LONGEVITY OF PLUTONIC SYSTEMS: SHRIMP EVIDENCE FROM AZTEC WASH AND SEARCHLIGHT PLUTONS, NEVADA

Issues of longevity of magma systems, residence time of magmas in chambers, and timing of episodes of crystallization have been addressed recently through studies of volcanic output of young systems (e.g. Brown & Fletcher, 1999; Vasquez & Reid, 2002), but few relevant quantitative data exist that provide constraints from a plutonic perspective. We are using the Stanford/USGS SHRIMP to investigate the ages of zircon growth in two shallow-level Miocene plutons in the Eldorado Mountains, Nevada. Although both record mafic and felsic recharging and vertical accumulation of solidified products, they differ in that Searchlight pluton is much thicker (~10 km) and is dominated by relatively felsic cumulates, whereas most of the ~3 km thick Aztec Wash pluton records repeated interaction between mafic and felsic magma. So far, we have determined U-Pb ages of 123 zircon spots from 5 samples from Aztec Wash and 73 spots from 3 Searchlight samples. These data demonstrate that the method can successfully identify and elucidate protracted magmatic histories for plutons.

Zircon populations from the Aztec Wash pluton suggest that it was active for a minimum of 200 ky prior to tilting and intrusion of later, related dikes. Three samples from the lower middle of the pluton are all 15.8 Ma (1s errors for individual samples ~0.15 Ma; MSWD's ~1.0), and a single sample at the top is 15.6 ± 0.2 Ma (MSWD 0.3); a late dike is 15.5 ± 0.1 Ma (MSWD 1.0). Two samples have distinguishable older populations between 16.0 and 16.4 Ma that are either xenocrystic (this is a common age range in regional volcanic and plutonic rocks) or indicate an earlier stage of pluton growth. Zircons from Searchlight pluton document a more extended history. A sample from the lower cumulate zone yields an age of 16.9 ± 0.2 Ma (MSWD 1.3). A granite from a thick felsic sheet and a fine-grained quartz monzonite from the roof both yield two populations, one at 16.7-17.0 Ma (n=20) and the other at 15.7-15.8 Ma (n=28)(all ± 0.2-0.4 Ma). The data suggest that this system lasted for at least a million years with at least two major episodes of zircon growth - either magma resided in this chamber for a very long time, or the pluton was "defrosted" by renewed thermal input after it initially solidified. Zircons from the earlier episode were entrained in magma from which the younger population grew.