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Description
The Carboniferous Peale Formation, located in the complex and disrupted northern Sierra terrane, Sierra Nevada, California, is composed of two distinct members. The upper Member; composed of rhythmically layered beds of chert and argillite, is informally referred to as the Peale chert. Cropping out east of Bowman Lake, the Peale chert can be traced from outside to inside the metamorphic aureole of the Middle Jurassic Emigrant Gap composite pluton. Outside the aureole, it is metamorphosed to chlorite-grade greenschist facies while inside it is metamorphosed to a biotite + white mica ± cordierite ± andalusite assemblage. These relationships suggest temperatures between 515°C and 630°C during contact metamorphism of the Peale chert. Aluminum-in-hornblende geobarometric estimates indicate emplacement pressures for the Emigrant Gap pluton between ~1.3 and ~2.7 kbars. Two stratigraphic sections of the Peale chert were chosen for detailed study, and are referred to as the northern and southern sections. The northern section, lying outside the aureole, is ~150 m thick and consists ot: in ascending order, the black and white (~104 m thick), mottled (~16 m thick), and zebra striped (~30 m thick) lithofacies. The southern section is located within the aureole and is ~110 m thick. It consists of: in ascending order, a lower (~64 m thick) and upper (~46 m thick) lithofacies. Depositional setting was estimated from the Fe2O3/TiO2 versus Al2O3/(Al2O3 + Fe2O3) and Lan/Cen versus Al2O3/(Al2O3 + Fe2O3) discrimination diagrams developed by Richard Murray of Boston University. Data from the northern section plot within the continental margin/island arc field. Similarly, Ce-anomaly values exhibit a continental margin affinity, but are consistently less than one throughout the black and white lithofacies, jumping to positive values (i.e., > 1.0) in the mottled lithofacies, and then returning to values less than one in the zebra striped lithofacies. Th/Sc and Zr/Sc ratios, display an inverse pattern to that exhibited by the Ce-anomaly data, reaching minimum values within the mottled lithofacies and maximum values within the zebra striped and lower part of the black and white lithofacies. Maximum Th/Sc and Zr/Sc values indicate an increase of terrigenous sediments based on comparisons to data from average mudstones derived from the interior of North America. A similar interpretation is apparent when data derived from the northern section are plotted on a La-Th-Sc ternary diagram. When chemical data derived from the southern section are compared to the patterns described above, broad chemostratigraphic similarities are evident. Thus, these observed chemostratigraphic trends appear to be little affected by contact metamorphism. In the western Cordillera, erosion of the Mesozoic Sierra Nevada batholith has exposed a terrane dominated by granitoid plutons surrounded by overlapping contact metamorphic aureoles. Tracing key Paleozoic tectonostratigraphic intervals from their positions in SE Nevada into this region has proven to be a difficult task. However, data presented in this thesis indicate that the chemostratigraphy of chert-argillite units can be preserved under such conditions, and thus may offer a method for resolving extensions of tectonostratigraphic terranes into more complex regions such as the Sierra Nevada batholith.
