New Mexico Geological Society Annual Spring Meeting — Abstracts

The Proterozoic rocks exposed in the Rincon Range (north of Mora, New Mexico) display progressive north to south changes in structural style that reflect depth and proximity to syntectonic granitoid plutons. At the north end of the range, overturned sections of the 1km thick quartzite of the Ortega formation are repeated several times by north-vergent thrusts. In the vicinity of the town of Guadalupita further south, the quartzite defines the limbs of a large-scale synformal anticline cored dominantly by fine-grained foliated felsic gneiss. This felsic gneiss is complicated internally by smaller scale folding and contains layers of amphibolite and quartzite. Further south, in the vicinity of the small village of EI Turquillo, the quartzite limbs of the synformal anticline become drastically attenuated and entirely surrounded by gneiss. This suggests that the quartzites are screens within an intrusive fine-grained gneiss. Additional evidence for an intrusive relationship comes from the existence of ~10 cm diameter 'nodules' of musc-sill ± kspar within the gneiss that are interpreted to be the be xenoliths of nearby quartzite within the felsic gneiss. A transition exists from thin tabular foliated quartzite layers to blocky nodules to more equant nodules. Similar 'nodules' are seen in both the Cimarron and Taos Ranges within the regionally extensive felsic gneiss in contact with quartzite.

Still further south, along La Canada del Carro, a Phanerozoic fault marks a fairly abrupt change in the character of the granitic gneiss. The gneiss south of the fault, and on to the town of Mora, is much more homogenous and the foliation is regionally sub horizontal over some 40 km². Within the gneiss, particularly in the vicinity of Tierra Amarilla canyon, considerable volumes of pegmatites are dominantly concordant with S1 but are locally seen to cross-cut S1 and, in one case, intrude the axial plane of an F2 fold. Within these southern gneisses is an S1 parallel zone of musc-kspar±grt schist which is several meters thick and is traceable over most of the area of gneiss exposure, This zone contains high proportions of muscovite, oxides, and zircon in addition to quartz and remnant kspar which define highly strained ribbons separated by muscovite layers. This muscovite rich rock likely represents a shear zone and fluid conduit which experienced metasomatic alteration within the parent gneiss.

From north to south, this progression can be summarized as follows: moderately steeply-dipping north vergent thrust duplex structures, more open folding, areas where supracrustal rocks are caught up in the intrusion as screens and xenoliths, to a more homogeneous fine-grained granite gneiss with subhorizontal foliation cut by concordant pegmatites and subhorizontal high strain fluid conduits.

The favored model is that pluton emplacement apparently thermally eroded the mechanical strength of the middle crust near the brittle-ductile transition and localized subhorizontal flow, both magmatic and solid state, along and within granitoid sills. Differences in metamorphic grade (~500º vs. 700° at ~4-6 kb) reflect steep field gradients around the pluton. The granite gneiss displays microtextural evidence for high temperature solid-state deformation such as grain boundary migration in kspar. This is in apparent contrast with Al₂SiO₅ triple point metamorphic conditions within the nearby quartzite. Additionally, the presence of a thick quartzite in the northern part of the range may accentuate the structural style changes and metamorphic grade contrast due to the high thermal conductivity of quartzite.