Silicic Volcanism and Crustal Evolution: Petrogenetic Studies in Central Chihuahua, Mexico, Uniquely Constrained by Mantle Isotopic Data

墨西哥西部的中新生代火成岩构成了世界上最大的流纹岩省，关于其成因的确凿证据必须记录在该地区的地壳深处和上地幔中。在以前对墨西哥和其他地方的大体积烟灰岩的岩石成因研究中，由于以下原因，不可能定量评价地壳和地幔组分的相对贡献：(1)深部地壳是不均匀的，几乎是不变的，它的年龄和组成只能推断火山场之下。(2)大陆弧玄武岩浆的地壳下同位素组成鲜为人知，因为任何穿过大陆地壳的岩浆都可能获得地壳同位素特征。墨西哥奇瓦瓦州的La Olivina地区提供了一个不同寻常的机会，通过利用地壳和地幔包体的数据来阐明钙碱性岩石的起源和演化，包括大量的火成岩。(1)利用SHRIMP离子探针和常规技术对麻粒岩相捕虏体中的锆石定年，确定了奇瓦瓦中部深部地壳的火成岩和变质史。虾的分析将由澳大利亚国立大学的罗伯塔·鲁德尼克进行。(2)通过主元素、同位素（Sr、Nd、Pb）和地热-地压研究，全面表征了该区深部地壳捕虏体和火山岩特征。(3)定量检验火山岩与特定深部地壳岩性的成岩模型。这些模型受到来自地幔变辉石岩捕虏体的数据的独特限制，这些数据记录了“未受污染”的大陆弧玄武岩岩浆的同位素组成。这项研究的最终结果应该是大量的硅岩浆活动对地壳深部的影响的一个特殊的文件。

The mid-cenozoic ignimbrites of western Mexico constitute the largest rhyolite province in the world, and definitive evidence regarding their genesis must be recorded in the deep crust and upper mantle beneath the region. In previous petrogenetic studies of voluminous ignimbrites from Mexico and elsewhere, it has not been possible to quantitatively evaluate the relative contributions of crustal and mantle components for the following reasons: (1) The deep crust is heterogeneous, and almost invariable, its age and composition beneath the volcanic field can only be inferred. (2) The sub-crustal isotopic compositions of continental arc basaltic magma are rarely known because any magma that passed through the continental crust may acquire a crustal isotopic signature. The La Olivina area of chihuahua, Mexico provides an unusual opportunity to elucidate the origin and evolution of calc-alkalic rocks, including voluminous ignimbrites, by drawing on data from crustal and mantle xenoliths. The primary goals of this research are as follows: (1) Determine the igneous and metamorphic history of the deep crust beneath central chihuahua by dating zircons from granulite facies xenoliths using the SHRIMP ion microprobe and conventional techniques. The SHRIMP analyses will be made by Roberta Rudnick at the Australian National University. (2) Completely characterize the deep crustal xenoliths and volcanic rocks from the region by major element, isotopic (Sr, Nd, Pb), and geothermic-geobaric studies. (3) Quantitatively test petrogenetic models relating the volcanic rocks to specific deep crustal lithologies. These models are uniquely constrained by data from mantle metapyroxenite xenoliths that record the isotopic compositions of "uncontaminated" continental arc basaltic magmas. The final result of this research should be an exceptional documentation of the impact of voluminous silicic magmatism on the deep crust.