DIVING DEEPER INTO THE PICURIS OROGEN: UPPER AMPHIBOLITE FACIES METAMORPHISM AND BRITTLE DEFORMATION ALONG THE PLOMO SHEAR ZONE

The Picuris Mountains of New Mexico are being actively reassessed as part of an extensive, trans-Laurentian Geon 14 orogenic belt. Well-preserved structures and metamorphic assemblages of the Picuris Mountains offer the opportunity to better understand the dynamics of this largely intracontinental event. Here, we describe new efforts to constrain the P-T-d history of a major architectural element of the Picuris orogen in New Mexico – the Plomo Shear Zone. Constraining the P­­–T–d history of long-lived, crustal-scale orogenic structures, like the Plomo, is often difficult, as polymetamorphism and polyphase deformation can obscure the chemical records petrologists rely on. This study employs methods that use mechanical rather than chemical equilibria – quartz-in-garnet Raman inclusion barometry (QuiG) and quartz c-axis opening-angle thermometry – in conjunction with garnet-biotite thermometry to determine the P­–T–d conditions of the Plomo Shear Zone.

The Plomo incorporates and transposes the Marqueñas Formation, an immature quartzite and metaconglomerate that provides a detailed record of strain but poses a challenge for chemical equilibria studies. We link the strain signature of the shear zone to deformation temperatures through quartz textures in metaconglomerate clasts and to pressures using QuiG barometry in matrix garnets. We constrain two stages of metamorphism: (1) peak P–T conditions at ~660 °C and 7.5 kbar associated with ductile deformation, and (2) a lower amphibolite facies event at ~550 °C and 6.5 kbar associated with transient brittle faulting. Peak metamorphic conditions determined in this study are ~100 ℃ hotter and 2-2.5 kbar greater than those reported for nearby schists in the footwall of the Plomo shear zone but are consistent with typical orogenic crustal geotherms. These new data suggest (1) the Plomo shear zone represents a metamorphic break between the hangingwall Marqueñas Formation and the footwall Trampas Group and classic three-aluminosilicate Hondo Group rocks, (2) the Plomo accommodated > 5 km of exhumation, and (3) the Marqueñas Formation preserves the deeper hotter conditions of the Picuris orogenic hinterland.