HETEROGENEOUS GLASSES PRESERVE CHEMICAL AND DYNAMIC PROCESSES IN PARTIALLY REMELTED AND REMOBILIZED CUMULATE OF THE PEACH SPRING TUFF (CA-NV-AZ), USA

The 10³ km³ Peach Spring Tuff (18.8 Ma) erupted from Silver Creek caldera in the southern Black Mountains, AZ (Ferguson et al. 2013). Ferguson & Cook (2015) identified five zones within outflow sections near Kingman, AZ, on the basis of welding and phenocryst, lithic and pumice abundances. The lower four zones (Tp1-Tp4) comprise early-erupted, phenocryst-poor (~5-15%), high-silica rhyolite tuff (HSR; ~74-76 wt % SiO₂)(Pamukcu et al. 2013; Barry et al. 2015). The transition into the capping unit is abrupt; Tp5 is crystal-rich (~>35%) and trachytic (65-69 wt % SiO₂); this facies matches intracaldera fiamme in texture and composition (Pamukcu et al. 2013; Foley et al. 2014). Phenocrysts include dominant sanidine + plag + bio±hbl + FeTi ox, zirc, sphene, ap, chevkinite and sparse qtz in HSR.

Whole-rock and zircon isotopic data (McDowell et al. 2016) and trace element modeling (Foley 2017) suggest that trachyte and HSR represent complementary crystal accumulation and melt extraction. All trachyte fiamme contain glass (TGB) with SiO₂ ~72 wt%, rarely reaching 75%. A subset of fiamme contains a second glass population (TGA, ~67%). TGA and TGB are complexly mingled but contacts are sharp on micrometer scale. Both are highly enriched in phenocryst-compatible trace elements and both show elevated zircon and apatite saturation Ts compared to HSR (Watson & Harrison 1983, Harrison & Watson 1984)(Table), TGA is most extreme.

	SiO2 %	Ba ppm	Sr ppm	Zr ppm	Ce ppm	Eu/Eu*	T Zrc, C	T Ap, C
TGA	66-68	1070-4100	180-350	600-870	280-320	0.8-1.0	920	870-900
TGB	72(-75)	410-1440	140-180	210-730	170-340	0.6-0.7	900-850	800-890
HSR	76-77	2-15	1-9	100-140	75-80	0.2-0.3	760-770	740-760

Resorbed and embayed phenocrysts, Ti-enrichment in zircon rims, and REE- and Zr-enrichment in sphene rims indicate that trachyte was heated after accumulation and prior to eruption (Pamukcu et al. 2013). Trachyte glass compositions document dissolution of cumulate phenocrysts during heating. Two distinct events separated in time and/or space are indicated by compositions of TGA and TGB: TGA reflects higher T and greater dissolution. The very sharp contact preserved between the mingled glasses demonstrates that they were mechanically stirred together just before, or during, eruption.