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    Precambrian crystalline basement in southern Mongolia asrevealed by SHRIMP zircon dating [查看] AntoineDemoux、AlfredKroner、DunyiLiu、GombosurenBadarch
    Single zircon ages determined by ion microprobe (SHRIMP II) for granitoid gneisses from the southern slope of the Baga Bogd massif (Gobi-Altai,southern Mongolia) reveal several episodes of zircon growth, ranging from late Palaeoproterozoic to late Cambrian.The oldest events are documented by a zircon crystallization age for a gneiss protolith at 1519 ± 11 Ma and by a xenocrystic zircon from a dark grey augen-gneiss yielding an age of c. 1701 Ma. Discrete igneous events are recorded in granite-gneisses with protolith emplacement ages of 983 ± 6, 956 ± 3 and 954 ± 8 Ma. These ages provide the first record of early Neoproterozoic magmatic activity in this region. A much younger and discrete magmatic event is recorded by several dioritic to granitic orthogneisses which are tectonically interlayered with the older gneisses and have protolith emplacement ages between 502 and 498 Ma. These late Cambrian granitoids of calc-alkaline affinity are likely to have been emplaced along an active continental margin and suggest that the Baga Bogd Precambrian crustal fragment was either docked against the southward (present-day coordinates) growing margin of the CAOB or was a large enough crustal entity to develop an arc along its margin. We speculate that the Precambrian gneisses of this massif may be part of a crustal fragment rifted off the Tarim Craton.
    Provenance of Meso- to Neoproterozoic cover sediments at the Ming Tombs, Beijing,North China Craton An integrated study of U–Pb dating and Hf isotopic measurement of detrital zircons and whole-rock geochemistry [查看] YushengWan、DunyiLiu、WeiWang、TianruiSong、AlfredKröner、ChunyanDong、HongyinZhou、XiaoyanYin
    The North China Craton (NCC)was subjected to an extensional regime after the Lüliangmovement at ~1.8 Ga and then was covered by an extensive Meso- to Neoproterozoic sedimentary succession, namely the Changcheng,Jixian and Qingbaikou Groups in ascending order. We report age spectra for detrital zircons and monazites, Hf isotopic systematics of detrital zircons, and whole-rock chemical and Nd isotopic compositions for sediments from the succession in the Ming Tombs area, Beijing, one of the typicalMeso- to Neoproterozoic areas in the NCC.Detrital zircons of six sedimentary samples have two distinct age peaks at ~2.52 Ga and ~1.85 Ga. There are some detrital zircons at 2.4–2.0 Ga but none at 2.3 Ga and only a few N2.7 Ga. The detrital zircon age spectra change with time. Sediments in the lower succession (Changcheng Group) and in the upper successions (Jixian and Qinbaikou Groups) are dominated by significant detrital zircon populations of late Neoarchean and late Paleoproterozoic ages, respectively. The ~2.5 Ga detrital zircons of the ChangchengGroup have εHf(2.5 Ga) values and tDM(Hf) model ages mainly ranging from −2 to +7 and 2.8 to 2.7 Ga, respectively. Detrital monazites of a sample fromthe JixianGroup exhibit amajor age peak between 1.95 and 1.80 Ga with somedata between 2.0 and 1.95 Ga. The sedimentary rocks of the Changcheng Group are characterized by high K2O contents (mostly 7.09–15.20%) and insignificant Eu anomalies (Eu/Eu*=0.71–1.16). They have tDM(Nd) model ages ranging from 2.70 to 2.43 Ga, being older than the tDM(Nd) ages (2.11 and 1.99 Ga) of sedimentary samples from the Qingbaikou Group. Based on a comparison with ages for the early Precambrian (N1.8 Ga) basement of the NCC, it can be concluded that (1) the sediments of theMeso- to Neoproterozoic coverwere undoubtedly derived fromthe NCC itself or once neighboring terranes; (2) variations in the detrital zircon age spectra from the lower to the upper successions reflect provenance evolution in that the lower crustal late Paleoproterozoic rockswere exposed at the surface after the upper crustal late Neoarchean rocks had already been eroded.
    Dating of prograde metamorphic events deciphered from episodic zircon growth in rocks of the Dabie-Sulu UHP complex,China [查看] DunyiLiu、PingJian、AlfredKröner、ShutongXu
    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 (N∼ 4 Gpa) at227.3±0.7 Ma (n=72), before exhumation (b∼ 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.
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