Prograde Metamorphic Stage Research Articles

Phase Equilibria Modeling and Zircon Dating for Precambrian Metapelites from the Xinghuadukou Complex in the Lulin Forest of the Erguna Massif, Northeast China

The nature and formation time of the Xinghuadukou complex in Northeast China are important for determining the tectonic evolution of the Precambrian geological evolution of the Erguna massif. In this study, we present the results of zircon U-Pb dating of two metapelites from the complex. Detrital and metamorphic zircons from the metapelites yield a depositional age of ~601 Ma and a metamorphic age of 496–509 Ma, indicating that the supracrustal rocks formed during the Neoproterozoic and recorded pan-African metamorphic events in the Erguna massif. Garnet porphyroblasts in Sil-Grt-Bt-Ms paragneiss show diffusion zoning, implying a decreasing P-T trend. Based on mineral transformation and P-T estimates using conventional geothermobarometers and phase equilibria modeling, three metamorphic stages were determined, including an early prograde metamorphic stage, a near peak upper amphibolite facies metamorphic stage, and a near-isobaric cooling stage with P-T conditions of 6.1 kb, 645 °C, 5–6 kb, 710–740 °C, and 4.4 kb, 625 °C, respectively. The metamorphic history of the Xinghuadukou complex is thus defined by a clockwise P-T trajectory, which implies that the metamorphism of the metapelites documented the subduction, subsequent uplift and post collision process.

Permo-Triassic high-pressure metamorphism in the central western Korean Peninsula, and its link to Paleo-Tethyan Ocean closure: Key issues revisited

Permo-Triassic high-pressure (HP) mafic granulites, together with the Bibong retrogressed eclogite, preserved along the central western Korean Peninsula provide important insights into the Late Permian to Triassic collisional orogeny in northeast Asia. The metamorphic pressure–temperature–time (P–T–t) paths of these rocks, however, remain poorly constrained and even overestimated, owing to outdated geothermobarometers and inaccurate isopleth techniques. Here we evaluate the metamorphic P–T conditions of Triassic HP mafic granulites including those in Baekdong, Sinri and Daepan and the Bibong Triassic retrogressed eclogite in the Hongseong area, and the Permo-Triassic Samgot mafic granulite in the Imjingang Belt of the central western Korean Peninsula through the application of modern phase equilibria techniques. The Baekdong and Samgot mafic granulites and the Bibong retrogressed eclogite yield a range of 12.0–16.0 kbar and 800–900 °C, representing HP granulite facies conditions. The Sinri and Daepan granulites from the Hongseong area show relatively lower grade metamorphic conditions between HP granulite and normal granulite facies, and are characterized by sub-isothermal decompression during exhumation. The similarities in the metamorphic ages and the post-collisional igneous activity from the central western Korean Peninsula indicate that the Triassic ages represent the retrograde stage of the metamorphic P–T paths. In contrast, the Late Permian metamorphic ages, which are older than protolith ages of the post-collisional igneous rocks, correspond to the possible prograde stage of metamorphism. The P–T–t paths presented in this paper, together with the metamorphic ages and post-orogenic igneous events reported from these areas suggest trace of the subduction, accretion and exhumation history, and indicate a tectonic linkage among the northeast Asian continents during the Paleo-Tethyan Ocean closure.

A mechanism for Nb incorporation in rutile and application of Zr-in-rutile thermometry: A case study from granulite facies paragneisses of the Mogok metamorphic belt, Myanmar

AbstractRutile grains occur extensively in host phases of biotite and quartz-feldspar aggregate in high-temperature paragneisses of the Mogok metamorphic belt of Myanmar. They occur as an isolated phase and sometimes show intergrowth texture with ilmenite. Most rutile grains contain up to 3.7 wt.% Nb2O5, which shows positive correlations with Fe and trivalent elements. Niobium substitutes for Ti by a coupled substitution with the trivalent cations (M3+) of Nb5+M3+Ti4+-2. Fine-grained rutile grains included in ilmenite are distinctly poor in Nb (<0.1 wt.% as Nb2O5) and contain Fe of 1.7–3.2 wt.% as Fe2O3, suggesting vacancybearing substitution of Fe3+4Ti4+-3□–1, where □ indicates a vacancy. The rutile grains in the felsic phases contain high Zr contents of up to 4200 ppm, suggesting equilibrium temperatures over 800°C using the Ti-in-rutile geothermometer. These high-temperature conditions are consistent with those estimated by conventional methods reported in the literature and suggest widespread occurrences of the upperamphibolite and granulite facies metamorphic rocks in the middle segment of the Mogok metamorphic belt. In contrast, the Zr contents of rutile grains in biotite are usually <1000 ppm, implying equilibrium temperatures lower than 750°C. Most of the rutile grains poorer in Zr might have been included in biotite and were isolated from the zircon-bearing system during an early stage of prograde metamorphism. Some other rutile grains poorer in Zr might have been an exsolved phase from Ti-rich biotite during retrograde metamorphism, which was furthered by the infiltration of metamorphic fluid under lower-amphibolite facies conditions.

Mid-Neoproterozoic amphibolite facies metamorphism at the northern margin of the Yangtze craton

The tectonic setting of the continental margin of the Yangtze craton during the Neoproterozoic has long been a subject of debate. The Douling Complex is a small segment of crystalline basement in the northern Yangtze craton; the study of which may provide useful insights into the tectonic evolution of this area. In this paper we report medium-grade regional metamorphism that occur in the Douling Complex. Petrographical textures and mineral compositions suggest that amphibolite facies rocks from the Douling Complex underwent a progressive metamorphic evolution from staurolite–kyanite grade to sillimanite grade. Conventional geothermobarometry defines P–T conditions to be 490–560 °C and 6–7.5 kbar for prograde metamorphic stage and 630–720 °C and 8–11 kbar for peak metamorphic stage. The post-peak evolution is characterized by decompression and thermal relaxation. Metamorphic zircons from garnet amphibolites and metapelites and magmatic zircons from a syn-deformed pegmatite pod yield U–Pb ages between 820 ± 5 Ma and 779 ± 3 Ma, with two populations at ca. 820–815 Ma and ca. 790–780 Ma, respectively. Biotites from metapelites also give two 40Ar/39Ar plateau ages of 814 ± 4 Ma and 790 ± 5 Ma. It is inferred that peak metamorphism occurred at ca. 820–815 Ma and subsequent cooling took place at ca. 790–780 Ma. Considering the distinct Precambrian crustal history of the Douling Complex, together with medium-grade amphibolite facies metamorphism accompanied by a clockwise P–T path, we propose that these observations provide evidence for the collision of the small Douling block with the northern margin of the Yangtze craton during the Mid-Neoproterozoic. Immediately after this collision, a newly accreted Andean-type active continental margin may have formed along the northern Yangtze craton.

Paleoproterozoic high-pressure-high-temperature pelitic granulites from Datong in the North China Craton and their geological implications: Constraints from petrology and phase equilibrium modeling

Paleoproterozoic granulite-facies metapelites are widespread in northern part of the North China Craton (NCC), and their metamorphic conditions and P-T paths provide important constraints for tectonic evolution of the NCC. However, it is still controversial on the metamorphic evolution of these rocks due to variations of retrograde modification. In combination with detailed petrological studies, geological mapping, and geochronology, we here present new evidence from Gushan, Datong, conjunction zone of the Ji’ning Complex and Huai’an Complex to constrain P-T evolution of the metapelites. Field investigations reveal that the pelitic granulites are spatially associated with high-pressure (HP) mafic granulites, showing concordant characteristics of deformation and metamorphism. Fine mineralogical and microstructural studies reveal that kyanite-bearing HP granulite-facies indicators preserved in garnet porphyroblasts of the pelitic granulites. It suggests the previously assumed moderate- to low-pressure (MP-LP) and high- to ultrahigh-temperature (HT-UHT) pelitic granulites have experienced HP granulite-facies metamorphism.Four metamorphic generation assemblages (M1–M4) are recognized in the studied samples. The pre-Pmax assemblages (M1) are characterized by mineral inclusions within garnet core, including Bt-Qz-Rt±Ms±Liq, representing the late stage of prograde metamorphism. The Pmax assemblages (M2, HP granulite-facies) are represented by garnet mantle and its inclusion-type minerals, i.e., Grt-Ky-Kfs-Bt-Rt-Liq-Qz±Ms±Zn Spl. It is followed by heating until the maximum of temperature stage (M3, Tmax), featured by disappearance of biotite and transition from kyanite into sillimanite, with mineral assemblages of Grt-Sil-Kfs-Liq-Qz-Ilm±Zn Spl. The cooling stage (M4) is featured by breakdown of garnet to Bt+Qz±Pl±Sil±Ilm. Phase equilibrium modeling and titanium-in-biotite thermometer are combined to constrain the P-T evolution of the studied samples. The P-T conditions for the M1–M4 are respectively estimated around ∼11–11.5kbar/∼770–790°C, ∼10.3–13.8kbar/∼820–835°C, ∼6.5–9.5kbar/∼850–925°C, ∼6kbar/∼645–761°C, which defined a clockwise P-T path. Zircon U-Pb ages of two representative samples demonstrated comparable age clusters of ∼1950Ma and ∼1850Ma, which are interpreted to represent the timing of HP and MP-LP granulite-facies stages, respectively. Combining with other studies of the NCC, we propose that the pelitic granulites and associated HP mafic granulites have experienced the common metamorphic processes from peak stage (HP-HT granulite-facies) to retrograde metamorphism. Due to intensive and pervasive overprint of sillimanite-type granulite-facies metamorphism, the HP granulite-facies mineral assemblages of the pelitic granulites are partially or completely reworked. The metamorphic peak conditions and relatively high thermal gradients of these granulite-facies rocks suggest that their formation is probably in response to a crustal-scale subduction-collision process during the late Paleoproterozoic.

The Schlema–Alberoda five-element uranium deposit, Germany: An example of self-organizing hydrothermal system

As a result of integrating geological, mineralogical, and geochemical data on the unique Schlema–Alberoda five-element uranium deposit situated in Federal Republic of Germany and explored in detail down to a depth of 2 km, it has been shown that its formation for more than 100 Ma has been caused by combination of internal and external factors. The latter comprise favorable metallogenic specialization of the region, injection of intrusive bodies bearing the necessary stock of energy, and periodic pulses of tectonic reactivation. The internal factors of self-development involve evolutionary processes, which occur in host rocks at the consecutive stages of prograde and retrograde metamorphism giving rise to alteration of rocks in consistence with physical and chemical laws at variable temperature and degree of system opening.

Constraints from monazite and xenotime growth modelling in the MnCKFMASH‐PYCe system on the P–T path of a metapelite from Shillong‐Meghalaya Plateau: implications for the Indian shield assembly

AbstractThe Palaeo‐Mesoproterozoic metapelite granulites from northern Garo Hills, western Shillong‐Meghalaya Gneissic Complex (SMGC), northeast India, consist of resorbed garnet, cordierite and K‐feldspar porphyroblasts in a matrix comprising shape‐preferred aggregates of biotite±sillimanite+quartz that define the penetrative gneissic fabric. An earlier assemblage including biotite and sillimanite occurs as inclusions within the garnet and cordierite porphyroblasts. Staurolite within cordierite in samples without matrix sillimanite is interpreted to have formed by a reaction between the sillimanite inclusion and the host cordierite during retrogression. Accessory monazite occurs as inclusions within garnet as well as in the matrix, whereas accessory xenotime occurs only in the matrix. The monazite inclusions in garnet contain higher Ca, and lower Y and Th/U than the matrix monazite outside resorbed garnet rims. On the other hand, matrix monazite away from garnet contains low Ca and Y, and shows very high Th/U ratios. The low Th/U ratios (<10) of the Y‐poor garnet‐hosted monazite indicate subsolidus formation during an early stage of prograde metamorphism. A calculated P–T pseudosection in the MnCKFMASH‐PYCe system indicates that the garnet‐hosted monazite formed at <3 kbar/600 °C (Stage A). These P–T estimates extend backward the previously inferred prograde P–T path from peak anatectic conditions of 7–8 kbar/850 °C based on major mineral equilibria. Furthermore, the calculated P–T pseudosections indicate that cordierite–staurolite equilibrated at ~5.5 kbar/630 °C during retrograde metamorphism. Thus, the P–T path was counterclockwise. The Y‐rich matrix monazite outside garnet rims formed between ~3.2 kbar/650 °C and ~5 kbar/775 °C (Stage B) during prograde metamorphism. If the effect of bulk composition change due to open system behaviour during anatexis is considered, the P–T conditions may be lower for Stage A (<2 kbar/525 °C) and Stage B (~3 kbar/600 °C to ~3.5 kbar/660 °C). Prograde garnet growth occurred over the entire temperature range (550–850 °C), and Stage‐B monazite was perhaps initially entrapped in garnet. During post‐peak cooling, the Stage‐B monazite grains were released in the matrix by garnet dissolution. Furthermore, new matrix monazite (low Y and very high Th/U ≤80, ~8 kbar/850–800 °C, Stage C), some monazite outside garnet rims (high Y and intermediate Th/U ≤30, ~8 kbar/800–785 °C, Stage D), and matrix xenotime (<785 °C) formed through post‐peak crystallization of melt. Regardless of textural setting, all monazite populations show identical chemical ages (1630–1578 Ma, ±43 Ma). The lithological association (metapelite and mafic granulites), and metamorphic age and P–T path of the northern Garo Hills metapelites and those from the southern domain of the Central Indian Tectonic Zone (CITZ) are similar. The SMGC was initially aligned with the southern parts of CITZ and Chotanagpur Gneissic Complex of central/eastern India in an ENE direction, but was displaced ~350 km northward by sinistral movement along the north‐trending Eastern Indian Tectonic Zone in Neoproterozoic. The southern CITZ metapelites supposedly originated in a back‐arc associated with subducting oceanic lithosphere below the Southern Indian Block at c. 1.6 Ga during the initial stage of Indian shield assembly. It is inferred that the SMGC metapelites may also have originated contemporaneously with the southern CITZ metapelites in a similar back‐arc setting.

P–T–t constraints of the Barrovian-type metamorphic series in the Khondalite belt of the North China Craton: Evidence from phase equilibria modeling and zircon U–Pb geochronology

The Barrovian metamorphic series represents a typical medium P/T regional metamorphism developed in pelitic lithologies, with metamorphic grades progressing from greenschist facies to granulite facies. As the metamorphic conditions are not too high in some lower-grade zones, some geological records of the prograde metamorphic stage may be well preserved. This is significant to the study of orogenic evolution. A sequence of Barrovian metamorphic series rocks from the garnet zone to the kyanite zone outcrops in the eastern part of Daqingshan terrane. An integrated study of petrography, phase equilibria modeling and zircon SIMS U–Pb geochronology is used to establish the P–T conditions and timing of this Barrovian metamorphism. Based on the petrology and mineral chemistry, minerals in each sample from different Barrovian metamorphic zones are divided into prograde, peak and retrograde metamorphic stages. Pseudosections are calculated using THERMOCALC for three samples from each zone, and the peak metamorphic conditions are confined to be 585°C and 0.59GPa for the garnet zone, 590°C and 0.60GPa for the staurolite zone, 645°C and 0.71GPa for the kyanite zone. The P–T paths of the Barrovian metamorphic rocks recorded a crustal over-thickening event during prograde metamorphic stage, and isobaric heating after reaching Pmax. The retrograde phase is characterized by cooling with decompression. Zircons from two higher metamorphic zones are dated using a CAMECA instrument. According to the results, the metamorphic ages of the different metamorphic zones are coeval and are estimated to be 1896Ma. This is quite similar to the protolith age of the nearby arc-related Halaqin volcanic rocks. We suggest that the Daqingshan terrane experienced crustal thickening at 1896Ma, with an arc magmatism superposition simultaneously.

Ruby–sapphire–spinel mineralization in marble of the middle and southern Urals: Geology, mineralogy, and genesis

Ruby and spinel occurrences hosted in marble on the eastern slope of the Urals are considered. Ruby- and spinel-bearing marble is a specific rock in granite-gneiss complexes of the East Ural Megazone, which formed at the Late Paleozoic collision stage of the evolution of the Urals. Organogenic marine limestone is the protolith of the marble. No relict sedimentary bedding has been retained in the marble. The observed banding is a secondary phenomenon related to crystallization and is controlled by flow cleavage. Magnesian metasomatism of limestone with the formation of fine-grained dolomite enriched in Cr, V, Ti, Mn, Cu, Zn, Ga, and REE took place at the prograde stage of metamorphism. Dedolomitization of rocks with the formation of background calcite marble also developed at the prograde stage. Mg-calcite marble with spinel and ruby of the first type formed in the metamorphic fluid circulation zone. Magnesian metasomatism with the formation of bicarbonate marble with ruby, pink sapphire, and spinel of the second type developed at the early retrograde stage. The formation of mica-bearing mineralized zones with corundum and spinel of the third type controlled by cleavage fractures is related to the pneumatolytic–hydrothermal stage. The data on ruby-bearing marble in the Urals may be used for forecasting and prospecting of ruby and sapphire deposits hosted in marble worldwide.

P–T–t–D paths of the North Himalayan metamorphic rocks: Implications for the Himalayan orogeny

Metamorphic P–T–t–D paths have been determined in the Yardoi gneiss dome of the eastern Himalaya that place new constraints on the genesis of the North Himalayan Gneiss Dome and tectonic evolution of the Himalayan orogen. Our study shows that the schists from the Yardoi dome underwent at least three stages of metamorphism and deformation. The first stage of prograde metamorphism is represented by the core of porphyroblastic garnet, showing distinct growth compositional zoning, and hosting mineral inclusions of biotite, muscovite, quartz and ilmenite; the curved inclusion trails define the first stage of deformation foliation (S1). The second stage of peak metamorphism is represented by the rim of porphyroblastic garnet and the aligned matrix minerals of biotite, muscovite, plagioclase, quartz, kyanite and staurolite, which define the main deformation foliation (S2). The third stage of retrograde metamorphism is characterized by occurrence of biotite, plagioclase, muscovite and sillimanite within the distributed shear bands, which define the late deformation foliation (S3). Phase equilibrium modeling reveals that Yardoi schists witnessed a peak metamorphism of upper amphibolite-facies under P–T conditions of 7–8kbar and 615–665°C, and records a clockwise P–T–t–D path with a prograde process of both temperature and pressure increase and subsequent retrogression of isothermal decompression. The zircon U–Pb dating indicates that the prograde process occurred during the Middle Eocene of 48–36Ma and the retrograde metamorphism probably lasted to ca. 16Ma. The present result shows that the formation of the Yardoi schists was related to the subduction and exhumation of Indian crust, the initial collision of Indian and Asian continents occurred at ca. 50Ma, the northeastern margin of the Indian continent was shallowly underthrusted beneath the Asian continent, and distinct tectonic discontinuity is present in the Greater Himalayan Sequence.

Petrology, geochemistry, and metamorphic evolution of meta-sedimentary rocks in the Diancang Shan–Ailao Shan metamorphic complex, Southeastern Tibetan Plateau

Meta-sedimentary rocks are widely distributed within the Diancang Shan–Ailao Shan metamorphic complex in the Southeastern Tibetan Plateau. Detailed geochemical analyses show that all of them have similar geochemical features. They are enriched in light rare-earth elements (LREEs) and depleted in heavy rare-earth elements (HREEs), with moderately negative Eu anomalies (Eu/Eu∗=0.55–0.75). Major and trace element compositions for the meta-sedimentary rocks suggest that the protoliths were probably claystone, siltstone, and greywacke and deposited in an active continental margin. Garnet porphyroblasts in meta-sedimentary rocks have distinct compositional zonation from core to rim. The zonation of garnet in St–Ky–Grt–Bt–Ms schist indicates an increasing P–T trend during garnet growth. In contrast, garnets from (Sil)–Grt–Bt paragneiss show diffusion zoning, implying a decreasing P–T trend. Based on mineral transformations and P–T estimates using conventional geothermobarometers and pseudosection calculations, four metamorphic stages have been determined, including an early prograde metamorphic stage (M1), a peak amphibolite–granulite facies metamorphic stage (M2), a near-isothermal decompression stage (M3), and a late amphibolites-facies retrograde stage (M4). The relic assemblage of Ms+St±Ky±Bt±Kfs+Qz preserved as inclusions in garnet porphyroblasts of the meta-sedimentary rocks belongs to prograde (M1) stage and records P–T conditions of 560–590°C and 5.5–6.3kb. Matrix mineral assemblages of Grt+Bt+Ky/Sil+Pl+Qz and Grt+Bt±Sil+Pl±Kfs+Qz formed at peak (M2) stage yield P–T conditions of 720–760°C and 8.0–9.3kb. M3 is characterized by decompression reactions, dehydration melting of assemblages that include hydrous minerals (e.g., biotite), and partial melting of felsic minerals. The retrograde assemblages is Grt+Bt+Sil+Pl+Qz formed at 650–760°C and 5.0–7.3kb. At the amphibolites-facies retrograde (M4) stage, fine-grained biotite–muscovite-bearing symplectites were formed, which occur at the grain boundaries of garnet and felsic minerals; such assemblage as Bt+Ms+Pl±Kfs±Grt+Qz were formed at 553–613°C and 4.0–5.0kb. The metamorphic history of the Diancang Shan–Ailao Shan metamorphic complex is thus defined by a clockwise P–T trajectory which includes a nearly isothermal decompression recording the subduction, collision and exhumation events between the Yangtze and Indochina blocks at Oligocene to Early Miocene, prior to the regional strong left-lateral shearing along the Ailao Shan-Red River Fault.

Metamorphism and P–T evolution of high pressure granulite in Chicheng, northern part of the Paleoproterozoic Trans-North China Orogen

It is widely accepted that the North China Craton (NCC) was finally formed by collision of the Eastern and the Western Blocks along the Trans-North China Orogen (TNCO) at approximately 1.85Ga. The continent–continent collision along the TNCO was partly defined by metamorphism of high pressure mafic granulites with clockwise P–T paths. However, the continental subduction process has not been well recorded by previously reported HP granulites due to lack of prograde metamorphism and their plutonic protoliths. In this paper, we report HP mafic granulites among a metamorphic volcanic-sedimentary sequence south to Chicheng, northern part of the TNCO, in which the prograde metamorphism was well preserved. The prograde metamorphic stage (M1) has been revealed by a distinct growth zone of garnet (Grt) porphyroblasts with an inclusion-rich core and inclusion-free mantle. The peak metamorphic mineral assemblage (M2) consists of Grt and clinopyroxene (Cpx) predominantly with minor plagioclase (Pl), Rutile (Ru), hornblende (Hb) and quartz (Q). The Pl+Hb symplectite or corona surrounding garnet and Cpx+Ilm (ilmenite) symplectite surrounding sphene provide evidence for the post-peak decompression (M3) stage. Ubiquitous Hb especially along cracks represents further retrogression (M4) within amphibolite facies. Phase equilibria modeling by the THERMOCALC program combined with thermobarometric calculations yielded a consecutive hairpin shaped clockwise P–T path, which indicate that the HP granulites experienced remarkable prograde metamorphism, during which the temperatures increased from 630°C to 740°C while the pressures increased from 8kbar to the barometric peak of 16kbar. After the barometric peak, the HP granulites subjected to decompression from 16kbar to 13kbar while the temperature continued to increased from 750°C to 790°C, suggesting a thermal relaxation. After the thermal peak, the HP granulites began their consecutive decompression with cooling from 790°C and 13kbar to 700°C and 7kbar through 740°C and 9kbar, avoiding access to the orthopyroxene (Opx) stability field as experienced by many other HP granulites in the TNCO. The hairpin-shaped P–T path of the Chicheng HP granulites suggests a continuous continental subduction and tectonic uplift, providing a better metamorphic constraint on the final assembly of the NCC along the TNCO.

Metamorphic P–T–t paths of the Zanhuang metamorphic complex: Implications for the Paleoproterozoic evolution of the Trans-North China Orogen

Garnet-bearing metapelites are cropped out in the Zanhuang metamorphic complex located in the central section of the N–S trending Trans-North China Orogen. Petrological studies indicate that these garnet-bearing metapelites preserve three distinct metamorphic assemblages (M1–M3). The cores of garnet porphyroblasts and fine-grained inclusions of plagioclase+quartz±biotite±apatite±magnetite±rutile±ilmenite define the prograde metamorphic stage (M1). The assemblage of the peak metamorphic stage (M2) consists of biotite+plagioclase+quartz±kyanite±hornblende±magnetite±ilmenite in equilibrium with the lowest-XMn garnet rims. The peak metamorphism was followed by a near-isothermal decompressional stage (M3) and the development of symplectitic coronas of biotite+plagioclase+quartz±hornblende±magnetite surrounding garnet porphyroblasts. Quantitative phase equilibria modeling and traditional thermobarometric estimations were applied to yield P–T conditions of 4.5–5.9kbar and 551–596°C for stage M1, 9.6–12.3kbar and 770–830°C for stage M2, and ∼4.7kbar and ∼635°C for stage M3. The combination of the mineral inclusions, mineral compositions, and metamorphic reactions in the Zanhuang metapelites defines a clockwise P–T path with nearly isothermal decompression, which is considered to have been related to the amalgamation of the Eastern and Western Blocks to form the North China Craton. Abundant U–Pb spot analyses of metamorphic zircons from the gneiss and amphibolite reveal two discrete, meaningful ages of metamorphism within the Zanhuang complex: the first age dates to ∼2507Ma, and the second age dates to ∼1839Ma. The first age group (∼2507Ma) was most likely associated with a regional Neoarchean granulite-facies metamorphic event within the North China Craton, whereas the second age group (∼1839Ma) is interpreted as the result of a Paleoproterozoic orogenic event that occurred in the Trans-North China Orogen (TNCO).

Metamorphic P-T trajectory and multi-stage fluid events of vein-bearing UHP eclogites from the Dabie terrane: insights from compositional zonations of key minerals

The multi-stage metamorphic evolution and high-pressure (HP) to ultra-high-pressure (UHP) fluid events of vein-bearing eclogites from the Ganghe in the Dabie UHP terrane have been established based on a detailed study of compositional zonations in garnet, omphacite, and epidote. Comparison of mineral assemblages and zoned garnet compositions with the predictions of phase equilibrium modelling results in six stages of metamorphism in eclogites: (i) prograde low-temperature (LT)-HP eclogite-facies metamorphism (2.0–2.3 GPa, <540°C); (ii) peak UHP lawsonite eclogite-facies metamorphism (~3.1 GPa, ~650°C); (iii) decompressional breakdown of UHP lawsonite during initial exhumation (2.8–3.0 GPa, 650–680°C), as we have previously reported; (iv) middle-temperature (MT) and HP eclogite-facies metamorphism (~2.6 GPa, ~720°C); (v) retrograde eclogite-facies metamorphism (1.5–1.7 GPa, 610–670°C); and (vi) amphibolite-facies metamorphism (<1.1 GPa, <600°C). A heating event with decompression is confirmed from metamorphic stages (iii) to (iv), indicating that the Ganghe eclogites underwent a relatively sluggish exhumation during the stages from the UHP to HP conditions. During prograde metamorphic stages (i) and (ii), fluid activities in the eclogites are limited because lawsonite growth consumed large amounts of early-stage hydrous minerals and potential free water. Dehydration reactions during metamorphic stage (iii) triggered a significant liberation of UHP fluid. Intense fluid-eclogite interaction and dissolution-precipitation processes during metamorphic stages (iii) to (iv) led to the occurrence of patchy zones in epidote cores and the systematic compositional change of epidote and omphacite cores near the vein. A late-stage infiltration of HP fluid into eclogites occurred in metamorphic stage (v), leading to the final-stage growth (mineral rims) of garnet, omphacite, and epidote. This study presents detailed mineral zonation patterns and mineral compositions of the vein-hosting eclogites to link the complicated metamorphic P-T evolution with episodic multi-stage HP-UHP fluid events in deeply subducted continental crust.

Metamorphic evolution and zircon U–Pb geochronology of the Mts. Huashan amphibolites: Insights into the Palaeoproterozoic amalgamation of the North China Craton

Exposed in the southernmost terminal of the Palaeoproterozoic Trans-North China Orogen (TNCO), the garnet-bearing amphibolites in the Mts. Huashan metamorphic complex preserve three generations of mineral assemblages including the prograde, peak and retrograde metamorphic stages. The prograde metamorphic stage (M1) is recognized as inclusion-type mineral assemblages (hornblende+plagioclase+quartz±biotite±ilmenite) preserved in the garnet porphyroblasts. The peak metamorphic assemblages (M2) are represented by the garnet porphyroblasts and matrix minerals (hornblende+plagioclase+quartz±biotite±ilmenite). The retrograde metamorphic assemblages (M3) are characterized by the symplectitic intergrowth minerals (hornblende+plagioclase+quartz±orthopyroxene±biotite±ilmenite) rimming decomposed, relict garnet porphyroblasts, known as the “white-eye socket” reaction textures. Calculated by geothermometers and geobarometers, the P–T conditions of these three metamorphic stages are estimated to be of 5.7–6.0kbar/660–760°C for the prograde stage (M1), 6.6–7.7kbar/700–820°C for the peak stage (M2) and 4.5–6.7kbar/720–760°C for the retrograde stage (M3), respectively. Therefore, these three metamorphic stages define clockwise P–T paths including nearly isothermal decompression (ITD) processes. Geochronological studies of zircons from the amphibolites indicate that the protolith of the amphibolites formed at ∼2.3Ga, and the metamorphism occurred at ∼1.87–1.85Ga, and the metamorphic period was further deciphered to be of 1.96–1.80Ga when considering our previous reports. It is therefore proposed that the southernmost terminal of the TNCO was also involved in the subduction and collision processes between the Eastern and Western Blocks of the North China Craton at least as early as 1.96Ga and continued as late as 1.80Ga, followed by rapid uplift and exhumation. The available data suggest that the Palaeoproterozoic tectono-metamorphic event of the southernmost terminal of the Trans-North China Orogen not only started earlier but also continued longer than the other terranes in the middle and northern sections elsewhere in the orogen.

Metamorphism and partial melting of high-pressure pelitic granulites from the Qianlishan Complex: Constraints on the tectonic evolution of the Khondalite Belt in the North China Craton

High-pressure pelitic granulites with a peak assemblage of garnet+kyanite+K-feldspar+plagioclase+biotite+quartz have been found from the Qianlishan Complex in the westernmost part of the Khondalite Belt, an east–west-trending Paleoproterozoic orogen between the Ordos and Yinshan Blocks in the North China Craton. Petrographic evidence indicates that high-pressure pelitic granulites underwent the prograde, peak and post-peak decompression stages. The early prograde metamorphic stage (M1) is represented by an inclusion-type mineral assemblage of quartz+plagioclase+muscovite+biotite+sillimanite, which occurs within the core of garnet porphyroblasts. The peak metamorphic stage (M2) is marked by the mantle growth of garnet porphyroblasts and matrix minerals kyanite, K-feldspar, biotite, plagioclase and quartz. The decompression stage (M3) is indicated by sillimanite replacing kyanite, the formation of the cordierite+sillimanite symplectite in the matrix (M3–1), and the cordierite corona replacing garnet (M3–2). These mineral assemblages and their P–T estimates based on the assemblage stability fields of P–T pseudosection constructed in NCKFMASHTO define a clockwise P–T path involving near-isothermal decompression. Combined with available metamorphic age data, the clockwise P–T paths reconstructed for high-pressure pelitic granulites from the Qianlishan Complex and medium-pressure pelitic granulites from other complexes in the Khondalite Belt are in accord with collision between the Yinshan and Ordos Blocks to form the Western Block at ∼1.95Ga, followed by the exhumation and associated decompressional partial melting at ∼1.88Ga.

Metamorphic P–T path and tectonic implications of pelitic granulites from the Daqingshan Complex of the Khondalite Belt, North China Craton

Pelitic granulites are cropped out in the Daqingshan Complex of the Khondalite Belt, a Paleoproterozoic tectonic belt in the Western Block of the North China Craton. Petrological studies show that these granulites contain four distinct metamorphic assemblages. The core of a garnet porphyroblast, along with fine-grained inclusions of biotite+plagioclase+K-feldspar+quartz±sillimanite±ilmenite, defines the prograde metamorphic (M1) stage. The peak (M2) assemblage consists of garnet (mantle)+biotite+K-feldspar+plagioclase+quartz±sillimanite±orthopyroxene±ilmenite±magnetite±rutile. Peak metamorphism was followed by a near-isothermal decompression (M3) and the development of coronae of garnet+biotite+cordierite+plagioclase+quartz±K-feldspar±sillimanite±spinel±ilmenite±magnetite (M3) in the Crd–Grt–Bt gneisses during the following garnet breakdown reactions: garnet+sillimanite+melt→cordierite+biotite+Fe-oxide and garnet+melt→biotite+quartz+plagioclase. Retrograde cooling (M4) assemblages are represented by biotite+muscovite+sillimanite+quartz+plagioclase+K-feldspar±ilmenite. Quantitative phase equilibria modeling in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 was applied to obtain P–T conditions of <780°C and <9kbar for M1, 840–880°C and 9–11kbar for M2, 800–870°C and 5.0–7.5kbar for M3, and <660°C and 4.1–6.9kbar for M4. The combination of the mineral assemblages, mineral compositions, and metamorphic reaction histories in the Daqingshan pelitic granulites defines a clockwise P–T path that involves periods of near-isothermal decompression and late cooling that followed the peak granulite-facies metamorphism. This result is consistent with the tectonic history of the Daqingshan Complex in the Khondalite Belt, which involved continent–continent subduction and collision followed by exhumation and cooling. This suggests a continent–continent collisional event between the Yinshan and Ordos blocks, which became amalgamated to form the Paleoproterozoic Khondalite Belt in the Western Block of the North China Craton.

Using P–T paths to interpret the tectonothermal setting of prograde metamorphism: An example from the northeastern Gawler Craton, South Australia

We present metamorphic analysis from a porphyroblastic garnet–cordierite–spinel pelite from the Palaeo-Mesoproterozoic Mount Woods Inlier in South Australia. Petrographic analysis combined with a series of P–T pseudosections representative of a progressively changing effective bulk composition due to early porphyroblast growth and creation of an Al-rich, Si-poor chemical environment in the porphyroblast cores provides evidence of a prograde path that reached granulite facies conditions. Andalusite porphyroblasts are interpreted to have grown during the early phases of prograde metamorphism at conditions of ∼2–3kbar and ∼550–610°C. Geothermal gradients associated with early stages of prograde metamorphism are estimated to be ∼50–90°Ckm−1. The aluminosilicate porphyroblasts subsequently reacted to form cordierite–spinel symplectites that are chemically restricted from the remainder of the bulk rock by earlier formed cordierite moats. Peak metamorphism is petrographically represented by a phase of garnet growth, and attained conditions of ∼4.7kbar and 750°C, equivalent to a geothermal gradient of 45–50°Ckm−1. Monazite CHIME analysis has yielded an age of ∼1615Ma for the Moonlight Hills samples, and is suggested to represent monazite growth during prograde metamorphism through temperatures of ∼500–600°C. Peak metamorphism was contemporaneous with high-temperature/low-pressure metamorphism elsewhere in the Gawler Craton (Fleurieu Peninsula) and eastern Proterozoic Australia (e.g. Broken Hill Block, Mount Isa Inlier) at ca. 1.61–1.59Ga. The Mount Woods Inlier was heated at low pressure conditions due to intrusion of voluminous magmas to mid- to upper-crustal levels, some of which have been removed during exhumation and erosion. The hot rocks were then buried, at which point peak metamorphic conditions were attained.

Metamorphic P–T paths of the Zanhuang amphibolites and metapelites: constraints on the tectonic evolution of the Paleoproterozoic Trans-North China Orogen

Garnet-bearing metapelites and amphibolites are exposed in the south and middle parts of the Zanhuang complex, which is located in the central segment of the nearly NS-striking Trans-North China Orogen. These rocks preserve three metamorphic mineral assemblages forming at the prograde, peak and post-peak decompression stages. The prograde metamorphic stage (M1) is represented by mineral inclusions within garnet porphyroblasts, the peak metamorphic stage (M2) is represented by garnet rims and matrix minerals, whereas the retrograde stage (M3) is represented by amphibole + plagioclase symplectite rimming garnet porphyroblasts in the amphibolites and biotite + plagioclase symplectite rimming garnet porphyroblasts in the metapelites. All garnet porphyroblasts in the metapelites preserve prograde chemical zoning except for the ubiquitous, quite narrow zones from the underwent post-peak decompression. It has been determined through thermobarometric computation that the metamorphic conditions are 650–710°C at 8.2−9.2 kbar for the M1 (inclusion) assemblages, >810°C at >12.5 kbar for the metamorphic peak M2 (matrix) assemblages, and 660–680°C at 4.4–4.5 kbar for the retrograde M3 (symplectite) assemblages. These rocks are thus determined to have undergone metamorphism with clockwise P–T paths involving nearly isothermal decompression (ITD) segments, which is inferred to be related to the amalgamation of the Eastern and Western Blocks to form the coherent basement of the North China Craton along the Trans-North China Orogen in the late Paleoproterozoic (1.88–1.85 Ga).

Petrology and thermobarometry of eclogite rocks in the Krasnaya Guba dike field, belomorian mobile belt

A sequence of mineral associations was examined in eclogitized basites of the Krasnaya Guba dike field in the Belomorian Mobile Belt. Two morphological types of eclogite and eclogite-like rocks were recognized: (1) eclogite rocks that developed after ferrogabbro dikes and completely replaced these dikes from contact to contact and (2) eclogite-like rocks that developed after gabbronorites in zones of ductile deformations and shearing. According to data mineral geothermobaromety, both rock types were formed within temperature and pressure ranges corresponding to high-pressure and high-temperature amphibolite facies at T = 700 ± 40°C and P = 10.0 ± 0.5 kbar. The peak metamorphic parameters of the host gneisses are analogous. The decompressional stage, which is unambiguously identified by reaction textures, occurred at 630–660°C and 7.9–8.2 kbar. As the temperature and, first of all, pressure decreased, the SiO2 activity in the fluid systematically varied. The eclogitization of the basites took place locally in relation to fluid fluxes, which were restricted to zones of intense deformations, at variable SiO2 activity. The rocks show evidence of two stages of post-eclogite amphibolization. Older amphibolization 1 was coeval with the late prograde metamorphic stage (T = 650°C, P = 10–11 kbar). Younger amphibolization 2 affected eclogitized basite dikes and unaltered gabbronorites (together with their host gneisses) over large areas. This process coincided with decompression (T = 580°C, P = 7–8 kbar) and was likely accompanied by the exhumation of deep zones of BMB to upper-middle crustal levels.