【作者】 丛亚楠；

【导师】 张铭杰；

【作者基本信息】 兰州大学 ， 矿物学、岩石学、矿床学， 2013， 硕士

【摘要】 华南克拉通岩石圈地幔自中新生代以来发生了减薄,其减薄造成的岩石圈地幔性质、介质条件等方面的认识需要地球化学多方面资料的制约。本文选择华南克拉通南部的福建福鼎大洋窠、白琳大嶂山和广东揭阳韭菜地等地新生代玄武岩和地幔捕虏体为研究对象,通过玄武岩全岩主微量分析测试及地幔捕虏体流体组分分析,探讨了华南克拉通新生代岩浆作用、岩石圈地幔性质及环境条件,获得如下主要认识：1.华南克拉通新生代玄武岩主要为碱性系列,包括碧玄岩、粗面玄武岩和碱性玄武岩。Si02含量为40.21%～49.15%;Na2O+K2O含量为2.08%-5.87%,大部分在4.0%-6.0%左右, Mg#值为52.45%-72.10%。Harker图解显示,MgO-Al2O3、TFe2O3、K2O、TiO2大致呈负相关关系,岩浆早期可能存在橄榄石分离结晶作用。从CaO/Al2O3-CaO图解中可以看到CaO/Al2O3和CaO呈现出正相关关系,且Cr和Mg#之间同时也存在正相关,说明岩浆经历了过单斜辉石的分离结晶。2.华南克拉通新生代玄武岩微量元素、稀土元素配分模式具有OIB特征,具有Nb和Ta明显正异常的特征,La-La/Sm图解中显示明显的正相关,在Zr/Nb-Ba/Nb投图中样品落入EMⅡ型的洋岛玄武岩附近。说明玄武岩微量元素成分变化主要受部分熔融作用的控制,玄武岩浆上升过程中发生的分离结晶作用和陆壳物质的混染程度较弱。3.在分步加热过程中华南克拉通地幔捕虏体中的流体组分在200-600℃、600-800℃和800-1100℃三个阶段释出。800-1100℃段释出流体组主要来源于矿物结晶过程中捕获的挥发份、矿物晶格缺陷和空隙中的流体组分,CO平均54.6%、CO2平均30.0%；600-800℃段释出流体组主要来源于后期交代,主要为CO2平均36.9%、CO平均32.9%；200-600℃段释出流体组主要来源于蚀变作用流体,主要为CO2平均41.5%、CO平均23.5%。4.华南克拉通新生代玄武岩不相容元素的比值与EMⅡ型的洋岛玄武岩类似,岩浆起源于软流圈地幔,并有EMII岩石圈地幔的混合组分。表明形成环境为拉张性-大陆裂谷环境,岩石圈减薄可能与软流圈地幔上涌有关。5.根据流体化学组成可以区分出初始流体组分、地幔交代作用流体组分及晚期流体挥发分3种流体组分,岩石圈地幔演化初期的流体组分以CO为主要成分,随后的交代作用流体组成由弱还原性转变为弱氧化性,可能与岩石圈减薄引起的氧化性组分有关。结合玄武岩主、微量元素特征,华南克拉通新生代玄武岩岩浆作用可能与太平洋板块俯冲造成的地壳物质再循环有关。更多还原

【Abstract】 The South China Craton lithospheric mantle has attenuated gradually since Cenozoic, and the understanding of lithospheric mantle characteristics and ambient condition desires constraints from geochemical data. This article studies Cenozoic basalts and mantle xenoliths, of which samples collected from Dayang Chao and Bailin, Fujian; Jiucai Di, Jie Yang, Guang Dong. The research contains major and trace elements analysis of basalt whole rocks and analysis of fluid compositions of mantle xenoliths, by which the magmatism, lithospheric mantle characteristics and environment conditions of South China Cenozoic Craton are discussed. The main conclusions are listed as follow:1.The cenozoic basalts in South China are mainly alkaline series, including basanite、trachybasalt and alkaline basalts.with SiO240.21%～49.15%; Na2O+K2O2.08%～5.87%, mainly lying between4.0%～6.0%; Mg#52.45%～72.10%, suggesting parts of the samples collected may be close to primary magma. In Harker diagram, MgO-Al2O3、TFe2O3、K2O、TiO2are negatively correlated, which may indicates a fractional crystallization process of olive, but an unobvious crystallization process of plagioclase and ilmenite.2.The distribution patterns of trace elements and REE of South China Cenozoic shows OIB characteristics, with Ta and Nb positive anomaly, La-La/Sm positively correlated in the diagram. The samples locate near EM II oceanic island basalt, indicating that trace element change in the basalt mainly depend on partial melting process, and fractional crystallization and continental matter contamination is low during basalt magma uplifting.3.Fluid compositions of the mantle xenoliths are released at200-600℃,600-800℃, and800-1100℃intervals in the stepwise heating process. The fluids released in800-1100℃mainly derive from volatiles and fluid compositions traped during mineral crystallization, with an average of CO54.6%, CO230.0%; in600-800℃, fluids mainly derive from subsequent metasomatism, with an average of CO236.9%, CO32.9%; and in200-600℃, mainly from alteration, with CO241.5%,CO23.5%.4.Incompatible element ratio in the basalts of South China Craton in Cenozoic is similar to that of EM II oceanic island basalt. The basalt derives from asthenospheric mantle and has compositions fo lithospheric mantle, suggesting that the formation environment is tension continental rift and that the attenuation of lithosphere may relate with the uplifting of asthenospheric mantle.5.According to the chemical compositions, the fluid can be divided into three groups:primary fluid, mantle metasomatism fluid and subsequent volatiles. The fluid of lithosphere in primary evolution stage is mainly CO, and metasomatism fluid changes from weak reductive to weak oxidative, which may be relate with the oxidative compositions caused by lithosphere attenuation. With the combination of trace and major element of basalt characteristics, South China Craton basalt magmatism in Cenozoic may be relate with the recycle of lithospheric matter, which is caused by Pacific plate subduction.更多还原