CONNECTION BETWEEN IGNEOUS ACTIVITY AND EXTENSION IN THE CENTRAL MOJAVE METAMORPHIC CORE COMPLEX, CALIFORNIA

The development of metamorphic core complexes and associated low-angle detachment faults commonly is intimately associated with synextension al igneous activity. In most areas studied to date, the relation of ma gmatism to extension is obscured by imprecise dating and by the overpr int of later tectonic events. We present data from the early Miocene c entral Mojave metamorphic core complex (CMMCC) which indicate that ext ension was accompanied by igneous activity, as reflected by prekinemat ic, synkinematic, and postkinematic plutons and coeval volcanic rocks deposited in the associated extensional basins. The principal intrusio n is an early Miocene granite pluton exposed in outcrops across an are a greater than 400 km(2). Dikes adjacent to the pluton are common in t he Mitchel Range, at The Buttes, and at Fremont Peak. The overall orie ntation of the pluton and associated dikes is west-northwest, roughly perpendicular to the extension direction. Results of U-Pb analyses on zircon from two pluton and two dike samples yield ages of 20 to 23 Ma. Two other dike samples yield inconclusive results. Synextensional bas ins formed by detachment faulting during the core complex development. Rocks in these basins compose the Jackhammer and Pickhandle formation s and filled an elongate, NW trending trough more than 50 km long. The Ar-40/Ar-39 ages for tuff beds are as old as 23.8 +/- 0.3 Ma near the base of the lower Pickhandle Formation and as young as 21.3 +/- 0.5 M a in the uppermost lower Pickhandle. Hence volcanism and plutonism are coeval. The diversity of intrusive relations relative to the timing a nd development of the mylonitic fabric in the CMMCC precludes any simp le cause-and-effect relationship between magmatism and extensional def ormation. Rather, magmatism and extension may have been localized at a releasing bend in a transfer-fault system which links extension in th e CMMCC with extension in the Colorado River area to the east.