Rutile U-Pb age for the ultrahigh pressure eclogite from the Dabie Mountains,central China: Evidence for rapid cooling

The U-Pb isotope compositions of rutile, omphacite and garnet in the eclogite from the Jinheqiao area in the Southern Dabie ultrahigh-pressure metamorphic zone were analyzed. The consistent high precision U-Pb age (218 ± 1.2) Ma of rutile in eclogite from the Dabie Mountains was obtained by two ways of isochron and common Pb correction based on the composition of omphacite. This proves that the omphacite in eclogite has a U/Pb ratio (m = 2.8) low enough to be used for common Pb correction in the analyses of rutile. Under the rapid cooling condition (40℃/Ma) the closure temperature for U-Pb diffusion in rutile is about 470℃. Thus, this U-Pb age of rutile proves that 218 Ma should be the cooling age of eclogite at 470℃ instead of the peak metamorphic age.     

SHRIMP dating of zircons from the Caledonian Xiongdian eclogite, western Dabie Mountains, China

This note reports the SHRIMP U-Pb data of zircons from the Caledonian Xiongdian eclogite, western Dabie Mountains. Zircons from the rock occur mainly in garnet and other metamorphic minerals with sharp boundaries and exhibit textures growing under metamorphic conditions. Analyses of 7 grains give ²⁰⁶Pb/²³⁸U ages ranging from 335 to 424 Ma, showing a certain degree of radiogenic lead loss. This suggests a minimum age of (424±5) Ma for the metamorphic zircons, as well as the high-pressure metamorphic event. The outer peripheral zone of a zircon gives ²⁰⁶Pb/²³⁸U age of about 300 Ma. Combined with Sm-Nd, ⁴⁰Ar-³⁹Ar, U-Pb and ²⁰⁷Pb/²⁰⁶Pb ages, the peak metamorphism of the Xiongdian eclogite is documented between 424—480 Ma.     

A combined study of SHRIMP U-Pb dating, trace element and mineral inclusions on high-pressure metamorphic overgrowth zircon in eclogite from Qinglongshan in the Sulu terrane

Methods recently advanced for discrimination on the genesis of metamorphic zircon, such as analysis of mineral inclusions and trace elements, provide us powerful means to distinguish zircon overgrowth during high-pressure metamorphism. Zircons in ultrahigh-pressure eclogite from Qinglongshan in the Sulu terrane were studied by the SHRIMP U-Pb method in combining with trace element and mineral inclusion analyses. No inherited core was identified in the analyzed zircons by means of cathodoluminescence images. The occurrence of high-pressure metamorphic mineral inclusions in zircon, such as garnet, omphacite, rutile, and the flat HREE pattern in zircon indicate that the zircon formed at high-pressure metamorphic conditions. Therefore, a weighted average U-Pb age of 227.4±3.5 Ma obtained from such a kind of zircon is interpreted to represent the timing of peak nietamorphism for the Qinglongshan eclogite.     

Zr-in-rutile thermometry in eclogite at Jinheqiao in the Dabie orogen and its geochemical implications

Trace element analysis was made by an electron microprobe on rutiles from ultrahigh-pressure ec-Iogite at Jinheqiao in the Dabie orogen. The results show Zr contents of 37 to 118 μg.g^-l, with signifi-cant variations between grains within the same thin section. Zr-in-rutile temperatures were calculated by the pressure-dependent thermometer, yielding 595 to 678℃ at 4.1 GPa for the upper limit of meta- morphic pressures. The temperatures are about 200 to 250℃ lower than temperatures of 846±70℃ calculated from garnet-omphacite Fe-Mg partition geothermometry, but 150 to 200℃ higher than -460℃ from quartz-ruUle O isotope temperatures and rutile Pb diffusion closure temperature for the same specimen. This indicates that the diffusion rate of Zr in ruUle is remarkably slower than that of Pb and O in rutile, and that the temperature of Zr-in-rutile is higher than the closure temperature corre-sponding to cooling age as dated by rutile U-Pb radiometric system. The latter is consistent with ex-perimental result of Zr diffusion in rutile. Diffusion and re-crystallization with intensive retrograde fluid during exhumation of UHP metamorphic rocks may be the major reason why the Zr contents in rutile decreased and thus did not provide the record of the peak metamorphic temperatures. The remarkable difference in Zr content between rutile grains suggests that the distribution of Zr in rutile was in the state of disequilibrium due to differential diffusion and retrograde resetting.     

Discovery of the eclogite and its petrography in the Northern Dabie Mountain

Eclogite from the Northern Dabie Mountain is a new finding by the authors. These eclogites in foliated perdotite are enveloped by banded gneiss and occur in the mafic-ultramafic rock belt. They are mainly composed of omphacite, garnet, diopside, orthopyroxene, amphibole, plagioclase and magnetite, and a small amount of rutile, spinel, olivin and 鏾rundum. The mineral association of peak metamorphism of the eclogite is omphacite+garnet+rutile. The existence of eclogite in the Northern Dabie Mountain implies that there was an eclogitic metamorphism prior to the granulitic facies one in the mafic-ultramafic rock belt.     

SHRIMP zircon U-Pb dating for two episodes of migmatization in the Dabie orogen

Zircon CL imaging and SHRIMP U-Pb dating were carried out for migmatite in the Dabie orogen. Zircons from the Manshuihe migmatite show clear core-rim structures. The cores display sector or weak zoning and low Th/U ratios of 0.01 to 0.17, indicating their precipitation from metamorphic fluid. They yield a weighted mean age of 137±5 Ma. By contrast, the rims exhibit planar or nebulous zoning with relatively high Th/U ratios of 0.35 to 0.69, suggesting their growth from metamorphic melt. They give a weighted mean age of 124±2 Ma. Zircons from the Fenghuangguan migmatite also display core-rim structures. The cores are weakly oscillatory zoned or unzoned with high Th/U ratios of 0.21 to 3.03, representing inherited zircons of magmatic origin that experienced different degrees of solid-state recrystallization. SHRIMP U-Pb analyses obtain that its protolith was emplaced at 768±12 Ma, consistent with middle Neoproterozoic ages for protoliths of most UHP metaigneous rocks in the Dabie-Sulu orogenic belt. By contrast, the rims do not show significant zoning and have very low Th/U ratios of 0.01 to 0.09, typical of zircon crystallized from metamorphic fluid. They yield a weighted 206Pb/238U age of 137±4 Ma. Taking the two case dates together, it appears that there are two episodes of zircon growth and thus migmati-tization at 137±2 Ma and 124±2 Ma, respectively, due to metamorphic dehydration and partial melting. The appearance of metamorphic dehydration corresponds to the beginning of tectonic extension thus to the tectonic switch from crustal compression to extension in the Dabie orogen. On the other hand, the partial melting is responsible for the extensional climax, resulting in formation of coeval migmatite, granitoid and granulite. They share the common protolith, the collision-thickened continental crust of mid-Neoproterozoic ages.     

Discovery of eclogite and its geological significance in Qiangtang area, central Tibet

Eclogites have been found in the margin of the Qinghai-Tibet Plateau respectively since the 1990s. First Eocene eclogite from Himalayan belt was discov- ered in Kaghan valley, northern Pakistan in 1991, in which coesite was identified[1,2]. Then two eclog…     

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 …     

Nd isotopic disequilibrium between minerals and Rb-Sr age of the secondary phengite in eclogite from the Yangkou area, Qingdao, eastern China

There are two generations of white micas in retrograded coesite-bearing eclogite from the Yangkou area near Qingdao, eastern China. The secondary phengite developed along the folliations in eclogite is the majority of the white micas. Nd and Sr isotopic disequilibriums between garnet and retrograded omphacite as well as secondary phengite have been observed. Consequently, the Rb-Sr ages ((193 ± 4) Ma―(195 ± 4) Ma) given by the tie lines of the secondary phengite + garnet or whole rock may predate the formation time of the phengite. The Rb-Sr age of (183 ± 4) Ma given by the secondary phengite + retrograde omphacite is much closer to the formation time of the phengite indicating the retrograde age of eclogite instead of a cooling age of eclogite at 500℃.     

Isotope composition of helium in eclogite from the Dabie Mountains, central China and its geological significance

Isotopic geochemical characteristics of helium in garnet and omphacite of eclogite in the Dabie Mountains are discussed. Concentrations of 3He and 4He in garnet and omphacite are 3.9×10 -14- 24.0×10 -14 and 0.48×10 -7-9.42×10 -7cm+3·g -1, respectively. Values of 3He/ 4He have a range of (1.19-4.63)×10 -7. Helium in the eclogite is derived from both mantle and crust. Isotopic geochemical data of helium indicate that eclogite in the Dabie Mountains might be formed in depleted mantle and the age of the cologite would be Indo-China epoch.     

Lower time limit on the UHPM rock exhumation: Discovery of eclogite pebbles in the Late Jurassic conglomerates from the northern foot of the Dabie Mountains, eastern China

Eclogite pebbles are first discovered in late Jurassic conglomerates of the Fenghuangtai Formation from the northern foot of the Dabie Mountains, eastern China. The eclogite shows relatively strong retrograde metamor- phism. The major remnant minerals are garnet, phengite, quartz, rutile, clinozoisite, fingerprint(or worm)-form sym- plectite and pseudomorphic enclosure of coesite in garnet. End-member components of the garnet mainly made up of grossular (27.7%-37.8%), almandinc (45.5%-49.0%) and pyrope (12.3%-25.7%). End-member components of the pyrope is less than 30%, i.e. equivalent to C type eclogite. Si-cations of the phengite are 3.44-3.54 p.f.u. (taking O=11 as standard). It is estimated that the pressures are 2.5-3.2 GPa, the temperatures are 600-900℃. Discovery of the eclogite pebbles in the late Jurassic conglomerates from the northern foot of the Dabie Mountains suggests that the HP-UHP metamorphic rocks of the Dabie Mountains, which were formed in Triassic, had been exposed to the surface in the late Jurassic or before it by exhumation.     

Multistage evolution of continental collision orogen: A case study for western Dabie orogen

The foramtion and evolution of collisional orogen is a prominent feature along convergent plate margins, and is generally a complex process. This article presents an integrated study of zircon genesis, U-Pb age and Lu-Hf isotope composition as well as geological characteristics for the western Dabie orogen to constrain its multi-stage evolution history. The results suggest that the formation of oceanic crust in the Huwan area was constrained at ca. 400--430 Ma, which was slightly later than the collision of the northern Qinling with the North China Block. It formed in a marginal basin in the northern margin of the Yangtze Block. The peak metamorphism of eclogite in the Huwan area occurred at ca. 310 Ma, and the timing of the initial exhumation of oceanic eclogite was about 270 Ma. The high to ultrahigh pressure （HP-UHP） metamorphic rocks in the Xinxian and the Hong＇an metamorphic zones have the same ages and natures as those of the HP-UHP metamorphic rocks in the other Dabie-Sulu terrains, and also have experienced multi-stage exhumation, and thus can be taken as a coherent part of the Dabie-Sulu orogen. Therefore, the Qinling-Dabie-Sulu orogen is a typical multi-stage continental collision orogen, with an amalgamation process extending more than 200 Ma.     

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…     

LA-ICP-MS zircon U-Pb dating for high-pressure basic granulite from North Qinling and its geological significance

Inrecentyears,discoveriesofhightoultrahighpressuremetamorphicrocksatthenorthandsouthsidesoftheQinlingGroupinthenorthernpartoftheQinlingMountains(hereafterNorthQinling)haveattractedfocusattentionofgeologistsworldwide.Thenorthhigh-pressure(HP)metamorphicbelt,distributedintheareafromGuanpotoShuanghuaishunorthwardtoShizipin,LushCountyinHenanProvince,consistsmainlyofeclogiteoutcroppedaslenticularmassesofdifferentsizesinthegneissesoftheQinlingGroupclosetothesouthsideofZhuyangguan-Xiaguanfault…     

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.     

SHRIMP U-Pb zircon ages for the UHP metamorphosed granitoid gneiss in Altyn Tagh and their geological significance

Different types of UHP metamorphic rocks havebeen recently discovered in the Altyn Tagh[1—4], the north-ern margin of Qadam Basin[5—7], the southwestern Tian-shan Mountains[8,9] and the northern Qinling Moun-tains[10,11] in Central and Western China. And these areashave attracted focus attention of geologists at home andabroad to the studying of UHP metamorphism and conti-nental deep subduction. However, as newly discoveredUHP metamorphic terranes, some questions have beenarisen abou…     

Characteristics and geological significance of the eclogites from the Tongbai Mountains, Henan Province

Some eclogites and the related high-P metamorphic belt have been found for the first time from the Tongbai Mountains, Henan Province, where the eclogites and their probable retrogressive products ——garnet amphibolites——occur as lenses in mica-schists. The fresh eclogites comprise garnet, omphacite, quartz, rutile, phengite, barroisite, etc. The compositional features of garnet and available geothermobarometers indicated that the eclogites belong to the low-T type. The mica-schists contain phengite, quartz, garnet and rutile that are also attributed to high-P assemblage. The eclogites and related high-P belt probably belong to the Early Paleozoic in age, which is distinct from the Indosinian Dabie high- and ultrahigh-P metamorphic belt.     

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.     

U-Pb zircon age of the foliated garnet-bearing granites in western Dabie Mountains, Central China

U-Pb zircon dating on two foliated garnet-bearing granite samples in the western Dabie ultra-high-pressure (UHP) metamorphic unit yields concordant ages of (234±4) Ma and (227±5) Ma, respectively. These ages, following the UHP peak metamorphism, represent the magma emplacement ages for the foliated garnet-bearing granites. This, for the first time, shows that there are the Triassic granites in the Dabie Mountains. The foliated garnet-bearing granites resemble A-type granite in geochemical characteristics, indicating that they were formed in extensional geodynamic setting. The magma formation reflects a reheating event in the Dabie orogenic belt and it enhances the transfer of tectonic regime from collision into extension and promotes the rapid exhumation into lower crust for the UHP metamorphic rocks.     

Isotope chronological trace of granite gravel in Hefei Basin

The discovery of granite gravel in Hefei Jurassic backland basin, which came from the Dabie Shan or North Huaiyang, has a great significance for probing into uplift and exhumation of the Dabie orogenic belt. Lithochemical and isotope chronological analyses on the granite gravel in the Sanjianpu Formation and the Fenghuangtai Formation in Jinzhai County prove that post-orogenic granite was developed in the major provenance of the Hefei Basin; that is, Dabie Shan or North Huaiyang. Discordant lower intersection point age of zircon U-Pb of granite gravel is about 214 Ma, and⁴⁰Ar/³⁹Ar age of muscovite about 196 Ma, K-Ar age of K-spar about 181 Ma. The former is the diagenetic age of the sample, and the latter two represent the sealed ages of muscovite and K-spar respectively. The age of sedimentary rocks in which a lot of granite gravels appear in the sedimentary section may represent the newest age of the exhumation of the granite body, so we assume that the age is 166 Ma. Therefore, we calculated the uplift rates of the granite body from Late Triassic to early Middle Jurassic; that is about 0.08 km/Ma and 0.4–0.3 km/Ma in the early slowly uplifting stage and later fast uplifting stage respectively, after the formation of the body at 214 Ma. These results are basically identical to the uplifting ages of the Dabie ultrahigh pressure metamorphic (UHPM) belt and the North Huaiyang thrust belt.