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.

• Main Author: Nelson, Wendy Rae
• Format: Others
• Published: BYU ScholarsArchive 2006
• Subjects: anorthite; contamination; high-alumina basalt; isotope; magma mixing; mantle; micro-sample; Nd; oxygen isotopes; plagioclase; pluton; Sierra Nevada batholith; Sr; trace elements; Yosemite Valley; Geology
• Online Access: https://scholarsarchive.byu.edu/etd/367; https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=1366&context=etd

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. Whole rock major element variation diagrams indicate the mafic rocks mixed with a more silicic component, but extrapolating to the silica end member does not yield the same result with each element. Trace element concentrations show a wide variation in concentration of 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 samples have the most primitive epsilon Nd values (-3.3, -3.5 at 100 Ma) analyzed thus far for the intrusive suite, indicating the suite has a larger range of isotopic values than previously thought. Delta 18 oxygen for Rockslides samples vary from 6.6 to 7.5 per mille (6 samples, average 7.03), higher than the 5.5 + 0.3 range for the mantle, indicating the presence of a crustal component in the system. Plagioclase phenocrysts within each unit display bimodal compositional populations. Subhedral to euhedral partially resorbed calcic cores (mode = An84-88) are reminiscent of a mafic magma, while sodic rims (mode = An48-50) are the product of a more silicic component. Very little to no intermediate zoning is present between cores and rims. Mineral-scale 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 plagioclase isotopic data show no direct evidence for a depleted mantle melt component contaminated by crustal assimilation. However, 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 provides an opportunity for magma contamination 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, especially since the bulk-rock oxygen isotopic values indicate a significant crustal component is present.