New Mexico Geological Society Annual Spring Meeting — Abstracts

The Monte Large Hills preserve outcrops of the roof and southeast aureole of the Sandia pluton. Recorded within deformed phases of the Sandia granite and thermally softened aureole is evidence of a dynamic interaction of pluton emplacement, metamorphism, and deformation. Deformation associated with the intrusion is recorded by variably developed high-temperatire solid-state deformation of both megacrystic (Sandia) and medium-grained (Cibola) granites, including S-C fabrics and dynamically recrystallized sigma feldspar porphyroclasts within the granite. In the aureole, syn-plutonic deformation is recorded by shallow lineations and reorientation of foliation surfaces relative to contact metamorphic porphyroblasts. Recent 1: 12000 mapping in the Sandia Park quadrangle supports the interpretation that the roof of the pluton is shallowly dipping to the southeast truncating the steeply dipping country rock foliation. The country rock exposed in the study area consist of a bimodal meta-volcanic package consisting of intimately interlayered rhyolite (60%), greenstone (25%), quartzite (10%), and pelitic schist(5%). Metamorphic grade within the 2 km wide aureole increases toward the pluton from greenschist faces (kyanite, actinolite), reflecting pre-pluton metamorphic conditions, to sillimanite grade proximal to granite exposures. Final crystallized melts of the granite appear relatively strain free suggesting emplacement of the Sandia pluton took place during the waning stages of some deformational event. A recent discovery in the Monte Largo Hills is the presence of a highly sheared granite which appears unrelated to the Sandia intrusion and may represent an earlier intrusion, possibly correlative with the Monzanita Granite. The presence of the roof and aureole of the Sandia pluton in the Monte Largo Hills with the same orientation as outcrops on the northwest side of the Tijeras-Gutierrez fault system suggests a relatively small strike slip component (less than several km) on this Laramide fault system.