PRIMING FOR SUPERERUPTION: THE HOT PRE-PEACH SPRING TUFF LAVA FLOWS AND PEACH SPRING TUFF MAFIC ENCLAVES, BLACK MOUNTAINS, ARIZONA

Supereruptions are some of the most cataclysmic events on Earth, ejecting greater than 450 km³ of volcanic material. The 18.8 Ma Peach Spring Tuff (PST) erupted in what is now the southern Black Mountains, Arizona (Ferguson et al., 2013), with outflow deposits covering an area greater than 35,000 km² (Glazner et al., 1986). The volcanic deposits erupted just prior to PST supereruption provide important insights on pre-supereruption magmatic conditions in the Black Mountains volcanic center. The pre-PST volcanic sequence consists of a ~1 km thick suite of trachyte lavas, a relatively thin section of more mafic lavas, and Cook Canyon Tuff: an ignimbrite distinct from and smaller than PST (Pratt et al., 2014 GSA). We took samples of pre-PST mafic lavas, one trachyte lava, and magmatic enclaves within PST. Bulk analyses of samples were obtained with XRF, phenocryst compositions were determined by SEM, and magmatic temperatures were estimated through use of the Excel MELTS program and apatite-saturation modeling. An atypically hot (~1015°C), aphyric trachyte lava, last of the thick trachyte sequence, contrasts with the rest of the sequence at temperatures near 850°C (Rice et al., 2014 GSA) and is followed by the eruption of relatively mafic magmas that suggest significant heat input. Mafic lavas range from trachybasalts to basaltic trachyandesites, with estimated temperatures ranging from 1020-1150°C (assuming water content of 2 weight % and constant pressure of 200 MPa). Magmatic enclaves within the PST itself also range from trachybasalt to trachyandesite, and are similar geochemically and in phenocryst assemblage to the mafic lavas. Estimated temperatures of enclave magmas range from 1010-1075°C, similar to those of the mafic lavas. The only definitive magmatic enclave identified previously within the PST is similar in composition and petrographic characteristics to our enclaves (Pamukcu et al., 2013). The presence of mafic enclaves, alone, is an indicator of hotter magma injected in the (PST) chamber and our data suggests that pre-PST lavas are related to these enclaves. The hot trachyte flow, followed by mafic lavas and related enclaves within the PST, indicate heat input into the Black Mountains magmatic system preceding PST supereruption and are possible evidence of the eruption trigger (cf. Pamukcu et al., 2013).