GEOCHEMICAL RELATIONS BETWEEN MAFIC INTRUSIONS IN POST-PEACH SPRING TUFF DEPOSITS IN THE SOUTHERN BLACK MOUNTAINS, ARIZONA

Mafic rocks intrude & overlie the rhyolite- & trachyte-dominated Miocene volcanic strat of Meadow Creek basin, Southern Black Mountains, AZ. The Meadow Creek sequence postdates formation of the nearby Silver Creek Caldera, source of the 18.8 Ma Peach Spring Tuff (PST) supereruption (Ferguson, et all, 2012). The mafic magmas intruded the volcanic center & formed capping lava. The chemistry of the intruding mafic magma may provide insights into the magmatic evolution of the system as it waned.

Three large, NNW-striking aphanitic dikes that cut the younger lavas & tuffs are compositionally & texturally similar trachyandesites with sparse phenocrysts of plag and cpx & a plag-dominated groundmass (SiO₂ 55-57 wt.%, Fe₂ O₃ (t) ~7%, MgO ~3%, K2O ~ 3%, Sr ~ 700 ppm, Ba ~1123ppm, Zr ~ 300ppm, Rb ~60 ppm [XRF]). Another parallel dike & a lopolith are diabase-textured (ol & laths of plag with interstitial cpx traces of apatite, Fe-Ti oxide, & interstitial amphibole) & more mafic (SiO₂ 49-51 wt.%, 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 a similar elemental composition. MELTS modeling indicates that these mafic magmas were emplaced at ~1200° C. The diabases & lava define a clear tholeiitic Fe-enrichment trend on an AFM ternary. The more silicic dikes are clearly distinct from the diabase & 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 & 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) & form an identical tholeiitic trend to mafic rocks in Meadow Creek basin. This compositional shift corresponds to the evolution in tectonic setting from early synextensional to late or post-extensional.