New Mexico Geological Society Annual Spring Meeting — Abstracts

A ~4 km-long dike in the central San Juan Basin, New Mexico, exhibits unique mineralogic, geochemical, and geochronologic characteristics that suggest a previously-unrecognized episode of magmatic activity. The north-trending dike, informally named the Pemada Canyon dike, crops out intermittently across sandstone mesas and mudstone canyons 15 km east of Aztec, San Juan County, New Mexico. Whole rock analyses collected along the dike have low SiO2 contents (24.5 to 37.6 wt%), elevated MgO contents (5.9 to 14.0 wt%) and elevated concentrations of transition metals (e.g., Fe2O3, MnO, NiO). The mineralogy of the dike is consistent throughout, but variations in mineralogy are observed depending on proximity to the dike margin. Samples collected immediately (<1 cm) adjacent to the dike margin have a trachytic, finely crystalline matrix composed of kalsilite, clinopyroxene, magnetite ± ilmenite, calcite and phlogopite (and possibly other silica undersaturated minerals). Samples slightly more distal from the dike margin (1-2 cm) contain abundant 1-3mm, faceted crystals of calcite, with obvious crystal boundaries in their interiors, 1-3mm crystals of euhedral clinopyroxene, 500µm to 1.5mm phlogopite, sparse, faceted kalsilite (±nepheline) crystals, with oxide phases in the groundmass. Away from the dike margin, the large crystals composed of recrystallized calcite also exhibit serpentinization, suggesting that the primary crystalline material was magnesian. The lack of abundant olivine, despite high MgO contents in the whole rocks, suggests that the faceted calcite crystals observed in the sample could be replacing olivine crystals. The replacement of olivine by serpentine and calcite is a common feature in Group 1 kimberlites. The mafic, carbonate-rich petrology and primitive bulk chemistry (e.g., Ni contents range from 442 to 1304 ppm in samples where wt% MgO >10) of the Pemada Canyon dike suggest that it originates from the mantle.

40Ar/39Ar dating yields a robust age of about 10 Ma, based on six samples collected along the dike, suggesting that we have identified a new interval of igneous activity in the southeastern Colorado Plateau.

The closest igneous features of similar age are ~90 km to the north in the Chicago Basin stock of the Needle Mountains, a quartz rhyolite porphyry. The closest igneous features of similar composition are in the Navajo volcanic field, approximately 130 km to the west-southwest, but these features are considerably older at 19 to 28 Ma. Preliminary petrography investigations show mineral fabrics in the dike, particularly near the dike margins, suggesting the crystallization occurred in a flowing fluid. The peculiar composition of the Pemada Canyon dike, combined with its geographic isolation, raise questions not only about its source, but also about the tectonic significance of deep-sourced magmatism in the interior of the Colorado Plateau in the late Miocene. The eastern Colorado Plateau and adjacent Southern Rocky Mountains have few geologic records from this epoch, warranting further investigation of the Pemada Canyon dike to elucidate the history of its emplacement