The behavior of zircon during partial melting in anatectic migmatites: Insights to metamorphism, deformation and tectonism in the central Santa Fe Range, northern New Mexico.

摘要

Three lithologic assemblages exposed in the central Santa Fe Range, northern New Mexico include (1) a septum of Proterozoic low temperature (∼700°C, 5.5 kbars), semi-pelitic anatectic migmatites surrounded by (2) deformed megacrystic biotite granite that is intruded by (3) discrete plutons of largely undeformed hornblende biotite tonalite. Metamorphic fabrics in the migmatite record three deformation events (D1 -- D3); a composite S1/S2 fabric (D1 and D2) that formed contemporaneous with anatexis and is folded about a southwest plunging map-scale (D3) antiform. The megacrystic granite exhibits a pervasive S1 subsolidus foliation that parallels the S1/S 2 fabric in the migmatite and is cut by discordant leucosomes. The hornblende tonalite cross-cuts both the S1 foliation of the megacrystic granite and the S1/S2 foliation of the migmatite. New U-Pb SIMS ages were collected on zircon from the two plutonic units and from a stromatic metasedimentary migmatite (leucosome-melanosome-paleosome). Megacrystic granite yielded a concordant U-Pb age of 1633 +/- 12 Ma (2) and hornblende tonalite yielded a concordant U-Pb age of 1395 +/- 7 Ma (2sigma); both plutonic units have Th/U >0.1. The evaluation of migmatite zircon from leucosome-melanosome-paleosome have highly variable Th/U values (>0.1 to 0.01) and uranium concentrations (200 -- 9,000ppm). Migmatite zircon with Th/U >0.1 yielded two age populations: protolith cores with a Pb-Pb age = 1634 +/- 16 Ma and new growth with a Pb-Pb age of 1398 +/- 17 Ma. Three zircon analyses yield older Pb-Pb ages of 1791 +/- 25 Ma. Migmatite zircon with Th/U values 0.1 yield mixed Pb-Pb ages between 1.6 -- 1.4 Ga that have elevated Ca, Fe, and Al concentrations, suggesting zircon recrystallization from fluids and/or melts; such zircon is primarily found in leucosome and may represent fluid released during melt crystallization. Older migmatite zircon ages are interpreted as being inherited from a metasedimentary protolith and younger migmatite zircon ages at 1399 Ma represents the age of anatexis and deformation (this includes deformation of the 1633 Ma megacrystic biotite granite). Statistical correlation of both the migmatite and tonalite ages suggests mafic magmatism enhanced regional metamorphism and supports studies documenting metamorphism, deformation and tectonism across the Laurentian margin at ∼1.4 Ga.