High-pressure Metamorphic Rocks Research Articles

Mesozoic tectonics of the Indo-Burmese wedge: a record from the high-pressure metamorphic rocks of the Nagaland-Manipur Ophiolite Belt, NE India

Mesozoic tectonics of the Indo-Burmese wedge: a record from the high-pressure metamorphic rocks of the Nagaland-Manipur Ophiolite Belt, NE India

Multiple sulfur isotopes evidence deep intra-slab transport of sulfate-rich fluids

Sub-arc mantle oxidation is often attributed to sulfates from seawater transported in aqueous fluids derived from slab dehydration, but sulfur speciation and origin(s) in high-pressure fluids remain weakly constrained. Here, we use the four sulfur stable isotopes (32S, 33S, 34S, 36S) to decipher processes, e.g. mixing of two end-member reservoirs and open-system reaction occurring during sulfur devolatilization, and deduce the nature and origin of sulfur species in aqueous slab-derived fluids. We analyzed high pressure metamorphic rocks from the Monviso massif Lower Shear Zone (LSZ; Western Alps), recording evidence of an interface-parallel, channelized fluid flow near peak burial metamorphic conditions (∼550 °C and 2.7 GPa). We sampled variably metasomatized Fe-Ti metagabbros, hybrid (talc) schists and serpentinites containing sulfides supposed to preserve the aqueous fluid composition, as well as other alpine metagabbro, metabasalt and serpentinites from greenschist to eclogite-facies devoid of the studied metasomatic overprint.The millimeter to centimeter-sized sulfide crystals found in abundance within LSZ lithologies are mainly pyrites containing inclusions of the peak burial mineral assemblage (e.g. garnet, omphacite, rutile) indicating their crystallization at peak conditions or during incipient exhumation. The sulfides in the non-metasomatic metagabbros and metabasalts cover a narrow range in δ’34S of 0.89 ± 0.63 ‰ and Δ’33S of 0.017 ± 0.01 (n = 6) that could be inherited from hydrothermal sulfides formed on the seafloor. In contrast, the LSZ sulfides from the Fe-Ti metagabbros, serpentinites and hybrid schists (n = 24) are enriched in 34S and 33S (bulk δ’34S from 4.91 to 21.32 ‰ and Δ’33S from 0.02 to 0.06 ‰) compared to the sulfides from the non-metasomatic rocks. Mixing between reduced sulfur species cannot explain all the data without requiring an end-member composition as high as δ34S ∼ 22 ‰ and Δ 33S ∼ 0.06 ‰, i.e. values rarely recorded by sulfides in the literature. Instead, we propose that this 33S-34S-enrichment results from open-system sulfate reduction to sulfides in aqueous fluids at 550 °C, with an initial δ34S ∼ 12 ± 3 ‰ and Δ 33S ∼ 0.025 ± 0.010 ‰. Because this signature is different from seawater sulfate, we suggest that sulfate was produced by hydrothermal sulfide oxidation during antigorite breakdown at 650 °C, in agreement with Mg, Ni and Cr enrichments in the metasomatized HP mafic lithologies. This model is consistent with relatively oxidized 34S-enriched arc lavas and can potentially explain recycled sulfur with positive δ34S signature recorded by ocean island basalts.

A Residual Middle Triassic Oceanic Island-Seamount in the Maoershan Area, Northwest Tibet: Implications for the Evolution of the Paleo-Tethys Ocean

The assemblage of oceanic islands and seamounts, arising from the widespread presence of mature oceans, plays a crucial role in reconstructing the evolutionary history of the paleoocean. Oceanic islands or seamounts within the Longmuco-Shuanghu metamorphic complex, a remnant of the Paleo-Tethys Ocean in the central Tibetan Plateau, have seldom been reported due to their remoteness. This study has identified an oceanic island-seamount in the Maoershan area, situated to the west of the Longmuco-Shuanghu metamorphic complex, composed of basalt, diabase, limestone, and siliceous rocks. Based on field observations, petrology, zircon U-Pb dating, whole-rock geochemistry, and Sr-Nd isotopes analyses, we have identified a suite of mafic rocks with OIB affinity. The youngest zircon U-Pb age cluster was concentrated at ~243–241 Ma. The geochemical characteristics of the siliceous rocks indicate a mixture of terrigenous material, suggesting that they formed in a continental margin. In combination with regional geological data, we conclude that the Longmuco-Shuanghu Paleo-Tethys Ocean remained open during the Middle Triassic. Furthermore, a fraction of the oceanic island-seamounts underwent scraping and transformed into a metamorphic complex, while other segments experienced deep subduction, resulting in the formation of high-pressure metamorphic rocks. Collectively, these processes gave rise to the distinctive high-pressure metamorphic complex within the central Qiangtang terrane.

Metamorphic P-T-t path of Triassic eclogite from the Rongcheng region, eastern China: implications for the tectonic evolution of Sulu orogenic belt

ABSTRACTInterpretation and inversion of P-T (Pressure-Temperature) paths of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks are critical to understanding the subduction and exhumation histories of an orogen. The Sulu orogen constitutes the eastern part of the Dabie-Sulu orogen, in which, HP metamorphic rocks are relatively poorly as studied compared to the well-documented UHP metamorphic rocks. This study presents the P-T paths of HP eclogites constructed by petrography, mineral chemistry, and pseudosection modelling by using the GeoPS software within the MnNCFMASKHTO (Mn-CaO-FeO-MgO-Al2O3-SiO2-K2O-H2O-TiO2-O2) system. Sample 19LR70–1 mainly consists of omphacite, garnet, quartz, minor rutile, ilmenite, and amphibole. Sample 19LR71–1 is mainly composed of omphacite, garnet, kyanite, and accessory minerals of rutile and amphibole. These two eclogite samples recorded clockwise P-T paths with peak P-T conditions of P ≥ 21.3 kbar, T ≥ 755°C and P ≥ 23.0 kbar, T ≥ 871°C, respectively. Well-correlated U-Pb ages and trace element data show that the protoliths of eclogites were 655–699 Ma and 676–754 Ma, the metamorphic ages were 212–232 Ma, and the time of partial melting were 221–225 Ma. Combined with regional geological data, we conjectured that before 240 Ma, the South China plate was subducted to the North China craton; subsequently, at 240–220 Ma, HP eclogite facies metamorphism occurred at a depth of 60–70 km. This was followed by the exhumation of these HP metamorphic rocks, coupled with concurrent partial melting, around 220–200 Ma.

Late orogenic high-temperature overprint in (ultra)high-pressure ([U]HP) metamorphic rocks of the North Qinling Orogen, Central China: Insights into the geodynamics of the exhumation of (U)HP metamorphic rocks

Exhumed (ultra)high-pressure ([U]HP) metamorphic rocks are markers of paleo-subduction zones and can provide crucial implications for the geodynamic evolution of subduction-exhumation. A variety of (U)HP metamorphic rocks from many collisional orogens have commonly documented a low-pressure high-temperature overprint, which appears to indicate a pervasive geodynamic process behind (U)HP metamorphic rock exhumation. A good case study of garnet amphibolite and associated garnet-biotite gneiss with an unambiguous low-pressure high-temperature overprint in the Xixia-Xiaguan area of the North Qinling Orogen, central China, provides a significant opportunity to uncover this geodynamic process. A detailed study of petrography, mineral chemistry, and phase equilibria modeling suggests that the investigated garnet amphibolite and garnet-biotite gneiss have jointly experienced multistage exhumation processes with near-isothermal decompression from the (U)HP eclogite facies (26.8−29.0 kbar/740−775 °C) to the medium-pressure (MP) amphibolite facies (8.7−10.0 kbar/740−775 °C), subsequent remarkable near-isobaric heating to the low-pressure (LP) granulite facies (6.8−8.2 kbar/832−835 °C), followed by final decompression and cooling to the LP amphibolite facies (5.8−6.8 kbar/740−757 °C). U-Pb age dating of zircons from the garnet amphibolite and garnet-biotite gneiss yielded metamorphic ages of ca. 500 Ma, 450 Ma, and 400 Ma, which we consider to represent (U)HP eclogite-facies, LP granulite-facies, and late LP amphibolite-facies metamorphism, respectively. According to the regional geology and the metamorphic history we have determined, this multistage exhumation process of two-step exhumation stages delimited by a phase of LP near-isobaric heating was associated with slab rollback. Accordingly, we propose a tectonic model of exhumation driven by slab rollback for the geodynamic evolution of (U)HP metamorphic rock exhumation in the North Qinling Orogen. Additionally, a combination of previous studies and our new results suggests that exhumation driven by slab rollback appears to be a fundamental mechanism for the (U)HP metamorphic rock exhumation within collisional orogens.

Determination of the early Paleozoic accretionary complex in Southwestern Yunnan, China: Implications for the tectonic evolution of the Proto-Tethys Ocean

Accretionary complex study provides important knowledge on the subduction and the geodynamic processes of the oceanic plate, which represents the ancient ocean basin extinction location. Nevertheless, there exist many disputes on the age, material source, and tectonic attribute of the Lancang group, located in Southwest Yunnan, China. In this paper, the LA-ICP-MS detrital zircon U-Pb chronology of nine metamorphic rocks in the Lancang group was carried out. The U-Pb ages of the three detrital zircons mainly range from 590–550 Ma, 980–910 Ma, and 1150–1490 Ma, with the youngest detrital zircons having a peak age of about 560 Ma. The U-Pb ages of the six detrital zircons mainly range from 440–460 Ma and 980–910 Ma, and the youngest detrital zircon has a peak age of about 445 Ma. In the Lancang group, metamorphic acidic volcanic rock, basic volcanic rocks, intermediate-acid intrusive rocks, and high-pressure metamorphic rocks are exposed in the form of tectonic lens in schist, rendering typical melange structural characteristics of “block + matrix”. Considering regional deformation and chronology, material composition characteristics, and the previous data, this study thinks the Lancang group may be an early Paleozoic tectonic accretionary complex formed by the eastward subduction of the Changning-Menglian Proto-Tethys Ocean, which provides an important constraint for the Tethys evolution.

The Redefinition of the “Bulunkuole Group” in the West Kunlun Orogenic Belt, Xinjiang (NW China): Implications for Tectonic Evolution of the Proto-Tethys

The formation ages and tectonic features of the Bulunkuole Complex (BKC) is critical for understanding the geological evolution of the western section of the West Kunlun Orogenic Belt (WKOB), and they are also critical for understanding the metallogenic background of the Taxkorgan Iron Belt. In this paper, we report new geochemical and in situ zircon U–Pb isotopes data for the most southwestern iron-bearing formation of the BKC. The petrography and sedimentation ages of the BKC reveal that the main part of the BKC was emplaced in the Early to Late Cambrian period as a giant accretionary wedge formed during the Proto-Tethys Ocean south-southwestward subduction. The high-pressure metamorphic rocks located at the margin of the Kangxiwa Fault should be further disintegrated from the Cambrian BKC to form a Triassic accretionary complex. Geochemical characteristics indicate that the metasedimentary rocks of the Cambrian BKC derived predominantly from the regional contemporary intermediate to felsic source rocks, and deposited in the fore-arc basin. Provenance studies further demonstrate that the detrital materials were mainly sourced from the Gondwana-affinity terranes, Mazar Terrane as well as the volcanic and magmatic rocks produced during the Tethys subduction. The metamorphism of the Cambrian BKC occurred at ca. 200 Ma in the western section of the WKOB. Proto-Tethys Ocean did not close until 230 Ma, possibly during the Early Mesozoic (200–180 Ma).

Late Ordovician High Ba-Sr Intrusion in the Eastern North Qilian Orogen: Implications for Crust–Mantle Interaction and Proto-Tethys Ocean Evolution

High Ba-Sr granitic rocks are widespread in Phanerozoic orogenic systems, and their petrogenesis is important for revealing the evolutionary process of the Proto-Tethys Ocean in the North Qilian orogenic belt. This paper presents a combination of zircon U-Pb age, whole-rock major and trace element concentrations, and Sr-Nd-Hf isotopic data for Caowa high Ba-Sr dioritic intrusion from the eastern part of the North Qilian orogenic belt, aiming to decipher its petrogenesis and tectonic setting. LA-ICP-MS zircon U-Pb dating yield an emplacement age of 450 ± 2 Ma for the Caowa intrusion, indicating a magmatic activity of the Late Ordovician. The Caowa quartz diorites contain moderate contents of SiO2, MgO, Mg#, and resultant high concentrations of Na2O + K2O, Fe2O3T, and Al2O3, displaying calc-alkaline and metaluminous characteristics. The studied samples have relatively elevated Ba (up to 1165 ppm) and Sr (561 to 646 ppm) contents, with obvious enrichment in LILEs (e.g., Ba, Th, U) and depletions in HFSEs (e.g., Nb, Ta, Ti), resembling those of typical high Ba-Sr granitoids in subduction zones. Together with enriched Sr-Nd isotopic composition [(87Sr/86Sr)i = 0.7082–0.7086, εNd(t) = −5.1 to −4.9], and the wide ranges of zircon εHf(t) values (−13.2 to +8.5), it suggests that these high Ba-Sr quartz diorites were derived from a mixture magma source between the ancient crust materials and the enriched lithospheric mantle metasomatized by fluid released from subducted oceanic crust or sediment. Taking into account the ophiolites, high pressure metamorphic rocks, and arc magmatic rocks in the region, we infer that due to the influence of the northward subduction of the Qilian Proto-Tethys Ocean, the Laohushan oceanic crust of the North Qilian back-arc basin was subducted during the Late Ordovician and resulted in extensive metasomatism of lithospheric mantle by fluids derived from oceanic crust or sediments, and the Caowa high Ba-Sr quartz diorites were generated in the process of crust–mantle interaction during the Late Ordovician.

Interrelated pressure-temperature-time-paths of medium to high pressure metamorphic rocks in the Sierra Pie de Palo (W-Argentina): Evolution of a “hard” collisional wedge during an Ordovician microcontinent-arc collision

The Sierra Pie de Palo (W-Argentina) is part of an Ordovician collisional wedge between the Cuyania microcontinent and the Ordovician Famatinian arc along the west Gondwana protomargin. The flatly E-dipping collisional wedge formed by polyphase shearing. The lower plate consists of Neoproterozoic-Cambrian metasedimentary rocks and Mesoproterozoic mafic-ultramafic basement of eastern Cuyania. The Famatinian forearc represents the upper plate composed of Mesoproterozoic metasedimentary rocks intruded by Mesoproterozoic and Ordovician plutons and overlying Neoproterozoic metasedimentary rocks. Pseudosection modeling on 20 samples distributed over the entire wedge yields a range of recorded maximum PT conditions of 7–14 kbar/480–590 °C along metamorphic gradients of 10–20 °C/km. Clockwise PT-paths are common starting from a midcrustal level with gradients of 20–30 °C to maximum depth followed by thermal relaxation. Local anticlockwise PT-paths are observed (1) in high-grade basement klippen of the overriding Famatinian arc (8–10 kbar/700–765 °C) caused by late emplacement onto the nappe stack and (2) within late upper crustal ductile zones with normal shear sense.Isotopic ages of (re)crystallisation of metamorphic minerals and assemblages comprise: 429 ± 2 to 434 ± 7 Ma (Lu-Hf; garnet), 460 ± 6 Ma (UPb monazite in garnet), 404 ± 7 to 422 ± 8 Ma (Rb-Sr; white mica) and 403 ± 4 to 440 ± 8 Ma (40Ar/39Ar white mica). Including previous data, ages cluster around major peaks at 464 ± 1 Ma, 436 ± 3 Ma, 427 ± 3 Ma, 415 ± 2 Ma and 405 ± 1 Ma related to deformation during three orogenic stages: (1) subduction of the lower plate (eastern Cuyania) to the basis of the wedge at 470–455 Ma and later burial of the upper plate (Famatinian forearc) to similar depths at 440–415 Ma, a characteristic of a “hard” collision; (2) exhumation mainly by erosion and (3) late dissection of the wedge by upper crustal thrusts and some normal faulting at the top of the wedge during arrival of the colliding microcontinent Chilenia (405–360 Ma) at the new Devonian leading margin.

Early cretaceous bimodal volcanic rocks in Wuga Co area, central tibet: The first identification of direct products derived from slab sinking in the Bangong-Nujiang suture zone

Introduction: The Bangong-Nujiang Suture Zone (BNSZ) in central Tibet is a remnant of the Bangong-Nujiang Ocean that records its entire Wilson Cycle. The model of divergent double-sided subduction (DDS) is crucial for elucidating the evolution of tectonomagmatic activity on both sides of the BNSZ and for understanding why no high-pressure metamorphic rocks occur in the BNSZ. However, the DDS geodynamics remain poorly constrained. In particular, there is a lack of reports on magmatic rocks directly associated with slab sinking in the DDS terminal stage.Methods: This study presents new geochronological, geochemical, and isotopic data for the Early Cretaceous bimodal volcanic rocks around the Wuga Co area.Results: The bimodal volcanic rocks are divided into the Wuga Co rhyolites (SiO2 = 77.0–79.0 wt%) and the Wuga Co basaltic andesites (SiO2 = 53.9–55.5 wt%). The isotopic values of the Wuga Co basaltic andesites with low (87Sr/86Sr)i values (+0.7040 to +0.7044) and high εNd(t) values (+3.8 to +4.1) lie among three endmembers (the BNO sediments in accretionary wedge, depleted mantle and the BNO slab).Discussion: These values indicate the partial melting of a mantle peridotite that interacted with the subducted slab and sediment in the accretionary wedge, which was caused by the sinking of the Bangong-Nujiang oceanic slab. The Wuga Co rhyolites (108 Ma) have low (87Sr/86Sr)i values (+0.703 to +0.706), high εNd(t) values (+2.25 to +2.49), and high εHf(t) values (+5.6 to +10.0). These values indicate that the rhyolite formed by partial melting of juvenile basaltic crust. This study also collected Hf isotope data from both sides of the BNSZ to constrain its evolution. Our results show that the εHf(t) values of magma on both sides of the BNSZ were elevated simultaneously at 130 Ma, which may be caused by the Bangong-Nujiang oceanic slab rupture. Based on these new data, we propose that the Bangong-Nujiang oceanic slab ruptured from the two overlying terranes at approximately 130 Ma and subsequently sank into the mantle at approximately 108 Ma.

Reaction-Induced Porosity in an Eclogite-Facies Vein Selvage (Monviso Ophiolite, W. Alps): Textural Evidence and In Situ Trace Elements and Sr Isotopes in Apatite.

Abstract Porosity generated during fluid–rock reaction can facilitate fluid transport and metasomatism in low permeability high-pressure metamorphic rocks. Evidence for reaction-induced porosity is found in an eclogite-facies clinopyroxene + apatite vein in an undeformed eclogitized Fe–Ti metagabbro from the Monviso Ophiolite (W. Alps) with a distinct garnet-rich selvage. Vein-forming fluids were sourced from adjacent metagabbros and reaction with the host rock removed Ca and P from the selvage and added Fe, REE, Pb and Cr. Textures at the selvage–host rock interface and in the host rock record local heterogeneity in reactivity and porosity during metasomatism linked to variable initial lawsonite abundance. These features reflect a hierarchy of pervasive-to-channelized porosity structures that facilitated widespread metasomatism of the host rock. Development of this metasomatic system in response to locally derived fluids suggests large-scale externally derived fluid transport is not required to drive extensive fluid–rock exchange. The production of porosity during metasomatic reactions could be important in facilitating further fluid–rock reaction and fluid transport in subducting slabs where permeability is low.

Cold subduction recorded by the 1.9 Ga Salma eclogite in Belomorian Province (Russia)

When plate tectonics emerged on Earth is one of the most fundamental questions in geosciences. Nowadays, different arguments from diverse research fields (petrology, structural geology, geochemical and numerical modeling) postulate that the lithospheric subduction began sometime in the Archean-Paleoproterozoic with different styles. This study provides robust petrologic evidence for the modern-style plate tectonics onset in the Paleoproterozoic featured by low geothermal gradients. We investigated the Salma eclogite association from the Precambrian Belomorian Province (BP) on the Fennoscandian Shield (Russia). Based on the garnet chemical annuli (major and trace elements) and therein mineral inclusions, plus comprehensive zircon isotopic-chemical analyses, we establish the prograde metamorphic history of the eclogite, leading to peak conditions of 2.2-2.3 GPa, 650-670°C prior to 1.93 Ga. We thus, for the first time, conclude cold subduction (303 ± 16°C/GPa) proceeded in the BP, connected with the Lapland-Kola Orogen (LKO). Such conclusions are clearly against the previously argued Archean subduction scenario and “warm” tectonic setting in the BP, and are very important for understanding the geodynamics of the early Earth. Furthermore, in the light of other scarce coeval “cold” Paleoproterozoic high pressure (HP) metamorphic rocks elsewhere, we posit that modern-style plate tectonics already operated in the form of the Columbia Supercontinent.

Late Cretaceous Transition From Calc-Alkaline to Alkaline Magmatism in the Eastern Anatolian Plateau: Implications for Microblock Collision Timing

Abstract In convergent belts, a collision between two blocks can reshape upper mantle geometry and processes responsible for a change in mechanisms of magma generation with distinct geochemical compositions. Therefore, identifying the turning point of a magma compositional shift can provide key constraints on collision timing, which is decisive in building a framework of regional tectonic evolution. The Eastern Anatolian Plateau is composed of a mosaic of rifted blocks assembled through successive collisional events, culminating with the terminal collision of Arabia with Eurasia and the closure of the southern branch of the Neotethys in the Cenozoic. The timing of the microblock collision of the Bitlis–Pütürge Massifs with the Eastern Taurides Block, the southern Eastern Anatolian Plateau, is a matter of debate due to limited constraints on the timing and petrogenesis of the (post-)collision-related magmatism during the Late Cretaceous. This study identifies three compositionally distinct intrusive suites aged from ~87 to ~69 Ma in the Eastern Taurides Block, the southern Eastern Anatolian Plateau. Group 1 intrusive rocks were emplaced in the southern Eastern Taurides Block at ~87–77 Ma and are characterized by high-K calc-alkaline compositions with predominantly depleted Hf and Nd isotope compositions [εHf(t) = 0.9 to +16.5 and εNd(t) = −2.3 to +6.9]. In contrast, the younger (~77–69 Ma) Group 2A comprises nepheline (Ne)-normative alkaline compositions, and Group 2B consists of shoshonitic compositions; both groups are mainly distributed in the central and northern Eastern Taurides Block. Groups 2A and 2B overlap in age between ~77 and 69 Ma and show relatively enriched Hf–Nd isotope compositions [Group 2A: εHf(t) = −1.0 to +4.7 and εNd(t) = −2.3 to +1.2; Group 2B: εHf(t) = −4.0 to +4.0 and εNd(t) = −6.5 to −1.3]. Group 1 intrusive rocks relate to the sub-arc asthenosphere-derived melts that differentiated toward the granite minimum by fractional crystallization. Parental melts of Group 2A intrusive rocks are interpreted to be derived from metasomatized subcontinental lithospheric mantle and differentiated toward the phonolite minimum. Crustal assimilation during magma ascent pushed the initially silica-undersaturated magma (Group 2A) into silica-(over)saturated compositions (Group 2B) through fractional crystallization that ultimately evolved toward the granite minimum. We propose that the Group 1 magmatism was related to northward subduction and closure of the Berit oceanic lithosphere, whereas the Group 2 magmatism results from collision-induced lithospheric delamination ± slab rollback. Together with the P–T–t evolution of the high-pressure metamorphic rocks from the Bitlis Massif and the spatiotemporal and geochemical variations of the Late Cretaceous magmatism in the Eastern Taurides Block, this study suggests that microblock collision of the Bitlis–Pütürge Massifs with the Eastern Taurides Block (Eurasia) most likely occurred at ~84–77 Ma. This study provides an example to constrain collision timing through a perspective of magmatic transition from calc-alkaline to alkaline series.

Petrographic, geochemical, and geochronological characteristics of metaplagiogranites from a high-pressure mélange in the Yuli belt, eastern Taiwan: Evidence for an early Miocene igneous precursor

Glaucophane-bearing metaplagiogranite occurs among the high-pressure (HP) metamorphic rock types poorly exposed in the Yuli belt. This lithologic association has been interpreted as scattered fragments of a metamorphosed ophiolite. Through field and petrographic investigations, we have identified several types of metaplagiogranite in the Chinshuichi area. This study reports the first U-Pb zircon dates by secondary ion mass spectrometry (SIMS) and whole-rock geochemical characterization of these metaplagiogranitic samples and associated metagabbro. The newly-obtained U-Pb zircon dates of 17.4–16.9 Ma are interpreted as magmatic crystallization ages. The δ 18 O averages of the dated zircon grains are 3.9–5.0 ‰ for the metaplagiogranites and 4.9 ‰ for the metagabbro, implying that the ultimate source rocks were of mantle origin. These metaigneous rocks are characterized by negative niobium tantalum anomalies in whole-rock composition. Protoliths of these HP metaigneous rocks may have formed in a nascent forearc setting related to an early Miocene subduction initiation. This finding infers that HP metamorphism of the Chinshuichi plagiogranite was caused by subduction erosion, and challenges some previous models that commonly postulated the South China Sea origin for the metaophiolitic fragments of the Yuli belt. • Four types of metaplagiogranite are newly identified. • First report of magmatic zircon ages of ~17 Ma by SIMS U-Pb dating on the metaplagiogranites. • Protoliths of the metaplagiogranites are likely forearc materials related to subduction initiation. • High-pressure metamorphism in the study area might have been partially caused by subduction erosion.

Favorable P–T–ƒO2 conditions for abiotic CH4 production in subducted oceanic crusts: A comparison between CH4-bearing ultrahigh- and CO2-bearing high-pressure eclogite

Subduction is a key element in the carbon cycle between the Earth’s surface and deep interior. In addition to pressure (P) and temperature (T), oxygen fugacity (ƒO2) is an intensive variable that can change the speciation, stability, and mobility of carbon-bearing phases in subduction zones. However, tracking redox evolution and associated carbon speciation in high-pressure metamorphic rocks from subduction zones remains challenging. In this study, we identified CH4-rich fluid inclusions in ultra-high-pressure (UHP; i.e., coesite-bearing) eclogites (the UHP-CH4 eclogite) and CO2-rich fluid inclusions in high-pressure (HP; i.e., quartz-bearing) eclogites (the HP-CO2 eclogite) from the Chinese southwestern (SW) Tianshan palaeosubduction zone, respectively. Combining the garnet-clinopyroxene oxybarometry and comprehensive thermodynamic calculations, we revealed that the ƒO2 of the UHP-CH4 eclogite decreases to a minimum of ∼FMQ − 3 at peak P–T conditions (2.8 GPa, 525 °C) and CH4 is expected to form in graphite-saturated CHO fluids. Whereas during the early-exhumation stage (2.5 GPa, 600 °C), the UHP-CH4 eclogite is predicted to buffer higher ƒO2 conditions of ∼FMQ − 0.9. In contrast, the HP-CO2 eclogite, which has a higher bulk CO2 content (∼10 wt%) and elevated Fe3+/ΣFe ratio (0.24) than the UHP-CH4 eclogite, suggesting a stronger oxidative seafloor alteration of the HP-CO2 eclogite protolith before subduction, stabilizes graphite and CO2-rich fluids at peak ƒO2 conditions between FMQ and FMQ + 1 (2.6 GPa, 500 °C). These data demonstrate that the thermal structure (P–T conditions) of the subduction zone and the composition of the slab are first-order controls on the redox evolution of subducting slabs and the changing speciation of carbon in slab fluids buffered by the metamorphic rock system. Therefore, we propose that the cold subduction of a weakly altered oceanic crust (e.g., UHP-CH4 eclogites from the SW Tianshan) provides ideal conditions for the formation of ultra-deep abiotic CH4.

Late Carboniferous extension-related magmatism in the southwestern Alxa block of NW China: geochronology, geochemistry, and tectonic implications

ABSTRACT Whether the northern margin of the North China Craton (NCC) was a continental arc or in a lithospheric extensional setting during the Carboniferous is the primary divergence among different tectonic models about the terminal evolutionary history of the southeastern Central Asian Orogenic Belt (CAOB). The Alxa block is the western segment of the northern NCC and contiguous to the southeastern CAOB, and therefore the tectonic setting of the Carboniferous magmatic intrusions in this block can shed light on this controversy. In this contribution, new zircon U–Pb ages, elemental geochemistry, and Sr-Nd isotopes are presented for thirteen Late Carboniferous (~318–300 Ma) magmatic intrusions in the southwestern Alxa block. Among them, seven calc-alkaline I-type granites and one aluminous A2-type granite were probably generated by the partial melting and crystallization differentiation of various crustal sources (perhaps including earlier orogenic materials) under different temperature conditions. Four quartz diorites and one hornblende gabbro may be the partial melts of subduction-modified basaltic lower crust sources and sub-continental lithospheric mantle sources, respectively. According to the integration of the new results and regional geological correlations, the Alxa block and the southeastern CAOB were most likely in a uniform tectonic regime during the Carboniferous. At that time, this region was characterized by the lithospheric extension-related magmatic rock associations, the increasing mantle inputs over time, the clockwise P–T paths of high temperature-low pressure metamorphic rocks, and the transgression sedimentary sequences. All these lines of evidence suggest an intra-continental lithospheric extensional setting rather than a continental arc setting for the southeastern CAOB during the Carboniferous.

Geology and genesis of the Silica-Listwaenite hosted Kaymaz gold deposit, Eskişehir, NW-Turkey: Implications from fluid inclusions and pyrite chemistry

The Kaymaz gold deposit comprises Damdamca, Karakaya, Küçük Mermerlik, and Kızılağıl ore zones within an area underlain by serpentinite and far-traveled Paleozoic-Mesozoic high-pressure metamorphic rocks. The Kızılağıl ore zone is hosted in silicified quartz schist, whereas the others are hosted in silica altered serpentinite. Pyrite-I, arsenopyrite, marcasite, magnetite, pentlandite, millerite, nickeline, bravoite, and fine-grained native gold and silver comprise the first stage, whereas pyrite-II and chalcopyrite represent the second ore stage. The Kaymaz gold deposit has been defined as a silica-listwaenite hosted gold deposit according to host rock relations and mineralogical properties.Gold, Ag, and As were found to be highly correlative in whole-rock silica-listwaenite analyses. Higher As content of the pyrite-I, coexisting native gold and silver together with the whole-rock analyses indicate that these metals were derived from the same source, possibly the Kaymaz granite. On the other hand, clathrate formations observed in some of the first stage fluid inclusions confirm that high Ni in the pyrite-I may have been derived from the serpentinites. However, a distinct lack of the separated carbonic phase and clathrate formations in the second stage fluid inclusions as well as their lower Th and salinities, reveal that higher Co in the pyrite-II possibly dissolved from the metabasites by meteoric solutions.

Mechanism of crustal thickening and exhumation of southern Lhasa terrane during the Late Cretaceous: Evidence from high-pressure metamorphic rocks of the Eastern Himalayan Syntaxis

AbstractThe mechanism of Late Cretaceous crustal thickening and exhumation of the southern Lhasa terrane is critical for understanding the tectonic evolution of the Tibetan Plateau. High-pressure metamorphic rocks from the lower crust are good candidates for addressing this issue. In this study, we focus on Late Cretaceous, high-pressure, garnet-bearing amphibolites from the Nyingchi Complex of the Eastern Himalayan Syntaxis and present an integrated study of geochronology, petrography, mineral chemistry, and thermodynamic modeling. Petrographic data determine three metamorphic stages (M1–M3). The M1 stage is characterized by a peak mineral assemblage of garnet + hornblende + albite + rutile + muscovite + quartz, which is followed by a post-peak (M2) assemblage of garnet + hornblende + plagioclase + epidote + biotite + rutile + quartz. The late retrograde stage (M3) is defined by hornblende + plagioclase symplectites surrounding garnet porphyroblasts. Mineral chemistry, with thermodynamic modeling, constrains the P-T conditions of the M1–M3 stages to 14–19 kbar/660–720 °C, 8–10 kbar/650–660 °C, and <7 kbar/<600 °C, respectively. Metamorphic zircons yield a concordant age at 90 Ma, which indicates the formation of garnet-bearing amphibolites. These results indicate a P-T-t path involving near-isothermal decompression for garnetbearing amphibolites, which suggests that the Nyingchi Complex underwent peak-pressure metamorphism (M1) at 90 Ma, followed by rapid exhumation to the depth of 32–26 km along the subduction channel. Moreover, the garnet-bearing amphibolites are considered to be the product of high-pressure metamorphism of mafic crust at the base of the Gangdese belt. Hence, the crust of the Gangdese belt experienced significant crustal thickening of up to 60 km at 90 Ma.

Episodic fluid flow in an eclogite-facies shear zone: Insights from Li isotope zoning in garnet

Abstract Episodic fluid overpressure and escape is invoked as a cause or consequence of many subduction-zone seismic phenomena but can be challenging to constrain in exhumed high-pressure metamorphic rocks. In situ measurements of lithium isotopes in garnet reveal evidence of episodic fluid transport in a subduction shear zone now exposed in the Monviso ophiolite (Western Alps). Garnet from an eclogite block and associated metasomatic reaction rind was analyzed by secondary ion mass spectrometry (SIMS). All analyzed garnet preserves core-rim zoning in δ7Li and large negative δ7Li excursions (NEs) in mantles. These excursions cannot be explained by instrumental mass fractionation during analysis, equilibrium fractionation, or intracrystalline diffusion of Li within garnet. Instead, NEs were produced by kinetic fractionation of Li isotopes during bulk diffusion through a pore fluid, and the fractionated isotopic compositions were incorporated into garnet by syn-diffusion growth. Disequilibrium garnet growth textures associated with negative δ7Li support this interpretation and suggest metasomatism drove rapid garnet growth. Four distinct NEs were identified requiring that at least four pulses of fluid were transported within the adjacent shear zone. This evidence of episodic fluid transport along a subduction shear zone at eclogite facies supports models of intermediate-depth seismicity that rely on cyclic fluid overpressure and escape.

Consistent detachment of supracrustal rocks from a fixed subduction depth in the Cyclades

While subduction zones recycle Earth's crust back into the mantle, high pressure metamorphic rocks exposed at the surface testify to occasional crustal subduction failure. Balancing the budget of Earth's recycling factory requires a fundamental comprehension of how and when some crustal rocks are returned to the surface. Here, we constrain the timing and conditions for rocks that were subducted, exhumed and now exposed on Syros and Sifnos islands (Cyclades, Greece). Detailed petrochronology on domains of supracrustal oceanic, passive margin and continental provenance shows that progressive subduction and exhumation has resulted in imbrication of a former continental margin. The consistency of age groups in 2–4 Myr intervals across two islands suggests episodic triggering of exhumation of coherent slices, similar to nappe stacking in the upper levels of orogens. Similar maximum burial conditions across the domains indicate that their highly variable material properties have not exerted a control on the depth from which they were recovered, and supracrustal exhumation was instead triggered by a steady state mechanism operating at ∼65–70km depth in the subduction zone. We propose that for mechanically strong rocks, consistent exhumation may have been triggered by thermal weakening at the depth of coupling between the subducting slab and mantle wedge corner flow in an extended back-arc. For weakly coupled supracrustal rocks, consistent exhumation may have been triggered by a stepped buoyancy increase for subducting rocks at the crust mantle boundary of a thickened overlying plate. These models help to understand the mechanisms and conditions by which supracrustal rocks may be systematically detached from a downgoing plate.