2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 11
Presentation Time: 8:00 AM-12:00 PM


LEE, Paula M.1, MIGGINS, Daniel P.2 and WHITTINGTON, Alan1, (1)Department of Geological Sciences, Univ of Missouri, Columbia, MO 65211, (2)U.S. Geol Survey, PO Box 25046, MS964, Denver, CO 80225, pml6vd@mizzou.edu

East-west trending basaltic and lamprophyric dikes and associated sills intrude sandstones, shales and coals throughout the Raton Basin. These are distinct from the radial dikes associated with the Spanish Peaks in the northern part of the basin. The purpose of this study is to evaluate magma flow directions in the E-W trending dikes and sills, and to determine the geochemical relationship between the basalts and lamprophyres. Microstructural fabrics along the chilled margins of intrusion/host rock contacts were evaluated in thin section to determine magma flow directions. Structures found include imbricated phenocrysts, ramps, bent or broken phenocrysts, and felsic stringers of host rock. Due to the highly weathered nature of the contacts, microscopic flow indicators were observed in only five of the sampled dikes. Four of these showed magma flow to the west, and one to the east. These data are consistent with a magma source to the east of the basin, which was suggested in a previous study of flow directions in the northern part of the basin. Flow directions in sills from six locations were variable, even among different sills exposed at the same outcrop. Both dikes and sills are frequently altered and contain carbonate-filled vesicles. Despite this, geochemical analyses of the samples show remarkable consistency in trace- and rare-earth element characteristics, regardless of location within the basin (north-south or east-west) and type of intrusion (dike or sill). In detail, dikes show slightly elevated LREE concentrations when compared with the sills, but there is no apparent difference between hydrous and anhydrous intrusions. 40Ar-39Ar dates from the new samples will provide additional constraints on the timing of mafic magmatism along the eastern and southern margins of the Raton Basin. Previous studies have shown that the E-W dikes and sills were mostly intruded between 20 and 25 Ma, the oldest of these overlapping with the intrusion of the Spanish Peaks. Combined with our flow direction data, this suggests that melt generation coincident with inception of the Rio Grande Rift also occurred at least 100 km to east of the present-day rift axis; an alternative but more complex scenario would require lateral magma migration (to the E) at depth, followed by migration along fractures (to the W) at shallow crustal levels.