Petrogenesis and tectonic implications of Neoproterozoic, highly fractionated A-type granites from Mianning, South China

The origin of Neoproterozoic intrusions (ca. 860–750 Ma) along the western part of the Yangtze Craton has been the subject of debate in recent years, with two competing models proposed. The plume model argues for an extensional setting and emphasizes the role of a superplume in the Rodinia breakup, whereas the arc model argues for the presence of a subduction zone in the Yangtze Craton. As a contribution to this animated dispute, geochronologic and geochemical analyses have been carried out on the Mianning granite, which is the largest pluton (700 km²) in the northern Kangdian rift of the western Yangtze Craton. It is shown that the Mianning granites were emplaced at ca. 780 Ma and display highly fractionated feature (i.e., SiO₂ > 75 wt%; Eu/Eu* = 0.03–0.50; enrichment of K, Rb, Th, U, Zr, Hf, Y and REEs; depletion of Nb, Ta, Ba, Sr, P, Eu and Ti). They are metaluminous to strongly peraluminous (A/CNK = 0.93–1.55) and contain abundant perthite and minor alkali riebeckite and sphene, sharing the petrological and geochemical characters of A₂-type granites. Positive _Nd (t) (2.97–5.24) and zircon _Hf (t) (9.2–12.1) values are consistent with a derivation by partial melting of a relatively young crust formed about 1000–900 Ma. Given the general absence of A-type granites in arc settings, the Mianning A-type granites are suggestive of an anorogenic, crustal extensional environment for the western Yangtze Craton during the Neoproterozoic. The data presented in this study are therefore consistent with an intracontinental rift model, but are not sufficient to identify plume involvement in the Neoproterozoic magmatism.     

高通量深海海水原位采样及分级过滤系统

深海悬浮颗粒物((含浮游微生物)的研究具有重要意义,但深海大洋中多数海域悬浮物的丰度很低,其研究需要较多水样(一次过滤水样超过200L,甚至达到1000L以上),传统方法用采水器采水后甲板过滤费时费力。基于此,本文研制了一种高通量深海海水采样及分级过滤装置,装置包括总成深海泵、过滤装置、数据采集与控制装置,以及支撑架体、供电部件、连接机构和流量计等辅助部件。过滤装置分三级过滤(分别放置0.22、1和5μm孔径的滤膜,并可根据需要更换组合),控制装置采用深度触发和时间触发两种工作模式,并可配套安装CTD、荧光计等海洋环境参数测量设备。针对调查站点,将若干套(如3套)高通量深海海水采样及分级过滤系统分一定间隔悬挂于深海绞车缆线上,可以同步进行相应水层的海水原位过滤,从而形成多水层同步高通量原位多级过滤,以获取浮游微生物等悬浮颗粒物样品。该原位取样系统具有多层同步高通量过滤和分级采样的特点,现已被应用于海洋科考航次中,为深海悬浮颗粒物(含浮游微生物等)的研究提供了一种简约高效的样品获取方法。     

东南亚红土镍矿床地质地球化学特征及成因探讨 ——以印尼苏拉威西岛Kolonodale矿床为例

Kolonodale矿床是东南亚红土镍矿带上一处典型矿床,位于印度尼西亚苏拉威西岛东部。矿床产自富镁超基性岩红土风化壳,矿化剖面自上而下出现红土层→腐岩层→基岩层垂向分带。红土镍矿石产在红土剖面上部,载镍矿物是铁质氧化物。硅镁镍矿石产在红土剖面中下部,载镍矿物为镍蛇纹石、镍滑石等含水层状硅酸盐矿物。地球化学分析显示,沿矿化剖面元素地球化学分异非常显著,Ni次生富集效应明显。超基性岩红土化过程的元素地球化学行为具有多样性,Fe、Al、Ti、Cr属残留富集组分,Si、Mg属淋滤缺失组分,Mn、Ca、Co、Ni属次生富集组分。通过典型矿床对比,Kolonodale矿床属原地自生硅酸盐型红土镍矿床,其发育受地质背景和地表环境条件的综合制约。富镁超基性岩、良好的构造组合、稳定的大地构造环境、湿热热带雨林气候、有利地形地貌等均是成矿有利条件。综合分析认为,Kolonodale矿床的成矿过程可划分为腐岩化、红土化和次生富集3个成矿阶段。     

Early Cretaceous gabbroic complex from Yinan,Shandong Province: petrogenesis and mantle domains beneath the North China Craton

Sensitive high resolution ion microprobe (SHRIMP) zircon U–Pb ages, geochemical and Sr-Nd-Pb isotopic data are reported for the gabbroic complex from Yinan (Shandong Province) with the aims of characterizing the nature of the Mesozoic mantle beneath the North China Craton. The Yinan gabbros contain alkali feldspar and biotite, and are characterized by moderate Mg#, high SiO₂, low FeO and TiO₂ contents and a strong enrichment of light rare earth elements [(La/Yb)_n=11–50], but no Eu anomaly. They have low Nb/La (0.07–0.29), radiogenic ⁸⁷Sr/⁸⁶Sr (0.710) and unradiogenic _Nd(t) (–15 to –13). These crustal fingerprints cannot be attributed to crustal contamination, given the lack of correlation between isotopic ratios and differentiation indices and the unreasonably high proportion of crustal contaminant (>20%) required in modeling. Instead, compositional similarities to contemporaneous basalts from nearby regions imply that the Yinan gabbros were not significantly affected by crystal cumulation. Isotopic data available for the Mesozoic mafic magmas reveal two distinct mantle domains beneath Shandong. While the EM1-like domain (with low ⁸⁷Sr/⁸⁶Sr) is confined to western Shandong, the mantle beneath eastern Shandong is dominated by EM2-type (with high ⁸⁷Sr/⁸⁶Sr) affinities. This aerial distinction suggests that the EM2-like signature of the Yinan gabbros may have been inherited from westerly-subducted Yangtze crust during the Triassic North China-South China collision. Emplacement of the Yinan gabbros (127 Ma) is likely affiliated with the widespread and protracted extension during the late Mesozoic in this region.     

Sources and Processes Affecting Nitrate in a Dam-Controlled Subtropical River,Southwest China

Excess nutrient (N and P) loads are recognized as the major cause of serious water quality problems in China. River systems play a very important role in nitrate (NO₃ ^?) transportation and transformation in the aquatic environment. To understand and clarify the sources and processes affecting NO₃ ^? in river basins, we have examined spatial and temporal variations of concentration and dual-isotopic composition of NO₃ ^? in the dam-controlled Jialing River, a major tributary of the Yangtze River where land use is dominated by agriculture. Water samples were collected in July 2008 and February 2009 from the main channel of the Jialing River and its major tributaries. The δ¹⁵N and δ¹⁸O of NO₃ ^? range from 1.5 to 11.0 ‰ (average 6.2 ‰) and ?5.0 to 11.1 ‰ (average, 1.6 ‰), respectively. NO₃ ^? isotope data and δ¹⁸O of water interpreted in combination with hydrological and chemical data suggest that most of the NO₃ ^? input is from nitrification during the rainy season, and discharge of sewage and manure in the upper course and from cities accounts for much of the NO₃ ^? load during the dry season. The construction of cascade dams has led to retention of Si and a decrease in the Si/N ratio, implying that assimilation and/or denitrification may significantly affect NO₃ ^? in the dam area, as demonstrated by NO₃ ^? and dissolved Si concentrations, and \(\updelta^{ 1 5} {\text{N}}_{{{\text{NO}}_{3} }}\) and \(\updelta^{ 1 8} {\text{O}}_{{{\text{NO}}_{3} }}\) values. This study indicates that dual-isotopic data can be used to identify NO₃ ^? pollution sources and the processes NO₃ ^? has undergone during its retention and transport in the watershed of the dam-controlled Jialing River.     

利用地震背景噪声成像技术反演三峡库区及邻近地区地壳剪切波速度结构

本文利用三峡库区重庆遥测台网及邻近地区台网2012年1月至12月期间的连续波形记录,通过噪声互相关,提取了这些台站间的经验格林函数.并通过获取的经验格林函数瑞利面波频散信息,反演出该区域5~40 s瑞利面波相速度分布.周期5 s及10 s的相速度分布结果显示：四川盆地具有较厚的沉积盖层,其中盆地中部沉积层相对偏浅.而周期20 s及35 s的相速度分布结果显示：中下地壳的扬子克拉通整体呈现较高的相速度分布,具有相对坚硬的中下地壳结构.这其中吉首—常德的武陵山区附近出现明显高速异常,可能与江南古陆古火山岛链（雪峰—九岭弧形古火山群）活动有关,火山活动导致大量的基性-超基性幔源物质侵入地壳,造成了武陵山重力异常,并一定程度上阻挡了云贵高原和湘鄂西部的物质向南逃逸;在地形上,此处恰好对应我国地形第二阶梯向第三阶梯的过渡带,并将华南地块分隔为东西两大部分.最后我们在瑞利面波相速度成像的基础上,反演出该区域三维剪切波速度分布,结果显示四川盆地周边的龙门山、米仓山、大巴山、七曜山和大娄山等地质构造与地壳剪切波速度结构具有明显对应关系.研究结果有助于深入研究该区的深部地质构造特征与地震发生机理.     

地震叠前逆时偏移算法的CPU/GPU实施对策

相较于单程波偏移算法而言,逆时偏移成像方法以其物理基础为依托优势,几十年来一直备受国内外地球物理学家的青睐.目前的逆时偏移(RTM)若直接采用双程波动方程进行延拓,尽管可以回避上下行波的分离处理,然就已有算法而言,其计算量和I/O(输入/输出)量却是最大的.针对此问题,本文在分析现行逆时偏移的多种算法基础上,提出利用CPU/GPU(中央处理器/图形处理器)作为数值计算核心,建立随机边界模型,从而克服存储I/O难题和提高计算效率.在实际的数据测试中,本文的方法可以大幅度的提高计算效率和减少存储单元,从而促使其高效地应用于生产实际.