TRANSITION FROM PLUTONISM TO VOLUMINOUS DIKING IN THE ELDORADO MOUNTAINS, NORTHERN COLORADO RIVER EXTENSIONAL CORRIDOR, NEVADA

The Aztec Wash pluton (AWP) was emplaced in the Colorado River extensional corridor as a series of mafic and felsic injections near the onset of rapid E-W extension (~15.6 to 16.1 Ma, SHRIMP U-Pb, Cates et al., 2003). Two sets of large N-S striking dikes intruded AWP a short time later (one analysis each, 15.5±0.2 Ma, Cates et al.; 15.5±0.1 Ma, Ar/Ar, Falkner et al., 1995). The western set (Eldorado swarm) comprises mostly felsic porphyries, some with sparse mafic enclaves, and less common mafic-intermediate dikes. This swarm extends southward through older country rock for 20 km, where it cuts the 15.8-17.7 Ma Searchlight pluton (Miller et al., 2003). Dikes are typically 2-5 m thick and cut both AWP and country rock with sharp, planar contacts. The eastern dikes are fine-grained phaneritic granites, locally choked with basaltic pillows. They are thicker than Eldorado dikes (5-200m), near vertical to E-dipping, and as a group extend ~5km entirely within AWP. The dikes splay, anastamose, and have locally lobate or irregular contacts; in rare exposures dike tips disaggregate into dispersed lenses within host granite. These relationships suggest that the host was hot and may have contained melt, either residual or as a consequence of heating by very large dikes. The dominant felsic magmas in both sets are almost identical to what are interpreted to be AWP input granitic magmas (73 wt% SiO2, K2O 5%, 87Sr/86Sr 0.710-0.712). Both sets also show elemental and isotopic evidence for contamination by mafic magma.

Paleomagnetic data indicate that both dikes and hosts for the eastern set are steeply E-tilted (60-90°), suggesting that they were emplaced as gently dipping sheets guided in part by sheeted anisotropies in the AWP. In contrast, the western dikes are tilted moderately NW and were emplaced subvertically. These data indicate that the pluton was deformed into a broad anticline, consistent with interpretations based on magmatic structures.

We propose that magmas transiting the crust switched from ponding (pluton formation) to diking as extension rates increased. The eastern dikes were emplaced when the AWP was still warm. Slightly later, increasing extension rates and a more rigid crust facilitated the larger Eldorado dike swarm.