The petrogenesis of andesites produced during regional extension: Examples from the northern McCullough Range, Nevada and Xitle Volcano, Mexico

Kelly Ann Boland, University of Nevada, Las Vegas


The northern McCullough Range andesites are calc-alkaline and contain phenocrysts of olivine, clinopyroxene, and plagioclase. Although the andesites are similar in major element chemistry, they exhibit wide variations in trace element chemistry. Resulting trace element Harker diagrams show considerable scatter and no discernible petrogenetic trends. On {dollar}\sp{87}{dollar}Sr/{dollar}\sp{86}{dollar}Sr and {dollar}\rm\varepsilon\sb{Nd}{dollar} isotope plots, however, andesites fall into 3 distinct groups: group A {dollar}\rm(\sp{87}Sr/\sp{86}Sr=0.70758{dollar} to 0.7077; {dollar}\rm\varepsilon\sb{Nd}=-6.2{dollar} to {dollar}-{dollar}6.9), group B {dollar}\rm(\sp{87}Sr/\sp{86}Sr=0.7081{dollar} to 0.7083; {dollar}\rm\varepsilon\sb{Nd}=-6.7{dollar} to {dollar}-{dollar}7.4), and group C {dollar}\rm(\sp{87}Sr/\sp{86}Sr=0.70897{dollar} to 0.70904; {dollar}\rm\varepsilon\sb{Nd}=-8.8{dollar} to {dollar}-{dollar}9.8). Geochemical variations, isotopic data and petrogenetic modeling suggest that the andesites in the northern McCullough Range formed primarily from mixing together several different andesitic sources (represented by isotope groups A, B, and C) that are similar in chemistry but isotopically distinct. This proposed model involving mixing of different andesitic end members explains the andesite dominated section observed in the McCullough Range and provides an alternative explanation for andesite petrogenesis in extensional terranes. A preliminary study of Xitle volcano in central Mexico shows similarities to the McCullough Range results. (Abstract shortened by UMI.).