贵州省大方地区峨眉山玄武岩基本特征及成因分析

发布时间：2018-08-16 10:49

【摘要】：峨眉山大火成岩省分为内带、中带、外带。贵州省大方地区的峨眉山玄武岩位于峨眉山大火成岩省的外带,作为峨眉山玄武岩的重要部分,虽受到众多学者的密切关注,但是对大火成岩省外带贵州境内的峨眉山玄武岩研究却相对较少。本文以野外实地调查为基础,通过研究大方地区峨眉山玄武岩的岩石学特征、矿物学特征和地球化学特征等,并与邻近的水城玄武岩为主要对比,分析了大方地区的峨眉山玄武岩的岩石学、矿物学和地球化学特征,探讨其岩浆源区性质、成岩温度压力及玄武岩形成的构造背景。通过野外地质调查和剖面测制查明了大方地区峨眉山玄武岩的时空分布、岩石类型及岩性特征,研究其喷发环境、喷发旋回及韵律。营盘、珠场两地虽相距不远,但火山喷发旋回、喷发韵律(次数)、玄武岩累计厚度以及玄武岩间的灰岩夹层和厚度等均有明显差异,表明本区二叠纪峨眉山玄武岩不仅横向上而且在垂向上的变化都较大,分布不是很稳定。玄武岩以巨厚层的块状构造为主,未见枕状熔岩,火山集块角砾岩中常见有灰红色、褐灰色火山碎屑物,且局部沉火山碎屑岩夹层中发现有双壳类化石、植物化石以及大量植物茎干化石碎片产出,这些特征均表明该套玄武岩总体上应形成于水上喷发环境。从小屯乡珠场剖面(PM11)和高店乡营盘剖面(PM38)上玄武岩岩石类型、岩石组合以及灰岩夹层的产出特征分析,研究区内二叠纪基性火山喷发作用由早到晚可划分为三个喷发旋回,并以第三旋回为主要喷发时期。大方县小屯乡珠场玄武岩剖面上三个岩浆喷发旋回均可划分为9个喷发韵律;而高店乡营盘玄武岩剖面仅划分出两个岩浆喷发旋回,但形成的火山岩地层累计厚度较大,可划分为14个喷发韵律。岩石类型主要有微晶玄武岩、隐晶玄武岩、杏仁状玄武岩及玄武质凝灰岩和火山角砾岩等。本区玄武岩造岩矿物为斜长石和单斜辉石,副矿物为磁铁矿、钛铁矿、榍石等。玄武岩具间隐结构、间粒结构、玻基交织结构等,局部杏仁体构造发育。通过单矿物电子探针分析,单斜辉石主要为普通辉石,其端元组分为Wo=37.45~40.80,En=36.54~42.89,Fs=17.06~23.22;斜长石主要为拉长石,少量中长石,其端元组分为An=41.34~58.71,Ab=38.79~54.29,Or=1.55~10.06;金属矿物主要为钛铁矿,其次为磁铁矿和钛磁铁矿。主量元素特征显示SiO_2=46.42%~51.28%,平均值48.70%;TiO_2=4.07%~4.66%,平均值4.34%,属高钛玄武岩;玄武岩的K_2O+Na_2O均值为3.33%,全碱含量并不高;大部分岩石的里特曼指数(σ)小于3.3,多介于1.02~2.93,平均值2.15,属钙碱性岩石;Mg~#值=37.20~51.70,平均值43.01;m/f=0.50~0.90,平均值0.64,属于铁质基性岩;AFM图解中显示其为拉班玄武岩系列。通过CIPW标准矿物计算,玄武岩标准矿物组合主要为:Q+An+Ab+Or+Di+Hy,属于SiO_2过饱和的正常岩石类型。玄武岩具有明显的轻稀土富集特征,轻稀土分馏程度较高,且有着与OIB相似的地球化学特征,在岩浆演化过程中,有斜长石的结晶及移出从而导致残余熔浆中Eu的不同程度亏损;微量元素和同位素特征显示大方地区峨眉山玄武岩的源区为与上地壳组分相关的富集地幔,岩浆在上升过程中受到了地壳物质的强烈混染。利用岩石地球化学数据和玄武岩构造环境判别图得知大方地区玄武岩的形成背景为大陆板内拉张环境,其形成与茅口组末期的峨眉山地幔柱活动有关。与邻近的水城地区对比表明,大方地区峨眉山玄武岩和水城玄武岩主要矿物成分都为辉石和斜长石,辉石都属普通辉石,斜长石主要属拉长石,但大方地区峨眉山玄武岩不含橄榄石,而水城玄武岩含少量橄榄石(5%);通过单斜辉石温度计估算大方地区峨眉山玄武岩的形成温度为1110~1253℃,平均为1162℃,而水城玄武岩的形成温度范围为1131~1231℃,说明两个地区玄武岩形成温度相差不大;通过地球化学特征分析可以看出大方地区峨眉山玄武岩属拉斑玄武岩系列,而水城玄武岩属碱性玄武岩系列。造成两地二叠纪玄武岩化学属性明显不一致的原因可能主要与所处位置、岩浆源区性质、部分熔融程度、地壳混染程度以及岩浆作用过程的不同有关。
[Abstract]:Emeishan basalt in Dafang area of Guizhou province is located in the outer belt of Emeishan igneous province. As an important part of Emeishan basalt, it has been paid close attention by many scholars, but the study of Emeishan basalt in Guizhou province is relatively few. On the basis of field investigation, this paper studies the petrological, mineralogical and geochemical characteristics of Emeishan basalt in Dafang area, and compares them with the Shuicheng basalt nearby. The petrology, mineralogy and geochemical characteristics of Emeishan basalt in Dafang area are analyzed, and the magma source area properties are discussed. Temperature and pressure of rocks and the tectonic setting of basalt formation.The spatial and temporal distribution,rock types and lithologic characteristics of the Emeishan basalt in Fangfang area have been ascertained through field geological survey and section survey.The eruption environment,eruption cycle and rhythm of the basalt have been studied.Although Yingpan and Zhuchang are not far apart from each other,volcanic eruption cycle,eruption rhythm(frequency),basalt have been discovered. The accumulative thickness of the basalt and the thickness of the limestone intercalation between the basalts are obviously different, indicating that the Permian Emeishan basalt is not only transversely but also vertically variable, and its distribution is not very stable. The occurrence of bivalve fossils, plant fossils and a large number of stem fossil fragments in partially submerged volcanic clastic rock interbeds indicate that the basalt suite should have been formed in a water eruption environment in general. The basalt rocks from the Xiaotunxiang Zhuchang section (PM11) and Gaodianxiang Yingpan section (PM38) were found. The eruption of Permian basic volcanic rocks can be divided into three eruption cycles from early to late, and the third cycle is the main eruption period. There are only two magmatic eruption cycles in the Pan basalt section, but the volcanic strata formed in this area are thick enough to be divided into 14 eruption rhythms.The main rock types are microcrystalline basalt, cryptocrystalline basalt, amygdaloid basalt, basaltic tuff and volcanic breccia.The basalt-forming minerals in this area are plagioclase and monoclinic pyroxene. Basalt has intercrystalline structure, intergranular structure, glass-based interwoven structure, and local amygdaloid structure is developed. According to single mineral electron microprobe analysis, clinopyroxene is mainly common pyroxene, and its end component is Wo=37.45~40.80, En=36.54~42.89, Fs=17.06~23.22; plagioclase is mainly elongated feldspar, with a small amount of medium pyroxene. Feldspar, whose end-members are An=41.34~58.71, Ab=38.79~54.29, Or=1.55~10.06; metal minerals are mainly ilmenite, followed by magnetite and titanomagnetite. The main element characteristics show that SiO_2=46.42%~51.28%, with an average value of 48.70%; TiO_2=4.07%~4.66%, with an average value of 4.34%, belongs to high-titanium basalt; the average value of K_2O+Na_2 of basalt is 3.33%, and the total alkali content is 3.33%. Most of the rocks are calc-alkaline with a Ritmann index (_) of less than 3.3, mostly between 1.02 and 2.93, with an average value of 2.15; Mg ~# value = 37.20 to 51.70, with an average value of 43.01; m/f = 0.50 to 0.90, with an average value of 0.64, belonging to iron-based rocks; and AFM diagrams show that they are Laban basalt series. Q+An+Ab+Or+Di+Hy belongs to SiO_2 supersaturated normal rock type.The basalt has obvious characteristics of light rare earth enrichment,high fractionation of light rare earth elements and geochemical characteristics similar to OIB.During the magma evolution,plagioclase crystallization and removal lead to different depletion of Eu in remnant magma. The source area of the Emeishan basalt in Dafang area is the enriched mantle related to the upper crustal components, and the magma was strongly contaminated by the crustal materials during the ascending process. The comparison with the adjacent Shuicheng area shows that the main mineral compositions of the Emeishan basalt and Shuicheng basalt in Dafang area are pyroxene and plagioclase, pyroxene is common pyroxene, plagioclase mainly belongs to anorthite, but the Emeishan basalt in Dafang area does not contain olivine and water. The formation temperature of the Emeishan basalt in Dafang area was estimated by clinopyroxene thermometer to be 1110~1253, with an average of 1162, while the formation temperature of the Shuicheng basalt ranged from 1131 ~1231, indicating that there was little difference in the formation temperature between the two areas. The Emeishan basalt in Fang area belongs to tholeiite series, while the Shuicheng basalt belongs to alkaline basalt series. The reasons for the obvious inconsistency of chemical properties of the Permian basalts between the two areas may be related to their location, magma source area property, partial melting degree, crustal contamination degree and magmatism process.
【学位授予单位】：成都理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P588.145

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