The Miocene Goldfield-Superstition volcanic province, Central Arizona, USA: Geochemically distinct rhyolite sources, 20.5 to 19 Ma

Abstract

Abstract The Miocene Goldfield-Superstition volcanic province (G-SVP) in central Arizona is composed of silicic pyroclastic and lava flows, and, to a small extent, silicic domes, and mafic and intermediate lavas. Volcanism began ~20.5\u202fMa with small-scale silicic domes, followed by ≤1.5\u202fm.y. of basaltic, andesitic, and dacitic lavas, after which large-scale pyroclastic flows and lavas created the bulk of this large province from ~19 to 18.5\u202fMa (~8000\u202fkm2). Rhyolite compositions dominate the G-SVP and our observations are that the earliest expressions, small-scale domes, had crustal sources geochemically different from those for the main mass of lavas and ash-flow tuffs. In particular, three early domes, while rhyolites, have notably low REE and HFSE abundances for high-SiO2 rocks (e.g., La 8–34\u202fppm; Zr 32–92; Nb 8–17, Th 6–7), generally lower than the abundances in overlying alkalic basalt lavas. They form two groups: one is higher in SiO2 (~77–79\u202fwt%) than the other (~70–73\u202fwt% SiO2). Both groups are crystal poor, from ~3 to 8\u202fvol%. Modal mineralogy is mainly oligoclase, alkali feldspar, and quartz in the high-SiO2 group, and andesine and biotite in the low-SiO2 group. The high-SiO2 rhyolite domes have REE patterns that are nearly flat (La/Yb(n) 9–16), whereas the low-SiO2 domes have steeper REE patterns (La/Yb(n) 46–56). In comparison, the lavas and tuffs that erupted beginning ~1.5\u202fm.y. later have REE patterns and higher REE and HFSE abundances (e.g., La largely 45–80\u202fppm; Nb 17–30; Th 9–20) characteristic of rhyolites in other western North American Oligocene-Miocene silicic large provinces. Their modal mineralogy (~2–13\u202fvol% tuffs; ~5 to 26\u202fvol% lavas) is andesine, oligoclase, alkali feldspar, quartz, biotite, edenitic amphibole, +/− pyroxene, titanite, zircon, and REE-rich perrierite. Our interpretation is that the earliest magmatism in the G-SVP – namely the domes – represents small percent (e.g., ≤10%) partial melts of lower crust amphibolite that had low trace element abundances, both with and without garnet (to influence the REE pattern slopes). Melting is attributed to heat from early stages of basaltic magmatism. The younger rhyolite lavas and pyroclastic flows are products of middle to upper granitoid crust anatexis (e.g., 40–60%) resulting from continual basalt magma injections up to 2\u202fm.y. following dome emplacement (i.e., from ~20.5 to 19–18.5\u202fMa). The G-SVP therefore chronologically and geochemically tracks the change in sources for rhyolites over ~2\u202fm.y. of volcanism to create a central Arizona large igneous province.