Dating Monazite in Migmatites Does Not Always Mean Dating Only Partial Melting: a Case Study from the CAGE District, Torngat Orogeny (Canada)

Migmatites, which represent zones of melt production and transfer, constitute a major component of the mid- to lower crust of orogenic systems. Exhumed migmatites preserve, through their mineralogy (rock-forming and accessory minerals) and structure, a wealth of information that can be used to decipher the thermo-mechanical evolution of orogenic continental crust trhough time.

Dating partially melted rocks is nowadays a common task thanks to the development of in-situ dating techniques like U-Th-Pb dating of monazite and zircon by LA-ICMPS, SIMS and EMP. However, the interpretation of single dates into a geological meaningful age is far to be trivial in migmatites: "Dating a migmatite" does not necessarily means "dating partial melting"! One of the main reason is that migmatites are open systems that are subject to intense melt-rock interactions during redistribution and transfer of the highly mobile melt from grain- to crustal-scale. Monazite and zircon are particularly sensitive to these melt-rock interactions because both phases can be dissolved into the melt. The solubility. During the P-T evolution and differentiation of migmatitic rocks, the solubility of monazite and zircon is likely to evolve, due to change in P-T and melt composition, resulting in complex zoning and geochronological record.

The aim of this contribution is to present an in-situ analysis of monazite carried out at the thin-section scale on a migmatite from the CAGE district in the Torngat orogeny (Canada). The studied sample consists in a garnet-bearing melanosome, a quartzo-feldspathic leucosomes surrounded by a quartz-depleted biotite and sillimanite rich selvage. Monazite shows complex chemical zoning that differs significantly for each domains. LA-ICPMS U-Th-Pb ages and isotopic disturbance is also function of the textural location of monazite. Three distinct ages have been determined in the various domains: 1.83 Ga interpreted as the main partial melting event, 1.79 Ga corresponding to the intrusion of the "leucosome" and 1.75 Ga corresponding to the retrograde formation of the selvage. These results suggest that the volume of rock and melt interacting with monazite during its growth and dissolution is much smaller than the cm3 and requires detailed in-situ anlaysis.