Reaction overstepping and re-evaluation of peak P‒T conditions of the blueschist unit Sifnos,Greece: implications for the Cyclades subduction zone

ABSTRACT

Equilibrium thermodynamic modelling, quartz in garnet (QuiG) Raman geobarometry, and modelling of garnet nucleation at overstepped conditions were applied to three garnet-bearing blueschists from a 1.5 km-long transect across the eclogite-blueschist unit in Sifnos, Greece, in order to evaluate the accuracy of P?T conditions calculated via equilibrium thermodynamics. QuiG barometry uses the Raman shift of quartz inclusions in garnet to estimate the pressure of garnet nucleation and is independent of chemical equilibrium. Garnet nucleation temperatures were estimated by determining the stability field of the palaeo-assemblage inferred from garnet inclusion suites on mineral assemblage diagrams calculated in the MnNCKFMASH system and on temperatures obtained from Zr in rutile thermometry. These conditions were then compared to P?T conditions calculated at the equilibrium garnet isograd, and the method of intersecting isopleths. The P?T conditions calculated with intersecting garnet isopleths over- and underestimated the temperature of nucleation in samples SPH99-1a and SPH99-7, respectively, whereas they significantly underestimated nucleation pressure in SPH99-5. Nucleation of garnet in SPH99-1a at 12 kbar and ~484°C requires overstepping of ~6 kbar and a reaction affinity of 2.2 kJ mol^?1 O. SPH99-5 requires overstepping of ~8 kbar with garnet reaction affinities of at least 2.0 kJ mol^?1 O at 15 kbar and ~520°C. SPH99-7 requires overstepping of approximately 15 kbar and affinities of about 2.0–2.4 kJ mol^?1 O at ~23 kbar and ~530°C. The geotherms calculated from SPH99-7 (~6.7°C km^?1) and SPH99-5 (9.8°C km^?1) are in accordance with previous studies. The geotherm calculated from SPH99-1a, however, is warmer (11.3°C km^?1), and could reflect changes in the rate of subduction or differences in structural position within the down-going slab. The 10 kbar pressure difference between SPH99-7 and SPH99-1a can be explained by thrusting and accretion of thin slices of underplated wedge material facilitated by slab rollback and gravitational collapse.     

Rare eclogite‐mafic granulite in felsic granulite in Blanský les: precursor of intermediate granulite in the Bohemian Massif?

Mafic granulite, generated from eclogite, occurs in felsic granulite at Kle?, Blanský les, in the Bohemian Massif. This is significant because such eclogite is very rare within the felsic granulite massifs. Moreover, at this locality, strong interaction has occurred between the mafic granulite and the adjacent felsic granulite producing intermediate granulite, such intermediate granulite being of enigmatic origin elsewhere. The mafic granulite involves garnet from the original eclogite, containing large idiomorphic inclusions of omphacite, plagioclase and quartz, as well as rutile. The edge of the garnet is replaced by a plagioclase corona, with the garnet zoned towards the corona and also the inclusions. The original omphacite–quartz–?plagioclase matrix has recrystallized to coarse‐grained polygonal (‘equilibrium’‐textured) plagioclase‐diopsidic clinopyroxene–orthopyroxene also with brown amphibole commonly in the vicinity of garnet. Somewhat larger quartz grains are embedded in this matrix, along with minor ilmenite, rutile and zircon. Combining the core garnet composition with core inclusion compositions gives a pressure of the order of 18 kbar from assemblage and isopleths on a P?T pseudosection, with temperature poorly constrained, but most likely >900 °C. From this P?T pseudosection, the recrystallization of the matrix took place at ~12 kbar, and from Zr‐in‐rutile thermometry, at relatively hot conditions of 900–950 °C. It is largely at these conditions that the eclogite/mafic granulite interacted with the felsic granulite to make intermediate granulite (see next paper).     

Partial melting due to breakdown of an epidote‐group mineral during exhumation of ultrahigh‐pressure eclogite: An example from the North‐East Greenland Caledonides

In the North‐East Greenland Caledonides, P–T conditions and textures are consistent with partial melting of ultrahigh‐pressure (UHP) eclogite during exhumation. The eclogite contains a peak assemblage of garnet, omphacite, kyanite, coesite, rutile, and clinozoisite; in addition, phengite is inferred to have been present at peak conditions. An isochemical phase equilibrium diagram, along with garnet isopleths, constrains peak P–T conditions to be subsolidus at 3.4 GPa and 940°C. Zr‐in‐rutile thermometry on inclusions in garnet yields values of ~820°C at 3.4 GPa. In the eclogite, plagioclase may exhibit cuspate textures against surrounding omphacite and has low dihedral angles in plagioclase–clinopyroxene–garnet aggregates, features that are consistent with former melt–solid–solid boundaries and crystallized melt pockets. Graphic intergrowths of plagioclase and amphibole are present in the matrix. Small euhedral neoblasts of garnet against plagioclase are interpreted as formed from a peritectic reaction during partial melting. Polymineralic inclusions of albite+K‐feldspar and clinopyroxene+quartz±kyanite±plagioclase in large anhedral garnet display plagioclase cusps pointing into the host, which are interpreted as crystallized melt pockets. These textures, along with the mineral composition, suggest partial melting of the eclogite by reactions involving phengite and, to a large extent, an epidote‐group mineral. Calculated and experimentally determined phase relations from the literature reveal that partial melting occurred on the exhumation path, at pressures below the coesite to quartz transition. A calculated P–T phase diagram for a former melt‐bearing domain shows that the formation of the peritectic garnet rim occurred at 1.4 GPa and 900°C, with an assemblage of clinopyroxene, amphibole, and plagioclase equilibrated at 1.3 GPa and 720°C. Isochemical phase equilibrium modelling of a symplectite of clinopyroxene, plagioclase, and amphibole after omphacite, combined with the mineral composition, yields a P–T range at 1.0–1. 6 GPa, 680–1,000°C. The assemblage of amphibole and plagioclase is estimated to reach equilibrium at 717–732°C, calculated by amphibole–plagioclase thermometry for the former melt‐bearing domain and symplectite respectively. The results of this study demonstrate that partial melt formed in the UHP eclogite through breakdown of an epidote‐group mineral with minor involvement of phengite during exhumation from peak pressure; melt was subsequently crystallized on the cooling path.     

Overstepping the garnet isograd: a comparison of QuiG barometry and thermodynamic modeling

The consequences of overstepping the garnet isograd reaction have been investigated by comparing the composition of garnet formed at overstepped P–T conditions (the overstep or “OS” model) with the P–T conditions that would be inferred by assuming garnet nucleated in equilibrium with the matrix assemblage at the isograd (the equilibrium or “EQ” model). The garnet nucleus composition formed at overstepped conditions is calculated as the composition that produces the maximum decrease in Gibbs free energy from the equilibrated, garnet-absent, matrix assemblage for the bulk composition under study. Isopleths were then calculated for this garnet nucleus composition assuming equilibrium with the matrix assemblage (the EQ model). Comparison of the actual P–T conditions of nucleation (the OS model) with those inferred from the EQ model reveals considerable discrepancy between the two. In general, the inferred garnet nucleation P–T conditions (the EQ model) are at a lower temperature and higher or lower pressure (depending on the coexisting calcic phase(s)) than the actual (OS model) nucleation conditions. Moreover, the degree of discrepancy increases with the degree of overstepping. Independent estimates of the pressure of nucleation of garnet were made using the Raman shift of quartz inclusions in garnet (quartz-in-garnet or QuiG barometry). To test the validity of this method, an experimental synthesis of garnet containing quartz inclusions was made at 800 °C, 20 kbar, and the measured Raman shift reproduced the synthesis conditions to within 120 bars. Raman band shifts from three natural samples were then used to calculate an isochore along which garnet was presumed to have nucleated. Model calculations were made at several temperatures along this isochore (the OS model), and these P–T conditions were compared to those computed assuming equilibrium nucleation (the EQ model) to estimate the degree of overstepping displayed by these samples. A sample from the garnet isograd in eastern Vermont is consistent with overstepping of around 10 degrees and 0.6 kbar (affinities of around 2 kJ/mole garnet). A sample from the staurolite–kyanite zone in the same terrane requires overstepping of around 50 °C and 2–5 kbar (affinities of around 10–18 kJ/mole garnet). A similar amount of overstepping was inferred for a blueschist sample from Sifnos, Greece. These results indicate that overstepping of garnet nucleation reactions may be common and pronounced in regionally metamorphosed terranes, and that the P–T conditions and paths inferred from garnet zoning studies may be egregiously in error.     

Quartz‐in‐garnet and Ti‐in‐quartz thermobarometry: Methodology and first application to a quartzofeldspathic gneiss from eastern Papua New Guinea

Mineral inclusions are ubiquitous in metamorphic rocks and elastic models for host‐inclusion pairs have become frequently used tools for investigating pressure–temperature (P–T) conditions of mineral entrapment. Inclusions can retain remnant pressures () that are relatable to their entrapment P–T conditions using an isotropic elastic model and P–T–V equations of state for host and inclusion minerals. Elastic models are used to constrain P–T curves, known as isomekes, which represent the possible inclusion entrapment conditions. However, isomekes require a temperature estimate for use as a thermobarometer. Previous studies obtained temperature estimates from thermometric methods external of the host‐inclusion system. In this study, we present the first P–T estimates of quartz inclusion entrapment by integrating the quartz‐in‐garnet elastic model with titanium concentration measurements of inclusions and a Ti‐in‐quartz solubility model (QuiG‐TiQ). QuiG‐TiQ was used to determine entrapment P–T conditions of quartz inclusions in garnet from a quartzofeldspathic gneiss from Goodenough Island, part of the (ultra)high‐pressure terrane of Papua New Guinea. Raman spectroscopic measurements of the 128, 206, and 464 cm^?1 bands of quartz were used to calculate inclusion pressures using hydrostatic pressure calibrations (), a volume strain calculation (), and elastic tensor calculation (), that account for deviatoric stress. values calculated from the 128, 206, and 464 cm^?1 bands’ hydrostatic calibrations are significantly different from one another with values of 1.8 ± 0.1, 2.0 ± 0.1, and 2.5 ± 0.1 kbar, respectively. We quantified elastic anisotropy using the 128, 206 and 464 cm^?1 Raman band frequencies of quartz inclusions and stRAinMAN software (Angel, Murri, Mihailova, & Alvaro, 2019,  234 :129–140). The amount of elastic anisotropy in quartz inclusions varied by ~230%. A subset of inclusions with nearly isotropic strains gives an average and of 2.5 ± 0.2 and 2.6 ± 0.2 kbar, respectively. Depending on the sign and magnitude, inclusions with large anisotropic strains respectively overestimate or underestimate inclusion pressures and are significantly different (<3.8 kbar) from the inclusions that have nearly isotropic strains. Titanium concentrations were measured in quartz inclusions exposed at the surface of the garnet. The average Ti‐in‐quartz isopleth (19 ± 1 ppm [2σ]) intersects the average QuiG isomeke at 10.2 ± 0.3 kbar and 601 ± 6°C, which are interpreted as the P–T conditions of quartzofeldspathic gneiss garnet growth and entrapment of quartz inclusions. The P–T intersection point of QuiG and Ti‐in‐quartz univariant curves represents mechanical and chemical equilibrium during crystallization of garnet, quartz, and rutile. These three minerals are common in many bulk rock compositions that crystallize over a wide range of P–T conditions thus permitting application of QuiG‐TiQ to many metamorphic rocks.     

Calcite–graphite isotope thermometry in amphibolite facies marble, Bancroft, Ontario

This study presents calcite–graphite carbon isotope fractionations for 32 samples from marble in the northern Elzevir terrane of the Central Metasedimentary Belt, Grenville Province, southern Ontario, Canada. These results are compared with temperatures calculated by calcite–dolomite thermometry (15 samples), garnet–biotite thermometry (four samples) and garnet–hornblende thermometry (three samples). Δ_cal‐gr values vary regularly across the area from >6.5‰ in the south to 4.0‰ in the north, which corresponds to temperatures of 525 °C in the south to 650 °C in the north. Previous empirical calibration of the calcite–graphite thermometer agrees very well with calcite–dolomite, garnet–biotite and garnet–hornblende thermometry, whereas, theoretical calibrations compare less well with the independent thermometry. Isograds in marble based on the reactions rutile + calcite + quartz =titanite and tremolite + calcite + quartz = diopside, span temperatures of 525–600 °C and are consistent with calculated temperature–X(CO₂) relations. Results of this study compare favourably with large‐scale regional isotherms, however, local variation is greater than that revealed by large‐scale sampling strategies. It remains unclear whether the temperature–Δ_cal‐gr relationship observed in natural materials below 650 °C represents equilibrium fractionations or not, but the regularity and consistency apparent in this study demonstrate its utility for thermometry in amphibolite facies marble.     

Garnet–clinopyroxene intermediate granulites in the St. Leonhard massif of the Bohemian Massif: ultrahigh‐temperature metamorphism at high pressure or not?

Garnet–clinopyroxene intermediate granulites occur as thin layers within garnet–kyanite–K–feldspar felsic granulites of the St. Leonhard granulite body in the Bohemian Massif. They consist of several domains. One domain consists of coarser‐grained coexisting ternary feldspar, clinopyroxene, garnet, quartz and accessory rutile and zircon. The garnet has 16–20% grossular, and the clinopyroxene has 9% jadeite and contains orthopyroxene exsolution lamellae. Reintegrated ternary feldspar and the Zr‐in‐rutile thermometer give temperatures higher than 950 °C. Mineral equilibria modelling suggests crystallization at 14 kbar. The occurrence and preservation of this mineral assemblage is consistent with crystallization from hot dry melt. Between these domains is a finer‐grained deformed matrix made up of diopsidic clinopyroxene, orthopyroxene, plagioclase and K‐feldspar, apparently produced by reworking of the coarser‐grained domains. Embedded in this matrix, and pre‐dating the reworking deformation, are garnet porphyroblasts that contain clinopyroxene, feldspar, quartz, rutile and zircon inclusions. In contrast with the garnet in the coarser‐grained domains, the garnet generally has >30% grossular, the included clinopyroxene has 7–27% jadeite and the Zr content of rutile indicates much lower temperatures. Some of these high‐grossular garnet show zoning in Fe/(Fe + Mg), decreasing from 0.7 in the core to 0.6 and then increasing to 0.7 at the rim. These garnet are enigmatic, but with reference to appropriate pseudosections are consistent with localized new mineral growth from 650 to 850 °C and 10 to 17 kbar, or with equilibration at 20 kbar and 770 °C, modified by two‐stage diffusional re‐equilibration of rims, at 10–15 and 8 kbar. The strong pervasive deformation has obscured relationships that might have aided the interpretation of the origin of these porphyroblasts. The evolution of these rocks is consistent with formation by igneous crystallization and subsequent metamorphism to high‐T and high‐P, rather than an origin by ultrahigh‐T metamorphism. Regarding the petrographic complexity, combination of the high grossular garnet with the ternary feldspar to infer ultrahigh‐T metamorphism at high pressure is not justified.     

Testing the fidelity of thermometers at ultrahigh temperatures

A highly residual granulite facies rock (sample RG07‐21) from Lunnyj Island in the Rauer Group, East Antarctica, presents an opportunity to compare different approaches to constraining peak temperature in high‐grade metamorphic rocks. Sample RG07‐21 is a coarse‐grained pelitic migmatite composed of abundant garnet and orthopyroxene along with quartz, biotite, cordierite, and plagioclase with accessory rutile, ilmenite, zircon, and monazite. The inferred sequence of mineral growth is consistent with a clockwise pressure–temperature (P–T) evolution when compared with a forward model (P–T pseudosection) for the whole‐rock chemical composition. Peak metamorphic conditions are estimated at 9 ± 0.5 kbar and 910 ± 50°C based on conventional Al‐in‐orthopyroxene thermobarometry, Zr‐in‐rutile thermometry, and calculated compositional isopleths. U–Pb ages from zircon rims and neocrystallized monazite grains yield ages of c. 514 Ma, suggesting that crystallization of both minerals occurred towards the end of the youngest pervasive metamorphic episode in the region known as the Prydz Tectonic Event. The rare earth element compositions of zircon and garnet are consistent with equilibrium growth of these minerals in the presence of melt. When comparing the thermometry methods used in this study, it is apparent that the Al‐in‐orthopyroxene thermobarometer provides the most reliable estimate of peak conditions. There is a strong textural correlation between the temperatures obtained using the Zr‐in‐rutile thermometer––maximum temperatures are recorded by a single rutile grain included within orthopyroxene, whereas other grains included in garnet, orthopyroxene, quartz, and biotite yield a range of temperatures down to 820°C. Ti‐in‐zircon thermometry returns significantly lower temperature estimates of 678–841°C. Estimates at the upper end of this range are consistent with growth of zircon from crystallizing melt at temperatures close to the elevated (H₂O undersaturated) solidus. Those estimates, significantly lower than the calculated temperature of this residual solidus, may reflect isolation of rutile from the effective equilibration volume leading to an activity of TiO₂ that is lower than the assumed value of unity.     

Experimental study of quartz inclusions in garnet at pressures up to 3.0 GPa: evaluating validity of the quartz-in-garnet inclusion elastic thermobarometer

Garnet crystals with quartz inclusions were hydrothermally crystallized from oxide starting materials in piston–cylinder apparatuses at pressures from 0.5 to 3 GPa and temperatures ranging from 700 to 800 °C to study how entrapment conditions affect remnant pressures of quartz inclusions used for quartz-in-garnet (QuiG) elastic thermobarometry. Systematic changes of the 128, 206 and 464 cm^?1 Raman band frequencies of quartz were used to determine pressures of quartz inclusions in garnet using Raman spectroscopy calibrations that describe the P–T dependencies of Raman band shifts for quartz under hydrostatic pressure. Within analytical uncertainties, inclusion pressures calculated for each of the three Raman band frequencies are equivalent, which suggests that non-hydrostatic stress effects caused by elastic anisotropy in quartz are smaller than measurement errors. The experimental quartz inclusions have pressures ranging from ??0.351 to 1.247 GPa that span the range of values observed for quartz inclusions in garnets from natural rocks. Quartz inclusion pressures were used to model P–T conditions at which the inclusions could have been trapped. The accuracy of QuiG thermobarometry was evaluated by considering the differences between pressures measured during experiments and pressures calculated using published equation of state parameters for quartz and garnet. Our experimental results demonstrate that Raman measurements performed at room temperature can be used without corrections to estimate garnet crystallization pressures. Calculated entrapment pressures for quartz inclusions in garnet are less than ~?10% different from pressures measured during the experiments. Because the method is simple to apply with reasonable accuracy, we expect widespread usage of QuiG thermobarometry to estimate crystallization conditions for garnet-bearing silicic rocks.     

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

The 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.     

Petrology of coesite-bearing eclogite from Habutengsu Valley, western Tianshan, NW China and its tectonometamorphic implication

Coesite inclusions in garnet have been found in eclogite boudins enclosed in coesite‐bearing garnet micaschist in the Habutengsu Valley, Chinese western Tianshan, which are distinguished from their retrograde quartz by means of optical characteristics, CL imaging and Raman spectrum. The coesite‐bearing eclogite is mainly composed of porphyroblastic garnet, omphacite, paragonite, glaucophane and barroisite, minor amounts of rutile and dotted (or banded) graphite. In addition to coesite and quartz, the zoned porphyroblastic garnet contains inclusions of omphacite, Na‐Ca amphibole, calcite, albite, chlorite, rutile, ilmenite and graphite. Multi‐phase inclusions (e.g. Czo + Pg ± Qtz, Grt II + Qtz and Chl + Pg) can be interpreted as breakdown products of former lawsonite and possibly chloritoid. Coesite occurs scattered within a compositionally homogenous but narrow domain of garnet (outer core), indicative of equilibrium at the UHP stage. The estimate by garnet‐clinopyroxene thermometry yields peak temperatures of 420–520 °C at 2.7 GPa. Phase equilibrium calculations further constrain the P–T conditions for the UHP mineral assemblage Grt + Omp + Lws + Gln + Coe to 2.4–2.7 GPa and 470–510 °C. Modelled modal abundances of major minerals along a 5 °C km^?1 geothermal gradient suggests two critical dehydration processes at ～430 and ～510 °C respectively. Computed garnet composition patterns are in good agreement with measured core‐rim profiles. The petrological study of coesite‐bearing eclogite in this paper provides insight into the metamorphic evolution in a cold subduction zone. Together with other reported localities of UHP rocks from the entire orogen of Chinese western Tianshan, it is concluded that the regional extent of UHP‐LT metamorphism in Chinese western Tianshan is extensive and considerably larger than previously thought, although intensive retrogression has erased UHP‐LT assemblages at most localities.     

Rutile R632 – A New Natural Reference Material for U‐Pb and Zr Determination

A new natural rutile reference material is presented, suitable for U‐Pb dating and Zr‐in‐rutile thermometry by microbeam methods. U‐Pb dating of rutile R632 using laser ablation ICP‐MS with both magnetic sector field and quadrupole instruments as well as isotope dilution‐thermal ionisation mass spectrometry yielded a concordia age of 496 ± 2 Ma. The high U content (> 300 μg g^?1) enabled measurement of high‐precision U‐Pb ages despite its young age. The sample was found to have a Zr content of 4294 ± 196 μg g^?1, which makes it an excellent complementary reference material for Zr‐in‐rutile thermometry. Individual rutile grains have homogeneous compositions of a number of other trace elements including V, Cr, Fe, Nb, Mo, Sn, Sb, Hf, Ta and W. This newly characterised material significantly expands the range of available rutile reference materials relevant for age and temperature determinations.     

Prograde and retrograde history of eclogites from the Eastern Blue Ridge, North Carolina, USA

Abstract The prograde metamorphism of eclogites is typically obscured by chemical equilibration at peak conditions and by partial requilibration during retrograde metamorphism. Eclogites from the Eastern Blue Ridge of North Carolina retain evidence of their prograde path in the form of inclusions preserved in garnet. These eclogites, from the vicinity of Bakersville, North Carolina, USA are primarily comprised of garnet–clinopyroxene–rutile–hornblende–plagioclase–quartz. Quartz, clinopyroxene, hornblende, rutile, epidote, titanite and biotite are found as inclusions in garnet cores. Included hornblende and clinopyroxene are chemically distinct from their matrix counterparts. Thermobarometry of inclusion sets from different garnets record different conditions. Inclusions of clinozoisite, titanite, rutile and quartz (clinozoisite + titanite = grossular + rutile + quartz + H₂O) yield pressures (6–10 kbar, 400–600 °C and 8–12 kbar 450–680 °C) at or below the minimum peak conditions from matrix phases (10–13 kbar at 600–800 °C). Inclusions of hornblende, biotite and quartz give higher pressures (13–16 kbar and 630–660 °C). Early matrix pyroxene is partially or fully broken down to a diopside–plagioclase symplectite, and both garnet and pyroxene are rimmed with plagioclase and hornblende. Hypersthene is found as a minor phase in some diopside + plagioclase symplectites, which suggests retrogression through the granulite facies. Two‐pyroxene thermometry of this assemblage gives a temperature of c. 750 °C. Pairing the most Mg‐rich garnet composition with the assemblage plagioclase–diopside–hypersthene–quartz gives pressures of 14–16 kbar at this temperature. The hornblende–plagioclase–garnet rim–quartz assemblage yields 9–12 kbar and 500–550 °C. The combined P–T data show a clockwise loop from the amphibolite to eclogite to granulite facies, all of which are overprinted by a texturally late amphibolite facies assemblage. This loop provides an unusually complete P–T history of an eclogite, recording events during and following subduction and continental collision in the early Palaeozoic.     

Integrated pressure–temperature–time constraints for the Tso Morari dome (Northwest India): implications for the burial and exhumation path of UHP units in the western Himalaya

Northward subduction of the leading edge of the Indian continental margin to depths greater than 100 km during the early Eocene resulted in high‐pressure (HP) quartz‐eclogite to ultrahigh‐pressure (UHP) coesite–eclogite metamorphism at Tso Morari, Ladakh Himalaya, India. Integrated pressure–temperature–time determinations within petrographically well‐constrained settings for zircon‐ and/or monazite‐bearing assemblages in mafic eclogite boudins and host aluminous gneisses at Tso Morari uniquely document segments of both the prograde burial and retrograde exhumation path for HP/UHP units in this portion of the western Himalaya. Poikiloblastic cores and inclusion‐poor rims of compositionally zoned garnet in mafic eclogite were utilized with entrapped inclusions and matrix minerals for thermobarometric calculations and isochemical phase diagram construction, the latter thermodynamic modelling performed with and without the consideration of cation fractionation into garnet during prograde metamorphism. Analysis of the garnet cores document (M1) conditions of 21.5 ± 1.5 kbar and 535 ± 15 °C during early garnet growth and re‐equilibration. Sensitive high resolution ion microprobe (SHRIMP) U–Pb analysis of zircon inclusions in garnet cores yields a maximum age determination of 58.0 ± 2.2 Ma for M1. Peak HP/UHP (M2) conditions are constrained at 25.5–27.5 kbar and 630–645 °C using the assemblage garnet rim–omphacite–rutile–phengite–lawsonite–talc–quartz (coesite), with mineral compositional data and regional considerations consistent with the upper P–T bracket. A SHRIMP U–Pb age determination of 50.8 ± 1.4 Ma for HP/UHP metamorphism is given by M2 zircons analysed in the eclogitic matrix and that are encased in the garnet rim. Two garnet‐bearing assemblages from the Puga gneiss (host to the mafic eclogites) were utilized to constrain the subsequent decompression path. A non‐fractionated isochemical phase diagram for the assemblage phengite–garnet–biotite–plagioclase–quartz–melt documents a restricted (M3) P–T stability field centred on 12.5 ± 0.5 kbar and 690 ± 25 °C. A second non‐fractionated isochemical phase diagram calculated for the lower pressure assemblage garnet–cordierite–sillimanite–biotite–plagioclase–quartz–melt (M4) documents a narrow P–T stability field ranging between 7–8.4 kbar and 705–755 °C, which is consistent with independent multiequilibria P–T determinations. Th–Pb SHRIMP dating of monazite cores surrounded by allanite rims is interpreted to constrain the timing of the M4 equilibration to 45.3 ± 1.1 Ma. Coherently linking metamorphic conditions with petrographically constrained ages at Tso Morari provides an integrated context within which previously published petrological or geochronological results can be evaluated. The new composite path is similar to those published for the Kaghan UHP locality in northern Pakistan, although the calculated 12‐mm a^?1 rate of post‐pressure peak decompression at Tso Morari would appear less extreme.     

Interpreting zirconium‐in‐rutile thermometric results

At first sight, experimental results and observations on rocks suggest that the Zr content in rutile, where equilibrated with quartz and zircon, should be a useful thermometer for metamorphic rocks. However, diffusion data for Zr in rutile imply that thermometry should not, for plausible rates of cooling, give the high temperatures commonly observed in high‐grade metamorphic rocks. It is suggested here that such observations can be accounted for by high‐T diffusive closure of Si in rutile, causing the interior of rutile grains to become insensitive to the thermometer equilibrium well above the temperature of Zr diffusive closure. Paired with comparatively slow grain boundary diffusion and problematic zircon nucleation, this allows for cases of Zr retention in rutile through temperatures where Zr is still diffusively mobile within rutile grains. Other observations that may be accounted for in this context are large inter‐grain ranges of rutile Zr contents uncorrelated with rutile grain size, and flat Zr profiles across individual rutile grains, counter to what would be expected from diffusive closure. A consequence is that it is unlikely that Zr‐in‐rutile thermometry will be useful for estimating rock cooling rates.     

Metamorphic evolution of low-T eclogite from the North Qilian orogen, NW China: evidence from petrology and calculated phase equilibria in the system NCKFMASHO

Low‐T eclogites in the North Qilian orogen, NW China share a common assemblage of garnet, omphacite, glaucophane, epidote, phengite, quartz and rutile with or without paragonite. Phase relations for the low‐T eclogites can be modelled well in the system NCKFMASHO with the updated solid‐solution models for amphibole and clinopyroxene. Garnet in the eclogite typically exhibits growth zonations in which pyrope increases while grossular somewhat decreases from core to rim, which is modelled as having formed mainly in the P–T conditions of lawsonite‐eclogite facies at the pre‐peak stage. Omphacite shows an increase in jadeite component as aegirine and also total FeO decrease in going from the inclusions in garnet to grains in the matrix, and from core to rim of zoned crystals, reflecting an increase in metamorphic P–T conditions. Glaucophane exhibits a compositional variation in X(gl) (= Fe²⁺/(Fe²⁺ + Mg)) and F(gl) (= Fe³⁺/(Fe³⁺ + Al) in M2 site), which decrease from the inclusions in garnet to crystals in the matrix, consistent with an increase in P–T conditions. However, for zoned matrix crystals, the X(gl) and F(gl) increase from core to rim, is interpreted to reflect a late‐stage decompression. Using composition isopleths for garnet rim and phengite in P–T pseudosections, peak P–T conditions for three samples Q5–45, Q5–01 and Q7–28 were estimated as 530–540 °C at 2.10–2.25 GPa, 580–590 °C at 2.30–2.45 GPa and 575–590 °C at 2.50–2.65 GPa, respectively, for the same assemblage garnet + omphacite + glaucophane + lawsonite (+ phengite + quartz + rutile) at the peak stage. The eclogites suggest similar P–T ranges to their surrounding felsic–pelitic schists. During post‐peak decompression of the eclogites, the most distinctive change involves the transformation of lawsonite to epidote, releasing large amount of water in the rock. The released fluid promoted further growth of glaucophane at the expense of omphacite and, in appropriate bulk‐rock compositions, paragonite formed. The decompression of eclogite did not lead to pronounced changes in garnet and phengite compositions. Peak P–T conditions of the North Qilian eclogite are well constrained using both the average P–T and pseudosection approaches in Thermocalc. Generally, the conventional garnet–clinopyroxene geothermometer is too sensitive to be used for constraining the temperature of low‐T eclogite because of the uncertainty in Fe³⁺ determination in omphacite and slight variations in mineral compositions because of incomplete equilibration.     

Garnet crystal plasticity in the continental crust,new example from south Madagascar

Garnet (10 vol.%; pyrope contents 34–44 mol.%) hosted in quartzofeldspathic rocks within a large vertical shear zone of south Madagascar shows a strong grain‐size reduction (from a few cm to ～300 μm). Electron back‐scattered diffraction, transmission electron microscopy and scanning electron microscope imaging coupled with quantitative analysis of digitized images (PolyLX software) have been used in order to understand the deformation mechanisms associated with this grain‐size evolution. The garnet grain‐size reduction trend has been summarized in a typological evolution (from Type I to Type IV). Type I, the original porphyroblasts, form cm‐sized elongated grains that crystallized upon multiple nucleation and coalescence following biotite breakdown: biotite + sillimanite + quartz = garnet + alkali feldspar + rutile + melt. These large garnet grains contain quartz ribbons and sillimanite inclusions. Type I garnet is sheared along preferential planes (sillimanite layers, quartz ribbons and/or suitably oriented garnet crystallographic planes) producing highly elongated Type II garnet grains marked by a single crystallographic orientation. Further deformation leads to the development of a crystallographic misorientation, subgrains and new grains resulting in Type III garnet. Associated grain‐size reduction occurs via subgrain rotation recrystallization accompanied by fast diffusion‐assisted dislocation glide. This plastic deformation of garnet is associated with efficient recovery as shown by the very low dislocation densities (10¹⁰ m^?3 or lower). The rounded Type III garnet experiences rigid body rotation in fine‐grained matrix. In the highly deformed samples, the deformation mechanisms in garnet are grain‐size‐ and shape‐dependent: dislocation creep is dominant for the few large grains left (>1 mm; Type II garnet), rigid body rotation is typical for the smaller rounded grains (300 μm or less; Type III garnet) whereas diffusion creep may affect more elliptic garnet (Type IV garnet). The P–T conditions of garnet plasticity in the continental crust (≥950 °C; 11 kbar) have been identified using two‐feldspar thermometry and GASP conventional barometry. The garnet microstructural and deformation mechanisms evolution, coupled with grain‐size decrease in a fine‐grained steady‐state microstructure of quartz, alkali feldspar and plagioclase, suggests a separate mechanical evolution of garnet with respect to felsic minerals within the shear zone.     

Very high-pressure (>4 GPa) eclogite associated with the Moldanubian Zone garnet peridotite (Nové Dvory, Czech Republic)

Equilibrium pressure–temperature (P–T) conditions were estimated for kyanite‐bearing eclogite from Nové Dvory, Czech Republic, by using garnet–clinopyroxene thermometry and garnet–clinopyroxene–kyanite–coesite (or quartz) barometry. The estimated P–T conditions are 1050–1150 °C, 4.5–4.9 GPa, which are mostly the same as previously estimated values for garnet peridotite from Nové Dvory (～1100–1250 °C, 5–6 GPa). Such very high‐P conditions, which correspond to about 150‐km depth, have been obtained for some garnet peridotites in the Gföhl Unit of the Bohemian Massif, but pressure conditions of eclogites associated with the garnet peridotites have not been so well constrained. This is the first substantial finding of eclogite that gives such very high‐P conditions in the Gföhl Unit of the Bohemian Massif. The Gföhl Unit mainly consists of felsic granulite or migmatitic gneiss, but these rock types do not display high‐P (>2.5 GPa) evidence. It is unclear whether both the peridotite body and surrounding felsic rocks in the Gföhl Unit were buried to very deep levels, but at least some garnet peridotites and associated eclogites in the Gföhl Unit have ascended from about 150‐km depth.     

Formation of eclogite, and reaction during exhumation to mid-crustal levels, Snowbird tectonic zone, western Canadian Shield

A re‐evaluation of the P–T history of eclogite within the East Athabasca granulite terrane of the Snowbird tectonic zone, northern Saskatchewan, Canada was undertaken. Using calculated pseudosections in combination with new garnet–clinopyroxene and zircon and rutile trace element thermometry, peak metamorphic conditions are constrained to ～16 kbar and 750 °C, followed by near‐isothermal decompression to ～10 kbar. Associated with the eclogite are two types of occurrences of sapphirine‐bearing rocks preserving a rich variety of reaction textures that allow examination of the retrograde history below 10 kbar. The first occurs as a 1–2 m zone adjacent to the eclogite body with a peak assemblage of garnet–kyanite–quartz interpreted to have formed during the eclogite facies metamorphism. Rims of orthopyroxene and plagioclase developed around garnet, and sapphirine–plagioclase and spinel–plagioclase symplectites developed around kyanite. The second variety of sapphirine‐bearing rocks occurs in kyanite veins within the eclogite. The veins involve orthopyroxene, garnet and plagioclase layers spatially organized around a central kyanite layer that are interpreted to have formed following the eclogite facies metamorphism. The layering has itself been modified, with, in particular, kyanite being replaced by sapphirine–plagioclase, spinel–plagioclase and corundum–plagioclase symplectites, as well as the kyanite being replaced by sillimanite. Petrological modelling in the CFMAS system examining chemical potential gradients between kyanite and surrounding quartz indicates that these vein textures probably formed during further essentially isothermal decompression, ultimately reaching ～7 kbar and 750 °C. These results indicate that the final reaction in these rocks occurred at mid‐crustal levels at upper amphibolite facies conditions. Previous geochronological and thermochronological constraints bracket the time interval of decompression to <5–10 Myr, indicating that ～25 km of exhumation took place during this interval. This corresponds to minimum unroofing rates of ～2–5 mm year^?1 following eclogite facies metamorphism, after which the rocks resided at mid‐crustal levels for 80–100 Myr.     

Discovery of diamond in the Tromsø Nappe,Scandinavian Caledonides (N. Norway)

We report the first finding of diamond in crustal rocks from the Tromsø Nappe of the North Norwegian Caledonides. Diamond occurs in situ as inclusions in garnet from gneiss at Tønsvika near Tromsø. The rock is composed essentially of garnet, biotite, white mica, quartz and plagioclase, minor constituents include kyanite, zoisite, rutile, tourmaline, amphibole, zircon, apatite and carbonates (magnesite, dolomite, calcite). The microdiamond, identified by micro‐Raman spectroscopy, is cuboidal to octahedral in shape and ranges from 5 to 50 μm in diameter. The diamond occurs as single grains and as composite diamond + carbonate inclusions. Diamond vibration bands show a downshift from 1 332 to 1 325 cm^?1, the majority of Raman peaks are centred between 1 332 and 1 330 cm^?1 and all peaks exhibit a full width at half maximum between 3 and 5 cm^?1. Several spectra show Raman bands typical for disordered and ordered graphite (sp²‐bonded carbon) indicating partial transformation of diamond to graphite. The calculated peak P–T conditions for the diamond‐bearing sample are 3.5 ± 0.5 GPa and 770 ± 50 °C. Metamorphic diamond found in situ in crustal rocks of the Tromsø Nappe thus provides unequivocal evidence for ultrahigh pressure metamorphism in this allochthonous unit of the Scandinavian Caledonides. Deep continental subduction, most probably in the Late Ordovician and shortly before or during the initial collision between Baltica and Laurentia, was required to stabilize the diamond at UHP conditions.