NEW U-PB ZIRCON AND 40AR/39AR CHRONOLOGY OF MAGMATISM IN THE LAKE MEAD AREA, NEVADA AND ARIZONA

We report 37 new U-Pb zircon and ⁴⁰Ar/³⁹Ar dates on Miocene volcanoes and plutons, along with a compilation of existing dates, for the Lake Mead area of Nevada and Arizona, and use them to provide a revised temporal model for magmatism in the Lake Mead area. Based on the new dates, we conclude: 1) Although previously thought to have occurred in three magmatic pulses, at ca. 13 Ma, 8-10 Ma, and 4-6 Ma, magmatism in the Lake Mead area occurred continuously from 15 to 11 or perhaps 10 Ma, and then was followed by the 4-6 Ma episode. 2) The three major plutons, Wilson Ridge, Boulder City, and Railroad Pass, were built incrementally over a timespan covering most of the period of volcanic activity (Plutonism occurred from 14.9-12.6 Ma compared to 15-11 Ma for volcanism). 3) Though geographically, lithologically, and chemically distinct, the three plutons have at least some overlap in crystallization ages (14.5-14.0 Ma), which not only indicates magmatism was coeval over large areas, but that it will be more difficult to correlate plutons with their volcanic piles. 4) The River Mountains volcanoes, previously thought to be linked entirely to the Wilson Ridge Pluton, may be more piecemeal in fashion. For instance, the Powerline Road volcanic section of the northern River Mountains (13.3-12.9 Ma) may be related to the youngest phase of the Wilson Ridge Pluton (13.0-12.6 Ma). The southern River Mountains, with the River Mountains Stratovolcano and Bootleg Wash (13.5-13.6 Ma), the Red Mountain and Casino Dacite volcanic sections (14.5-14.0 Ma), and the two small stocks (13.7-13.5 Ma) are more comparable in age to the Boulder City and Railroad Pass Plutons. 5) The age and overall lithologic similarity between the Bootleg Wash and River Mountains Stratovolcano suggest they may be the same unit, unlike previously described. Similarly, the Red Mountain and Casino Dacite sections may be equivalent, although the degree of alteration in each make correlation difficult. 6) The northern McCullough Range Stratovolcano formed at 15.0-13.9 Ma, while the adjacent Railroad Pass Pluton crystallized 14.5-13.9 Ma and shows some zircon populations at 15-16 Ma. The main point is that magmatism in the Lake Mead area occurred continuously over a ca. 5 m.y. period with the 3 main plutons and associated volcanic centers forming coevally, at least for part of their history.