Geology and geochemistry of mafic to felsic plutonic rocks in the Cretaceous intrusive suite of Yosemite Valley, California

The intrusive suite of Yosemite Valley provides an excellent example of coe val mafic and felsic magmatism in a continental-margin arc. Within the suit e, hornblende gabbros and diorites associated with the Cretaceous El Capita n and Taft Granites occur as scattered mafic enclaves, enclave swarms, smal l pods, synplutonic dikes, and a 2 km(2) mafic complex known as the "diorit e of the Rockslides." Field evidence suggests that most of the mafic rocks are temporally related to the El Capitan Granite and that significantly les s mafic magma accompanied the slightly later intrusion of the Taft Granite. Concordant zircon fractions from the diorite of the Rockslides yield an ag e of 103 +/- 0.15 Ma, which is the same age as the El Capitan Granite. Init ial isotopic compositions of the mafic and felsic rocks are similar; the ma fic rocks exhibit only slightly higher Sr-87/Sr-86, lower Nd-143/Nd-144, an d higher Pb-206/Pb-204 ratios than the granites. Because the mafic magmas a re only slightly more isotopically evolved than the granites, geochemical v ariation within the granites is not easily explained in terms of contaminat ion of a depleted-mantle component by partial melts of ancient, high-silica continental crust. Rather, these data are consistent with an interpretatio n that the El Capitan Granite was derived by partial melting of relatively young mafic sources broadly similar to the mafic rocks of the suite.