长白山天池火山粗面岩成因与岩浆房系统演化

Petrogenesis of Trachyte and the Felsic Magma System at Tianchi Volcano:Trace Elements and Isotopic constraints

长白山天池火山是典型的大陆板内碱性火山,岩石组合以玄武岩盾和粗面岩、碱流岩等碱性岩为主。为厘清天池火山粗面岩的形成过程,本文在综合前人研究的基础上,结合本文新发表数据,检验了粗面岩的成因。研究结果显示天池火山粗面岩不可能由古老的华北克拉通下地壳部分熔融形成,也不符合造盾玄武岩部分熔融的模式。通过主量元素、微量元素定性和定量计算后认为粗面岩是由进化玄武质岩浆(玄武粗安岩)经历了分离结晶作用所形成,粗面岩形成过程中岩浆遭受了地壳混染,并且具有高n(^(87)Sr)/n(^(86)Sr)和低n(^(87)Sr)/n(^(86)Sr)两种截然不同的混染趋势。结合岩相和层序信息,作者认为两种演化趋势是上地壳岩浆房系统变化的反映。早期玄武质岩(头道白山期)上侵至上地壳演化,因与上地壳直接接触,沿高n(^(87)Sr)/n(^(86)Sr)趋势演化。随着岩浆房接受补给,岩浆房逐渐增大,新侵入的岩浆与地壳被早期岩浆房结晶形成的岩浆房壁所隔挡,使得年轻粗面岩和碱流岩只与早期粗面质岩浆的残留体等进行物质交换,最终沿着低Sr同位素比值趋势演化。

Tianchi volcano located on the China / North Korea border is a composite volcano with typical bimodal suites（ trachybasalt shield,a trachyte stratocone and a comendite ignimbrite sheet）. The petrogenesis of trachyte and the relationship between the mafic and the felsic suite are ambiguous. In this paper,we combined both new and literature data examined the petrogenesis of Tianchi felsic rocks and developed a possible petrological and geochemical model. We found that Tianchi trachytes can neither be formed by partial melting of the old lower crust of the North China craton nor by partial melting of the newly intruded basalts. The results of qualitative and quantitative geochemical modeling suggest that the felsic rocks are formed by fractional crystallization from the shield basalts with assimilation the crust. In addition,at least two different evolve paths exist in Tianchi felsic rocks： the high and low^（87）Sr /^（86）Sr path. We believe that the two differentiation paths are related to the development of crustal plumbing systems. In the early stage,the evolved magma directly encountered with the upper crust,therefore,had high^（87）Sr /^（86）Sr. In contrast,at the late stage,the differentiation magma evolved along the low^（87）Sr /^（86）Sr trend since the fresh magma was isolated from the upper crust by the previous cooled magma body.