EXPLOSIVE SILICIC VOLCANISM PRECEDING THE PEACH SPRING TUFF SUPERERUPTION, PART 1: GEOCHEMICAL COMPARISON AND TENTATIVE CORRELATION OF FALL DEPOSITS IN THE SOUTHERN BLACK MOUNTAINS, KINGMAN AREA, AND ADJACENT COLORADO PLATEAU, AZ

Prior to supereruption of the 18.8 Ma, rhyolitic Peach Spring Tuff (PST) in the southern Black Mountains, AZ (Ferguson et al 2013), most volcanism in the Colorado River extensional corridor was effusive and intermediate in composition. The only large pyroclastic flow, the ~18.9 Ma Cook Canyon Tuff (CCT), was trachytic (Buesch & Valentine 1986; Lidzbarski 2014). Three fallout tuffs are exposed at Kingman, AZ, immediately below and above CCT and just beneath PST. On the basis of feldspar compositions, Buesch (1993) concluded that the lower two deposits were associated with eruption of CCT and that the upper tuff correlated with Fort Rock Creek Rhyodacite tuffs on the Colorado Plateau (Fuis 1973). Ferguson & Cook (2015) mapped the three fall deposits and associated sediment in the Kingman area as Unit of Fort Rock, based on Buesch’s interpretation of the uppermost deposit. Other prominent but undescribed fall deposit tuffs are present at similar stratigraphic levels in the southernmost Black Mountains (SBM). Elemental compositions of tuffs, pumices, glasses, and minerals from these deposits (XRF, SEM-EDS, LA-ICPMS) allow us to evaluate correlations among these units. Scheland et al (2016) present initial petrogenetic implications of ongoing work on this growing data set.

Fall deposits immediately below and above CCT at Kingman are similar to CCT in both WR (SiO2 62-66 wt%; Sr 500-800 ppm, Ba 1200-1300) and glass (SiO2 ~71 wt%, Sr 220 ppm, Ba 1000 )(Perry et al 2015). The uppermost fall deposit in the Kingman section is distinct from CCT and may correlate with Fort Rock tuffs as suggested by Buesch, with WR (SiO2 68 wt %, Sr 260 ppm, Ba 950) and glass (SiO2 ~76 wt%, Sr 50 ppm, Ba 150 ppm); Fort Rock tuff samples have WR SiO2 67-70 wt%, Sr 300-500 ppm, Ba 500-700) and glass SiO2 76 wt%, Sr 290 ppm, Ba 740) . The only true rhyolites among the fall deposits are those from SBM, with 73-75 wt% SiO₂ (Sr <200 ppm, Ba <400) in pumice and 77 wt% SiO2 (Sr 40 ppm Sr, Ba 230 ).

Our results verify that fall deposits enveloping CCT were derived from the CCT magma chamber, and that they may provide clues regarding that large eruption. They permit but are less conclusive reagarding correlation of the uppermost Kingman deposit with Fort Rock tuff. SBM deposits are clearly distinct from and do not correlate with any of the other deposits.