THE EMINENCE-VAN BUREN VOLCANIC SERIES: A RIFT-RELATED CALDERA

Volcanic rocks in the Eminence-Van Buren (EVB) area of southeastern Missouri preserve a major ignimbrite-air fall-effusive sequence centered on the Missouri Gravity Low (MGL). Collapse of at least one caldera is indicated by a 7000-8000 m thick lower sequence of 1.47 Ga ignimbrites, lavas, and air-fall tuffs exhibiting steep-dipping (65-90°) fabric parallel to the MGL axis. This lower sequence is overlain with angular unconformity by a post-collapse series of gently dipping effusive rocks approximately 300 m thick; the unconformity is marked by widely distributed air-fall tuff and volcaniclastic sediment. The lower sequence is composed of six map units ranging in composition from dacite to rhyolite. Two units are recognized within the upper effusive series, both predominantly rhyolite. Despite obvious alteration features common to glassy rocks, e.g., inverse K vs Na and (K+Na) vs Si, individual units are distinguished by phenocryst traits and certain major and trace element ratios. Most EVB rocks are metaluminous but rare peralkaline types exist. The EVB suite resembles volcanic rocks of the St. Francois Terrane in age and chemical features (e.g., REE and spider diagram patterns) but the EVB exhibits a wider compositional range (67.28-78.44 wt% SiO2, excluding air fall and volcaniclastic material) and lacks a major coeval basaltic partner. Major and trace element data indicate that the EVB volcanic series is anorogenic (A2 subtype) and compatible with continental rift origin.