HR: 1340h
AN: V13B-0546    [Abstracts]
TI: Mineral-Scale Sr Isotopic Study of Plagioclase in the Mafic Dikes of the North American Wall and the Diorite of the Rockslides, Yosemite Valley, California
AU: * Nelson, W R
EM: wnelson@geosc.psu.edu
AF: Pennsylvania State University, Dept. of Geosciences, 503 Deike Building, University Park, PA 16802 United States
AU: Dorais, M J
EM: mike_dorais@byu.edu
AF: Brigham Young University, Dept. of Geology, S335 ESC, Provo, UT 84602 United States
AU: Hart, G
EM: ghart@wsu.edu
AF: Washington State University, Dept. of Geology, PO Box 642812, Webster 1228, Pullman, WA 99164 United States
AB: The North American Wall mafic dikes and the diorite of the Rockslides mafic complex in the intrusive suite of Yosemite Valley show evidence of mixing with their host granites as well as with earlier components. Rocks sampled are mostly gabbro to gabbroic diorite in composition. Whole rock major element variation diagrams indicate the mafic magma mixed with a more silicic component, but extrapolation to the silica end member does not yield the same result for each element. Mafic rocks show a wide variation in trace element concentrations, especially in Cr and Ni, with two samples showing enrichment in Cr (>300 ppm) and Ni (~44 ppm) compared to other samples (Cr =13-94 ppm; Ni = 5-26 ppm). These two samples also have the most primitive whole rock isotopic values determined to date for Yosemite Valley rocks, with ε_Nd = -3.3 to -3.5 compared to -5 to -7 for other mafic rocks of the suite. Plagioclase phenocrysts within each unit display bimodal compositional populations. Subhedral to euhedral partially resorbed calcic cores (mode = An_84-88) are reminiscent of a mafic magma, while sodic rims (mode = An_48-50) are the product of a more silicic component. Very little to no intermediate zoning is present between cores and rims. Mineral-scale LA-ICP-MS ⁸⁷Sr/⁸⁶Sr analysis of plagioclase cores and rims are consistent with previously published enriched bulk-rock ratios for the suite (0.7065-0.7078), but are unable to distinguish between mixing components. The data show no direct evidence for a depleted mantle melt component contaminated by crustal assimilation. The mafic rocks are comparable to high-alumina basalts, whose generation involves crystal fractionation and magma mixing/crustal assimilation. The evolution of these high-alumina basalts may have provided an opportunity for assimilation to take place before plagioclase crystallization, thus explaining why plagioclase core-rim analysis could not distinguish between mixing components. Therefore, it is possible but not necessary to derive the rocks from an enriched mantle source.
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005