【作者】 于涛；

【导师】 王宗起； 马昌前；

【作者基本信息】 中国地质大学 ， 矿物学、岩石学、矿床学， 2021， 博士

【摘要】 华南板块是欧亚大陆东南部的重要组成部分,由扬子与华夏地块组成,二者之间的江南造山带是理解华南构造演化、岩浆过程及成矿效应的关键。关于扬子与华夏地块的最终拼合时间与机制,以及扬子地块东南缘早古生代沉积盆地的构造属性,有多种模型被提出,主要基于火成岩与变质岩研究,而对沉积岩的研究有限。本文以浙西北地区下古生界为研究对象,进行详细的沉积相解剖,分析其沉积环境时空演化规律,重建扬子地块东南缘早古生代古地理格局,对盆地内沉积岩进行碎屑组分、砂岩重矿物组合、全岩地球化学、碎屑锆石U-Pb年代学及Hf同位素研究,结合盆地内发育的多层斑脱岩的年龄及性质,讨论沉积物的物质组成,源区方向、性质及构造背景;综合前人的区域地质研究,探讨盆地原型,从而为华南板块的构造演化提供证据。沉积特征表明,浙西北地区的寒武系-中奥陶统,盆地内广泛发育正断层,以发育碳硅质岩、碳酸盐岩浊积岩、滑塌堆积为特征,表明此时为深水斜坡环境;上奥陶统黄泥岗组、砚瓦山组发育大量碎屑流成因的砾岩及瘤状灰岩,长坞组则发育代表深海环境的巨厚海底扇沉积,上述特征均反映了强烈的构造活动以及盆地持续拉张的状态。上奥陶统文昌组-下志留统发育潮坪相及浅海陆棚相沉积,至中上志留统,发育三角洲沉积。沉积相时空展布及古流向表明,盆地整体呈现出SE高NW低的古地貌特征,物源来自SE方向。岩石学特征表明,研究区砂岩样品主要由棱角状-次棱角状的岩屑(杂)砂岩及长石岩屑砂岩组成,成熟度低,为近源快速堆积形成;岩屑成分主要由流纹岩、安山岩、花岗岩、石英片岩、板岩、千枚岩、粉砂岩以及燧石组成,与砾岩成分一致。重矿物组合显示以金红石、电气石、铬铁矿为主,同时出现少量石榴子石、磁铁矿、辉石、锐钛矿、榍石。地球化学特征表明沉积分选及再循环作用未影响到样品组分,岩石微量元素主要受到母源区性质及构造背景控制,研究区物源主要为中性-长英质的岛弧环境下的岩石,以及少量镁铁质岩石。上述碎屑岩组成特征共同表明物源主要为中酸性火山岩,以及少量镁铁质岩石、变质岩、沉积岩,源区构造背景为岛弧。碎屑锆石U-Pb测试结果显示438Ma及818Ma的主峰年龄,东南方向的浙江中部为主要物源区,陈蔡群、河上镇群、双溪坞群及八都群为研究区提供了主要的碎屑物质。结合区域地质资料,438Ma的峰值年龄来自双溪坞岛弧、陈蔡增生杂岩及侵入其中的镁铁质岩石。古地理特征及物源分析结果表明,浙西北地区早古生代为一弧后盆地,经历了先持续拉张,经巨厚海底扇等碎屑物质堆积填满之后,逐步萎缩的过程。盆地与双溪坞弧、陈蔡增生杂岩及其中弧相关镁铁质侵入体配套,证明早古生代扬子地块东南缘为一活动大陆边缘。上述认识为华南板块早古生代的构造演化提供了新的资料与思路。更多还原

【Abstract】 The South China Block forms part of the south-eastern segment of the Eurasian continent,and the Jiangnan Orogen separates the Yangtze Block from the Cathaysia Block to the southeast.As a tectonic-magmatic-metallogenic belt,the Jiangnan Orogen is the key to understand the tectonic evolution,magmatic process and mineralization effect of the South China.Several competing models have been proposed for the evolution of both blocks and the early Paleozoic sedimentary basin in the SE margin of Yangtze Block,which mainly focus on aspects of igneous and metamorphic rocks,limited research has been conducted on the sedimentary rocks in this region.In this paper we take the lower Paleozoic in NW Zhejiang as the research object to explain the sedimentary facies in detail,analyse the temporal and spatial evolution of the depositional environment,reconstruct the paleogeographic pattern of the SE margin of the Yangtze Block.Provenance of the late Ordovician sedimentary rocks in the northeastern Jiangnan Orogen has been constrained based on systemic analyses of petrology,heavy mineral assemblages,geochemistry,detrital zircon U-Pb ages and Hf isotope,combined with the properties of the multi-layered bentonite developed in the basin,were used for joint discussion the direction,distance,property and tectonic background of the provenance of the sediments.The above studies combined with previous regional geological research were used to explore the prototype of the basin and provide vital clues for reconstructing the evolution of South China Block during the early Paleozoic.Sedimentary characteristics show that in the Cambrian-middle Ordovician of the NW Zhejiang,normal faults are widely developed in the basin,the formation is characterized by carbonaceous siliceous rocks,carbonate turbidite and slump accumulation,indicating the slope environment of deep water at this time.The Huangnigang and Yanwashan formations of the upper Ordovician developed a large number of conglomerates and nodular limestones of detrital flow origin,and the Changwu Formation developed huge submarine fan deposits representing the deep-sea environment.These characteristics represent strong tectonic activity and continuous basin extension of the basin.Tidal flat and shallow marine facies were developed in Wenchang Formation of the upper Ordovician-lower Silurian,and delta sedimentary facies were developed in the middle-upper Silurian.The temporal and spatial distribution of sedimentary facies and paleo-current direction indicate that the basin is characterized by high SE and low NW palaeogeomorphology,and the provenance is from SE direction.The characteristics of petrology indicate that the sandstone contains lithic graywackes and feldspar-lithic graywackes,with low compositional and textural maturities,which is the result of rapid accumulation from near sources.The lithic fragments are angular,including mainly rhyolites,andesites,granites,quartz-schist,slates,phyllites,siltstones,and chert,which also occur in conglomerate form.Minor rutile,tourmaline,anatase,sphene,pyroxene,chromite,magnetite,and garnet grains occur in the sandstone.Geochemical analyses show that the composition of the samples has not been affected by sedimentary sorting and recycling,the trace elements of the clastic rocks are mainly controlled by the property of the provenance and tectonic background,and the provenance in the study area is mainly medium-felsic with some mafic rocks in the island arc.The composition characteristics of the clastic rocks above indicate that the provenance is mainly intermediate-acidic volcanic rocks with a small amount of mafic rocks,metamorphic rocks and sedimentary rocks,and the tectonic background of the provenance was an arc setting.U-Pb ages of detrital zircons demonstrate a mixed source from ca.818 Ma and ca.438 Ma,the central Zhejiang in the southeast direction is the main provenance area,and the Chencai,Heshangzhen,Shuangxiwu and Badu groups provide the main detrital materials to the study area.Combined with regional geological data,the peak age of438 Ma comes from Shuangxiwu island arc,Chencai accretionary complex with the mafic intrusions.The palaeogeographic characteristics and provenance analysis results show that the NW Zhejiang region was a back-arc basin in the early Paleozoic,which first experienced continuous extension,and then gradually shranked after being filled with clastic materials such as huge submarine fans.The basin was matched with the Shuangxiwu arc,the Chencai accretion complex and arc-related mafic intrusions,which demonstrate that early Paleozoic sedimentary deposition occurred in an active continental margin along the southeastern Yangtze Block.This understanding provide new data and ideas for the tectonic evolution of South China Block in the early Paleozoic.更多还原