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- W12402893 abstract "Geochemical and Nd-Sr isotope tracer studies of ultrahigh-pressure (UHP) eclogites and associated ultramafic rocks from the Su—Lu and Dabie terranes of central China provide important constraints on tectonic models of continental subduction, collision, and subsequent exhumation in the Dabie orogen. UHP eclogites have three types of occurrence: I) enclaves in ortho- and paragneisses gneisses of granitic composition, II) enclaves in or interlayers with marbles or metaclastic sediments (e.g., garnet-mica schists), and III) enclaves and interlayers with ultramafic rocks. The eclogites are mainly of basaltic composition, but show a wide range of major and trace element abundances, suggesting multiple origins and heterogeneous sources, and in some cases, metamorphic segregation.Types I and II eclogites show LREE- (light rare earth element) enriched patterns and highly negative εNd(220 Ma) values (–6 to –20). The overall geochemical and isotopic data indicate that most of the eclogite protoliths resemble basalts/gabbros and their metamorphic equivalents (amphibolites or basic granulites) of Precambrian gneiss terranes, and they constitute an integral part of ancient continental craton. A minimum age of 1.7 Ga was established for those from the Weihai area (northeastern Su—Lu). Sm-Nd model ages (TDM) are highly variable, ranging from >3 Ga to 800 Ma. This indicates that the protoliths were emplaced in Precambrian times. The eclogites do not represent subducted Tethys Ocean crust. Type III eclogites are genetically related to the associated ultramafic rocks (lherzolite, harzburgite, or pyroxenites). Sr-Nd isotopic data indicate minor contamination by lower crustal materials for the Bixiling, whereas a greater contamination by upper crustal rocks is recorded in the Maowu layered intrusions. Isotopic signatures of ultramafic rocks (e.g., Rizhao, Rongcheng, Donghai) indicate that some rocks evolved from an enriched reservoir (Rongcheng) whereas others are from a slightly depleted mantle source (Rizhao).The present study provides the following constraints to tectonic models of the Dabie Orogen. 1) Type I and II eclogites have continental protoliths, and are not subducted Tethyan oceanic crust. This is significantly different from the eclogites of oceanic origin from the Alpine and Hercynian orogenic belts. Sm-Nd model age data suggest that the eclogite protoliths were produced at different Precambrian times; some Su—Lu eclogites could have formed in the late Archean, whereas Dabie eclogites are Proterozoic. 2) Type III eclogites are generally of mantle derivation. Different degrees of crustal contamination are observed in the Bixiling and Maowu layered intrusions. The UHP parageneses of these intrusions indicate deep continental subduction, whereas those of some ultramafic rocks (e.g., Rizhao) do not. However, they clearly indicate exhumation of deep-seated mantle slices in the collision zone. 3) The UHP metamorphism took place at ~220 Ma based on a variety of reliable age data (U-Pb, Sm-Nd, Ar-Ar) obtained in different laboratories. Other “aberrant” ages can be interpreted as due to a) lack of isotope equilibrium in UHPM conditions (Sm-Nd, U-Pb systems), or b) excess Ar, particularly in phengite. No ages can be determined from the Rb-Sr systems because most eclogites experienced Rb depletion relative to Sr, leading to unsupported high 87Sr/86Sr ratios. However, the lowest 87Sr/86Sr (~0.7036) for the Bixiling complex and Rizhao clinopyroxenites may represent the Sr isotopic composition of the subcontinental lithosphere of the Dabie orogen. 4) Identical U-Pb zircon ages for UHP eclogites and apparently non-UHP host gneisses are consistent with the in situ origin of the UHP complex of the Dabie orogen." @default.
- W12402893 created "2016-06-24" @default.
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- W12402893 date "1998-01-01" @default.
- W12402893 modified "2023-10-18" @default.
- W12402893 title "Geochemical and Isotopic Characteristics of UHP Eclogites and Ultramafic Rocks of the Dabie Orogen: Implications for Continental Subduction and Collisional Tectonics" @default.
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- W12402893 doi "https://doi.org/10.1007/978-94-015-9050-1_8" @default.
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