MICROPROBE MONAZITE GEOCHRONOLOGY FROM THE MANZANO MOUNTAINS, NEW MEXICO: DISTINGUISHING STAGES IN A LONG-LIVED AND REACTIVATED OROGEN

Proterozoic rocks in the Manzano Mountains experienced a series of deformation events in the Paleo- and Mesoproterozoic, including the 1.65 Ga Mazatzal orogeny, 1.5-1.3 Ga reactivation, and younger events possibly including the Grenville orogeny. Resolving and characterizing deformation and metamorphism at each stage is critical for understanding the tectonic evolution and ultimate stabilization of North American continental crust. In-situ electron microprobe dating is ideally suited to constraining the age of overprinted tectonic fabrics and metamorphic assemblages. In the Manzano Mountains, metamorphic grade increases adjacent to both 1.65 and ~1.4 Ga plutons, and peak grades at both times were upper greenshist to amphibolite facies. Monazite from the contact aureole of the 1.66 Ojito pluton have ca. 1.65 Ga cores and distinct slightly younger rims that overgrow the matrix foliation, indicating 1.65 Ga metamorphism and deformation. Monazite from the high-grade, (syn-plutonic?) shear zone below the 1.64 Ga Manzanita pluton have 1.65 Ga cores and 1.4 Ga overgrowths, aligned with the shear zone. We suggest that the 1.65 Ga syn-plutonic shear zone was reactivated during 1.4 Ga ductile thrusting. Monazite from the sheared margin of the Monte Largo granite (Monte Largo thrust) have 1.45 Ga cores and 1.38 Ga rims suggesting that the thrust is dominantly a 1.4 Ga feature, but may have several stages. Finally, monazite from the contact aureole of the 1.4 Ga Priest pluton yield 1.35-1.4 Ga dates. Taken together, the data document two major cycles of mid-crustal, pluton-influenced metamorphism and deformation at ca. 1.65 Ga and ca. 1.4 Ga, but each cycle may include a number of events. Tectonism at 1.4 Ga locally reactivated and overprinted some, but not all, older foliations, shear zones and assemblages. The combined structural and geochronologic data highlight our improving knowledge of the processes and resulting fabrics that develop during long-term residence of rocks in the middle crust during the progressive southward accretion of the North American margin.