Abstract

The intrusive suite of Yosemite Valley provides an excellent example of coeval mafic and felsic magmatism in a continental- margin arc. Within the suite, hornblende gabbros and diorites associated with the Cretaceous El Capitan and Taft Granites occur as scattered mafic enclaves, enclave swarms, small pods, synplutonic dikes, and a 2 km2 mafic complex known as the “diorite of the Rockslides.” Field evidence suggests that most of the mafic rocks are temporally related to the El Capitan Granite and that significantly less mafic magma accompanied the slightly later intrusion of the Taft Granite. Concordant zircon fractions from the diorite of the Rockslides yield an age of 103 ± 0.15 Ma, which is the same age as the El Capitan Granite. Initial isotopic compositions of the mafic and felsic rocks are similar; the mafic rocks exhibit only slightly higher 87Sr/86Sr, lower 143Nd/ 144Nd, and higher 206Pb/204Pb ratios than the granites. Because the mafic magmas are only slightly more isotopically evolved than the granites, geochemical variation within the granites is not easily explained in terms of contamination of a depleted-mantle component by partial melts of ancient, high-silica continental crust. Rather, these data are consistent with an interpretation that the El Capitan Granite was derived by partial melting of relatively young mafic sources broadly similar to the mafic rocks of the suite.

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