MELT INCLUSIONS IN GRANULITE XENOLITHS FROM THE GERONIMO VOLCANIC FIELD: EVIDENCE FOR THE NATURE OF CRUSTAL MELTING PROCESSES

While the presence of melt inclusions in phenocrysts in volcanic rocks is not uncommon, they are rare in granulites and migmatites that have equilibrated under the high pressure and temperature conditions of the lower crust. Melt inclusions in lower crustal granulite xenoliths from the Geronimo Volcanic field (GVF), SE Arizona, therefore, provide a rare opportunity to study the compositions of potentially pristine, in situ anatectic melts.

The granulite xenoliths are medium to coarse grained with granoblastic textures. Lithologically, they comprise several groups , including mafic metacumulates (pl + cpx ± ol + sp), metadiorites (pl + kspar + cpx + opx), and quartzofeldspathic rocks (pl + kspar + qtz ± cpx ± opx). U-Pb zircon geochronology [1] yields three main age peaks for the granulites at ~1.64-1.65 Ga, ~1.48-1.42 Ga, and ~76-2 Ma (with a peak between 35 – 23 Ma for the latter). The oldest ages are consistent with the timing of formation of the Mazatzal terrane in which GVF is located. The mid-Tertiary peak (35-23 Ma) is consistent with the age of rhyolitic ignimbrites in the nearby Chiricahua Mts and the mid-Tertiary ignimbrite flare-up across western North America in general. This raises the question as to whether the melt inclusions found within the GVF granulites are related to this major crustal melting event.

Melt inclusions have only been found in the metadiorites and quartzofeldpathic rocks. They vary in size from 0.5 mm to 30 mm. Glass rims occur on some of the melt inclusions, and in at least one sample, this colorless glass can be traced to interstitial areas where it connects with a brown glass that is common along grain boundaries. Some inclusions appear to have multiple phases that include glass, quenched crystals (unidentified), and vapor bubbles. In this presentation we will report on new electron microprobe major element analyses and Raman spectroscopy to characterize the compositional range of the observed melt inclusions, providing new insights into the melt generation processes in the lower continental crust beneath the southern Basin and Range.

[1] Rader et al., 2021, GSA abstract, Portland, OR.