Ultrahigh-pressure Metamorphic Rocks Research Articles

Subduction polarity reversal facilitated by plate coupling during arc-continent collision: Evidence from the Western Kunlun orogenic belt, northwest Tibetan Plateau

Abstract Subduction polarity reversal usually involves the break off or tearing of the downgoing plate (DP) along the continent-ocean transition zone, in order to initiate subduction of the overriding plate (OP) with opposite polarity. We propose that subduction polarity reversal can also be caused by DP-OP coupling and can account for the early Paleozoic geological relationships in the Western Kunlun orogenic belt in the northwestern Tibetan Plateau. Our synthesis of elemental and isotopic data reveals transient (~2 m.y.) changes in the sources of early Paleozoic arc magmatism in the southern Kunlun terrane. The early-stage (ca. 530–487 Ma) magmatic rocks display relatively high εNd(t) (+0.3 to +8.7), εHf(t) (−3.6 to +16.0), and intra-oceanic arc-like features. In contrast, the late-stage (485–430 Ma) magmatic rocks have predominantly negative εNd(t) (−4.5 to +0.3), εHf(t) (−8.8 to +0.9), and higher incompatible trace elements (e.g., Th), similar to the sub-continental lithospheric mantle beneath the Tarim craton. This abrupt temporal-spatial variation of arc magmatism, together with the detrital zircon evidence, indicate that subduction polarity reversal of the Proto-Tethys Ocean occurred in a period of ~10 m.y., consistent with the time interval reflected by ophiolite age. This rapid polarity reversal corresponds with the absence of ultrahigh-pressure (UHP) metamorphic and post-collisional magmatic rocks, features normally characteristic of slab break-off or tearing. Numerical modeling shows that this polarity reversal was caused by plate coupling during arc-continent collision. This coupling modified the normal succession of arc-continent collision events, preventing slab break-off or tearing-induced buoyant rock rebound and asthenosphere upwelling. Our model successfully explains early Paleozoic orogenesis in the Western Kunlun orogenic belt and may be applied elsewhere where post-collisional magmatic and UHP rocks are absent.

Crustal thickness variation of the Dabie orogenic belt: Insights from detrital zircon evidence and geological significance

The Dabie orogenic belt is characterized by widely distributed Mesozoic ultrahigh-pressure metamorphic rocks. Together with the Qinling orogenic belt, it constitutes one of the most important collisional orogens in eastern Asia. Because of intense continental subduction and collision during the Mesozoic and subsequent denudation, it remains controversial whether pre-Mesozoic convergent events happened in the region of the Dabie orogenic belt. In this study, we reconstruct the crustal thickness variation of the Dabie orogenic belt based on detrital zircon data. Results reveal three stages of crustal thickening at ca. 900-800 Ma, ca. 530-400 Ma, and ca. 250-210 Ma that are likely associated with three major convergent events that happened between the North and South China Blocks since the Neoproterozoic. The first thickening stage is possibly related to long-term subduction-accretion-collision processes along the northern margin of the South China Block. The second one corresponds to early Paleozoic ocean closure and the arc-continent collision that happened between the North and South China Blocks. The third one is a most significant thickening that represents the final collision between the North and South China Blocks during the Mesozoic. The turning point (ca. 210 Ma) of the crustal thickness curve represents the cessation of ultradeep subduction and constrains the completion of Mesozoic continental collision. Late Archean crustal thinning may be related to the onset of plate tectonics. Overall, the fluctuations of crustal thickness and detrital zircon age populations from sediments of the Dabie orogenic belt suggest interaction among Chinese continental blocks is consistent with the evolution of global supercontinent cycles.

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.

Sedimentary record of Variscan unroofing of the Bohemian Massif

The Bohemian Massif in the eastern Variscan Orogen is the host of globally significant ultrahigh-pressure metamorphic rock suites that experienced enigmatic unroofing processes. In the eastern Bohemian Massif, the Moravosilesian Culm Basin (The Czech Republic) with ≤7.5 km thick Lower Carboniferous siliciclastic turbidities has recorded crustal unroofing associated with the late Paleozoic amalgamation of Gondwana and Laurussia. This study presents data from sandstone petrography, heavy mineral assemblages, and detrital zircon U-Pb geochronology from Moravosilesian Culm sediments. Sandstone modal compositions imply continental/recycled orogen affinities. High-grade metamorphic lithic fragments and high garnet contents in upper Culm sediments indicate the exhumation of the middle to lower crustal materials, consistent with those of the Bohemian Massif. Detrital zircon U-Pb data constrain maximum depositional ages of sediments in the Drahany Upland to 335–326 Ma (late Viséan–middle Serpukhovian). Detrital, igneous and metamorphic zircon U-Pb data compiled from surrounding regions support that source regions for lower Culm sediments (Moldanubian Unit and Sudetes) remained the same. Whereas, upper Myslejovice sediments might have derived from exhumation and erosion of the root of the Moldanubian Unit and/or Moravosilesian Nappes. Rapid exhumation of the Moldanubian Unit within the Bohemian Massif during the deposition of upper Culm sediments is inferred.

The origin of the ultrahigh-pressure Tso Morari complex, NW Himalaya: implication for early Paleozoic rifting

AbstractThe origins and age distribution of the Himalayan high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks are critical for understanding the pre-Himalayan history. Although the protoliths to the UHP Tso Morari eclogites in Ladakh, NW Himalaya are believed to be the Permian Panjal volcanics, the geochronological evidence is absent. Here, we demonstrate that the protoliths of the UHP Tso Morari Complex formed in a continental rift setting at the Indian margin associated with the northern East Gondwana during the Early Paleozoic. Zircon U–Pb dates from eight gneisses and one garnet amphibolite indicate the Early Paleozoic bimodal magmatism of 493–476 Ma, which could be associated with the separation of South China from North India. Except for arc-related eclogites found in the Nidar ophiolite, the eclogites and amphibolites are rift-related, exhibiting enriched light rare earth elements and high concentrations of incompatible elements, along with evidence for crustal contamination. Our findings support the previously reported diversity in the sources and ages of the protoliths of the Himalayan HP–UHP metamorphic rocks along the orogen.

Multiphase-solid fluid inclusions in HP-LT eclogite facies rock (Zavkhan Terrane, Western Mongolia): evidence for the evolution from saline to hypersaline fluids during metamorphism in subduction zone

Fluid inclusions in high- and ultrahigh-pressure metamorphic rocks provide direct information on the composition of the fluids that evolved during metamorphism and fluid-rock interactions in deep subduction zones. We investigate the fluid inclusions in the Khungui eclogite of the Zavkhan Terrane, Central Asian Orogenic Belt. Fluid inclusions are observed in garnet and quartz in the eclogite samples that underwent metamorphism during subduction. The primary fluid inclusions in quartz are composed of liquid and vapor with high salinities (15.7–16.4 wt.% NaCl eq.), whereas the secondary fluid inclusions in quartz are classified as: relatively high salinity (Type I:12.5–16.3 wt.% NaCl eq.) and low salinity (Type II:6.7–10.6 wt.% NaCl eq.). The garnet shows compositional zoning from Ca-poor cores to Ca-rich rims, and the rims that grew during the eclogite-stage metamorphism (2.1–2.2 GPa at 580–610 °C) preferentially contain numerous primary fluid inclusions. The primary fluid inclusions in garnet are commonly bi-phases (liquid and vapor); however, some are multiphase-solid fluid inclusions composed of fluids (liquid and vapor) and combinations of several minerals (halite, quartz, apatite, calcite, biotite, chlorite, and actinolite). Bi-phase fluid inclusions preferentially occur in the inner parts of the Ca-rich garnet rim, whereas multiphase-solid fluid inclusions occur along the margins of the Ca-rich rim. We hypothesize that the multiphase-solid fluid inclusions are formed via interactions between trapped fluids, trapped minerals, and the host garnet during exhumation. By combination of FIB–SEM and synchrotron X-ray CT analyses, the detailed occurrences, volumes, and compositions of the solid phases in the fluid inclusion was analyzed. We then conduct mass balance analysis to reconstruct accurate fluid compositions using data from the FIB–SEM and synchrotron X-ray CT images of the multiphase-solid fluid inclusion. The results of these analyses reveal that (1) fluid changed from an H2O-dominated saline fluid (13–16 wt. % NaCl eq.) at the prograde to the earlier eclogite stage to H2O–CO2-dominated hypersaline fluid at later eclogite stage (~ 32 wt. % NaCl eq., 7.3 wt. % CO2 and ~ 19 molal dissolved cations); (2) a variety of mineral assemblages in multiphase-solid fluid inclusions are produced by post-entrapment reactions between the trapped hypersaline fluid, trapped minerals and the fluid host mineral. The evolution of fluids from saline to hypersaline during the eclogite facies stage is probably caused by the formation of hydrous minerals (i.e., barroisite) under a near-closed system.

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.

Exhumation mechanism and crustal anatexis of ultrahigh-pressure metamorphic rocks in continental collision zones

Exhumation mechanism and crustal anatexis of ultrahigh-pressure metamorphic rocks in continental collision zones

A revisit to the phase transition behavior of K-feldspar at high-pressure and high-temperature: Implications on metastable K-feldspar in cold subduction

Abstract Felsic gneiss is a major type of ultrahigh-pressure metamorphic rock in continental subduction zones. As an important mineral of felsic gneiss, the phase behavior of K-feldspar is of great significance for studying subducting seismic activities, crust-mantle interactions, and plate tectonics in the Earth’s interior. In this study, we investigated the phase relations in K-feldspar using Raman spectroscopy combined with externally heating diamond-anvil cell (DAC) under high-pressure (28 GPa), high-temperature (400 °C), and simultaneous high pressure-temperature (P-T) (14 GPa, 430 °C) conditions. The phase diagram of K-feldspar at relatively low temperatures has been constructed, and together with previous results, a more comprehensive P-T phase diagram of K-feldspar is established. K-feldspar undergoes a reversible phase transition from C1 to P1 (metastable K-feldspar) symmetry at 10.3 GPa. Metastable K-feldspar is an intermediate phase from K-feldspar to K-holl-I (KAlSi3O8 with a hollandite-I structure) in extremely cold subduction slabs (<2 °C/km) or the center of some old, cold, and rapidly subducting slabs. Metastable K-feldspar is stable even at 11.4 GPa and 400 °C in enriched hydroxyl group (OH) environments, which shows greater high-P-T stability than K-feldspar. Thus metastable K-feldspar could withstand subduction-zone fluids, low-temperature metamorphism, and survive to deeper than previously expected. These results enhance our understanding of the formation paths and conditions of K-holl-I, the subducted depth of K-feldspar, the effect of subduction-zone fluids on continental subduction, and provide a possible reason for the origin of intermediate- and deep-focused seismicity.

Two generations of crustal anatexis in association with two-stage exhumation of ultrahigh-pressure metamorphic rocks in the Dabie orogen

Two generations of crustal anatexis in association with two-stage exhumation of ultrahigh-pressure metamorphic rocks in the Dabie orogen

In Situ High-Pressure Raman Spectroscopic, Single-Crystal X-ray Diffraction, and FTIR Investigations of Rutile and TiO2II

In ultrahigh-pressure (UHP) metamorphic rocks, rutile is an important accessory mineral. Its high-pressure polymorph TiO2II can be a significant indicator of pressure in the diamond stability field. In the present study, in situ high-pressure Raman spectroscopic measurements of natural rutile in UHP eclogite from the main hole of the Chinese Continental Scientific Drilling Project (CCSD) have been conducted up to ~16 GPa. Rutile and recovered TiO2II have also been analyzed via single-crystal X-ray diffraction and FTIR spectroscopy. The results indicate that (1) the phase transition from rutile to baddeleyite-type TiO2 terminates at about 16 GPa under compression at ambient temperature; (2) the metastable TiO2II in the exhumated UHP rocks formed during deep continental subduction can be characterized by a highly distorted octahedral site in the crystal structure. X-ray powder diffraction analyses (with Cu Kα radiation) at ambient conditions are sufficient for identifying the lamellae of TiO2II within natural rutile based on the angles (2θ) of two strong peaks at 25.5° and 31.5°; (3) rutile and recovered TiO2II in the continental slabs can contain certain amounts of water during deep subduction and exhumation. The estimated water contents of rutile in the present study range from 1590 to 1780 ppm of H2O by weight. In the crystal structure of TiO2II, hydrogen can be incorporated close to the long O-O edges (>2.5143 Å) of the TiO6 octahedra. Further studies on the pressure–temperature stability of hydroxyls in rutile and TiO2II may help to understand the transportation and release of water in subducted continental slabs.

Recycling of continental crust materials: Geochemical constraints from post-collisional alkaline intrusive rocks in the Dabie orogen

Post-collisional alkaline intrusive rocks from the Dabie orogen were studied for their whole-rock major-trace elements and Sr-Nd-Hf-Pb isotopes, as well as zircon U-Pb ages and Hf-O isotopes. The results provide geochemical constraints on the nature of their mantle sources and thus insight into crust-mantle interaction in the continental collision zone. The alkaline intrusive rocks are composed of syenite and nepheline syenite. Syn-magmatic zircon U-Pb dating by LA-ICP-MS for them yielded Early Cretaceous ages of 131.3 ± 1.4 Ma to 122.6 ± 0.6 Ma, coeval with the post-collisional magmatism in the Dabie orogen. One relict zircon with U-Pb age of 211 Ma is consistent with the timing of metamorphism for the ultrahigh-pressure (UHP) metamorphic rocks in this orogen. They have arc-like trace element distribution patterns, such as enrichment in LILE (large ion lithophile element) and LREE (light rare earth element) but depletion in HFSE (high field strength element), and enriched whole-rock Sr-Nd-Hf isotope compositions with high (87Sr/86Sr)i ratios of 0.7077–0.7131 but negative εNd(t) values of −16.0 to −9.4 and εHf(t) values of −17.5 to −12.7. Their syn-magmatic zircons have three groups of Hf-O isotope compositions comparable to those of UHP metamorphic rocks in Central-South Dabie and North Dabie, which represent the upper and middle continental crust of the subducted South China Block, respectively. In this regard, slab–mantle interaction is evident during the Triassic continental collision. We suggest that the melts derived from the subducted South China Block reacted with the lithospheric mantle wedge of the North China Block, resulting in phlogopite-bearing metasomatites, whose partial melting would generate the post-collisional alkaline intrusive rocks during the Early Cretaceous.

Magnetotelluric constraints on the tectonic evolution beneath the Luotian dome in the North Dabie complex zone, Central China

The Dabie orogen, formed by a Triassic continent-continent collision between the Yangtze and the North China craton, contains one of the largest ultrahigh-pressure (UHP) metamorphic terranes in the world. However, compared to the widespread UHP metamorphic rocks in the Central Dabie, little evidence shows the North Dabie Zone (NDZ) has experienced UHP metamorphism. To understand the cause of these variances and constrain the post-collision evolution of the NDZ, we conducted a magnetotelluric profile across the Luotian dome, southwestern NDZ. Our electrical resistivity model exhibits intriguing post-collision evolution-related characteristics. On both sides of the profile, the resistive features extend to ∼10 km, suggesting the UHP metamorphic or magmatic rocks are mainly concentrated in the upper crust; the underlying crustal high conductive features are interpreted to be the graphite from the mantle-derived or related magma, which were interconnected by the shearing. The middle resistive features extend deeper to ∼20 km, which may represent the magmatic rocks that contributed to the formation of the Luotian dome during the early Cretaceous. The lithosphere mantle generally behaves as high conductive features related to graphite, indicating a fertile mantle. Combining geological and geophysical observations previously, we suggest the Luotian region experienced doming in response to the hot asthenosphere upwelling resulting from the delamination of the lithospheric keel of the Dabie orogen during the early Cretaceous as triggered by synchronous tectonic events like the development of the Tan-Lu fault. These processes may fertilize the lithosphere, uplift and erode the Luotian or the NDZ, resulting in the observed resistivity structures and the lack of UHP metamorphic rocks.

Water solubility in coesite at realistic temperatures of subduction zones

To constrain the water solubility of coesite (Coe) at typical temperatures of subduction zones, a series of Coe coexisting with an aqueous fluid was synthesized at 3–6 GPa and at 600–800 °C. Most experiments were performed in the system SiO2-H2O, and some were performed with small amounts of boron addition. With very long heating durations (120–336 h), all these experiments successfully produced Coe crystals of large grain size, ranging from ∼100 to 1300 μm. For every experimental product, multiple unpolarized FTIR spectra were collected on randomly-selected Coe crystals. We have found that type-I hydrogarnet substitution (Si4+(Si2) + 4O2− = [4]□(Si2) + 4OH−) is the major water-incorporation mechanism whereas B-related defect (H+ + B3+ ↔ Si4+) makes no much contribution. The water solubility of Coe, ranging from 3(1) to 47(12) wt ppm, positively correlates with both P and T. It can be well described by the empirical equation cH2O = −49(17) + 6.0(21) × P + 0.06(2) × T (cH2O representing water content in wt ppm, P pressure in GPa and T temperature in °C) and by the thermodynamic expression cOH=exp∆S1barRfH2O2exp−∆H1bar−∆VsolidPRT (cOH representing water content in H/106 Si, fH2O water fugacity in GPa, R the gas constant, P pressure in GPa, T temperature in K, ∆S1 bar reaction entropy as 18.5(509) J/mol/K, ∆H1 bar reaction enthalpy as −10.7(516) kJ/mol, and ∆Vsolid the volume change of Coe during hydroxylation as 23.6(42) cm3/mol). Consequently, the water solubility of Coe in subduction zones should be ∼0–123 wt ppm. When Coe with deep origin becomes metastable (i.e., approaching the P-T locus of the Coe-quartz (Qz) reaction), its water content is likely less than ∼10 wt ppm. This trace water may be quickly lost along with further exhumation process, and metastable Coe becomes completely dry, as observed in the Coe discovered in all exhumed ultrahigh-pressure (UHP) metamorphic rocks. Since structurally-bonded water substantially speeds up the Coe-to-Qz phase transition, zero water in metastable Coe may be the key to the preservation of Coe in the UHP metamorphic rocks.

Serpentinization and Deserpentinization of the Mantle Wedge at a Convergent Plate Margin: Evidence of Orogenic Peridotites from a Composite Oceanic–Continental Subduction Zone

AbstractSerpentinites of the mantle wedge were identified from a composite oceanic–continental subduction zone that occurs as the Hong’an orogen in east–central China. They were comprehensively investigated by an integrated approach of whole-rock major and trace elements, mineral major and trace elements, whole-rock and olivine O isotopes, and phase equilibrium modeling as well as zircon U–Pb ages, trace elements, and Hf-O isotopes. These serpentinites show high Mg# and MgO/SiO2 ratios but low Al2O3/SiO2 ratios as well as low Ti contents, resembling those from the mantle wedge rather than the subducting oceanic slab. Petrographic observations were combined with CIPW norm calculations to suggest that the protoliths of these serpentinites are refractory peridotites in the mantle wedge. Whole-rock and mineral compositions indicate that these serpentinites experienced multiple episodes of metamorphism and metasomatism during the tectonic development from oceanic subduction in the Carboniferous to continental collision in the Triassic. The enrichments of Th, U, light rare earth elements and high field strength elements in the whole rock and the finding of zircon inside them indicate their formation through metasomatism of the mantle wedge peridotites by subducting crust-derived fluids. The ca. 310 Ma zircon domains exhibit positive εHf(t) values of 4.5 to 19.1 and variable δ18O values similar to those of oceanic-type eclogites in the Hong’an orogen but different trace element compositions, indicating their formation through metasomatism by fluids from the subducting Paleotethyan oceanic crust. The ca. 430 Ma zircons exhibit U–Pb ages, trace elements and Hf-O isotopes similar to those of the protolith magmatic zircons in the oceanic-type eclogites. The older zircon domains also exhibit similar element and isotope compositions to inherited zircons from the high- to ultrahigh-pressure metamorphic rocks in the Hong’an orogen. Thus, these relict zircons would be physically transferred into the serpentinites by metasomatic fluids originating from the subducting Paleotethyan oceanic crust. High Th/U ratios for the ca. 310 Ma zircons imply their formation through the metasomatic reaction rather than direct precipitation from the subducting oceanic crust-derived fluids. The metasomatic zircons of Triassic age show negative εHf(t) values of −16.0 to −2.7, indicating their formation through metasomatic overprinting by deeply subducting continental crust-derived fluids in the continental subduction channel during the Triassic continental collision. In this context, the mantle wedge peridotites were first metasomatized in the Carboniferous by the subducting oceanic crust-derived fluids. Afterwards, part of the metasomatized peridotites were off-scrapped into the oceanic subduction channel, where they were hydrated by further subducting oceanic crust-derived fluids to form antigorite serpentinite at forearc depths. The antigorite serpentinite was carried to deeper depths during subsequent continental subduction and underwent dehydration there. This gave rise to metamorphic olivine with extremely high Fo values of 96.7 to 97.6 and higher MnO but lower NiO contents than common mantle olivine. The coexistence of lizardite, brucite and magnesite in the serpentinites indicates their retrograde processes of serpentinization and carbonation on the subsurface. Therefore, the present observations and interpretations of orogenic serpentinites provide insights into the multistage fluid metasomatism at different depths during the tectonic transition from oceanic subduction to continental collision.

Geochemistry, zircon U-Pb age and Hf isotope for the Huatugou granitoid in western Qaidam: Petrogenesis and tectonic implications

The Qaidam Precambrian block is located in the northeastern Tibetan Plateau and was intruded by numerous Ordovician-Devonian granitoids during and after the closure of the Proto-Tethys Ocean. In the past 20 years, the granitoids within early Paleozoic subduction-collision belts have been investigated in detail. However, the granitoids intruding the inner part of the Qaidam block, bearing the tectonic significance for the whole block, still need to be understood. This study presents new whole-rock geochemical and zircon U-Pb-Hf isotopic data for the Huatugou granitoids in the Qaidam Precambrian block. The investigated granitoids include granodiorite, monzogranite, biotite granite, and muscovite granite, which intruded the Precambrian basements during 451–400 Ma. The granodiorites (451 ± 6 Ma) display adakitic geochemical features and syn-tectonic textures, and their magmas were generated by the partial melting of the lower mafic crust within a thickened continent. The muscovite granites (410 ± 6 Ma), with negative zircon εHf(t) values of −14.5 to −10.4, were crystallized from fractionated S-type magmas, which were derived from the partial melting of ancient crustal materials. The biotite granite (410 ± 3 Ma) and monzogranites (400 ± 4 Ma) are high-temperature A2-type granites. The biotite granite displays positive zircon εHf(t) values of +1.7 to +5.6. Its magma was generated by the high-temperature partial melting of juvenile crustal rocks in a thinned lower crust. The monzogranites exhibit higher SiO2 contents and lower εHf(t) values, and their magmas were derived from the same source but underwent assimilation and fractional crystallization during emplacement. From the thickened to the thinned continent during 451–410 Ma, the western Qaidam block experienced a tectonic transition from compression to extension. Combined with regional geological data, this study suggests that the Qaidam block consisted of the thickened continental crust during subduction processes until the detachment of the subducted slab during the continental collision. The regional extension of the Qaidam block commenced at ∼420 Ma, soon after the exhumation of ultrahigh-pressure metamorphic rocks within the northern Qaidam subduction-collision complex belt.

Discovery of the mid-Cretaceous sedimentary rocks from the ultrahigh-pressure terrane, Dabie Orogenic Belt, and its tectono-paleogeographic implications

The Dabie Orogenic Belt, located in east-central China, is known for the greatest outcrop area of the ultrahigh-pressure metamorphic rocks in the world and is typical for studying deep continental subduction and continent–continent collision. However, since lower-grade metamorphic rock occurrence in the core of the ultrahigh-pressure terrane was reported, it has become a new point of concern. This study reported new results. The so-called lower-grade metamorphic rocks are divided into sedimentary rocks and meta-volcanics. The sedimentary rocks consist mainly of fine-grained turbidites formed under an anoxic deep-water lacustrine basin. Laser Ablation–Inductively Coupled Plasma–Mass Spectrometry U–Pb detrital zircon dating indicates that their maximum deposit age is 119.0 ± 1.1 Ma belonging to the mid-Cretaceous Aptian. U–Pb detrital zircon ages from the sedimentary rocks cluster in three age groups: 1) 2773.8–1550.0 Ma (45.0%); 2) 868.8–622.3 Ma (23.0%); 3) 147.0–100.0 Ma (31.5%). And the provenance and tectonic settings of these sedimentary rocks are discussed by synthesizing U–Pb detrital zircon ages, initial Hf isotope ratios, and trace elements of the zircons. Source rocks of the first group were mainly composed of the Paleoproterozoic basement derived from the reworking of the Paleoarchean–Neoarchean Yangtze continental crust. For the second group, its source rocks consisted of Neoproterozoic volcanics mainly from an island arc system, which derived from the reworking of older continental crust in the northern Yangtze Block. The third group's source rocks were chiefly composed of Cretaceous rift igneous rocks derived from the derivation through melting of middle-aged continental crust in the Dabie Orogenic Belt. The meta-volcanics have weighted mean U–Pb ages from 751.0 ± 16 Ma to 786.3 ± 5.9 Ma, and their protoliths formed under a Middle Neoproterozoic island arc system in the northern Yangtze Block. The meta-volcanics underwent a Triassic subduction of different depths and then an exhumation, but the sedimentary rocks did not. The findings suggest that the ultrahigh-pressure terrane had suffered violent subsidence during the mid-Cretaceous, and a four-stage evolution model of the Dabie Orogenic Belt was established accordingly.

Barium isotope fractionation during slab dehydration: Records from an eclogite-quartz vein system in Dabie orogen

Barium isotopes are a potential tracer of fluid-mediated recycling of subducted crustal materials into the mantle wedge. However, the behavior of Ba isotopes during metamorphic breakdown of Ba-rich hydrous minerals is not well understood. High-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks and associated veins in convergent orogens can provide important insight into Ba transport and Ba isotope fractionation during metasomatic fluid activities. This study reports Ba isotope data for whole rocks and mineral separates of a HP-UHP eclogite-quartz vein system from Bixiling in the Dabie orogen. The samples include epidote-phengite-bearing quartz veins and two types of eclogites. The Type I eclogites are the host of the quartz veins and were not affected by the metasomatic fluids, whereas the Type II eclogites are hosted in the veins and were strongly modified by the fluids. The results show that the Type I eclogites have relatively homogeneous δ138/134Ba ranging from −0.12 to −0.04‰. In contrast, the quartz veins have highly variable δ138/134Ba ranging from 0.16 to 0.51‰. The Type II eclogites have δ138/134Ba from 0.32 to 0.40‰, similar to those from 0.25 to 0.35‰ of the quartz separates within the veins, which may represent the average Ba isotopic composition of the fluid inclusions trapped in the quartz during vein formation.These results demonstrate that the vein-forming fluid has much higher δ138/134Ba than the host eclogites, indicating significant Ba isotope fractionation during dehydration of eclogites. Mineralogical evidence suggests that the fluid was probably derived from the breakdown of phengite at the HP eclogite-facies to amphibolite-facies stage during slab exhumation. The higher δ138/134Ba of the phengite separates (from −0.16 to −0.05‰) than those of the coexisting biotite separates (from −0.34 to −0.22‰) in the Type I eclogites indicate that the fluid released from the breakdown of phengite to biotite would be enriched in heavy Ba isotopes. This explains well the observed heavier Ba isotopic compositions of the quartz veins relative to the host eclogites. In summary, our results reveal that the breakdown of phengite during continental slab exhumation could produce Ba-rich fluids with remarkably higher δ138/134Ba than that of the host rocks, and thus might potentially influence the Ba isotopic systematics at convergent plate margins.

Origin and metamorphic evolution of Chachahe eclogites, North Qaidam UHP metamorphic Belt, NW China: Implications for fate of overriding plate material in subduction channel

• A clockwise P–T path was established for Chachahe eclogites in North Qaidam UHP belt, NW China. • The Chachahe eclogites experienced a 1.92 Ga metamorphic event prior to eclogitization at ∼ 450 Ma. • Overriding plate material can enter subduction channel via tectonic erosion and be carried to mantle depths of ≥ 85 km. Although numerical modelling and marine seismic data show that overriding plate material can enter subduction channel and even recycle to mantle, and finally contribute to mantle heterogeneity and arc magmas, protoliths of the majority of high pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks on the earth’s surface originate from the subducting plate. Whether overriding plate material can enter subduction channel and their fate in subduction channel remain enigmas. The North Qaidam UHP Metamorphic Belt (NQUB), located in northwestern China, was formed by north-dipping subduction of the Qaidam Block beneath the Quanji block in the Early Paleozoic. In this paper, we report that some eclogites in Chachahe area, NQUB experienced a 1.92 Ga low-grade metamorphism prior to eclogitization at ∼ 450 Ma. This Paleoproterozoic metamorphic event is not reported in the subducted Qaidam Block but widespread in the overriding Quanji Block, indicating that protoliths of Chachahe eclogites originated from the overriding plate. A clockwise P–T path from 12 kbar/520 °C to near UHP conditions of 25.5 kbar/690 °C, followed by isothermal decompression, was estimated for the Early Paleozoic metamorphism using a combination of garnet major and trace element zoning, phase equilibrium modelling and thermobarometry. This P–T path pattern is similar to other eclogites in this area, protoliths of which originated from the subducted plate. These results clearly show that overriding plate material can enter subduction channel and be carried to mantle depths of at least 85 km and exhume fast along subduction channel with other slices from subducted slab.

Crustal melting in orogenic belts revealed by eclogite thermal properties

Partial melting in the continental crust may play a critical role on the behavior of continents during collision. However, the occurrence of partial melt in orogenic continental crust is not well understood. Since the temperature of the orogen is controlled by the thermal properties of constituent rocks, we measured the thermal conductivity and diffusivity of eclogite, the most important ultrahigh pressure metamorphic rocks, as a function of pressure, temperature, composition, and water content, and simulated the thermal structure of the Sulu and Himalaya-Tibet orogens in eastern and southwestern China, respectively. Our results show that the temperature at ~30-km depth beneath the orogens reaches the solidus of wet granite and phengite (~940 K), therefore, the partial melting in the orogenic continental crust is well explained. The melt may facilitate the exhumation of subducted crust, produce the low seismic-velocity zone, and cause the high-conductivity anomaly in the shallow depth of orogenic belts.