【作者】 李波；

【导师】 颜佳新；

【作者基本信息】 中国地质大学 ， 古生物学与地层学， 2013， 博士

【摘要】 中二叠世是地史时期中的一个关键时段,峨眉山大火成岩省在此时喷发,导致了古-中生代之交双幕式生物大灭绝启动。在中二叠世建立高分辨率年代标尺对了解生物灭绝和峨眉山玄武岩喷发的时间及原因均具有重要意义。然而,由于精确的放射性同位素年龄数据较少(唯一的精确锆石年龄来自于卡匹敦阶底界之下-37.2m的火山灰层),中二叠世年代标尺的精度相对于其它时代严重落后,急需提高。精确绝对年龄的缺乏,使其它定年方法的应用显得十分必要。近几十年来,随着对轨道参数变化影响地球气候变化理论(米兰科维奇理论)的认识和研究,在利用天文周期定年、建立高分辨率地质年代标尺方面取得了丰硕成果,而定年精度比传统放射性同位素定年更高。因此,本文选择了华南地区两个典型剖面,在岩石磁学、沉积学研究基础上,利用磁化率数据的频谱分析结果对中二叠统卡匹敦阶和沃德阶进行高分辨率天文周期标定。早二叠世,华南发生了晚古生代以来最大规模的海侵作用,为大型孤立台地型海洋环境,在地质历史时期极为罕见。本文对广西来宾铁桥剖面下-中二叠统碳酸盐岩进行了详细的岩石磁学研究。铁桥剖面早中二叠世碳酸盐岩的磁化率大部分为负值,表明抗磁性方解石占主导,顺磁性、亚铁磁性和反铁磁性矿物含量较少。岩石磁学实验结果表明,铁桥剖面样品中主要磁性矿物是顺磁性矿物(伊利石为主)以及少量磁铁矿、钛磁铁矿、赤铁矿和针铁矿。从底部到顶部,携磁矿物中硬磁组分(赤铁矿和针铁矿)所占比例逐渐增加,在上部的卡匹敦阶,完全以硬磁性矿物为主。在瓜德鲁普-乐平统界线附近,岩石磁学特征发生明显变化,磁化率先增大再减小,携磁矿物成分呈硬磁性矿物→软磁性矿物→硬磁性矿物的变化趋势,而携磁矿物含量先减少再增多,这些转变仅在界线上下大约4m的岩层内完成,与瓜德鲁普晚期海平面变化、古海水温度变化同步,可能指示中-晚二叠之交短暂(-0.1myr)的特殊地质事件。另外,在栖霞组底部深水相中发现大量草莓状黄铁矿,证实磁性矿物还原成岩作用的存在。基于详细的岩石磁学分析和生物地层工作,本文对铁桥剖面卡匹敦阶进行天文周期标定。铁桥剖面卡匹敦阶样品的磁化率大部分为负值(抗磁性),表明来宾地区在该段沉积期孤立于古陆,与卡匹敦期来宾地区古地理格局吻合。通过生物地层对比,我们认为卡匹敦阶上部牙形石J.altudaensis带中磁化率的突然增加与峨眉山玄武岩喷发有关,这次喷发事件向黔桂盆地输送了更多的碎屑物质。K-T曲线测试表明,样品中主要的磁性矿物为顺磁性矿物(伊利石)及少量磁铁矿。利用多窗谱法(MTM)和傅里叶变换法(FT)对原始质量磁化率(未平滑)进行频谱分析,提取出五个米兰科维奇周期，分别是：长偏心率周期(～405kyr)、短偏心率周期(-100kyr)、长地轴斜率周期(～44.Ikyr)、长岁差周期(～20.95kyr)和短岁差周期(～17.7kyr)。基于频谱分析结果,利用图形对比方法(Graphic Comparison),以半个E2周期(-200kyr)为单位在卡匹敦阶建立了高分辨率浮点年代标尺。计算出卡匹敦阶的时限为～3.85Ma,而整个沉积序列(包括沃德阶上部、整个卡匹敦阶和吴家坪阶下部)的平均沉积速率为～2.9cm/kyr。计算出卡匹敦阶内部七个牙形石带的时限,从最短的～10.3kyr到最长的～2.43myr。另外,根据峨眉山玄武岩喷发与卡匹敦阶顶部磁化率增大的对应关系,估算出峨眉山大火成岩省喷发的启动时间,为～262.67Ma,位于瓜德鲁普-乐平统界线之下～1.42myr,同时这也是古中生代之交双幕式生物大灭绝事件的启动时间。前人对广元上寺剖面沃德阶生物地层的研究比较精细,因此,在本次研究中选择了广元上寺剖面沃德阶作为天文周期定年对象。上寺剖面活德阶样品的磁化率同样绝大多数为负值,表明广元地区在沃德期孤立于古大陆,输送到沉积盆地中的陆源碎屑较少,自生抗磁性方解石主导了磁信号。K-T曲线测试表明，上寺剖面样品中主要磁性矿物为顺磁性矿物(伊利石)及少量磁铁矿。利用MTM和FT分析,从高分辨率磁化率数据中提取出五个米兰科维奇旋回,分别是：长偏心率周期(～405kyr)、长地轴斜率周期(～44.0kyr)、短地轴斜率周期(～35.0kyr)、长岁差周期(～20.9kyr)和短岁差周期(～17.6kyr)。通过图形对比方法,对整个沉积序列建立了分辨率为～200kyr的浮点年代标尺。计算出沃德阶的时限为～2.84Ma,整个沉积序列(包括罗德阶上部，整个沃德阶和卡匹敦阶下部)的平均沉积速率为。～1.65cm/kyr。更多还原

【Abstract】 Middle Permian is a critical period in geological history. The eruption of Emeishan large igneous province which was considered as the trigger of the Paleozoic-Mesozoic double mass extinction launched at this time. The establishment of high-resolution time scale in Middle Permian is all important for understanding the onset time and cause of the mass extinction and the eruption of Emeishan large igneous province. However, the precision of time scale for Middle Permian is seriously limited due to the lack of high precision radiometric ages (The only zircon age currently available, comes from an ash bed that～37.2m below the base of the Capitanian stage). According to the fact that it is difficult to obtain accurate absolute ages, it’s necessary to consider other dating methods. In recent decades, as the understanding and research of the theory that Earth’s orbital parameters can affect climate change, a great number of achievements have been achieved in aspects of dating and establishment of high-resolution geological time scale by astronomical cycles. In this study, a high resolution float point time scale(FPTS) has been established for the Wordian stage in Shangsi section and the Capitanian stage in Tieqiao section by astronomical cycles dating based on the research of sedimentology and rock magnetism.The largest transgression since Late Paleozoic had begun in South China during the Early Permian. That formed a rigorously isolated platform-type marine environment which was rare in the geological history. We investigated the characteristics of rock magnetism and sedimentology for Lower-Middle Permian carbonates at Tieqiao section, Guangxi area, South China. The magnetic susceptibility (MS) data sets of Tieqiao section are mostly negative, indicating the dominant magnetic components are diamagnetic calcite and chert, with subordinate paramagnetic, ferrimagnetic and antiferromagnetic minerals. Laboratory measurements on rock magnetism suggest that the magnetic minerals are mainly paramagnetic minerals (mainly are illite) with a small amount of ferrimagnetic minerals and antiferromagnetic minerals, including soft magnetic magnetite and titanomagnetite, hard magnetic hematite and goethite. The proportion of hard magnetic component increased from bottom to top in the section profile. Hard magnetic minerals are dominant in the upper part of the Capitanian stage. The characteristics of rock magnetism near the Guadalupian-Lopingian boundary showed a pronounced shift. The magnetic minerals are changed from hard magnetic minerals to soft magnetic minerals and then returned to hard magnetic minerals. The MS increased first and reduced later, but the content of magnetic particles showed a contrary tendency. All these transitions were finished in4m strata around the Guadalupian-Lopingian (G-L) boundary that synchronizes with the regressive and subsequent transgressive at Later Guadalupian, also the shift of paleotemperature of seawater. These transitions probably indicated a short-lived (0.1myr) special event at the G-L boundary. In addition, a large number of pyrite framboids were found in Lower Chihsia formation, confirming the existence of reduction of magnetic minerals.Astronomical cycles are used to dating the Capitanian stage based on detailed research of rock magnetism and biostratigraphic work at Tieqiao section. The MS data sets of Capitanian stage are mostly negative (diamagnetic), suggesting that the Laibin area was isolated from terrigenous sources during most of the Capitanian. A sudden increase in the MS signal at the top of J.altudaensis just below the Guadalupian-Lopingian (G-L) boundary is recorded. The coincidence of the increase with the onset of the Emeishan Large Igneous Province (LIP) eruptions argues strongly for volcanic effects controlling the MS in this part of the section. The thermomagnetic susceptibility measurements indicate that the dominant magnetic constituents in this study are paramagnetic and subordinate ferrimagnetite minerals. Spectral (time-series) analysis of the MS data was performed using Multi Taper Method (MTM) and Fourier Transform (FT) analysis. Five frequencies that are consistent with Milankovitch-band orbital forcing were compared to this data set:eccentricity E1and E2(～‖100and～405kyr, respectively), obliquity02(-44.10kyr), and precession P1(～17.70kyr) and P2(-20.95kyr). Interpretation of these results indicates that the duration for the Capitanian in the Tieqiao section was～3.85myr. In turn, the mean sediment accumulation rate (SAR) for the whole Capitanian was～2.9cm/kyr. In addition, graphic comparison of the Tieqiao MS data to a floating point time scale(FPTS) for the Upper Wordian throughout Capitanian time allows estimates of the duration for conodont zones identified from the section, these ranging from～10.3kyr to～2.43myr before the extinction of individual conodont species. In turn, timing of the onset age of Emeishan LIP eruptions was estimated, yielding an age of～262.67Ma, with the start of these eruptions at～1.42myr below the G-L boundary, and it is also the onset time of Paleozoic-Mesozoic mass extinction event.We chose the Wordian stage of Shangsi section in Guangyuan, Sichuan province as the object of astronomical cycles dating because biostratigraphic works were well done by previously researchers. The MS data sets of Wordian in Shangsi section also are mostly negative, suggesting that the Guangyuan area was isolated from terrigenous sources during most of the Wordian. Thermomagnetic susceptibility measurements indicate that the magnetic carriers in this study mainly are paramagnetic with subordinately magnetite. Spectral (time-series) analysis of the MS data was performed using MTM and FT analysis. Five frequencies that are consistent with Milankovitch-band orbital forcing were compared to this data set:eccentricity E2(～405kyr,～100kyr), obliquity01(-35.00kyr) and O2(～44.00kyr), and precession P1(～17.60kyr) and P2(-20.9kyr). A FPTS was built for Wordian of Shangsi section by graphic comparison, with resolution of～200kyr. Interpretation of spectral analysis indicates that the duration for the Wordian in the Shangsi section was～2.84myr. In turn, the mean sediment accumulation rate (SAR) for the whole section (from upper Roadian to lower Capitanian) was～1.65cm/kyr.更多还原