NEW GEOCHRONOLOGIC DATA FROM THE SANDIA PLUTON, NEW MEXICO

The Sandia pluton, in central New Mexico, figures prominently in models for deformation at ca. 1.4 Ga inboard of an active continental margin. Published ages for crystallization of the Sandia pluton rely on either U-Pb zircon and sphene data (ca. 1.44 Ga) or the concordance of ⁴⁰Ar/³⁹Ar ages between hornblende and muscovite (ca. 1.42 Ga), inferred to reflect rapid cooling following emplacement. Here we discuss: 1) the 'age' of the Sandia pluton and 2) its thermal evolution subsequent to emplacement.

The Seven Springs Shear Zone is a 1-2 km wide zone of high-temperature, solid-state deformational fabrics developed in the contact zone between leucocratic granitic rocks (Cibola gneiss) and the south margin of the megacrystic Sandia granite. Field relationships suggest that the fabrics developed in the presence of melt, and thus record syn-emplacement deformation. New U-Pb analyses of single zircon crystals from both the Cibola gneiss and from a deformed megacrystic granite (mapped as Sandia) yield upper intercept ages of ca. 1.65 Ga. These ages suggest the presence and/or incorporation of a significant percentage of older granite within the shear zone, and raise the question of the timing of shearing and fabric development within the shear zone.

⁴⁰Ar/³⁹Ar analysis of biotite and muscovite using a laser microprobe shows significant apparent age gradients within single crystals. Apparent ages from muscovites are somewhat older than corresponding biotite. Gradients within biotites exhibit individual spot ages that range up to 200 m.y. We interpret these gradients as reflecting a protracted period of slow cooling below ~350° C. We estimate cooling rates (from apparent age gradients in biotite) to have been on the order of 1°/m.y. Given a 2.5-3 kbar emplacement depth estimated from mineral assemblages and thermobarometry in the aureole, we infer a post-emplacement geotherm of ~25-30°/km. Slow cooling following pluton emplacement implies, at least locally, a return to lithospheric stability in the southwestern U.S. after ca. 1.4 Ga magmatism.