Search Results (46)

Eclogites exhumed from subduction channels are pivotal for deciphering the thermal structure of continental subduction zones. However, heterogeneities in bulk-rock composition and evolutionary history within the subduction channel can lead to variations in petrographic textures and elemental characteristics among eclogites. Therefore, investigating the pressure–temperature (P-T) evolution of eclogites from different outcrops is crucial for refining dynamic models of convergent plate boundaries. The Western Dabie Mountain represents an ideal locality for studying the petro-thermodynamics of continental subduction channels. This study focuses on samples collected from the Dongyuemiao area, situated at the boundary between the high-pressure and ultrahigh-pressure metamorphic belts in the Western Dabie. We integrate petrographic observations, mineral chemistry, phase equilibrium modeling, Zr-in-rutile thermometry and hornblende-plagioclase thermobarometry to constrain the P-T evolution of the eclogite. The samples exhibit a consistent mineral assemblage: garnet + omphacite + amphibole + quartz + phengite, with accessory minerals including rutile and titanite. Garnet grains display characteristic “cloudy-core” and “atoll” textures. Major and trace element analyses of large garnet porphyroblasts reveal pronounced growth zoning in divalent cations, with cores showing enrichment in light rare earth elements (LREEs). Based on phase equilibrium modeling and calculated isopleths for garnet (Ca, Mg) and phengite (Si content), we interpret that the garnet core mineral assemblage (glaucophane + rutile + sphene) records a blueschist-facies metamorphic stage, situated near the rutile-titanite transition. A prograde P-T path is reconstructed, comprising an initial stage of isobaric heating (from ~480 °C at 20 kbar to ~550 °C at 21 kbar), followed by an isothermal compression to the Pmax stage (from ~550 °C at 21 kbar to ~575 °C at 26 kbar). Subsequent retrograde evolution is characterized by decompression and cooling, with symplectite formation recording conditions of ~570 °C and 13 kbar. This study demonstrates that the reconstructed P-T path for the Dongyuemiao eclogites shows stepped geothermal gradient for the prograde stage, and that fluid activity during exhumation resulted from a combination of internal and external factors. Full article

Maw-sit-sit jade resembles kosmochlor-jadeitite in appearance and is spatially associated with it in the Myanmar Jade Belt. However, the mineral composition, microstructure, and petrogenesis of this type of jade remain unclear. To address this gap, this study investigated high-quality Maw-sit-sit jade using a range of analytical techniques, including conventional gemological tests, infrared spectroscopy, petrographic observations, electron probe microanalysis (EPMA), and backscattered electron (BSE) imaging. Results show that Maw-sit-sit jade primarily consists of albite and chromium-omphacite, with minor amphibole (eckermannite and richterite). Jadeite and relict chromite are absent in the studied samples. Its high albite content gives it lower refractive index (RI: 1.55–1.56) and specific gravity (SG: 2.69–2.73) compared to kosmochlor-jadeitite and jadeite jade. Additionally, Maw-sit-sit jade exhibits punctate or banded fluorescence under ultraviolet (UV) light, distinguishing it from kosmochlor-jadeitite and jadeite jade (both inert). Petrographically, euhedral albite fills interstices between early-formed Cr-omphacite and eckermannite, which is textural evidence of its late-stage origin. Eckermannite and Cr-omphacite occur as enclosed grains with embayed boundaries and dissolution pores, indicating they experienced mechanical disruption and chemical dissolution during subsequent geological processes. Petrogenetically, Maw-sit-sit jade (defined as “Cr-omphacite-albitite”) forms via a two-stage process: (1) Under high-pressure/low-temperature (HP/LT) conditions in the subduction zone, Na-Al-Si-rich fluids metasomatize chromite-bearing serpentinite protoliths, generating an early assemblage of jadeite, Cr-omphacite and amphiboles; (2) During subsequent plate exhumation and decompression, jadeite underwent retrograde metamorphism under low-pressure/low-temperature (LP/LT) conditions involving residual Na-Al-Si fluids, resulting in the formation of albite. This process led to the replacement of early-formed minerals by euhedral albite, ultimately generating the Ab+Cr-Omp+Eck symplectic texture. This study elucidates the mineralogical, gemological identity and petrogenesis of high-quality Maw-sit-sit jade, advancing our understanding of fluid evolution within a subduction zone. Full article

The Varnous Mt. area in the northern Pelagonian Nappe is characterized by the intrusion of an Early Permian pluton, with its tectonic setting and igneous petrology well constrained in earlier studies. The metamorphic basement rocks warrant further detailed investigation due to their complex history. These rocks are polymetamorphosed, preserving a sequence of overprinting metamorphic and deformational events. The metapelitic rocks have undergone an initial, pre-Carboniferous regional metamorphism of unknown grade before or during Hercynian Orogeny, followed by a thermal metamorphic event associated with the intrusion of the Varnous pluton at 297 Ma. The assemblage attributed to this event is And + Crd + Bt + Ms (west), while the first assemblage identified at the eastern part is Sil + Bt + Gt. Additionally, three regional tectonometamorphic events occurred during the Alpine Orogeny. For the Alpine events, the assemblages are as follows: first, the development of St + Gt + Chl + Kfs + Pl + Qtz at 150–130 Ma; second, retrograde metamorphism of these assemblages with Cld + Gt + Ser + Mrg + Chl ± Sil (Fi) at 110–90 Ma; and finally, mylonitization of all previous assemblages at 90–70 Ma with simultaneous annealing and formation of Cld + Chl + Ms. Full article

We report information on the protolith, the Triassic retrograde metamorphism, and anatexis recorded in zircons extracted from granitic gneiss and biotite schist in the Sulu orogenic zone, central China. The schist is enclosed within the granitic gneiss in the form of a lens. Zircon grains from the schist sample indicate anatexis occurred at 214.6 ± 3.6 Ma (MSWD = 5.1), with ε_Hf (t) values ranging from −22.6 to −18.3, corresponding to T_DM^C(Hf) ages between 2675 Ma and 2407 Ma. The granitic gneiss originated from magmatic rock formed at 774 ± 32 Ma (MSWD = 5.7) and subsequently underwent metamorphism at ~211 Ma. Three zircon cores from the granitic gneiss exhibit ε_Hf (t) values ranging from −13.6 to −6.3, with T_DM ^C(Hf) ages spanning 2487–2075 Ma. Six zircon rims from the gneiss yield ε_Hf (t) values of −14.7 to −13.3, and T_DM ^C(Hf) ages ranging from 2176 to 2092 Ma. We believe that the protolith of granitic gneiss is the Neoproterozoic magmatic rock, whose tectonic affinity is the northern margin of the Yangtze craton. The granitic gneiss experienced Triassic collisional orogeny-related metamorphism and subsequent retrograde metamorphism, with the timing of retrograde overprinting consistent with zircon-recorded anatexis in the schist. In addition, the protoliths of both the gneiss and schist exhibit close affinity to Archean-Paleoproterozoic crustal sources. Full article

The Biggenden gold-bearing Fe skarn deposit in southeast Queensland, Australia, is a calcic magnetite skarn that has been mined for Fe and gold (from the upper portion of the deposit). Skarn has replaced volcanic and sedimentary rocks of the Early Permian Gympie Group, which formed in different tectonic settings, including island arc, back arc, and mid-ocean ridge. This group has experienced a hornblende-hornfels grade of contact metamorphism due to the intrusion of the Late Triassic Degilbo Granite. The intrusion is a mildly oxidized I-type monzogranite that has geochemical characteristics intermediate between those of granitoids typically associated with Fe-Cu-Au and Sn-W-Mo skarn deposits. The skarn mineralogy indicates that there was an evolution from prograde to various retrograde assemblages. Prograde garnet (Adr_11-99Grs_1-78Alm_0-8Sps_0-11), clinopyroxene (Di_30-92Hd_7-65Jo_0-9), magnetite, and scapolite formed initially. Epidote and Cl-bearing amphibole (mainly ferropargasite) were the early retrograde minerals, followed by chlorite, calcite, actinolite, quartz, and sulfides. Late-stage retrograde reactions are indicated by the development of nontronite, calcite, and quartz. Gold is mainly associated with sulfide minerals in the retrograde sulfide stage. The fluids in equilibrium with the ore-stage calcites had δ¹³C and δ¹⁸O values that indicate deposition from magmatically derived fluids. The calculated δ¹⁸O values of the fluids in equilibrium with the skarn magnetite also suggest a magmatic origin. However, the fluids in equilibrium with epidote were a mixture of magmatic and meteoric water, and the fluids that deposited chlorite were at least partly meteoric. δD values for the retrograde amphibole and epidote fall within the common range for magmatic water. Late-stage chlorite was deposited from metasomatic fluids depleted in deuterium (D), implying a meteoric water origin. Sulfur isotopic compositions of the Biggenden sulfides are similar to other skarn deposits worldwide and indicate that sulfur was most probably derived from a magmatic source. Based on the strontium (⁸⁷Sr/⁸⁶Sr) and lead (²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb) isotope ratios, the volcanic and sedimentary rocks of the Gympie Group may have contributed part of the metals to the hydrothermal fluids. Lead isotope data are also consistent with a close age relationship between the mineralization at Biggenden and the crystallization of the Degilbo Granite. Microthermometric analysis indicates that there is an overall decrease in fluid temperature and salinity from the prograde skarn to retrograde alterations. Fluid inclusions in prograde skarn calcite and garnet yield homogenization temperatures of 500 to 600 °C and have salinities up to 45 equivalent wt % NaCl. Fluid inclusions in quartz and calcite from the retrograde sulfide-stage homogenized between 280 and 360 °C and have lower salinities (5–15 equivalent wt % NaCl). In a favored genetic model, hydrothermal fluids originated from the Degilbo Granite at depth and migrated through the shear zone, intrusive contact, and permeable Gympie Group rocks and leached extra Fe and Ca and deposited magnetite upon reaction with the adjacent marble and basalt. Full article

The Alattu–Päkylä gold occurrence is located in the Northern Lake Ladoga area, in the Raaha-Ladoga suprasubduction zone, at the Karelian Craton (AR)—Svecofennian foldbelt (PR₁) boundary. Its gold ore mineral associations are of two types of mineralization: (1) copper–molybdenum–porphyry with arsenopyrite and gold (intrusion-related) and (2) gold–arsenopyrite–sulfide in shear zones. Optical and scanning electron microscopy, X-ray fluorescence spectrometry, inductively coupled plasma mass spectrometry (ICP-MS), instrumental neutron activation analysis (INAA) and fire analysis with AAS finishing were used to study them. Type 1 was provoked by shallow-depth tonalite intrusion (~1.89 Ga) and type 2 by two stages of Svecofennian metamorphism (1.89–1.86 and 1.83–1.79 Ga) with the possible influence of the impactogenesis of the Janisjärvi astrobleme (age ~1 Ga). Intrusive and host rocks were subjected to shearing accompanied by the formation of ore-bearing metasomatic rocks of the propylite-beresite series (depending on substrate) and quartz–sericite, quartz and sericite–tourmaline veins and streaks. Ore mineralization is present as several consecutive mineral associations: pyritic–molybdenite with arsenopyrite and gold; gold–arsenopyrite; quartz–arsenopyrite with antimony sulfosalts of lead; gold–polysulfide with tetrahedrite –argentotetrahedrite series minerals and gold–antimony with Pb–Sb–S system minerals and native antimony. Arsenopyrite contains invisible (up to 234 ppm) and visible gold. Metamorphosed domains in arsenopyrite and rims with visible gold around it are usually enriched in As, indicating higher (up to >500 °C) temperatures of formations than original arsenopyrite with invisible gold (<500 °C). A paragenetic sequence associated with the deposition of invisible and visible gold established at the Alattu–Päkylä ore occurrence: pyrrhotite + unaltered arsenopyrite (with invisible gold) → altered arsenopyrite (As-enriched) + pyrite ± pyrrhotite + visible gold. Gold, associated with gudmundite, sphalerite and native antimony, seems to be due to cainotypic rhyodacitic porphyry cutting tonalite intrusion or with a retrograde stage in post-Svecofennian metamorphism. The isotopic composition of Pb and ²³⁸U/²⁰⁴Pb (9.4–9.75) for the feldspar of the tonalite intrusion and the pyrite of gold mineralization, εNd (−4 up to −5) for tonalites and ẟ³⁴S values of −2.10–+4.99 for arsenopyrite, indicate the formation of gold occurrence provoked by Svecofennian magmatic and tectono-thermal processes with the involvement of matter from a mantle-lower crustal reservoir into magma formation and mineralization. Full article

We report the petrogenesis of arfvedsonite granites from the Dimra Pahar pluton in the Chhotanagpur Gneissic Complex based on petrology, whole-rock chemistry, mineral chemistry, and La-ICP-MS zircon U-Pb ages and Hf-Lu isotopic analyses. These granites are dominantly peralkaline, occasionally peraluminous, and demonstrate features of A1-type granites. The magma was emplaced at a shallow depth and had a high liquidus temperature, fO₂ (>NNO), and water saturation. The zircons exhibit three distinct U-Pb isotopic ages. The oldest (1324 ± 6 Ma), large-sized inherited zircons (εHf(t) = +1.65 to +7.64), show complex zoning and signs of partial resorption. The euhedral, prismatic-bipyramidal zircons displaying oscillatory zoning (εHf(t) = −3.43 to +1.43) reveal a crystallization age of 1046 ± 7 Ma. Their thin periphery (εHf(t) = −3.23 to +0.27) grew during retrograde metamorphism (995 ± 6 Ma). The whole-rock geochemistry and the Hf-isotope values imply that the parental magma of these granites resulted from the anatexis of metasomatized lithospheric mantle sources. These granites intruded in a syn-orogenic (syn-collisional exhumation) stage of the orogeny. Full article

The B6 metamorphic buried hill condensate gas reservoir exhibits a highly compact matrix, leading to a rapid decline in bottom-hole pressure during initial production. The minimal difference between formation and saturation pressures results in severe retrograde condensation, with multiphase flow further increasing resistance. Conventional numerical simulations often overestimate reservoir energy supply due to their failure to account for this additional resistance, leading to inaccuracies in bottom-hole pressure predictions and gas–oil ratio during history matching. To address these challenges, this study conducted research on nonlinear numerical simulation for buried hill condensate gas reservoirs and established a method for calculating a multiphase pressure sweep range based on the well testing theory. By correcting and fitting the pressure propagation boundaries with numerical simulation, the nonlinear flow parameters applicable to the B6 gas field were inversed. This study revealed that conventional Darcy flow is inadequate for predicting pressure propagation boundaries and that it is possible to reasonably characterize the pressure sweep range through nonlinear flow. This approach resulted in an improvement in the accuracy of historical matching for bottom-hole pressure and gas–oil ratio, which improve the historical fitting accuracy to 85%, providing valuable insights for the development of similar reservoirs. Full article

An enrichment in nickel (Ni) or platinum group elements (PGE) is seldom observed in ores of the iron oxide–copper–gold (IOCG) type. This phenomenon is, however, known from a few deposits and prospects in the Carajás Mineral Province, Brazil. The Ni-PGE enrichment is explained, in part, by the spatial association of the IOCG-type ores with altered mafic-ultramafic lithologies, as well as by reworking and remobilization of pre-existing Ni and PGE during multiple mineralization and tectonothermal events across the Archean-Proterozoic. One such example of this mineralization is the Jatobá deposit in the southern copper belt of the Carajás Domain. This is the first detailed study of the Ni and PGE mineralization at Jatobá, with implications for understanding ore genesis. Petrographic and compositional study of sulfides shows that pyrite is the main Ni carrier, followed by pyrrhotite and exsolved pentlandite. Measurable concentrations of palladium (Pd) and platinum (Pt), albeit never more than a few ppm, are noted in pyrite. More importantly, however, the trace mineral signature of the Jatobá deposit features several platinum group minerals (PGM), including merenskyite, naldrettite, sudburyite, kotulskite, sperrylite, and borovskite. These PGM occur as sub-10 µm-sized grains that are largely restricted to fractures and grain boundaries in pyrite. All Pd minerals reported contain mobile elements such as Te, Bi, and Sb and are associated with rare earth- and U-minerals. This conspicuous mineralogy, differences in sulfide chemistry between the magnetite-hosted ore and stringer mineralization without magnetite, and microstructural control point to a genetic model for the sulfide mineralization at Jatobá and its relative enrichment in Ni and PGE. Observations support two alternative scenarios for ore genesis. In the first, an initial precipitation of disseminated or semi-massive Ni-PGE-bearing sulfides took place within the mafic rock pile, possibly in a VHMS-like setting. Later partial dissolution and remobilization of this pre-existing mineralization by mineralizing fluids of IOCG-type, possibly during the retrograde stage of a syn-deformational metamorphic event, led to their re-concentration within magnetite along structural conduits. The superposition of IOCG-style mineralization onto a pre-existing assemblage resulted in the observed replacement and overprinting in which PGE combined with components of the IOCG fluids like Sb, Bi, and Te. An alternative model involves leaching, by the IOCG-type fluids, of Ni and PGE from komatiites within the sequence or from ultramafic rocks in the basement. The discovery of PGM in Jatobá emphasizes the potential for additional discoveries of Ni-PGE-enriched ores elsewhere in the Carajás Domain and in analogous settings elsewhere. Full article

The East Kunlun Orogenic Belt (EKOB), northwestern China, recording long-term and multiple accretionary and collisional events of the Tethyan Ocean, belongs to a high-pressure to ultra-high-pressure (HP-UHP) metamorphic belt that underwent complex metamorphic overprinting in the early Paleozoic. In this contribution, we carry out an integrated study, including field investigations, petrographic observations, whole-rock analyses, zircon U-Pb dating, and P-T condition modeling using THERMOCALC in the NCKFMASHTO system for the eclogites, especially for the newly discovered UHP eclogite in the eastern part of EKOB. The eclogites exhibit geochemistry ranging from normal mid-ocean ridge basalt (N-MORB) to enriched mid-ocean ridge basalt (E-MORB). Zircons from the eclogites yield metamorphic ages of 416–413 Ma, indicating the eclogite facies metamorphism. Coesite inclusions in garnet and omphacite and quartz exsolution in omphacite and pseudosection calculation suggest that some eclogites experienced UHP eclogite facies metamorphism. The eclogites from the eastern part of EKOB record peak conditions of 29–33 kbar/705–760 °C, first retrograde conditions of 10 kbar at 9.5–12.5 kbar/610–680 °C, and second retrograde conditions at ~6 kbar/<600 °C. New evidence of the early Paleozoic UHP metamorphism in East Kunlun is identified in our study. Thus, we suggest that these eclogites were produced by the oceanic crust subducting to the depth of 100 km and exhumation. The presence of East Gouli and Gazhima eclogites in this study and other eclogites (430–414 Ma) in East Kunlun record the final closure of the local branch ocean of the Proto-Tethys and the evolution from subduction to collision. Full article

Titanites can be excellent markers of element transfer in medium-temperature retrograde metamorphism. Euhedral titanites from several alpine fissures from Mont Blanc, particularly those of Périades and Courtes, crystallised at the end of the main quartz stage and are synchronous with the formation of green biotites and albite before chlorite formation. Micro-XRF, SEM, electron probe, and LA-ICP-MS analyses show that these titanites have a wide range of Al₂O₃ content from 1 to 8%, are dominated by -OH versus F, and have a wide range of Nb (up to 4500 ppm), Y (up to 3000 ppm), Zr (up to 1800 ppm), and Sn (up to 1400 ppm) concentrations. The allanite from the granite, partly destabilised into epidote, is the most likely source of Nb, Y, Zr, Sn, and REE. Titanites are enriched in HREE and show variations in LREE depending on the studied sites. Like quartz, they formed at around 400 ± 20 °C, which is compatible with the formation of green biotites after the destabilisation of granite Fe-Mg silicates. This early stage of fluid circulation, synchronous with the Mont Blanc massif uplift, is therefore marked by the titanite formation at the transition between the biotite and chlorite stability fields. Full article

Spinel- and orthopyroxene-bearing metapelitic granulites exposed in the Bunger Hills, East Antarctica, have been intensively studied in recent years because they are supposed to record evidence for UHT metamorphism. Detailed petrographic observations, as well as whole rock and mineral chemistry, together with SIMS trace element data on quartz, garnet, and orthopyroxene, are presented for these rocks. Mineral thermobarometry, including Al-in-orthopyroxene, ternary feldspar, Ti-in-quartz, and Fe-Ti oxide solvus, has been used to quantify the UHT conditions. Based on phase equilibrium modeling, a tight clockwise P-T path has been deduced, which involves near-isobaric heating at 6–7 kbar to ~950 °C followed by near-isobaric to slightly up-pressure cooling at 5–6 kbar to ~750 °C. It is concluded that the outlined metamorphic history is characteristic of an extensional crustal regime which is also evidenced by the correlation of prograde and retrograde metamorphism with the extensional and compressional phases of major ductile deformations recognized in the region. In order to constrain the tectonic setting of the granulites, this result is discussed in the context of current views on the Mesoproterozoic evolution of the Albany-Fraser Orogen, the westernmost part of which the Bunger Hills are considered to be. Full article

In order to assess the geochemical effects of retrograde metamorphic rehydration, fluid metasomatism, and the fluid-mobile elements (FMEs) budget in the case of oceanic and continental subduction, we report the petrography, bulk, and in situ LA-ICP-MS trace-element data for the two poorly studied ophiolites in the northern (Khara-Nur, Eastern Sayan, Russia) and central (Alag-Khadny accretionary wedge, SW Mongolia) parts of the peri-Siberian orogenic framing. Both complexes are relics of the ancient oceanic mantle, which was subjected to processes of partial melting, metasomatism, and retrograde metamorphism. Typical mineral assemblages include olivine + orthopyroxene + chlorite + tremolite ± secondary olivine (640–800 °C), olivine + antigorite ± secondary clinopyroxene (<640 °C), and olivine + chrysotile ± secondary clinopyroxene (<250 °C) and are stable at pressures up to 2 GPa. Hydration and partial serpentinization of mantle peridotites lead to tremolite formation after orthopyroxene, followed by olivine replacement by antigorite. Serpentine-group minerals (antigorite and chrysotile) were distinguished by Raman spectroscopy, and the contents of incompatible elements (mobile and immobile in fluids) in metamorphic minerals (tremolite, antigorite, and chrysotile) were examined in situ by LA-ICP-MS. The behavior of conservative HFSE (Zr, Nb, Ta, and Ti) and—in part—HREE does not distinguish between the two types (oceanic and continental) of subduction environments. Different patterns of FMEs (Cs, Rb, Ba, U, Sb, Pb, Sr, and LREE) enrichment in metaperidotites reflect variations in the slab fluid composition, which was primarily governed by the contrasting nature of subducted lithologies. The affinity of Alag-Khadny to the subduction of a continental margin is recorded by increased FME contents and selective enrichment by some moderately mobile elements, such as U, Th, and LREE, with respect to the oceanic-type subduction environment of Khara-Nur. Distinct patterns of FME enrichment in tremolite and antigorite from two complexes indicate different sequences of fluid-induced replacement, which was controlled by Opx composition. We demonstrate that evaluation of the initial composition of precursor minerals affected by multi-stage melting and melt metasomatism should be considered with care to estimate the differential fluid overprint and associated elemental uptake from subduction fluids. Full article

The discovery of eclogite outcrops in the East Kunlun Orogen Belt (EKOB) has confirmed the existence of an Early Paleozoic HP-UHP metamorphic belt. However, the protoliths and metamorphic histories of widespread metabasites remain poorly constrained. We collected three types of metabasites from the central part of EKOB. We present an integrated study of petrography, whole-rock geochemistry, Sr-Nd isotopes, estimated P–T conditions, and zircon U-Pb isotope ages. The results show that amphibolites and retrograde eclogites have clockwise P–T paths with peak conditions of, respectively, 11–12 kbar and 675–695 °C, and 21.5–22.2 kbar and 715–750 °C. Zircon dating of metabasites from Dagele yields Late Ordovician (~449 Ma) to Early Silurian (~440 Ma) protolith ages and Early Devonian (~414 Ma) amphibolite facies metamorphic ages. Retrograde eclogites from east Nuomuhong have a protolith age of ~902 Ma and metamorphic ages of ~418 Ma, consistent with other eclogites from East Kunlun. Our data suggest that the protoliths of Dagele metabasites represent arc-type magmatism during the subduction of a small back-arc oceanic basin. Instead, the protoliths of retrograde eclogites are Neoproterozoic tholeiitic basalts emplaced into continental crust and subsequently deeply subducted. We develop a new model for Early Paleozoic subduction and collision in the East Kunlun region, emphasizing the role of ‘dominant’ and ‘secondary’ suture boundaries. This model helps explain the ages and metamorphic histories of the metabasites studied here and offers new perspectives on the evolution of the Proto-Tethys Ocean. Full article

Granulite facies rocks provide important keys to evaluating collisional metamorphism in orogenic belts. The mafic granulites of Baoding in the Fuping Complex of the North China Craton occur within the Trans-North China Orogen (TNCO), a major Paleoproterozoic collisional orogen. Here, we present results from detailed investigations on newly discovered garnet pyroxenite, garnet two-pyroxene granulite, and garnet-bearing-plagioclase amphibolite using petrographic, mineralogical, geochemical, and zircon U-Pb dating methods. Our results show that the Fuping Complex metamorphic evolution in this study evolved in four stages: prograde (M₁), high-pressure granulite facies (M₂), granulite facies (M₃), and retrograde (M₄) stages. The mineral assemblage of the prograde stage (M₁) consists of Amp + Pl + Q within garnet cores. The mineral assemblage of high-pressure granulite facies at the peak stage (M₂) consists of Gt + Cpx + Pl + Q ± Amp, forming the garnet pyroxenite. The granulite facies stage M₃ is characterized by the occurrence of orthopyroxene, with a mineral assemblage of Gt + Cpx + Opx + Amp+ Pl + Q. The early retrograde stage M_4-1 includes clinopyroxenes scattered inside amphiboles, following the breakdown of garnet and clinopyroxene. The mineral assemblage of this stage comprises Amp + Pl + Q + Ilm ± Cpx. Later, in the late retrograde stage M_4-2, the composition of amphiboles changed to actinolite, and epidote and chlorite started to appear in the matrix. Traditional geothermobarometry yielded P-T conditions of 700~706 °C and 6.0~6.2 kbar for prograde stage M₁, 854~920 °C and 13.0~13.8 kbar for high-pressure granulite facies stage M₂, 912~939 °C and 8.1~9.9 kbar for M₃, 661~784 °C and 3.1~4.4 kbar for M_4-1, and 637~638 °C, 1.1~1.3 kbar for M_4-2, along a clockwise P-T path with a nearly isothermal decompression (ITD) and slight heating. Zircon LA-ICP-MS U-Pb dating constrains the timing of the high-pressure granulite facies metamorphic event to be between 1.83 and 1.86 Ga. Geochemical features suggest that the protoliths of the high-pressure granulites may have formed in an island arc environment within a convergent margin setting. Together with results from previous studies, our data suggest that the ~1.85 Ga metamorphic age recorded in the Fuping Complex represents a regional metamorphism in the TNCO, associated with the subduction–collision and assembly of the Eastern and Western Blocks of the NCC. Full article