GEOCHEMICAL AND PETROGRAPHIC RELATIONS BETWEEN MAFIC INTRUSIONS IN POST-PEACH SPRING TUFF DEPOSITS IN THE SOUTHERN BLACK MOUNTAINS, MEADOW CREEK BASIN, NEAR OATMAN, ARIZONA

Mafic rocks intrude and overlie the rhyolite- and trachyte- dominated Miocene volcanic stratigraphy of the Meadow Creek basin in the Southern Black Mountains, AZ. The Meadow Creek sequence postdates formation of the nearby Silver Creek Caldera, the source of the 18.8 Ma Peach Spring Tuff (PST) supereruption (Ferguson, et all, 2013). Mafic magmas that intruded this volcanic center and formed capping lavas can provide insights into the evolution of magmatism as the silicic system that fed the supereruption waned.

Three large, NNW-striking aphanitic dikes that cut the younger lavas and tuffs are compositionally and texturally similar trachyandesites with sparse phenocrysts of plagioclase and cpx and a plag-dominated groundmass (SiO2 55-57 wt%, Fe2O3 (t) ~7%, MgO ~3%, K2O ~ 3%, Sr ~ 700 ppm, Ba ~1123ppm, Zr ~ 300ppm, Rb ~60 ppm[by XRF]). Another parallel dike and a lopolith are diabase-textured (olivine and laths of plag with interstitial cpx; traces of apatite, Fe-Ti oxide, and interstitial amphibole) and more mafic (SiO2 49-51%, Fe2O3(t) 11%, MgO 5-8%, K2O 0.3-0.7%, Sr 350-530ppm, Ba 320-430ppm, Zr 110-135ppm, Rb 20-30ppm). Capping lava has similar elemental composition. MELTS modeling indicates that these mafic magmas were emplaced at ~1200° C. The diabases and lava define a clear tholeiitic Fe-enrichment trend on an AFM ternary. The more silicic dikes are clearly distinct from the diabases and cluster tightly near the calc-alkaline:tholeiitic boundary.

The diabase intrusions are likely part of a system that fed the ~14-16 Ma capping lavas of the region, which have been interpreted to have been late- to post extensional (Thorson, 1971; Faulds et al., 2001; Varga et al 2004). Proximity and orientation of the trachyandesite dikes suggests that they are part of the same intrusive episode, but their distinct characteristics indicate that they not closely related petrogenetically.

Mafic to intermediate volcanics that predate the PST in the southern Black Mountains form a continuous calc-alkaline compositional trend (Flansburg et al 2014), while younger capping lavas are uniformly basalt (Spencer et al 2007) and form an identical tholeiitic trend to mafic rocks in Meadow Creek basin. This compositional shift corresponds to evolution in tectonic setting from early synextensional to late or post-extensional.