Identifying relationships among silicic magma batches by polytopic vector analysis: A study of the Topopah Spring and Pah Canyon ash-flow sheets of the southwest Nevada volcanic field

Abstract

The chemically and mineralogically zoned Topopah Spring silicic ash-flow sheet (12.8 Ma, 1200 km 3) in the southwest Nevada volcanic field (SWNVF) contains two pumice fragment populations, a lower silica (LS) and a high-silica rhyolite (HSR) composition. Recent interpretations are that these magmas, represented by the pumice fragments, were not related by AFC processes within a single magma chamber. The emplacement of the Pah Canyon ash-flow sheet followed the Topopah Spring ash-flow sheet eruption. This smaller volume (∼ 35 km 3) ash-flow sheet contains pumice fragments that were previously interpreted as a hybrid magma formed by mixing of residual Topopah Spring LS and HSR magmas. Pumice fragments from these ash-flow sheets were analyzed by polytopic vector analysis (PVA), a multivariate statistical program that describes each sample in a dataset in terms of some proportion of each end member generated by the program. Therefore each sample, or pumice fragment, is uniquely described by some amount of each of the end members so that graphical analysis of the dataset allows the immediate recognition of separate magma batches, as pumice fragment samples cluster in discrete groups and show different variations in end member proportions. Recognition of hybrid magmas is also immediately apparent, as pumice fragments representing mixed magmas must plot between the parent magmas (pumice fragment groups). Our results confirm the existence of independently generated magmas involved in the formation of the Topopah Spring ash flow. However, PVA of Pah Canyon pumice fragments reveals that Pah Canyon cannot be described solely as a hybrid magma derived from Topopah Spring LS and HSR magmas. Furthermore, Pah Canyon and the Topopah Spring HSR pumice fragments have identical trends in the variation of end member proportions that most likely represent crystal fractionation or accumulation in both magmas that was previously undetected. This pattern is inconsistent with simple mixing between the TS-LS and TS-HSR magmas. Rather it is consistent with magma mixing followed by fractional crystallization.