中祁连东段兴隆山群基性火山岩锆石U-Pb定年及岩石成因研究  

Zircon U-Pb dating and petrogenesis of Xinglongshan Group basic volcanic rocks at eastern segment of Middle Qilian Mts.

作　　者：徐学义[1] 王洪亮[1] 陈隽璐[1] 何世平[1] 武鹏[2] 高婷[2] 

机构地区：[1]中国地质调查局西安地质调查中心(西安地质矿产研究所),西安710054 [2]长安大学,西安710069

出　　处：《岩石学报》2008年第4期827-840,共14页Acta Petrologica Sinica

基　　金：科技部科技支撑课题(2006BAB01A11);国土资源大调查项目(1212010610319);国家自然科学基金(40773044)资助成果.

摘　　要：兴隆山群主体为一套浅变质、变形较强的碎屑岩与基性火山岩建造组合,可分为下、中、上三个组。岩石学、主量元素和微量元素地球化学研究揭示:下组基性熔岩为玄武岩或玄武安山岩,SiO_2含量高(50.97%～54.70%),TiO_2含量小于1%(0.74%～0.75%),MgO含量较低(3.76%～4.20%);稀土总量高,轻稀土富集,重稀土亏损;微量元素原始地幔标准化分配型式显示出岛弧火山岩或受地壳混染的大陆玄武岩特有的Nb-Ta和Ti负异常。中、上组基性火山熔岩为细碧玢岩或细碧岩,SiO_2含量较低(45.71%～49.58%),TiO_2含量多大于1%,MgO含量较高(5.75%～9.88%);轻稀土亏损,重稀土富集;微量元素原始地幔标准化分配型式为平坦型。兴隆山群基性火山岩均为亚碱性拉斑系列火山岩,源岩浆在演化过程中经历了plag+cpx[±ol]的分离结晶作用。下组基性熔岩形成于大陆板内拉伸环境,岩浆上升过程中经历了较强的地壳混染作用;中、上组基性熔岩形成于拉张较为强烈的构造环境,具N-MORB型地球化学特征,代表洋壳的出现。上组基性熔岩LA-ICPMS锆石U-Pb测年结果显示最新的岩浆锆石年龄为713±53Ma,结合区域地质特征,推断兴隆山群形成年龄极有可能为新元古代。具明显岩浆结晶特征的锆石年龄构成1.0～1.2Ga的最大峰值,可能是与Rodinia超大陆形成有关的岩浆事件在该地区的响应。兴隆山群提供了Rodinia超大陆形成后一段不完整的由大陆伸展至洋壳发育阶段的地质历史记录。The Xinglongshan Group lays down in Eastern Qilian Mts. in Yuzhong County, Gansu Province, consisting mainly of detrital rocks and basic volcanic rocks, which can be divided into lower, middle and upper formations according to its lithological formation, Based on petrographic, major and trace element data, the Xinglongshan basic volcanic rocks can be classified into two magma types. These are： （ 1 ） basalt or basaltic andesite in the lower formation that exhibits high SiO2 （50. 97% - 54.70% ）, low TiO2 （0. 74% -0. 75% ） and MgO （3.76% - 4.20% ） with LREE enriched and HREE depleted, also showing distinct Nb-Ta and Ti depletion in trace element patterns; （2） spilite or spilitic porphyry in middle and upper formations that has low SiO2 （45.71% - 49.58% ）, high TiO2（ 〉 1% except one sample ） and MgO（5.75% -9.88% ） with LREE depleted and HREE enriched, their primitive-mantle normalized diagram show a ＂ flat-type＂ patterns. The Xinglongshan Group basic volcanic rocks are dominantly tholeiitic in composition and their parent magma subjected to shallow level gabbroic （ palg ＋ cpx[ ＋ ol ] ） fractionation. The basic lavas in lower formation occurs within plate rift setting and are most likely originated from asthenosphere mantle in garnet stability field deep more than 80km, crustal contamination has also contributed significantly to the formation of the basalt. In contrast, the basic volcanic rocks in middle and upper formations derived from depleted mantle source similar to N-MORB in spinel stability field in deep about 60km, indication the initial ocean crust occurred in strong rift setting, The result of U-Pb zircon LA-ICPMS dating for upper fromation basic lava is 713 ＋53Ma, together with the regional geological characteristics, we suggest that Neoproterozoic is the formation age of Xinglongshan and the cluster ages between 1.2Ga and 1.0Ga maybe the response of Mesoproterozoic magmatic events related to Rodinia supercontinent evolution. The study on Xinglongshan Group

关 键 词：兴隆山群基性火山岩 锆石U—Ph定年 岩石地球化学 岩石成因 中祁连东段 

分 类 号：P588.145[天文地球—岩石学] P597.3[天文地球—地质学]