Understanding the origin of granitic rocks is an essential part of reconstructing earth history. It is likely that several processes (fractional crystallization, restite unmixing, and magma mixing) are important. Isotopic and chemical data have suggested that mixing of mafic and felsic magmas is widespread, producing commingled rocks to more thoroughly hybridized rocks, depending on compositions, temperatures and relative proportions of the interacting magmas. Rheological modeling predicts that commingling should predominate, and small-scale composites dikes support this prediction. In larger plutons, however, hybridi- zation has apparently occurred under conditions that models suggest are prohibitive of mixing. This dichotomy suggests that mechanisms other than two fluid mixing may be operative in the large plutons. Murray and Reid will perform a coordinated field, petro- graphic, geochemical and isotopic study of localities in Yosemite National Park to test the viability of two additional hybridization mechanisms, disintegration of chilled globules and contamination of basalts with wallrock to more easily mixed intermediate compositions. Sites include a suite of partially disaggregated pillows in the El Capitan and Tuolumne granites and a series of contaminated basalts in the North America Wall, El Capitan. This project will enhance geologic education at both insti- tutions by involving students in all aspects of the study: field and analytical work, participation in research seminars and professional meetings.
