229-4 Melt Inclusions in Granulite Xenoliths of the Geronimo Volcanic Field, SE Arizona: Evidence for Lower Crustal MASH Processes During the Mid-Tertiary?

Session: Crustal Petrology (Posters)

Presenting Author:

Authors:

Abstract:

The preservation of melt inclusions (MI) within granulites and migmatites is a rare phenomenon, as they are typically destroyed by high PT post-entrapment petrogenetic processes. As such, instances of their preservation provide a rare look into the complex process of crustal anatexis. Pleistocene-age alkaline volcanism of the Geronimo Volcanic Field (GVF), SE Arizona, has entrained lower crustal granulite xenoliths that preserve such melt inclusions. Consequently, they present the opportunity to glean unique insights into the tectonomagmatic history of the southern Basin and Range province. U-Pb zircon ages obtained from the GVF xenoliths exhibit peaks at ~1.6 Ga, ~1.48 – 1.42 Ga, and 35 – 23 Ma (Rader, 2021). The youngest group of ages indicate a thermal event associated with Farallon shallow-slab rollback and overlap in time with the two pulses of caldera-forming magmatism in the adjacent Boot Heel Volcanic Field (35.3 – 32.7 Ma and 27.7 - 26.9 Ma). By extension, this suggests a possible genetic link between the formation of MI entrapped within the GVF granulites and the melting event that gave rise to this mid-Tertiary ignimbrite-forming volcanism. Prior research has identified melt inclusions within xenoliths of metadiorite (pl + kspar + cpx +opx) and quartzofeldspathic (pl + kspar + qtz ± cpx ± opx) composition (Kempton et al., 1990). Melt inclusions are most abundant in the metadiorites, vary from 0.5 – 30 μm in diameter, and are commonly entrapped within plagioclase. These MI have been classified into three groups: glassy MI with single phase core (Type I), glassy MI with single phase core and a shrinkage bubble (Type II), and multi-phase crystalline inclusions (Type III). In this presentation, we will report continued petrographic investigation of these melt inclusions, new results from SEM & BSE imaging, and preliminary electron microprobe major element analyses. On-going work will focus on continued major and trace element analyses of melt inclusions, geochemical comparison between GVF granulite, MI, and BHVF ignimbrite compositions, and thermodynamic modelling of those compositions to determine P-T conditions during anatexis.

Rader (2021) MSc thesis, Kansas State University.

Kempton et al. (1990) Geochemica et Cosmochimica Acta v. 54, p. 3401 – 3426.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7986

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.