New Mexico Geological Society Annual Spring Meeting — Abstracts

~1.1 Ga diabase dike swarms cut Paleoproterozoic units throughout the southwestern region of Laurentia. Howard (1991) noted that many of these intruded as horizontal sheets cutting existing subvertical fabrics. Exposures of these intrusions in the southern Mazatzal Province in New Mexico have not been dated, but recent dating of similar dikes in Arizona yielded ages of ~1080–1094 Ma (Bright et al., 2012). The source of this late stage Grenville magmatism remains controversial.

The Burro Mountains, located in southwestern New Mexico, exposes an abundance of cross-cutting diabase intrusions cutting ~1.4 Ga intrusive rocks and ~1.6 Ga metamorphic rocks. In this study, we measured the planar contact orientations of the diabase dikes, and then analyzed orientations in an attempt to determine the structural significance of the diabase swarm and the post-emplacement tilting. Dike orientations were rotated 25° SW about a horizontal axis of 300°, based on orientation of NE-dipping Cretaceous Beartooth Quartzite which uncomformably overlies 1.46 Ga granite. We postulated that the intrusions would be intruded as either vertical dikes, related to horizontal extension, or horizontal sheets as described by Howard (1991).

After rotation, a large percentage of the dikes have a subhorizontal dip, with most having dips of <30° and no consistent strike orientation. This suggests that the majority of the intrusions were originally intruded as horizontal sheets. Some of the dikes have steeper orientations with E-W strike and these might be feeder dikes to the subhorizontal sheets. No regional folding of the dikes was observed based on the plotted orientations, but some of the scatter is likely related to tilting on faults related to Tertiary extension in the region.

Intrusions with distinct zoning represent syntaxial dike intrusions suggesting that the emplacement process was not geologically instantaneous. Chilled margins are not present in dikes cutting ~1.4 Ga intrusive rocks, indicating that the country rock was at an elevated temperature at the time of diabase intrusion. However, the diabasic texture indicates hypabyssal intrusive depths. These intrusions are likely coeval with 1.08 Ga felsic intrusions mapped in the nearby Little Hatchet Mountains (Amato and Mack, 2012) and coeval with Grenville-age magmatism throughout the southwest U.S.

References:

1. Amato, J. M., and Mack, G. H., 2012, Detrital zircon geochronology from the Cambrian-Ordovician Bliss Sandstone, southwest New Mexico: Evidence for contrasting source regions on opposite sides of the Transcontinental Arch: GSA Bulletin, v. 124, p. 1826-1840.
2. Bright, R., Amato, J. M., Denyszyn, S. W., and Ernst, R. E., 2012, U-Pb geochronology and isotope geochemistry of ~1.1 Ga diabase dikes and sills in the southwest U.S.: Implications for Rodinia reconstructions, GSA Rocky Mountain Section Meeting, Abstracts with Programs, v. 44, no. 6, p. 9.
3. Howard, K. A., 1991, Intrusion of horizontal dikes: Tectonic significance of Middle Proterozoic diabase sheets widespread in the upper crust of the southwestern United States: Journal of Geophysical Research, v. 96, p. 12461-12478.