UNDERSTANDING PRE-SUPERERUPTION MAGMATIC PROCESSES: MULTIPLE VARIETIES OF ENCLAVES IN PRE-PEACH SPRING TUFF TRACHYTE LAVAS

The northern Colorado River Extensional Corridor of northwestern Arizona, southeastern California, and southern Nevada was the site of repeated effusive and explosive eruptions from 16-19 Ma (e.g. Faulds et al. 2001). By far the largest of these was the Peach Spring Tuff (PST) supereruption (e.g. Glazner et al. 1986), whose source was the Silver Creek caldera in the southern Black Mountains of northwestern Arizona. In this area, the PST was preceded by eruption of ~1000 km³ of trachytic lavas. Understanding these pre-supereruptive lavas may provide insight into the processes that lead to supervolcanoes. The pre-PST trachytes contain a variety of enclaves that illuminate the history of the magmatic system. Field reconnaissance and sampling followed by XRF and SEM analysis revealed three different types of enclave included in the pre-PST trachytes: (1) Mafic porphyry with aphanitic groundmass, (~20% phenocrysts: biotite, hbl, plag, 59%wt SiO₂), (2) trachyte porphyry with aphanitic groundmass, (~30% phenocrysts: biotite, plag, several samples 62-69 %wt SiO₂), and (3) fine grained phaneritic monzodiorite (plag, biotite, oxides, 58%wt SiO₂). Apatite saturation thermometry suggests temperatures near to or above 1000°C for all three types of enclaves .

The presence of these multiple types of enclaves indicates various phases of magmatism. Trachyte porphyry enclaves have crenulate margins, porphyritic texture and a similar composition to the host lavas (mostly ~62-66 wt% SiO₂), suggest repeated injection of similar magmas into the system. The somewhat less silicic composition of phaneritic enclaves than trachyte host suggests entrainment of crystallized, possibly cumulate, remnants of an earlier-formed magma chamber wall. The porphyritic texture and crenulate margins on the magmatic enclaves suggest that the mafic magma was coeval with the trachyte, indicating mafic (and potentially) thermal input to the system.