INVESTIGATION OF MAGMATIC FEATURES AND GEOCHEMICAL PROPERTIES OF THE AZTEC WASH PLUTON, SOUTHERN NEVADA

The Miocene-aged Aztec Wash Pluton is located in southern Nevada, where tectonic extension and rotation have revealed a large vertical slice of the intrusion from the roof downwards. This exposure makes it an excellent body to study the spatial construction and petrogenesis of a granitic pluton and hence, continental crust. Stratigraphically, from top down, the Aztec Wash consists of relatively homogeneous upper granite (the Granite Zone) underlain by the Heterogeneous Zone, made up of mafic, felsic and intermediate rocks (Miller and Miller, 2002). The ubiquitous exposures of intermingling mafic and felsic rocks lead to questions about the relationship of mafic magmas to granite genesis. We utilized a handheld X-Ray Fluorescence Analyzer to examine these small-scale chemical changes in the field. Preliminary data show significant changes in some element concentrations leading up to and across these contacts. Further investigation will involve SEM analysis of larger hand samples containing contacts between granites, intermediate and mafic rocks.

Bulk samples collected along spatial transects through the pluton are being analyzed for iron isotope ratios to test for thermal gradient effects. Transects aligned along the approximate paleo-horizontal and paleo-vertical axes of the pluton varied in length from tens of meters to kilometers in scale. MC-ICPMS analysis was performed in high-resolution mode using a bracketed double spike method with a 2s precision of +/-0.04‰. Felsic samples range in δ⁵⁶Fe from +0.13‰ to +0.30‰ while mafic samples have δ⁵⁶Fe between +0.08‰ and +0.16‰ and intermediate samples have δ⁵⁶Fe values between +0.13‰ and +0.16‰, thus following previous patterns of isotope ratio correlating with degree of differentiation. So far, no clear trend appears in iron isotope data along a paleohorizontal transect from the country rock in to the Granite Zone and ending in the Homogeneous Zone.

Miller C.F. and Miller J.S. (2002) Contrasting stratified plutons exposed in tilt blocks, Eldorado Mountains, Colorado River Rift, NV. USA Lithos 61, 209 – 224.