Cerro Toledo Rhyolite, Jemez Volcanic Field, New Mexico: 40Ar/39Ar geochronology of eruptions between two caldera-forming events
Cerro Toledo Rhyolite, Jemez Volcanic Field, New Mexico: 40Ar/39Ar geochronology of eruptions between two caldera-forming events
复制标题
新墨西哥州杰梅斯火山田托莱多山流纹岩：两次破火山口形成事件之间喷发的 40Ar/39Ar 地质年代学
DOI：
--
复制
发表时间：
1996
期刊：
影响因子：
--
通讯作者：
A. Doulgeris
中科院分区：
文献类型：
--
作者：
T. Spell;I. Mcdougall;A. Doulgeris
Cerro Toledo流纹岩由一组穹丘和火山灰组成，它们是在新墨西哥州Jemez火山区的Bandelier凝灰岩的Tshirege段和Otowi段这两个形成破火山口的熔结凝灰岩之间喷发的。为了提供一个年代学框架的地球化学和同位素研究这些韵律，记录了班德尔岩浆系统的演化，在此期间，进行了40 Ar/ 39 Ar年代学研究。流纹岩火山灰和流纹岩圆顶的样品内的主要火山碎屑落沉积物的浮石进行了日期以及地层括号Bandelier凝灰岩。分析的晶体种群范围从相当均匀的少年材料非常异质混合少年和捕虏晶组合。在大多数情况下，占主导地位的集团的青年透长石晶体定义40 Ar/ 39 Ar年龄同意地层的限制。斜长石分析明显更加分散，通常不能确定合理的年龄。班德利尔凝灰岩两段的40 Ar/ 39 Ar年龄为380 ± 20 k.y.在这些火山口形成的喷发之间。在此期间，9个主要的火山碎屑浮石单位沉积在研究的部分，其中6个产量等时线年龄，一个加权平均年龄，一个最大年龄，和一个没有可靠的年龄，由于缺乏透长石。对Cerro Toledo流纹岩火山喷发岩中的浮石坠落单元进行的40 Ar/ 39 Ar年龄测定表明，火山喷发活动发生在>1.59、1.54、1.48、1.37和1.22 Ma。Cerro Toledo流纹岩圆顶的40 Ar/ 39 Ar测年表明，这些圆顶在1.54、1.45、1.38-1.34和1.27 Ma时在破火山口内喷发。获得的日期表明，整个380 k. y的喷发活动发生。间隔之间的两个成员的Bandelier凝灰岩，但建议喷发生产火山灰和圆顶发生在离散的时间间隔约。1.54、1.48和1.38-1.34 Ma。从1.34到1.38 Ma的时间段特别活跃; 18个单位中有7个是这些年龄。
The Cerro Toledo Rhyolite comprises a group of domes and tephra which were erupted during the interval between two caldera-forming ignimbrites, the Tshirege Member and Otowi Member of the Bandelier Tuff, in the Jemez Volcanic Field, New Mexico. To provide a chronologic framework for geochemical and isotopic studies on these rhyolites, which record the evolution of the Bandelier magma system during this interval, a 40 Ar/ 39 Ar geochronology study was undertaken. Pumice from major pyroclastic fall deposits within the rhyolite tephra and samples from the rhyolite domes were dated as well as the stratigraphically bracketing Bandelier Tuff. Analyzed crystal populations range from being fairly homogeneous juvenile material to very heterogeneous mixed juvenile and xenocrystic assemblages. In most cases dominant groups of juvenile sanidine crystals define 40 Ar/ 39 Ar ages which agree with stratigraphic constraints. Plagioclase analyses are distinctly more scattered and do not typically define reasonable ages. The 40 Ar/ 39 Ar ages for the two members of the Bandelier Tuff yield an interval of 380 ± 20 k.y. between these caldera-forming eruptions. During this interval nine major pyroclastic pumice units were deposited in the sections studied, for which six yield isochron ages, one a weighted mean age, one a maximum age, and one no reliable age due to lack of sanidine. 40 Ar/ 39 Ar dates on pumice fall units within the Cerro Toledo Rhyolite tephra indicate that eruptive activity occurred at >1.59, 1.54, 1.48, 1.37 and 1.22 Ma. 40 Ar/ 39 Ar dating of Cerro Toledo Rhyolite domes indicates these were erupted within the caldera at 1.54, 1.45, 1.38–1.34, and 1.27 Ma. The dates obtained indicate that eruptive activity occurred throughout the 380 k.y. interval between the two members of the Bandelier Tuff, but suggest that eruptions producing both tephra and domes occurred during discrete intervals at ca. 1.54, 1.48, and 1.38–1.34 Ma. The interval from 1.34 to 1.38 Ma was particularly active; 7 of 18 units dated are these ages.