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    Mafic dykes derived from Early Cretaceous depleted mantle beneath the Dabie orogenic belt_ implications for changing lithosphere mantle beneath eastern China [查看] LINGCHEN、CHANGQ.MA、JINY.ZHANG、ROGERMASON、CHAOZHANG
    SHRIMP zircon U–Pb geochronological, elemental and Sr–Nd isotopic data from Early Cretaceous mafic dykes in North Dabie orogenic belt elucidate a change of Mesozoic lithospheric mantle in eastern China. The dykes are predominantly dolerite with the major mineral assemblage clinopyroxeneþhornblendeþplagioclase and yield a SHRIMP zircon U–Pb age of 111.65.3 Ma. They have a narrow range of SiO2 from 46.16% to 49.78%, and relative low concentrations of K2O (1.072.62%), Na2O (2.453.54%), Al2O3 (13.0414.07%), and P2O5 (0.420.55%) but relatively high concentration of MgO (5.94–6.61%) with Mg# 52–54. All the samples are characterized by enrichment of large ion lithophile elements (LILE, e.g., Ba, Th) and high field strength elements (HFSE, e.g., Nb, Ti). (87Sr/86Sr)i ratios from 0.704 to 0.705, eNd values from 3.36 to 4.33 and mantle-depletion Nd model ages (T2DM) in the range 0.56–0.64 Ga indicate that the magma of the Baiyashan mafic dykes was derived from a young depleted mantle source. This finding is different from previous research on mafic dykes in the age range 120–138 Ma that revealed enrichment of LILE and depletion of HFSE, high initial Sr isotopic ratios and negative eNd, value which represents an old enriched mantle source. Ours is the first report of the existence of Early Cretaceous depleted mantle in eastern China and it implies that changing of enriched mantle to depleted mantle occurred at ca. 112 Ma, associated with back-arc extension which resulted from the subduction of the Palaeo-Pacific Plate towards the Asian Continent.
    Implications of subduction and subduction zone migration of the Paleo-Pacific Plate beneath eastern North China, based on distribution, geochronology, and geochemistry of Late Mesozoic volcanic rocks [查看] ChaoZhang、Chang-QianMa、Qun-AnLiao、Jin-YangZhang、Zhen-BingShe
    Several major volcanic zones are distributed across the eastern North China Craton, from northwest to southeast: the Greater Xing’an Range, Jibei-Liaoxi, Xishan,and Songliao Basins, and the Yanji, Huanghua, and Ludong volcanic zones. The Huanghua depression within the Bohai Bay Basin was filled by middle Late Mesozoic volcanic rocks and abundant Cenozoic alkaline basalts. Zircon LA-ICP-MS and SHRIMP U–Pb dating show that basic–intermediate volcanic rocks were extruded in the Early Cretaceous of 118.8 ± 1.0 Ma (weighted mean 206Pb/238U age), before Late Cretaceous acid lavas at 71.5 ± 2.6 Ma.An inherited zircon from andesite has a Paleoprotoerozoic core crystallization age of 2,424 ± 22 Ma (206Pb/207Pb age) indicating that the basement of the Bohai Bay Basin is part of the North China Craton. Early Cretaceous basic and intermediate lavas are characterized by strong enrichments in LREE and LILE and depletions in HREE and HFSE,indicating a volcanic arc origin related to oceanic subduction.Depletion in Zr only occurs in basic and intermediate volcanic rocks, while depletions in Sr and Ti exist only in acid samples, indicating that the acid series is not genetically related to the basic–intermediate series. Formation ages and geochemical features indicate that the Late Cretaceous acid lavas are products of crustal remelting in an extensional regime. Combined information from all these volcanic zones shows that subduction-related volcanic rocks were generated in the Jibei-Liaoxi and Xishan volcanic zones during the Early Jurassic, about 60 Ma earlier than their analogues extruded in the Huanghua and Ludong volcanic zones during the Early Cretaceous. This younging trend also exists in the youngest extension-related volcanism in each of these zones: Early Cretaceous asthenosphere-derived alkaline basalts in the northwest and Late Cretaceous in the southeast. A tectonic model of northwestward subduction and continuous oceanward retreat of the Paleo-Pacific Plate is proposed to explain the migration pattern of both arc-related and post-subduction extensionrelated volcanic rocks. As the subduction zone continuously migrated, active continental margin and backarc regimes successively played their roles in different parts of North China during the Late Mesozoic (J1–K2).
    Further SHRIMP Geochronology on the Early Cambrian of South China [查看] W.COMPSTON、ZICHAOZHANG、J.A.COOPER、GUOGANMA、R.J.F.JENKINS
    ABSTRACT. Zircon 206Pb/238U ages are reported for bentonites from two Cambrian sections in South China, the Meishucun and the Fandian. The analyzed zircons vary widely in age because of the presence of detrital and altered grains. Spot ages were corrected for calibration drift during analysis, and the age of reference zircon SL13 was calibrated as 580 Ma against other standards using SIMS. Use of mixture modeling was essential to resolve the different age components and to identify the volcanic ages. Two age groups occur within the Meishucun Bed 5 bentonite, of which the larger at 518.7±2.5 Ma (σ) is interpreted as a post depositional zircon growth event and the other at 539.4±2.9 Ma as the volcanic age. Bentonites from the Meishucun Bed 9 and its Fandian equivalent have two age groups, 526.5±1.1 Ma and 540.2±1.0 Ma. We interpret the younger of these as the tuff magmatic age and the older as detrital. Small shelly fossils indicate Meischucunian Bed 5 is an equivalent of the late Nemakit Daldynian, and its 539.4±2.9 Ma age both closely times the base of the Cambrian and is a maximum for the basal Tommotian. The 526.5±1.1 Ma for Bed 9 and its equivalents approximates the close of the Tommotian.
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