Proterozoic magmatism and regional contact metamorphism in the Sandia-Manzano Mountains, New Mexico, USA

A bstrAct — The Sandia-Manzano and adjacent Los Piños mountains, the eastern uplift of the Rio Grande rift along the Albuquerque basin, expose a series of Paleoproterozoic and Mesoproterozoic plutons that intrude the 1.70-1.60 Ga Manzano Group within the Manzano thrust belt. This paper summarizes the age and tectonic setting of two distinct pulses of magmatism: one at 1.67-1.64 Ga and one at 1.46-1.45 Ga. The 1.67-1.64 Ga granite plutons are as follows, from north to south, the 1669±13 Ma Monte Largo Hills, 1653±16 Ma Cibola, 1655±14 Ma Manzanita, 1659±5 Ma Ojito, and 1655±1 Ma Los Piños/Sepultura granites. Circa 1.45 Ga magmatism is recorded in the 1453±12 Ma Sandia Pluton, and the 1456±13 Ma Priest Pluton. The plutons range from monzogranite to granodiorite. The older plutonic suite is closely associated with a period of voluminous bimodal rhyolite-basalt volcanism. The temporal, spatial, and chemical proximity of these volcanics to the 1.65 Ga plutons suggests a tectonic relationship between intrusive and extrusive products of the same magmatic system. This relationship is explained in terms of rhyolitic, caldera-style eruptions that were intruded by plutons of similar age. Metamorphic assemblages and fabrics in aureoles of both the 1.65 Ga and 1.46 Ga plutons indicate syntectonic (syncontractional) emplacement at middle crustal depths of 3-4 km. Contact aureoles of 1.65 plutons have andalusite-sillimanite assemblages, and those of 1.46 plutons have triple point aluminosilicate assemblages (550°C, 3.5 kbars). The 1.45-1.46 Ga suite has no known proximal extrusive activity and is associated with late-stage aplite and pegmatite dikes, which frequently crosscut the outer margins of the plutons. Extrusive equivalents may be present east of the Rocky Moun- tain front in the western Granite-Rhyolite provinces. The plutons and their aureoles record two episodes of deformation and metamorphism: the 1.65 Ga Mazatzal orogeny, and the 1.45 Ga Picuris orogeny. Collectively, these two events produced the observed polyphase strain fabric in the Manzano Group rocks of the Manzano Mountains.