TUFF OF BONELLI HOUSE, PART 1: PETROLOGIC CHARACTERIZATION IN TYPE AREA AT KINGMAN, REGIONAL CORRELATION AND CONSTRAINTS ON EXTENT

New mapping in and along the margins of the Colorado River extensional corridor (Spencer et al 2006; Ferguson & Cook, 2015; Ferguson & Howard, in prep) identifies a pair of thin rhyolite ignimbrites that overlie the Peach Spring Tuff (PST) in at least three localities: Kingman, the southern Black Mountains (BM), AZ, and the Sacramento Mountains (SM), CA. The ignimbrites are petrographically similar enough to the PST to warrant investigation into how they might relate to that ignimbrite’s 18.8 Ma super-eruption and source caldera near Oatman, AZ (Ferguson et al, 2013). At each locality, the pair are separated by a thin (<15 cm) ultraplinian ash layer. At Kingman and SM, sandstone and conglomerate separate the pair from underlying PST; basalt lava intervenes in BM. We present field, petrographic, and geochemical characteristics of the ignimbrites in the three areas.

At Kingman, the type area for the ignimbrite pair, where it has been named collectively the tuff of Bonelli House (TB), an Afs Ar date of the upper unit yields an age of 17.72 ± 0.01 Ma (Ferguson & Cook, 2015). The ignimbrites are dominated by 1-7% <4mm alkali feldspar (Na-sanidine–anorthoclase), with minor Pl and traces of Bt, Mag, Ilm, Zrn, and chevkinite, and small, sparse lapilli of pumice (<5%) and lithics (<1%). Whole rock compositions cluster at 72.5-74 wt% SiO₂, 1.5-1.8 Fe₂O₃, 5.1-5.9 K₂O, 1.5-2.2 CaO, 0.20-0.23 TiO₂; 150-200 ppm Rb, 30-90 Sr. Zr is high and more variable (480-930 ppm).

In the BM and SM, the apparent TB sequence is similarly poor in phenocrysts, pumice, and lithics. Whole rock elemental composition of one sample from BM is identical to type TB, and glass in a pumice fragment is very similar elementally to glasses in type TB pumices. SM samples are all highly altered (e.g. 0.5-1.5% Na₂O, 1.9-4.8% CaO, 4-9.5% K₂O) and cannot be confidently compared with type TB. One SM sample is similar in many respects to type TB (74% SiO₂, 1.5% Fe₂O₃, 1.9% CaO, 45 ppm Sr); other SM samples range from 71 to 78% SiO₂, are highly variable in most other elemental concentrations, and considered unreliable.

The BM tuffs are confidently correlated with type TB, and thus that TB extends at least 35km from Kingman. Based on general characteristics, it is likely that the tuffs in the Sacramento Mountains are also correlative, but our geochemical data do not provide conclusive ties.