Inclusions In Garnet Research Articles

Парагенезисы включений гранатов в алмазах из кимберлитов Якутии по данным рамановской и ИК-спектроскопии

Парагенезисы включений гранатов в алмазах из кимберлитов Якутии по данным рамановской и ИК-спектроскопии

Bauxite to eclogite: Evidence for late Permian supracontinental subduction at the Red River shear zone, northern Vietnam

We have investigated the geological processes recorded in aluminous granulites from the Red River shear zone in northern Vietnam using mineral and whole-rock chemistries, fluid inclusions, metamorphic pressure–temperature paths, and geochronology. The granulites are extremely rich in Al2O3 (36.3–50.9wt%), TiO2, and total Fe2O3, and poor in SiO2 (7.9–24.1wt%), MgO, CaO, Na2O, and K2O. The granulites are enriched in high-field-strength elements and rare earth elements, and severely depleted in large-ion lithophile elements. These features strongly suggest the protolith was lateritic bauxite. Moreover, the other elemental concentrations and the Zr/Ti ratios point to basaltic rock as the precursor of the bauxite. Some of the aluminous granulites contain high-pressure mineral inclusions of kyanite, staurolite, siderite, and rutile, none of which are observed in the matrix. Abundant primary carbonic fluid inclusions are observed in garnet, corundum, and staurolite, but are rare in quartz and zircon. The average densities of fluid inclusions in garnet, corundum, staurolite, quartz, and zircon are 1.00±0.06, 1.07±0.04, 1.09±0.03, 0.29±0.07, and 1.15±0.05g/cm3, respectively. The mineral features not only in the matrix and but also in garnet from all rock types, isochemical phase diagrams obtained for each bulk rock composition, and Zr-in-rutile thermometry indicate an early eclogite-facies metamorphism (~2.5GPa at 650°C) and a subsequent nearly isothermal decompression. Zircons yield a wide range of U–Pb ages from 265 to 36Ma, whereas the dark luminescent cores of the zircons, which contain high-density CO2 inclusions, yield a concordia age of 257±8Ma. These observations suggest that the dark luminescent zircon cores were formed at the same time as the garnet, corundum, and staurolite that contain high-density CO2 fluid inclusions. Based on the carbonic fluid inclusion isochore and the densities as well as calculated phase diagram, the concordia age can be regarded as recording a prograde stage of metamorphism under conditions lower than 600°C and 0.7GPa. Our new data provide the following geological and tectonic constraints: 1) the eruption of basalt occurred before the Permian, possibly related to subduction of the Paleo-Tethys Plate beneath the Indochina craton near the paleo-equator in the Devonian–Carboniferous; 2) strong weathering transformed the basalt to bauxite before the late Permian; 3) the uppermost continental crust, including the bauxites, was subducted in the late Permian due to the collision of the Indochina and South China cratons, leading to eclogite-facies metamorphism; 4) the rocks were then exhumed; and 5) shearing-related thermal events took place until the Paleogene.

Neoproterozoic reworking of the Ubendian Belt crust: Implication for an orogenic cycle between the Tanzania Craton and Bangweulu Block during the assembly of Gondwana

The Paleoproterozoic pelitic gneisses from the Ubendian Belt host Neoproterozoic eclogites (590–520 Ma) that formed in a subduction zone. The Neoproterozoic subduction event in the Ubendian Belt was followed by a collision between the Tanzania Craton and the Bangweulu Block and tectono-metamorphic reworking of the Ubendian Belt rocks. This study focuses to characterize the P-T-t conditions of the Neoproterozoic collisional event in the Ubendian Belt and its regional geodynamic significance.Despite hosting Neoproterozoic eclogite, the Ubendian Belt pelitic gneisses have zircons that yield concordant metamorphic ages between 1949 ± 16 and 1901 ± 37 Ma (U-Pb zircon, 2σ errors) having neither clear younger metamorphic ages nor Neoproterozoic rim overgrowth. However, monazite crystals from the same rocks yield clear younger rims with ages between 566 ± 8 and 556 ± 5 Ma overgrowing old cores.Monazite inclusions in garnet yields an age of 566 ± 8 Ma, which implies garnet growth during the Neoproterozoic tectonometamorphic event. Garnet and the associated minerals yield high-pressure metamorphic P-T conditions (P = 9–12 kbar; T = 760–820°C) which indicates a low geothermal gradient of 17–22 °C/km. A clockwise P-T-t path (kyanite to sillimanite reactions) with nearly isothermal decompression (growth of symplectites) after the peak of metamorphism was deduced.The subduction of an oceanic basin between the Tanzania Craton and the Bangweulu Block started at about 590 Ma and resulted in a major collisional event between the Tanzania Craton and Bangweulu Block between 570 and 550 Ma. The collision ended at about 500 Ma during the final stages of Gondwana Supercontinent amalgamation. For this collisional event we therefore propose the term Ufipa orogeny.During the collision, the Ubendian Belt gneisses and the margins of the Bangweulu Block formed part of a passive continental margins that subducted to attain high-pressure metamorphic conditions. The absence of zircons with Neoproterozoic metamorphic ages from older reworked crusts, the lack of pre-collisional volcanic arc, and low geothermal gradient support the metacratonitization process of the Ubendian Belt and the Bangweulu Block.

The reaction history of kyanite in high‐P aluminous granulites

AbstractCathodoluminescence (CL) mapping of kyanite in high pressure, aluminous granulites from the central Grenville Province reveals internal structures that are linked to their metamorphic reaction history. In two samples, individual kyanite crystals are shown to be composite porphyroblasts comprising three distinct generations, defined by their CL intensity and Cr (±V, Ti, Fe and Ga) content, and each separated by resorbed interfaces. In contrast, a sub‐aluminous sample contains two types of kyanite, one as resorbed inclusions in garnet and another in the groundmass or replacing garnet. These textural variants of kyanite are interpreted within the framework of phase equilibria modelling. In P–T pseudosections, a first generation of kyanite, which is only present in the most aluminous samples, is potentially linked to staurolite breakdown, and its resorption is consistent with a subsequent increase in pressure. This kyanite represents the earliest remnant of prograde metamorphism identifiable in these rocks. The second generation, present in the porphyroblasts in the same samples and as inclusions in garnet in the sub‐aluminous sample, is interpreted to be the peritectic product of muscovite dehydration melting. Resorption of this kyanite is consistent with subsequent continuous dehydration melting of biotite, which is also inferred based on microstructural considerations. The final generation of kyanite, present as rims on the prograde kyanite porphyroblasts in aluminous samples and as part of the groundmass or replacing garnet in the sub‐aluminous rock, is interpreted to have grown during melt crystallization upon retrogression. The presence of retrograde kyanite implies that the melt crystallized over a wide range of temperatures, and provides an important constraint on the P–T conditions of the metamorphic peak and on the retrograde P–T path. CL mapping is crucial for identifying retrograde kyanite in aluminous samples, as it preferentially overgrows existing kyanite rather than replacing other prograde phases. The scarcity of kyanite in sub‐aluminous rocks allows retrograde kyanite to grow as discrete crystals that can be identified by optical microscopy. This work attests to the potential of unconventional tools such as CL imaging for deciphering the metamorphic history of rocks.

Coupled Lu–Hf and Sm–Nd geochronology on a single eclogitic garnet from the Huwan shear zone, China

We present coupled Lu–Hf and Sm–Nd dates from multiple growth zones within a single large garnet porphyroblast in an eclogite from the Huwan shear zone in the Hong'an orogen, central China. The eclogite sample contains large garnet porphyroblasts up to several centimetres in diameter and a second major population of smaller, millimetre-sized garnet grains. Elemental compositions and mineral inclusions in garnet suggest that the large garnet crystal formed during an early episode of metamorphism, with the abundant small-sized garnets in the matrix growing during a late metamorphic period contemporaneous with the overgrowth of the rim of the large garnet crystal. Ten coupled Lu–Hf and Sm–Nd dates determined from five micro-sawed garnet sections of a ~2cm garnet coherently decrease from core to rim. Lu–Hf dates from each section are consistently older than the corresponding Sm–Nd dates. Petrographic and chemical observations of the eclogite show that three generations of garnet growth occurred, with the latter two generations coincident with growth of the rim of the large garnet, and that of the smaller matrix garnets. Five micro-sawed growth zones were analysed for Lu–Hf and Sm–Nd isotopes. The calculated Lu–Hf dates from the mega garnet record garnet growth initiation at about 397Ma, during prograde to peak metamorphic conditions of the first orogenic episode, lasting about 11Ma. Sm–Nd garnet dates are systematically younger than corresponding Lu–Hf dates, with the Sm–Nd garnet rim date >25Ma younger than the Lu–Hf date. We suggest that the Sm–Nd dates represent a combination of 1) growth and simultaneous diffusion at elevated temperatures during the first metamorphic episode, 2) diffusive age resetting as a result of continued elevated temperatures during residence at mid-crustal depths and/or slow cooling between the two orogenic episodes, and 3) mixing of growth generations. U–Pb dates on three zircon inclusions from the garnet separates range from 386 to 351Ma, possibly suggesting zircon crystallization during exhumation and cooling. This study highlights that coupled microsampling Lu–Hf and Sm–Nd chronology on a single garnet (down to ~cm in diameter) can help decipher the timescales of burial and heating (for Lu–Hf) and cooling from elevated temperatures (for Sm–Nd) from a single orogenic cycle, and that zoned Lu–Hf and Sm–Nd garnet can be utilized as a geo (thermo) -chronometer. Lastly, these Lu–Hf garnet, Sm–Nd garnet and U–Pb zircon dates confirm an early high-pressure metamorphic episode during the Devonian before the final convergence of the South China and North China Blocks in the Triassic.

High‐pressure metamorphic evolution of eclogite and associated metapelite from the Chuacús complex (Guatemala Suture Zone): Constraints from phase equilibria modelling coupled with Lu‐Hf and U‐Pb geochronology

AbstractAs is common in suture zones, widespread high‐pressure rocks in the Caribbean region reached eclogite facies conditions close to ultrahigh‐pressure metamorphism. Besides eclogite lenses, abundant metapelitic rocks in the Chuacús complex (Guatemala Suture Zone) also preserve evidence for high‐pressure metamorphism. A comprehensive petrological and geochronological study was undertaken to constrain the tectonometamorphic evolution of eclogite and associated metapelite from this area in central Guatemala. The integration of field and petrological data allows the reconstruction of a previously unknown segment of the prograde P–T path and shows that these contrasting rock types share a common high‐pressure evolution. An early stage of high‐pressure/low‐temperature metamorphism at 18–20 kbar and 530–580°C is indicated by garnet core compositions as well as the nature and composition of mineral inclusions in garnet, including kyanite–jadeite–paragonite in an eclogite, and chloritoid–paragonite–rutile in a pelitic schist. Peak high‐pressure conditions are constrained at 23–25 kbar and 620–690°C by combining mineral assemblages, isopleth thermobarometry and Zr‐in‐rutile thermometry. A garnet/whole‐rock Lu‐Hf date of 101.8 ± 3.1 Ma in the kyanite‐bearing eclogite indicates the timing of final garnet growth at eclogite facies conditions, while a Lu‐Hf date of 95.5 ± 2.1 Ma in the pelitic schist reflects the average age of garnet growth spanning from an early eclogite facies evolution to a final amphibolite facies stage. Concordant U‐Pb LA‐ICP‐MS zircon data from the pelitic schist, in contrast, yield a mean age of 74.0 ± 0.5 Ma, which is equivalent to a U‐Pb monazite lower‐intercept age of 73.6 ± 2.0 Ma in the same sample, and comparable within errors with a less precise U‐Pb lower‐intercept age of 80 ± 13 Ma obtained in post‐eclogitic titanite from the kyanite‐bearing eclogite. These U‐Pb metamorphic ages are interpreted as dating an amphibolite facies overprint. Protolith U‐Pb zircon ages of 167.1 ± 4.2 Ma and 424.6 ± 5.0 Ma from two eclogite samples reveal that mafic precursors in the Chuacús complex originated in multiple tectonotemporal settings from the Silurian to Jurassic. The integration of petrological and geochronological data suggests that subduction of the continental margin of the North American plate (Chuacús complex) beneath the Greater Antilles arc occurred during an Albian‐Cenomanian pre‐collisional stage, and that a subsequent Campanian collisional stage is probably responsible of the amphibolite facies overprint and late syncollisional exhumation.

Mineral textural evolution and PT-path of relict eclogite-facies rocks in the Paleoproterozoic Nagssugtoqidian Orogen, South-East Greenland

The Nagssugtoqidian Orogen in South-East Greenland is a deeply eroded, Paleoproterozoic collision orogen. It consists of a variety of Archean and Paleoproterozoic rocks, most notably TTG gneiss, a variety of supracrustal rocks and basic dykes. This study aims at providing new insight into the geodynamic processes and subduction depth of this orogen by investigating the metamorphic evolution of garnet pyroxenite, retrogressed eclogite and amphibolite-facies rocks that are exposed within the Kuummiut Terrane of the Nagssugtoqidian Orogen. The garnet-pyroxenite has a dominant mineral assemblage of garnet, orthopyroxene, clinopyroxene and hornblende, while garnet-amphibolite and garnet-kyanite schist are made up of garnet, hornblende, plagioclase and quartz, and garnet, kyanite, biotite and quartz, respectively. Relicts of, and pseudomorphs after, eclogite-facies mineral assemblages are frequently found within basic metavolcanic rocks and Paleoproterozoic discordant basic dykes. In the retrogressed eclogite, the retrograde mineral reactions ceased prior to completion, resulting in the formation of two domains. A clinopyroxene domain consists of diopside-plagioclase symplectites, which are interpreted to have grown at the expense of omphacite. The symplectites are surrounded and partly replaced by hornblende and plagioclase. Omphacite (XJd 25–42) is preserved in a Na-rich sample, where it occurs in the core of large clinopyroxene and as inclusion in garnet and hornblende. In a garnet domain, garnet is variably replaced by an inner corona of plagioclase and an outer corona of amphibole +/− orthopyroxene and clinopyroxene. The degree of retrogression as well as the type of the retrograde assemblage in both domains appears to be dependent on fluid activity. Large garnet grains preserve Ca-rich cores, interpreted as prograde in origin, while Mg-rich garnet rims formed during eclogite-facies metamorphism and later re-equilibration. Pseudosection modelling combined with conventional geothermobarometry reveals a clockwise PT-evolution, involving eclogite-facies conditions of 17–19kbar and 740–810°C, followed by near-isothermal decompression to medium-pressure granulite-facies conditions (13.8–15.4kbar, 760–880°C) and subsequent decompression with minor cooling to high-pressure amphibolite-facies grades (8.8–10.9kbar, 660–840°C). These data show that rocks of the Kuummiut Terrane were exhumed from 70 to about 30km into the mid- and lower crust. The PT-path implies that exhumation initially was rapid and tectonically-controlled.

Conditions of metamorphism of garnet-bearing aluminous gneisses in the Orekhov-Pavlograd zone of the Ukrainian Shield

Garnet-bearing aluminous gneisses in the Vasil’kovka area of the Orekhov-Pavlograd zone of the Ukrainian Shield were studied using the THERIAK-DOMINO, THERMOCALC, and WINTWQ software and mineral geothermobarometry methods. The stability fields of parageneses formed at the initial, peak, and post-peak metamorphic stages have been estimated: T = 500-650 °C and P > 7 kbar, T = 800-850 °C and P - 8 kbar, and T = 600-670 °C and P = 4.0-5.5 kbar, respectively. A “clockwise” PT path has been established. Based on the aluminous gneiss, the age of metamorphism is determined as Paleoproterozoic (2014 ± 11 Ma). Unique regularly oriented acicular goethite microinclusions were described in garnet. In contrast to other known oriented Ti-bearing mineral inclusions in garnet, these goethite inclusions suggest not only extremely high P and T values (necessary for the formation of oriented Ti-bearing inclusions) but also a high oxygen fugacity. The obtained data broaden the concepts of the ways and conditions of formation of regularly oriented mineral phases in garnet.

Petrography and phase equilibria modeling of mid-pressure aluminous gneisses derived from hydrothermally altered protoliths, Grenville Province, Canada

A large portion of the hinterland of the central Grenville Province is characterized by mid-pressure granulite-facies metamorphic rocks. In the Manicouagan – Lac du Milieu region of Quebec, aluminous gneisses derived from hydrothermally altered felsic protoliths provide a record of anatectic processes and of the pressure–temperature (P–T) evolution during orogenesis. Samples collected in areas separated by several tens of kilometres consist of garnet + biotite + quartz + K-feldspar ± plagioclase ± sillimanite, with retrograde cordierite in some, and most display microstructural evidence of partial melting. However, they have a wide range of bulk compositions and textures, and may be grouped into two types: sillimanite-rich rocks, with a high alumina index and sillimanite-poor rocks with low alumina index and in which sillimanite is not part of the peak assemblage. Phase equilibria modeling with THERMOCALC software constrained the P–T field of the peak mineral assemblage(s) at 800–900 °C and 6–11 kbar (1 kbar = 100 MPa), with melt solidification in the range of 800–865 °C and 6–8 kbar. The presence of sillimanite inclusions in garnet, and the scarcity of retrograde cordierite are consistent with moderate dP/dT gradient “hairpin” P–T paths. The data suggest that this part of the mid-P hinterland in the central Grenville experienced a rather uniform high-temperature metamorphic evolution, with large temperature variations relative to pressure. This is consistent with the thermal evolution of middle crust beneath an orogenic plateau.

Rare earth elements and Sm-Nd isotope redistribution in apatite and accessory minerals in retrogressed lower crust material (Bergen Arcs, Norway)

In the Bergen Arcs (Norway), Grenvillian granulite retrogressed under eclogite- and amphibolite-facies conditions during Caledonian subduction/collision offer a unique opportunity to investigate rare earth elements (REE) and Sm-Nd isotope redistribution in accessory minerals during fluid-assisted metamorphism. Our sampling targeted apatite-bearing REE-rich protoliths (mangerite and jotunite) that preserve distinct mineral assemblages, depending on the external fluid availability and metamorphic conditions. REE concentrations in apatite are the highest in the granulite. Two populations are present: magmatic apatite (Ap1) relics that occur as inclusions in ilmenite-hematite, and intergranular apatite (Ap2) formed under granulite-facies conditions. The presence of abundant needle-like monazite and sulphide inclusions in Ap2 indicate that granulite reactions were fluid assisted. A thin (typically <10μm) rim of REE-rich epidote (Ep1) commonly surrounds Ap2. In these accessory minerals, U and Th contents are too low, or grains are too small, for in situ U-Th-Pb dating. Sm-Nd isotope data of Ap2, monazite and Ep1 give an isochron age of 601±69Ma, which is interpreted to represent a partially reset Grenvillian age, affected by Caledonian fluid-assisted mineral growth. In amphibolitized samples, granulite Ap2 is replaced by apatite (Ap3) with lower REE contents and no monazite inclusions. The REE released by this replacement are redistributed in a corona of epidote group minerals (Ep2) surrounding Ap3. The in situ Sm-Nd isotope data for Ep2 and titanite, found in replacement of ilmenite-hematite, return an isochron age of 395±65Ma, recording the timing of amphibolite-facies mineral growth when fluids were introduced into the rock. In eclogitized samples, eclogitic apatite (Ap4) occurs as polycrystalline aggregates, suggesting for a complex replacement process during deformation. REE contents of Ap4 are low, as REE originally contained in the precursor apatite were redistributed mainly into zoisite. Apatite shielded as inclusions in ilmenite and garnet preserve the REE-rich signature of the initial magmatic (Ap1) and granulite (Ap2) apatite, indicating these grains did not undergo further re-equilibration during Caledonian metamorphism. The resistance of apatite to compositional re-equilibration in this case confirms the petrological potential of apatite inclusions shielded in chemically inert minerals to track early magmatic, or metamorphic, crystallisation stages.

Archaean and Proterozoic diamond growth from contrasting styles of large-scale magmatism

Precise dating of diamond growth is required to understand the interior workings of the early Earth and the deep carbon cycle. Here we report Sm-Nd isotope data from 26 individual garnet inclusions from 26 harzburgitic diamonds from Venetia, South Africa. Garnet inclusions and host diamonds comprise two compositional suites formed under markedly different conditions and define two isochrons, one Archaean (2.95 Ga) and one Proterozoic (1.15 Ga). The Archaean diamond suite formed from relatively cool fluid-dominated metasomatism during rifting of the southern shelf of the Zimbabwe Craton. The 1.8 billion years younger Proterozoic diamond suite formed by melt-dominated metasomatism related to the 1.1 Ga Umkondo Large Igneous Province. The results demonstrate that resolving the time of diamond growth events requires dating of individual inclusions, and that there was a major change in the magmatic processes responsible for harzburgitic diamond formation beneath Venetia from the Archaean to the Proterozoic.

Crystallization temperature of anatectic melt: An example of biotite–muscovite granite in the Rikolatvi structure, Belomorian Mobile Belt

Zircon hosted in granite, which crystallized from local pools of anatectic melt among migmatites, in the Rikolatvi structure, Belomorian Mobile Belt, contains minute inclusions of various minerals, biotite and garnet among others. The compositions of the biotite and garnet in the microinclusions differ from those of the same minerals in the granite containing the zircon. The crystallization temperature of the anatectic melt was estimated by the biotite–garnet geothermometer and the composition of the biotite and garnet inclusions at ~800°C.

Pressures of skarn mineralization at Casting Copper, Nevada, USA, based on apatite inclusions in garnet

Pressures of skarn mineralization at Casting Copper, Nevada, USA, based on apatite inclusions in garnet

Primary spinel + chlorite inclusions in mantle garnet formed at ultrahigh-pressure

Primary spinel + chlorite inclusions in mantle garnet formed at ultrahigh-pressure

Geochronology of accessory allanite and monazite in the Barrovian metamorphic sequence of the Central Alps, Switzerland

The formation of accessory allanite, monazite and rutile in amphibolite-facies rocks across the Barrovian sequence of the Central Alps (Switzerland) was investigated with a combination of petrography and geochemistry and related to the known structural and metamorphic evolution of the Lepontine dome. For each of these minerals a specific approach was adopted for geochronology, taking into account internal zoning and U–Th–Pb systematics. In-situ U–Th–Pb dating of allanite and monazite by ion microprobe revealed systematic trends for the ages of main deformation and temperature in the Lepontine dome. Isotope dilution TIMS dating of rutile returns dates in line with this picture, but is complicated by inheritance of pre-Alpine rutile and possible Pb loss during Alpine metamorphism.Allanite is generally a prograde mineral that is aligned along the main foliation of the samples and found also as inclusions in garnet. Prograde allanite formation is further documented by rutile inclusions with formation temperatures significantly lower than the maximum T recorded by the rock mineral assemblage. Allanite ages vary from 31.3±1.1Ma in orthogneisses in the East to 31.7±1.1Ma for a Bündnerschiefer and 28.5±1.3Ma for a metaquartzite in the central area, to 26.8±1.1Ma in the western part of the Lepontine dome. These ages are interpreted to date the main deformation events (nappe stacking and isoclinal deformation of the nappe stack), close to peak pressure conditions.The timing of the thermal peak in the Lepontine dome is recorded in monazite that grew at the expense of allanite and after a main episode of garnet growth at temperatures of ~620°C. Monazite in the central area yields an age of 22.0±0.3Ma, which is indistinguishable from the age of 21.7±0.4Ma from a metapelite in the western part of the Lepontine dome. In the central area some of the classical kyanite‐staurolite-garnet schists directly underlying the metamorphosed Mesozoic sediments contain monazite that records only a pre-Alpine, Variscan metamorphic event of upper greenschist to lower amphibolite-facies conditions dated at ~330Ma.The new age data provide evidence that nappe stacking at prograde amphibolite-facies conditions and refolding of the nappe stack occurred between 32 and 27Ma, only a few million years after eclogite-facies metamorphism in the Adula-Cima Lunga unit. Amphibolite-facies metamorphism lasted for about 10My to ~22Ma, allowing for multiple ductile deformation and recrystallization events. The long lasting amphibolite-facies metamorphism requires fast cooling between 20 and 15Ma in the Central Alps. This fast cooling was not related to an increase in sedimentation rates in the foreland basins, suggesting that tectonic exhumation was responsible for termination of amphibolite-facies metamorphism in the Lepontine dome.

Pseudosection modelling and garnet Lu–Hf geochronology of HP amphibole schists constrain the closure of an ocean basin between the northern and southern Lhasa blocks, central Tibet

AbstractThe P–T–t path of high‐P metamorphic rocks in subduction zones may reveal valuable information regarding the tectonic processes along convergent plate boundaries. Herein, we present a detailed petrological, pseudosection modelling and radiometric dating study of several amphibole schists of oceanic affinity from the Lhasa Block, Tibet. The amphibole schists experienced an overall clockwise P–T path that was marked by post‐Pmax heating–decompression and subsequent isothermal decompression following the attainment of peak high‐P and low‐T conditions (~490°C and 1.6 GPa). Pseudosection modelling shows that the amphibole schists underwent water‐unsaturated conditions during prograde metamorphism, and the stability field of the assemblage extends to lower temperatures and higher pressures within the water‐unsaturated condition relative to water‐saturated model along the prograde path. The high‐P amphibole schists were highly reduced during retrograde metamorphism. Precise evaluation of the ferric iron conditions determined from the different compositions of epidote inclusions in garnet and matrix epidote is crucial for a true P–T estimate by garnet isopleth thermobarometry. Lu–Hf isotope analyses on garnet size separates from a garnet‐bearing amphibole schist yield four two‐point garnet–whole‐rock isochron ages from 228.2 ± 1.2 Ma to 224.3 ± 1.2 Ma. These Lu–Hf dates are interpreted to constrain the period of garnet growth and approximate the timing of prograde metamorphism because of the low peak metamorphic temperature of the rock and the well‐preserved Mn/Lu growth zoning in garnet. The majority of zircon U–Pb dates provide no constraints on the timing of metamorphism; however, two concordant U–Pb dates of 222.4 ± 3.9 Ma and 223.3 ± 4.2 Ma were obtained from narrow and uncommon metamorphic rims. Coexistence of zircon and sphene in the samples implies that the metamorphic zircon growth was likely assisted by retrogression of rutile to sphene during exhumation. The near coincident radiometric dates of zircon U–Pb and garnet Lu–Hf indicate rapid burial and exhumation of the amphibole schists, suggesting a closure time of c. 224–223 Ma for the fossil ocean basin between the northern and southern Lhasa blocks.

Primary crustal melt compositions: Insights into the controls, mechanisms and timing of generation from kinetics experiments and melt inclusions

We explore the controls, mechanisms and timing of generation of primary melts and their compositions, and show that the novel studies of melt inclusions in migmatites can provide important insights into the processes of crustal anatexis of a particular rock. Partial melting in the source region of granites is dependent on five main processes: (i) supply of heat; (ii) mineral–melt interface reactions associated with the detachment and supply of mineral components to the melt, (iii) diffusion in the melt, (iv) diffusion in minerals, and (v) recrystallization of minerals. As the kinetics of these several processes vary over several orders of magnitude, it is essential to evaluate in Nature which of these processes control the rate of melting, the composition of melts, and the extent to which residue–melt chemical equilibrium is attained under different circumstances. To shed light on these issues, we combine data from experimental and melt inclusion studies. First, data from an extensive experimental program on the kinetics of melting of crustal protoliths and diffusion in granite melt are used to set up the necessary framework that describes how primary melt compositions are established during crustal anatexis. Then, we use this reference frame and compare compositional trends from experiments with the composition of melt inclusions analyzed in particular migmatites. We show that, for the case of El Hoyazo anatectic enclaves in lavas, the composition of glassy melt inclusions provides important information on the nature and mechanisms of anatexis during the prograde suprasolidus history of these rocks, including melting temperatures and reactions, and extent of melt interconnection, melt homogenization and melt–residue equilibrium. Compositional trends in several of the rehomogenized melt inclusions in garnet from migmatites/granulites in anatectic terranes are consistent with diffusion in melt-controlled melting, though trace element compositions of melt inclusions and coexisting minerals are necessary to provide further clues on the nature of anatexis in these particular rocks.

Petrogenetic relations among titanium‐rich minerals in an anatectic high‐P mafic granulite

AbstractThe petrogenetic relations among Ti‐rich minerals in high‐grade metabasites is illuminated here through a detailed petrological investigation of an anatectic garnet–clinopyroxene granulite from the Grenville Province, Ontario, Canada containing rutile, titanite and ilmenite in distinct microtextural settings. Garnet porphyroblasts exhibit zoned Ti concentrations (up to 0.15 wt% TiO2 in their cores), as well as a variety of rutile inclusion types, including clusters of small, variably elongate grains and thin (≤1 μm) oriented needles. Calcite inclusions in garnet, commonly observed surrounding garnet cores containing quartz and clinozoisite, indicate the presence of evolving C–O–H fluids during garnet growth and suggest that the rutile clusters may have formed from subsequent Ti diffusion and rutile precipitation within existing fluid inclusions. Titanite forms large subhedral crystals and typically occurs where the primary garnet–clinopyroxene assemblage is in contact with leucosome containing megacrystic hornblende, silvialitic scapolite and calcic plagioclase. Many titanite crystals exhibit marginal subgrains that correspond with sharp changes in their major and trace element composition, likely related to a dissolution–precipitation or recrystallization process following primary crystallization. Clinopyroxene–ilmenite symplectite coronas surround titanite in most locations, likely forming from reaction with the hornblende‐plagioclase matrix (±fluids/melt). Integration of multi‐equilibria thermobarometry and Zr thermometry in rutile and titanite with phase equilibrium modelling allows definition of a clockwise P–T path evolving to peak pressures of ~1.5 GPa at ~750°C during garnet and rutile growth, followed by peak temperature conditions of ~1.2 GPa and ~820–880°C associated with melt‐present titanite growth, and finally cooling and decompression to regional amphibolite facies conditions (~1.0 GPa and ~750°C) associated with the formation of clinopyroxene–ilmenite symplectites surrounding titanite. P–T pseudosections calculated for the pristine (leucosome‐ and titanite ‐free) metabasite bulk composition reproduce much of the prograde phase relations, but predict rutile as the stable Ti‐rich mineral at the peak thermal conditions associated with melt‐present titanite growth. The P–M(CaO) and T–M(CaO) models show that bulk CaO concentrations have a significant effect on the stability ranges of titanite and rutile. Increased bulk CaO tends to stabilize titanite to higher pressure and temperature at the expense of rutile, with a ≥15% increase in CaO producing the observed titanite‐bearing assemblage at high‐P granulite facies conditions. Thus, the model results are consistent with the textural observations, which suggest that titanite stability is associated with a chemical exchange between the host metabasite and a Ca‐rich melt.

Forbidden mineral assemblage coesite‐disordered graphite in diamond‐bearing kyanite gneisses (Kokchetav Massif)

Confocal 2D and 3D Raman imaging was applied to SiO2 inclusions in kyanite porphyroblasts from diamond‐bearing kyanite gneisses of the Kokchetav Massif (Northern Kazakhstan). The study reveals a mineral assemblage of coesite‐disordered graphite which is forbidden in ultrahigh pressure‐high temperature (UHP‐HT) metamorphic complexes. Disordered graphite in coesite inclusions is optically undetectable and has been identified exclusively by Raman spectroscopy. Raman imaging of the forbidden coesite‐disordered graphite assemblage fails to detect bands assigned to fluid components (e.g. CO2, N2, CH4, H2Oliq), but the Raman spectra of fluid inclusions in kyanite, garnet and quartz demonstrate the presence of CO2, N2, CH4, H2Oliq and disordered graphite. The disordered graphite most likely formed by precipitation from a C―O―H fluid during the retrograde stage. The fluid was entrapped by kyanite with coesite inclusions near the peak metamorphic conditions. The lack of disordered graphite around diamond crystals in our samples does not support the generally accepted origin of disordered graphite in ultra high pressure metamorphic rocks by partial diamond graphitization during exhumation. Copyright © 2017 John Wiley &amp; Sons, Ltd.

CO2‐bearing fluid inclusions associated with diamonds in zircon from the UHP Kokchetav gneisses

CO2‐bearing fluid inclusions coexisting with diamonds were identified in zircons from diamondiferous gneiss in the Kokchetav Massif. This discovery provides evidence for the presence of CO2 in UHP fluids and diamond formation in moderately oxidized conditions in the Kokchetav gneiss. Fluid and multiphase solid inclusions coexisting in zircons represent immiscible melt and fluid captured close to the peak metamorphic conditions for the Kokchetav UHP gneiss. Most of CO2‐bearing inclusions are CO2+H2O mixtures except for some cases when they also contain daughter phases (e.g. muscovite, calcite and quartz) tracing the presence of aqueous and solute‐rich fluids at different phases of UHP metamorphism. Decrease of pressure and temperature may have been responsible for the reduction of solutes in the CO2‐bearing fluid. The lack of CO2‐bearing inclusions in garnet porphyroblasts from diamond‐bearing gneiss, as well as the common coexistence of aqueous CO2‐bearing inclusions with calcite, testify that most likely all CO2 in fluid was consumed by the calcite‐forming reaction and hydrous melt was the only remaining growth medium during retrograde metamorphism of the Kokchetav UHPM gneisses. Neither K‐cymrite nor kokchetavite was identified among daughter phases in the hydrous melt inclusions in garnet, which indicates that they hardly could originate in a metapelitic system. Copyright © 2017 John Wiley &amp; Sons, Ltd.