Petrogenesis of Mid-Tertiary Synextensional Volcanic Rocks in the Basin and Range Province, East-central Nevada and Adjacent Utah

本项目将对盆地和山脉东部中第三纪大震级伸展带火山岩进行详细的岩石学和同位素研究。目标是：1)完全表征这种中等至高钾的钙组；2)套组岩石学模式组分变化与持续伸展；3)评估大陆地壳和下伏地幔的相对化学和同位素贡献。伸展期开始时（-39 Ma），火山作用大致为双峰型（硅安山岩-英安岩和高硅流纹岩），并以分散喷口的小喷发为特征。在-35 Ma，火山作用转变为英安岩到低硅流纹岩组成，喷发出大量的，真正广泛的凝灰岩和熔岩。该地区现有的岩浆活动模型表明，地幔岩浆的贡献从零到高达50%。该套岩浆岩的组成和同位素多样性表明，该套岩浆岩形成历史复杂，在岩浆活动早期涉及多个岩浆源，随后这些岩浆在中地壳中大规模混合，并持续伸展和热输入。拟议的工作是多方面的，包括野外、岩石学和同位素研究，这将补充正在进行的研究工作，包括基底岩石的结构、变质作用和同位素组成。

This project will undertake a detailed petrologic and isotopic study of volcanic rocks associated with mid-Tertiary, large- magnitude extension in the eastern Basin and Range Province. The goals are to: 1) completely characterize this moderate- to high-K calcalkalic suite; 2) petrogenetically model compositional changes in the suite with continued extension; and 3) evaluate the relative chemical and isotopic contributions of the continental crust and underlying mantle. At the onset of extension (-39 Ma), it appears that volcanism was crudely bimodal (silicic andesite to dacite, and high-silica rhyolite) and was characterized by small eruptions from scattered vents. At -35 Ma, volcanism shifted to dacite to low-silica rhyolite compositions erupted as voluminous, areallly extensive tuffs and lavas. Existing models for magmatism in the region suggest a contribution ranging from zero to as much as 50% of mantle- derived magmas. The compositional and isotopic diversity of the suite suggest a complex petrogenetic history involving several magma sources during early stages of magmatism, followed by large-scale mixing of these magmas in the middle (?) crust with continued extension and heat input. The proposed work is multifaceted, including field, petrologic and isotopic studies which will compliment ongoing research efforts in the area dealing with the structure, metamorphism, and isotopic composition of the basement rocks.