EVOLUTION OF LAYERING IN A MIGMATITE SAMPLE: IMPLICATIONS FOR THE PETROGENESIS OF MULTIDOMAIN MONAZITE AND ZIRCON

The timing of partial melting in high-grade metamorphic rocks is critical for constraining tectonic histories and processes. However, uncertainties exist about the behavior of monazite and zircon during partial melting, especially about the timing of crystallization with respect to melting reactions. Some experimental and theoretical studies have suggested that monazite and zircon dissolve into partial melt and precipitate on retrograde cooling and melt crystallization; other studies have interpreted monazite and zircon crystallization during prograde melting. We analyzed a single sample (16TG143) of finely layered, migmatitic gneiss from the Adirondack Highlands, NY, interrupted to have undergone extensive biotite dehydration melting (i.e., Bt + Pl + Als + Qz = Grt + Kfs + melt). The rock contains one distinct leucosome layer. The non-leucosome (gray gneiss) portion of the migmatite has mm-scale sublayers with distinct differences in modes and mineralogy. The layers are interpreted to reflect differential preservation of reactants and products formed during forward and reverse progress of the melting reaction. Monazite and zircon modes, and to some degree, texture, composition, and geochronology all vary from layer to layer in the migmatitic rock. Both minerals have up to three domains: ca. 1150 Ma anhedral cores, ca. 1050 Ma monazite mantles/fir tree textured zircon, and ca. 1030 Ma rims. Although the leucosome layer might be expected to provide firm constraints on the timing of melting, it proved to be the most enigmatic because of difficulty in distinguishing inherited chronometers vs. newly crystallized grains. Instead, the heterogeneous layered gray gneiss provided robust constraints on the timing of melting (ca. 1050 Ottawan orogenesis), melt crystallization, and post-melting retrogression in addition to information about earlier metamorphic events. Earlier monazite and zircon grains were largely dissolved during progressive melting, except where preserved as relicts or inclusions. Monazite mantles and fir tree zircon grains both precipitated upon cooling and progressive melt crystallization between temperatures of 800 to 750 ºC. Rims are interpreted to have precipitated during subsolidus, solid-state retrogression after ca. 1050 Ma.