临洪河口现代沉积环境及重金属元素的分布特征

对江苏北部临洪河口不同区域的表层与柱状沉积物进行了粒度分析、石英颗粒表面微细结构和沉积物的测年分析以及沉积物中的重金属含量分析．将河口划分为不同的沉积环境区．探讨了不同沉积环境中沉积物的粒度分布、石英颗粒表面微细结构、沉积速率以及重金属元素分布差异的原因。结果显示,临洪河口可以划分为低潮滩上的波浪与潮流强烈混合作用区、高潮滩上的弱潮流作用区、口门附近的潮流与河流强烈混合作用区和河道内河流与潮流弱混合作用区。重金属的含量在柱样的30～40cm深度呈现向表层递增的趋势,Hg的污染比较严重．其他元素基本无污染,整个河口区域的生态危害性严重;沉积物粒度分布不是该区域重金属分布的主要影响因素,城市污水排放量增加、氧化还原层位、沉降方式和不同沉积区的水动力环境是影响本区域重金属元素分布和累积的主要原因。     

成都金沙遗址青铜器的化学特征及矿质来源

在总结前人对商周遗址中青铜器研究的基础上,通过对成都金沙遗址出土青铜器的铅同位素和主要金属元素含量的测定,分析讨论了金沙青铜器的化学特征及可能的矿质来源。通过对测定结果的分析认为：青铜器的冶炼过程中人为进行铜铅锡含量配比的迹象并不明显,更多体现出直接利用多金属矿的结果;金沙遗址中青铜器的铅同位素值大部分异常高,表现出N（206Pb）/N（204Pb）大于20。根据区域地质资料和前人对多金属矿床的研究结果,认为金沙青铜器的铜矿石极有可能来自于距离较近的会理拉拉厂铜矿。     

黄龙钙华210Pb计年与现代沉积的环境变化研究

本文以黄龙钙华岩芯为主要研究对象, 利用210Pb定年技术在测定年轻碳酸盐样品短时间尺度上的优势建立了研究区近100年来高精度的年代标尺及环境记录; 通过碳氧稳定同位素的分析, 讨论了钙华稳定同位素记录与器测气候数据的相关性特征, 并与其他定年技术所获得的结果进行了相互验证。研究区钙华δ18O在1908~2004年时段内的变化范围为?12.13‰~?10.42‰, 平均值为?11.23‰, 变化幅度达到了1.7‰。同属西南季风区、由于海拔效应和距离水汽来源远近不同等因素的影响, 在同时段内黄龙钙华δ18O的变化范围和变化幅度都比董歌洞和Kahf Defore洞的δ18O记录要大, 说明处于中纬度、高海拔的黄龙钙华相对于低海拔的董歌洞和Kahf Defore洞石笋对气候环境的响应可能更加敏感。     

珠江口沉积物210Pb分布特征及环境意义

根据珠江口及其邻近海域中的14个沉积柱样的210Pb随深度分布的测量数据以及210Pb过剩衰减法的原理,建立了基于傅立叶级数的变沉积速率模型,对珠江河口近50年来河口沉积物的沉积序列进行了分析。分别对各柱样的210Pb数据使用变沉积速率模型进行计算,获得了柱样更切实的沉积速率变化过程并分析沉积速率、年代计算结果与同时期内沉积环境之间的关系,结果表明:珠江口不同站位的沉积速率差异很大,几个站位的整体平均沉积速率分别是,7.25 cm/a,5.36 cm/a,3.26 cm/a,3.03 cm/a,1.58 cm/a;沉积速率的变化与同时期发生在区域内的洪水、风暴潮、人类活动、河口演变等一系列综合因素有较好的对应关系,这说明使用基于傅立叶级数的变沉积速率模型得到的210Pb沉积速率变化的垂直分布能够解释其对应时段内的沉积环境,此模型亦可为其它河口区域的210Pb沉积速率计算提供借鉴。     

Towards a complete magmatic barcode for the Zimbabwe craton: Baddeleyite U–Pb dating of regional dolerite dyke swarms and sill complexes

We present baddeleyite U–Pb ages of Neoarchaean to Palaeoproterozoic dyke swarms and the Mashonaland sill province in Zimbabwe. The 2575.0 ± 1.5 Ma age of the Umvimeela dyke is indistinguishable from the 2575.4 ± 0.7 Ma result (Oberthür et al., 2002) for a pyroxenite layer of the Great Dyke and testifies to synchronous emplacement of the Great Dyke and its satellites. Three samples of WNW- to NNW-trending dykes of the Sebanga swarm yielded ages of 2512.3 ± 1.8 Ma, 2470.0 ± 1.2 Ma and 2408.3 ± 2.0 Ma, the latter of which dates the Sebanga Poort Dyke of this swarm. These results suggest that emplacement took place over a protracted period which involved at least three generations of dykes within the swarm and, more importantly, invalidate previous inferences of a genetic link between the Sebanga swarm and the Mashonaland sills. Crystallisation ages of 1877 ± 2.2 Ma, 1885.9 ± 2.4 Ma and 1875.6 ± 1.6 Ma for three dolerite samples of the extensive Mashonaland sills from different parts of the Zimbabwe craton were also obtained. This is the oldest common igneous event that is recorded in the Zimbabwe and Kaapvaal cratons. Collectively with previous published geochronological and petrological evidence in favour of a major 2.0 Ga event within the Limpopo Belt, these results suggest that the Zimbabwe and Kaapvaal cratons did not form a coherent unit (Kalahari) until ca. 2.0 Ga.     

Precise U–Pb dating of Paleoproterozoic mafic dyke swarms of the Dharwar craton,India: Implications for the existence of the Neoarchean supercraton Sclavia

We report seven high precision U–Pb age determinations for mafic dykes from a number of major Precambrian swarms located in the Dharwar craton, south India. These new age results define two previously unrecognized widespread Paleoproterozoic dyking events at 2221–2209 and 2181–2177 Ma, and confirm a third at 2369–2365 Ma. Three parallel E–W trending mafic dykes from the petrographically and geochemically variable Bangalore dyke swarm, the most prominent swarm in the Dharwar craton, yield indistinguishable U–Pb baddeleyite ages of 2365.4 ± 1.0, 2365.9 ± 1.5 and 2368.6 ± 1.3 Ma, indicating rapid emplacement in less than five million years. A compilation of Paleoproterozoic U–Pb ages for mafic magmatic events worldwide indicates that the 2369–2365 Ma Bangalore dyke swarm represents a previously unrecognized pulse of mafic magmatism on Earth.     

Two cycles of voluminous pyroclastic volcanism and sedimentation related to episodic granite emplacement during the late Archean: Eastern Yilgarn Craton,Western Australia

The thick piles of late-Archean volcaniclastic sedimentary successions that overlie the voluminous greenstone units of the eastern Yilgarn Craton, Western Australia, record the important transition from the cessation in mafic-ultramafic volcanism to cratonisation between about 2690 and 2655 Ma. Unfortunately, an inability to clearly subdivide the superficially similar sedimentary successions and correlate them between the various geological terranes and domains of the eastern Yilgarn Craton has led to uncertainty about the timing and nature of the region's palaeogeographic and palaeotectonic evolution. Here, we present the results of some 2025 U–Pb laser-ablation-ICP-MS analyses and 323 Sensitive High-Resolution Ion Microprobe (SHRIMP) analyses of detrital zircons from 14 late-Archean felsic clastic successions of the eastern Yilgarn Craton, which have enabled correlation of clastic successions. The results of our data, together with those compiled from previous studies, show that the post-greenstone sedimentary successions include two major cycles that both commenced with voluminous pyroclastic volcanism and ended with widespread exhumation and erosion associated with granite emplacement. Cycle One commences with an influx of rapidly reworked feldspar-rich pyroclastic debris. These units, here-named the Early Black Flag Group, are dominated by a single population of detrital zircons with an average age of 2690–2680 Ma. Thick (up to 2 km) dolerite bodies, such as the Golden Mile Dolerite, intrude the upper parts of the Early Black Flag Group at about 2680 Ma. Incipient development of large granite domes during Cycle One created extensional basins predominantly near their southeastern and northwestern margins (e.g., St Ives, Wallaby, Kanowna Belle and Agnew), into which the Early Black Flag Group and overlying coarse mafic conglomerate facies of the Late Black Flag Group were deposited. The clast compositions and detrital-zircon ages of the late Black Flag Group detritus match closely the nearby and/or stratigraphically underlying successions, thus suggesting relatively local provenance. Cycle Two involved a similar progression to that observed in Cycle One, but the age and composition of the detritus were notably different. Deposition of rapidly reworked quartz-rich pyroclastic deposits dominated by a single detrital-zircon age population of 2670–2660 Ma heralded the beginning of Cycle Two. These coarse-grained quartz-rich units, are name here the Early Merougil Group. The mean ages of the detrital zircons from the Early Merougil Group match closely the age of the peak in high-Ca (quartz-rich) granite magmatism in the Yilgarn Craton and thus probably represent the surface expression of the same event. Successions of the Late Merougil Group are dominated by coarse felsic conglomerate with abundant volcanic quartz. Although the detrital zircons in these successions have a broad spread of age, the principal sub-populations have ages of about 2665 Ma and thus match closely those of the Early Merougil Group. These successions occur most commonly at the northwestern and southeastern margins of the granite batholiths and thus are interpreted to represent resedimented units dominted by the stratigraphically underlying packages of the Early Merougil Group. The Kurrawang Group is the youngest sedimentary units identified in this study and is dominated by polymictic conglomerate with clasts of banded iron formation (BIF), granite and quartzite near the base and quartz-rich sandstone units containing detrital zircons aged up to 3500 Ma near the top. These units record provenance from deeper and/or more-distal sources. We suggest here that the principal driver for the major episodes of volcanism, sedimentation and deformation associated with basin development was the progressive emplacement of large granite batholiths. This interpretation has important implication for palaeogeographic and palaeotectonic evolution of all late-Archean terranes around the world.     

Paleomagnetic study of NeoArchean–Paleoproterozoic dykes in the Kaapvaal Craton

431 oriented samples were collected from 27 dolerite dykes at 17 sites, belonging to 2.95, 2.65, and 1.90 Ga swarms, that trend SE, E and NE, respectively from the Bushveld Igneous Complex into the eastern Kaapvaal Craton (ages determined by Olsson et al., 2010; Olsson in Söderlund et al., 2010). Samples were analyzed for paleomagnetism and also anisotropy of magnetic susceptibility (AMS). For the 2.95 Ga SE-trending dykes high temperature/coercivity ‘P’ component has unblocking temperatures up to 590 °C and coercivity 40–90 mT and demonstrate SSW declination and intermediate positive inclination. Based on positive contact and conglomerate tests we argue for a primary origin of this component. The paleopole (BAD), calculated from ‘P’ component, does not correspond to any of the previously obtained Archean–Paleoproterozoic paleopoles for the Kaapvaal Craton, and represents a new key pole for 2.95 Ga. The high-coercivity ‘H’ component for the 2.65 Ga-old E-trending dykes has a SSW declination and steep positive inclination. Paleomagnetic pole (RYK), recalculated from this component, is close to the paleopoles, obtained by Wingate (1998) and Strik et al. (2007) for 2.78 Ga Ventersdorp volcanics. The third group, NE-trending dykes of the 1.90 Ga Black Hill swarm demonstrate an ‘M’ component with dual polarity high-coercivity component with SSE-declination and negative intermediate inclination. The paleopole (BHD), calculated from this component is close to the 1.87 Ga pole of the Kaapvaal Craton obtained by Hanson et al. (2004). Overprint directions include a very well developed thermo-chemical overprint (Dec = 329° Inc = −36°), which is believed to be associated with a ∼0.18 Ga regional ‘Karoo’ thermal event.     

黄龙洞年轻石笋的~(210)Pb测年研究

利用210Pb测年法对来自亚洲季风系统相互作用的青藏高原边缘区、高海拔的黄龙洞两根年轻石笋进行了测年研究,数据分析结果显示黄龙洞石笋顶部的210Pb放射性活度随距离呈指数衰变,衰变的趋势逐渐减缓,表明所研究的石笋中含有过剩的210Pb,样品顶部年龄小于100a。对黄龙洞石笋过剩210Pb数据进行非线性拟合,计算出两根石笋的平均沉积速率分别为0.104mm/a和0.143mm/a,与利用230Th测年法得到的沉积速率在定年误差范围之内一致,表明黄龙洞的210Pb测年数据是可靠的,通过计算石笋的沉积速率,可以建立近百年的石笋记录。研究表明,利用210Pb测年方法可以判断石笋(或石笋顶部)是否为100a内沉积的年轻石笋,弥补230Th法测定年轻石笋年龄的不足,有利于建立更加精确的石笋年代标尺,对探讨洞穴现代沉积记录和现代气象数据之间的相关性,明确洞穴石笋δ18O值所指示的气候环境含义具有重要意义。     

Penglai Zircon Megacrysts: A Potential New Working Reference Material for Microbeam Determination of Hf–O Isotopes and U–Pb Age

We introduce a potential new working reference material – natural zircon megacrysts from an Early Pliocene alkaline basalt (from Penglai, northern Hainan Island, southern China) – for the microbeam determination of O and Hf isotopes, and U–Pb age dating. The Penglai zircon megacrysts were found to be fairly homogeneous in Hf and O isotopes based on large numbers of measurements by LA-multiple collector (MC)-ICP-MS and SIMS, respectively. Precise determinations of O isotopes by isotope ratio mass spectrometry (IRMS) and Hf isotopes by solution MC-ICP-MS were in good agreement with the statistical mean of microbeam measurements. The mean δ¹⁸O value of 5.31 ± 0.10‰ (2s) by IRMS and the mean ¹⁷⁶Hf/¹⁷⁷Hf value of 0.282906 ± 0.0000010 (2s) by solution MC-ICP-MS are the best reference values for the Penglai zircons. SIMS and isotope dilution-TIMS measurements yielded consistent ²⁰⁶Pb/²³⁸U ages within analytical uncertainties, and the preferred ²⁰⁶Pb/²³⁸U age was found to be 4.4 ± 0.1 Ma (95% confidence interval). The young age and variably high common Pb content make the Penglai zircons unsuitable as a primary U–Pb age reference material for calibration of unknown samples by microbeam analysis; however, they can be used as a secondary working reference material for quality control of U–Pb age determination for young (particularly < 10 Ma) zircon samples.