Ubiquitous post-peak zircon in an eclogite from the Kumdy-Kol,Kokchetav UHP-HP Massif (Kazakhstan): Significance of exhumation-related zircon growth and modification in continental-subduction settings

U–Pb geochronological, trace-element and Lu–Hf isotopic studies have been made on zircons from ultrahigh-pressure (UHP) mafic eclogite from the Kumdy-Kol area, one of the diamond-facies domains of the Kokchetav Massif (northern Kazakhstan). The peak eclogitic assemblage equilibrated at > 900 °C, whereas the bulk sample composition displays light rare-earth element (LREE) and Th depletion evident of partial melting. Zircons from the eclogite are represented by exclusively newly formed metamorphic grains and have U–Pb age spread over 533–459 Ma, thus ranging from the time of peak subduction burial to that of the late post-orogenic collapse. The major zircon group with concordant age estimates have a concordia age of 508.1 ±4.4 Ma, which corresponds to exhumation of the eclogite-bearing UHP crustal slice to granulite- or amphibolite-facies depths. This may indicate potentially incoherent exhumation of different crustal blocks within a single Kumdy-Kol UHP domain. Model Hf isotopic characteristics of zircons (ε_Hf(t) +1.5 to +7.8, Neoproterozoic model Hf ages of 1.02–0.79 Ga) closely resemble the whole-rock values of the Kumdy-Kol eclogites and likely reflect in situ derivation of HFSE source for newly formed grains. The ages coupled with geochemical systematics of zircons confirm that predominantly late zircon growth occurred in Th–LREE-depleted eclogitic assemblage, that experienced incipient melting and monazite dissolution in melt at granulite-facies depths, followed by amphibolite-facies rehydration during late-stage exhumation-related retrogression.     

Mineral parageneses and metamorphic P-T paths of ultrahigh-pressure eclogites from Kyrghyzstan Tien-Shan

Abstract Eclogites occur in three districts of the northern and southern parts of Tien-Shan. Three eclogites collected from the Aktyuz, Makbal and Atbashy districts were analyzed; the P-T paths of three eclogites were estimated by analyzing compositional growth zoning and retrograde reaction of garnet and omphacite. Aktyuz and Makbal eclogites have not preserved the prograde path. An Aktyuz eclogite that underwent a quartz eclogite facies metamorphism (about T = 600°C, P = 12 kbar) has recorded three stages of retrograde metamorphism. Four stages of retrograde metamorphism were recognized in a Makbal eclogite; the garnet-omphacite geothermometer gave about T = 560°C at 20 kbar as the highest metamorphic condition. Garnet from a garnetchloritoid-talc schist of the Makbal district includes quartz pseudomorphs after coesite; some units evidently underwent a low-temperature part of coesite eclogite fades metamorphism. Prograde and retrograde paths were recognized in an Atbashy eclogite; five stages of metamorphic reaction were observed in the Atbashy sample. The prograde path from stage I to stage III has been recorded in garnet and omphacite in which quartz pseudomorphs after coesite are included. The peak metamorphism of stage III took place at about 660°C at 25 kbar. The stages IV and V are retrograde. UHP eclogite facies metamorphism took place twice in Kyrghyzstan. The Aktyuz and Atbashy eclogites gave Rb-Sr mineral-isochron ages of about 750 Ma and 270 Ma, respectively. The K-Ar age of paragonite from the Makbal eclogite is about 480 Ma.     

Mineral inclusions in zircon domains and geological significance of SHRIMP U-Pb dating for coesite-bearing zircons of paragneiss in Sulu terrane,eastern China

Laser Raman spectroscopy and cathodoluminescence (CL) image reveal that zircons separated from paragneisses in the southwestern Sulu terrane (eastern China) preserve multi-stage mineral assemblages in different zircon domains. In the same paragneiss zircon sample, some zircon grains retain inherited (detrital) cores with abundant low-pressure mineral inclusions of Qtz + Phe + Ap + impurities and Qtz + Phe + impurities. The ultrahigh-pressure (UHP) metamorphic overgrowths mantles of these zircons preserve Coe, Coe + Phe and other UHP mineral inclusions, indicating that these inherited (detrital) zircons from protoliths experienced metamorphic recrystallization during the Sulu UHP metamorphic event. However, other zircon grains preserve UHP mineral inclusions of Coe, Coe + Ap and Coe + Phe in the cores and mantles, whereas the outmost rims contain quartz (Qtz) and other low-pressure mineral inclusions. These phenomena prove that the second group zircons were crystallized at UHP metamorphic stage and overpr     

SHRIMP U-Pb zircon age,geochemistry and Nd isotope of the Guandaoshan pluton in SW Sichuan: Petrogenesis and tectonic significance

SHRIMP U-Pb zircon age, geochemical and Nd isotopic data are reported for the Neo-proterozoic Guandaoshan pluton in the Yanbian region, SW Sichuan. This pluton is of typical I-type granite and emplaced at (857 ± 13) Ma. Geochemical and Nd isotopic characters suggest that the pluton was generated by partial melting of pre-existing, young (late Mesoproterozoic to early Neo-proterozoic) low-K tholeiitic protolith within an intraplate anorogenic setting. The Guandaoshan pluton probably records the earliest magmatism induced by the proposed ca. 860–750 Ma mantle superplume beneath the supercontinent Rodinia.

     

Metamorphic zircon from Xindian eclogite,Dabie Terrain: U-Pb age and oxygen isotope composition

~~Metamorphic zircon from Xindian eclogite,Dabie Terrain: U-Pb age and oxygen isotope composition@E. Deloule$CRPG-CNRS Nancy,54501,France1. Vavra, G, Gebauer. D., Schmid. R. et al., Multiple zircon growth and recrystallization during polyphase Late Carboniferous to Tri-assic metamorphism in granulites of the Ivrea Zone (Southern Alps): an ion microprobe (SHRIMP) study, Contrib. Mineral Petrol., 1996, 122:337-358 2. Vavra, G, Schmid, R., Gebauer, D., Internal morphology, ha…     

The chemical Th-U-total Pb isochron ages of Jiaodong and Jiaonan metamorphic rocks in the Shandong Peninsula, eastern China

Abstract The chemical Th-U-total Pb isochron method (CHIME) was applied to determine the age of monazite and thorite in five gneisses and zircon in an ultra high-pressure (UHP) phengite schist from the Su-Lu region, eastern China. The CHIME ages and isotopic ages reported in the literature show that gneisses in the Su-Lu region are divided into middle Proterozoic (1500–1720 Ma) and Mesozoic (100–250 Ma) groups. The Proterozoic group includes paragneiss and orthogneiss of the amphibolite-granulite facies, and their protolith age is late Archean-early Proterozoic. The Mesozoic group is mainly composed of orthogneiss of the greenschist-epidote amphibolite facies, and the protolith age is Middle Paleozoic-Early Proterozoic. The Proterozoic and Mesozoic gneisses occupy northern and southern areas of the Su-Lu region, respectively, which are divided by a major Wulian-Qingdao-Yantai fault. Ultra high-pressure metamorphic rocks occur as blocks in the Mesozoic gneisses, and form a UHP complex.
The UHP phengite schist in the Mesozoic orthogneiss contains detrital zircons with late Proterozoic CHIME age ( ca 860 Ma). Age of the UHP metamorphism is late Proterozoic or younger, and protolith age of the UHP metamorphic rocks is probably different from that of the surrounding Mesozoic gneisses.     

Eclogitic metapelites in the western segment of the north Qaidam Mountains: Evidence on “in situ” relationship between eclogite and its country rock

In Yuka-Luofengpo area of the north Qaidam Mountains, eclogitic metapelites are recognized. The metapelites enclosed lenses of eclogites, and locally intercalated with eclogites. Their typical mineral assemblages are garnet+kyanite+chloritoid+phengite+quartz+rutile. Strong growth zoning is preserved in garnets of metapelites, and phengite contains up to 3.4 Si per formula units. The petrographic observations and textural relations testify to the following sequence of mineral assemblages connected to three metamorphic stage: (1) Grt+ChlI+CldI +PheI ± StI+Qtz; (2) Grt+Ky+PheII±CldII+Qtz; and (3) Grt+CldIII+ChlII+PheIII+StII ± ky+ Qtz. Applying THERMOCALC Program, Grt-Phe thermometer and Grt-Ky-Phe-Qtz barometry, P-T conditions for three metamorphic stages were obtained: P=1.07±0.31GPa, T=564±22? (prograde stage); P=2.3–3.1GPa, T=615–700°C-(peak stage); and P =1.22 ± 0.26GPa, T=581±20°C (retrograde stage). A hairpin shape P-T path similar to that of adjacent eclogite is inferred. In combination with eclogitic mineral relics in marbles and orthogneisses enclosing eclogites, we thought that the relationship between eclogites and country rocks is “in situ” rather than “tectonic emplacement”.     

Ultra-high pressure metamorphic rocks in the Dabie-Su-Lu region, China: Their formation and exhumation

Abstract Based on petrological, structural, geological and geochronological research, the authors summarize the progress of ultra-high pressure (UHP) metamorphic rock study since 1989 by Chinese geoscientists and foreign geoscientists in the Dabie-Su-Lu region. The authors introduce and discuss a two-stage exhumation process for the UHP metamorphic rocks that have various lithologies; eclogite, ultramafics, jadeitic quartzite, gneiss, schist and marble. The metamorphic history of UHP metamorphic rocks is divided into three stages, that is, the pre-eclogite stage, coesite eclogite stage, and retrograde stage. Prior to UHP metamorphism, the ultramafics had a high temperature environment assemblage of mantle and others had blueschist facies assemblages. The granulite facies assemblages, which have recorded a temperature increase event with decompression, have developed locally in the Weihai basaltic rocks. Isotopic ages show a long range from > 700 Ma to 200 Ma. The diversity in protoliths of UHP metamorphic rocks may be related to the variation of isotopic ages older than 400 Ma. The Sm-Nd dating of ~ 220 Ma could reflect the initial exhumation stage after the peak UHP metamorphism in relation to the collision between the Sino-Korean and Yangtze blocks and subsequent events. Petrological and structural evidence imply a two-stage exhumation process. During the initial exhumation, the UHP metamorphic rocks were sheared and squeezed up in a high P/T regime. In the second exhumation stage the UHP metamorphic rocks were uplifted and eventually exposed with middle crustal rocks.     

NanoSIMS imaging method of zircon U-Pb dating

We report an imaging method of zircon U-Pb dating with NanoSIMS 50 L, which overcomes the significant U-Pb fractionation as the pit was sputtered deeper during conventional spot mode analysis and can be applied to irregular small grains or heterogeneous areas of zircon. The U-Pb and Pb-Pb ages can be acquired simultaneously for 2 μm×2 μm(for small grains) or 1 μm×9 μm(for zoned grains), together with Zr, Y and other trace elements distributions. Using zircon M257 as standard, the U-Pb ages of other zircon standards, including Qinghu, Plesovice, Temora and 91500, were measured to(2σ) as158.8±0.8, 335.9±3.4, 412.0±12 and 1067±12 Ma, respectively, consistent with the recommended values within the analytical uncertainties. Tiny zircon grains in the impact melt breccia of the lunar meteorite SaU 169 were also measured in this study,with a Pb-Pb age of 3912±14 Ma and a U-Pb age of 3917±17 Ma, similar to previous results reported for the same meteorite.The imaging method was also applied to determine U-Pb age of the thin overgrowth rims of Longtan metamorphic zircon, with a Pb-Pb age of 1933±27 Ma and a U-Pb age of 1935±25 Ma, clearly distinct from the Pb-Pb age of 2098±61 Ma and the U-Pb age of 2054±40 Ma for detrital cores.     

Metamorphic zoning and thermal structure of the Dabie ultrahigh- pressure–high-pressure terrane, central China

The ultrahigh- and high-pressure (UHP–HP) metamorphic belt of the Dabie Mountains, central China, formed by the Triassic continental subduction and collision, is divided into four metamorphic zones; from south to north, the greenschist facies zone, epidote amphibolite to amphibolite facies zone, quartz eclogite zone, and coesite eclogite zone, based on metabasite mineral assemblages. Most of the coesite-bearing eclogites consist mainly of garnet and omphacite with homogeneous compositions and have partially undergone hydration reactions to form clinopyroxene + plagioclase + calcic amphibole symplectites during amphibolite facies overprinting. However, the least altered eclogites sometimes contain garnet and omphacite that preserve compositional zoning patterns which may have originated during their growth at peak temperature conditions of ∼ 750 °C, suggesting a short duration of UHP metamorphic conditions and/or consequent rapid cooling during exhumation. Systematic investigation on peak metamorphic temperatures of coesite eclogite have revealed that, contrary to the general trend of metamorphic grade in the southern Dabie unit, the coesite eclogite zone shows rather flat thermal structure (T = 600 ± 50 °C) with the highest temperature reaching up to 850 °C and no northward increase in metamorphic temperature, which is opposed to the previous interpretations. This feature, along with the preservation of compositional zonation, implies complicated differential movement of each eclogite mass during UHP metamorphism and the return from the deeper subduction zone at mantle depths to the surface.     

Low temperature eclogite facies metamorphism in Western Tianshan, Xinjiang

According to the field occurrences and petrological study, the low temperature eclogite facies metamorphic rocks in Western Tianshan of Xinjiang can be divided into five types: (i) massive glaucophane-epidote eclogites and glaucophane-paragonite eclogites; (ii) schistose or gneissic mica eclogites; (iii) banded calcite eclogites; (iv) pillow glaucophane eclogites; (v) garnet-omphacite quartzites. Their eclogite facies metamorphism has undergone four stages of evolution: (i) pre-peak lawsonite-blueschist facies stage,T = 350–4000°C,P = 0.7–0.9 GPa; (ii) peak eclogite facies stage,T = 530 ± 20°C,P = 1.6–1.9 GPa; (iii) retrograde epidote-blueschist facies stage, T=500–530°C,P = 0.9–1.2 GPa and (iv) retrograde blueschist-greenschist facies stage,T= 450–550°C,P= 0.7–0.8 GPa. The metamorphic PT path of Western Tianshan eclogites is characterized by clockwise ITD resulting from the subduction of Tarim plate northward to Yili-Central Tianshan plate followed by fast uplift to the surface. But there were at least two stages of blueschist facies retrograde metamorphism overprinted during their uplift.     

Thermobaric structure of the Kokchetav ultrahigh-pressure–high-pressure massif deduced from a north–south transect in the Kulet and Saldat–Kol regions, northern Kazakhstan

Abstract To investigate the regional thermobaric structure of the diamondiferous Kokchetav ultrahigh‐pressure and high‐pressure (UHP–HP) massif and adjacent units, eclogite and other metabasites in the Kulet and Saldat–Kol regions, northern Kazakhstan, were examined. The UHP–HP massif is subdivided into four units, bounded by subhorizontal faults. Unit I is situated at the lowest level of the massif and consists of garnet–amphibolite and acidic gneiss with minor pelitic schist and orthogneiss. Unit II, which structurally overlies Unit I, is composed mainly of pelitic schist and gneiss, and whiteschist locally with abundant eclogite blocks. The primary minerals observed in Kulet and Saldat–Kol eclogites are omphacite, sodic augite, garnet, quartz, rutile and minor barroisite, hornblende, zoisite, clinozoisite and phengite. Rare kyanite occurs as inclusions in garnet. Coesite inclusions occur in garnet porphyroblasts in whiteschist from Kulet, which are closely associated with eclogite masses. Unit III consists of alternating orthogneiss and amphibolite with local eclogite masses. The structurally highest unit, Unit IV, is composed of quartzitic schist with minor pelitic, calcareous, and basic schist intercalations. Mineral assemblages and compositions, and occurrences of polymorphs of SiO₂ (quartz or coesite) in metabasites and associated rocks in the Kulet and Saldat–Kol regions indicate that the metamorphic grades correspond to epidote–amphibolite, through high‐pressure amphibolite and quartz–eclogite, to coesite–eclogite facies conditions. Based on estimations by several geothermobarometers, eclogite from Unit II yielded the highest peak pressure and temperature conditions in the UHP–HP massif, with metamorphic pressure and temperature decreasing towards the upper and lower structural units. The observed thermobaric structure is subhorizontal. The UHP–HP massif is overlain by a weakly metamorphosed unit to the north and is underlain by the low‐pressure Daulet Suite to the south; boundaries are subhorizontal faults. There is a distinct pressure gap across these boundaries. These suggest that the highest grade unit, Unit II, has been selectively extruded from the greatest depths within the UHP–HP unit during the exhumation process, and that all of the UHP–HP unit has been tectonically intruded and juxtaposed into the adjacent lower grade units at shallower depths of about 10 km.     

Ultrahigh-pressure metamorphic records hidden in zircons from amphibolites in Sulu Terrane, eastern China

Abstract The amphibolites occur sporadically as thin layers and blocks throughout the Sulu Terrane, eastern China. All analyzed amphibolite from outcrop and drill cores from prepilot drill hole CCSD‐PP1 and CCSD‐PP2, Chinese Continental Scientific Drilling Project in the Sulu Terrane, are retrograded eclogites overprinted by amphibolite‐facies retrograde metamorphism, with characteristic mineral assemblages of amphibole + plagioclase + epidote ± quartz ± biotite ± ilmenite ± titanite. However, coesite and coesite‐bearing ultrahigh‐pressure (UHP) mineral assemblages are identified by Raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from these amphibolites. In general, coesite and other UHP mineral inclusions are preserved in the cores and mantles of zircons, whereas quartz inclusions occur in the rims of the same zircons. The UHP mineral assemblages consist mainly of coesite + garnet + omphacite + rutile, coesite + garnet + omphacite, coesite + garnet + omphacite + phengite + rutile + apatite, coesite + omphacite + rutile and coesite + magnesite. Compositions of analyzed mineral inclusions are very similar to those of matrix minerals from Sulu eclogites. These UHP mineral inclusion assemblages yield temperatures of 631–780°C and pressures of ≥2.8 × 10³ MPa, representing the P–T conditions of peak metamorphism of these rocks, which are consistent with those (T = 642–726°C; P ≥ 2.8 × 10³ MPa) deduced from adjacent eclogites. These data indicate that the amphibolites are the retrogressive products of UHP eclogites.     

Multi-isotopic system geochronology of low temperature eclogite from Huangzhen,Southern Dabie Terrain

Zircon U-Pb thermal ionization mass-spectrometer (TIMS) and secondary ion mass-spectrometer (SIMS) dating, mica and amphibole 40Ar-39Ar dating and mineral Sm-Nd isotopic compositions of Huangzhen Iow temperature eclogite and country granitic gneiss are carried out. The zircon U-Pb weighted average SIMS age is (231.6±9.7) Ma for one eclogite.The mica 40Ar-39Ar isochron age is (232.6±2.1) Ma and the lowest plateau age is (221.7±2.4)Ma from same sample. U-Pb TIMS concordant ages from other eclogite zircons are from (221.3± 1.4) Ma to (222.5±2.3) Ma. U-Pb SIMS low intercept age from country granitic gneiss is (221±35) Ma. The retrograde amphibole 40Ar-39Ar isochron age is (205.9± 1.0) Ma. Except for mica,which may contain excess 40Ar, all the ages represent peak and retrograde metamorphism of low temperature eclogites. It is indicated that the Huangzhen low temperature eclogites differ from Xiongdian low temperature eclogites of north of the Northern Dabie Terrain in metamorphic ages.Huangzhen low temperature eclogites share one coherent HP-UHP terrain with high temperature eclogites from Southern Dabie Terrain and they may have differences in subduction depth and cooling rates during exhumation.     

Age and nature of eclogites in the Huwan shear zone,and the multi-stage evolution of the Qinling-Dabie-Sulu orogen,central China

In situ LA-ICPMS U-Pb, trace element, and Hf isotope data in zircon demonstrate a Carboniferous age for eclogite-facies metamorphism in Siluro-Devonian protoliths in the Huwan shear zone, Dabie Mountains, Central China. This age contrasts with the more prevailing Triassic age for high- to ultrahigh pressure (HP to UHP) metamorphism in the Qinling-Dabie-Sulu orogen. Metamorphic zircon in two eclogite samples from Sujiahe is characterized by low Th/U ratios, small negative Eu anomalies, flat HREE patterns, and low ¹⁷⁶Lu/¹⁷⁷Hf ratios. These geochemical signatures suggest that the zircon crystallized in the presence of garnet and in the absence of plagioclase feldspar. Furthermore, temperatures of ~ 655 and ~ 638 °C, calculated using the Ti content of zircon, are consistent with their formation during eclogite-facies metamorphism. The weighted mean ²⁰⁶Pb/²³⁸U age of 309 ± 4 Ma (2δ) for this zircon improves previous age estimates for eclogite-facies metamorphism in the Huwan shear zone, ranging from 420 to 220 Ma. Metamorphic zircon from one eclogite sample from Hujiawan, most likely formed during prograde metamorphism, yields an equivalent age estimate of 312 ± 11 Ma. Magmatic zircon cores in the three samples yield ages for the magmatic protoliths of the eclogites ranging from 420 ± 7 to 406 ± 5 Ma, and post-dating the middle Paleozoic collision of the North China and the Qinling terrain. The zircon crystals in the three eclogite samples display a large variation of ε_Hf (t) values of ? 4.9 to 21.3. The metamorphic zircon overgrowths show the same range of ε_Hf (t) values as those of the inherited magmatic crystal interiors. This suggests that the metamorphic zircon overgrowths may have formed by dissolution-reprecipitation of pre-existing magmatic zircon thereby preserving their original Hf isotopic composition. The high ε_Hf (t) values suggest that the protoliths were derived from depleted mantle sources, most likely Paleotethyan oceanic crust; while the low ε_Hf (t) values are attributed to crustal contamination. Some eclogites in the Huwan shear zone have a distinctive signature of continental crust most probably derived from the Yangtze Craton. The coexistence of Paleozoic oceanic crust and Neoproterozoic continental crust with similar metamorphic ages in the Huwan shear zone implies that Paleozoic Paleotethyan oceanic crust was produced within a marginal basin of the northern Yangtze Craton. The opening of the Paleo-Tethyan ocean was slightly younger than the collision of the North China Craton and the Qinling terrain during the Late Paleozoic in the Qinling-Dabie-Sulu orogen. Subduction of the Paleo-Tethyan oceanic crust and associated continental basement resulted in the 309 ± 2 Ma (2σ) eclogite-facies metamorphism in the Huwan shear zone. The subsequent Triassic continent-continent collision led to the final coalescence of the Yangtze and Sino-Korean cratons. Amalgamation of the Yangtze and North China cratons was, therefore, a multistage process extending over at least 200 Ma.     

Coexisting different types of zoned garnet in kyanite‐quartz eclogites from the Sanbagawa metamorphic belt: Evidence of deformation‐induced lithological mixing during prograde metamorphism

Garnet grains in Sanbagawa quartz eclogites from the Besshi region, central Shikoku commonly show a zoning pattern consisting of core and mantle/rim that formed during two prograde stages of eclogite and subsequent epidote–amphibolite facies metamorphism, respectively. Garnet grains in the quartz eclogites are grouped into four types (I, II, III, and IV) according to the compositional trends of their cores. Type I garnet is most common and sometimes coexists with other types of garnet in a thin section. Type I core formed with epidote and kyanite during the prograde eclogite facies stage. The inner cores of types II and III crystallized within different whole‐rock compositions of epidote‐free and kyanite‐bearing eclogite and epidote‐ and kyanite‐free eclogite at the earlier prograde stage, respectively. The inner core of type IV probably formed during the pre‐eclogite facies stage. The inner cores of types II, III, and IV, which formed under different P–T conditions of prograde metamorphism and/or whole‐rock compositions, were juxtaposed with the core of type I, probably due to tectonic mixing of rocks at various points during the prograde eclogite facies stage. After these processes, they have shared the following same growth history: (i) successive crystal growth during the later stage of prograde eclogite facies metamorphism that formed the margin of the type I core and the outer cores of types II, III, and IV; (ii) partial resorption of the core during exhumation and hydration stage; and (iii) subsequent formation of mantle zones during prograde metamorphism of the epidote–amphibolite facies. The prograde metamorphic reactions may not have progressed under an isochemical condition in some Sanbagawa metamorphic rocks, at least at the hand specimen scale. This interpretation suggests that, in some cases, material interaction promoted by mechanical mixing and fluid‐assisted diffusive mass transfer probably influences mineral reactions and paragenesis of high‐pressure metamorphic rocks.     

Poisson's ratio of eclogite: Implications for lower crustal delamination of orogens

Eclogite is essentially a bi-mineralogic high-grade metamorphic rock consisting of garnet and omphacite and is the product of high-to-ultrahigh pressure metamorphism of basaltic rocks due to the subduction of oceanic crust or the thickening/subduction of …     

Dating of prograde metamorphic events deciphered from episodic zircon growth in rocks of the Dabie–Sulu UHP complex, China

The timing of ultra-high pressure (UHP) metamorphism has been difficult to determine because of a lack of age constraints on crucial events, especially those occurring on the prograde path. New Sensitive High-Resolution Ion Microprobe (SHRIMP) U–Pb age and rare-earth element (REE) data of zircon are presented for UHP metamorphic rocks (eclogite, garnet peridotite, garnet pyroxenite, jadeite quartzite and garnet gneiss) along the Dabie–Sulu UHP complex of China. With multiphase metamorphic textures and index mineral inclusions within zircon, the Dabie data define three episodes of eclogite-facies metamorphism, best estimated at 242.1 ± 0.4 Ma, 227.2 ± 0.8 Ma and 219.8 ± 0.8 Ma. Eclogite-facies zircons of the Sulu UHP complex grew during two major episodes at 242.7 ± 1.2 and 227.5 ± 1.3 Ma, which are indistinguishable from corresponding events in the Dabie UHP complex. A pre-eclogite metamorphic phase at 244.0 ± 2.6 Ma was obtained from two Sulu zircon samples which contain low pressure–temperature (plagioclase, stable below the quartz/Ab transformation) and hydrous (e.g., amphibole, stable below  2.5 Gpa) mineral inclusions. In terms of Fe–Mg exchange of trapped garnet–clinopyroxene pairs within zircon domains, we are able to determine the Pressure–Temperature (PT) conditions for a specific episode of metamorphic zircon growth. We suggest that mineral phase transformations and associated dehydration led to episodic eclogite-facies zircon growth during UHP metamorphism ( 2.7 Gpa) began at 242.2 ± 0.4 Ma (n = 74, pooling the Dabie–Sulu data), followed by peak UHP metamorphism (>  4 Gpa) at 227.3 ± 0.7 Ma (n = 72), before exhumation (<  220 Ma) to quartz stability (~ 1.8 Gpa). The Dabie–Sulu UHP metamorphism lasted for about 15 Ma, equivalent to a minimum subduction rate of 6 mm/year for the descending continental crust.     

Regional-Scale Excess Ar wave in a Barrovian type metamorphic belt, eastern Tibetan Plateau

Laser step heating ⁴⁰Ar/³⁹Ar analysis of biotite and muscovite single crystals from a Barrovian type metamorphic belt in the eastern Tibetan plateau yielded consistent cooling ages of ca. 40 Ma in the sillimanite zone with peak metamorphic temperatures higher than 600°C and discordant ages from 46 to 197 Ma in the zones with lower peak temperatures. Chemical Th‐U‐Total Pb Isochron Method (CHIME) monazite (65 Ma) and sensitive high mass‐resolution ion microprobe (SHRIMP) apatite (67 Ma) dating give the age of peak metamorphism in the sillimanite zone. Moderate amounts of excess Ar shown by biotite grains with ages of 46 to 94 Ma at metamorphic grades up to the high‐grade part of the kyanite zone probably represent incomplete degassing during metamorphism. In contrast, the high‐grade part of the kyanite zone yields biotite ages of 130 to 197 Ma. The spatial distribution of these older ages in the kyanite zone along the sillimanite zone boundary suggests they reflect trapped excess argon that migrated from higher‐grade regions. The most likely source is muscovite that decomposed to form sillimanite. The zone with extreme amounts of excess argon preserves trapped remnants of an ‘excess argon wave’. We suggest this corresponds to the area where biotite cooled below its closure temperature in the presence of an elevated Ar wave. Extreme excess Ar is not recognized in muscovite suggesting that the entrapment of the argon wave by biotite took place when the rocks had cooled down to temperatures lower than the closure temperature of muscovite. The breakdown of phengite during ultrahigh‐pressure (UHP) metamorphism may be a key factor in accounting for the very old apparent ages seen in many UHP metamorphic regions. This is the first documentation of a regional Ar‐wave spatially associated with regional metamorphism. This study also implies that resetting of the Ar isotopic systems in micas can require temperatures up to 600°C; much higher than generally thought.     

Eclogites from the Chinese continental scientific drilling borehole, their petrology and different P-T evolutions

Abstract Four phengite‐bearing eclogites, taken from different depths of the Chinese continental scientific drilling (CCSD) borehole in the Sulu ultrahigh pressure terrane, eastern China, were studied with the electron microprobe. The compositional zonations of garnet and omphacite are moderate, whereas phengite compositions generally vary significantly in a single sample from core to rim by decrease of the Si content. Various geothermobarometric methods were applied to constrain the P‐T conditions of these eclogites on the basis of the compositional variability of the above minerals. The constrained P‐T path for sample B218 is characterized by pressure decrease from ca 3.0 GPa (ca 600°C) to 1.3 GPa (ca 550°C). Eclogite B310 yielded P‐T conditions of 3.0 GPa and 750°C. The path for eclogite B1008 starts at about 650°C and 3.6–3.9 GPa (stage I) followed by a pressure decrease to 2.8–3.0 GPa and a significant temperature rise (stages II and IIIa, 750–810°C). Afterwards, this rock cooled down to 620–660°C at still high pressures (2.5–2.7 GPa, stage IIIb). Retrograde conditions were about 670°C and 1.3 GPa (stage IV). Eclogite B1039 yielded a P‐T path starting at ca 600°C and 3.3–3.9 GPa (stage I). A pressure decrease to about 3.0 GPa (stage II, 590–610°C) and then a moderate isobaric temperature increase to ca 630°C (stage III) followed. Stage IV is characterized by temperatures of 650°C at pressures close to 1.3 GPa. During and after this stage (hydrous) fluids partially rich in potassium penetrated the rocks causing minor changes. Relatively high oxygen fugacities led to andradite and magnetite among the newly formed minerals. We think that the above findings can be best explained by mass flow in a subduction channel. Thus, we conclude that the assembly of UHP rocks of the CCSD site, eclogites, quartzofeldspathic rocks, and peridotites, cannot represent a crustal section that was already coherent at UHP conditions as it is the common belief currently. The coherency was attained after significant exhumation of these UHP rocks.