大兴安岭北段富克山岩浆弧的组成:对蒙古—鄂霍茨克洋南向俯冲的制约

富克山岩浆弧呈北东向分布于大兴安岭北段富克山—古莲河一带,其物质组成、形成时代及其空间展布规律对于研究蒙古—鄂霍茨克洋晚三叠世地质构造演化具有重要意义。本文对其内的辉长岩、闪长岩、花岗闪长岩进行了岩石学、地球化学及锆石U-Pb定年分析研究,探讨大兴安岭北段晚三叠世时期的构造背景。LA-ICP-MS锆石U-Pb定年获得辉长岩、花岗闪长岩的结晶年龄分别为(205.7±2.2) Ma、(203.2±2.5) Ma,反映了晚三叠世的构造岩浆事件。岩石学和地球化学研究表明,辉长岩、闪长岩具有富钠、高铝、高钙、高镁、高Mg^#值、低钛等特征,属于拉斑系列与钙碱性过渡系列岩石,无明显的Eu负异常,相对富集LILE和LREE,亏损HFSE。花岗闪长岩具有高硅、富钠、高铝、低镁等特征,属于钙碱性系列岩石,高Sr,低Y、Yb,无明显的Eu负异常,显示了O型埃达克质岩石的地球化学特征。富克山岩浆弧的空间展布,具有由北向南的分布规律,指示了蒙古—鄂霍茨克洋具有往南的俯冲极性。     

全球海洋硅循环及研究中面临的主要挑战

海洋硅循环是海洋生物地球化学循环的关键过程之一,对调控全球二氧化碳浓度、海洋酸碱度和多种元素(氮、磷、铁、铝等)的循环具有重要作用。在当今气候变化和人类活动影响日益增强的背景下,硅循环与“生物泵”及碳循环的紧密联系,是其成为地球科学领域研究热点的主要原因。海洋中硅的外部来源主要为河流、地下水、大气沉降、海底玄武岩风化作用和海底热液输送5个途径,在全球气温变暖趋势的影响下,极地冰川融化成为高纬度海域不可忽视的硅源。生物硅在沉积物中的埋藏、硅质海绵和生物硅的反风化作用是重要的海洋硅移除过程。海洋硅循环过程复杂,受生物(生物吸收、降解)、物理(吸附、溶解)和化学(矿化分解和反风化作用)多重因素的影响,针对海洋硅循环关键过程的研究有助于综合评估海洋硅的“源-汇”和收支。本文总结了海洋硅循环的主要过程及海洋硅的收支,根据国际和国内研究现状讨论了当前海洋硅循环研究中面临的主要问题和挑战。现有研究成果显示,海洋硅的外源输入和输出通量比以往的评估分别增加了2.4和2.2倍。在短时间尺度内(<8 ka),全球海洋中硅的收支大致平衡,海洋硅循环基本处于稳定状态。气候变化和人类活动导致河流输送至陆架边缘海的硅通量发生变化,可能影响硅藻等海洋浮游植物种群结构,是未来海洋硅循环研究需要关注的问题之一。陆架边缘海较高沉积速率和强烈的反风化作用提高了该区域生物硅的埋藏效率,准确评估该区域生物硅的埋藏通量仍是亟须解决的难题。目前的研究评估了全球海洋浮游硅藻、硅质海绵以及放射虫生产力,而海洋底栖硅藻生产力的贡献受到忽视,未来需要关注底栖硅藻对生物硅的贡献及其在海洋硅的生物地球化学过程中的作用。     

Source Lithology and Magmatic Processes Recorded in the Mineral of Basalts from the Parece Vela Basin

Since the Early Cenozoic, the Philippine Sea Plate (PSP) has undergone a complex tectonic evolution. During this period the Parece Vela Basin (PVB) was formed by seafloor spreading in the back-arc region of the proto-Izu-Bonin-Mariana (IBM) arc. However, until now, studies of the geological, geophysical, and tectonic evolution of the PVB have been rare. In this study, we obtained in situ trace element and major element compositions of minerals in basalts collected from two sites in the southern part of the PVB. The results reveal that the basalts from site CJ09-63 were likely formed via ~10% partial melting of spinel-garnet lherzolite, while the basalts from site CJ09-64 were likely formed via 15%–25% partial melting of garnet lherzolite. The order of mineral crystallization for the basalts from site CJ09-64 was olivine, spinel, clinopyroxene, and plagioclase, while the plagioclase in the basalts from site CJ09-63 crystallized earlier than the clinopyroxene. Using a plagioclase-liquid hygrometer and an olivine-liquid oxybarometer, we determined that the basalts in this study have high H2O contents and oxygen fugacities, suggesting that the magma source of the Parece Vela basalts was affected by subduction components, which is consistent with the trace element composition of whole rock.     

角闪石高温高压实验研究进展及其地球物理意义

高温高压实验作为地球科学研究的重要方向之一,通过模拟地球深部的温度和压力条件,了解地球深部物质的物理化学性质、地球内部结构和动力学演化。角闪石属于双链硅酸盐矿物,为地幔岩石圈的重要组成,广泛分布在海洋地壳、俯冲板块、变质岩和火成岩中。作为俯冲带的重要含水矿物,角闪石的广泛分布和高温高压下的脱水对于理解俯冲带水含量以及水迁移具有重要作用,同时在俯冲带的地震活动、高电导率异常、地震波速异常和岩浆活动中扮演重要角色。在过去的近百年时间里,国内外学者对角闪石高温高压物理化学性质进行了大量的研究。角闪石具有非常复杂的元素组成和结构特征,由此也导致了不同角闪石物理化学性质存在显著不同,包括脱水与脱羟基反应中元素迁移的差异、角闪石形成与分解过程中碱性元素(K+Na)和H2O含量对热稳定的影响、不同空间群结构下的高压结构相变、原位条件下不同结晶方向的电导率异常、不同结晶学优选方位(CPO)下的波速异常等。已有的研究对于角闪石的物理化学性质以及其在俯冲带中发挥的作用有了比较清楚的认识,但仍然有许多问题需要进一步研究,如角闪石的高压脱水动力学、热物性和变形机制等。     

试论现行盐湖卤水矿勘查规范的适用性——兼与钱学溥先生商榷

笔者等对我国盐湖矿床的勘查工作及盐湖矿产勘查规范的制修订过程进行了梳理、总结,认为盐湖卤水矿作为一种液体矿床,水只是其载体,其本质属性是矿,在开展水文地质工作的同时,按照矿产的勘查方法查明卤水及共伴生矿产资源储量,开展综合评价是一种适合的勘查方法;盐湖卤水矿勘查规范的制定和修订是对多年来盐湖卤水勘查工作的总结和完善,符合卤水矿的勘查实际,勘查成果被矿山的设计及开采所验证,总体是一部适用的规范。钱先生提出的按照地下水的勘查、评价方法去勘查盐湖卤水矿产,忽略了卤水矿的属性及固液共生的特点,夸大了周边水补给对盐湖矿产的影响,提出的可采储量与矿山设计部门计算的可采储量的约束条件不同,故不能用淡水资源的评价方法去评价卤水矿。     

定量表征河流砂颗粒的形状——在西藏帕隆藏布的尝试

河流搬运体系中,颗粒的形状属性可以反映其来源、搬运、沉积的过程。长期以来沉积学界缺乏相对统一的定量的、多维的颗粒形状参数。本研究以西藏雅鲁藏布江支流帕隆藏布为例,运用图像分析、软件分析等计算机辅助技术,对8个天然河流砂样品中的2276个颗粒开展碎屑成分鉴定和形状参数量化等工作。重点评估了11个颗粒形状参数,通过因子分析等统计方法提取并定义了3个形状信号,包括圆形度因子、规则度因子和平滑度因子。这些参数可用于表征颗粒宏观形状上接近圆形的程度、中观尺度上轮廓规则的程度及轮廓平滑的程度。挖掘天然河流砂的重要形状特征后,进一步探索了这些形状参数与颗粒成分、粒度的关系。本研究成果为解释河流系统中颗粒的形状信号、理解颗粒自身微观属性和外部物理过程等提供重要借鉴。     

东北那丹哈达岭中—新生代构造-热演化史:来自(U-Th)/He和裂变径迹热年代学的证据

中国东北的那丹哈达岭地区位于中亚造山带最东部,它的中新生代热演化史是认识陆内造山活动的关键,但该地区相关研究比较薄弱,其中—新生代的热演化史缺乏有效的约束.因此本文应用磷灰石裂变径迹、锆石和磷灰石(U-Th)/He等多种低温热年代学方法,对东北那丹哈达岭地区的侵入岩开展构造热演化历史研究.热年代学数据和热史模拟结果表明,该地区存在早白垩世晚期—晚白垩世(110～80Ma)、古新世—始新世(60～40 Ma)两期快速冷却事件,其冷却速率分别为3.42～4.81℃/Ma和1.43～1.83℃/Ma.结合区域构造和应力分析,我们认为两期冷却事件均受构造活动控制.第一期快速冷却事件是古太平洋板块北西向俯冲引发的构造叠加到鄂霍特莫茨克地块并与东亚大陆边缘碰撞引起;而第二期快速冷却事件是古太平洋俯冲的板片后撤使东亚陆缘处于伸展环境,造成东北大面积的剥露作用引起.这次研究增强了对东亚陆缘中新生代构造-热演化历史的认识,对于理解大陆内部造山带的构造变形过程与机理具有重要意义.     

Zircon U-Pb ages,geochemistry, and Sr-Nd isotope ratios for early cretaceous magmatic rocks,southern Saqqez,northwestern Iran

Sanandaj-Sirjan Zone (SaSZ) is one of the most dynamic structural zones of Iran, which is divided into three main parts: Northern, Central and Southern. The northern SaSZ has been affected by deformation due to fault activities near the Zagros suture zone, and mylonitic structures have overprinted these rocks and was affected by three episodes of magma injection during the Permian-Carboniferous, Early Cretaceous and Cenozoic. In this study, the rock units investigated that have been considered Precambrian-Paleozoic basement on geological maps. This paper considers zircon U-Pb dating, whole-rock chemistry and Sr-Nd isotope ratios of Cretaceous magmatic rocks in the N-SaSZ to develop a new geodynamic model for the evolution of these magmatic rocks. The new zircon U-Pb ages obtained in this study show that the magmatic rocks crystallized at 115–107 Ma in the Early Cretaceous (Aptian-Albian) and are much younger than the supposed ages presented on geological maps. This complex classified into two main groups of basic-intermediate and acidic rocks based on SiO₂ contents. The whole-rock chemistry of the basaltic and andesitic rocks, which are interbedded with marine shallow-water sedimentary deposits, shows their typical calc-alkaline affinity and subordinate tholeiitic series on an active margin. The positive ε_Nd(t) of approximately +4 for some undifferentiated basalts with negative Ti and Nb anomalies shows the relation of these rocks to calc-alkaline magmatism and was generated by the partial melting of subcontinental lithospheric mantle (SCLM). Granitoid rocks with some affinity to the peraluminous group with a negative ε_Nd(t) value (-3.2) mainly and negative Ti and Nb anomalies plot in an active margin tectonic setting. Simultaneous mafic calc-alkaline volcanism and the generation of granitic intrusions in the Early Cretaceous could have occurred on an active margin. Due to the absence of Jurassic arc related magmatic rocks in northern SaSZ and presence of Cretaceous calc alkaline magmatic activity, which are not observed in the central SaSZ, support the idea that the subduction of the Neotethys beneath the northern SaSZ started in the Early Cretaceous.     

Distribution of contourite drifts on convergent margins: Examples from the Hikurangi subduction margin of New Zealand

Contourite drift systems form a significant component of the marine clastic sedimentary record. Although contourites form in all tectonic settings, few studies have described their development along convergent margins; such characterization is needed to underpin oceanographic and palaeoenvironmental studies in active settings. This study is the first to document contourite drift development along the Hikurangi subduction margin of New Zealand. Integration of bathymetric, seismic and well data enables five classes of drift to be recognized around the subduction wedge, occurring in three principal associations: (i) an upper slope drift association of giant elongate mounded (ca 150 km long, 50 km wide and up to 1100 m thick) and plastered drifts (ca 300 km long, 8 km wide and <600 m thick), which occurs upon and inboard of a major intrabasinal thrust‐cored high, whose long axis parallels the coast; shallow bottom currents disperse sub‐parallel to this axis; (ii) a spatiotemporally discontinuous association of confined and mounded hybrid drifts (ca 500 m long, <2 km wide and up to 500 m thick) that occurs along the mid‐to‐outer slope domain of the wedge, recording the interaction of along‐slope and downslope currents within trench‐slope basins; and (iii) a trench fill assemblage that implies the passage of abyssal bottom currents across a 40 km reach of the trench‐axial Hikurangi Channel‐levée, with associated modification of the channel form and of overbank sediment waves. The fundamental presence of contourites along this margin appears to depend on the orientation and strength of oceanographic bottom currents. However, drift type and evolution vary depending on the slope gradient and the presence of irregular seafloor topography created by tectonic structures. The documented drifts are generally smaller, less continuous, and develop more intermittently than similar styles of drifts documented on passive margins; this mode of occurrence may be characteristic of contourite development on convergent margins.     

华北克拉通丰镇碳酸岩中榴辉岩捕虏体岩石学研究:现今板块体制古元古代开始启动证据

关于现今板块构造体制何时启动是目前地球科学研究的焦点问题。本文在原报道的古元古代丰镇火成碳酸岩中发现的榴辉岩捕虏体基础上,开展了详细的岩石学研究。该榴辉岩捕虏体分为两种类型:即相对富石榴石的Fz-2和贫石榴石的Fz-16,它们产于同一地点,且具有相同的矿物成分和结构构造特征。Xu et al.(2018)的研究表明该捕虏体具有1839±26Ma和1766±7Ma的独居石U-Th-Pb年龄且具有大洋辉长岩原岩的全岩成分特征。本文通过进一步的岩相学研究发现该榴辉岩至少经历了两期变质阶段:M1,角闪石/绿帘石-榴辉岩阶段;M2,硬柱石-榴辉岩阶段。具有放射状裂纹包裹特征的柯英石假象在石榴石变斑晶和基质绿辉石中以包体形式出现。以蓝晶石与黝帘石共存为特征的柱状硬柱石假象,也偶尔以包体形式存在于石榴石中。变斑晶石榴石分为富含包体的核部和比较干净的边部。石榴石从中心到边部具有明显的镁铝榴石含量增加和钙铝榴石含量降低的环带特征,通过相平衡模拟和等值线投图得到其温压范围为2.6~3.7GPa和655~670℃,记录了从M1到M2的近等温增压的进变质过程。通过石榴石边部-绿辉石-蓝晶石-石英的地质温压计计算得到温压条件为3.0GPa、734℃。金红石中的锆含量温度计也给出了相似的温度条件,即在2.6~3.7GPa压力时为601~685℃。石榴石边部的柯石英假象和硬柱石假象支持了M2硬柱石-榴辉岩阶段的存在,这表明丰镇古元古代榴辉岩可能是目前发现的世界上最古老的低温超高压变质岩。同时,我们得到该榴辉岩代表的进变质过程中的地温梯度为216±35℃/GPa,证明至少在~1.8Ga以来代表现今板块构造体制的板块冷俯冲作用就开始启动了。