IDENTIFYING RELATIONSHIPS AMONG MAGMA BATCHES BY POLYTOPIC VECTOR ANALYSIS: A STUDY OF THE TOPOPAH SPRING AND PAH CANYON ASH-FLOW SHEETS OF THE SOUTHWEST NEVADA VOLCANIC FIELD

The chemically and mineralogically zoned Topopah Spring (TS) ash-flow sheet (12.8 Ma, 1200 km³) in the southwest Nevada volcanic field (SWNVF) was originally inferred to represent the eruptive products of a zoned magma body¹. Recent studies have shown that the lower silica (LS) and the high-silica rhyolite (HSR) pumice fragments from TS cannot be related by AFC processes occurring within a single magma chamber². The two compositions of TS represent separate magmas emplaced into high-level magma chambers. Following the emplacement of TS was the eruptive event that produced the Pah Canyon (PC) ash-flow sheet. This smaller volume (35 km³) ash-flow sheet contains pumice fragments that were previously thought to be consistent with a hybrid magma formed by mixing of residual TS LS and HSR magmas based on trace-element ratio plots and multiple linear regression analyses^3,4.

Polytopic Vector Analysis (PVA) is a multivariate statistical method of analyzing a dataset using multiple variables. The current version of this program describes each sample in a dataset in terms of some proportion of each end member generated by the program^5-7. As a result, each sample in the dataset is described as the sum of some fraction of each end member; therefore each 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). PVA of pumice fragments from TS confirms the existence of independently generated magmas involved in the formation of the ash-flow sheet; however, PVA of PC reveals that it is not a hybrid magma derived from TS LS and HSR magmas. PVA is a powerful tool for petrologists, as this program analyses multiple variables simultaneously; interpretations on magma relationships based on this approach are not constrained to methods based on 3 or 4 variables chosen to represent magma compositions.

References Cited: 1. Lipman et al., 1966. 2. Bindeman and Valley, 2003. 3. Flood et al., 1989a. 4. Flood et al., 1989b. 5. Full et al., 1981. 6. Full et al., 1982. 7. Ehrlich, et al., 1987.