UHP Metamorphic Belt Research Articles

Protolith origin of eclogites from the North Qaidam UHP metamorphic Belt, NW China: Implications for the breakup of the Rodinia supercontinent

The North Qaidam ultrahigh-pressure metamorphic belt (NQUB) is distinguished by the widespread distribution of eclogites. However, the protolith nature of these eclogites remains controversial. This study reports new zircon LA-ICP-MS U–Pb ages and whole-rock geochemistry as well as Sr–Nd isotopic data on the Yuka and Dulan eclogites in the NQUB. Zircon U–Pb dating results of these eclogites indicate that the protolith and metamorphic ages are 831–809 Ma and 439–433 Ma. Based on distinct geochemical features, the samples can be classified into high-Ti/Y type (Group 1) and low-Ti/Y type (Group 2 and Group 3). The Group 1 eclogite protoliths are geochemically similar to oceanic island basalts (OIB) and likely originated from garnet-spinel peridotite mantle sources. The protoliths of Group 2 eclogites have a similar mantle source to those of the Group 1 protoliths, but experienced a higher degree of fractional crystallization and crustal contamination. The Group 3 eclogites have N-MORB-like affinity, and may be derived from shallower mantle sources in the spinel stability field. The differences in mantle sources of enriched (Group 1 and Group 2) and depleted (Group 3) eclogite protoliths can be ascribed to varying extents of mixing between enriched plume and depleted MORB-type mantle components at various melting depths. Our results suggest that the Neoproterozoic protoliths of eclogites in the NQUB belonged to a part of widespread Neoproterozoic rift-related mafic magmatism in the Altun-Qilian-Kunlun Orogenic Belt. These rocks could be the products of mantle plume activities that caused the breakup of Rodinia. Most Neoproterozoic mafic rocks in the NQUB experienced early Paleozoic HP/UHP metamorphism.

Mineralogy and Geochemistry of “Laoshan Jade” from Shandong Province, China: Implications for Petrogenesis

“Laoshan Jade” is located in the Yangkou area of the Sulu UHP metamorphic belt. This is the first time a systematic research study is performed on its conventional gemological characteristics, mineral compositions, geochemical characteristics, and petrogenesis using a polarized light microscope, an electron probe microanalyzer (EMPA), an X-ray fluorescence spectrometer (XRF), and a Raman spectrometer. The gemological characteristics of “Laoshan Jade” are consistent with serpentine jade. It can be named serpentinite, mainly composed of serpentine (antigorite), chlorite (clinochlore), and magnetite. Some samples contain magnesite and chromite. Only small amounts of unaltered forsterite, chrysolite, and diopside can be found in poor-quality serpentine jade. The protolith of “Laoshan Jade” is metamorphic peridotite. Antigorite is altered from olivine. Clinochlorite is altered from the diopside. Disseminated magnetite is the metamorphic product. Unaltered olivine and diopside belong to protolith. Chromite with zonal texture also belongs to protolith. The average temperature and pressure of its core are calculated to be 1348 °C and 3.1 GPa, which indicates that the protolith comes from the mantle asthenosphere. The formation temperature of its edge is lower than 500 °C, with metamorphic origin. XRF experiments show that “Laoshan Jade” samples are relatively rich in Mg and more depleted in fusible elements (Al, Ca) than the original mantle peridotite, indicating that serpentinite protolith has undergone a certain degree of partial melting. Combined with EPMA results, the findings of this study reveal that protolith belongs to the depleted mantle peridotite in the subduction zone, partially melted in the mantle wedge, and reacted with subduction zone fluid. Pyroxene is altered into chlorite, olivine is altered into serpentine, and magnet is generated. The edge of chromite in the protolith is altered by the iron-rich fluid into magnetite. Finally, serpentinite is formed.

Mobilization and fractionation of Ti-Nb-Ta during exhumation of deeply subducted continental crust

The behavior of Ti-Nb-Ta is crucial to reveal the genesis of island arc magmatism. However, mobilization and fractionation of Ti-Nb-Ta in subduction zone settings remain poorly understood. The discovery of felsic veins rich in coarse-grained rutile within retrograde eclogite of the North Qaidam UHP metamorphic belt provides a unique and novel opportunity to study age variation during rutile formation and alteration, as well as Ti-Nb-Ta mobility and fractionation during fluid/melt-rock interaction. Rutile high-resolution elemental mapping, and U-Pb bulk grain (ID-TIMS), and in-situ U-Pb geochronology have been utilized to focus on the properties of rutile in both, felsic vein and retrograde eclogite host to gain insight into possible similarities and differences. Three groups of rutile were distinguished according to its host rock, trace elements signature, and genetical connection to ilmenite: eclogite-hosted rutile (Rt-1), felsic vein-hosted rutile not associated with ilmenite (Rt-2a), and associated with ilmenite (Rt-2b). Field evidence and rutile trace elements characteristics document the source of vein-hosted rutile to be mainly derived from the eclogite during fluid/melt-rock interaction. Principal Component Analysis reveals that Nb, Ta, Sn, and W are more enriched in Rt-2a compared to Rt-1; Rt-2b has higher Nb, U, and Hf than Rt-2a. High-resolution mapping across large rutile grains shows the enrichment of high field strength elements (HFSEs) in rutile near to ilmenite, which indicates a HFSEs back diffusion from the rutile–ilmenite boundary during the replacement of rutile by ilmenite. The Nb/Ta ratios of Rt-2a are lower than those of Rt-1, which result from different partition coefficients of Nb and Ta during fluid/melt-rock interaction. The diffusion-influenced rutile exhibits suprachondritic Nb/Ta ratios and demonstrates that diffusion of Nb in rutile is higher than that of Ta under identical P-T conditions. Rutiles Rt-1 and Rt-2a yield consistent 206Pb/238U ages of 426–423 Ma, which is similar to the 433 ± 3 Ma determined by ID-TIMS results of bulk rutile grains. This indicates that Ti-Nb-Ta must have been mobilized during the exhumation of deeply subducted continental crust. However, the diffusion-influenced rutile shows a large variation of ages compared to the rutile not associated with ilmenite, demonstrating that the back-diffusion may affect the U-Pb system in rutile. Therefore, when rutile is partially altered into ilmenite or titanite, its dating should be used with caution. Thus, this study demonstrates volume diffusion is a very important geological process to result in extreme HFSEs fractionation and age variation of rutile on the mineral scale. The rutile aggerates that occur in the felsic veins in 3–5 m distance to the adjacent retrograde eclogite suggest that Ti-Nb-Ta-rich melts/fluids were transported over a distance of at least several meters and that rutile does not represent a residual phase of the Na-Si-Al-, F- and CH4-bearing fluid/melt environment that formed during anatexis of the subducted continental crust. The formation of rutile-rich aggregates during the generation, transport, and crystallization of subducted continental crust-derived melts/fluids in the deep roots of orogenic belts may be a critical trigger for the depletion of HFSEs in arc magmatic rocks during the formation of the continental crust.

Formation process of Al-rich calcium amphibole in quartz-bearing eclogites from The Sulu Belt, China

AbstractAluminum-rich and Si-poor calcium amphibole [~3.9 Al atoms per formula unit (apfu) and ~5.5 Si apfu for 23 O] occur in the quartz-bearing eclogites from the Donghai area, Sulu ultrahigh-pressure metamorphic belt, eastern China. Most of the aluminous amphibole phases are retrograde products from the exhumation and hydration stage and are texturally divided into a mantle phase around a porphyroblastic garnet and a crack-filling (vein) phase of a garnet. Less aluminous amphibole occurs as symplectite phase with plagioclase after omphacite. The formation process of the aluminous amphibole in the quartz-bearing samples is discussed on the basis of the analytical data by EPMA, FIB-TEM, and EBSD.The mantle amphibole occurs between garnet and symplectite or quartz. A set of plagioclase and aegirinediopside/argirine-hedenbergite thin monomineralic bands forms at the boundary between the mantle amphibole and matrix quartz. However, these monomineralic bands do not occur at the mantle amphibole-symplectite boundary. These textural differences indicate that the recrystallization of the aluminous amphibole around garnet was controlled by significant local reactions, and the size of equilibrate domains was probably several tens of micrometers or less.The mantle amphibole is composed of inner (garnet-side) and outer (matrix-side) zones. The inner zone is compositionally homogeneous, and its atomic Al/Si value is ~0.63–0.66 and similar to that of garnet. Atomic Ca/Si value in the inner zone is also almost uniform and is generally identical to that of garnet. The outer zone exhibits a monotonic decrease in the Al/Si and Ca/Si values outward, and its composition at the outermost margin is similar to that of the symplectitic amphibole. The crack-filling amphibole has a composition similar to the inner zone of the mantle amphibole. The CPO pattern of the crack-filling amphibole is different from that of the adjacent mantle amphibole, showing that the crack-filling amphibole is cut by the mantle amphibole. The textural relationship between the mantle and crack-filling amphibole phases and their compositional characteristics imply that: (1) the mantle type is a slightly later stage product than the crack-filling type, and (2) the boundary between the inner and outer zones of the mantle aluminous amphibole probably corresponds to the initial surface of the porphyroblastic garnet. The inner zone is considered to have grown inward by simple substitution of garnet, using the tetrahedral and octahedral cations of the garnet as the basic framework. On the other hand, most of the outer zone of the mantle-type amphibole grew outward in the matrix from the initial surface of the garnet porphyroblast. The mantle amphibole shows a CPO similar to that of amphibole in the adjacent symplectite domain, suggesting that these two types of amphibole formed almost simultaneously, sharing crystallographic orientation with each other.The formation of crack-filling aluminous amphibole was probably promoted by the hydraulic microfrac-turing process at an early stage of exhumation and hydration. The mantle and symplectitic amphibole phases formation was promoted by the subsequent infiltration of metamorphic fluid. The aluminous amphibole in the SiO2 phase-bearing eclogites probably recrystallized with the formation of a localized SiO2-undersaturated reaction domain because of rapid exhumation and subsequent rapid cooling of the Sulu UHP metamorphic belt.

Petrogenesis and tectonic implication of the Late Jurassic Kunyushan granitic complex in the Jiaodong Peninsula, eastern China

The Kunyushan granitic complex in the Jiaodong Peninsula, East China, is located in the northeast part of the Sulu UHP metamorphic belt. It is the transition belt of the North China Craton and Sulu UHP metamorphic belt. Field relationship and dating results revealed five intrusive phases of the Kunyushan granitic complex were modified into gneissic fine‐grained garnet‐bearing monzogranite (G1), gneissic medium‐ to fine‐grained monzogranite (G2), medium‐ to coarse‐grained monzogranite (G3), medium‐grained monzogranite (G4), and porphyritic coarse‐ to medium‐grained monzogranite (G5). All these phases have similar rock assemblages of peraluminous, calc‐alkaline, I‐type monzogranites. The monzogranites had primitive mantle‐normalized patterns enriched in light rare earth elements (LREE) and lithophile elements (Sr, Ba, and Rb), but depleted in heavy rare earth elements (HREE) and high‐field‐strength elements (Nb, P, and Ti). LA‐ICP‐MS zircon U–Pb dating shows that the crystallization age of the Kunyushan granitic complex extends at least from 157.7–142.0 Ma. The weighted average ages of G1‐G5 were 145.6 ± 1.9 Ma, 145.8 ± 0.5 Ma, 147.2 ± 0.7 Ma, 154.2 ± 0.6 Ma, and 155.0 ± 0.7 Ma, respectively. G5 to G1, from the middle to the edge shows a trend of gradually becoming younger. Zircon grains from different phases have similar 176Hf/177Hf values from 0.281917 to 0.282298, and εHf(t) values from −27.6 to −14.0, with a weighted average of −22.6 ± 0.5, typical of crustal‐derived origin. The corresponding two‐stage model ages TCDM are from 2,910 to 2,066 Ma, with a weighted average of 2,603 ± 31 Ma. Hf–Sr–Nd–Pb isotopes and inherited zircons suggest that the parental magma of the Kunyushan granitic complex was derived from melting of basement rocks of the North China Craton and the Yangtze Craton. The magmatism presumably took place under the background of subduction and back‐arc extension of the Palaeo‐Pacific plate. We infer that the Kunyushan granitic complex was produced by partial melting of the thickened lower crust, through delamination of the thickened lower crust during the Late Jurassic. This magmatic event may mark the beginning of the extensive Mesozoic thinning of the lithosphere in East China.

Unravelling slab δ34S compositions from in-situ sulphide δ34S studies of high-pressure metamorphic rocks

ABSTRACT Sulphur is of particular importance in the subduction zone system, as it is thought to be intimately related to the arc redox state and metallogenesis above the slab. Knowing the sulphur isotope compositions of the subducted slab, and identifying the δ34S variations between the pre-subduction oceanic lithosphere and its high-pressure (HP) equivalents is of great importance for understanding the sulphur behaviour in subduction zones and the long-term sulphur recycling of the Earth. In this study, we undertook secondary ion mass spectroscopy (SIMS) in-situ δ34S analyses of sulphides from metasediments, mafic blueschists/eclogites, and ultramafic slab serpentinites from the Southwestern Tianshan HP–UHP metamorphic belt, which represents one of the deepest fossil subduction zones on Earth. By integrating the in-situ δ34S compositions of sulphides studied here and those reported from worldwide HP–UHP metamorphic terranes, we found that prograde to peak metamorphic sulphides of metasediments, metabasites/metagabbros, and slab HP serpentinites have δ34S values ranging from –33 to –6‰, –4 to +4‰, and +2 to +18‰, respectively. However, retrograde metasomatism of HP rocks during exhumation may alter the δ34S compositions of sulphides. The integrated dataset proposes that sulphides in subducting slab sequences retain their protolith’s δ34S signatures during HP metamorphism. The δ34S inheritance indicates that no large isotope fractionation is expected to occur in subduction zones. It further suggests that evaluations of slab subduction effects on the δ34S signature of arc magmatism and the deep mantle should consider pre-subduction processes such as ridge geodynamic setting and hydrothermal alteration.

The tectonic affinity of the Meso-Neoproterozoic low-grade metamorphic mafic rocks in the northern margin of the Sulu UHP metamorphic belt and its tectonic significance

The tectonic affinity of the Meso-Neoproterozoic low-grade metamorphic mafic rocks in the northern margin of the Sulu UHP metamorphic belt and its tectonic significance

Multistage CO2 sequestration in the subduction zone: Insights from exhumed carbonated serpentinites, SW Tianshan UHP belt, China

Climate is regulated by the carbonate–silicate cycle in which slab outgassing of C into deep fluids and volcanic degassing of CO2 into the atmosphere are an important part. However, the mechanisms of C mobility in subduction zones remain largely unresolved. Previous research has focused mainly on investigating the upward transfer of slab-derived carbonic fluids for the forearc mantle metasomatism and partial melting. Furthermore, percolation of CO2-bearing fluids parallel to the downgoing plates can potentially drive carbonation of subducted rocks, which influences the global estimates of C fluxes at convergent margins. Nevertheless, the geological conditions and processes leading to the carbonation of subduction-zone lithologies by fluid–rock interactions are still poorly understood. Here, we present new field, petrological, and isotopic results of carbonated serpentinites—high-pressure (HP) ophidolomites and low-pressure (LP) ophimagnesites and listvenites—from the Chinese southwestern Tianshan HP–UHP metamorphic belt. These rocks recorded the carbonation of subduction-zone serpentinites at HP and LP conditions during exhumation, reflecting the multistage transfer and infiltration of carbonic fluids along the plate boundary. The HP ophidolomites are characterized by the growth of carbonates (dolomite, aragonite, and Mg-calcite) at the expense of silicates in the host serpentinites. Integrated Sr–C–O isotopic data and thermodynamic modelling suggest that carbonic fluids (containing a CO2,aq concentration of up to 1.9 molal) emanating from carbonate-bearing metamafic rocks (e.g., eclogites) likely contributed to HP carbonation of serpentinites at about 15–25 kbar and 550–600 °C. The close contact of ophidolomites and carbonated metasedimentary rocks in the field as well as their similarities in Sr isotope compositions suggests that the latter could also have acted as the possible C source. Alternatively, both lithologies may have formed by coupled HP carbonation along the lithological interface between serpentinites and metasedimentary rocks. Subsequent fluid–rock interactions at relatively shallow crustal levels resulted in a second stage of serpentinite carbonation to form LP ophimagnesites and listvenites, during which the metasomatic CO2-bearing fluids may have originated from metasedimentary rocks. The multistage CO2 sequestration in subduction-zone serpentinites implies that hydrated ultramafic rocks in subducted slabs are highly effective reactants to capture and store slab-released C over a wide range of P–T conditions, with the potential to substantially control the C distribution between shallow and deep reservoirs and thus modulate C fluxes in subduction zones.

Metamorphic Evolution and Tectonic Implications of the Granulitized Eclogites from the Luliangshan Terrane in the North Qaidam Ultrahigh Pressure Metamorphic Belt, NW China: New Constraints from Phase Equilibrium Modeling

The granulitized eclogites from the Luliangshan terrane of the North Qaidam UHP metamorphic belt occur as lenses within pelitic gneisses and orthogneisses. Combined petrologic data and phase equilibrium modeling indicate a multi-stage metamorphic history of the granulitized eclogites: (1) an earlier eclogite facies metamorphism (P>18.5 kbar, T> 830 °C) is deduced from omphacite relics in the matrix and rare omphacite inclusions within garnet. The possible assemblage is garnet+omphacite+rutile+ quartz; (2) the early stage of high pressure granulite facies assemblages (garnet+clinopyroxene+ plagioclase+rutile+quartz+liquid) developed in the early decompression process has a P-T regime of 17.5 kbar and 852–858 °C, constrained by plagioclase and clinopyroxene inclusions in garnet. The late stage of high pressure granulite assemblages (garnet+clinopyroxene+amphibole+plagioclase+rutile+quartz+liquid) records an isothermal decompression process with the pressure successively declining from 17.5 to 14.7 kbar and to 11.3 kbar at 858 °C; (3) the later medium pressure granulite facies assemblage (garnet+ orthopyroxene+ clinopyroxene+amphibole+plagioclase+ilmenite+liquid+quartz) indicates a drop in pressure and rise in temperature at P-T conditions of 7.6–7.7 kbar and 878–883 °C; (4) retrogressive amphibolite facies stage, which is represented by amphibole+plagioclase kelyphitic rims around garnet, formed under conditions of <5 kbar and <650 °C. The preservation of medium pressure granulite facies assemblage and the garnet composition feature constrain a following isobaric cooling path during late exhumation. This process suggests a clockwise P-T path and indicates that the granulitized eclogites record a high grade “Barrovian” metamorphic overprint at the middle-lower crust during exhumation. The present data show that the Luliangshan terrane is a “hot” HP-UHP terrane.

The Discovery of Mesoproterozoic Oceanic Crust In North Qaidam UHP Belt, NW China

AbstractThe North Qaidam is an Early Paleozoic UHP metamorphic belt located at the north margin of the Tibet plateau. Eclogites in this belt contain both continental‐and oceanic‐type ones. In which, the continental‐type eclogites have protolith ages of 750–850 Ma and WPB or CFB geochemical signatures and are believed to have formed in a continental rift or an incipient oceanic basin setting related to the breakup of the Rodinia supercontinent, their metamorphic ages (421–458 Ma) and P–T paths are comparable to their host gneisses; oceanic‐type eclogites have cumulate gabbro or E‐MORB geochemical signatures, their protolith and metamorphic ages are 510–516 Ma and 425–450 Ma, respectively(Zhang et al., 2008). Therefore, the North Qaidam UHP belt was thought to record the whole Neoprotoerozoic–Paleozoic Wilson cycle (Song et al., 2014). In this study, we reported three new kinds of eclogites: kyanite‐bearing eclogite, lawsonite pseudomorph‐bearing eclogite and double mineral eclogite. They occur as big lentoid blocks in regional granitic gneiss in the western part of the belt. Phase equilibrium modelling and zircon LA‐ICPMS U‐Pb dating show that all these three eclogites experienced a clockwise P–T path with peak metamorphic conditions close to or fall in the coesite stability field, and their peak metamorphic age were around 436‐439 Ma, similar to those continental‐type eclogites in this belt. But their protolith ages are between 1273 and 1070 Ma, and some of them recorded an amphibolite facies metamorphic age of 927 Ma, and geochemical data and zircon Lu‐Hf and O isotope analysis indicate these eclogites have features of present day N‐MORB. Combined with the existing results, we propose that the North Qaidam is a polycyclic composite orogenwhich recorded tectonic evolution of Mesoproterozoic ocean floor spreading, assembly and breakup of Rodinia supercontinent, Early Paleozoic oceanic deep subduction and subsequently continental deep subduction.

Deformation-enhanced HP-UHP metamorphic transformation of two-pyroxene granite in ductile shear zones at Yangkou, Sulu UHP metamorphic belt, China

We report that high strain deformation in ductile shear zones greatly enhanced high to ultrahigh pressure (HP-UHP) metamorphic transformation of a two-pyroxene granite in deep continental subduction zone. Integrated structural and petrologic study of rocks from the two-pyroxene granite block and shear zones in the block in the UHP shear complex at Yangkou, Sulu UHP metamorphic belt, reveals that the undeformed granite has preserved granitic texture and primary mineral assemblage of hypersthene, augite, plagioclase, K-feldspar, quartz and biotite. Metamorphic transformation of the granite is limited to grain boundaries due to low fluid activity and low reaction kinetics. In contrast rocks from the shear zones have a mylonitic texture and most of the primary minerals are transformed to HP-UHP phase minerals. Dislocation creep accommodated by dynamic recrystallization results in significant grain size reduction of the primary minerals in the mylonite, which increases reaction surface area and enhances metamorphic transformation. Fluid released by biotite transformation to HP-UHP minerals leads to diffusion mass transfer that greatly accelerates the metamorphic process in the shear zone. Deformation conditions are estimated ca 546 °C and pressure 2.5 GPa.

HP–UHP Metamorphic Belt in the East Kunlun Orogen: Final Closure of the Proto-Tethys Ocean and Formation of the Pan-North-China Continent

The East Kunlun Orogen, the northwestern part of the Central China Orogenic Belt, is a long-lived accretionary orogenic belt that records the evolution and eventual destruction of branches of the Tethys Ocean, from the Cambrian to the Triassic. Here we report an Early Paleozoic eclogite belt that extends for ∼ 500 km within the East Kunlun Orogen. This belt consists of eclogite blocks, meta-sedimentary rocks and minor serpentinite blocks, accompanied by ophiolites (530-460 Ma) and concurrent arc volcanic sequences and granitic plutons. Geochemical data show that the eclogites have N-MORB- to OIB-like compositions. U-Pb dating of metamorphic zircons from eclogites and their surrounding rocks gave peak and retrograde metamorphic ages of 430-410 Ma. Coesite pseudomorphs in garnet, quartz exsolution rods in omphacite, as well as P-T calculations, suggests that some eclogites experienced UHP metamorphic conditions at 29-30 kbar and 610-675 °C; these could represent oceanic crust subducted to and exhumed from coesite-forming depths (100–120 km). The UHP metamorphic eclogite belt in the East Kunlun Orogen may represent the final closure of the Proto-Tethys Ocean (opening at ∼580 Ma, subduction initiating at ∼520 Ma) at ∼ 430-410 Ma in the East Kunlun, with the formation of Pan-North-China Continent in the Early Paleozoic and expansion of Paleo-Tethys Ocean in the south.

Two contrasting accretion v. collision orogenies: insights from Early Paleozoic polyphase metamorphism in the Altun–Qilian–North Qaidam orogenic system, NW China

Abstract The Altun–Qilian–North Qaidam (AQQ) orogenic system in northern Tibet is considered to be the northernmost orogenic collage of the Prototethyan domain. It is regarded as resulting from collisions between various continental terranes derived from the northern margin of Gondwana, although the AQQ orogenic system also includes abundant ophiolites, arc magmatic rocks and subduction–accretion complexes. Some researchers regard the orogenic system of north Tibet as a typical accretionary orogen built by the development of an evolving arc–accretion complex growing southwards along the margin of the Tarim and North China cratons during the Paleozoic. We propose, based on both published data and our new data, that two distinct accretion and collision orogenies developed in the AQQ during the early Paleozoic. The diagnostic marks are HP–LT metamorphic rocks in the North Altun–North Qilian Mountains and UHP metamorphic rocks in the South Altun–North Qaidam Mountains. A review of metamorphic, geochronological, geochemical and structural data indicates that the North Altun–North Qilian HP–LT metamorphic belt is related to early Paleozoic subduction–accretion and, together with ophiolite mélanges and arc metamorphic–magmatic complexes, forms an early Paleozoic accretionary orogen. By contrast, the South Altun–North Qaidam UHP metamorphic belt is associated with continental subduction and collision, accompanied by Barrovian-type metamorphic overprinting and collision-related magmatism, reflecting an early Paleozoic collisional orogeny.

Crustal structure of the Dabie orogenic belt (eastern China) inferred from gravity and magnetic data

In order to better characterize the crustal structure of the Dabie orogen and its tectonic history, we present a crustal structure along a 500km long profile across the Dabie orogenic belt using various data processing and interpretation of the gravity and magnetic data. Source depth estimations from the spectral analysis by continuous wavelet transform (CWT) provide better constraints for constructing the initial density model. The calculated gravity effects from the initial model show great discrepancy with the observed data, especially at the center of the profile. More practical factors are then incorporated into the gravity modeling. First, we add a high density body right beneath the high pressure metamorphic (HPM) and ultrahigh pressure metamorphic (UHPM) belt considering the exposed HPM and UHPM rocks in the mid of our profile. Then, the anomalous bodies A, B, and C inferred from the CWT-based spectral analysis results are fixed in the model geometry. In the final crustal density structure, two anomalous bodies B and C with high density and low magnetization could possibly be attributed to metasomatised mantle materials by SiO2-rich melt derived from the foundering subducted mafic lower crust. Under the extensional environment in the early Cretaceous, the upwelling metasomatised mantle was partially melted to produce the parental magma of the post-collisional mafic-ultramafic intrusive rocks. As for the low density body A with strong magnetization located in the lower crust right beneath the HP and UHP metamorphic belt, it is more likely to be composed of serpentinized mantle peridotite (SMP). This serpentinized mantle peridotite body (SMPB) represents the emplacement of mantle-derived peridotites in the crust, accompanying the exhumation of the UHP metamorphic rocks.

Discovery and Geological Significance of Neoproterozoic Metamorphic Granite in Jimo, Shandong Province, Eastern China

AbstractDuring the 1:50000 regional geological survey in Jimo, east Shandong Province, Paleoproterozoic metamorphic supracrustal rocks and Neoproterozoic metamorphic plutonite were newly discovered. These rocks displayed inclusions which had occurred in the Mesozoic granite, and the main lithologies are schist, granulite, marble, and granitic gneiss. Geochemical analyses suggest that Neoproterozoic metamorphic plutonite are characterized by high‐K, metaluminous to weakly peraluminous. They are enriched in LILE and depleted in HFSE, with moderately enrichment of LREE, weak fractionation of LREE from HREE and negative Eu anomalies. The surface age of plutonic rocks in the survey area is 770.2±2.4 Ma, representing the age of magma crystallization, which is agreement with the the Neoproterozoic magmatic event after Rodinia supercontinent in the northern margin of Southern China continental block. In addition, the age of sporadic distribution (298 Ma and 269 Ma) is mixed zircon age, representing the rocks experienced metamorphism in Indosinian period. According to the associated mineral assemblages, and the characteristic metamorphic minerals and temperature pressure conditions, four metamorphic facies were identified, including amphibolitic, epidote amphibolite, greenschist, and mid‐high pressure greenschist. Analysis of tectonic setting suggests that granitic gneiss is formed in an extensional environment and was involved from the continental margin magmatic arc to intraplate environment. Jimo is distributed in the east of Zhuwu fault, and has the same Spatial distribution location with the Weihai uplift UHP metamorphic belt rocks. The metamorphic rocks in Jimo area have similar geochemical characteristics of elements, tectonic setting and retrograde metamorphism with that in the Sulu UHP metamorphic belt. Therefore, Zhuwu fault may be the boundary fault of Sulu UHP metamorphic belt.

He Grenville Orogenesis Recorded by Monazite from the Paragneiss of North Qaidam UHP Metamorphic Belt, Western China

He Grenville Orogenesis Recorded by Monazite from the Paragneiss of North Qaidam UHP Metamorphic Belt, Western China

Differential Evolution of High‐Pressure and Ultrahigh‐Pressure Metapelites from Habutengsu, Chinese Western Tianshan: Phase Equilibria Modelling and 40Ar/39Ar Geochronology

AbstractMetapelite is one of the predominant rock types in the high‐pressure–ultrahigh‐pressure (HP–UHP) metamorphic belt of western Tianshan, NW China; however, the spatial and temporal variations of this belt during metamorphism are poorly understood. In this study, we present comparative petrological studies and 40Ar/39Ar geochronology of HP and UHP pelitic schist exposed along the Habutengsu valley. The schist mainly comprises quartz, white mica, garnet, albite and bluish amphibole. In the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (MnNCKFMASH) system, P–T pseudosections were constructed using THERMOCALC 333 for two representative pelitic schists. The results demonstrate that there was a break in the peak metamorphic pressures in the Habutengsu area. The northern schist has experienced UHP metamorphism, consistent with the presence of coesite in the same section, while the southern one formed at lower pressures that stabilized the quartz. This result supports the previous finding of a metamorphic gradient through the HP–UHP metamorphic belt of the Chinese western Tianshan by the authors. Additionally, phengite in the northern schist was modelled as having a Si content of 3.55–3.70 (a.p.f.u.) at the peak stage, a value much higher than that of oriented matrix phengite (Si content 3.32–3.38 a.p.f.u.). This indicates that the phengite flakes in the UHP schist were subjected to recrystallization during exhumation, which is consistent with the presence of phengite aggregates surrounding garnet porphyroblast. The 40Ar/39Ar age spectra of white mica (dominantly phengite) from the two schists exhibit similar plateau ages of ca. 315 Ma, which is interpreted as the timing of a tectonometamorphic event that occurred during the exhumation of the HP–UHP metamorphic belt of the Chinese western Tianshan.

The early Cretaceous orogen-scale Dabieshan metamorphic core complex: implications for extensional collapse of the Triassic HP–UHP orogenic belt in east-central China

The Dabieshan massif is famous as a portion of the world’s largest HP–UHP metamorphic belt in east-central China that was built by the Triassic North–South China collision. The central domain of the Dabieshan massif is occupied by a huge migmatite-cored dome [i.e., the central Dabieshan dome (CDD)]. Origin of this domal structure remains controversial. Synthesizing previous and our new structural and geochronological data, we define the Cretaceous Dabieshan as an orogen-scale metamorphic core complex (MCC) with a multistage history. Onset of lithospheric extension in the Dabieshan area occurred as early as the commencement of crustal anatexis at the earliest Cretaceous (ca. 145 Ma), which was followed by primary (early-stage) detachment during 142–130 Ma. The central Dabieshan complex in the footwall and surrounding detachment faults recorded a consistently top-to-the-NW shearing. It is thus inferred that the primary detachment was initiated from a flat-lying detachment zone at the middle crust level. Removal of the orogenic root by delamination at ca. 130 Ma came into the extensional climax, and subsequently isostatic rebound resulted in rapid doming. Along with exhumation of the footwall, the mid-crustal detachment zone had been warped as shear zones around the CDD. After 120 Ma, the detachment system probably experienced a migration accommodated to the crustal adjustment, which led to secondary (late-stage) detachment with localized ductile shearing at ca. 110 Ma. The migmatite–gneiss with HP/UHP relicts in the CDD (i.e., the central Dabieshan complex) was product of the Cretaceous crustal anatexis that consumed the deep-seated part of the HP–UHP slices and the underlying para-autochthonous basement. Compared with the contemporaneous MCCs widely developed along the eastern margin of the Eurasian continent, we proposed that occurrence of the Dabieshan MCC shares the same tectonic setting as the “destruction of the North China craton”. However, geodynamic trigger of the Cretaceous continental-scale NW–SE extension in eastern Asia is still a mystery.

郯庐断裂带形成演化与大别–苏鲁造山带的关系探讨

大别–苏鲁造山带是三叠纪华北板块和扬子板块碰撞形成的高压–超高压变质带，其形成演化大致经历了强烈碰撞、俯冲和折返三个阶段，在三叠纪晚期完成最终拚合。作为中国东部巨型断裂构造系统的郯庐断裂带，受控于特提斯构造域向滨太平洋构造域转换和太平洋板块俯冲机制的动力学背景，发生大规模左行平移并将大别–苏鲁超高压变质带错开，左行平移的最大距离在300 km左右，同时将郯庐左行剪切走滑断裂发育历史划分为五个发育阶段，重现了从特提斯体制向太平洋体制的复杂历史转化过程，也在相当程度上反映了我国东部大陆边缘的动力学特征。 Dabie-Sulu orogenic belt is a HP-UHP metamorphic belt formed by the collision of Triassic North China plate and the Yangtze plate. Its formation and evolution have experienced three stages, such as intense collision stage, subduction stage and exhumation stage. In the late Triassic, final regis-tration has been completed. As a giant fault structure system in eastern China, Tanlu fault belt is controlled by dynamic background of transformation from Tethyan tectonic domain to the marginal Pacific tectonic domain, and subduction of the Pacific plate. Thus, large scale sinistral strike- slip happened and staggered Dabie-Sulu UHP metamorphic belt. The maximum distance of sinistral strike-slip is about 300 km. At the same time, development history of Tanlu sinistral shear strike-slip fault is divided into five development stages. It not only reappears complex history of transformation process from Tethys system to the Pacific system, but also reflects dynamics cha-racteristics of continental margin in Eastern China.

Ultra-deep subduction of Yematan eclogite in the North Qaidam UHP belt, NW China: Evidence from phengite exsolution in omphacite

Phengite exsolution in omphacite from the Yematan eclogite, North Qaidam UHP belt, NW China, is described. Mineralogical investigations show that the precursor omphacite in the Yematan eclogite contains up to 1.16 wt% K 2 O and ~10 000 ppm H 2 O. Experimental studies document this omphacite to be formed at pressures higher than 6 GPa (at 900 °C). The pressure-temperature conditions of 3.68 GPa and 892 °C for phengite exsolution in omphacite associated with garnet during the exhumation were obtained by using Grt-Omp-Ph geothermobarometer. We conclude that omphacitic-clinopyroxene in subducted eclogites may act as a robust medium to transport H 2 O and potassium deep into the interior of the Earth. This study suggests that the Yematan eclogite in North Qaidam UHP metamorphic belt, NW China, may have been subducted and exhumed from depths of more than 200 km.