EVALUATION OF MAGMA FLOW AND EMPLACEMENT MECHANISMS OF THE MAFIC DULCE-PLATORO DIKE SWARM, NW NEW MEXICO AND SW COLORADO USING MAGNETIC SUSCEPTIBILITY FABRICS

Mafic dikes of the Oligocene Dulce-Platoro intrusive complex extend from the northeastern San Juan Basin north-northwest to the Platoro Caldera complex. The distribution of the dike swarm has an aerial exposure extent of nearly 100km (N-S) by 25km (E-W). The swarm can be segmented into the N10E trending Dulce dikes, which merge in trend and composition with the N40E trending Platoro dikes. Preliminary new and previously reported ⁴⁰Ar/³⁹Ar sanidine and mica dates of several Dulce-Platoro dikes bracket their emplacement to between ca. 26.8 (26.77 +/-0.03) and 25.0 (24.97 +/-0.06 Ma). Significant volcanic and tectonic activity occurred during this time interval in the general area, including initiation of Rio Grande rift extension, emplacement of the oldest Rio Grande rift basalts, formation of the Questa Caldera, late-stage Platoro Caldera volcanism, and emplacement of the Navajo volcanic field dikes and diatremes. How the Dulce-Platoro dike complex was formed and how these intrusions relate to other regional events is unknown. As part of a multifaceted study, we have initiated a systematic sampling of these dikes for paleomagnetic and anisotropy of magnetic susceptibility (AMS) study. AMS data often serve as a proxy for magma flow during emplacement, and comparisons can be made with general dike trend and sparsely exposed dike wall lineation orientations. Preliminary AMS fabric data have been obtained for a total of 30 sites and show Dulce dikes with well-defined magnetic fabrics, yielding both oblate and prolate susceptibility ellipsoids. Intermediate principal susceptibility (K2) axes in the dikes generally coincides with the observed trend of the dikes, and where they can be observed, dike wall lineations. The dikes sampled closer to Platoro Caldera are often coarser grained and more diabasic in texture and they yield magnetic fabrics that are less well-defined than those to the south; this may be attributed to compositional and textural differences. Paleomagnetic, rock magnetic, petrographic, and geochemical analysis are underway to better characterize this large but relatively poorly studied yet regionally significant dike swarm.