U-Pb Monazite and 40Ar/39Ar data supporting polyphase tectonism in the Manzano Mountains: a record of both the Mazatzal (1.66-1.60 Ga) and Picuris (1.45 Ga) Orogenies

Proterozoic basement rocks of the Sandia-Manzano-Los Pinos Mountains record two phases of plutonism, at 1.66-1.65 Ga and 1.46-1.45 Ga. These episodes correspond to two episodes of pluton-enhanced regional metamorphism and deformation that collectively resulted in the development of top-to-the-north thrust sense shear zones referred to as the Manzano thrust belt. U-Pb monazite ages indicates that prograde aureole metamorphism took place at ~1.68-1.66 Ga and again at 1.45 Ga, and both are tied to regional fabric-forming episodes. 40Ar/39Ar dating of muscovite reveals a variable 1.45 Ga thermal overprint. “Cold blocks” (at 1.45 Ga) were not heated above 375°C at 1.45 Ga; they have muscovite ages of ~1.66 Ga and record cooling of 1.65 Ga pluton aureoles through 350-400°C shortly after pluton emplacement. “Hot” blocks are in thrust sheets containing 1.45 Ga plutons and these record more extensive 1.45 Ga monazite rim growth and muscovite cooling ages of 1.44-1.42 Ga. The presence of sillimanite and andalusite in contact aureoles of 1.65 Ga and 1.45 Ga plutons suggests both were emplaced at 2-4 kbar middle crustal depths. This area does not contain 1.5 Ga rocks correlative with the Trampas Group. The 1.60-1.50 Ga “tectonic gap” in magmatism and metamorphism in New Mexico is best explained by a 100-million-year period of erosion following the 1.65-1.60 Ga Mazatzal orogeny. Plate tectonic models to explain these and other regional data require two orogenic pulses: the 1.65-1.60 Ga Mazatzal orogeny and 1.45-1.35 Ga Picuris orogeny. Total observed strain reflects shortening that occurred during both events. The Picuris orogeny resulted in formation of the Monte Largo and other thrust-sense shear zones in New Mexico. Additional studies are needed to parse the metamorphic and deformational products of each event across New Mexico. 177