CONSTRAINING THE AGE OF PLUTON DEFORMATION THROUGH MICROTEXTURAL ANALYSIS: A CASE FOR THE SYN-TECTONIC EMPLACEMENT OF THE MESOPROTEROZOIC TRES PIEDRAS AND TUSAS METAGRANITES

Multiple metamorphic and deformation events (D1-D3) in Mesoproterozoic rocks of the southwestern United States have been attributed to the Mazatzal orogeny at 1.7-1.6 Ga., and/or an event at 1.4 Ga. The Tres Piedras granite (TP) and the slightly younger Tusas granite (TG) plutons exposed in the Tusas Mountains of northern New Mexico crystallized at ~1.69 Ga. and intruded Yavapai aged (1.8-1.7 Ga.) metasedimentary and metaigneous rocks of the Moppin Complex and Vadito group. These host rocks represent juvenile island arc and supracrustal material accreted to the margin of the North American craton. In order to assess the relative timing of D1 and D2 in the granites, samples were collected across the plutons to evaluate their microtextural evolution using the criteria of Patterson et al. 1989 and others. Microstructural textures are preserved in the muscovite, feldspar and quartz assemblage and record evidence of a magmatic fabric as well as solid-state deformation synchronous with the Mazatzal orogeny. The coarse- to fine-grained TP granite contains cm thick spaced cleavage (S1) that is roughly parallel to S0/S1 in the host rock. Observed textures are interpreted as a progression from magmatic flow overprinted by both higher- and lower-temperature solid-state deformation and recrystallization. A mylonite containing S1-parallel foliation found along the southern margin lacks static recrystallization textures and is folded by open to isoclinal F2 folds that are pervasive in the host rock. Apalitic layers contain F2 folds, but lack S1 and a strong secondary cleavage (S2), suggesting these deformed in a partially molten state. The core of the TG pluton contains 1-5 mm phenocrysts of quartz and magmatic feldspar aligned in a fine-grained matrix, and exhibits little to no evidence for fabrics formed in the solid-state. Within 300 meters of the pluton's margin, a planar fabric intensifies towards the margin, however the TG lacks folds observed in the host rock. Evidence from the plutons suggest D1 and D2 occurred during pluton emplacement and cooling at 1.69 Ga. Intrusion of TP was accompanied by the development of an S1 fabric. Subsequent F2 folding observed in TP is manifested in TG by a fabric parallel to regional S2. The lack of S1 or S2 in folded apalite layers suggests that the parts of the system were still partially molten at 1.69 Ga.