秦巴地区泥盆纪地层及重要含矿层位形成环境的研究

ON STRATIGRAPHY AND SEDIMENTARY ENVIRONMENT OF IMPORTANT ORE-BEARING HORIZON IN DEVONIAN PERIOD,QINLING-DABASHAN AREA,CHINA

泥盆纪时,奏岭古海盆是介于华北古陆和扬子古陆之间的一个复杂的陆表海,故应作独立地层区。根据基底性质、层序和沉积特征由北而南划为3个分区:北秦岭分区,以海陆交替相上泥盆统不整合于早古生代变质沉积、火山岩系之上;南秦岭分区,海相泥盆系由南而北,由西向东超覆于志留系或寒武、奥陶系之上;摩天岭分区,泥盆系不整合于碧口群不同层位之上。又根据分区内构造活动、古地理变化、沉积类型和相的分异,进一步划分了11个小区和11个相带。同时编制了包括39个自然单元的泥盆系划分、对比表。区域性生物带及层序的建立,基本上概括了9个主要化石门类在区内的时空分布。为本区各岩石地层单位时代的厘定及与区内外同类地层的对比,提供了重要依据。对泥盆系顶、底及系以下的界线进行了探讨,概述了3种不同类型的石炭、泥盆系界线的位置及特征,着重介绍了西秦岭志留泥盆系界线研究的新成果。根据区域生物群的分析结合海水进退规程,提出早泥盆世早、中期属华南西部亚区,早泥盆世晚期之后为华南东部亚区。并按照生物的生活方式,将本区划分为动、植物混生、浅海底栖生物及浮游生物3种类型,在浅海底栖生物类型内结合沉积物性质又划分为3个亚类。并以普通沟、当多沟及热尔至洛大为范例,分别代表区内纵向和横向上的群落交替,基本上反映了一个陆表海内的生态环境。沉积相的分析和划分,以现有的剖面为基础,总结了泥盆纪海侵由南向北,由西向东,沉降中心、沉积中心由南向北逐渐迁移,古海盆地形由南低北高到南高北低的变化和各时期岩相、古地理格局及各相带的特征,确认这一陆表海是加里东运动和印支运动两期强烈活动期间的一个相对稳定环境,具有过渡性质。秦岭在泥盆纪处于北纬低一中纬度。早、中泥盆世海盆收缩,古地磁南、北近于一致,晚泥盆世呈明显的东西向分带,西部开裂强烈,往东逐渐减弱,致使古海域呈现一向西扩展的漏斗状。与区域性地壳运动有关的地质事件发生于早泥盆世,即由早期的聚敛运动转向晚期的张裂运动,大规模的海退变为海侵,海平面由上升趋于下降、海水盐度由高变低气候、由干热变为湿热,生物由衰亡至兴盛。另一种属全球性事件,如弗拉斯与法门阶之间生物绝灭事件。秦岭地区泥盆纪古海盆演化的研究取得了如下新的认识:①泥盆纪古海盆是由残留海发展为陆表海;②加里东运动在秦岭各地块内表现为抬升活动,地块边缘有褶皱运动;③泥盆纪秦岭海盆内有3期由南而北逐渐迁移的沉降活动及由此形成的3个沉降带;④根据构造活动性、沉积作用、火山活动的差异,划分出断块沉积和断陷沉积两种不同的沉积-构造单元,其古构造格局由3个断陷带和2个断块相间组成;⑤秦岭泥盆纪古海盆的演化分为早期聚合阶段及晚期张裂阶段。秦岭泥盆系赋存有丰富的各类矿产,是以海底热水沉积作用为主导的区域成矿作用,以铅、锌、汞,锑、铁和金矿为主。本区根据热水沉积含矿建造划分了一主一次两个成矿期。并从成矿区划和矿产类型两方面对成矿远景和找矿方向进行了探索。

During Devonian Period, the old sea basin of Qinling range was a continental sea, which was situated between Northern China and Yangtz old land. Its stratigraphical features and paleotectonic properties are differ- ent from those of Northern China and Yangtz old land, therefore, it is an independent stratigraphical region. According to the properties of basement, stratigraphical sequences and sedimentary features, this region may be divided into three subregions: The first, Northern Qinling subregion with the Upper Devonian strata of paralic facies, being unconformable on the metamorphic sediments and igneous rock systems of Early Palaeozoic Era; the second, Southern Qinling subregion with the marine Devonian system overlaping on Silurian or Cambrian, Ordo- vician systems from south to north and west to east, respectively; the third, Motianling subregion with Devonian system being unconformity on the Bikou Group and its correspondence strata Furthermore, eleven little districts and 11 phase zones have also been subdivided in this region in the light of stru- ctural activity, change of palaeogeography, sedimentary type and phase. Devonian stratigraphic table containg 39 natural units for division and correlation has been made by the writers, which summarizes lithological characters and its thickness, fossil, age, correlation and naming etc. The establishment of region biozones and stratigraphic sequences basica- lly outlines time and space distribution of nine main fossil classes in the investigated region, which provides an important foundation for age determi- nation of each lithological stratigraphic unit and correlation with inside and outside of the region. The writers discuss the top and bottom of Devonian and the boundary under the Devonian system. The position and feature of three different types Carboniferous-Devonian boundary have been pointed out with emphasis on the new achievement of Devonian-Silurian boundary research. A new understanding that the early and middle period of Early Devonian Epoch belong to subregion in the west of Southern China and that they are closer to western Yunnan Province and northern Tibet (belonging to a subregion in the east of Southern China, Yangtz subregion after late of Early Devonian Period), is put forward by the writers according to the analysis of areal biota characters in combination with regulation of transgre- ssion and regression. The biota in the investigated region have been divided into mixture-lived types of animal and plant, benthos in shallow sea and plankton, according to the mode of living of biota. Three subtypes has been divided within benthos of shallow sea based on sedimentary characters there. Taking Putonggou, Dangduogou and Reer to Luoda as examples, they repre- sent longitudinal and horizonal biocoenose evolution respectively, which shows basically the ecological environment of continental sea. With respect to lithofacies and palaeogeography, analysis and division of sedimentary facies in the investigated region have been made systematic- cally on the basis of available profiles, and found that transgression during Devonian Period migrated from south to north and west to east, the centre in sedimentation and settle has been shifted from south to north, the topo- graphy of old sea basin had changed from lower level in south to higher level in north. It is concluded that Qinling range is located between south and north old land, being a continental sea in Devonian Period and under rela- tively stable stage in the interval of Caledonian movement and Indosinian movement, which denotes the characters of transitional type. Palaeomagnetic results show that Qinling range was located in lower- middle N° during Devonian Period, corresponding to those of Northern China and Yangtz block. The Qinling sea basin had been shrunken in Early and Middle Devonian Epoch, being in keeping with palaeomagnetism south to north approximately, but separate zone from east to west was presented obviously in Late Devonian Period, that is, a spolit occured between the north and south subregion within western Qinling range and Motianling subregion, differing by 8°—9°at latitude. The splitting degree in eastern Qinling where there is Motianling subregion and up to east of Baoji-Chengdu Railway was getting weak. A horizontal funnel shape that open faces toward west appea- red in whole Qinling old sea area. At present, two events can be distinguished in the investigated region, one is related with areal continental movement, taking place in early and middle period of Early Devonian Epoch as a result of the convergence of the block, large-scale regression, led to the dropping of sea level, transforing of the climate from damp and hotness into aridity, increasing salinity in the sea and biota becoming feeble and death. The block movement was transformed from covergence to splitting in late of Early Devonian Epoch, resulting in large-scale transgression, rising up in the sea level, dropping salinity in the sea and restoration of damp and hot climate at last, a lot of Yangtz types biota in association with the transgression were brought in Another event belongs to global character, for example, biota extiction event between Frasnian stage and Famennian stage. Various benthos and spore fossil were flourishing in western Qinling under gigas zone Suddenly, Anthozoa, Cephalopoda, Bivalvia, Trilobita and Tentaculita disppeared and genus of Brachiopoda decreased one half with extiction ratio of biota species being over 80%. Not only spore fossil declined, but also appeared intensive carbonization, perforation and coking crack. The writers infer that reson resulting in the event mentioned above was neither sea level nor climate change, but it may be more possible that it was related with the impact of a little planet to the earth. Evolution research for old sea basin in Qinling is made by may of analysis vicissitudinous sea area at different period, migration of sedimentary facies and sedimentary centre, movement of earth crust and volcanic activity etc. Some new understandings obtained are as follows: 1, Devonian old sea basin was a continental sea. 2, There was a widespread Caledonian movement in Qinling, appearance in the form of lifting up movement in the block, and some folding movement can occur in edge of the block. 3, There exist three gradually vicissituding settle activities and three settle zones which contributed to the settle activities from south to north during Devonian Period. 4, Both the block sedimentation and graben sedimentation, two kinds of different sedimentation-structure unit have been divided on the basis of difference in tectonic activity, sedimentation and volcanic activity. Palaeotectonic pattern in Devonian Period of Qinling is composed of interval three graben zones and two blocks. 5, Evolution of old sea basin during Devonian Period in Qinling can be divided into convergence stage in the early period and splitting open stage in late period. There exist lots of mineral resources in Devonian system. Hot water sedimentation in the sea bottom as a guide for areal mineralization has been put forward, Pb and Zn deposits belong to hot water sedimentary genesis, which can be called Qinling-type. Hg and Sb deposits are hot water sedimentation also, except for Fe deposit that can be divided into cold water and hot water types respectivily. Gold deposit in Ertaizi and Shuangwang is hot water sedimentation or hot water-transforing type in its genesis. The writers suggest that ore-bearing sedimentary formation should be replaced by ore-bearing formation in hot water sedimentation and they can be divided into two mineralization periods, one is main, another comes next. Finaly, mine-ralization perspective and direction of prospecting have been probed, with respect to mineralization divisions and mineral resouces types on the basis of analysis of mineralization regularity and ore controled factors in this professional paper.