MELT INCLUSIONS IN GRANULITE XENOLITHS FROM THE GERONIMO VOLCANIC FIELD: EVIDENCE FOR THE CRUSTAL MELTING EVENT GIVING RISE TO MID-TERTIARY RHYOLITES OF THE SOUTHERN BASIN AND RANGE?

Plio-Pleistocene age alkali basalts from the Geronimo Volcanic Field (GVF), SE Arizona, USA, entrained granulite xenoliths derived from the lower crust of the southern Basin and Range. The granulites yield U-Pb zircon ages (Rader et al. 2021) that include three main age peaks 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 mid-Tertiary peak (35-23 Ma) overlaps with the ages of rhyolitic ignimbrites in the nearby Chiricahua Mts and the mid-Tertiary rhyolitic magmatic event that occurred across much of western North America. This temporal similarity raises questions as to whether there is a genetic link between the magmatic event that produced the widespread rhyolitic ignimbrites and genesis of the lower crustal xenoliths.

GVF granulites are unusual in containing physical evidence for crustal melting in the form of melt inclusions, many of which occur in the xenolith types that yield mid-Tertiary ages. Such inclusions are extremely rare in lower crustal rocks; hence, the GVF xenoliths provide a unique opportunity to (i) study the compositions of potentially pristine, in situ anatectic melts and (ii) to test whether there is a genetic link between the granulite xenoliths and the exposed rhyolitic ignimbrites.

Lithologically, the granulites comprise several groups, including mafic metacumulates (pl + cpx ± ol + sp), metadiorites (pl + kspar + cpx + opx), and quartzofeldspathic rocks (pl + kspar + qtz ± cpx ± opx). Melt inclusions have only been found in the metadiorites and quartzofeldpathic rocks. The melt inclusions range in size from 0.5 mm to 30 mm and four types have been identified: (1) clear, colorless glass ± a single mineral inclusion (usually quartz); (2) clear glassy inclusions with a vapor bubble ± mineral inclusions; (3) clear glassy inclusion that show leakage or connection to interstitial grain boundary brown glass; and (4) glassy inclusions that contain multiple mineral phases. The latter group can also be described as ‘nanogranites’. New electron microprobe analyses of glasses and associated mineral phases will be presented and compared with the compositions of Turkey Creek mid-Tertiary rhyolites from the adjacent Chiricahua Mountains.

Rader, M., Kempton, P.D., & Möller, A. (2021). GSA Abstr Prog, Portland, OR