U-PB ZIRCON AGES OF PROTEROZOIC PLUTONS AND MIGMATITE, SANTA FE RANGE, NEW MEXICO: EVIDENCE OF MAFIC MAGMATISM AND SYN-KINEMATIC METAMORPHISM AT 1.4 GA

Three lithologic assemblages are present in the central Santa Fe Range: (1) a septum of anatectic migmatites (700°C, 5.5 kbars) surrounded by (2) deformed megacrystic granite intruded by (3) discrete plutons of largely undeformed tonalite-diorite. In the septum a composite S₁/S₂ foliation developed contemporaneous with anatexis and was folded about a SW plunging upright antiform. At the septum margin an S₁ fabric in megacrystic granite is coplanar with S₁/S₂ in the migmatite and cut by discordant leucosome. New U-Pb SIMS ages were collected on zircon from the two plutonic units and a stromatic metasedimentary migmatite (paleosome, melanosome and leucosome). Megacrystic granite yielded a U-Pb concordant age of 1633 ±12Ma, tonalite yielded a concordant U-Pb age of 1395 ±13 Ma; both have Th/U >0.1. Migmatite zircons have highly variable Th/U ratios (>0.1 to <0.01) and U contents (200-9000 ppm). A subset of the migmatite zircon with Th/U >0.1 were used for age determinations and yielded two age populations, cores with U-Pb age of 1635 ±17 Ma (Pb-Pb age = 1634 ±16) and overgrowths with U-Pb age of 1399 ±15Ma (Pb-Pb age = 1398 ±14 Ma); two zircons yielded older Pb-Pb ages of 1792 ±24Ma. Migmatite zircon with Th/U <0.1 exhibit mixed Pb-Pb ages between 1.6-1.4 Ga with detectable Ca, Fe and Al suggesting re-equilibration with aqueous fluids; such zircon is primarily found in leucosome and may record fluid release during melt crystallization. We interpret the 1635 Ma and older migmatite zircon as detrital grains and the 1399 Ma as the age of anatexis and deformation (including deformation of 1635 Ma megacrystic granite). The similarity of the metamorphic age with that of undeformed tonalite-diorite suggests pluton enhanced regional metamorphism. Subalkaline, metaluminous diorite trace element chemistry points to an enriched mantle source carrying a subduction zone signature. Diorite may record the basaltic under plate hypothesized as the thermal driver for 1.4 Ga granite magmatism, metamorphism and deformation recorded across the Laurentian margin.