Independently generated magma batches in the compositionally zoned ash-flow sheets from the southwest Nevada volcanic field

Compositionally zoned ignimbrites have been inferred to represent the eruptive product of zoned magma chambers. Topopah Spring (12.8 Ma), Tiva Canyon (12.7 Ma), Rainier Mesa (11.6 Ma), and Ammonia Tanks (11.45 Ma) are four compositionally zoned ash-flow sheets within the southwest Nevada volcanic field, SW Nevada. These large volume ash-flow sheets have been extensively studied with the goal of understanding the formation of large volumes of high silica magmas that are, in this case, rapidly generated and erupted within short time intervals (150,000 years between the youngest of these ash flows). Previous studies have concluded, based on major and trace element geochemistry and isotopic analyses, that the lower silica magmas and high-silica rhyolite magmas within and among each ash-flow sheet cannot be related by assimilation/fractional crystallization processes occurring within a single magma chamber.; The purpose of this current study is to evaluate this conclusion using Polytopic Vector Analysis (PVA). Based on these analyses we conclude that not only can unrelated magma types be identified, but that magmas related by mixing processes can also be determined. Using PVA, it can be shown that the coevally erupted lower silica (≤73 wt% SiO2) and high-silica rhyolite magmas (≥74 wt% SiO2) within Topopah Spring, Tiva Canyon, Rainier Mesa, and Ammonia Tanks are unrelated, and must represent independent magma batches. An intermediate magma type identified in Tiva Canyon was found not to be the result of mixing between the lower silica and high-silica rhyolites of Tiva Canyon; however, a similar intermediate magma type of Ammonia Tanks can be explained as the result of mixing between more evolved portions of the lower silica magma and the coevally erupted high-silica rhyolite. Rainier Mesa is unique among these ash-flow sheets in that three high-silica rhyolite magmas (HSR-1, HSR-2, and HSR-3) can be identified based on trace element geochemistry (in particular Th/Nb, and La). PVA results show that the HSR-1 magma type is unrelated to the coevally erupted lower silica magma and the other two high-silica rhyolites. However, HSR-2 and HSR-3 are related and may be considered as one magma type.; End members determined by PVA for each of the high-silica rhyolite magmas of Rainier Mesa overlap in composition, which may be interpreted as the result of mixing. Indeed, sanidine and melt inclusion, and glass matrix trace-element compositions support mixing among these high-silica magmas and also with a less evolved magma type. However, mixing is limited, such that standard geochemical modeling fails. PVA is more sensitive than typical major and trace element least squares linear regression models in recognizing mixing systems.