Relation of peralkaline magmatism to heterogeneous extension during the Middle Miocene, southeastern Nevada

Abstract

Volcanism migrated southward in the northern Basin and Range province in the Oligocene and early Miocene to produce voluminous calcalkaline silicic ash flow tuffs. Alkaline volcanism became dominant by middle Miocene (17–14 Ma) as smaller volumes of rhyolite-trachyte-basalt suites were erupted from the relatively small Kane Springs Wash caldera complex including the Narrow Canyon, Boulder Canyon, and Kane Springs Wash calderas in southeastern Nevada. Only minor extension affected the Kane Wash area before the end of calcalkaline activity, but extension expressed by rate of progressive stratal tilt peaked (15–13.5 Ma) with peralkaline magmatism (14.7–14.4 Ma). Variations in distribution, degree, style, and timing of deformation demonstrate heterogeneous extension in the Kane Wash area. Only minor extension and tilting persisted post-middle Miocene (<12 Ma). All major eruptive sources overlap domains of rapid extension. Most of the eruptive volumes from the two oldest calderas of the complex apparently pooled within their calderas, creating outflow deficits. Denudation faulting associated with magmatic tumescence may have followed preexisting active extensional fault systems to unload magma chambers, thus triggering eruptions into structural depressions. Evolution of alkaline magmas is demonstrated by progressive increases in peralkalinity and high field strength elements such as Zr, Y, and Nb. Nd, Pb, and Sr isotopic compositions provide evidence that significantly less crustal interaction affected middle Miocene peralkaline magmas than pre-middle Miocene calcalkaline magmas. ε Nd values are −5 to −7 for peralkaline magmas and −7 to −11 for calcalkaline magmas; 208 Pb/ 204 Pb ratios are 38.2–38.6 for peralkaline magmas and 38.5–38.9 for calcalkaline magmas. Regional cooling, short duration of magmatism, small volumes of magma, and local extension caused less crustal interaction in peralkaline Kane Wash magmas than in earlier magmas. North of the Kane Wash area, older more voluminous calcalkaline magmas intruded hotter crust for a longer period and thus interacted with the crust to a greater degree in spite of synvolcanic extension.