Coesite inclusions in zircon from gneisses identified by laser Raman microspectrometer in ultrahigh pressure zone of Dabie Mountains, China

The mineral inclusions in zircon from gneisses in ultra-high pressure (UHP) zone of the Dabie Mountains were identified by using a laser Raman microspectrometer. Coesite occurs as inclusions in zircons from all types of gneiss. Other important minerals, such as jadeite, omphacite, aragonite, barite, and anhydrite were also found as inclusion minerals. These discoveries indicate that (ⅰ) gneissic country rocks had metamorphosed at the same time as the enclosed eclogites; and (ⅱ) SO₄^-2-bearing fluids were present in the UHP metamorphic process, which is manifested by occurrence of barite and anhydrite coexisting with coesite.     

Multi-stage evolution of gneiss from North Dabie： evidence from zircon U-Pb chronology

TTG gneiss is a common rock to outcrop in the northern part of the Dabie orogen, a few of which are closely associated with eclogites that experienced the Triassic ultrahigh pressure metamorphism. Although they were thermally metamorphosed by a large-scale magma activity in this region at the Early Cretaceous, it is unclear whether or not they are also affected by the Triassic metamorphism during continental subduction and exhumation. In order to resolve this issue, SHRIMP zircon U-Pb dating was carried out for the host gneiss of eclogites in North Dabie. The results show that cores from the gneiss have an age of 746~.31 Ma, consistent with the protolith ages of granitic gneisses in the Dabie orogen. Zircon overgrowing with different U and Th concentrations give concordant ages of 212~21 and 120~11 Ma, respectively. Th/U ratios of overgrown zircons are both lower than 0.1, suggesting a metamorphic genesis. The present resuits suggest that the gneiss in North Dabie has the similar protolith ages of Neoproterozoic to those granitic gneisses elsewhere in the Dabie orogen, and experienced not only the Triassic metamorphism but also the thermal metamorphism due to the Early Cretaceous magmatism. This provides an important insight into the geodynamic evolution of gneissic rocks in the Dabie orogen.     

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.``     

Fluid inclusions evidence for differential exhumation of ultrahigh pressure metamorphic rocks in the Sulu terrane

Since the discovery of coesite and diamond inclusions in eclogites from the Dabie-Sulu orogen, east-central China[1―3], this largest ultrahigh pressure (UHP) metamor- phic terrane in the world has attracted extensive scientific interests. A number of hydrous minerals such as zoisite, phengite, magnesite and talc have been found in the UHP rocks, showing that fluids have played an important role in this type of extreme metamorphic evolution[4―8]. Sev-eral techniques have been applied to th…     

Oxygen isotope compositions of eclogites in Rongcheng,Eastern China

Collected from the Rongcheng region,Shandong Province,the three types of eclogites suffering the UHP (i.e.ultra-high pressure )metamorphism have obviously different oxygen isotope compositions.The eclogites occurring in regional orthogneisses and ultramafic rocks have the oxygen isotope compositions of normal eclogites in the world,but the eclogites existing in marbles are extremely enriched in ^18 O.By applying oxygen isotope geothermometry ,for the all types of exlogites,the temperature estimates of quartz-garnet pair,in principle,indicate the formation temperatures of eclogites,so that the peak-metamorphic temperatures are estimated to be averagely little higher than 800℃ ,which are consistent with the estimates by using other geothermometers according to cation partitioning between coesisting phases ,The δ^18O values of eclogitic inclusions hosted in marbles from Yangguantun,Rongcheng region ,are far higher than the values of eclogites (including eclogitic inclusions in marbles ) from the Dabieshan and other places of the Sulu,The oxygen isotope compositions of various minerals indicate that the formation temperatures of eclogites from the Rongcheng region ,in genceral ,are higher than that from the Dabieshan and the southwestern part of the Sulu ,but the dispersive temperatures estimated by different mineral pairs probably refect that the UHP eclogites from the Rongcheng region generally suffered overprinted metamorphisms during the exhumation.     

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.     

Lawsonite-bearing eclogites in the north Qilian and north Altyn Tagh： Evidence for cold subduction of oceanic crust

Laboratory experiments and thermal models predic that lawsonite-bearing eclogite should be the dominan rock types for typical oceanic subduction zone[1,2] However, eclogite containing unaltered lawsonite is rare in nature and has been described only from …     

Mineral inclusions of zircon and UHP metamorphic evidence from paragneiss and orthogneiss of pre-pilot drillhole CCSD-PP2 in north Jiangsu Province, China

Coesite inclusions, together with omphacite, jadeite, garnet and phengite inclusions, were identified in zircons separates from almost all gneissic core samples of pre-pilot drillhole CCSD-PP2 by the Laser Raman spectroscopy and the cathodoluminescence method. These data indicate that gneissic rocks consisting of paragneisses and orthogneisses ubiquitously experienced UHP metamorphism. This research may be of great significance for an in-depth study of the subduction-exhumation mechanism of the Sulu UHP metamorphic belt and selecting the drilling site for the Chinese Continental Scientific Drilling Project.     

The recognition of adakite-type gneisses in the North Dabie Mountain and its implication to ultrahigh pressure metamorphic geology

Adakite[1] is an igneous rock generated by the partial melting of subducting ocean slab in an island arc setting. Meanwhile, residuals of the slab-melting are transformed to eclogite or garnet-amphibolite during a slab subduction process. This note reports the adakite-type grey gneisses discovered in the North Dabie Mountain. These adakitic rocks are characterized by high Al2O3 (14.97%-17.56%), Sr (545-941 μg/g), Sr/Y (44.71-218.98), La/Yb (16.70-97.50), and low Y (3.32-12.19?μg/g), Yb (0.31-1.34 μg/g), Sc (0.92-4.2 μg/g), as well as positive Sr anomaly but absent or positive Eu anomaly. They are different from the low aluminum grey gneisses with low Al2O3 (13.73%-14.38%), Sr (262-409μg/g), Sr/Y (6.46-15.97) and La/Yb (1.90-22.60), high Y(21.79- 36.24 μg/g), Yb (2.00-5.59 μg/g) , Sc (8.30%-12.70%), and negative Sr, Eu anomaly in the North Dabie Mountain. The adakite-type grey gneisses in the North Dabie Mountain were most probably formed by the partial melting of subducting ocean slab before the Indo-Chinese epoch. We suggest that the subducting of an ocean slab took place prior to the incorporation between the Yangtze and North China plates in the Indo-Chinese epoch, resulting in formation of adakite-type igneous rocks and some ultrahigh pressure eclogites related to the subducted ocean slab. Therefore, besides the ultrahigh pressure metamorphism of the Indo-Chinese epoch, some ultrahigh pressure ecoglits were most likely formed prior to the Indo-Chinese epoch in the Dabie Mountain. In addition, the adakite-type grey gneisses with arc origin occur in the North Dabie Mountain, inferring that the Yangtze plate subducted down below the North China plate and the main suture zone between the two plates was likely along the southern side to the North Dabie Mountain.     

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 …     

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.     

Mineralogical evidence for continental deep subduction

Diamond is an index mineral to prove ultrahigh pressure (UHP) metamorphic conditions because it is only stable at the pressures above 3.3 GPa. Its occurrence in eclogite-facies metamorphic rocks suggests plate subduction to depths over 120 km assuming the normal gradient of lithostatic pressure. Because UHP eclogites are the metamorphic products of basaltic rocks, the occurrence of diamond in the eclogites demonstrates a complete geodynamic cycle in that mafic crustal rocks were subducted t…     

The ages of U-Pb and Sm-Nd for eclogite from the western segment of Altyn Tagh tectonic belt

The eclogites of Altyn Tagh tectonic belt occur as lens within gneisses characterized by amphibolite-facies mineral parageneses. The well-preserved eclogite is selected for Sm-Nd and U-Pb isotopic dating. The Sm-Nd isotopic data yield a whole rock-garnet-omphacite isochron of (500 ± 10) Ma. The U-Pb isotopic measurements of zircons show that the four grain populations are nearly concordant and well plotted on concordia curves, giving a weighted mean age (503.9 ± 5.3) Ma. Two kinds of methods obtain a similar age which reveals the peak metamorphic age of eclogites, and reflect the existence of orogenic root related to Caledonian subduction and continent-continent collision.     

The ages of U-Pb and Sm-Nd for eclogite from the western segment of Altyn Tagh tectonic belt——Evidence for existence of Caledonian orogenic root

The eclogites of Altyn Tagh tectonic belt occur as lens within gneisses characterized by amphibolite-facies mineral parageneses. The well-preserved eclogite is selected for Sm-Nd and U-Pb isotopic dating. The Sm-Nd isotopic data yield a whole rock-garnet-omphacite isochron of (500 ± 10) Ma. The U-Pb isotopic measurements of zircons show that the four grain populations are nearly concordant and well plotted on concordia curves, giving a weighted mean age (503.9 ± 5.3) Ma. Two kinds of methods obtain a similar age which reveals the peak meta-morphic age of eclogites, and reflect the existence of orogenic root related to Caledonian sub-duction and continent-continent collision.     

Extreme crustal oxygen isotope signatures preserved in coesite in diamond

The anomalously high and low oxygen isotope values observed in eclogite xenoliths from the upper mantle beneath cratons have been interpreted as indicating that the parent rock of the eclogites experienced alteration on the ancient sea floor. Recognition of this genetic lineage has provided the foundation for a model of the evolution of the continents whereby imbricated slabs of oceanic lithosphere underpin and promote stabilization of early cratons. Early crustal growth is thought to have been enhanced by the addition of slab-derived magmas, leaving an eclogite residuum in the upper mantle beneath the cratons. But the oxygen isotope anomalies observed in eclogite xenoliths are small relative to those in altered ocean-floor basalt and intermediate-stage subduction-zone eclogites, and this has hindered acceptance of the hypothesis that the eclogite xenoliths represent subducted and metamorphosed ocean-floor basalts. We present here the oxygen isotope composition of eclogitic mineral inclusions, analysed in situ in diamonds using an ion microprobe/secondary ion mass spectrometer. The oxygen isotope values of coesite (a polymorph of SiO2) inclusions are substantially higher than previously reported for xenoliths from the subcratonic mantle, but are typical of subduction-zone meta-basalts, and accordingly provide strong support for the link between altered ocean-floor basalts and mantle eclogite xenoliths.     

Zircon U-Pb dating of basement gneisses in the super-large Bayan Obo REE-Fe-Nb deposit,Inner Mongolia

Zircon U-Pb ages have been determined for basement gneisses in the Bayan Obo REE-Fe-Nb deposit. On the U-Pb concordia diagram data of three samples yield upper intercept ages of 1948-1917 Ma. Thus, these basement gneisses are of Paleoproterozoic. Rock association of these basement gneisses includes tonalite, syenite and paragneiss, not belonging to a sedimentary package. It is more appropriate to consider this association as a basement complex rather than a stratigraphic "Group".     

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.     

辽北新宾?苇子峪地区太古宙花岗质岩石的形成年代、成因及其地质意义

辽北新宾?苇子峪地区的花岗质岩石主要由英云闪长质?奥长花岗质片麻岩和二长花岗岩?正长花岗岩岩体等组成。为确定其形成年代及成因, 对这些花岗质岩石进行锆石U-Pb-Hf同位素和全岩地球化学分析。锆石LA-ICP-MS U-Pb同位素定年结果表明这些花岗质岩石均形成于新太古代, 英云闪长质和奥长花岗质片麻岩的岩浆结晶年龄分别为2588±4 Ma (MSWD=1.3)和2587±6 Ma (MSWD=1.8), 二长花岗岩?正长花岗岩则侵位于2555±4 Ma (MSWD=0.51)。全岩地球化学和锆石Lu-Hf同位素研究表明, 英云闪长质?奥长花岗质片麻岩形成于俯冲板片的部分熔融, 其原始岩浆在上升过程中受到地幔楔岩石的交代; 而二长花岗岩?正长花岗岩中一部分岩浆起源于变质杂砂岩的部分熔融, 其余形成于以变质玄武岩与变质沉积岩为主要成分的混合源区的部分熔融。结合近年的研究成果, 认为新宾?苇子峪地区的新太古代花岗质岩石可能形成于活动大陆边缘的动力学背景。     

Eclogites in the interior of the Tibetan Plateau and their geodynamic implications

Eclogites have been recently reported in the interior of the Tibetan Plateau, including in the central Qiangtang metamorphic belt, in the Basu metamorphic massif of the eastern Bangong-Nujiang suture zone, and at Songdo and Pengco in the eastern Lhasa terrane. Some typical ultrahigh-pressure （UHP） metamorphic phenomena, e.g., garnet exsolution from clinopyroxene, were documented in the Basu and Pengco eclogites. The UHP metamorphism in the interior of the Tibetan Plateau marked by these eclogites generally took place in the Early Mesozoic. Along with exhumation of these eclogites, （post-） collision-related magmatism extensively occurred around the central Qiangtang belt, the eastern Bangong-Nujiang suture zone, and the eastern Lhasa terrane. The occurrence of these Early Mesozoic eclogites manifests an out-of-sequence evolution of the Tethys, and they could be a product of diachronous collision between the eastern Qiangtang terrane and the irregular continental margin of the united western Qiangtang-Lhasa plate, along the linked eastern Bangong-Nujiang-central Qiangtang zone. The collision-related magmatic rocks could have been originated from lithospheric thickening, melting, or detachment due to the collision. The presence of UHP metamorphic rocks in central Qiangtang and Basu implies likely continental deep-subduction, and the denudation of these two metamorphic zones could have served as the source of the Triassic turbidites in the Songpan-Garze complex and the Jurassic turbidites in the western Bangong-Nujiang zone, respectively. However, studies of the eclogites in the interior of the Tibetan Plateau just began, and many principal aspects still remain to be explored, such as their distributions, typical lithologies and minerals, temperature-pressure conditions, timing of formation and exhumation, protoliths and tectonic setting, and relationship with the evolution of the Tethys and large-scale basins in Tibet.     

Call in question and discussion: Are there sandwiched low-grade metamorphic slabs within UHP metamorphic rocks in the Dabieshan terrane

Some geologists reported their discovery of sandwiched low-grade metamorphic slabs within UHP metamorphic Complexes in Changpu, Yuexi County, Anhui Province. They also suggested that some coesite-bearing eclogites are igneous veins, which intruded low-grade metamorphic slabs and other related rocks. Moreover they further called in question to UHP metamorphic process and continental collision tectonism in the Dabieshan terrane. Based on our recent study, so-called low-grade metamorphic slabs are strongly deformed fabric, fine-grained and tectonic recrystallized mylonites and tectonites. Their protolith rocks are garnet-bearing orthogneiss, eclogite and marble, as well as a few amounts of strongly deformed acid and basic veins. Their metamorphic, geochemical and geochronological characteristics are also identical with UHPM rocks and regional country orthogneisses. Therefore we conclude that there are not low-grade metamorphic slabs sand wiched with UHP metamorphic rocks in the Dabieshan terrane.