宁芜盆地娘娘山早白垩世黝方石响岩中黑云母逆反应边结构:岩浆混合触发火山喷发的矿物学证据

Inverse reaction rim of biotite in early Cretaceous nosean phonolite of Niangniangshan,Ningwu Basin:mineralogical evidence of magma mixing triggered volcanic eruption

矿物的不平衡结构是岩浆混合、不平衡结晶以及岩浆-流体交代等过程的重要指标。在长江中下游宁芜盆地娘娘山破火山口的早白垩世娘娘山组黝方石响岩中,黑云母斑晶发育了罕见的逆反应边结构,即在黑云母边部出现由角闪石及辉石等镁铁质矿物组成的反应边,但对其成因长期以来缺少深入研究。文章通过详细的岩相学研究和电子探针分析等手段,查明宁芜盆地娘娘山组黝方石响岩中具逆反应边结构的黑云母具有核-幔-边结构,其核部为原生黑云母,靠近逆反应边的幔部为高温熔蚀再平衡结晶成因的黑云母,边部为角闪石。黑云母及角闪石的温压计算及其地球化学特征表明,黑云母结晶温度为807~862℃,压力为231~369 MPa;相比之下,“逆反应边”中的角闪石结晶温度为920~949℃,压力为271~309 MPa,其寄主岩浆化学组成类似于亚碱性系列的基性岩浆,较黑云母寄主岩浆更偏基性。黑云母与“逆反应边”中角闪石结晶的物理化学条件差异表明,这种特殊的“逆反应边”结构是早期碱性岩浆与晚期基性亚碱性岩浆混合的结果。矿物CSD计算表明,反应边的形成可能仅先于火山喷发一个月左右,暗示岩浆混合可能是导致火山喷发的重要诱因。

The disequilibrium texture of minerals can significantly indicate magmatic processes such as magma mixing,disequilibrium crystallization,and magma-fluid interaction.In the early Cretaceous nosean phonolite in the Niangniangshan Formation of the Ningwu Basin along the middle-lower reaches of Yangtze River.Biotite phenocrysts developed a rare inverse reaction rim texture,that is,a reaction rim composed of mafic minerals such as amphibole and pyroxene at the edge of biotite,but it’s been insufficient to in-depth research the origin of this texture.Based on detailed petrographic study and electron probe analysis,this paper identifies a core-mantle-rim texture of the inverse reaction rimmed biotite in the Niangniangshan Formation nosean phonolite,in which the core is primary biotite,the mantle near the inverse reaction rim is biotite formed by high-temperature melt erosion and re-equilibration crystallization,and the rim is amphibole.The temperature-pressure calculation and geochemical characteristics of biotite and amphibole indicate that the crystallization temperature and pressure of biotite is 807~862℃and the crystallization pressure is 231~369 MPa;in contrast,the crystallization temperature of amphibole is 920~949℃and the crystallization pressure is 271~309 MPa.The chemical composition of the host magma is similar to that of the subalkaline basic magma,and is more mafic than the biotite host magma;The difference in the physicochemical conditions of biotite and amphibole crystallization in the"inverse reaction rim"indicates that this special“inverse reaction rim”texture results from the mixing of early alkaline magma and late basic subalkaline magma.Meanwhile,the crystal size distribution(CSD)theory shows that the formation of the reaction rim may only precede the volcanic eruption by around one month,suggesting that magma mixing may be an important trigger for the volcanic eruption.