Metamorphic Pressure Temperature Research Articles

Tectono-metamorphic evolution of paragneiss in the Chengde metamorphic Complex, northern Paleoproterozoic Trans-North China Orogen

Metasedimentary rocks can provide important information for reconstructing tectonic evolution of an orogen. In this study, we report the newly identified metapelite and metagraywacke samples from the Chengde Metamorphic Complex, the Trans-North China Orogen in the North China Craton. The garnet-bearing paragneiss generally occurs as country rocks of mafic granulite and amphibolite, and displays compositional layering with distinct gneissosity. The metapelite mainly consists of garnet, sillimanite, biotite, muscovite, plagioclase, and quartz, while the metagraywacke commonly comprises garnet, plagioclase, K-feldspar, quartz, and biotite/chlorite. Application of conventional geothermobarometers and phase equilibrium modelling constrains diverse peak metamorphic pressure–temperature (P-T) conditions (CD35, ∼800 ℃/10.5 kbar determined by phase equilibrium modelling; CD37, 677−716 ℃/9.6−9.8 kbar determined by geothermobarometers; CD45, 665−691℃/9.1−9.6 kbar and ∼ 745 ℃/11.2 kbar determined by geothermobarometers and phase equilibrium modelling respectively). The metapelite sample CD35 also records two retrograde stages, defining a clockwise P-T path with isothermal decompression (ITD) process. U-Pb dating of zircon and monazite yields metamorphic ages of 1910−1796 Ma, implying the timing of subduction to the lower crust. Combining previous studies in the Chengde Metamorphic Complex, we conclude that this complex is composed of imbricate rocks with different peak metamorphic P-T conditions, metamorphic grades, metamorphic ages, protolith origins, or protolith ages, indicating that these rocks were subducted to different depths in the subduction channel and then exhumed together to shallower crustal levels.

Neoarchean clockwise P-T trajectories with evolution sequence from HP to UHT metamorphism during decompression: Evidence from petrology, mineralogy, and phase equilibria modeling of mafic granulites in the Sittampundi Complex, southern India

The dispute of tectonic environments such as subduction-collision setting versus extension tectonic setting, is a common phenomenon in the Precambrian. Despite the ongoing controversy, the pressure–temperature (P–T) paths recorded by metamorphic rocks are often used as an indicator for distinguishing between subduction-collision and extension tectonic settings in the early Precambrian. It is widely acknowledged that high-pressure (HP) granulites may have formed in crustal-thickening-dominated collision/subduction environment. Conversely, ultra-high temperature (UHT) granulites may have formed in the elevated mantle heat flux environment. The Sittampundi Complex (SC), where HP granulites and UHT granulites coexist, plays a crucial role in unraveling the late-Neoarchean to early-Paleoproterozoic tectonic evolution of northern South Granulite-facies Terrane (NSGT) of India. In this study, we present new P–T paths from three HP mafic granulites overprinted by UHT granulite facies metamorphism in the SC and provide more data to unravel metamorphic evolution of NSGT. Phase equilibria modeling and traditional thermobarometry calculations illustrated that clockwise metamorphic pressure–temperature (P–T) paths were yielded, passing from 3.3 to 9.3 kbar/631–669 °C (prograde stage) through 11.9–18.5 kbar/828–986 °C (pressure-peak stage), and through 6.6–11.1 kbar/1019–1085 °C (temperature-peak stage) to 4.4–8.1 kbar/845–930 °C (retrograde stage). The HP mafic granulites exhibit clockwise P–T paths with a considerable heating process (ca. 150–180 °C) from the pressure-peak stage (HP-granulite sub-facies) to the temperature-peak stage (UHT-granulite sub-facies). Zircon U-Pb dating yields an age of 2513 ± 29 indicates that the metamorphism took place in the Neoarchean. The uncommon P–T paths from Neoarchean HP granulite in the NSGT suggest that during late Neoarchean, the NSGT had experienced the crustal-thickening dominated collision/subduction (assembly of micro-blocks of Dharwar Craton) and subsequently mantle diapirism process.

Paleoarchean metamorphism in the Acasta Gneiss Complex: Constraints from phase equilibrium modelling and in situ garnet Lu–Hf geochronology

AbstractThe oldest known evolved (felsic) rocks on Earth (c. 4.03 Ga) are found in the Acasta Gneiss Complex (AGC) in north‐western Canada and represent a fundamental keystone in unravelling the geological processes governing crustal growth and differentiation during the Hadean and early Archean. Although the timing of multiple episodes of magmatism, metamorphism and deformation in these tonalitic gneisses has been investigated extensively, the metamorphic pressure–temperature (P–T) conditions recorded by the rocks are poorly constrained. Here, we use phase equilibrium modelling coupled with in situ garnet Lu–Hf geochronology and trace element analysis for two garnet‐bearing tonalitic gneisses to decipher the metamorphic history of the AGC. The observed mineral assemblages are consistent with peak metamorphic conditions of T = 725–780°C and P = 4.5–6.2 kbar and the generation of a small amount of melt (<7 vol.%). Garnet geochronology constrains the age of metamorphism to 3.3–3.2 Ga, consistent with previous evidence for a late Paleoarchean tectono‐metamorphic event in the AGC. Subsequent isotopic disturbance of garnet at c. 1.9 Ga is interpreted to correspond to a modification of the primary Lu–Hf systematics in response to garnet resorption/recrystallization during the Paleoproterozoic Wopmay orogeny, resulting in significant scatter between these two age components. Our study adds to the small number of published P–T data for metamorphic rocks older than 2.8 Ga and shows that tonalitic gneisses in the AGC record a high apparent thermal gradient of ~140°C/kbar in the late Paleoarchean. This thermal gradient is the highest among the limited dataset, but is broadly similar to data from other Paleoarchean‐Mesoarchean crustal rocks in recording high T/P ratios (>77.5°C/kbar).

Early Neoproterozoic magmatism and Caledonian metamorphism recorded by the Mårma terrane, Seve Nappe Complex, northern Swedish Caledonides

Petrology, geochronology and bulk-rock chemistry are combined to investigate the early Neoproterozoic magmatism and Cambrian–Ordovician metamorphism in the northern Swedish Caledonides. This work includes several lithologies of the Mårma terrane in the Seve Nappe Complex exposed in the Kebnekaise region. U–Pb zircon geochronology yielded crystallization ages of 835 ± 8 Ma for a mylonitic orthogneiss, 864 ± 3 Ma for the Vistas Granite and 840 ± 7 Ma for an intruded granitic dyke, whereas a gabbro and a granodiorite intrusion gave U–Pb zircon crystallization ages of 856 ± 3 Ma and 850 ± 1 Ma, respectively. U–Pb monazite dating of the mylonitic orthogneiss gave an upper intercept age of 841 ± 7 Ma and a lower intercept age of 443 ± 20 Ma. Pressure–temperature estimates derived for the mylonitic orthogneiss reveal metamorphic peak-pressure and peak-temperature of 10.5–11.75 kbar and 550–610°C and 7.4–8.1 kbar at 615–675°C, respectively. Metamorphic pressure–temperature estimates for the Vistas Granite yield 6.5–7.5 kbar at 600–625°C. Whole-rock chemistry coupled with U–Pb geochronology indicates that bimodal magmatism was related to attempted rifting of Rodinia between 870 and 840 Ma. Supplementary material: Supplementary figures and tables are available at https://doi.org/10.6084/m9.figshare.c.6675075 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle

分光学と地質学の融合：光を通じて視る地球内部環境

Spectroscopy has been widely used in geology since the 1990s because it is non-destructive and easy to analyze. Raman spectroscopy has generally been used to identify mineral phases in geology, but recent studies have proposed new methods to quantitatively estimate the metamorphic pressure (quartz Raman barometry) and peak temperature (Raman carbonaceous material geothermometry). Studies using infrared spectroscopy are also underway to advance our understanding of hydration processes in subduction zones and mantle through the analysis of water in rocks. In addition to the development of new methods using spectroscopy, new tools are also being developed to analyze huge amounts of data through iterative processing, which will enable the extraction of more informative and quantitative results in a shorter time. This paper introduces examples of spectroscopy applications in geology and examines future developments.

P–T–t–D records of Early Palaeozoic Andean‐type shortening of a hot active margin: The Dunhuang block in NW China

AbstractHigh‐pressure (HP) granulites form either in the domain of the subducted plate during continental collision or in supra‐subduction systems where the thermally softened upper plate is shortened and thickened. Such a discrepancy in tectonic setting can be evaluated by metamorphic pressure–temperature–time‐deformation (P–T–t–D) paths. In the current study, P–T–t–D paths of Early Palaeozoic HP granulite facies rocks, in the form of metabasic lenses enclosed in migmatitic metapelite, from the Dunhuang block, NW China, are investigated in order to constrain the nature of the HP rocks and shed light on the geodynamic evolution of a modern hot orogenic system in an active margin setting. The rocks show a polyphase evolution characterized by (1) relics of horizontal or gently dipping fabric (S1) preserved in cores of granulite lenses and in garnet porphyroblasts, (2) a N‐S trending sub‐vertical fabric (S2) preserved in low‐strain domains and (3) upright folds (F3) associated with a ubiquitous steep E‐W striking axial planar foliation (S3). Garnet in the granulites preserves relics of a prograde mineral assemblage M1a equilibrated at ~11.5 kbar and ~770–780°C, whereas the matrix granulite assemblage (M1b) from the S1 fabric attained peak pressure at ~13.5 kbar and ~850°C. The granulites were overprinted at ~8–11 kbar and ~850–900°C during crustal melting (M2) followed by partial re‐equilibration (M3) at ~8 kbar and ~625°C. A garnet Lu–Hf age of 421.6 ± 1.2 Ma dates metamorphism M1, while a garnet Sm–Nd age of 385.3 ± 4.0 Ma reflects M3 cooling of the granulites. The mineral assemblage, M1, of the host migmatitic metapelite formed at ~9–12.5 kbar and ~760–810°C, partial melting and migmatization (M2) occurred at ~7 kbar and ~760°C and re‐equilibration (M3) at ~5–6 kbar and ~675°C. A garnet Lu–Hf age of 409.7 ± 2.3 Ma dates thermal climax (M2) and a garnet Sm–Nd age of 356 ± 11 Ma constrains M3 for the migmatitic metapelites. The timing of this late phase is also bracketed by an emplacement age of syntectonic granite dated at c. 360 Ma. Decoupling of M1 and M2 P–T evolutions between the mafic granulites and migmatitic metapelites indicates their different positions in the crustal column, while the shared pressure–temperature (P–T) evolution M3 suggests formation of a mélange‐like association during the late stages of orogeny. The high‐pressure event D1‐M1 is interpreted as a result of Late Silurian–Early Devonian moderate crustal thickening of a thermally softened and thinned pre‐orogenic crust. The high‐temperature (HT) re‐equilibration D2‐M2 is interpreted as a result of Mid‐Devonian shortening of the previously thickened crust, possibly due to ‘Andean‐type’ underthrusting. The D3‐M3 event reflects Late Devonian supra‐subduction shortening and continuous erosion of the sub‐crustal lithosphere. This tectono‐metamorphic sequence of events is explained by polyphased Andean‐type deformation of a ‘Cascadia‐type’ active margin, which corresponds to a supra‐subduction tectonic switching paradigm.

Secular changes in metamorphism and metamorphic cooling rates track the evolving plate-tectonic regime on Earth

When disturbed, dynamic emergent systems, such as tectonics on Earth, may transition from one stable state to another if the perturbation is sufficiently large. Here, we identify such state shifts through examination of statistically significant change points in the time series of metamorphic pressure–temperature and cooling rate data. Change points occur in the mid-Paleoproterozoic, the Mesoproterozoic, the early and late Paleozoic and the Cenozoic. To compare the timing of change points with mantle geodynamics, we interrogate the time series of calculated mantle potential temperature, which yields a statistically significant drop of >130°C in the mid-Paleoproterozoic, indicating a change in the mechanism of mantle cooling. We interpret changes in the mid-Paleoproterozoic to relate to the global emergence of stable subduction, orogenesis associated with the formation of the Nuna megacontinent, and slab breakoff, consistent with the operation of a distinctive style of Proterozoic plate tectonics. By contrast, changes from the dawn of the Cambrian onward may relate to the large volume of sediments supplied to trenches following multiple Cryogenian and Paleozoic glaciations. Sediments acted as a lubricant, allowing deeper subduction, transport of lower continental plates to mantle depths and faster metamorphic cooling rates, features characteristic of modern plate tectonics. Supplementary material: A dataset of metamorphic cooling rates is available at https://doi.org/10.6084/m9.figshare.c.5943418

Diachronous Redistribution of Hf and Nd Isotopes at the Crystal Scale—Consequences for the Isotopic Evolution of a Poly-Metamorphic Crustal Terrane

In metamorphic rocks, mineral species react over a range of pressure–temperature conditions that do not necessarily overlap. Mineral equilibration can occur at varied points along the metamorphic pressure–temperature (PT) path, and thus at different times. The sole or dominant use of zircon isotopic compositions to constrain the evolution of metamorphic rocks might then inadvertently skew geological interpretations towards one aspect or one moment of a rock’s history. Here, we present in-situ U–Pb/Sm–Nd isotope analyses of the apatite crystals extracted from two meta-igneous rocks exposed in the Saglek Block (North Atlantic craton, Canada), an Archean metamorphic terrane, with the aim of examining the various signatures and events that they record. The data are combined with published U–Pb/Hf/O isotope compositions of zircon extracted from the same hand-specimens. We found an offset of nearly ca. 1.5 Gyr between U-Pb ages derived from the oldest zircon cores and apatite U–Pb/Sm–Nd isotopic ages, and an offset of ca. 200 Ma between the youngest zircon metamorphic overgrowths and apatite. These differences in metamorphic ages recorded by zircon and apatite mean that the redistribution of Hf isotopes (largely hosted in zircon) and Nd isotopes (largely hosted in apatite within these rocks), were not synchronous at the hand-specimen scale (≤~0.001 m3). We propose that the diachronous redistribution of Hf and Nd isotopes and their parent isotopes was caused by the different PT conditions of growth equilibration between zircon and apatite during metamorphism. These findings document the latest metamorphic evolution of the Saglek Block, highlighting the role played by intra-crustal reworking during the late-Archean regional metamorphic event.

Clockwise P–T–t paths with considerable heating processes during decompression from high-pressure mafic granulites in the Wuhe Complex, southeastern North China Craton

The Wuhe Complex lies in the southern section of the Jiao–Liao–Ji mobile/orogenic belt (JLMB/JLJB), critical for understanding the Proterozoic tectonic evolution of the southeastern North China Craton (NCC). Detailed microstructural observations, mineral chemistry, phase equilibria modeling, and zircon and titanite chronological dating were conducted on the mafic granulites from the Wuhe Complex. Five stages of mineral assemblages were identified: the prograde stage (Pl1 + Cpx1 + Amp1 + Ilm1 + Ap1 ± Qz1 ± Kfs1 ± Ttn1 ± Py1), the pressure–peak stage (the core of matrix plagioclase and Grt2a + Cpx2a + Rt2a + Qz2a + Liq2a ± Amp2a ± Kfs2a ± Ttn2a), the temperature–peak stage (the rim of garnet porphyroblast and Pl2b + Cpx2b + Amp2b + Liq2b ± Ilm2b ± Qz2b ± Rt2b), the retrograde stage (Pl3 + Cpx3 + Opx3 + Amp3 + Ilm3 + Ap3 ± Grt3 ± Qz3 ± Scp3), and the cooling stage (Mag4 ± Chl4 ± Py4 ± Ccp4). Traditional thermobarometry calculations and phase equilibria modeling illustrated that clockwise metamorphic pressure–temperature (P–T) paths were retrieved, passing from 9.4 to 13.4 kbar/701 ℃–781 ℃ (prograde stage) through 16.3–17.2 kbar/807 ℃–846 ℃ (pressure–peak stage), and through 10.2–10.8 kbar/895 ℃–912 ℃ (temperature–peak stage) to 5.6–9.1 kbar/803 ℃–883 ℃ (retrograde stage). Hence, the high-pressure (HP) granulites show clockwise P–T paths with heating processes (∼80 ℃) from the lower crust (pressure–peak stage) to the middle-lower crust (temperature–peak stage). Therefore, HP mafic granulites might be buried to the depth of ∼ 57 km, with a slow exhumation rate. The metamorphic zircon and titanite grains from HP mafic granulites yield U–Pb ages of 1991–1758 Ma and 1832–1810 Ma, respectively. The 1832–1810 Ma might represent the retrograde time when HP mafic granulites cooled to 820 ℃–800 ℃. Therefore, the HP mafic granulites of the Wuhe Complex record the Late Paleoproterozoic subduction/collision orogenic event of the southeastern NCC; therefore, the JLMB/JLJB is a prolonged orogenic belt.

Pressure–temperature–time–deformation (P–T–t–d) path for Devonian forearc deposits in the Imjingang Belt, South Korea: Implications for Permian–Triassic collisional orogenesis on the eastern margin of Eurasia

AbstractThe Taean Formation on the central western margin of the Korean Peninsula contains layered metapelite, quartzo‐feldspathic schist, amphibolite, calcsilicate, and impure marble. The layering was formed by tectonic transposition of the original bedding. The formation has lithologic and isotopic age signatures similar to those of metasedimentary rocks of the Imjingang Belt, which is the eastern extension of the Qinling–Dabie–Sulu Belt, and the depositional age of the formation can be constrained to the Late Devonian. The metamorphic pressure–temperature (P–T) path for the formation is characterized by a high ΔP/ΔT prograde part and subsequent near‐isothermal decompression with peak P–T conditions of 600–620°C and 9.0–10.5 kbar, based on observations of mineral assemblages and phase equilibria modelling. Strong compressional deformation (Dn) produced a N–S‐trending Sn foliation that transposed the earlier Sn–1 and isoclinal Fn folds and was coeval with an episode of medium‐ to high‐pressure amphibolite‐facies metamorphism. The NNE–SSW‐trending left‐lateral Ryeseonggang Fault played a key role in producing the N–S distribution of the metamorphosed forearc deposits along the western coast of the Korean Peninsula. The peak metamorphism and deformation occurred during the early Permian to Early Triassic (276–250 Ma), with retrogression taking place during the Late Triassic (c. 230 Ma), as inferred from new U–Pb zircon age data and previous titanite and muscovite ages for the formation. The clockwise P–T–t (time)–d (deformation) path for the Taean Formation indicates rapid burial during Dn followed by rapid exhumation during the Permian–Triassic collision between the Qinling microcontinent and the North China Craton.

Clay mineralogy and geochemistry of pelitic rocks in the middle Permian Fanjiatun formation, East Changchun, China: implications for metamorphism and provenance

Metamorphic pressure–temperature conditions and sedimentary provenance were determined for the middle Permian Fanjiatun Formation in east Changchun, China, based on clay mineralogy, vitrinite reflectance and geochemical data. The pelitic rocks in the Fanjiatun Formation contain illite + kaolinite + chlorite ± mixed-layer chlorite/smectite and detrital quartz + plagioclase. Illite crystallinity in the formation varies from 0.38 to 0.49 (average = 0.44), and consists dominantly of the 2M1 and mixed 2M1 and 1Md polytypes. The illite b0 dimension is 8.9860–9.0130 A (average = 9.0010 A), indicating the Fanjiatun Formation experienced very-low-grade metamorphism at <1.5 kbar. The metamorphic temperatures obtained based on vitrinite reflectance are 191–308 °C, indicating the formation experienced anchizone-zone facies metamorphism. Al2O3/TiO2 ratios vary from 23.93 to 24.22 (average = 24.11), and the chemical index of alteration is 64–69 (average = 67). Rare earth element contents are high and exhibit enrichment in light rare earth elements and negative Eu anomalies. Trace element characteristics indicate that the source of the pelitic rocks was moderately weathered silicic rocks. The sediments were deposited in a reducing continental setting.

Ultrahigh‐temperature mafic granulites from the Madurai Block, southern India: Constraints from conventional thermobarometry, pseudosection analysis, and rare earth element‐based thermometry

The determination of metamorphic pressure–temperature (P–T) conditions of granulites provides us a natural window into the composition, structure, and P–T conditions of the lower crust and process of continental growth. Here, we constrain the P–T evolution of a suite of mafic granulite from the eastern part of Madurai Block. Integrated results from mineral reactions, conventional thermobarometry, and pseudosection analysis suggest that the studied mafic granulites, with a peak assemblage of coarse‐grained garnet + coarse‐grained clinopyroxene + plagioclase + quartz + [rutile], were metamorphosed under granulite facies conditions of 830 ± 50°C and 9.5 ± 1 kbar. Subsequently, the rocks underwent near isothermal decompression leading to the formation of symplectic assemblages of clinopyroxene + orthopyroxene + plagioclase and orthopyroxene + ilmenite, within a P–T range of 9.0–5.5 kbar and 750–800°C. The estimated peak temperature conditions are somewhat lower than those obtained using characteristic ultra‐high temperature (UHT) mineral assemblages and non‐conventional thermometers (~1,000°C at ~10 kbar) from the co‐metamorphosed metapelitic granulites of the Madurai Block. This may be attributed to the high diffusion rate of divalent Fe and Mg, which often results in retrograde re‐equilibration leading to considerable underestimation of peak temperatures in mafic granulites. To overcome this problem, we have additionally determined peak metamorphic temperatures of the studied rocks using a recently developed garnet–clinopyroxene rare earth element (REE)‐based thermometer that takes advantage of much lower diffusion rate of trivalent REEs. Using the REE thermometer, we have obtained a peak temperature of 1,050 ± 50°C for the studied rocks, which is in reasonable agreement with the peak metamorphic temperature obtained from previous studies. The constrained P–T conditions and the clockwise P–T path suggest that the studied rocks were buried to lower crustal depths at UHT conditions in a convergent margin setting. The near isothermal decompression suggests rapid exhumation, most likely due to an extensional event. The peak UHT metamorphic conditions could have been attained at the core of a long‐lived orogenic plateau, as suggested by other studies.

Ultra-High Pressure Metamorphism and Geochronology of Garnet Clinopyroxenite in the Paleozoic Dunhuang Orogenic Belt, Northwestern China

Ultra-high pressure (UHP) metamorphism is recorded by garnet clinopyroxenite enclaves enclosed in an undeformed, unmetamorphosed granitic pluton, northeastern Paleozoic Dunhuang orogenic belt, northwestern China. The protoliths of the garnet clinopyroxenite might be basic or ultrabasic volcanic rocks. Three to four stages of metamorphic mineral assemblages have been found in the garnet clinopyroxenite, and clockwise metamorphic pressure–temperature (P-T) paths were retrieved, indicative of metamorphism in a subduction environment. Peak metamorphic P-T conditions (790–920 °C/28–41 kbar) of garnet clinopyroxenite suggest they experienced UHP metamorphism in the coesite- or diamond-stability field. The UHP metamorphic event is also confirmed by the occurrence of high-Al titanite enclosed in the garnet, along with at least three groups of aligned rutile lamellae exsolved from the garnet. Secondary ion mass spectrometry (SIMS) U-Pb dating of metamorphic titanite indicates that the post-peak, subsequent tectonic exhumation of the UHP rocks occurred in the Devonian period (~389–370 Ma). These data suggest that part of the Paleozoic Dunhuang orogenic belt experienced UHP metamorphism, and diverse metamorphic facies series prevailed in this Paleozoic orogen. It can be further inferred that most of the UHP rocks in this orogen remain buried.

Clockwise hairpin‐type metamorphic pressure–temperature (P–T) path recorded in the Shangla blueschist along the Indus Suture Zone, Pakistan Himalaya

The Shangla blueschist, exposed along the Indus Suture Zone, Pakistan Himalaya, have been petrologically investigated in order to elucidate their tectono‐metamorphic evolution, and to better understand the geodynamic scenario of intra‐oceanic subduction between the Indian Plate and Kohistan‐Ladakh Arc. Representative mineral assemblages identified from the Shangla blueschist, contain amphiboles (glaucophane, magnesioriebeckite, winchite, actinolite, edenite, and magnesiohornblende), epidote, phengite, albite, chlorite, titanite, rutile, quartz, and haematite. Three distinct metamorphic stages are recognized for the Shangla blueschist. An early prograde (pre‐peak) metamorphic stage (M1) is characterized by the presence of winchite as the core part of zoned amphiboles, together with inclusions of epidote, albite, chlorite, quartz, titanite, and haematite in epidote porphyroblasts. This stage roughly reveals a temperature range of 300–450°C and pressures of 0.4–0.7 GPa. The equilibrium minerals of peak (M2) metamorphic stage consist of matrix, schistosity‐forming minerals including amphiboles (glaucophane, magnesioriebeckite), epidote and phengite, with fine‐grained rutile, titanite, and haematite in albite porphyroblasts, suggesting P–T condition of 350–520°C and 0.9–1.2 GPa. The retrograde (M3) metamorphic stage involved overgrowth of winchite and actinolite along glaucophane rims, and chlorite replacement of amphibole as well as growth of albite porphyroblasts, representing the greenschist‐facies metamorphism. Our obtained peak P–T metamorphic condition (epidote‐blueschist facies) is significantly higher than previously estimated P–T ranges of 400°C/0.7 GPa. Moreover, the retrograde approximately reversed prograde trajectory defines a hairpin‐type P–T path for the Shangla blueschist, similar to the Franciscan‐type metamorphism that is characterized by relatively slow exhumation after peak stage with the decompression accompanied by cooling process. Taking the published geochronological data into account, these new results provide insights into the metamorphic evolution of the Shangla blueschist during ca. 80 Ma (Late Cretaceous) from burial by cold subduction of the Neo‐Tethys to subsequent post‐peak exhumation. Furthermore, the exhumation mechanism of the Shangla blueschist is most likely driven by a combined effect of both underplating and later thrust faulting, probably during the collision of the Indian Plate and Kohistan‐Ladakh Arc.

Metamorphic evolution and 40Ar/39Ar geochronology of the Wuguan complex, eastern Qinling area, China: Implications for the late Paleozoic tectonic evolution of the Qinling orogen

The late Paleozoic tectonic evolution of the Qinling orogen is poorly constrained. The Wuguan Complex, which lies between the North Qinling and South Qinling tectonic belts, represents a late Paleozoic metamorphic belt of the Qinling orogen. In this contribution, we report new metamorphic pressure–temperature (P–T) constraints and 40Ar/39Ar ages of garnet amphibolites and garnet-bearing metapelites from the Wuguan Complex. Textural observations and mineral compositions, especially garnets with two stages of growth, indicate the Wuguan Complex experienced two distinct metamorphic events. Conventional geothermobarometry and phase equilibria modelling indicate that the earlier metamorphism had peak P–T conditions of 660–725 °C and 8.0–12.0 kbar with a clockwise P–T path, whereas the later metamorphic overprint records P–T conditions of 560–610 °C and 7.5–9.0 kbar. Hornblendes from amphibolites yield 40Ar/39Ar ages of ~306–299 Ma, whereas muscovites from garnet-bearing metapelites yield 40Ar/39Ar ages of ~252–251 Ma. The two groups of 40Ar/39Ar ages correspond to the cooling time of the two distinct metamorphic events, respectively. Combining with previously published zircon UPb ages from the Wuguan Complex and two groups of metamorphic ages from the Guishan Complex and the Xiongdian eclogite belt, we infer that the earlier metamorphism probably occurred at the hangingwall of a subduction zone during Carboniferous subduction of oceanic crust. The later metamorphic overprint might have been related to the initial collision between the Sino-Korean and Yangtze cratons during the late Permain.

The absence of high-pressure metamorphism in the inverted Barrovian metamorphic sequences of the Arun area, eastern Nepal and its tectonic implication

The metamorphic pressure–temperature (P–T) conditions across the Main central thrust (MCT) in the Arun area have been investigated. The MCT marks the tectono-metamorphic boundary between the overlying high-grade High Himalaya crystalline sequences (HHCS) and the underlying low-grade Lesser Himalaya sequences (LHS). The metamorphic rocks regionally preserve an inverted Barrovian sequence (i.e., intermediate P/T type metamorphism) devoid of previously reported high-pressure metamorphism. The metamorphic grade increases upwards from 670–740 °C and 6.9–9.4 kbar in the MCT zone and lower HHCS to 760–835 °C and 10.0–11.1 kbar in the middle HHCS. Orthoamphibole gneisses in the middle HHCS yield prograde Barrovian-type metamorphism, such as staurolite inclusions in garnets, showing an intermediate P/T gradient. The differences in the tectonic setting and metamorphic evolution imply that the metamorphic units in the Arun area do not correspond to the other high-pressure units in eastern Himalaya. Zircon and monazite U–Pb ages from kyanite gneiss of the lower HHCS reveal the MCT activity, associated with fluid-present anatexis, at ca. 20–14 Ma. Furthermore, similar K–Ar white mica ages (ca. 13–7 Ma) in the hanging wall and footwall of the MCT could represent the timing of later deformation events in shear zones or cooling, possibly associated with exhumation accompanied by activities on younger, structurally lower thrust faults such as the lower MCT. The similar P–T conditions near the MCT in this area could result from recrystallization during syn-metamorphic thrusting, whereas the middle HHCS away from the MCT preserve the original Barrovian metamorphic sequences related to crustal thickening. This and previous studies imply that different P–T profiles near the MCT according to each transect observed in Nepal could be apparent and the cumulative result of polyphase metamorphism.

Metamorphic Age Comparison and Its Implications between the Zuoquan and Zanhuang Complexes in the Central North China Craton, Based on LA-ICP-MS Zircon U–Pb Dating

The Trans-North China Orogen (TNCO) is well-known as an early Precambrian orogenic belt in the North China Craton (NCC). Three meaningful groups of metamorphic ages have been reported in the metamorphic complexes throughout the TNCO, including c. 1.85 Ga, c. 1.95 Ga, and c. 2.5 Ga. The spatial distributions and relationships of these ages provide notable insight into the formation timing and tectonic evolution of the NCC basement. The Zuoquan and Zanhuang complexes are exposed in the south–central TNCO and are adjacent to the Eastern Block. In order to place new constraints on the timing of two phases of metamorphism that occurred in the complexes, combined U–Pb and rare earth element analyses were performed on zircons from different types of metamorphic rocks. Uranium–Pb zircon dating in this study shows that two groups of metamorphic ages of 1.88–1.85 Ga and 2.48–2.46 Ga were commonly recorded by metamorphic rocks in the Zanhuang and Zuoquan complexes, respectively. Our previous geochronological studies showed that metamorphic ages of c. 2.51 Ga and c. 1.90 Ga were locally recorded in the Zanhuang and Zuoquan complexes, respectively. These data indicate that metamorphic rocks in the two complexes underwent at least two phases of metamorphism, i.e., 2.51–2.46 Ga (Phase I) and 1.90–1.85 Ga (Phase II). In combination with previous studies regarding reaction microstructures, metamorphic pressure–temperature paths, and geochronology, the Phase II metamorphic ages are interpreted to be linked to the collision between the Western and Eastern Blocks along the TNCO between 1.97 Ga and 1.80 Ga, whereas the Phase I metamorphic ages, as a result of an earlier and extensive tectono-thermal event that occurred in the Eastern and Western Blocks of the NCC, were related to underplating of mantle-derived magma. It is inferred that the rocks with c. 2.51–2.46 Ga metamorphic ages in the two complexes formed in the Eastern Block and underwent regional metamorphism during that period, and then were tectonically involved in the TNCO and experienced c. 1.90–1.85 Ga metamorphism. Metamorphic peaks occurred at different crustal levels in the orogen, resulting in distinct metamorphic ages and peak conditions preserved by metamorphic rocks in the two complexes.

About this title - HP–UHP Metamorphism and Tectonic Evolution of Orogenic Belts

High pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks play a key role in understanding the tectonic evolution of orogenic belts. They have typically experienced complex changes during subduction and exhumation processes arising from recrystallization, deformation, fluid–rock interactions and even partial melting, and may therefore carry valuable records of evolving geodynamic systems in an orogenic belt. This special publication addresses the current work on HP–UHP metamorphism and its relation to the tectonic evolution of orogenic belts. This special publication contains fifteen papers covering the important orogenic belts of the Himalaya, Dabie–Sulu, Tian Shan, North Qaidam and others that have been grouped into three parts: (I) new developments in the determination of metamorphic pressure–temperature (PT) conditions and their timing, (II) overview papers of well-known HP–UHP metamorphic belts and (III) research papers for some newly discovered HP–UHP belts.

Anticlockwise metamorphic pressure–temperature paths and nappe stacking in the Reisa Nappe Complex in the Scandinavian Caledonides, northern Norway: evidence for weakening of lower continental crust before and during continental collision

Abstract. This study investigates the tectonostratigraphy and metamorphic and tectonic evolution of the Caledonian Reisa Nappe Complex (RNC; from bottom to top: Vaddas, Kåfjord, and Nordmannvik nappes) in northern Troms, Norway. Structural data, phase equilibrium modelling, and U-Pb zircon and titanite geochronology are used to constrain the timing and pressure–temperature (P–T) conditions of deformation and metamorphism during nappe stacking that facilitated crustal thickening during continental collision. Five samples taken from different parts of the RNC reveal an anticlockwise P–T path attributed to the effects of early Silurian heating (D1) followed by thrusting (D2). At ca. 439 Ma during D1 the Nordmannvik Nappe reached the highest metamorphic conditions at ca. 780 ∘C and ∼9–11 kbar inducing kyanite-grade partial melting. At the same time the Kåfjord Nappe was at higher, colder, levels of the crust ca. 600 ∘C, 6–7 kbar and the Vaddas Nappe was intruded by gabbro at &gt; 650 ∘C and ca. 6–9 kbar. The subsequent D2 shearing occurred at increasing pressure and decreasing temperatures ca. 700 ∘C and 9–11 kbar in the partially molten Nordmannvik Nappe, ca. 600 ∘C and 9–10 kbar in the Kåfjord Nappe, and ca. 640 ∘C and 12–13 kbar in the Vaddas Nappe. Multistage titanite growth in the Nordmannvik Nappe records this evolution through D1 and D2 between ca. 440 and 427 Ma, while titanite growth along the lower RNC boundary records D2 shearing at 432±6 Ma. It emerges that early Silurian heating (ca. 440 Ma) probably resulted from large-scale magma underplating and initiated partial melting that weakened the lower crust, which facilitated dismembering of the crust into individual thrust slices (nappe units). This tectonic style contrasts with subduction of mechanically strong continental crust to great depths as seen in, for example, the Western Gneiss Region further south.

Metamorphic pressure–temperature conditions of the Lützow–Holm Complex of East Antarctica deduced from Zr–in–rutile geothermometer and Al&lt;sub&gt;2&lt;/sub&gt;SiO&lt;sub&gt;5&lt;/sub&gt; minerals enclosed in garnet

The metamorphic pressure–temperature (P–T) conditions of high–grade pelitic gneisses from Akarui Point, Skarvsnes, Skallen, and Rundvågshetta in the Lützow–Holm Complex (LHC), East Antarctica are re–examined by applying the Zr–in–rutile geothermometer to rutile inclusions in garnet enclosing Al2SiO5 minerals and to matrix rutile grains. By utilizing the P zoning of garnet to indicate isochronous surface, samples from Akarui Point, Skarvsnes, and Skallen were shown to have experienced almost the same P–T conditions around the kyanite/sillimanite transition boundary (~ 830–850 °C/~ 11 kbar). From Rundvågshetta, higher–T condition (850 ± 15 °C/0.1 kbar to 927 ± 16 °C/12.5 kbar) was confirmed from rutile inclusions in garnet enclosing sillimanite. Matrix rutile yielded similar temperature as inclusion rutile for Akarui Point, Skarvsnes, and Skallen samples. Therefore, the traditional metamorphic zone mapping based on matrix mineral assemblages, which classified Akarui Point as belonging to the transitional zone between the upper–amphibolite and the granulite facies zones, does not reflect the highest metamorphic conditions attained. The P–T–t evolution of the LHC needs to be re–evaluated utilizing detailed petrochronological approaches.