The Butler Hill Caldera: a mid-Proterozoic ignimbrite-granite complex

Abstract

Abstract Uplift along rift-related faults in the eastern St. Francois igneous terrane of southeastern Missouri exposes a high-level anorogenic igneous complex consisting of 1.5 km of intracaldera ignimbrites, a zoned central pluton, and late-stage ring plutons. A collapse structure, named the Butler Hill Caldera (BHC), formed in response to eruption of comenditic ignimbrites of the Lake Killarney Formation which attains an intracaldera thickness of 400 m. Continued peralkaline volcanism resulted in an additional 1000 m of Grassy Mountain Ignimbrite within the caldera and a thin, but widespread, outflow sheet. Following degassing of the magma chamber, the central pluton forcefully intruded the intracaldera volcanic sequence producing arcuate folds concentric to the caldera axis. Remnants of the original magma chamber roof are not preserved. The final igneous event related to the BHC involved intrusion of amphibole-granitoids in an annular pattern within, and concentric to, the caldera walls. The latter bodies probably merge, with depth, into a single ring pluton. The petrographic zoning exhibited by the mildly peraluminous central pluton of the BHC consists of: (1) a roof-zone of fine-grained, brick-red, hypersolvus granophyre with abundant miarolitic cavities which is nearly devoid of hydrous minerals; (2) a transitional zone of pink fine- to medium-grained seriate granite in which granophyric intergrowths are common and plagioclase and biotite are present in trace amounts only; and (3) a border zone of medium- to coarse-grained pink to grey subsolvus biotite granite with rapakivi texture. The paradoxical association of peralkaline and peraluminous suites with hypersolvus and subsolvus petrographic traits at the same structural level is recognized in other high-level anorogenic complexes. The common factor in these terranes, regardless of their age, appears to be that initial magmas were hot, relatively dry, low in CaO and high in F. These compositional features favor ascent of hypersolvus magmas to subvolcanic levels where saturation is accomplished by influx of meteoric water. Field, chemical, isotopic, and petrographic evidence indicate that BHC rocks have experienced at least two periods of subsolidus metasomatism. The resulting metasomatic features are typical of anorogenic ignimbrites and high-level granitoids elsewhere and must be quantified before existing chemical data can be applied to questions of igneous petrogenesis.