南大别变质块体温压结构的初步探讨

根据对太湖地区大坝附近榴辉岩的峰期变质P-T条件的研究,以及前人对南大别榴辉岩的P-T条件的分析,同时结合榴辉岩的空间分布状态的研究,文章认为南大别变质块体自北向南可分为金刚石、柯石英榴辉岩、石英/柯石英榴辉岩和石英榴辉岩4个变质单元;变质P-T条件总体上显示逐渐降低的趋势,反映了南大别变质块体俯冲时的连续性,而现今榴辉岩的P-T分布型式则代表南大别多板片的折返状态。     

A huge oceanic-type UHP metamorphic belt in southwestern Tianshan, China: Peak metamorphic age and P-T path

Recent progress in the study of the UHP metamorphic belt in southwestern Tianshan, China, is summarized in this paper. This about 80-kin-long and over 10-km-wide UHP belt has been recognized by the discovery of coesite, coesite pseudomorphs and other UHP minerals. It is the largest oceanic-type UHP metamorphic belt reported so far. It has formed due to northward subduc- tion of the Tianshan Paleo-Ocean. U-Pb dating of metamorphic rims of zircons from a coesite-bearing garnet-phengite schist yields a peak UHP metamorphic ages of 320±3.7 Ma. Combined with ages of 233-226 Ma obtained from rims of zircons from retrograded eclogites, a long retrograde metamorphic evolution （〉70 Ma） has been revealed. According to phase equilibria mod- eling, the P-T paths of both coesite-bearing eclogites and garnet-phengite schists are characterized by thermal relaxation, i.e., the metamorphic temperature peak lags behind the pressure peak, indicating that the UHP rocks experienced slow and long heating and decompression during exhumation in the subduction channel. On the basis of the field observation that a small amount of eclogite lenses is wrapped in large volumes of metapelites, and the similar P-T paths of both rock types, we propose that the ex- humation of the UHP eclogites from southwestern Tianshan, China, may have resulted from the exhumation of large volumes of low-density metapelites, which carried the denser eclogites to the Earth＇s surface.     

Metamorphic solid salt (KCl-NaCl) in quartzo-feldspathic polyphase inclusions in the Sulu ultrahigh-pressure eclogite

Unusual polyphase inclusions of K-feldspar+quartz+titanite+solid salt and K-feldspar+albite+quartz+epidote with textures similar to the other K-feldspar+quartz inclusions were found in omphacite grains from the Sulu ultrahigh pressure (UHP) eclogites. One of these inclusions contain square to round solid salt inclusions of KCl-NaCl composition. Such a mineral assemblage within K-feldspar-bearing inclusions hosted by UHP metamorphic phases suggests that (1) potassium granitic melts enriched in Cl components were presented during UHP metamorphism or at the early stage of rapid exhumation of deeply subducted continental slab; (2) they were resulted from reactions between the incoming granitic melts and quartz (or coesite); and (3) solid salt inclusions of NaCl-KCl were derived from dehydration and desiccation of Cl-bearing melts. Our new observations further demonstrate that during the tectonic evolution of UHP rocks, fertile components within deeply subducted continental materials could undergo partial melting, leading to the formation of Cl-bearing potassium granitic melts and substantial migration of fluid-conservative elements (e.g. Ti, Hf) within the UHP slab.     

Fluid activity during exhumation of deep-subducted continental plate

It is well known that a great deal of fluid wasreleased during subduction of oceanic crust, resulting in arcmagmatism, quartz veining and metamorphic mineralizationof syn-subduction. In contrast, the process of continentalsubduction is characterized by the relative lack of fluid andthus no arc magmatism has been found so far. During exhu-mation of deep-subducted continental crust, nevertheless,significant amounts of aqueous fluid became available fromthe decomposition of hydrous minerals, the decrepitation ofprimary fluid inclusions, and the exsolution of structuralhydroxyls. This kind of metamorphic fluid has recently at-tracted widespread interests and thus been one of the mostimportant targets in deciphering the geological processesconcerning metamorphism, magmatism and mineralizationin collisional orogens. A large number of studies inlvolvingstable isotopes, fluid inclusions and petrological phase rela-tionships have been accomplished in past a few years withrespect to the mobility and amount of met     

The geological characteristics of oceanic-type UHP metamorphic belts and their tectonic implications: Case studies from Southwest Tianshan and North Qaidam in NW China

The geological characteristics of ultrahigh-pressure （UHP） metamorphic belts formed by deep subduction of oceanic crust are summarized in this paper. Oceanic-type UHP metamorphic belt is characterized by its protolithlc assemblage of typical oceanic crust, the peak metamorphic temperature 〈600℃, P-T path undergoing blueschist facies during prograde and retrograde metamorphic evolution, reepectively, with low geothermal gradient of cold subduction. The further study of oceanic-type UHP metamorphic belt is very significant for constructing metamorphic reaction series of cold subduction zone, for understanding how aqueous fluids were transported into deep mantle and for classifying the types of UHP metamorphism in cold subduction zone. The uplift and exhumation mechanism of oceanic UHP metamorphic rocks is one of the most challenging problems in the study of UHP metamorphism, which is very important for understanding the geodynamic mechanism of solid Earth. As a traveler eubducted into the mantle depth end then uplifted to the surface, oceanic-type UHP metamorphic belts witness the bulk process from the subduction to exhumation and is an ideal target to study the geochemical behavior end cycling of elements in subduction zones. The tectonic evolution of one convergent orogenic belt can be usually divided into two stages of oceanic subduction and followed continental subduction and collision, and the two best-established examples of orogenic belts are Alpa and Himalaya. Therefore, the study of oceanic-type UHP metamorphic belt is the frontier of the current plate tectonic theory. As two case studies, the current status and existing problems of oceanic-type UHP metamorphic belts in Southwest Tianshan and North Qaidam, NW China, are reviewed in this paper.     

Detachment within subducted continental crust and multi-slice successive exhumation of ultrahigh-pressure metamorphic rocks: Evidence from the Dabie-Sulu orogenic belt

Although tectonic models were presented for exhumation of ultrahigh-pressure （UHP） metamorphic rocks during the continental collision, there is increasing evidence for the decoupling between crustal slices at various depths within deeply subducted continental crust. This lends support to the multi-slice successive exhumation model of the UHP metamorphic rocks in the Dabie-Sulu orogen. The available evidence is summarized as follows： （1） the low-grade metamorphic slices, which have geotectonic affinity to the South China Block and part of them records the Triassic metamorphism, occur in the northern margin of the Dabie-Sulu UHP metamorphic zone, suggesting decoupling of the upper crust from the underlying basement during the initial stages of continental subduction; （2） the Dabie and Sulu HP to UHP metamorphic zones comprise several HP to UHP slices, which have an increased trend of metamorphic grade from south to north but a decreased trend of peak metamorphic ages correspondingly; and （3） the Chinese Continental Science Drilling （CCSD） project at Donghai in the Sulu orogen reveals that the UHP metamorphic zone is composed of several stacked slices, which display distinctive high and low radiogenic Pb from upper to lower parts in the profile, suggesting that these UHP crustal slices were derived from the subducted upper and middle crusts, respectively. Detachment surfaces within the deeply subducted crust may occur either along an ancient fault as a channel of fluid flow, which resulted in weakening of mechanic strength of the rocks adjacent to the fault due to fluid-rock interaction, or along the low-viscosity zones which resulted from variations of geotherms and lithospheric compositions at different depths. The multi-slice successive exhumation model is different from the traditional exhumation model of the UHP metamorphic rocks in that the latter assumes the detachment of the entire subducted continental crust from the underlying mantle lithosphere and its subsequent exhumation as a whol     

Water contents and hydrogen isotopes in nominally anhydrous minerals from UHP metamorphic rocks in the Dabie-Sulu orogenic belt

A continuous flow method, by a combination of thermal conversion elemental analyzer （TC/EA） with isotope ratio mass spec- trometry （MS）, was developed to determine both H isotope composition and H2O concentration of ultrahigh-pressure （UHP） metamorphic rocks in the Dabie-Sulu orogenic belt. By using the developed step-heating technique, we have studied H2O concen- tration and H isotope composition of the different forms of water （structural OH and molecular H2O） in garnet. The quantitative measurements of HzO concentration and H isotope composition of minerals in UHP metamorphic rocks from several typical out- crops indicate that the gneisses can release more amounts of water than the eclogites during exhumation of the deeply subducted continental crust. Therefore, by decompression dehydration at the contact between eclogite and gneiss, the released water could flow from the gneiss to the eclogite and result in significant hydration of the eclogite adjacent to the gneiss. The measured maxi- mum water contents of minerals in eclogites indicate that garnet and omphacite have the maximum water solubilities of 2500 and 3500 ppm, respectively, under the peak UHP metamorphic conditions.     

Partial melting of deeply subducted continental crust and the formation of quartzofeldspathic polyphase inclusions in the Sulu UHP eclogites

Two types of quartzofeldspathic inclusions hosted by omphacite and garnet were identified in the Sulu UHP eclogites. The first consists of albite, quartz, and various amounts of K-feldspar. In contrast, the second consists predominantly of K-feldspar and quartz without any albite. The presence of quartzofeldspathic inclusions within the UHP mafic eclogites indicates that partial melting occurred in deeply subducted continental crust via mica dehydration melting reactions at an early stage of rapid exhumation. Such a melting event generated hydrous Na-K-Al-Si melts. These melts infiltrated into the mafic eclogite and were captured by recrystallizing garnet or omphacite, which together followed by dehydration and crystallization to form feldspar-bearing polyphase inclusions. Formation of silicate melts within the deeply subducted continental slab not only provides an excellent medium to transport both mobile （LILE） and immobile （HFSE） elements over a large distance, but also induces effective changes in the physical properties of the UHP slab. This process could be a major factor that enhances rapid exhumation of a deeply subducted continental slab.     

Whole-rock Ar-Ar dating for low-grade metavolcanics within the Dabie orogen and its geological significance

Greenschist-facies metasedimentary and metaigne- ous rocks are frequently found to occur continuously along convergent plate margins where high pressure (HP) or ultrahigh pressure (UHP) metamorphic rocks also crop out[1-7]. Geological investigations of co…     

Discovery and geological implication of rodingites derived from eclogites of ophiolites at Changawuzi, western Tianshan, China

We here report the first case of rodingites that were formed at the expense of eclogite enclosed in the ultramafic rocks of Changawuzi ophiolites. The rodingites contain a mineral assemblage of grossular, diopside, prehnite, chlorite, and relict omphacite and Fe-Mg-Al garnets. The presented data indicate that the formation of the rodingites resulted from a phase of secondary serpentinization during exhumation of the subducted oceanic plate. The rodingitization started at 370～410 ℃/6.5～8.5 kbar, while pervasive rodingitization took place under the condition of 200～350 ℃/4～6 kbar. The established PT path shows a retrograde track from eclogites to rodingites. We conclude that the process of rodingitization could also take place under subduction conditions in addition to its more common occurrence under ocean-floor metamorphic conditions.     

Discovery of Late Paleozoic retrograded eclogites from the middle part of the northern margin of North China Craton

The northern margin of the North China Craton (NCC), located between the Paleo-Asian Ocean tectonic region on the north and the NCC on the south, is a key region for studying the tectonic evolution of NCC. A Pre-cambrian retrograded eclogite (2500 Ma or 1800 Ma) was reported in Baimashi near Hengshan Mountain in the NCC, which is characterized by the vermicular symplec-tite of diopside and plagioclase with absence of ompha-cite[1,2]. In Hongqiyingzi Group from the middle part of the …     

Finding of high-pressure mafic granulites in the Amdo basement,central Tibet

High-pressure mafic granulites with a peak mineral assemblage of garnet + clinopyroxene + rutile + quartz were found in the Amdo basement, central Tibet. Two kinds of symplectites were identified that are composed of orthopyroxene + plagioclase ± spinel and hornblende + plagioclase around garnet, which were interpreted to develop during the retrogressing stages in the granulites. P-T estimates suggested that peak metamorphic conditions were about 860–920°C and 1.46–1.56 GPa, which retrogressed from post-peak phase at 820–890°C and 0.88–1.15 GPa to amphibolite facies at 550–670°C and 0.52–0.65 GPa. These three stages define a clockwise P-T path with near-isothermal decompression and cooling following the peak high-pressure metamorphism. This suggests that the Amdo granulites underwent an initial subduction to a deep crustal level of ~50 km and then were rapidly exhumed to a shallow crustal level (~20 km). The formation of Amdo granulites is considered to result from the arc-continent collision between the Amdo basement and the Qiangtang terrane in the middle Jurassic, which is a crucial step to the tectonic evolution of the Tibetan Plateau.     

Science Letters： Discovery of ultrahigh-T spinel-garnet granulite with pure CO2 fluid inclusions from the Altay orogenic belt, NW China

We first report discovery of the spinel-garnet-orthopyroxene granulite with pure CO2 fluid inclusions from the Fuyun region of the late Paleozoic Altay orogenic belt in Central Asia, NW China. The rock is characterized by an assemblage of garnet, orthopyroxene, spinel, cordierite, biotite, plagioclase and quartz. Symplectites of orthopyroxene and spinel, and orthopyroxene and cordierite indicate decompression under UHT conditions. Mineral chemistry shows that the orthopyroxenes have high XMg and Al2O3 contents (up to 9.23 wt%). Biotites are enriched in TiO2 and XMg and are stable under granulite facies conditions. The garnet and quartz from the rock carry monophase fluid inclusions which show peak melting temperatures of around -56.7℃, indicating a pure CO2 species being presented during the ultrahigh-T metamorphism in the Altay orogenic belt. The inclusions homogenize into a liquid phase at temperatures around 15.3-23.8℃ translating into CO2 densities of the order of 0.86-0.88g/cm^3. Based on preliminary mineral paragenesis, reaction textures and petrogenetic grid considerations, we infer that the rock was subjected to UHT conditions. The CO2-rich fluids were trapped during exhumation along a clockwise P-T path following isothermal decompression under UHT conditions.     

<Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar isochron ages of lawsonite blueschists from Jiuquan in the northern Qilian Mountain,NW China,and their tectonic implications

Using laser ⁴⁰Ar/³⁹Ar dating method, we have gotten the metamorphic ages of lawsonite blueschist and epidote blueschist from Jiuquan, northern Qilian Mountain, NW China. The high quality laser ⁴⁰Ar/³⁹Ar dating of glaucophane from lawsonite blueschist gives an isochron age of 413±5 Ma. The isochron age obtained from phengite in epidote blueschist is 415±7 Ma. These data, combining with peak metamorphic P-T conditions and regional geological setting, allow us to infer that the lower limit of the ages of the prograde subduction metamorphism from lawsonite blueschist facies to epidote blueschist facies occurred at ca. 413–415 Ma, which also suggests that the formation of lawsonite blueschist in the northern Qilian Mountain maybe resulted from the corner flow in the cold subduction zone. This study shows that the final closing time of the northern Qilian remnant oceanic basin is about 413–415 Ma, which also represents the convergent age between the North China Craton and the Qaidam block.     

Phase equilibria of HP-UHP mica-schists in Jiangling,Eastern Dabieshan orogen

The HP-UHP metamorphic terrane of Jiangling, eastern Dabieshan comprises extensively distributed granitic gneisses, mica-schists and numerous eclogite blocks. The mica-schists generally contain garnet, phengitic muscovite, biotite, plagioclase, quartz, rutile and a small amount of epidote and hornblende. Study on petrography and phases equilibria in the NCKMnFMASH system indicates that the present mineral assemblages in mica-schists are not in equilibrium. The earlier stage of mineral assemblage represented by garnet and phengite reflects a HP-UHP condition. The garnet compositions and the phengite Si contents give a PT condition of 580-00 ℃ at 2.6-2.8 GPa. The garnet zonation records an earlier progressive metamorphic process which may be associated with the appearance of glaucophane, jadeite and lawsonite. The later stage of mineral assemblage characterized by the presence of biotite and plagioclase reflects a PT condition of 620-635℃ at 0.9-1.1 GPa, belonging to the HP amphibolite facies. The main mineral assemblage in mica-schists from the Jiangling region has recorded a complete HP-UHP metamorphic process.     

Heterogeneity of water in UHP eclogites from Bixiling in Dabieshan： Evidence fromgarnet

Garnets in ultrahigh pressure (UHP) eclogites from Bixiling in Dabieshan were investigated by Fourier transform infrared spectrometer (FTIR). The results demonstrate that all garnets contain structural water which occurs as hydroxyl (OH), with contents ranging from 164 to 2034 ppm (H2O wt.) and mostly higher than 500 ppm. Like omphacite which is another major OH-rich mineral in eclogites, garnet is an important carrier that can recycle the surface water into deep mantles. Heterogeneity of water in garnets exists not only among different samples of the same outcrop (～150 m), but also among different crystals of the same sample (～1 cm). This indicates that the mobility of fluids during UHP metamorphism is very limited (possibly on centimeter scales), and that both subduction and exhumation processes of UHP rocks are very fast.     

The Dabie-Sulu UHP rocks belt: review and prospect

The new results in the studies of the Dabie-Sulu UHP rocks belt during the past 5 years were summarized and discussed. The discussion included the following key points: ( i ) UHP eclogite has two kinds of country rocks, with one being UHP eclogite facies rocks and the other non-UHP granitic gneiss. ( ii ) The FeTiO₃ in olivine indicated exsolution at depth of 300–400 km. However, the key point is to prove the peridotite in which the FeTlO₃ in olivine was found once had been subducted down that depth. ( iii ) UHP hydrous phase evidenced that fluids had taken part in the UHP metamorphism, while the meter-scale inhomogeneous distribution of O-, C-isotope indicated no fluid activity in the deep subduction environment. ( IV ) No agreement has been arrived on many problems related to the tectonic background of the UHP rocks, such as “whether or not ophiolitic rocks there exist now?”, “when did UHP metamorphism proceed?”, “what is the subdution polarity?”, etc. ( V ) How did the UHP rocks exhume from mantle depth? The future studies will focus on the following three subjects: ( i ) thermal dynamics of the UHP metamorphism, ( ii ) relationship between UHP metamorphism and collision orogeny, as well as their geodynamics, and ( iii ) interactions between crust and mantle, and between continental lithosphere and asthenosphere during the collision orogenic process, as well as their constraints to the evolution of continental lithosphere.     

Discovery of eclogite inclusions and its geological significance in early Jurassic intrusive complex in Xuzhounorthern Anhui,eastern China

The early Jurassic intrusive complex is chiefly made of monzodioritic porphyry in northern Anhui and northern Jiangsu, which emplaced in 191 Ma. The intrusive complexes contain a lot of eclogite inclusions which belong to eclogite and garnet-pyroxenite. The inclusions had undergone eclogite facies high-pressure metamorphism and amphibolite facies retrogressive metamorphism. The garnets in eclogite inclusions are mainly almandine varieties and clinopyroxenes are omphacite and augite. The mineral assemblage and P-T estimation results show that P-T conditions of eclogite facies metamorphism and amphibolite facies retrogressive metamorphism are over 1.2—1.5 GPa, 709—861℃ and 0.7—1.03 GPa, 666—738℃, respectively. The discovery of the highpressure xenoliths not only is of important significance to understand the composition and structure of deep crust in southern edge of North China Platform, but also can be of important influence on realizing the subduction-collision-exhumation evolutional process of the DabieSulu ultrahigh-pressure (UHP) metamorphic belt.     

Evidence for UHP metamorphism of eclogites from the Altun Mountains

Ultrahigh pressure (UHP) metamorphism refers to metamorphism that has occurred at pressures for the stability of coesite. The polycrystalline quartz inclusions showing the characteristic texture within garnets of eclogites indicates the pre-existence of coesites under the peak metamorphic condition. The unusual exsolution textures in ompacites and apatites, and the pressure estimations of phengite-bearing eciogites have been taken to provide further proof of eclogite formation under the UHP conditions.Combined with the fact that coesites have been observed in country rocks of eclogites in North Qaidam Mountains, another UHP metamorphic belt cut by the large-scale strikeslip fault in the AItun-North Qaidam area of China is confirmed.``     

Pliocene eclogite exhumation at plate tectonic rates in eastern Papua New Guinea

As lithospheric plates are subducted, rocks are metamorphosed under high-pressure and ultrahigh-pressure conditions to produce eclogites and eclogite facies metamorphic rocks. Because chemical equilibrium is rarely fully achieved, eclogites may preserve in their distinctive mineral assemblages and textures a record of the pressures, temperatures and deformation the rock was subjected to during subduction and subsequent exhumation. Radioactive parent-daughter isotopic variations within minerals reveal the timing of these events. Here we present in situ zircon U/Pb ion microprobe data that dates the timing of eclogite facies metamorphism in eastern Papua New Guinea at 4.3 +/- 0.4 Myr ago, making this the youngest documented eclogite exposed at the Earth's surface. Eclogite exhumation from depths of approximately 75 km was extremely rapid and occurred at plate tectonic rates (cm yr(-1)). The eclogite was exhumed within a portion of the obliquely convergent Australian-Pacific plate boundary zone, in an extending region located west of the Woodlark basin sea floor spreading centre. Such rapid exhumation (> 1 cm yr(-1)) of high-pressure and, we infer, ultrahigh-pressure rocks is facilitated by extension within transient plate boundary zones associated with rapid oblique plate convergence.