Evolutionary stages of crystallization of weakly peralkaline syenites: evidence from ejecta in the plinian deposits of Agua de Pau volcano (São miguel, Azores Islands)
Evolutionary stages of crystallization of weakly peralkaline syenites: evidence from ejecta in the plinian deposits of Agua de Pau volcano (São miguel, Azores Islands)
复制标题
弱过碱性正长岩结晶的演化阶段：来自阿瓜德波火山（亚速尔群岛圣米格尔）普林尼式沉积物中喷射物的证据
作者：
F. Ridolfi;A. Renzulli;P. Santi;B. Upton
阿瓜德帕火山(亚速尔群岛S米格尔群岛)普里尼亚瀑布沉积中的弱过碱性正长岩喷出物可分为(1)硅不饱和和(2)硅饱和/过饱和两类。第一组产于前Fogo-A火山碎屑瀑布(15.2ka)，第二组产于Fogo-A浮岩(5ka)。岩石学研究表明，两组正长岩均为浅层结晶，全岩地球化学显示其成分反映了阿瓜德堡粗面岩浆的成分。正长岩推测来自母岩浆室边缘的凝固带。这两个正长岩群中的矿物共生矿物允许区分三个岩浆结晶阶段：(1)早期结晶阶段；(2)主要结晶阶段，在此阶段大部分长石生长；(3)晚期间隙阶段。粗面岩岩浆的结晶产生了富含挥发分、碱、高场强元素和稀土元素的过碱性残渣。在硅质不饱和正长岩中，残留岩浆间隙结晶出复杂的锆钛硅酸盐矿物--透辉石和类L脉石。与之相反，锆硅酸盐英安岩和富稀土矿物(雪佛金石-(Ce)、橄榄石-(Ce)和焦绿石)与针铁矿一起，是硅质饱和/过饱和正长岩的晚期间隙产物。
Abstract Weakly peralkaline syenite ejecta within the plinian fall deposits of Agua de Pau volcano (São Miguel, Azores Islands) are divisible into (1) silica-undersaturated and (2) silica-saturated/oversaturated groups. The first group occurs in the pre-Fogo-A pyroclastic falls (15.2 ka) whereas the second occurs in the Fogo-A pumices (5 ka). The petrography indicates that the syenites of both groups crystallized at shallow depths and whole-rock geochemistry shows that they have compositions reflecting those of the Agua de Pau trachyte magmas. The syenites are inferred to be derived from solidification zones around the margins of the parental magma chamber. Mineral parageneses in both syenite groups permit distinction of three magmatic stages of crystallization: (1) an early crystallization stage; (2) a main crystallization stage during which most of the feldspar growth occurred; and (3) a late interstitial stage. Crystallization of the trachytic magmas yielded peralkaline residuals, highly enriched in volatiles, alkalis, HFSE and REE. In the silica-undersaturated syenites, the complex Zr-Ti silicate minerals, eudialyte and a låvenite-like mineral, crystallized interstitially from the residual magmas. In contrast, the zircono-silicate dalyite, and REE-rich minerals (chevkinite-(Ce), britholite-(Ce) and pyrochlore), together with thorite, were late-stage interstitial products of the silica-saturated/oversaturated syenites.