THE MONARCH DIVIDE INTRUSIONS: TRANSITIONAL MAGMATISM IN THE SIERRAN ARC

Building on the modern geochronologic framework of the Sierran arc, including abundant evidence of incrementally assembled plutons, we have examined a series of small-volume plutons of the Monarch Divide region on the north rim of Kings Canyon. This remote area has not been examined in detail since the regional geochemical study of the Marion Peak quadrangle by Moore (1991; USGS Bull 1986); however, these plutons are now recognized to sit (spatially and temporally) at an important transition arc, typified by smaller-volume plutons to the west, and the voluminous intrusions of the 95–85 Ma Sierra Crest plutons to the east. Thuse, these plutons represent some of the youngest, small-volume plutons in the central Sierra and are hypothesized to have chemical and textural traits that may record conditions leading to the Sierra Crest magmatic “flare-up”.

The Monarch Divide plutons includes 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 members from the Paradise Granodiorite of the Whitney Intrusive Suite. 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. The data show similar geochemical variety of analogous magmas but are generally more felsic in older members and become more mafic in younger members, with greater concentrations of mafic bodies and enclave swarms at edges of younger plutons.

U-Pb zircon ages show Kennedy Lakes and Tehipite Dome plutons as oldest (ca. 97 Ma) members of the intrusions; the White Divide pluton cross-cuts both Tehipite Dome and Kennedy Lakes and is 96 ± 2 Ma. One sample from the Pyramid pluton is 93.5 ± 2.0 Ma. New samples in the North Dome pluton are ca. 93 Ma and the mafic and felsic plutonic phases of the Dougherty Peak area are ca. 90 Ma and several additional ages are pending. Collectively, the data show that the older members of the suite, true granites, progressed to granodiorite and diorite magmas at <94 Ma with some of the youngest phases in the eastern areas of the Monarch Divide being similar in age to Sierra Crest magmas. Overall, the ~7 million year range of intrusive age shows a broad transition into granodiorite typical of Sierra Crest magmas.