摘要
西天山昭苏北部早石炭纪大哈拉军山组火山岩由碱性橄榄玄武岩和少量的拉斑玄武岩组成。火山岩TiO2（1．10～1．99％）和P2O5（0．22～0．70％）含量较高，富集大离子亲石元素（Rb，Ba，K）并强烈亏损高场强元素（Nb，Ta，Ti）（（Nb／La）N=0．20～0．36），轻重稀土分异，其地球化学特征总体上与美国盆岭地区新生代（中新世中期之前）火山岩以及中美洲和南美洲的弧后“过渡型”基性火山岩相似，但与典型火山弧岩浆岩有些不同。大量元古代继承锆石的出现以及主量、微量元素和Nd同位素地球化学特征均表明火山岩经历了相当程度的结晶分异和陆壳混染（AFC）。与碱性玄武岩相比，拉斑玄武岩的SiO2含量和K2O／Na2O比值较高，而εNd（t），MgO和Mg^#值较低，可能由碱性玄武岩浆经AFC演化形成。碱性橄榄玄武岩中含富Fe，Ti贫Si的火山岩夹层（Fe-Ti玄武岩），是碱性玄武质岩浆依Fenner趋势发生较高程度结晶分异的产物。火山岩的元素地球化学特征指示岩浆源区可能为俯冲流体交代富集的岩石圈地幔，然而，Fe-Ti玄武岩的出现以及地球化学特征随时间的规律性变化均指示它们形成于拉张的构造环境。我们提出，昭苏北部大哈拉军山组火山岩可能形成于具有元古代陆壳基底的活动大陆边缘弧后拉张环境。这一认识与前人提出的大哈拉军山组火山岩为地幔柱活动影响的大陆裂谷岩浆岩的认识有所不同。
Early Carboniferous volcanic rocks from north Zhaosu in western Tianshan Mountains are composed of alkaline olivine basalts and minor tholeiitic basalts. These rocks are characterized with moderate TiO2 （ 1. 10% - 1.99% ） and P2O5 （0.22% - 0.70% ） contents, enriched large-ion-lithophile elements （LILE） such as Rb, Ba and K, strongly depleted high-field-strength elrments （HFSE） such as Nb, Ta and Ti （（Nb/La） N = 0.20 -0.36 ）, as well as fractionated rare earth elements （REE）. The geochemical characteristics in general resemble that of the Cenozoic （ before middle Miocene） volcanic rocks from Basin and Range, western America, and that of the back-arc ＂transitional＂mafic rocks in Central and South America, but are some different from that of typical volcanic arc magma. Abundant inherited zircon of Proterozoic age, and major and trace elemental and neodymium isotopic geochemistry together prove that the magma experienced extensive fractional crystallization and crustal contamination （ AFC ） before eruption. The tholeiites are likely to be derived from the parental alkaline magma through higher degree of AFC processes, as is inferred from their higher SiO2 contents and K2O/Na2O ratios and lower εNd（ t）, MgO and Mg^# values relative to the alkaline basalts. Fe-Ti basalts, typically titanium- and iron-rich and silica-poor, occur as thin layer within the alkaline basalts, and are interpreted to be produced from the parental alkaline basalts through moderate to high degree of Fenner trend differentiation. Elemental geochemistry of the volcanic rocks is attributed to a lithospheric mantle source that had been enriched by subduction fluids. However, occurrence of the Fe-Ti basalts and regularly temporal geochemical variation of the whole volcanic stratum are consistent with a rifting tectonic environment. It is proposed that the volcanic rocks from north Zhaosu were formed in a back-arc extensional environment of active continental margin with Proterozoic continental basement. This conclusion also argues against the previous suggestion that the Dahalajunshan group volcanic rocks in western Tianshan Mountains represent continental rift magmatism induced by mantle plume.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2006年第5期1307-1323,共17页
Acta Petrologica Sinica
基金
本文得到国家重点基础研究发展规划项目(编号2001CB409803)
国家自然科学基金项目(编号40421202
40472047
40002006)
中国科学院百人计划的资助。
关键词
早石炭世
大哈拉军山组火山岩
Fe-Ti玄武岩
地球化学
构造环境
西天山
Early Carboniferous, Dahalajunshan group volcanic rocks, Fe-Ti basalts, geochemistry, tectonic setting, western Tianshan
