GEOCHEMICAL EVIDENCE FOR A COMMON MAGMATIC SOURCE OF SOME DIFFERENT POST-MARKAGUNT MEGABRECCIA VOLCANIC ROCKS (MIOCENE), HIGH PLATEAUS, SOUTHWESTERN UTAH

Recent studies of several different volcanic rocks exposed within a small area of the Markagunt Plateau, including the group of spherulitic dikes at Sandy Creek (SD), the immense Sandy Peak volcanic mass (SP), the purported Sandy Creek pseudotachylyte (SC), and the enigmatic Haycock Mountain Tuff (HMT), suggest that they are all derived from the same parental to more evolved magmatic source and that they all postdate the Markagunt Megabreccia (MM).

Striking similarities and trends in concentrations of elements, including rare earth elements (REE), and trends in major oxides used for total alkali - silica classification (TA & S; volatile-free, weight % normalized to 100%) suggest that the SP (5.5% & 59.7% = andesite) evolved from the same magmatic source as the SD (4.4% & 55.2% = basaltic andesite). In addition, x-ray diffraction (XRD) data show that the mineralogy of both of these rocks is mostly a plagioclase (SD, 48.3% and SP, 57.5%) with the same chemical composition. Furthermore, the glassy nature of the SD is similar to the glassy SC and both of the rocks occur in fractures, preexisting joints and intra-MM contact planes which suggest that they are derived from the same generation and intrusion of melt, thus genesis of the SC also is related to the same magmatic source as the SD and the SP.

The proposed relationship of the highly evolved HMT (TA & S = 9.4% & 71.1% = rhyolite) with the other rocks is based upon consistent continuation of the same geochemical trends noted above and is further supported by the shape, position, and clockwise rotation of its chondrite-normalized REE concentration plot relative to those of both the SD and the SP, combined with the negative europium anomaly of the HMT plot. In addition, some of the ubiquitous xenoliths in the HMT are petrographically similar to the lithology of the SP north ridge.

The narrow age range of these rocks also supports their apparent comagmatic relationship: the SD is 22.59 ± 0.18 Ma (whole-rock, but XRD indicates the main mineral phase is plagioclase); the SP is 21.71 ± 0.31 Ma (amphibole; a cooling age); and the HMT is 22.7 ± 0.6 Ma (plagioclase), 22.75 ± 0.12 Ma (sanidine), and 21.63 ± 0.73 Ma (zircon). Overall the evidence suggests that all of these volcanic rocks are related to a single parental to more evolved magmatic source, perhaps located near Sandy Peak, and all of them postdate the MM.