GEOCHEMISTRY OF PSEUDOTACHYLYTES FROM THE MEGA-SCALE MARKAGUNT AND SEVIER GRAVITY SLIDES (UTAH)

The Marysvale volcanic field (MVF) in southwest Utah contains three mega-scale gravity slides: the Sevier Gravity Slide (SGS), the Markagunt Gravity Slide (MGS), and the Black Mountains Gravity Slide (BMGS). Pseudotachylyte (PST), a glass generated from frictional heat between rock surfaces during the high-velocity slip events, is found within the MGS and SGS at multiple locations within various lithologies. Jiang et al. (2015) suggests that the melt composition of the glass evolves toward its host rock lithology during slip events, indicating that short-duration events will generate more mafic PST. The primary minerals which comprise PST melt are feldspars and micas, due in part to their lower thermal conductivity and fracture toughness. Geochemical controls on PST generation remain enigmatic in the Marysvale Gravity Slide Complex (MGSC). Here we present laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) elemental concentrations, x-ray fluorescence (XRF) data, and corresponding petrology studies which investigate the range of PST compositions found in the MGS and SGS.

Five samples of PST were collected from the MGSC. Four samples are from the MGS. Three of these come from the Sandy Wash (SW) section, and the fourth is from a newly recognized location known as Hellhole (HH). One sample is from the SGS, known as Cherry Creek (CC). Backscattered electron images demonstrate that the selected glass shards are heterogeneous, consisting of subrounded to angular relict clasts of quartz or plagioclase surrounded by glass. These locations host PST in different lithologies. At SW, the three samples show a range of compositions, particularly in Fe, K, Al, and Mg. These compositions are distinct from those recorded in the HH and SGS samples. Minor elements, such as Zr, Mn, and Ba further support the notion of distinct chemistries among sites; however, rare earth and other trace elements, and their ratios, are not distinct among the samples. Recent literature states that biotite in the host rock is preferentially melted to form PST. The concentration of Fe and K are positively correlated and display an obvious divergence between SW and the other collection sites, which may be directly linked to the incorporation of biotite or K-feldspars derived from the host rock. Our findings support the melting models proposed by Spray (2010) and suggest that frictional melting of the host rock is localized at the micro-scale.