PETROLOGIC CONSTRAINTS FROM A DISCONTINUOUS REGIONAL METAMORPHIC FIELD GRADIENT DURING THE PICURIS OROGENY IN THE RINCON RANGE, NORTH-CENTRAL NEW MEXICO

Petrologic observations from pelitic schists, collected from two localities in the Rincon Range, North-Central New Mexico, provide evidence for regional metamorphism during a single tectonic cycle resulting in different P-T-t histories in different parts of this deformed crustal block. Based on recent and published monazite and zircon U-Th-Pb geochronology, this regional metamorphism is attributed to the Mesoproterozoic Picuris Orogeny, with ages clustering near 1370 Ma and 1420 Ma. A kyanite zone schist from the northern Rincon Range contains Grt + Bt + St + Chl + And + Ms + [Ky] + Qtz + Pl + accessory minerals + oxides. Kyanite porphyroblasts are entirely replaced by intergrown retrograde biotite and andalusite. 0.5–1.0 mm garnet grains occur as small matrix phases and as inclusions in poikiloblastic staurolite. Petrologic constraints and garnet + biotite thermometry for this rock suggest peak conditions near 590 ± 40ºC and 5–6 kbar. A central Rincon Range metapelite containing Grt + Bt + Sil + Qtz + Ms + Pl [± St] + accessory minerals + oxides [± melt] experienced garnet growth at similar conditions but growth proceeded above staurolite stability and during partial melting close to ~700ºC. Garnet growth into higher temperature conditions, accompanying partial melting above staurolite stability, produced larger porphyroblasts in which zoning wass truncated by retrograde garnet resorption during crystallization of partial melt upon cooling. Differences in trace element zoning (Y, Ti, Sc) in garnet from these metapelitic rocks suggest different accessory mineral parageneses. Therefore the crustal block exposed in the Rincon Range may have been assembled by Mesoproterozoic compressional deformation overlapping with episodic regional metamorphism as a response to an extended period of crustal thickening in the Picuris Orogenic wedge.