AGE AND GEOCHEMISTRY OF THE MONARCH DIVIDE INTRUSIONS: TRANSITIONAL MAGMATISM IN THE SIERRAN ARC

This study updates examination of a series of small granite, granodiorite, and some mafic stocks of the Monarch Divide region on the north rim of Kings Canyon, central Sierra Nevada, CA. The area has not been examined in detail since the regional geochemical study of the Marion Peak quadrangle by Moore (1991; USGS Bull 1986). The plutons studied include the granites of Tehipite Dome and Kennedy Lakes, and granodiorites of White Divide, Lookout Peak, North Dome, and Muro Blanco. The east-west trending Pyramid pluton separates other plutons from the Paradise Granodiorite member of the Whitney Intrusive Suite, the youngest of the four voluminous Sierra Crest magmatic suites that punctuated magmatism in the Sierran Arc. In aggregate, these plutons are hypothesized to contain information about the magmatic arc state preceding the Sierra Crest magmatic “flare-up,” which makes them an important target to better contextualize temporally and geochemically. In this effort, representative samples of the plutons were analyzed for whole-rock major and trace elements, and zircons were analyzed for U-Pb ages via LA-ICP-MS. Limited radiogenic isotope analyses were also performed. U-Pb zircon ages of multiple samples each of the granites of Kennedy Lakes and Tehipite are ca. 96 Ma and 95 Ma, respectively, making them the oldest intrusions studied. The White Divide pluton cross-cuts both the Tehipite Dome and Kennedy Lakes plutons is ca. 94 Ma. The Pyramid pluton is ca. 92 Ma; North Dome ca. 91 Ma in its center and southern margin; and the mafic and felsic phases of the Dougherty Peak intrusion are ca. 90 Ma. Collectively, the data show that the older members of the suite, true granites, gave way to granodiorite and diorite magmas in eastern areas, but with the North Dome pluton being a notable compositional exception. Values of εNd are highest in the 94–96 Ma granites ( –3.57 to –4.42), and lowest in the Pyramid pluton (εNd = ­–6.6 to –8.4), with other intrusions having intermediate values of εNd. Thus, the 6 million year history of magmatism recorded in the Monarch Divide plutons shows a broad transition into granodiorite typical of the Sierra Crest event. Paradoxically, the older granites studied have the lowest amount of aged crust in their sources, and the most mafic pluton has the highest amount of aged crust in its source.