http://shrimp.anu.edu.au/people/tri/Publications/2003RatschP1.pdf

Tectonics of the Qinling (Central China): tectonostratigraphy, geochronology, and deformation history

Stable isotope geochemistry of ultrahigh pressure metamorphic rocks from the Dabie–Sulu orogen in China: implications for geodynamics and fluid regime

The golden transformation of the Cretaceous plate subduction in the West Pacific

Tectonic evolution of a composite collision orogen: An overview on the Qinling–Tongbai–Hong'an–Dabie–Sulu orogenic belt in central China

Metamorphic chemical geodynamics in continental subduction zones

SHRIMP U–Pb dating and laser ablation ICP-MS trace element analyses of zircon from four eclogite samples from the north-western Dabie Mountains, central China, provide evidence for two eclogite facies metamorphic events Three samples from the Huwan shear zone yield indistinguishable late Carboniferous metamorphic ages of 312 ± 5, 307 ± 4 and 311 ± 17 Ma, with a mean age of 309 ± 3 Ma One sample from the Hong'an Group, 1 km south of the shear zone yields a late Triassic age of 232 ± 10 Ma, similar to the age of ultra-high pressure (UHP) metamorphism in the east Qinling–Dabie orogenic belt REE and other trace element compositions of the zircon from two of the Huwan samples indicate metamorphic zircon growth in the presence of garnet but not plagioclase, namely in the eclogite facies, an interpretation supported by the presence of garnet, omphacite and phengite inclusions Zircon also grew during later retrogression Zircon cores from the Huwan shear zone have Ordovician to Devonian (440–350 Ma) ages, flat to steep heavy-REE patterns, negative Eu anomalies, and in some cases plagioclase inclusions, indicative of derivation from North China Block igneous and low pressure metamorphic source rocks Cores from Hong'an Group zircon are Neoproterozoic (780–610 Ma), consistent with derivation from the South China Block In the western Dabie Mountains, the first stage of the collision between the North and South China Blocks took place in the Carboniferous along a suture north of the Huwan shear zone The major Triassic continent–continent collision occurred along a suture at the southern boundary of the shear zone The first collision produced local eclogite facies metamorphism in the Huwan shear zone The second produced widespread eclogite facies metamorphism throughout the Dabie Mountains–Sulu terrane and a lower grade overprint in the shear zone

Carboniferous and Triassic eclogites in the western Dabie Mountains, east-central China: evidence for protracted convergence of the North and South China Blocks

A combined study of major and trace elements, fluid inclusions and oxygen isotopes has been carried out on garnet pyroxenite from the Raobazhai complex in the North Dabie Terrane (NDT). Well-preserved compositional zoning with Na decreasing and Ca and Mg increasing from the core to rim of pyroxene in the garnet pyroxenite indicates eclogite facies metamorphism at the peak metamorphic stage and subsequent granulite facies metamorphism during uplift. A P–T path with substantial heating (from c. 750 to 900 °C) after the maximum pressure reveals a different uplift history compared with most other eclogites in the South Dabie Terrane (SDT). Fluid inclusion data can be correlated with the metamorphic grade: the fluid regime during the peak metamorphism (eclogite facies) was dominated by N2-bearing NaCl-rich solutions, whereas it changed into CO2-dominated fluids during the granulite facies retrograde metamorphism. At a late retrograde metamorphic stage, probably after amphibolite facies metamorphism, some external low-salinity fluids were involved. In situ UV-laser oxygen isotope analysis was undertaken on a 7 mm garnet, and impure pyroxene, amphibole and plagioclase. The nearly homogeneous oxygen isotopic composition (δ18OVSMOW = c. 6.7‰) in the garnet porphyroblast indicates closed fluid system conditions during garnet growth. However, isotopic fractionations between retrograde phases (amphibole and plagioclase) and garnet show an oxygen isotopic disequilibrium, indicating retrograde fluid–rock interactions. Unusual MORB-like rare earth element (REE) patterns for whole rock of the garnet pyroxenite contrast with most ultra-high-pressure (UHP) eclogites in the Dabie-Sulu area. However, the age-corrected initial eNd(t) is − 2.9, which indicates that the protolith of the garnet pyroxenite was derived from an enriched mantle rather than from a MORB source. Combined with the present data of oxygen isotopic compositions and the characteristic N2 content in the fluid inclusions, we suggest that the protolith of the garnet pyroxenite from Raobazhai formed in an enriched mantle fragment, which has been exposed to the surface prior to the Triassic metamorphism.

Geochemical constraints of the eclogite and granulite facies metamorphism as recognized in the Raobazhai complex from North Dabie Shan, China

Although ultrahigh-pressure (UHP) metamorphic rocks are present in many collisional orogenic belts, almost all exposed UHP metamorphic rocks are subducted upper or felsic lower continental crust with minor mafic boudins. Eclogites formed by subduction of mafic lower continental crust have not been identified yet. Here an eclogite occurrence that formed during subduction of the mafic lower continental crust in the Dabie orogen, east-central China is reported. At least four generations of metamorphic mineral assemblages can be discerned: (i) hypersthene + plagioclase ± garnet; (ii) omphacite + garnet + rutile + quartz; (iii) symplectite stage of garnet + diopside + hypersthene + ilmenite + plagioclase; (iv) amphibole + plagioclase + magnetite, which correspond to four metamorphic stages: (a) an early granulite facies, (b) eclogite facies, (c) retrograde metamorphism of high-pressure granulite facies and (d) retrograde metamorphism of amphibolite facies. Mineral inclusion assemblages and cathodoluminescence images show that zircon is characterized by distinctive domains of core and a thin overgrowth rim. The zircon core domains are classified into two types: the first is igneous with clear oscillatory zonation ± apatite and quartz inclusions; and the second is metamorphic containing a granulite facies mineral assemblage of garnet, hypersthene and plagioclase (andesine). The zircon rims contain garnet, omphacite and rutile inclusions, indicating a metamorphic overgrowth at eclogite facies. The almost identical ages of the two types of core domains (magmatic = 791 ± 9 Ma and granulite facies metamorphic zircon = 794 ± 10 Ma), and the Triassic age (212 ± 10 Ma) of eclogitic facies metamorphic overgrowth zircon rim are interpreted as indicating that the protolith of the eclogite is mafic granulite that originated from underplating of mantle-derived magma onto the base of continental crust during the Neoproterozoic (c. 800 Ma) and then subducted during the Triassic, experiencing UHP eclogite facies metamorphism at mantle depths. The new finding has two-fold significance: (i) voluminous mafic lower continental crust can increase the average density of subducted continental lithosphere, thus promoting its deep subduction; (ii) because of the current absence of mafic lower continental crust in the Dabie orogen, delamination or recycling of subducted mafic lower continental crust can be inferred as the geochemical cause for the mantle heterogeneity and the unusually evolved crustal composition.

Ultrahigh‐pressure eclogite transformed from mafic granulite in the Dabie orogen, east‐central China

Handling Editor: Doug Robinson ABSTRACT Garnet, as a major constitutive mineral of eclogite, is important for Sm–Nd dating of eclogite due to its high Sm/Nd ratio and its stability during retrogression. However, a comprehensive study of the petrography, mineral chemistry, garnet water content, and Sm–Nd isotopic composition of eclogites from the Bixiling massif, Central Dabie Zone (CDZ), reveals significant modification of the Sm–Nd isotopic system in garnet as a result of retrogression. This problem constitutes a challenge for Sm–Nd dating of the Bixiling eclogites, with the Sm–Nd isochron ages of 218 4 to 210 9 Ma reported in the literature being younger than 226 3 Ma, which is the generally accepted peak metamorphic age of the CDZ. Petrographic analysis reveals heterogeneity in colour within single fractured garnet grains. There are light-pink garnet (Grt-P) and red garnet (Grt-R) types that possess distinct chemical compositions. Compared to Grt-P, Grt-R has higher Fe and andradrite contents but lower Al and grossular contents. Grt-P also has lower water contents (15–35 ppm) than Grt-R (34–65 ppm), which, together with the spatial association between Grt-R and fractures, suggests that the colour change is related to fluid alteration. Grt-P is an ultra-high-pressure (UHP) mineral, and GrtR is the product of the interaction between Grt-P and a fluid during retrogression. Moreover, Grt-R features lower Sm and Nd contents but higher Sm/Nd ratios than Grt-P. The Sm–Nd isochrons defined by UHP minerals (Grt-P+Omp+Rt or GrtP+Cpx+WR) from three eclogite samples yield consistent ages of 226.0 3.8 Ma, 225.0 3.9 Ma and 226.2 6.9 Ma, which are identical to the peak metamorphic age of 226 3 Ma for the CDZ. The retrogressed garnet (i.e., Grt-R), omphacite and rutile, together define a pseudoisochron with younger ages of 218.9 5.9 to 202.8 4.8 Ma, which are geologically meaningless. The increase in the Sm/Nd ratio with constant or lower Nd/Nd ratios during the transformation of Grt-P to Grt-R was probably the cause of these younger ages.

Shu-Guang Li

Papers

Carboniferous and Triassic eclogites in the western Dabie Mountains, east-central China: evidence for protracted convergence of the North and South China Blocks

Geochemical constraints of the eclogite and granulite facies metamorphism as recognized in the Raobazhai complex from North Dabie Shan, China

Ultrahigh‐pressure eclogite transformed from mafic granulite in the Dabie orogen, east‐central China

Modification of the Sm–Nd isotopic system in garnet induced by retrogressive fluids