摘要
碧口群分布于扬子板块西北缘陕、甘、川三省毗邻地区,地质构造复杂。长期以来,由于其时代和层序问题得不到解决,直接影响了对该区的进一步找矿和构造性质的认识,并成为秦巴地区引人瞩目的重大基础地质问题之一。通过近几年的工作,取得了重大进展。建立了碧口群的层序和统一地层系统。主要根据接触关系、火山活动演化规律、物质成分来源、变形变质特点及时代依据等建立了碧口群的基本层序。并根据岩石建造特征、沉积旋回、生物群、冰川作用、地球化学、同位素年龄及含矿层位等综合分析,自下而上将其划分为前震旦系、震旦系和寒武系。寒武系和震旦系时代依据比较充分,已从碧口群内解体出来,前震旦系由于下限尚不明确,仍保留碧口群(狭义)名称。火山岩为解体后“碧口群”的重要组成部分,进行了较系统地研究和总结。从分布情况和发育程度来看,火山作用横向东强西弱;纵向上,自下而上是弱—强—弱的演化规律。大体可分为3个大的喷发旋回。该火山岩为一套以细碧岩、角斑岩、石英角斑岩及次火山岩相的辉绿岩为主的建造。主要岩石类型有辉绿岩、辉绿玢岩、细碧岩、细碧玢岩、玄武岩、玄武玢岩、安山玢岩、角斑岩、石英角斑岩、流纹岩、流纹斑岩及同质火山碎屑岩等。根据大量岩石化学分析结果及稀土元素配分模式判断,大部分属大洋拉斑玄武岩系列,部分为钙碱玄武岩系列。主要形成于大洋板块洋中脊构造环境,部分为岛弧—洋岛环境下的产物。碧口群具有多期变质和变形作用。根据大量同位素资料分析,碧口群大体经历了晋宁、澄江、加里东、华力西、印支—燕山五期热事件及构造运动。变质作用主要表现为区域动力变质作用所形成的绿片岩相。部分地区受区域动力热流和区域高温变质作用影响达高绿片岩相和角闪岩相。空间上具有由北往南、由上部层位向下部层位变质程度逐渐加深的特点。沿青川—勉县大断裂带的北侧尚伴有高压低温蓝闪片岩带。变形作用在不同层位表现强弱不同,根据变形特点,可将碧口群划分为前震旦系和震旦—寒武系两个大的构造层和四个变形期次。而后一构造层又可分为下震旦统秧田坝组和下震旦统南沱组—下寒武统两个次一级构造层。上述构造层都具有独特的构造变形序列和构造样式。早期构造变形以线状褶皱为主。后期构造产生倒转以及次一级褶皱的叠加、韧性剪切、片理、劈理,以至脆性断裂的产生。反映出各构造层处于不同的构造环境,经历了不同的演化历史。对碧口群的有关矿产进行了较系统地总结。碧口群分布区内目前已知矿产地共193处,其中以铁、锰、金、镍、重晶石、磷、石棉等矿产为主。成因类型包括火山沉积(变质)型、沉积(变质)型、热液型、岩浆型和风化淋滤型。并从层位、岩石、沉积坏境、构造及火山机构等方面,分析探讨了不同矿种和成因类型的成矿规律,提出了进一步找矿的意见。
The Bikou Group is distributed at the northwestern margin of the Yangtzeplate,which is the adjacency of Shaanxi,Gansu and Sichuan three provencesand the geological structure is very complicated.Because of problomes of ageand sequence in the group have not been settled for long time,so it influencesdirictly on the works how to discover the mineral resources in the districtand how to recognize the structural character there correctly.Some advances have been gained by the researchers in recent several yearssuch as establishment of the sequence of Bikou Group and unity of stratigra-phic system:The division of AnZ,Z,and ∈,among them the age evidence ofCambrian and Sinian being relatively reliable.According to the contact relationof the strata,evolutional rule of the Volcanic activity,the material source,deformational-metamorphic character,age evidence,compositive analysis ofrock formation,sedimentary cycle,biota,glaciation,geochemistry,isotopicage and the ore-bearing horizon etc,the Cambrian and Sinian are dismenberedfrom the Bike Group,the pre-Sinian,because its lower boundary is not yetclear,still retains the term of Bikou Group(narrow sense).The volcanic rocks,as the important constituent of the dismenbered BikouGroup,have been systematically studied and summrized.In view of their distri-bution and evolutionary regularities:Laterally,effuslve process was strongerin the east than in the west,stratigraphically,it displays weak-strong-weakfrom bottom to top.Generally it is divisible into three large effusive cycles.This volcanic formation mainly consists of spilite,keratophyre,quartz-kera-tophre and diabase which belong to the sub-volcanic rock.The main rock typesare diabase,diabase-porphyrite,spilite,spilite-porphyrite,basalt,basalt-porphyrite,andesite-porphyrite,keratophyre,quartz-keratophyre,rhyolite,rhyolite-porphyre and their volcanic clastic rocks.On the basis of quantitycalculation of petrochemistry data and from the judge of distributed model ofrare-earth elements,these,rocks mainly belong to the oceanic tholeiite seriesand a part belongs to the lime-alkali series.They mainly formed at tectonicenvironment of the middle ridge of the oceanic plate,partial are the productsof island arc and oceanic islands.The Bickou Group is characterised by multi-epoch metamorphism and de-formation.They had been subjected to such five epoches thermoevents and stru-ctural movement as Jinning,Chenjiang,Caledonian,Hercynian and Indosi-nian-Yanshannian periods on thd basis of the field observation,laboratorystudy and the analysis of a great quantity of isotopic data.The metamor-phism is mainly marked by greenschist facies underregional dynamic metamor- phism,partial district had undergone regional dynamic heat flow metamor-phism or regional mid-high temperature metamorphism to form high greeschistfacies and amphibolite facies respectively.Spatially from north to south,andfrom upper to lower horizon,the metamorphic degree increases gradually,Along the northern side of the Qingchuan-Mianxian regional fracture zone,there appears a high pressure-low temperature glaucopgane-greeschist belt.Deformation presents relatively strong or weak in the differant horizon.According to the feature of deformation,the Bikou Group may be divided intotwo large tectonic formations,i.e.prf-Sinian and Sinian-Cambrian,includingfour deformational epoches.The latter tectonic formation may still be subdi-vided into two secondary order teetonic formation i.e.lower Sinian series Yong-tianba Formation and lower Sinian series Nuantuo Formation-lower Cambrian.The above mentioned tectonic formation shows individual structural deforma-tional sequence and structural style.The early structure deformation is mainlymade up of linear folds,whereas the later one is marked by inverted foldsas well as superposition by the second order folds,ductile shear structure,schistosity,cleavege as well as brittle faults,reflecting the differences of te-ctonic environment and evolutionary histories where the tectonic formation wasformed.The mineral resources of Bikou Group have been rather systematically stu-died and summarized.At present,there are 11 large scale,14 medium scaleand 23 small scale ore ceposits:Eight kinds utlizable mineral resources havebeen discovered,including iron,manganese,gold,nickel,barium phosphate,asbestos etc.The prospeting reserves attains the important position in north-west district.The genetic types of them are of volcano-sedimentary(meta-morphic),sedimentary(metamorphic)hydrothermal,magmagene and wea-thering-eluvial types.The metallogenetic regularities of the different ore sor-ts and genetic types are studied and discussed,according to horizon,rocks,sedimentary environment,structure and volcano-mechanism etc.Since certain intricate problem of basic geology still remains unsolved,only some suggestions for further exploration of the ore deposit are offeredby the authors in this professional paper.
