Sulu Ultrahigh-pressure Research Articles

SHRIMP U–Pb ages of ultrahigh‐pressure and retrograde metamorphism of gneisses, south‐western Sulu terrane, eastern China

AbstractLaser Raman spectroscopy and cathodoluminescence (CL) images reveal that most zircon separated from paragneiss and orthogneiss in drillhole CCSD‐PP2 at Donghai, south‐western Sulu terrane, retain low‐P mineral‐bearing inherited cores, ultrahigh‐pressure (UHP) mineral‐bearing mantles and low‐P mineral‐bearing (e.g. quartz) rims. SHRIMP U–Pb analyses of these zoned zircon identify three discrete and meaningful age groups: Proterozoic protolith ages (> 680 Ma) are recorded in the inherited cores, the UHP metamorphic event in the coesite‐bearing mantles occurred at 231 ± 4 Ma, and the late amphibolite facies retrogressive overprint in the quartz‐bearing rims was at 211 ± 4 Ma. Thus, Neoproterozoic supracrustal protoliths of the Sulu UHP rocks were subducted to mantle depths in the Middle Triassic, and exhumed to mid‐crustal levels in the Late Triassic. The exhumation rate deduced from the SHRIMP data and metamorphic P–T conditions is 5.0 km Ma−1. Exhumation of the Sulu UHP terrane may have resulted from buoyancy forces after slab break‐off at mantle depths.

K-rich lamellar exsolution in clinopyroxene: Constraint on the depth of peridotite source at Zhimafang

The oriented lamellar K-rich exsolutions within clinopyroxene host were recognized in garnet peridotite massif at Zhimafang in the Sulu ultrahigh pressure metamorphic terrane, eastern China. EMP and TEM analyses revealed that the composition and the index of electron diffraction for this exsolution are corresponding with phlogopite, and the shape is the lamellae with 1 μm in width and 5 to 30 μm in length. Moreover, the topotaxy is that the (001) of K-rich exsolution parallels to (100) of the clinopyroxene host. The electron diffraction analyzing shows the exsolution is of a 1M polymorphism. The high-resolution lattice fringe image displays the stack features along [001], which only a layer mineral shows. We conclude that the clinopyroxene with 1M and K-rich exsolution was probably derived from mantle depths over 240 km, thus resulting from decompressional exsolution in the initial stage of slab exhumation.

Pb and Nd isotopic composition of the Jigongshan granite: constraints on crustal structure of Tongbaishan in the middle part of the Qinling–Tongbai–Dabie orogenic belt, Central China

The Qinling–Dabie–Sulu belt is the world's largest ultrahigh pressure (UHP) metamorphic belt. The UHP metamorphism is well dated at 220–245 Ma in the Dabie–Sulu belt but at ∼507 Ma in the Qinling belt. The Tongbaishan is located between the Qinling orogenic belt to the west and the Dabie–Sulu UHP metamorphic belt to the east. It is the key area for studying the tectonic relation between the Qinling and Dabie–Sulu belts and the diachronous UHP metamorphism. The Jigongshan granitic pluton ( t=128 Ma) with a total area of 1200 km 2, composed of monzogranite, was mostly emplaced into the Tongbai complex, an exposed basement in the Tongbaishan. The Jigongshan granites have SiO 2=69.85–72.35%, K 2O/Na 2O=0.87–1.13, A/CNK=0.91–1.03, Rb/Sr=0.14–0.25 and Th/U=3.3–12. Their REE compositions show strongly fractionated patterns with (La/Yb) N=14–58 and Eu*/Eu=0.79–1.05. The granites are characterized by low radiogenic Pb isotopic composition. The present-day whole-rock Pb isotopic ratios are 20 6Pb/ 204Pb=16.707–17.055, 207Pb/ 204Pb=15.239–15.326 and 208Pb/ 204Pb=37.587–37.853, which are similar to that of the continental lower crust. Their ε Nd( t) values range from −16 to −20, and depleted-mantle Nd model ages ( T DM) from 1.8 to 2.2 Ga. The above evidence indicates that the magma of the Jigongshan granites was derived from the partial melting of the continental crust. The Pb and Nd isotopic compositions of the Jigongshan granites resemble those of the Dabie core complex in the Dabieshan but are distinct from those of the Tongbai complex in the Tongbaishan. Thus, the Dabie core complex would be the magma source of the Jigongshan granites. The result implies that the Dabie core complex is extended to the west and constitutes the unexposed basement underlaying the Tongbai complex in the Tongbaishan.

High‐ to ultrahigh‐pressure (UHP) ductile shear zones in the Sulu UHP metamorphic belt, China: implications for continental subduction and exhumation

AbstractHigh‐ to ultrahigh‐pressure (HP‐UHP) metamorphic rocks that resulted from deep continental subduction and subsequent exhumation in the Sulu orogenic belt, China, have experienced multiphase deformation and metamorphic overprint during its long journey to the mantle and return to the surface. HP‐UHP shear zones are strain‐localized weak zones on which the UHP slab is transported over long distances. HP‐UHP shear zones are well exposed along a 200‐km belt in the Sulu UHP metamorphic belt. The shear zones lie structurally below the UHP rocks and above the non‐UHP rocks, suggesting the early exhumation of the UHP rocks by thrusting. The large area distribution, HP‐UHP nature, high strain and structural association of the shear zones with the UHP rocks suggest that the shear zones are probably a regional detachment developed during the early stage of exhumation of the UHP rocks. Kinematic indicators suggest top‐to‐the N–NW motion of the UHP slab during the exhumation, which, combined with isotope signature in Mesozoic igneous rocks, leads us to the interpretation that the subduction polarity is the North China plate down to the south rather than the Yangtze plate down to the north in the Sulu region.

Chromian dissakisite-(Ce) in a garnet lherzolite from the Chinese Su-Lu UHP metamorphic terrane: Implications for Cr incorporation in epidote-group minerals and recycling of REE into the Earth’s mantle

Chromian dissakisite-(Ce), a Cr-enriched Mg analogue of allanite-(Ce), is found in a garnet lherzolite from the Su-Lu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. Major minerals of the lherzolite are olivine [ X Mg = Mg/(Mg + Fe) = 0.92], pyrope-rich garnet (Prp70.1–73.2Alm11.3–13.4Grs6.9–8.3Uvr3.6–5.1Anr0–0.4Sps0.7–1.7), orthopyroxene ( X Mg = 0.92–0.93), and clinopyroxene (XMg = 0.92–0.95). The chromian dissakisite-(Ce) occurs as a single-crystal inclusion in a clinopyroxene neoblast that also contains several irregularly shaped olivine grains. It is partly metamict, and there is intensive radial cracking in the surrounding clinopyroxene. Its composition is represented by the averaged formula: (Ca0.86Na0.03)0.89(REE0.86Th0.04)0.90(Mg0.98Fe2+0.20Fe3+0.21Cr0.36Al1.39Ti0.04)Si3.03O12(OH), with Ce2O3, MgO and Cr2O3 contents up to 13.1, 7.3, and 5.4 wt%, respectively. Because the M3 site in the chromian dissakisite-(Ce) is fully occupied by Mg and Fe2+, Fe3+ and Cr can only occupy the smaller M1 site. Similar site occupancy is noted for a chromian allanite from the Sanbagawa metamorphic belt, Japan, which contains up to 0.547 Cr atoms per formula unit (apfu). Such compositional characteristics of the chromian dissakisite/allanite and existing polarized absorption data on Cr-bearing epidote suggest the existence of possible end-members, “CeCa(Mg,Fe)CrAlSi3O12(OH),” for the epidote-group minerals, which may form solid solution with Ca2Al3Si3O12(OH) through coupled substitutions of REEMgCa−1Al−1, REEFeCa−1Al−1, and CrAl−1. The rare earth elements (REE), Ca, and Na total 1.79 apfu, implying that Mn and Fe2+ are also present on the nine-coordinated A1 site. The high REE abundance, the presence of minor Na, and the Mg and Cr-enriched characteristics of the mineral suggest that it was formed as a result of reaction between the host lherzolite and crust-derived melt/fluid at UHP conditions. Textural relations suggest that the chromian dissakisite-(Ce) was unstable in the garnet lherzolite and reacted with olivine to form clinopyroxene. Dissakisite-(REE) in subducted ultramafic rocks can transport rare earth elements into the Earth’s mantle.

U-Pb zircon ages for a collision-related K-rich complex at Shidao in the Sulu ultrahigh pressure terrane, China.

The Shidao complex at the eastern extremity of the Sulu ultrahigh pressure (UHP) terrane is composed of oldest pyroxene syenite, quartz syenite and youngest granite intrusives of the K-rich shoshonitic series. U-Pb zircon dating yields nearly concordant ages of 225 ± 2, 211 ± 3 and 205 ± 5 Ma for pyroxene syenite, quartz syenite and granite, respectively. The ages closely postdate the 240 to 220 Ma UHP metamorphism and correspond to the rapid cooling and exhumation of UHP rocks. A close genetic relation may exist between the formation of the Shidao intrusives and the continental collision and UHP metamorphism. However, the K-rich Shidao intrusive rocks are different from common syn-collisional granites in association of K-rich and granitic magmatism. A breakoff model is postulated to explain the formation of the complex. The breakoff of the subducting slab caused the rapid exhumation of the UHP terrane. Mantle upwelling resulted in basaltic magmatism and formation of the K-rich complex. Formation of the Shidao complex marked the cessation of the UHP metamorphism and the oldest ages of 225 ± 2 Ma of the complex is the minimum timing for the UHP metamorphism.

Ultrahigh-pressure (UHP) low-Al titanites from carbonate-bearing rocks in Dabieshan-Sulu UHP terrane, eastern China

Titanites previously reported from high-pressure rocks commonly contain high Al and F contents. This finding has led previous workers to conclude that high Al and F in titanite can expand its stability field to high-pressure and ultrahigh-pressure (UHP) conditions. In this study, a coesite inclusion was identified in titanite of aragonite- and jadeite-bearing gneiss from the Dabieshan UHP metamorphic terrane of eastern China. We also found UHP titanite in carbonate-bearing garnet clinopyroxenite from the Sulu UHP terrane. Interestingly, both types of the UHP titanite mentioned above contain low Al and F contents. Therefore, our data indicate that the high-Al and F contents are not necessary criteria for stabilizing titanite to UHP conditions. It is inferred that the UHP titanite formed via the reaction TiO 2 + CaCO 3 + SiO 2 = CaTiSiO 5 + CO 2 , and that the presence of the UHP assemblage aragonite + rutile + clinopyroxene + coesite in the host rock and a very low X CO₂ value in the coexisting fluid are crucial conditions for the formation of these UHP titanites

The Lanshantou Kyanite‐bearing Eclogite with Coesite Inclusions in the Sulu Ultrahigh‐Pressure Metamorphic Belt and Its PTt Path

Abstract Coesite inclusions are found in kyanite from the Lanshantou eclogite in the Sulu ultrahigh‐pressure (UHP) metamorphic belt. This discovery extends the stable region of kyanite to over 2.4 GPa. As an important UHP metamorphic belt in China, the Sulu eclogite belt is the product of A‐subduction induced by strong compression of the Yellow Sea terrane to the Jiaodong‐northereastern Jiangsu terrane during the interaction of the Eurasian plate and Palaeo‐Pacific plate in the Indosinian. It stretches about 350 km and contains over 1000 eclogite bodies. Most eclogites in this belt belong to Groups B and C in the classification of Coleman et al., and commonly contain kyanite, while the Lanshantou eclogite belongs to Group A and contains coesite. The MgO, CaO and FeO contents in garnet and pyroxene show regular variation from the core to the rim, which reveals the PTt paths of progressive metamorphism during the Early Mesozoic (240‐200 Ma) and retrogressive metamorphism during the Late Mesozoic and Cenozoic exhumation.

Diamond, former coesite and supersilicic garnet in metasedimentary rocks from the Greek Rhodope: a new ultrahigh-pressure metamorphic province established

We report here the first discovery of ultrahigh-pressure (UHP) indicator minerals and textures from the Rhodope Metamorphic Province (RMP), northern Greece. In particular, we document the exsolution of quartz, rutile and apatite in sodic garnet from metapelites (garnet–biotite–kyanite gneisses), which attests to the presence of a Si–Ti–Na–P-rich precursor garnet phase. Similar textures in garnet have been reported in the literature only once before for eclogites from the Su Lu UHP metamorphic province, China. We also document the presence of microdiamonds and multicrystalline quartz pseudomorphs after coesite, included in garnet from both eclogites and metapelites. We then argue that these rocks had once been transported to depths exceeding 220 km, well into the upper mantle, thus establishing the RMP as another important UHP metamorphic belt in the world.

The possible subduction of continental material to depths greater than 200 km

Determining the depth to which continental lithosphere can be subducted into the mantle at convergent plate boundaries is of importance for understanding the long-term growth of supercontinents as well as the dynamic processes that shape such margins. Recent discoveries of coesite and diamond in regional ultrahigh-pressure (UHP) metamorphic rocks has demonstrated that continental material can be subducted to depths of at least 120 km (ref. 1), and subduction to depths of 150-300 km has been inferred from garnet peridotites in orogenic UHP belts based on several indirect observations. But continental subduction to such depths is difficult to trace directly in natural UHP metamorphic crustal rocks by conventional mineralogical and petrological methods because of extensive late-stage recrystallization and the lack of a suitable pressure indicator. It has been predicted from experimental work, however, that solid-state dissolution of pyroxene should occur in garnet at depths greater than 150 km (refs 6-8). Here we report the observation of high concentrations of clinopyroxene, rutile and apatite exsolutions in garnet within eclogites from Yangkou in the Sulu UHP metamorphic belt, China. We interpret these data as resulting from the high-pressure formation of pyroxene solid solutions in subducted continental material. Appropriate conditions for the Na2O concentrations and octahedral silicon observed in these samples are met at depths greater than 200 km.

Carbon concentrations and isotopic ratios of eclogites from the Dabie and Sulu terranes in China

Both concentration and isotope composition of bulk carbon in apatite and host eclogites from the Dabie–Sulu ultrahigh pressure (UHP) terranes in China have been determined along with their oxygen isotope composition. The results show significant 13C-depletion in the apatite (δ 13C=−27.7‰ to −20.8‰) with the carbon concentrations of 0.59 to 1.65 wt.% CO 2 despite a large variation in δ 18O (−6.5‰ to +9.5‰). The bulk carbon in 21 of the 24 eclogites has low δ 13C values of −26.1‰ to −17.9‰ with low carbon content of 500 to 1000 ppm, whereas the other three samples show high δ 13C values of −7.1‰ to −2.8‰ with high carbon contents of 2400 to 4300 ppm. Noncarbonate carbon was measured by treating the all eclogites with 5 N HCl solution, yielding uniformly low δ 13C values of −27.9‰ to −24.2‰ and carbon contents of 200 to 300 ppm. Carbonate carbon is thus calculated by mass balance to have also low δ 13C values of −25.6‰ to −15.1‰ for the 21 samples but high δ 13C values of −4.3‰ to −1.2‰ for the remaining three samples. Secondary carbonate was identified in the three eclogites that are also depleted in 13C primarily, but subjected to overprint of 13C-rich CO 2-bearing fluid subsequent to the UHP metamorphism. The isotopically light carbon in both eclogite and apatite is interpreted to represent the isotope composition of carbon in eclogite precursors before plate subduction, and thus has an origin of organic carbon from the Earth's surface. The uniformly low δ 13C values of apatite suggest that the CO 2 of metamorphic fluid in equilibrium with the host-eclogites would be derived from the oxidation of organic carbon rather than the decarbonation of underthrust carbonates during progressive metamorphism. Protoliths of the eclogites are inferred to be of igneous origin, which underwent more extensive interaction with organic matter than with meteoric-hydrothermal fluid on the subsurface of the continental crust. The break off of the subducted plate containing the isotopically light carbon and subsequent interactions with the surrounding mantle could produce the mafic and/or silicic magmas that are significantly depleted in 13C relative to the primary mantle carbon. This may provide evidence for a linkage of the 13C-depleted mantle carbon to a surficial source via plate subduction.

Large areal extent of ultrahigh-pressure metamorphism in the Sulu ultrahigh-pressure terrane of East China: new implications from coesite and omphacite inclusions in zircon of granitic gneiss

Coesite and omphacite inclusions have been identified for the first time as minute inclusions in zircon from amphibolite-facies granitic orthogneiss in the Sulu ultrahigh-pressure (UHP) metamorphic terrane of eastern China by Raman spectroscopy and microprobe analyses. The occurrences of these minerals in the voluminous granitic gneiss of Sulu support a regional and pervasive UHP metamorphic event that predated regional amphibolitic retrogression. Taking into account the widespread discoveries of coesite in other lithologies, we thus conclude that a substantial crustal component in the Sulu UHP metamorphic terrane appears to have shared a common history of Triassic subduction to mantle depths and later exhumation.

Prograde P–T path of kyanite eclogites from Junan in the Sulu ultrahigh‐pressure province, eastern China

AbstractKyanite‐bearing ultrahigh‐pressure (UHP) eclogites occur as blocks in orthogneisses at Yangzhuang, in the Junan area of the southwestern Sulu province, eastern China. Eclogites have variable bulk rock compositions, with Al2O3 = 16–27 wt%, FeO* + MgO = 6–22 wt% and CaO = 9–13 wt%. Major minerals are garnet, omphacite, phengitic white mica, zoisite, kyanite, rutile and an SiO2 phase. Fe‐rich staurolite (Mg ⇒ Mg# = 0.24 ± 0.01) and paragonite–margarite aggregates are rarely included in the cores of prograde zoned garnet. Metamorphic conditions ranged from 520 to 650°C and <1.4 GPa at an early prograde stage, and mostly reached 660–830°C and 2.7–3.5 GPa at the peak UHP stage. The estimated dP/dT of the prograde P–T path is less than 0.25 GPa/100°C at earlier stages and increases to 0.7–1.4 GPa/100°C just before the UHP stage. The kink of the prograde P–T path closely resembles the steady‐state P–T paths proposed, assuming a two‐parameter brittle‐plastic shear stress model. The estimated P–T path adequately explains the absence of prograde lawsonite and sodic amphibole and the common occurrence of coexisting zoisite, kyanite and sodic‐calcic amphibole in the UHP eclogites throughout the Sulu province. Simple clockwise prograde P–T paths for Sulu UHP eclogites proposed in earlier studies should be carefully re‐examined.

The Sulu UHP Terrane: A Review of the Petrology and Structural Geology

Petrological and isotopic evidence suggests that the protolith of the Sulu ultrahigh-pressure (UHP) terrane was Precambrian continental crust consisting of granite, granodiorite, gabbro, marble, and basic dikes, with local granulite-facies assemblages. Around 220 Ma this unit of continental rocks was buried to depths up of ∼120 km within the mantle. Structures formed during exhumation suggest highly mobile behavior of acidic rocks, even under conditions of very low water activity. Petrological studies show that the Sulu terrane underwent isothermal decompression, which implies relatively rapid exhumation, and suggests that the role of melting during exhumation may have been underestimated. The later stages of exhumation are associated with NW-SE-directed tectonic transport and the formation of at least one major normal detachment.

Tectonic evolution of the Tancheng‐Lujiang (Tan‐Lu) fault via Middle Triassic to Early Cenozoic paleomagnetic data

The north‐striking Tancheng‐Lujiang (Tan‐Lu) fault is a conspicuous and controversial feature of the eastern Asian landscape. Near the southeast extremity of the fault in Anhui Province, we collected paleomagnetic samples at 17 Middle Triassic (T2) and 10 Upper Cretaceous (K2) to lower Cenozoic (E1) sites. T2 remanent magnetizations are interpreted as primary in two of three areas. The three areas are rotated 37° to 137° counterclockwise with respect to the South China Block (SCB) reference direction. K2‐E1 remanent magnetization directions pass regional fold and reversals tests and are not rotated with respect to surrounding areas. Counterclockwise rotation of T2 strata therefore ended before K2 and is attributed to left lateral shear acting along Tan‐Lu during the North China Block (NCB)‐SCB collision. In Shandong Province, 700 km north of the Anhui sites, four areas containing 33 Upper Jurassic (J3) and Cretaceous sites have negligible declination differences, except for one which has dispersed directions. The fold test is inconclusive for this latter area and positive for the other three. Regional concordance of the J3‐E1 paleomagnetic data (including paleolatitudes) together with observed deformation patterns suggest that an extensional regime prevailed in the Late Cretaceous and Cenozoic. Euler pole positions that constrain the North‐South China collision and account for Tan‐Lu motion suggest at least 500 km of sinistral shear took place along the fault, and either (1) subduction and related ultrahigh pressure (UHP) metamorphism occurred near the present location of the Qinling‐Dabieshan and Sulu UHP belts while Tan‐Lu acted as a transform fault that connected the two subduction zones, or (2) Tan‐Lu and Sulu were parts of the same transform fault system and no UHP rocks formed in situ at Sulu. In either case, UHP rocks originally exhumed near Dabieshan could have been transported by plate capture toward Sulu along Tan‐Lu. After North and South China impacted near Dabieshan, the Tan‐Lu fault grew within the SCB as the Dabieshan corner indented the SCB, causing folds in SCB cover rocks to conform to the NCB margin. Late Cretaceous to Cenozoic reactivation of Tan‐Lu, with both right lateral strike‐slip and normal fault motion, occurred as the SCB extruded east relative to the NCB under the influence of the India‐Asia collision.

How mafic is the lower continental crust?

Crustal structures for nine broad tectonic units in China, except the Tarim craton, are derived from seismic data for 18 geophysical refraction profiles, which include twelve geoscience transects and have a total length of 16 216 km. All the tectonic units except the Qinling orogen show a four-layered crustal structure, consisting of upper, middle, upper lower, and lowermost crust. If corrections are made to room temperature and 600 MPa, the upper lower crust has P-wave velocities of 6.7–6.8 km s −1 and the lowermost crust 7.0–7.2 km s −1. Velocities of the bulk lower crust and total crust are 6.8–7.0 and 6.4–6.5 km s −1, respectively. They are slower by 0.2–0.4 km s −1, than the global averages. By correlation with experimental results on velocities of typical lower crustal rocks, the upper lower crust is suggested to be intermediate, while the lowermost crust is mafic in composition. Due to dominance of the former, the bulk lower crust is still intermediate for most tectonic units. However, inter-unit compositional variations are also evident, as indicated by large variations in the thickness ratio (0.3–3.0) of the upper lower to lowermost crust. The lower crust in East China as a whole is suggested to have ∼57% SiO 2. The results contrast with generally accepted models of mafic lower crust. Our estimates of the total crust composition in central East China show a more evolved character compared to models of Rudnick and Fountain, and Taylor and McLennan and are characterized by a prominent negative Eu anomaly (Eu/Eu * = 0.80), higher SiO 2 (61.8%), and lower Sr/Nd (∼10) as well as lower Sr/Nd, Cr/Nd, Ni/Nd, Co/Nd, V/Nd and Ti/Nd ratios. This, together with slower crustal velocities and remarkably thin crustal thicknesses (34 km) for the Paleozoic–Mesozoic Qinling–Dabie orogenic belt, leads to the suggestion that lower crustal delamination played an important role in modification of the East China crust. Mass balance modeling further suggests that eclogite from the Dabie–Sulu ultrahigh pressure metamorphic belt is the most likely candidate as the delaminated material, and that a cumulative 37–82 km thick eclogitic lower crust is required to have been delaminated in order to explain the relative Eu, Sr and transition metal deficits in the crust of central East China. Delamination of eclogites can also explain the significantly higher than eclogite Poisson's ratio in the present Dabie lower crust and upper mantle and lack of eclogite in Cenozoic xenolith populations of the lower crust and upper mantle in East China.

Sr-bearing zoisite and epidote in ultra-high pressure (UHP) metamorphic rocks from the Su-Lu province, eastern China; an important Sr reservoir under UHP conditions

Sr-bearing zoisite and epidote are common constituents of eclogites and associated paraschists throughout the Su-Lu ultra-high pressure (UHP) province, eastern China. The SrO content of prograde zoisite and epidote reaches 3.2 wt% in crystal cores and generally decreases toward crystal margins. Retrograde epidote is poorer in SrO (,0.1 wt%). Preliminary rare earth element (REE) analyses of epidote give La2O3 (up to 2.9 wt%), Ce2O3 (5.9 wt%), and Nd2O3 (3.0 wt%). REE contents of zoisite are distinctly lower (La 2O3 up to 0.16 wt%, Ce2O3 up to 0.26 wt%, and Nd2O3 up to 0.16 wt%) than coexisting epidote. Apatite is always more depleted in SrO (0.10‐0.59 wt% on average) than coexisting zoisite and epidote, and Sr-Ca partition coefficients for zoisite and epidote and apatite [(Sr/Ca)zo/ep-ap] range from 5 to 20. SrO content of K-white mica (0.012‐0.044 wt%) is an order of magnitude lower than that of apatite. An evaluation of the SrO content in zoisite and epidote and their modal abundances in seven samples indicates that .70% of the whole-rock SrO is contained in these minerals. Apatite and K-white mica are only minor reservoirs for SrO in these rocks. Zoisite and epidote are thus regarded as the most important Sr reservoirs at UHP conditions where calcic plagioclase and titanite are unstable.

Ultrahigh-ϵ Nd eclogites from an ultrahigh-pressure metamorphic terrane of China

The SuLu ultrahigh-pressure (UHP) metamorphic terrane is part of the Qinling-Dabie suture zone formed by the collision between the Sino-Korean and Yangtze cratonic blocks. Geochemical and isotopic investigation of the UHP metamorphic rocks has revealed a spectacular history for some eclogitic rocks. Retrograded coesite-bearing eclogites from the Weihai area in the eastern end of the SuLu terrane have recorded an extraordinary Nd isotopic growth with a world-record high ϵ Nd-value of + 264 (range = + 170 to + 264). Such isotopic characteristics are consistent with the observed extreme LREE fractionation and very high Sm/Nd ratios. The SmNd isotopic data allow us to calculate the minimum age of protolith formation at ∼ 1.7 Ga, assuming their evolution from a depleted or a chondritic mantle. Geochemical data indicate that the eclogite protoliths were of basaltic compositions originally, but underwent metasomatism (water-rock interaction) at high pressures and temperatures leading to the present apparent andesitic composition and extreme LREE depletion. This process took place in the Proterozoic (∼ 1.7 Ga) and was not related to the Mesozoic continental collision and ultrahigh-pressure metamorphism at ∼ 210−220 Ma ago. Because of a protracted retrograde metamorphism, garnet SmNd systems were not closed until ∼ 160 Ma ago, ∼ 50 Ma after the peak eclogite metamorphism. This implies that the exhumation history is significantly different in different parts of the Dabie and SuLu UHP metamorphic terranes. From geochemical analyses and isotopic age determinations we argue that at least two ancient tectonic events in the middle and late Proterozoic can be identified prior to the last Mesozoic continental collision and accompanying UHP metamorphism in the SuLu and Dabie terrane.

Eclogitized Metagranitoid from the Su-Lu Ultra-High Pressure (UHP) Province, Eastern China.

Lenses of weakly deformed metagranitoid and coesite-bearing eclogite are both found within mylonitic orthogneiss of the Jiaodong Group in the Su-Lu ultrahigh pressure (UHP) province, eastern China. In the metagranitoid, omphacite overgrowing host clinopyroxene, and garnet coronas developed along the grain boundaries between igneous plagioclase and mafic phases suggest that the metagranitoid suffered an eclogite facies metamorphism. Quartz aggregates after igneous quartz in the metagranitoid show a distinctive polycrystalline, granoblastic texture. This texture is similar to quartz aggregates of the Dora-Maira metagranite which suffered coesite-eclogite facies metamorphism, but is dissimilar to the Mount Mucrone metagranite which recrystallized under the less extreme quartz eclogite facies. These features of the Su-Lu metagranitoid suggest that the UHP metamorphism may have affected not only the eclogite but also the surrounding metagranitoid.