22 Advances In Mineralogy, Petrology, and Economic Geology (Posters)

Monday, 2 April 2012
8:00 AM - 12:00 PM
Grand Ballroom

PETROGENESIS OF THE ANDESITIC ELDORADO DIKE SWARM: LAST SURGE OF MAGMATISM AT THE SEARCHLIGHT PLUTON, NEVADA?

Paper number: 22-9
Program: Technical Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

Several plutons located within the Colorado River extensional corridor (CREC), a 50-100 km wide extensional belt that includes parts of Nevada, Arizona, and California, are tilted steeply to the west, which has revealed most of the entire thickness of these plutons. Plutonism in the region was restricted to a geologically brief interval around 16 Ma, and the last surge of magmatism may be related to the emplacement of the extensive ~15.5 Ma Eldorado dike swarm that cuts several plutons in the CREC and is interpreted to postdate these intrusions by as little as 0.2 Myr.

Fe-Ti geothermometer/barometer phase relations indicate a low-temperature /high oxygen-fugacity equilibration of binary oxides found in Eldorado dikes samples from the mafic pod at the Searchlight Pluton. The magnetite and ilmenite phases from sample APML-02 plot at high-oxygen fugacity (Δlog fO2 ~4), and at a low temperature of ~400-450 ºC, indicative of extremely slow cooling, perhaps due to the higher temperature, recent nature of the plutonism. This trend indicates a sub-solidus, late-stage exsolution of ilmenite in the binary oxides. Calc-alkaline crystallization trends and Eldorado dike mineral assemblages suggest a highly-differentiated calc-alkaline petrogenesis for the andesitic Eldorado dikes.

Hornblende mineral chemistry of synplutonic diorite samples from the mafic pod at Searchlight Pluton and from andesitic dike samples that cut the mafic pod indicate distinct compositional differences and reveal evidence for fractional crystallization and evolution of the magma source for this region. The diorite hornblendes are enriched in the mobile element K relative to the dike hornblendes, while the dike samples have a higher Mg#, pointing to a higher degree of differentiation for the andesitic Eldorado dikes. These relationships suggest that the Eldorado dike swarm represents a final late-stage surge of magmatism at Searchlight Pluton, NV.