SITGREAVES TUFF, SOUTHERN BLACK MOUNTAINS, NW AZ: PHASE CHEMISTRY AND AR DATING ILLUMINATE THE EVOLUTION OF A DYNAMIC SILICIC MAGMATIC SYSTEM

The Sitgreaves Tuff (SGT; Ransom 1923) is one of the youngest and most voluminous silicic units in the southern Black Mountains. We present ⁴⁰Ar/³⁹Ar (Sa) ages and glass, mineral, and whole-pumice elemental chemistry from the largest exposure, below Sitgreaves Pass in Meadow Creek Basin, 6 km east of Silver Creek Caldera (source of the 18.8 Ma Peach Spring Tuff (PST) supereruption; Ferguson et al. 2013). Elucidating processes that contributed to the evolution of SGT magma may provide insights into the waning stages of this giant magmatic system.

We distinguish two distinct SGT units. The lower unit, SGT1, a ≤30 m unit of interbedded pyroclastic flow and fall deposits, yields an ⁴⁰Ar/³⁹Ar age of 17.65 Ma. The upper unit, SGT2, a ~140 m unit of pyroclastic flow and minor fall deposits, yields an ⁴⁰Ar/³⁹Ar age of 17.41 Ma.

SGT1 (Plag>Afs>Bt >Amp>Qtz + Ttn, Zrn, Ap, Chev, Mag) contains two mingled glasses: (1) 74 wt% SiO₂, 340 ppm Sr, 720 ppm Ba, Eu/Eu* 0.7, strong HREE depletion; T_{zrc sat} ~820 ºC; (2) 77 wt% SiO₂, 100 ppm Sr, 210 ppm Ba, Eu/Eu* 0.7, strong depletion in both MREE and HREE; T_{zrc sat} ~720 ºC. REE patterns indicate amphibole fractionation and, for glass (2), Ttn fractionation. These data, together with ubiquitous, abundant amph+bio phenocrysts, suggests evolution under relatively wet and cool conditions. Al-in-amph barometry for SGT1 suggests storage pressures of ~200 MPa. SGT1 Afs (avg. Ca₃Na₂₈K₇₁) are oscillatory zoned and contain multiple Ba rich zones, reflecting fluctuating growth conditions.

SGT2 (Afs>Qtz>Am + Ttn, Zrn, Ap, Chev, Mag) has a single glass: 77 wt% SiO₂, very low Sr (4 ppm), Ba (3 ppm), and Eu/Eu* (0.06), enriched in HREE; T_{zrc sat} ~760ºC. REE, Sr, and Ba indicate extensive feldspar fractionation and delayed Ttn and amph saturation, suggesting a relatively hot, dry environment. R-MELTS and amph barometry indicate storage pressures of ~150-200 MPa. Afs (avg. Ca₁Na₅₇K₄₂) display monotonic trace element core to rim profiles (12 to 4 ppm Ba) reflecting stable growth conditions; patchy sodic zones (avg. Ca₃Na₇₄K₂₃) indicate near- or subsolidus exsolution.

SGT data suggest that the magmatic system following the PST supereruption was dynamic, transitioning from open-system, cool–wet conditions (SGT1) to closed system, hot–dry conditions (SGT2), producing more evolved magma compositions over a 240 ky period.