Pressure Temperature Time Research Articles

Meta-rodingite dikes as recorders of subduction zone metamorphism and serpentinite dehydration: Voltri Ophiolite, Italy

The metamorphic devolatilization of serpentinites during subduction represents the largest potential source of fluids from the subducting slab and influences a range of important processes from subduction zone seismicity to arc magmatism. Obtaining a record of metamorphic dehydration directly from serpentinites, however, is challenging, as serpentinite mineral assemblages are less conducive to extracting a detailed pressure-temperature-time (P-T-t) history. In this study, we use meta-rodingites (metasomatized gabbros) from the central portion of the Voltri Ophiolite in the Ligurian Alps (Italy), to investigate the P-T and geochronological record of subduction metamorphism and dehydration within the adjacent serpentinites. These meta-rodingites underwent eclogite-facies metamorphic recrystallization to a garnet-clinopyroxene-chlorite-ilmenite assemblage, and are cross-cut by several generations of garnet-bearing veins. A comparison of the Voltri meta-rodingites with un-subducted seafloor rodingites from the Apennines reveals textural and geochemical evidence of complete metamorphic recrystallization of the original seafloor metasomatic assemblage during Alpine subduction. Phase equilibria modeling of the Voltri meta-rodingite sample suggests that the peak metamorphic assemblage recorded was stable in the range of ~450–600 °C, which is consistent with previous P-T estimates for gabbroic and ultramafic lithologies from the Voltri Ophiolite. A late prograde to peak metamorphic age of 40.6 ± 2.9 Ma was obtained from the meta-rodingite using SmNd garnet geochronology, which closely matches the ages of late prograde to peak metamorphism estimated by companion studies of associated eclogites from the central part of the Voltri Ophiolite. The overlap in P-T conditions and timing of peak metamorphism for samples across the area suggests subduction and exhumation of the central part of the Voltri Ophiolite as a coherent lithospheric slice. Dating of a garnetite vein cross-cutting the main meta-rodingite body suggests an influx of fluid at 37.7 ± 1.4 Ma, at or near peak eclogite-facies conditions. Enrichment in the vein selvage of both compatible transition metals (Cr, Ni, Cu) and fluid-mobile elements (Li, Rb, Cs, Ba) suggests the fluid carried both serpentinite and sedimentary geochemical signatures and interacted with the main meta-rodingite body. We suggest that the most probable explanation for this fluid composition, and thus the source of the fluid, is the dehydration of the Voltri serpentinites. These serpentinites likely experienced an influx of sedimentary-derived, fluid-mobile-element-bearing fluids earlier in their history, either on the seafloor or most likely during the early stages of subduction. The presence of a sedimentary signature in this serpentinite-derived fluid emphasizes the importance of subducted serpentinites and their dehydration to the geochemical cycling of fluid-mobile elements in subduction zones. The widespread development of such vein systems within multiple meta-rodingites across the Voltri Ophiolite points to a large-scale fluid release event, likely resulting from the partial dehydration of brucite and antigorite within the surrounding serpentinite hosts. By utilizing direct dating of vein mineralogy formed during serpentinite dehydration, we provide one of the first geochronological records of serpentinite dehydration and fluid release occurring during subduction.

Pressure–temperature–time–composition (P–T–t–x) of paleo–fluid in Permian organic–rich shale of Lower Yangtze Platform, China: Insights from fluid inclusions in fracture cements

Recently, much attention has been paid to Permian shale from the Yangtze Platform because of the high–quality hydrocarbon source rock. This research investigates fracture veins derived from conventional cores of the Permian Leping Formation in the Pingle Depression, Lower Yangtze Platform. Based on petrographic observations, micro–Raman spectroscopy, and fluorescent spectrometry, hybrid CH4–C2H6 gas inclusions, pure CH4 inclusions, dissolved CH4 aqueous inclusions, and different abundant blue-green fluorescence petroleum inclusions were discovered in quartz and calcite. The pressure–temperature–time–composition properties of the CH4–bearing inclusions were obtained by quantitative Raman analysis and thermodynamic models. The pore fluid pressure (59.05–87.78 MPa) calculated from Raman shift of C–H symmetric stretching (v1) band of methane indicates inclusions trapped at close to lithostatic pressures, which can provide a key evidence for understanding the formation and evolution of overpressure. PT entrapment conditions, reconstruction of hydrocarbon generation and simulation of burial history permit to constrain accumulation time sequence of paleo-fluid in shale veins. Blue-green fluorescence with high-mature petroleum fluid trapped in late Cretaceous (~81–71 Ma); hybrid CH4–C2H6 fluid trapped in early Paleogene (~68–61 Ma); and CH4 fluid trapped in Paleogene (~62–51 Ma). Dry gas generation period close to the age of maximum burial depth at early Himalayan uplift period.

Crustal material recycling induced by subduction erosion and subduction-channel exhumation: A case study of central Tibet (western China) based onP-T-tpaths of the eclogite-bearing Baqing metamorphic complex

AbstractSubduction and exhumation processes, interacting with each other, play a key role in crustal recycling. Downgoing oceanic lithosphere constitutes the dominant input at subduction margins, but subduction erosion, the removal of crustal material from the overriding plate, may add additional ingredients and complexity to the subduction factory. Different exhumation models have been proposed to explain how subducted materials are exhumed and therefore contribute to crustal recycling, e.g., exhumation up the subduction channel versus diapiric rise through the mantle wedge that overlies the subducted plate. The recently discovered Baqing eclogite-bearing high-pressure metamorphic complex, central Tibet, China, provides an excellent opportunity to decode the exhumation process, the origin of subduction-related magmatism, and the crustal structure of the North Qiangtang block, in addition to elucidating processes of crustal recycling. Pressure-temperature-time (P-T-t) paths and zircon U-Pb ages and trace-element compositions for Baqing high-pressure rocks were used to evaluate exhumation processes and to determine the geochemical and tectonic affinity of the Baqing metamorphic complex.The Baqing metamorphic complex is mainly composed of eclogite, gneiss, and schist. It is located between two geologically distinct terranes—the South Qiangtang block, which has early Paleozoic basement, and the North Qiangtang block, which has Proterozoic basement. In the schist, zircon cores with steep heavy rare earth element (HREE) slopes and oscillatory zoning yielded inherited ages that are similar to detrital zircon ages for the South Qiangtang block schist; in contrast, zircon rims with flat HREE slopes yielded metamorphic ages of 224 Ma that are similar to the metamorphic ages obtained for the Baqing eclogite. In contrast, zircons from the gneiss yielded an upper-intercept age of 1033 ± 32 Ma (interpreted as the crystallization age) and a lower-intercept metamorphic age of 198 ± 4 Ma. Field relations indicate that gneiss and eclogite/amphibolite were exhumed together, so the ∼20 m.y. gap between the gneiss and the metabasite metamorphism may indicate a long exhumation duration. In the region, Proterozoic ages of ca. 1000 Ma are known only from the North Qiangtang block; we thus propose that the Baqing gneiss originated from North Qiangtang block Proterozoic basement, which, along with North Qiangtang block Triassic arc magmatic rocks and the discrepancies between ancient and current arc-trench distances, results in estimates of ∼20–170 km of Triassic subduction erosion. Results of P-T analyses show that most eclogite, amphibolite, and schist shared a similar clockwise P-T path, different from that of the gneiss, which records a higher geothermal gradient. The clockwise P-T trajectory, long exhumation duration, lack of significant heating during exhumation, and the South Qiangtang block affinity of the schist (host rock of the Baqing eclogite) are consistent with subduction-channel exhumation rather than diapiric rise through the mantle wedge. Geochemical similarities between the North Qiangtang block Triassic subduction-related rocks and the Baqing gneiss may signal the involvement of unexhumed Baqing metamorphic complex in the recycling of the Qiangtang crust.

Variations in the P‐T‐t of Deformation in a Crustal‐Scale Shear Zone in Metagranite

AbstractDeformation in crustal‐scale shear zones occurs over a range of pressure‐temperature‐time (P‐T‐t) conditions, both because they may be vertically extensive structures that simultaneously affect material from the lower crust to the surface, and because the conditions at which any specific volume of rock is deformed evolve over time, as that material is advected by fault activity. Extracting such P‐T‐t records is challenging, because structures may be overprinted by progressive deformation. In addition, granitic rocks, in particular, may lack syn‐kinematic mineral assemblages amenable to traditional metamorphic petrology and petrochronology. We overcome these challenges by studying the normal‐sense Simplon Shear Zone (SSZ) in the central Alps, where strain localization in the exhuming footwall caused progressive narrowing of the shear zone, resulting in a zonation from high‐T shearing preserved far into the footwall, to low‐T shearing adjacent to the hanging wall. The Ti‐in‐quartz and Si‐in‐phengite thermobarometers yield deformation P‐T conditions, as both were reset syn‐kinematically, and although the sheared metagranites lack typical petrochronometers, we estimate the timing of deformation by comparing our calculated deformation temperatures to published thermochronological ages. The exposed SSZ footwall preserves evidence for retrograde deformation during exhumation, from just below amphibolite‐facies conditions (∼490°C, 6.7 kbar) at ∼24.5 Ma, to lower greenschist‐facies conditions (∼305°C, 1.5 kbar) at ∼11.5 Ma, with subsequent slip taken up by brittle faulting. Our estimates fall within the P‐T‐t brackets provided by independent constraints on the maximum and minimum conditions of retrograde ductile deformation, and compare reasonably well to alternative approaches for estimating P‐T.

From static alteration to mylonitization: a nano- to micrometric study of chloritization in granitoids with implications for equilibrium and percolation length scales

Strain accommodation in upper crustal rocks is often accompanied by fluid-mediated crystallization of phyllosilicates, which influence rock strength and shear zone formation. The composition of these phyllosilicates is frequently used for pressure–temperature–time constraints of deformation events, although it is often highly heterogeneous, even in mylonites. This study investigates the reactions producing a phyllosilicate, chlorite, in and below greenschist-facies conditions and the variations in chlorite composition along a strain gradient in the Variscan Bielsa granitoid (axial zone, Pyrenees). Compositional maps of chlorite including iron speciation are compared to nanostructures observed by transmission electron microscopy in increasingly-strained samples and related to mechanisms of fluid percolation. In the Bielsa granitoid, altered at the late-Variscan, Alpine-age shear zones are found with high strain gradients. The undeformed granitoid exhibits local equilibria, pseudomorphic replacement, and high compositional heterogeneities in chlorite. This is attributed to variable reaction mechanisms at nanoscale and element supply, little interconnected intra- and inter-grain nanoporosity, and isolation of fluid evolving in local reservoirs. In samples with discrete and mm-sized fractures, channelized fluid triggered the precipitation of homogeneous Alpine chlorite in fractures, preserving late-Variscan chlorite within the matrix. In low-grade mylonites, where brittle–ductile deformation is observed, micro- and nanocracks and defects allow the fluid percolating into the matrix at the scale of hundreds of µm. This results in a more pervasive but incomplete replacement of late-Variscan chlorite by Alpine chlorite, despite the high strain. In studied granitoids deformed under greenschist-facies conditions, local equilibria and high compositional heterogeneities in phyllosilicates as chlorite are therefore preserved according to reaction mechanisms and element mobility controlled by (1) matrix-fracture porosity contrasts at nanoscale and (2) the location and interconnection of nanoporosity between crystallites of phyllosilicates. This preservation influences our ability to reconstruct the pre- and syn-kinematic metamorphic history of granitic rocks in low-grade units of orogens.

Magma Pressure-Temperature-Time Paths During Mafic Explosive Eruptions

We have constrained syneruptive pressure-temperature-time (P-T-t) paths of mafic magmas using a combination of short-timescale cooling and decompression chronometers. Recent work has shown that the thermal histories of crystals in the last few seconds to hours of eruption can be constrained using concentration gradients of MgO inside olivine-hosted melt inclusions, produced in response to syneruptive cooling and crystallization of olivine on the inclusion walls. We have applied this technique to the study of melt inclusions erupted by arc and ocean island volcanoes, including the 1974 subplinian eruption of Fuego volcano; the 1977 fire-fountain eruption of Seguam volcano; and three eruptions of Kilauea volcano (episode 1 of the 1959 Kilauea Iki fire-fountain eruption, the 1500 CE vigorous fire-fountain eruption, and the 1650 CE subplinian eruption). Of the eruptions studied so far, melt inclusions from the 1959 Kilauea Iki eruption record the highest syneruptive cooling rates (3–11°C/s) and the shortest cooling durations (4–19 s), while inclusions from the 1974 Fuego eruption record the slowest cooling rates (0.1–1.7°C/s) and longest cooling durations (21–368 s). The high cooling rates inferred for the Kilauea Iki and Seguam fire fountain eruptions are consistent with air quenching over tens of seconds during and after fragmentation and eruption. Melt inclusions sampled from the interiors of small (∼6 cm diameter) volcanic bombs at Fuego are found to have cooled more slowly on average than inclusions sampled from ash (with particle diameters < 2 mm) during the same eruption, as expected based on conductive cooling models. We find evidence for a systematic relationship between cooling rates and decompression rates of magmas, in which rapidly ascending gas-bearing magmas experience slower cooling during ascent and eruption than slowly ascending magmas. Our magma P-T-t constraints for the Kilauea Iki eruption are in broad agreement with isentropic models that show that the dominant driver of cooling in the conduit is adiabatic expansion of a vapor phase; however, at Fuego and Seguam, our results suggest a significant role for latent heat production and/or open-system degassing (both of which violate assumptions required for isentropic ascent). We thereby caution against the application of isentropic conduit models to magmas containing relatively high initial water concentrations (e.g., arc magmas containing ∼4 wt% water). We note that several processes that have been inferred to occur in volcanic conduits such as magma stalling, magma mingling, open- and closed-system degassing, vapor fluxing, and vapor accumulation (in foam layers or as slugs of gas) are associated with different implied vapor volume fractions during syneruptive ascent. Given the sensitivity of magma P-T-t paths to vapor volume fraction, the syneruptive thermometer presented here may be a means of identifying these processes during the seconds to hours preceding the eruption of mafic magmas.

Estimation of the influences of air density on wind energy assessment: A case study from China

The distribution characteristics of air density in China were modelled on a high-spatial-resolution scale (4000 m × 4000 m) and the absolute error was calculated by comparing with constant air density. Daily mean air pressure and air temperature time series from 2403 meteorological stations for the period 1998–2018 were used to compute air density at the hub height of a typical wind turbine. The statistical air density distributions were applied in the study area and the influence of elevation on changes in air density was quantified. Mean annual energy production in the study area was estimated using extrapolated parameters of the Weibull distribution and adjusted power curves. Compared with energy production under the assumption of constant air density, the results demonstrate that over 33% of the study area had a mean air density less than 1.000 kg/m3 due to the effect of elevation. Values as low as 0.347 kg/m3 were observed in the Tibetan Plateau. Energy production was underestimated (0.04–1.76%) in the north-east and north when the variability of air density was considered, and wind resources were overestimated by more than 20% in approximately 13.1% of the study area. The results of this study will contribute to identifying suitable locations for developing the wind energy industry. The influence of air density on wind energy production at different elevations can now be quantified, allowing more efficient utilization of wind resources.

Constraints on retrograde metamorphism of UHP eclogites in North Qinling, Central China, from 40Ar/39Ar dating of amphibole and phengite

Coesite- and microdiamond- bearing ultra-high pressure (UHP) eclogites in the North Qinling terrane have been widely retrogressed to amphibolites. Previous geochronological studies on these UHP rocks mainly focused on the timing of peak eclogite facies metamorphism. The Kanfenggou UHP metamorphic domain is one of the best-preserved coesite-bearing eclogite occurrences in the North Qinling terrane. In this study, mafic amphibolites and host schists from this domain were collected for 40Ar/39Ar dating to constrain their retrograde evolution. Two generations of amphibole are recognized based on their mineral parageneses and 40Ar/39Ar ages. A first generation of amphibole from garnet amphibolites yielded irregularly-shaped age spectra with anomalously old apparent ages. Isochron ages of 484–473 Ma and initial 40Ar/36Ar ratios of 3695–774 are obtained from this generation of amphibole, indicating incorporation of excess argon. Second generation amphibole occurs in epidote amphibolites yielded flat age spectra with plateau ages of 464–462 Ma without evidence for excess argon. These ages suggest that the amphibolite-facies metamorphism has taken place as early as 484 Ma and lasted until 462 Ma for the North Qinling UHP metamorphic rocks. Phengite from the country-rock schists yielded 40Ar/39Ar plateau ages of 426–396 Ma, with higher phengite Si contents associated with the older the plateau ages. Based on our new 40Ar/39Ar ages and previous zircon UPb geochronological data, we construct a new detailed pressure-temperature-time (P-T-t) path illustrating the retrograde metamorphism and exhumation rate of the North Qinling eclogites and host schists. The P-T-t path suggests that these UHP metamorphic rocks experienced initial medium-to-high exhumation rates (ca. 8.7 mm/yr) during the Early Ordovician (489–484 Ma), which was mainly derived from buoyancy forces. Subsequently, the exhumation rate decreased gradually from ~0.8 to 0.3 mm/yr from 484 to 426 Ma, which was probably governed by extension and/or erosion.

An experimentally validated numerical model for bubble growth in magma

Volcanic eruptions are driven by the growth of gas bubbles in magma. The timing and rate of bubble growth are important because they determine whether enough gas pressure can develop to fragment the melt. Bubbles grow in response to decompression and diffusive transport of dissolved volatiles (predominantly H2O) that exsolve into the bubbles. Growth is resisted by the viscosity of the melt. Both melt viscosity and H2O diffusivity have non-linear dependence on the concentration of H2O dissolved in the melt, which necessitates a numerical approach to modelling bubble growth. Several bubble growth models have previously been published and applied, but none of them has been validated against continuous, in situ experimental data or provided as a user-friendly tool. Here we present a numerical bubble growth model, implemented in MATLAB, which allows for arbitrary temperature and pressure pathways, and accounts for the impact of spatial variations in dissolved H2O concentration on viscosity and diffusivity. We validate the model against two sets of experimental data: (1) New continuous data for gas-volume fraction as a function of time, collected using optical dilatometry of vesiculating hydrous obsidian samples which were heated from 930 °C to 1000 °C at atmospheric pressure. This dataset captures isobaric, isothermal bubble growth under strongly disequilibrium conditions. (2) Discrete data from published decompression experiments at 825 °C and pressures from 200 MPa to ~5 MPa with decompression rates from 0.1 MPa s−1 to 10 MPa s−1. These experiments represent isothermal, decompression-driven bubble growth spanning equilibrium to strongly disequilibrium conditions. The numerical model closely reproduces the experimental data across all conditions, providing validation against contrasting bubble growth scenarios. The validated model has a wide range of potential volcanological applications, including forward modelling of bubble growth phenomena, and inverse modelling to reconstruct pressure–temperature–time pathways from textures and volatile contents of eruptive products.

U–Pb geochronology and REE geochemistry of zircons in mafic granulites from the Lützow-Holm complex, East Antarctica: Implications for the timing and P–T path of post-peak exhumation and Antarctica–Sri Lanka correlation

We report new petrological and geochronological data, together with zircon REE patterns, of mafic granulites from the Lützow-Holm Complex (LHC), East Antarctica, and investigate the pressure-temperature-time (P-T-t) evolution of granulite-facies metamorphism in order to unravel the timing and tectonics of Neoproterozoic collisional processes related to the amalgamation of Gondwana. The LHC is composed of three tectonic units: a ca. 2.5 Ga continental fragment (Shirase Microcontinent), a ca. 1.0 Ga magmatic arc (Prince Olav–Vijayan Block; PVB), and a latest Neoproterozoic suture zone (Lützow-Holm Bay Suture Zone; LHSZ) separating the two. Mafic granulites from Tenmondai Rock and Sudare Rock, which belong to the PVB and the LHSZ, respectively, contain orthopyroxene + plagioclase symplectites around garnets, probably formed during post-peak decompression. The application of phase equilibrium modeling and geothermobarometry indicates similar peak P-T ranges of 850–860 °C/7.8–8.4 kbar (Tenmondai Rock) and 800–810 °C/8.5–9.0 kbar (Sudare Rock), together with clockwise P-T paths for both localities. LA-ICP-MS zircon U-Pb dating of the studied granulites yielded Mid-Neoproterozoic magmatic ages of >808 Ma (Tenmondai Rock) and >783 Ma (Sudare Rock) from igneous cores, and Late Neoproterozoic to Cambrian metamorphic ages of ca. 630–481 Ma from overgrown rims and/or structureless grains. Zircon REE analyses indicate that younger 560–510 Ma metamorphic zircons from the localities exhibit heavy-REE-enriched patterns and negative Eu anomalies, suggesting that the consumption of garnet and growth of plagioclase took place during zircon growth. This process was probably related to the formation of post-peak decompressional textures (orthopyroxene + plagioclase symplectite around garnet) observed in the mafic granulites. The results of this study therefore indicate that post-peak exhumation of the LHC started at ca. 560 Ma. Available U-Pb and REE data from 560 to 510 Ma metamorphic zircons of the Highland Complex, Sri Lanka, show similar heavy-REE-enriched patterns and Eu negative anomalies, suggesting that post-peak exhumation of both the LHC and the Highland Complex took place almost simultaneously at ca. 560 Ma. The results of this study further support the proposed tectonic model in which the Highland Complex and the LHSZ are parallel collisional sutures formed during the Late Neoproterozoic-Cambrian Gondwana amalgamation.

Simultaneous operation of opposing reaction mechanisms: The influence of matrix heterogeneity on post‐kinematic garnet crystallization in an inverted metamorphic sequence

AbstractMetapelites from the inverted Barrovian sequence in the Sikkim Himalaya (northeast India) are shown to be largely continuous with respect to their bulk rock compositions, microstructures and pressure–temperature–time–deformation (P–T–t–D) histories. However, the upper garnet–lower staurolite zone demarcates a region of microstructurally anomalous post‐kinematic garnet populations contained within strongly segregated matrices. The different microstructures within samples from this region cannot be attributed to differences in their thermobarometric histories or bulk compositions, but are instead interpreted to represent an otherwise unexposed level of the Daling Group that is now exposed along a post‐metamorphic thrust splay. These heterogeneous samples contain several discrete garnet populations that progressively crystallized with increasing P–T. Garnet populations that experienced the most protracted growth now form complex polycrystals that exhibit crystallographically controlled and morphologically irregular interfaces adjacent to micaceous and quartzofeldspathic domains respectively. Electron backscatter diffraction indicates that these polycrystalline garnet structures contain numerous coalesced porphyroblasts that are structurally uncorrelated across their grain boundaries. However, a crystallographically preferred orientation at the polycrystal scale is interpreted to derive from epitaxial crystallization of early‐formed garnet porphyroblasts on precursor mica. Later‐nucleated porphyroblasts within polycrystals preferentially concentrated towards quartzofeldspathic domains, with the overall nucleation distribution likely controlled by a complex interplay between chemical heterogeneities, strain partitioning and epitaxial crystallization. The subsequent growth of these polycrystals was equally spatially heterogeneous; it was moderated by differences in the efficiency of grain boundary transfer between quartzofeldspathic and micaceous domains that precluded thin section‐scale chemical equilibration. In contrast to samples from Sikkim containing more typical porphyroblastic populations in continuous and disseminated matrices, heterogeneous availability of garnet‐forming components within this strongly layered matrix is shown to have resulted in grain‐scale variations in growth rates and the spatial juxtapositioning of interface‐controlled microstructures and locally equilibrated chemical compositions with those that were transport controlled.

Reply to comment by S.H. Büttner on “Evidence for Mesoproterozoic collision, deep burial and rapid exhumation of garbenschiefer in the Namaqua Front, South Africa ˮ

Abstract We thank Prof. Buttner for his comments on our paper (van Schijndel et al., 2020), which allow us to clarify our work and the tectonic model we proposed for the eastern margin of the Namaqua Sector of the Namaqua Natal Metamorphic Province. The Kaaien Terrane forms a tectonic transition between the Kaapvaal Craton, Rehoboth Province, Kheis belt and the Archeachap Terrane of the Namaqua Sector and forms part of an NW-vergent foreland thrust sequence in the Namaqua Front (e.g. Cornell et al., 2006). The exact tectonic extent of the Kaaien Terrane is still unclear and it has recently been proposed to be part of the newly defined Kheis Terrane (Van Niekerk and Beukes, 2019). Buttner (2020) states that the tectonic model for the Mesoproterozoic evolution of Namaqua Sector which envisages the collision of multiple exotic terranes after the subduction of multiple oceanic basins (e.g. Miller 2012, and references therein) is outdated and newer tectonic models have been discussed in the literature in recent years. Bial et al. (2015a, 2015b, 2016) and Macey et al. (2018) propose a continental back-arc mobile belt setting to explain the high temperature and low pressure history of several terranes of the Namaqua Sector. For example, Pressure-Temperature time (P-T-t) paths describing the crustal evolution for the Kakamas Terrane by Bial et al. (2015a) suggest along-lasting high-temperature history (>650°C) at pressure not exceeding 5.5 kbar between ∼1350 Ma and

High‐P granulites of the Songshugou area (Qinling Orogen, east‐central China): Petrography, phase relations, and U/Pb zircon geochronology

AbstractHigh‐pressure (HP) granulites provide telling records of mineral reactions at upper mantle to lower crustal levels and key information on the fate of material in subduction systems. The latter especially applies when they abut eclogite and mantle dunite because such rock associations are crucial for understanding the incompletely known processes at the interface of converging plates. A continental arc, active c. 520–395 Ma ago, formed an enigmatic example of such a rock association in the Songshugou area, Qinling Orogen. To unravel the juxtaposition of the distinct rocks, this study combines petrography, phase equilibria modelling, conventional thermobarometry, and zircon U–Th–Pb–Ti–REE analysis. Two mafic HP granulites, which contain the mineral assemblages garnet–clinopyroxene–plagioclase–rutile–mesoperthite–quartz and garnet–clinopyroxene–plagioclase–rutile, experienced peak metamorphic conditions of ≤1.4 GPa, 860°C and ~1.3 GPa, ≥910°C, respectively. During decompression and cooling, at 489 ± 4 Ma, amphibole lamellae unmixed from a clinopyroxene solid solution and orthopyroxene in part replaced garnet. A felsic HP granulite shows equilibration of garnet, perthite, antiperthite, kyanite, quartz, and rutile at 810–860°C, ~1.2 GPa, sillimanite growth during decompression, and upper amphibolite facies cooling at 510 ± 4 Ma. Though the thermobarometric data are just within the methodological errors, the U/Pb zircon ages imply the HP granulites did not evolve coherently. The HP granulites either represent foundered lower arc crust or originated from subduction erosion because their geochemistry is indistinguishable from that of the hanging‐wall plate. Published and new pressure–temperature–time–deformation paths converge at ~710°C, ~0.9 GPa, and ≲470 Ma, implying exhumation tectonics juxtaposed the HP granulites with a mélange of eclogite and mantle dunite at lower crustal levels. This study highlights that lower arc crust can comprise material of diverse evolution.

CH4 accumulation characteristics and relationship with deep CO2 fluid in Lishui sag, East China Sea Basin

Co-accumulation of carbon dioxide and hydrocarbon gas is common and widely existing in oil and gas basins. Such two natural pools are located in Lishui sag (East China Sea Shelf Basin), as a favorable prospecting area for exploration. CO2 gas derived from inorganic mantle sources and CO2 accumulation times have been investigated. In this study, petrographic observation shows pure CH4, CH4-rich, H2O-rich and hybrid CH4-CO2 fluid inclusions exist in healed micro-fractures and overgrowths of quartz mineral in Palaeocene reservoirs of Lishui sag. The pressure–temperature–time–composition (PVT–x) properties of CH4-bearing inclusions were obtained by Raman quantitative analysis and thermodynamic models. These results provide important constraints on the P-T entrapment conditions and predict relative accumulation time of CH4 fluid. Combination of carbon isotope analyses, inclusion petrography, microthermometry, PT entrapment conditions of CH4-bearing inclusions, hydrocarbon generation and simulation of burial history permit to assign periods of CH4 fluid activity and its sequence with CO2 fluid. Methane charging time occurs during two stages in LS-A: low density CH4 fluid (~0.10 g/cm3 or lower) were trapped during late Oligocene (30-20 Ma) and high density (>0.115 g/cm3) during a Pliocene (8-1 Ma) CH4 accumulation event. CO2 fluid accumulation (Miocene, 21-4 Ma) displaced the methane gas trapped in the early stage. Pliocene CH4 fluid partial displaced Miocene CO2 fluid. In well B, only high density (0.144–0.153 g/cm3) recorded a late Oligocene to early Miocene (36-19 Ma) CH4 accumulation event, Pliocene (5-3 Ma) CO2 fluid displaced early CH4 fluid. Micro-thermometry, composition of fluid inclusion, features of reservoir, thermal maturity reflect hydrothermal fluid vertical migration and the existence of Supercritical CO2 extraction.

Prolonged Neoproterozoic high‐grade metamorphism of the Wanni Complex, Sri Lanka: New insights from petrology, phase equilibria modelling, and zircon U–Pb geochronology of partially melted cordierite gneiss from Walpita

We report new petrological and geochronological data of garnet‐cordierite (Grt‐Crd) gneiss as well as two types of leucocratic rocks (garnet‐bearing and cordierite‐bearing leucosomes) crystallized during prograde and near‐peak metamorphisms of the Wanni Complex (WC) in Sri Lanka, and investigate the pressure–temperature–time evolution of granulite‐facies metamorphism for unravelling the timing and tectonics of the Neoproterozoic collisional processes related to Gondwana amalgamation. Grt‐Crd gneiss, garnet‐biotite (Grt‐Bt) gneiss, garnet‐bearing leucocratic rock (Grt‐L), and cordierite‐bearing leucocratic rock (Crd‐L) discussed in this study were collected from Walpita in the margin of the WC along the boundary with the Highland Complex (HC). Grt‐L occurs as thin layers parallel to the foliation of matrix Grt‐Bt and Grt‐Crd gneisses. The occurrences of porphyroblastic garnet in quartzo‐feldspathic matrix and crystallised melt inclusions within the garnet suggest that the Grt‐L could have formed by partial melting of host Grt‐Bt gneiss during prograde to peak metamorphism. Crd‐L is coarse‐grained and massive, lacks obvious foliation, and crosscuts the foliation of host Grt‐Bt gneiss. These observations suggest that the Crd‐L could have intruded after the crystallization of the Grt‐L. Application of phase equilibria modelling in the system NCKFMASHTO for the rocks yielded three discrete thermal events: 600–800°C/4.0–6.0 kbar (Stage 1 from Grt‐L), 700–840°C/5.5–7.0 kbar (Stage 2 from Grt‐Crd gneiss), and 840–880°C/4.5–5.5 kbar (Stage 3 from Crd‐L), suggesting a clockwise P–T evolution. LA‐ICP‐MS zircon U–Pb dating of the leucosomes yielded several metamorphic events: &gt;577 Ma (partial melting of Grt‐L), ~562 Ma (crystallization of Grt‐L), 537 ± 14 Ma (peak metamorphism and crystallization of Crd‐L), and 507–503 Ma (post‐peak cooling event). The results of this study therefore confirmed prolonged (&gt;70 Myr) high‐grade metamorphism of the WC, although the duration might be shorter than that of the HC (&gt;100 Ma). The continuous heat supply necessary for such long‐lived high‐grade metamorphism is explained by a recent tectonic model of Sri Lanka. The model suggests double‐sided subduction and continent‐continent collision, which could have given rise to subsequent slab delamination and differential heating of lower to middle crust. Such complex subduction–collision processes possibly controlled the difference in the style of P–T path and the duration of high‐grade metamorphism between the HC and the WC.

Melt Segregation and Depletion During Ascent of Buoyant Diapirs in Subduction Zones

AbstractCold, low‐density diapirs arising from hydrated mantle and/or subducted sediments on the top of subducting slabs have been invoked to transport key chemical signatures to the source region of arc magmas. However, to date there have been few quantitative models to constrain melting in such diapirs. Here we use a two‐phase Darcy‐Stokes‐energy model to investigate thermal evolution, melting, and depletion in a buoyant sediment diapir ascending through the mantle wedge. Using a simplified 2‐D circular geometry, we investigate diapir evolution in three scenarios with increasing complexity. In the first two scenarios we consider instantaneous heating of a diapir by thermal diffusion with and without the effect of the latent heat of melting. Then, these simplified calculations are compared to numerical simulations that include melting, melt segregation, and the influence of depletion on the sediment solidus along pressure‐temperature‐time (P‐T‐t) paths appropriate for ascent through the mantle wedge. The high boundary temperature induces a rim of high porosity, into which new melts are focused and then migrate upward. The rim thus acts like an annulus melt channel, while the effect of depletion buffers additional melt production. Solid matrix flow combined with recrystallization of melt pooled near the top of the diapir can result in large gradients in depletion across the diapir. These large depletion gradients can either be preserved if the diapir leaks melt during ascent, or rehomogenized in a sealed diapir. Overall our numerical simulations predict less melt production than the simplified thermal diffusion calculations. Specifically, we show that diapirs whose ascent paths favor melting beneath the volcanic arc will undergo no more than ~40–50% total melting.

Metamorphic Indicators for Collision, Extension, and Shear Zone Geodynamic Settings of the Earth’s Crust

An effective method for identifying the tectonic-magmatic causes of metamorphism is the reconstruction and analysis of P–T–t paths, which can be used to develop a model of rock evolution with time in the “pressure–temperature–time” coordinates. This approach is most widely used in interpreting the development of orogens with a polycyclic history, where different types of metamorphism are combined in contrasting geodynamic settings. The progress in this direction in recent years is due to the improvement of the instrumental base, precision methods for the study of matter – in situ local microanalysis and dating of zonal metamorphic minerals, and the computing apparatus of geothermobarometric procedures using multi-equilibria. The article discusses these issues on the example of geological complexes of various geodynamic nature, which are typical for different types of metamorphism. Reaction microtextures, chemical zoning in minerals, geochronological dating as well as the trajectory of the Р-Т paths provide evidence for the polymetamorphic history. A generalized P-T diagram was built using data on well-studied metamorphic complexes, which are characterized by the simultaneous presence of prograde and retrograde segments of a P-T path. Diagnostic P-T-t paths characteristic of rocks formed in zones of active tectogenesis—under collision, extension and in shear zones of the earth’s crust—have been established. This paper discusses some of the ambiguous interpretations of P-T-t paths in areas with simultaneous manifestation of different metamorphic processes, which requires further research.

Late Orogenic Heating of (Ultra)High Pressure Rocks: Slab Rollback vs. Slab Breakoff

Some (ultra)high-pressure metamorphic rocks that formed during continental collision preserve relict minerals, indicating a two-stage evolution: first, subduction to mantle depths and exhumation to the lower-crustal level (with simultaneous cooling), followed by intensive heating that can be characterized by a β-shaped pressure–temperature–time (P–T–t) path. Based on a two-dimensional (2D) coupled petrological–thermomechanical tectono-magmatic numerical model, we propose a possible sequence of tectonic stages that could lead to these overprinting metamorphic events along an orogenic β-shaped P–T–t path: the subduction and exhumation of continental crust, followed by slab retreat that leads to extension and subsequent asthenospheric upwelling. During the last stage, the exhumed crustal material at the crust–mantle boundary undergoes heating from the underlying hot asthenospheric mantle. This slab rollback scenario is further compared numerically with the classical continental collision scenario associated with slab breakoff, which is often used to explain the late heating impulse in the collisional orogens. The mantle upwelling occurring in the experiments with slab breakoff, which is responsible for the heating of the exhumed crustal material, is not related to the slab breakoff but can be caused either by slab bending before slab breakoff or by post-breakoff exhumation of the subducted crust. Our numerical modeling predictions align well with a variety of orogenic P–T–t paths that have been reported from many Phanerozoic collisional orogens, such as the Variscan Bohemian Massif, the Triassic Dabie Shan, the Cenozoic Northwest Himalaya, and some metamorphic complexes in the Alps.

Metamorphism and geochronology of garnet amphibolite from the Beishan Orogen, southern Central Asian Orogenic Belt: Constraints from P-T path and zircon U-Pb dating

Numerous lenses of garnet amphibolite occur in the garnet-bearing biotite-plagioclase gneiss belt in the Baishan area of the Beishan Orogen, which connects the Tianshan Orogen to the west and the Mongolia-Xing’anling Orogen to the east. The study of metamorphism in Beishan area is of great significance to explain the tectonic evolution of Beishan orogen. According to the microstructures, mineral relationships, and geothermobarometry, we identified four stages of mineral assemblages from the garnet amphibolite sample: (1) a pre-peak stage, which is recorded by the cores of garnet together with core-inclusions of plagioclase (Pl1); (2) a peak stage, which is recorded by the mantles of garnet together with mantle-inclusions of plagioclase (Pl2) ​+ ​amphibole (Amp1) ​+ ​Ilmenite (Ilm1) ​+ ​biotite (Bt1), developed at temperature-pressure (P-T) conditions of 818.9–836.5 ​°C and 7.3–9.2 ​kbar; (3) a retrograde stage, which is recorded by garnet rims ​+ ​plagioclase (Pl3) ​+ ​amphibole (Amp2) ​+ ​orthopyroxene (Opx1) ​+ ​biotite (Bt2) ​+ ​Ilmenite (Ilm2), developed at P-T conditions of 796.1–836.9 ​°C and 5.6–7.5 ​kbar; (4) a symplectitic stage, which is recorded by plagioclase (Pl4) ​+ ​orthopyroxene (Opx2) ​+ ​amphibole (Amp3) ​+ ​biotite (Bt3) symplectites, developed at P-T conditions of 732 ​± ​59.6 ​°C and 6.1 ​± ​0.6 ​kbar. Moreover, the U-Pb dating of the Beishan garnet amphibolite indicates an age of 301.9 ​± ​4.7 ​Ma for the protolith and 281.4 ​± ​8.5 ​Ma for the peak metamorphic age. Therefore, the mineral assemblage, P-T conditions, and zircon U-Pb ages of the Beishan garnet amphibolite define a near-isothermal decompression of a clockwise P-T-t (Pressure-Temperature-time) path, indicating the presence of over thickened continental crust in the Huaniushan arc until the Early Permian, then the southern Beishan area underwent a process of thinning of the continental crust.

Metamorphic pressure variation in a coherent Alpine nappe challenges lithostatic pressure paradigm

Pressure–temperature–time paths obtained from minerals in metamorphic rocks allow the reconstruction of the geodynamic evolution of mountain ranges under the assumption that rock pressure is lithostatic. This lithostatic pressure paradigm enables converting the metamorphic pressure directly into the rock’s burial depth and, hence, quantifying the rock’s burial and exhumation history. In the coherent Monte Rosa tectonic unit, Western Alps, considerably different metamorphic pressures are determined in adjacent rocks. Here we show with field and microstructural observations, phase petrology and geochemistry that these pressure differences cannot be explained by tectonic mixing, retrogression of high-pressure minerals, or lack of equilibration of mineral assemblages. We propose that the determined pressure difference of 0.8 ± 0.3 GPa is due to deviation from lithostatic pressure. We show with two analytical solutions for compression- and reaction-induced stress in mechanically heterogeneous rock that such pressure differences are mechanically feasible, supporting our interpretation of significant outcrop-scale pressure gradients.