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    In situ determination of U–Pb ages and Sr–Nd–Hf isotopic constraints on the petrogenesis of the Phalaborwa carbonatite Complex, South Africa [查看] Fu-YuanWu、Yue-HengYang、Qiu-LiLi、RogerH.Mitchell、J.BarryDawson、GüntherBrandl、MasakiYuhara
    The Phalaborwa carbonatite Complex, situated in the northeastern part of South Africa, is characterized by copper and zirconium mineralization, and is composed principally of pyroxenites, phoscorite and carbonatite(banded and transgressive). The complex is transected by mafic dykes, and is geographically associated with a satellite syenite and minor granite intrusions. Zircon and baddeleyite U–Pb isotopic age determinations using CAMECA 1280 secondary ion mass spectrometry have shown that the outer pegmatitic pyroxenite at the Loolekop pipe was emplaced at 2060±4 Ma, and the main phoscorite at 2062±2 Ma. Both ages are identical to those of 2060±2 and 2060±1 Ma for the banded and transgressive carbonatites, respectively. The satellite syenite, which forms plug-like bodies outside of the border of the main complex, and the later mafic dyke have “similar” emplacement ages of 2068±17 and 2062±53 Ma, indicating that these intrusions were apparently near-synchronously emplaced. In contrast to other carbonatites, the Phalaborwa Complex is characterized by high initial Sr and low initial Nd and Hf isotopic compositions. In situ isotopic analyses of apatite,calcite, zircon and baddeleyite indicate that the primary magma was derived from an enriched mantle.As the complex was emplaced slightly earlier at ~2060 Ma than the nearby mafic phase of the Bushveld Complex (~2055 Ma), it is proposed that the Phalaborwa carbonatite magmatism was triggered by the same mantle plume activity, which partially melted the overlying lithospheric mantle. This contribution also highlights that isotopic studies used to constrain the genesis of ancient igneous complexes should concentrate on minerals with low parent/daughter elemental ratios, such as apatite and calcite for Sr isotopes, and zircon and baddeleyite for Hf isotopes.
    Precisely dating Paleozoic kimberlites in the North China Craton and Hf isotopic constraints on the evolution of the subcontinental lithospheric mantle [查看] Qiu-LiLi、Fu-YuanWu、Xian-HuaLi、Zhi-LiQiu、YuLiu、Yue-HengYang、Guo-QiangTang
    Kimberlite, a deep-sourced ultramafic potassic rock, carries not only diamond, but also invaluable mantle xenoliths and/or xenocrysts,which are important for tracking the evolution of subcontinental lithosphericmantle(SCLM). However, it is challenging to accurately determine the emplacement age of kimberlite and its compositions of primary magma because of modifications by crustal and/or mantle contamination and postemplacement alteration. This paper reports emplacement ages of diamondiferous kimberlites in Mengyin and Fuxian of the North China Craton (NCC) using three different dating methods. For Mengyin kimberlite, singlegrain phlogopite Rb–Sr dating yields an isochron age of 485±4 Ma, U–Th–Pb analyses on perovskite give a 238U–206Pb age of 480.6±2.9 Ma and a 232Th–208Pb age of 478.9±3.9 Ma, and baddeleyite yields a 207Pb–206Pb age of 480.4±3.9 Ma. For Fuxian kimberlite, baddeleyite gives a 207Pb–206Pb age of 479.6±3.9 Ma, indicating that the Paleozoic kimberlites in the NCC were emplaced at ~480Ma. Numerous lines of evidence indicate that the studied baddeleyites are xenocrysts fromthe SCLM, and can be used to constrain Hf isotope compositions (εHf(t)~−6) of the SCLM when kimberlite erupted. Combined with data from Mesozoic–Cenozoic mantle-derived rocks and xenoliths, the Hf isotope evolution trend of the SCLM beneath NCC before craton destruction was tentatively constructed, which suggested that the Archean SLCM was enriched by metasomatism at ~1.3 Ga.Further Hf isotope investigations on additional SCLM-derived materials could be used to compare with the constructed Hf isotope evolution trend before craton destruction to determine when lithospheric thinning occurred.
    Origin of postcollisional magmatic rocks in the Dabie orogen Implications for crust-mantle interaction and crustal architecture [查看] Zi-FuZhao、Yong-FeiZheng、Chun-ShengWei、Fu-YuanWu
    Zircon Hf and whole-rock Sr–Nd isotopic compositions were determined for postcollisional mafic–ultramafic and felsic intrusive rocks in the Dabie orogen. The results provide not only insights into the character of their source rocks and the nature of crust–mantle interaction, but also constraints on the crustal architecture of continental collision orogen. SHRIMP zircon U–Pb dating gave concordant ages of 121±6 to 131±2 Ma for the bimodal intrusives. Zircon Hf isotope analyses gave negative εHf(t) values of−26.3±0.6 to−7.0±0.5 for the mafic–ultramafic rocks, with two-stage Hf model ages of 1.62 to 2.83 Ga. Zircons from felsic granitoids also gave negative εHf(t) values of −31.6±0.5 to −16.9±0.9 with two-stage Hf model ages of 2.25 to 3.16 Ga.Both the mafic–ultramafic and felsic rocks have high initial 87Sr/86Sr raios of 0.7065 to 0.7084 and very low εNd(t) values of −21.7 to −11.7 for whole-rock. The crust-like geochemical signatures in the mafic– ultramafic rocks suggest their derivation from partialmelting of an orogenic lithospheric mantle source that is enriched in radiogenic isotopes as well as incompatible trace elements such as LILE and LREE. It would be generated by reaction of the overlying subcontinental lithospheric mantle (SCLM) wedge peridotite with the felsic melt derived from the subducted continental crust during the continental collision. Therefore, these postcollisional mafic–ultramafic rocks record recycling of the subducted continental crust and consequent crust–mantle interaction in the continental subduction zone. The granitoids have Hf model ages as old as Paleoarchean, which cannot be derived from partial melting of surrounding orthogneisses alone, but requires involvement of more ancient Archean crust in their source region. Thus, their source represents a mixture of crustal rocks from Paleoproterozoic and Paleoarchean basement. In combination of the present and previous studies, the Dabie orogen is suggested to have a three-layer crustal structure prior to the postcollisional magmatism: Central Dabie in the upper with dominantly young Hf model ages of late Mesoproterozoic to Neoproterozoic, North Dabie in the middle with dominantly middle Paleoproterozoic Hf model ages, and the source region of the postcollisional granitoids in the lower with Paleoproterozoic to Paleoarchean Hf model ages.
    Zircon U-Pb ages, Hf and O isotopes constrain the crustal architecture of the ultrahigh-pressure Dabie orogen in China [查看] Zi-FuZhao、Yong-FeiZheng、Chun-ShengWei、Fu-KunChen、XiaomingLiu、Fu-YuanWu
    The crustal structure of the Dabie orogen was reconstructed by a combined study of U–Pb ages, Hf and O isotope compositions of zircons from granitic gneiss from North Dabie, the largest lithotectonic unit in the orogen. The results were deciphered from metamorphic history to protolith origin with respect to continental subduction and exhumation. Zircon U–Pb dating provides consistent ages of 751±7 Ma for protolith crystallization, and two group ages of 213±4 to 245±17 Ma and 126±4 to 131±36 Ma for regional metamorphism. Majority of zircon Hf isotope analyses displays negative εHf(t) values of −5.1 to −2.9 with crust Hf model ages of 1.84 to 1.99 Ga, indicating protolith origin from reworking of middle Paleoproterozoic crust. The remaining analyses exhibit positive εHf(t) values of 5.3 to 14.5 with mantle Hf model ages of 0.74 to 1.11 Ga, suggesting prompt reworking of Late Mesoproterozoic to Early Neoproterozoic juvenile crust. Zircon O isotope analyses yield δ18O values of −3.26 to 2.79‰, indicating differential involvement of meteoric water in protolith magma by remelting of hydrothermally altered low δ18O rocks. North Dabie shares the same age of Neoproterozoic low δ18O protolith with Central Dabie experiencing the Triassic UHP metamorphism, but it was significantly reworked at Early Cretaceous in association with contemporaneous magma emplacement.The Rodinia breakup at about 750 Ma would lead to not only the reworking of juvenile crust in an active rift zone for bimodal protolith of Central Dabie, but also reworking of ancient crust in an arc-continent collision zone for the North Dabie protolith. The spatial difference in the metamorphic age (Triassic vs. Cretaceous) between the northern and southern parts of North Dabie suggests intra-crustal detachment during the continental subduction. Furthermore, the Dabie orogen would have a three-layer structure prior to the Early Cretaceous magmatism: Central Dabie in the upper, North Dabie in the middle, and the source region of Cretaceous magmas in the lower.
    Zircon U-Pb age and Hf-O isotope evidence for Paleoproterozoic metamorphic event in South China [查看] Shao-BingZhang、Yong-FeiZheng、Yuan-BaoWu、Zi-FuZhaoa、ShanGaob、Fu-YuanWu
    To understand the connection between continental cratonization and global tectonothermal event is essential for recognizing the formation and evolution of continental crust. Paleoproterozoic is an important era with occurrence of megascale tectonomagmatism in the world, but it has been intriguing whether they also influenced the oldest continent in South China. In order to decipher the nature of Paleoproterozoic event in South China, a combined study of zircon U-Pb dating, Hf and O isotope analyses was carried out for metasediments and amphibolite from the Kongling terrane, the only Archean microcontinent outcropped in South China. U-Pb ages of 1.97±0.03 Ga were obtained with low Th/U ratios of 0.01–0.14, indicating that the ages are a record of Paleoproterozoic metamorphic event.18O values of ∼11‰ and ∼8‰ were measured for quartz from the metasediments and garnet from the amphibolite, respectively, suggesting that their sources experienced supracrustal recycling. εHf(t) values of about −6.5 and model Hf ages of about 3.0 Ga were acquired for zircons from the metapelites, suggesting an Archean source. Thus a response to the Paleoproterozoic global tectonothermal event in South China is reworking of Archean continental nucleus. Compared with Archean rocks at Kongling, abrupt changes in K2O/Na2O, REE and other trace elements are observed in the Paleoproterozoic metasedimentary rocks. This is interpreted to reflect a change in upper crustal composition at the Archean–Proterozoic boundary.A survey of Paleoproterozoic ages throughout the Yangtze Block suggests that metamorphic event and subsequent magmatic activity occurred in the north, but only magmatic activity in the south. Both metamorphic and magmatic activities are associated with formation of a unified basement responsible for cratonization of the Yangtze Block. This provides a geodynamic connection between the formation of this craton and the global tectonomagmatism in the Paleoproterozoic, marking continental accretion by arc-continent collision orogeny during assembly of the supercontinent Columbia.
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