zone. Structural analysis combined with the thermobarometry indicate that this unit recorded a polyphase deformation history developed under High Pressure-Low Temperature metamorphic conditions (D1: 1.2–1.0 GPa and Tpeak: 330–260 °C; D2: 0.4–0.7 GPa and 230–170 °C) during its underthrusting, accretion into the Alpine wedge and subsequent exhumation up to the shallower crustal levels.

Petrology and zircon U–Pb geochronology of pelitic gneisses and granitoids from the Dai Loc Complex in the Truong Son Belt, Vietnam: Implication for the Silurian magmatic-metamorphic event

The Dai Loc Complex, located in the southern Truong Son Belt, is separated from the northern Kontum Massif by the early Paleozoic Tam Ky–Phuoc Son suture zone. This study presents new zircon U–Pb ages and petrographic observations of the migmatitic pelitic gneisses and mylonitized granite intrusions from the Dai Loc Complex in order to constrain the Early Paleozoic thermal history of the eastern Indochina Block. The identification of metamorphic zircon with sillimanite inclusions, euhedral cordierite, and garnet in the migmatitic pelitic gneisses indicates high temperature metamorphic conditions at ca. 430–440 Ma. The metamorphism was followed by an isobaric cooling stage defined by the replacing of cordierite by kyanite–staurolite–quartz symplectite. The Dai Loc granite likely intruded the metapelites at ca. 430 Ma and was subsequently mylonitized. The thermal event in the Dai Loc Complex may be correlated with Ordovician–Silurian low pressure/high temperature metamorphism caused by subduction related arc magmatism in the Kannack and Ngoc Linh Complexes of the Kontum Massif. However, the frequency distribution of detrital zircon ages suggests the metasedimentary rocks from Dai Loc Complex have a unique provenance that was distinct from those of the neighboring A Vuong Formation and South China Block.

Validation of clinopyroxene-garnet magnesium isotope geothermometer to constrain the peak metamorphic temperature in ultrahigh-temperature ultramafic-mafic granulites

Abstract Conventional Fe-Mg exchange geothermometers generally are not effective means to measure the peak metamorphic temperatures of granulites because of Fe-Mg diffusion during the re-equilibration during the exhumation and cooling. Therefore, it is essential to find alternative geothermometers. In this study, we investigated the magnitude of Mg isotope fractionation between the coexisting clinopyroxene and garnet in garnet pyroxenites and high-pressure mafic granulites from southern India. The clinopyroxene and garnet from the garnet pyroxenites have δ26Mg values of −0.04 to −0.07‰, and −0.65 to −0.64‰, respectively, with Δ26MgCpx-Grt = δ26MgCpx − δ26MgGrt = 0.62 and 0.57‰. The δ26Mg values of the coexisting clinopyroxene and garnet in the high-pressure mafic granulites are 0.03 to 0.07‰ and −0.54 to −0.55‰, respectively, with Δ26MgCpx-Grt = 0.57 and 0.62‰. The inter-mineral Mg isotope fractionations between the clinopyroxene and garnet of the garnet pyroxenites are similar to those of the high-pressure mafic granulites and more or less fall within the equilibrium fractionation lines at peak metamorphic temperatures. The measured peak temperatures for all four samples indicate equilibrium Mg isotope fractionations between the clinopyroxene and garnet. The clinopyroxene-garnet Mg isotope geothermometer of Li et al. (2016) yields temperatures of 994 ± 60 °C and 1048 ± 89 °C for the garnet pyroxenites, and 1048 ± 89 °C and 994 ± 65 °C for the high-pressure mafic granulites, which are slightly lower than those at both 10 and 12 kbar from the clinopyroxene-garnet Mg isotope thermometer of Huang et al. (2013). Compared with the peak metamorphic conditions from phase equilibrium modeling, the clinopyroxene-garnet Mg isotope thermometry yielded temperatures corresponding to the peak metamorphic conditions of the garnet pyroxenites and the high-pressure mafic granulites, whereas the conventional clinopyroxene-garnet Fe-Mg exchange thermometry yielded lower temperatures corresponding to the retrograde metamorphism. These results underscore the inter-mineral Fe-Mg exchange between clinopyroxene and garnet during the retrograde cooling of the ultrahigh temperature (>900 °C), which, however, did not disturb their Mg isotope fractionation equilibrium attained during the ultrahigh-temperature metamorphic condition. Therefore, we conclude that clinopyroxene-garnet Mg isotope thermometry, together with phase equilibrium modeling, is a valid tool to constrain the peak metamorphic temperature conditions even for ultramafic-mafic granulites that have undergone ultrahigh-temperature metamorphism.

Progressive Low-Grade Metamorphism Reconstructed from the Raman Spectroscopy of Carbonaceous Material and an EBSD Analysis of Quartz in the Sanbagawa Metamorphic Event, Central Japan

Low-grade metamorphic temperature conditions associated with the Sanbagawa metamorphic event were estimated by the Raman spectroscopy of carbonaceous material (RSCM) in pelitic rocks and an electron backscatter diffraction (EBSD) analysis of the quartz in siliceous rocks. Analytical samples were collected from the Sanbagawa metamorphic complex, the Mikabu greenstones, and the Chichibu accretionary complex in the eastern Kanto Mountains, central Japan. Previously, low-grade Sanbagawa metamorphism was only broadly recognized as pumpellyite–actinolite facies assigned to the chlorite zone. The RSCM results indicate metamorphic temperatures of 358 °C and 368 °C for the chlorite zone and 387 °C for the garnet zone of the Sanbagawa metamorphic complex, 315 °C for the Mikabu greenstones, and 234–266 °C for the Chichibu accretionary complex. From the EBSD analyses, the diameter of the quartz grains calculated by the root mean square (RMS) approximation ranges from 55.9 to 69.0 μm for the Sanbagawa metamorphic complex, 9.5 to 23.5 μm for the Mikabu greenstones, and 2.9 to 7.3 μm for the Chichibu accretionary complex. The opening angles of the c-axis fabric approximate 40–50°, presenting temperatures of 324–393 °C for the Sanbagawa metamorphic complex and the Mikabu greenstones. The temperature conditions show a continuous increase with no apparent gaps from these low-grade metamorphosed rocks. In addition, there exists an empirical exponential relationship between the estimated metamorphic temperatures and the RMS values of the quartz grains. In this study, integrated analyses of multiple rock types provided valuable information on progressive low-grade metamorphism and a similar approach may be applied to study other metamorphic complexes.

Chapter 3 Archean (>2.6 Ga) and Paleoproterozoic (2.5–1.8 Ga), pre- and syn-orogenic magmatism, sedimentation and mineralization in the Norrbotten and Överkalix lithotectonic units, Svecokarelian orogen

Abstract Two lithotectonic units (the Norrbotten and Överkalix units) occur inside the Paleoproterozoic (2.0–1.8 Ga) Svecokarelian orogen in northernmost Sweden. Archean (2.8–2.6 Ga and possibly older) basement, affected by a relict Neoarchean tectonometamorphic event, and early Paleoproterozoic (2.5–2.0 Ga) cover rocks constitute the pre-orogenic components in the orogen that are unique in Sweden. Siliciclastic sedimentary rocks, predominantly felsic volcanic rocks, and both spatially and temporally linked intrusive rock suites, deposited and emplaced at 1.9–1.8 Ga, form the syn-orogenic component. These magmatic suites evolved from magnesian and calc-alkaline to alkali–calcic compositions to ferroan and alkali–calcic varieties in a subduction-related tectonic setting. Apatite–Fe oxide, including the world's two largest underground Fe ore mines (Kiruna and Malmberget), skarn-related Fe oxide, base metal sulphide, and epigenetic Cu–Au and Au deposits occur in the Norrbotten lithotectonic unit. Low- to medium-pressure and variable temperature metamorphic conditions and polyphase Svecokarelian ductile deformation prevailed. The general northwesterly or north-northeasterly structural grain is controlled by ductile shear zones. The Paleotectonic evolution after the Neoarchean involved three stages: (1) intracratonic rifting prior to 2.0 Ga; (2) tectonic juxtaposition of the lithotectonic units during crustal shortening prior to 1.89 Ga; and (3) accretionary tectonic evolution along an active continental margin at 1.9–1.8 Ga.

The Ligurian oceanic successions in southern Italy: The key to decrypting the first orogenic stages of the southern Apennines-Calabria chain system

In this paper, we present a review about stratigraphy, petrology, deformation and metamorphism of the oceanic, sedimentary, magmatic and metamorphic successions exposed from northern Calabria to the Campania region (southern Italy), referred to the paleogeographic Southern Ligurian Domain. These successions experienced different tectonic episodes, with those located in the westernmost sector of the Southern Ligurian Domain firstly subducted, reaching high pressure/low temperature metamorphic conditions (Eocene) and subsequently exhumed and exposed (Tortonian). On the contrary, the deep-basin successions located in the easternmost sector (close to the continental Adria margin) were obducted and frontally accreted (Aquitanian-Burdigalian). The tectonic transport for the obducted successions was dominantly to SE (in the present geographic coordinates), whereas new data about tectonic transport directions and vergences of the subducted ophiolites, exposed in northern Calabria, indicate a mean eastward-directed transport during the tectonic exhumation. The petrochemical comparison between the mafic rocks of the Southern Ligurian Domain successions with the corresponding rocks exposed in the Alps, Corsica and northern Apennines, representing the northern sector of the Ligurian Domain, suggests an ocean-continental transition paleotectonic setting for the entire Ligurian Domain. The early orogenic stages of the southern Apennines-northern Calabria system, defined by the subduction of Ligurian lithosphere, were characterised by a complex kinematic evolution of the subduction system, including the migration of the basal and roof decollements upward, downward and toward the foreland within the subduction channel.

Metamorphic processes preserved in early Archean supracrustal rocks of the Barberton Greenstone Belt, South Africa

AbstractThe Palaeoarchean supracrustal rocks of the Barberton Greenstone Belt (BGB) consist of hydrothermally altered volcanic and silicified sedimentary rocks that form the dramatic topography in the Barberton Mountain Land of Mphumalanga Province in South Africa and Swaziland. Previous metamorphic studies are limited and used mainly empirical chlorite geothermometry in the BGB sequence regionally, or Raman thermometry, both proposing a narrow range in metamorphic conditions of around 320°C regional for the BGB. This study demonstrates that a range of petrological and thermodynamic modelling techniques are required in an integrated approach to unravel the very low- to medium-grade metamorphic conditions preserved in the oldest part of the BGB. The study investigates low-temperature metamorphic processes in the Komati, Hooggenoeg, Kromberg and Mendon formations of the c. 3530–3298 Ma Onverwacht Group. Chlorite thermodynamic modelling indicates metamorphic conditions of between 250 and 445°C, in the c. 3482 Ma Komati Formation, but uncertainties on these conditions are large (c. 80 and 100°C) due to the protoliths being mostly ultramafic in composition with uncertainty in XFe3+ estimates. Raman thermometry on carbonaceous material in the c. 3472 Ma volcano-sedimentary Middle Marker chert (HC1) at the base of the Hooggenoeg Formation indicates metamorphic conditions of between 281 and 301°C, lower than in previous studies. Raman thermometry on a carbonaceous chert (HC5) in the 3472–3432 Ma Hooggenoeg Formation in conjunction with δ18O oxygen isotope values on underlying metabasalts, indicate metamorphic temperature conditions of less than 340°C, preserved in the Hooggenoeg Formation. Application and critical evaluation of chlorite thermodynamic modelling results confirm the presence of an inverted metamorphic field gradient beneath a fuchsite–chlorite–quartz shear zone in the c. 3334 Ma Kromberg Formation. Thermodynamic modelling using a di-octahedral mica model indicates that retrograde metamorphism in the Kromberg ultramafic shear zone occurred along a kyanite-type geothermal gradient, providing evidence in support of tectonic thrusting at c. 3227–3223 Ma. In the c. 3298 Ma Mendon Formation seawater alteration involving metasomatism occurred at conditions of T = 150°C with log aCO2(aq) of −2.74. This study sheds new light on the metamorphic history of the BGB supracrustal sequence and early Earth metamorphic processes. It also demonstrates that combining different petrological and thermodynamic methods across the BGB supracrustal stratigraphy allows a robust critical assessment of metamorphic conditions for the first time, with implications for the early Archean geodynamic evolution of the BGB.

Sequential kinetic modelling: A new tool decodes pulsed tectonic patterns in early hot orogens of Earth

Tectonic styles in an early hot Earth were different from the present-day situation governed by plate tectonics. Processes in such hot settings remain poorly understood because they often occur on timescales that are below the resolution of conventional isotopic clocks, the rock records are fragmentary, and these have been superposed by later high-temperature events. We have developed a tool based on diffusion kinetics to overcome these difficulties and reconstruct sequences of short-lived episodes. Application of the method to a rock from the ultra-hot c.1.6 Ga orogenic domain of the Central Indian Tectonic Zone, where additional data are available to verify the results, shows that pulses of approach and roll-back of colliding plates preceded the final closure and collision. We demonstrate that cooling from ultra-high temperature metamorphic conditions in the orogen took place in multiple pulses that occurred with a periodicity of about 10 Myr at rates that vary between 100's to 10's °C/Myr, and burial-/exhumation-rates that vary between 30 and 2 km/Myr, respectively. Such details of tectonic processes in the Precambrian, with quantification of variable heating-, cooling-, burial-, and exhumation-rates of individual stages, have not been accessible until now. Application of this method to other regions would provide a means of exploring the thermal viability of the inferred long durations (>100 Myr) for some ultra-high temperature orogenies.

How fluids infiltrate through fractures and change metamorphic rocks – a case study from northern Spitsbergen [abstract

Metasomatism is a process leading to changes in the chemical composition of a rock or its portion. It involves introduction or removal of chemical components due to the interaction of the rock with aqueous fluid (Zharikov et al. 2007). Here we present a case study of metasomatic changes in metapelites from northern Spitsbergen. There is a visible metasomatic zonation in the studied samples. The most intense changes are observed close to the contact with fractures, whereas the zones farther away from them are less altered or not affected by fluids at all. Thin sections were prepared from all of the aforementioned zones. The mineral chemistry was obtained using a JEOL SuperProbe 8230 electron microprobe at Faculty of Geology, Geophysics and Environmental Protection, AGH University of Science and Technology. Mineral assemblage varies from Grt-Bt-Qtz-Pl in zones that were not affected by fluids to Grt-Chl-Bt-Qtz-Pl in altered portions of the rock. Garnet forms subhedral crystals with abundant quartz inclusions. Its composition is changing from Alm0.56Sps0.11Prp0.05Grs0.28 in the core to Alm0.64Sps0.03Prp0.09Grs0.24 in the rim. It shows bell-shaped Sps profile and opposite Alm trend, indicating one-step progressive garnet growth. Plagioclase composition is characterized by Ab74.16An24.39Or1.44, K-feldspar also occurs along with Fe-chlorite. Point counting technique was used to estimate volume percentage of minerals. For each thin section one thousand points were counted. Plagioclase, quartz and biotite are the main phases, building more than 95% of the rocks. Amount of garnet oscillates around 2% and chlorite varies from 2.38% to 0%. Point counting technique was compared with image analysis, which was performed using the Matlab software. The image analysis revealed that the amount of biotite is changing in different zones from 35.42% to 37.38%, whereas chlorite from 1.54% to 0%, respectively. The redistribution of elements between biotite and chlorite was investigated by volume and mass-balance using Gresens (1967) approach. During chloritization of biotite amount of H2O in the system must have increased by ca. 8% in order to form 2.4% chlorite. Potassium released during this reaction was bonded to secondary K-feldspar, which is apparent on BSE images, and it is closely associated with biotite and chlorite. This study reveals that metasomatic zones significantly differ in mineralogical composition and fluid induced alterations are more pervasive closer to the fractures. It has also been concluded that replacement of biotite by chlorite required aqueous fluids, with inflow and outflow of metallic ions and formation of K-feldspar as a product of biotite breakdown reaction. Further investigations will involve X-ray mapping using the XMap Tools software in order to obtain more in depth information on volume of minerals and their chemistry. Pressure and temperature conditions of metamorphism will be determined using garnet-biotite geothermometry and thermodynamic modelling. Isocon method will be used to track changes in whole rock chemistry due to interaction with fluid (Grant 1986, 2005). Acknowledgements:The study was financed from the statutory research project No 11.11.140.319 and 11.11.140.613, AGH University of Science and Technology.

Dating thin zircon rims by NanoSIMS: the Fengtien nephrite (Taiwan) is the youngest jade on Earth

Nephrite in the Fengtien area of the eastern part of the Central Mountain Range, Taiwan, is associated with antigorite-serpentinite within the Yuli belt, a late Cenozoic subduction–accretionary complex related to the eastward subduction of the South China Sea plate forming the Luzon arc. Diopsidite and clinozoisite rock are two other metasomatic components accompanying nephrite between serpentinites and the greenschist-facies country rock (carbonaceous material-)quartz-mica schist. Detrital zircons were separated from one clinozoisite rock sample, formed through metasomatic replacement after mica-quartz schist at temperatures of 320–420°C or slightly lower, which is lower than the metamorphic temperature conditions of the Yuli belt. Most of the detrital zircons have thin zircon rims less than 15–20 μm wide. These zircon rims, considered as newly formed during metasomatism leading to nephrite/diopsidite/clinozoisite rock formation, were dated by a high lateral resolution secondary ion mass spectrometer (CAMECA NanoSIMS NS50). The resulting 238U/206Pb-204Pb/206Pb inverse isochron gave an age of 3.3 ± 1.7 Ma. The collision of the Eurasian continental margin with the Luzon arc has been suggested to have begun at ca. 6.5 Ma in the Taiwan area. The nephrite formation processes therefore clearly post-dated South China Sea plate subduction. The present date, substantiated by the metamorphic and metasomatic temperature information, demonstrates that the fluid–rock interaction forming Fengtien nephrite would have taken place during a Barrovian-type metamorphic overprint resulting from arc-continent collision, leading to the exhumation of the Yuli belt. This conclusion on nephrite formation with regard to regional tectonics can serve as a working model for future studies on other nephrite deposits with similar occurrences, mostly embedded within Mesozoic or older subduction-accretionary complexes. The Fengtien nephrite deposit is therefore the youngest one of its kind exposed on Earth’s surface.

The ultrahigh temperature granulites of southern Madagascar in a polymetamorphic context: implications for the amalgamation of the Gondwana supercontinent

We examined the petrological characteristics of the Graphite group and Androyan group in Southern Madagascar, south of the prominent Ranotsara shear zone, and we performed U-Pb SHRIMP dating on zircon and U-Th-total Pb dating on monazite. Widespread high-temperature metamorphism is evidenced by Spl–Qtz assemblages occurring over ca . 75,000 km 2 in the whole Androyan group. The occurrence of symplectites consisting of Crd + Kfs + Qtz + Opx or Crd + Kfs + Q333tz + Bt, which are interpreted as pseudomorphs after osumilite, is restricted to a smaller area of about 250 km 2 . Furthermore, in some pelites Spr + Qtz + Sil or Opx + Sil + Qtz formed the peak-metamorphic assemblage, which broke down to Crd ± Spl. Orthopyroxene in metapelites is aluminous with Al 2 O 3 = 9–10 wt%. Peak-metamorphic conditions of T = 950–1000 °C and P = 8–11 kbar are followed by decompression at high temperatures, as shown by the formation of Crd + Opx 2 (Opx with 6–8 wt% Al 2 O 3 ) symplectites from Grt–Qtz–Opx 1 (8–9 wt% Al 2 O 3 ). The pressure decrease is furthermore constrained by Spr–Crd symplectites in SiO 2 -undersaturated metapelites, and extensive formation of late-stage cordierite in the whole Androyan group. During subsequent cooling, cordierite broke down to form And + Qtz + Carbonate/Chl. Throughout the Androyan group, ages of 560–530 Ma have been obtained from monazite (M 2 metamorphism). Samples which do not contain ultrahigh-temperature assemblages provide evidence for an earlier metamorphic event at 650–600 Ma (M 1 ) in monazite cores. Zircon generally shows both metamorphic ages. Therefore, the deduced clockwise P-T evolution of the UH T metamorphism is interpreted to correspond to the M 2 stage, which affected the whole Androyan group. P-T conditions of the older M 1 metamorphism are generally unrecognisable. High temperature metamorphic conditions during M 2 are likely caused by intense charnockite emplacement. The near-isothermal decompression points to subsequent rapid exhumation of the formerly overthickened crust during the M 2 metamorphism at 560–530 Ma. We interpret this metamorphic stage to reflect the assembly of the Gondwana supercontinent, most likely related to the collision of the Tanzania Craton with the Azania microcontinent subsequent to closure of the Mozambique Ocean.

Deformation mechanisms of plagioclase and seismic anisotropy of the Acebuches metabasites (SW Iberian massif)

Abstract Samples of the Acebuches metabasites (SW Spain), deformed under low-pressure/medium-to-high temperature metamorphic conditions, have been analysed via electron backscattered diffraction (EBSD) to obtain their plagioclase crystal lattice preferred orientations (LPO). Plagioclases from the highest temperature amphibolites show moderate LPO and a good correlation between 180° misorientation angles and both the crystal and the kinematic coordinate systems, which is attributed to dislocation glide accommodated by mechanical albite+pericline twinning. Plagioclases from medium-temperature amphibolites exhibit well-developed LPO, suggesting that dislocation creep was active during plagioclase deformation. Plagioclases from the more intensively deformed mafic schists exhibit weak LPO, indicating the activity of LPO-destroying deformation mechanisms. Evidence points to grain-boundary sliding accompanied by limited fracturing. The observed LPO are characterized by the alignment of [100] parallel to the kinematic X -direction. This association suggests that [100] was the preferential slip direction during dislocation creep of plagioclase, with (010) and/or (001) appearing to have acted as the dominant slip planes. The observed plagioclase LPO is combined with hornblende LPO to define the seismic fabric of the Acebuches metabasites. In samples with strong plagioclase LPO, the resulting seismic fabrics are highly influenced by this phase.

Geochemistry and mineral chemistry of Pan-African amphibolites of South Sinai, Egypt

In the Wadi Feiran area, amphibolites occur as inclusions, bands, linear bodies of variable thickness and irregular lenses in para-geneisses. Chemical evidence indicates that these amphibolites display an igneous origin and were derived from magma essentially of tholelitic rather than alkaline composition; transitional in character between continental and island-arc. The chemistry of amphiboles, related to pressure and temperature conditions of metamorphism, showed that they were formed under low pressure and high temperature conditions.

Magmatic and metamorphic evolution of the Shotur Kuh metamorphic complex (Central Iran)

Metamorphic basement rocks, that are exposed beneath the very low-grade to unmetamorphosed Upper Jurassic-Eocene formations north of the Torud fault zone within the Great Kavir Block, were investigated to elucidate the origin of their protoliths and the pressure and temperature conditions of metamorphism. The basement, previously assumed as a pre-Cambrian metamorphic complex, is mostly formed by amphibolite-facies orthogneisses (tonalite, granodiorite, and granite) with amphibolites and small amounts of metasediment-micaschists. Major- and trace-element geochemistry in combination with U–Pb age dating of zircon showed that the protoliths formed during Late Neoproterozoic continental arc magmatism that has also been identified in other tectonic blocks of Central Iran. In addition to quartz, feldspar(s), micas in orthogneisses, and amphibole + plagioclase in amphibolite, all rocks may contain garnet that shows prograde zoning. Kyanite was found only in some Al-rich amphibolite together with gedrite. The PT conditions of the rocks, based on conventional geothermobarometry and the pseudosection method, show a medium-pressure amphibolite-facies metamorphism. Ar–Ar age dating of muscovite reveals that this metamorphism occurred in the Middle Jurassic (166 Ma) and related to the closure of the Neotethyan basin.

How low can you go? - Extending downwards the limits of plastic deformation in pyrite

AbstractUnderstanding the deformation mechanisms that may operate in pyrite (FeS2) across a range of P-Tconditions is important in deciphering the history of deformed ore deposits. Pyrite has frequently been considered a hard mineral, which deforms by cataclastic flow or diffusive processes, if at all, at temperatures <425°C. However, utilizing SEM-based orientation-contrast (OC) imaging and electron-backscatter diffraction (EBSD) techniques, plastic deformation can now be readily identified within pyrite grains. In this study, a series of pyrite-rich polymetallic ore deposits, deformed at low temperature metamorphic conditions (∼200—420°C), have been investigated. Results indicate that pyrite grains in all the ore deposits preserve internal lattice ‘distortion’ or ‘bending’ and therefore plastic deformation mechanisms have operated. Many pyrite grains in the ore deposits also contain low-angle (∼2°) sub-grain boundaries or ‘dislocation walls', indicating that both dislocation glide and creep have been the dominant deformation mechanisms at peak metamorphic conditions within the pyrite grains. These results suggest that the brittle-ductile transition in pyrite occurs at temperatures potentially as low as ∼200°C, far lower than implied from previous studies or the current pyrite deformation-mechanism map.

Sapphirine+quartz association in garnet: implication for ultrahigh-temperature metamorphism at Rundvågshetta, Lützow-Holm Complex, East Antarctica

Abstract We report the occurrence of sapphirine+quartz association within garnet porphyroblast in the garnet–orthopyroxene–sillimanite granulite (Grt–Opx–Sill granulite) from Rundvågshetta in the Lützow-Holm Complex, East Antarctica. The granulites in the study area show a characteristic mineral assemblage consisting of orthopyroxene+sillimanite+quartz. The presence of sapphirine and quartz inclusions within garnet in the sapphirine-bearing Grt–Opx–Sill granulite suggests that metamorphic conditions changed from the stability field of orthopyroxene+sillimanite+quartz to that of sapphirine+quartz during the garnet growth. Peak metamorphic temperature conditions of about 1000–1100 °C are obtained by ternary feldspar thermometry for these granulites. Similar temperatures were also estimated from the Al-in-orthopyroxene geothermometer. The granulites are also characterized by coarse-grained garnet, being partly surrounded by a fine-grained symplectite composed of orthopyroxene and cordierite, whereas sapphirine+cordierite symplectitic intergrowth occurs in the matrix. These textures imply that the area underwent isothermal decompression subsequent to ultrahigh-temperature metamorphism. The sapphirine-bearing Grt–Opx–Sill granulite is likely to be the restitic product of partial melting and shows signs of segregation and movement of melt.

Fluid characteristics of high- to ultrahigh-temperature metamorphism in southern India: A quantitative Raman spectroscopic study

We report the first quantitative compositional data on fluid inclusions in Late Neoproterozoic to Cambrian ultrahigh-temperature (UHT) granulites from major crustal blocks and shear zones in southern India. Fluid inclusions in various high-grade minerals such as garnet, orthopyroxene, plagioclase, cordierite, and quartz from these rocks were studied in terms of petrography, microthermometry and laser Raman spectroscopy. Measured melting temperatures of the inclusions indicate that the trapped fluid phase is dominantly carbonic. Raman analyses confirmed a CO 2-rich composition with only minor dilutants such as N 2 (≤5.0 mol%), H 2O (≤0.50 mol%), and CH 4 (≤0.47 mol%). The range of most CO 2 isochores computed from density measurements in fluid inclusions from the UHT granulites pass below the peak pressure–temperature conditions of metamorphism indicating considerable resetting of fluid densities during decompression. In contrast, garnetiferous granulites from the Salem area along the Palghat-Cauvery Shear Zone System, which underwent high-pressure prograde metamorphism, contain very high-density syn-metamorphic pure CO 2 inclusions. Such dense carbonic fluid inclusions in garnet and plagioclase coexist with carbonate minerals (calcite or dolomite), indicating probable derivation from deep-seated primary magmatic sources. Although models on the origin of CO 2 are debatable, the ubiquitous association of carbonic fluids in the UHT mineral assemblages is in favour of large-scale infiltration of CO 2-rich fluids during extreme crustal metamorphism associated with collisional orogeny and subsequent extensional collapse at the final stage of amalgamation of the Gondwana supercontinent.

Development of lattice preferred orientation in clinoamphiboles deformed under low-pressure metamorphic conditions. A SEM/EBSD study of metabasites from the Aracena metamorphic belt (SW Spain)

The mechanical properties of the lower continental crust are intimately related to the rheology of calcic clinoamphibole. Naturally deformed clinoamphibole typically shows brittle behaviour under a wide range of deformation conditions, whereas crystal plasticity is rarely observed. In this study, the microfabric of clinoamphiboles from metabasites deformed under low-pressure/medium-to-high temperature metamorphic conditions is presented. Amphiboles from all samples have developed LPO that can be attributed to different deformation mechanisms depending on deformation temperature, fluid content, structure and phase-strength contrasts. Rigid body rotation of amphibole prisms within a weaker plagioclase matrix corresponds to metabasites deformed under medium deformation temperatures that show a weak layer structure and a high phase strength-contrast between plagioclase and clinoamphibole. Lower temperatures and the presence of fluids mean that one of the studied rocks was affected also by dissolution-precipitation creep and cataclastic flow. In the final sample, dislocation creep was accommodated by recovery and subgrain rotation dynamic recrystallization in monominerallic hornblende layers at higher temperatures, which could be representative of the rheology of amphiboles in a lower continental crust at high temperature conditions.