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

内华达山脉南部提供了一个倾斜剖面，从表面（火山）到深处（粒状），穿过年轻的岩基生成的唾液壳。区域填图和Pb/U锆石年代学显示该地壳剖面具有100 Ma的火成岩-变质组合的结构连续性，是世界上最年轻的硅质地壳剖面之一。本节观测到的成分和构造模式结合地球物理资料，对硅质地壳成因和下大陆地壳性质提出了重要的问题。国家科学基金正在为PI和两名研究生寻求资金，以便对这些问题进行同位素、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（美国国家科学基金会-加州理工学院费用分摊）多收集器热电离质谱仪的主要基础。提出的研究目标包括：(1)对100 Ma时明显的幔源火成岩套进行同位素、地球化学和岩石学表征，并考虑其在粒状下地壳形成中的作用；(2)根据区域图关系，研究地幔火成岩套作用于各种已知的前旧石器时代框架特征的地壳物质上的地壳熔融现象；(3)将适当的温度计应用于深部暴露，作为对总地壳结构模型的检验，以及对峰值变质作用和地壳熔融物理条件的约束；(4)建立年轻地幔成分与已有地壳物质的混合模型，并将其应用于大尺度岩基成岩与侵位问题。混合现象将主要通过Nd-Sr同位素技术在已熟知的深部区域进行研究，这些区域在选定的氧同位素和微量元素数据的帮助下表现出熔化和混合关系。本研究的大尺度目标是模拟导致南塞拉硅质地壳柱形成的主要成分和物质搬运模式。这项研究还将为研究内华达山脉岩基西壁广泛分布的含金矿石的形成提供新的线索。

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.