Isotopic and Petrologic Investigations Into the Development of a Young (~100 Ma) Sialic Crustal Section-Southern Sierra Nevada, California

内华达山脉南部从表层(火山)层到深层(花岗岩)层提供了一个斜面，穿过年轻的岩基生成的硅质地壳。区域填图和Pb/U锆石年代学研究表明，100 Ma的火成岩-变质岩组合在该地壳剖面上具有构造连续性，使其成为世界上最年轻的硅质地壳剖面之一。本节中观察到的成分和构造模式，结合地球物理数据，对唾液地壳成因和下大陆地壳的性质提出了重要问题。NSF为PI和两名研究生寻求资金，用于对此类问题进行同位素、岩石学和实地研究。所寻求的资金构成了支持和科学利用最近获得的(NSF-Caltech成本分摊)多收集热电离质谱计的主要基础。这项研究的一些具体目标包括：(1)100 Ma时添加到地壳中的一个明显的主要地幔起源的火成岩的同位素、地球化学和岩石学特征，以及它在花岗质下地壳形成中的作用；(2)根据区域地图关系，研究该地幔火成岩套系对多种已存在的地壳物质施加的地壳熔融现象，这些地壳物质是石器时代前的大部分结构的特征；(3)应用适当的温压计对深部暴露进行检验，作为对地壳总体结构模型的检验，并对峰期变质和地壳熔融的物理条件进行约束；(4)建立年轻地幔成分与先前存在的地壳物质之间的混合模型，并将这些模型应用于大型岩基岩石成因和侵位问题。混合现象的研究将主要在人们熟知的深能级区域内利用Nd-Sr同位素技术进行研究，这些区域表现出熔融和混合关系，并辅之以选定的氧同位素和微量元素数据。这项研究的大规模目标是模拟促成南塞拉利昂地壳柱产生的主要组成部分和物质运输模式。这项研究还将为广泛分布在内华达山脉岩基西壁的含金矿石的生成提供新的线索。

The southern Sierra Nevada offers an oblique section through young batholith-generated sialic crust from surface (volcanic) levels to deep (granulitic) levels. Regional mapping and Pb/U zircon geochronology reveal structural continuity through this crustal section for igneous-metamorphic assemblages of 100 Ma, making it one of the youngest sialic crustal sections in the world. Compositional and structural patterns observed in this section in conjunction with geophysical data pose important problems on sialic crustal genesis and the nature of lower continental crust. NSF funds are sought for the PI and two graduate students for isotopic, petrologic and field studies into such problems. The funds sought constitute the main basis of support and scientific utilization of a recently acquired (NSF-Caltech cost-share) multicollector thermal ionization mass spectrometer. Some specific aims of the proposed research include (1) isotopic, geochemical and petrographic characterization of an apparent major mantle-derived igneous suite that was added to the crust at 100 Ma, and consideration of its role in the formation of a granulitic lower crust; (2) study of crustal melting phenomena imposed by this mantle igneous suite on a variety of pre- existing crustal materials that are known to characterize much of the prebatholithic framework based on regional map relations; (3) application of appropriate thermobarometers to deep-level exposures as a test for a gross crustal structure model and for constraints on the physical conditions of peak metamorphism and crustal melting; and (4) construction of mixing models between the young mantle component and pre-existing crustal materials and the application of these models to large-scale problems of batholith petrogenesis and emplacement. The mixing phenomena will be studied mainly by Nd-Sr isotopic techniques in well- understood deep-level domains which exhibit melting and mixing relations with the aid of selected oxygen isotope and trace element data. Large-scale goals of this research are to model the major components and material transport patterns that contributed to the production of the southern Sierra sialic crustal column. This research will also shed new light on auriferous ore generation that was widespread along the western wall of the Sierra Nevada batholith.