PETROLOGIC INVESTIGATION OF THE ENCLAVE-BEARING TRACHYDACITE TO RHYOLITE ANTELOPE LAVA, SOUTHERN BLACK MOUNTAINS, AZ

Antelope Lava (An) is an 18.50 Ma (McDowell et al 2016) trachydacite-rhyolite flow exposed in Meadow Creek basin within the Colorado River Extensional Corridor. Post-dating the Peach Spring Tuff (PST) supereruption (18.8 Ma, Ferguson et al 2013) and located within 4 km of its caldera, the An provides insights into post supereruption magmatism. The abundance of magmatic enclaves within An suggests that open system processes that may have contributed to the PST supereruption (Pamukcu et al 2013) continued into post-PST silicic eruptions. Characterization of magmatic enclaves and host An elucidates these processes and magma chamber conditions.

The An is a glassy flow containing mafic enclaves with crenulate margins, capped by a horizon of crystal-rich lava. The An contains bio, plag, alkali feld, and hbl phenocrysts within a glassy matrix; magmatic enclaves have plag + bio + cpx phenocrysts in a matrix of bio, hbl, and plag microlites. Plag phenocrysts in the An exhibit sieve texture, resorbed rims, rounding, and embayment, suggesting heating. Enclave whole rock compositions (55-57 wt% SiO₂, 1000-1100 ppm Sr [XRF]) are chemically distinct from host An (68-71 wt% SiO₂, 300-400 ppm Sr). Whole rock compositions of glassy and crystal-rich An are nearly identical. Al-in-hbl barometry (Schmidt 1992) of phenocryst rims by SEM-EDS suggests a pressure of ~5 Kbar for the crystal-rich An, notably higher than that of the glassy An (~3 Kbar), suggesting that the crystal-rich horizon magma was stored at greater depth. The zircon saturation temperature of glassy An, based on glass compositions (SEM, LA-ICP-MS) is ~720 (Boehnke et al, 2013). Similar age intrusive rocks and their enclaves (McDowell et al. 2016) that intrude the PST caldera show compositions that range between those of rhyolitic An and its enclaves, as well as somewhat more silicic granites. Linear compositional patterns for both major and trace elements suggest that intrusive and extrusive rocks may have been related, and that compositional variability may have resulted from mixing. The geochemical and petrological characteristics of the An and magmatic enclaves, and those of the possible intrusive equivalents, may indicate widespread rhyolitic-trachyandesite interaction that contributed to eruptions of both the PST and post-PST silicic flows.