Miocene rhyolitic volcanism in the Socorro area of New Mexico
— Danny J. Bobrow, Philip R. Kyle, and Glenn R. Osburn

During the post-20 my. development of the Rio Grande rift, basaltic volcanism has been widespread, both within the rift and in surrounding areas of New Mexico and Colorado (Christiansen and Lipman, 1972; Leeman, 1982; Elston and Bornhorst, 1979; Baldridge, 1979). These basaltic lavas are variable in composition and both tholeiitic and alkalic varieties have been described (Lipman, 1969; Aoki and Kudo, 1976; Baldridge, 1979; Warren and others, 1979). Rhyolitic rocks younger than 20 my. occur in only a few places in the Rio Grande rift. These areas include the Taos Plateau (Lipman and Mehnert, 1979), Jemez volcanic field (Smith, Bailey, and Ross, 1970), and the Socorro area (Chapin and others, 1978). This paper is a progress report on a detailed chemical study of the Miocene, rift-related rhyolites of the Socorro area. K-Ar ages cited herein are from Osburn and Chapin (1983) unless otherwise noted.

 
Tertiary volcanism has been widespread in the Socorro area since about 39 my. ago. The earliest phase of volcanism consists of wide- spread calc-alkaline andesites, with rhyodacite and rhyolite ash-flow tuffs locally interbedded in the upper third of the sequence (Osburn and Chapin, this guidebook). Bimodal rhyolite and basaltic andesite vol- canism replaced the calc-alkaline volcanism about 32 m.y. ago (Elston and Bornhorst, 1979; Osbum and Chapin, this guidebook). This sequence consists of interbedded rhyolite to high-silica rhyolite ash-flow tuffs and basaltic-andesite lava flows. Ash-flow volcanism ended about 27 m.y. ago and basaltic-andesite volcanism about 24 my. ago; 24- to 18-my-old volcanic rocks are sparse in the Socorro area. Following this volcanic lull, moderately voluminous silicic lavas began to be erupted near Magdalena. Rhyolitic volcanism continued sporadically at several eruptive centers until about 7 my. ago. Basaltic volcanism began prior to the rhyolitic eruptions and continued until at least Pliocene time (4 my., Bachman and Mehnert, 1978) as scattered small- volume basalt and basaltic andesite flows.

The location of vents and domes for the 18 to 7 my. old silicic and mafic lavas is believed to be controlled largely by the Morenci lineament (called the transverse shear zone in this area, Chapin and others, 1978), and caldera-margin ring fracture zones (Chapin and others, 1978; Chamberlin, 1980; fig. 1). At lower- to middle-crustal depths the Morenci lineament provides a less resistant path for rising magma, while at shallow, upper crustal depths, the caldera ring fracturezones are a zone of weakness allowing magmas easier access to the surface.