华北地块南部及东部中新元古代地层序列中钾质斑脱岩等火山沉积物的确认及其测年研究

This project will focus on the recognition of K-bentonites and other volcanic deposits in the typical Meso- and Neoproterozoic succession from the southern and eastern parts of the North China Block (NCB). The petrology, mineralogy, and petrochemical compositions of these deposits will be studied systematically. Further, magmatic zircons and other possible uranium-rich phenocrysts from these volcanic deposits will be thoroughly analyzed and include: in situ high-precision U-Pb dating, Hf-isotope study and related REE fingerprints, as well as the components of some mineral inclusions. Consequently, two areas of crucial achievement can be expected: .First, the eruption ages of the K-bentonites and other related volcanic deposits will be precisely dated, which may necessitate a further revision in the conventional chronostratigraphic subdivision of Meso- and Neoproterozoic successions from southern and eastern parts of the NCB, in order to establish a new and more precise litho- and chronostratigraphic framework across the middle and east NCB for comparison with the recently revised framework in the Yanshan Mountains, northern NCB. Obviously, it will also provide more precise chronostratigraphic constraints for the NCB in the global paleo-continental reconstruction during the Meso- and Neoproterozoic Era, which is roughly correlated with the stages of break-up of the supercontinent Columbia (~1.8Ga) to the assemblage of Rodinia (~1.0Ga)..Second, the nature of the parental magma, as well as the tectono-magmatic setting of these volcanic deposits will be reasonably determined. Furthermore, combined with ash-distribution potential and the likely flexural-response information from the depositional succession on the southern and eastern NCB, such as regional unconformities and black-shales, we will infer the possible location or orientation of the volcanic vent and the related volcanic arc, as well as the location of ancient orogens along the Proterozoic margin of NCB. Hence, constrained by a revised Meso- and Neoproterozoic chronostratigraphic framework from this study, regional tectonic signals from the depositional successions will more clearly relate the basin-range coupling on the NCB to the timing and organization of Meso- and Neoproterozoic supercontinent cycles.

本项目拟在分析其岩石矿物学及地球化学特征、特别是稀土及微量元素示踪基础上，识别华北南部及东部中新元古界典型剖面中可能存在的钾质斑脱岩等火山沉积物，精选锆石等富铀矿物斑晶，开展以U-Pb年龄为核心的高精度微区地质测年。这将精确标定华北南部和东部中新元古界关键层位的地质年代，确立其与燕山地区近年已厘定地层序列的对比关系，建立新的华北中东部中新元古代年代地层格架。在此基础上，通过分析锆石斑晶的Hf同位素组成、矿物包裹体成分等，确认这些火山沉积物的母岩岩浆类型及来源、构造-岩浆背景等；结合其时空分布趋势、相关沉积序列可能存在的板块负载效应（flexural -response）的共生组合特征，探讨重塑该时期华北地块沉积-构造古地理格局及演化。这将从"盆山耦合"的角度，为此间华北地块与Columbia裂解-Rodinia聚合过程关系等重大命题，初步剖析其有效的构造-岩浆活动沉积响应信息。

项目学术目标首先是在华北南部及东部原“中新元古界”及燕山地区未获精确定年的有关层段中，识别钾质脱班岩等火山沉积物并获其锆石U-Pb测年，厘定并建立新的年代地层格架；同时通过上述火山沉积物地球化学组成、锆石Hf 同位素等分析，结合沉积序列成因分析，揭示该阶段华北地块与Columbia-Rodinia 超大陆演化之间关系。目前已取得的主要进展包括：（1）在华北南缘不同剖面上，识别并获得汝阳群洛峪口组层凝灰岩锆石高精度U-Pb测年均为~1620Ma，在验证笔者等稍早工作基础上，更精确地标定了该组及其余下伏汝阳群的时代应属于国际古元古代固结纪末期。同时还可确认，中条山地区汝阳群北大尖组以Tappania为代表的具刺大型疑源类生物群的时代应为~1650Ma，为目前全球真核生物遗存最早出现层位。这对于早期生命演化研究具有十分重要的科学意义；（2）在华北东部原“新元古界”徐淮群中部第一次发现并确认了斑脱岩等火山沉积物夹层，并获得了~1025Ma锆石U-Pb测年，直接标定了该群主要层段的形成时代为中新元古代之交。与格林威尔造山运动相对应的此期火山-岩浆活动，实为Rodinia超大陆聚合在华北地台的直接响应，并指示华北与扬子等古陆应同属该超大陆的一部分；（3）蓟县剖面雾迷山组第三段斑脱岩的识别与测年（~1485Ma）是燕山地区的第一次；其铁岭组斑脱岩（~1440Ma）及高于庄组层凝灰岩（~1580Ma）测年，则在精确标定该剖面中元古界巨厚碳酸盐岩序列形成时代的同时，进一步夯实其华北中元古界标准剖面的科学性；（4）京津地区大红峪组、团山子组碱性火山岩及侵入串岭沟组基性岩脉的系统分析表明，其时代为1635~1625Ma，微量元素组成与OIB相似，锆石Hf同位素显示弱亏损特征。表明其原始岩浆可能起源于同一软流圈地幔源区的低程度部分熔融，并经历强烈结晶分异，应与哥伦比亚超大陆裂解相关；（5）根据上述进展重新厘定的华北和扬子中元古代年代地层格架，可涵盖GTS2012新建议中元古代（1780-850Ma）。鉴于其“中元古代”只划分一个“罗迪尼亚纪”，提出可根据该阶段一系列可能影响全球各层圈的地幔羽-LIPs事件沉积纪录、将其划分为更易操作的“一代（界）四纪（系）”的中国方案，通过开展其“系”级单位全球界线层型“金钉子”工作，可从地球系统科学角度推动中国中元古代及整个前寒武纪研究的飞越发展。