ANATECTIC ALUMINOUS GNEISSES FROM THE MID-P SEGMENT OF THE CENTRAL GRENVILLE PROVINCE: PETROGRAPHY, P-T EVOLUTION AND U-PB MONAZITE AGES

Anatectic aluminous gneisses, some derived from broadly pelitic sediments and others from hydrothermally altered felsic volcanic rocks, are exposed in the mid-P Canyon domain of the hinterland in the central Grenville Province. These rocks contain the mid-P granulite facies mineral assemblage garnet–sillimanite–biotite–quartz–K-feldspar–plagioclase, and microstructural evidence of partial melting by mica dehydration, most widespread in the metasedimentary samples.

Microstructural and mineral chemical data, integrated with P–T pseudosections calculated with THERMOCALC using measured and melt-reintegrated rock compositions, predict a P–T path with a strong T gradient and maximum P–T at ~9.5 kbar –850^oC, restricted between the sillimanite-kyanite transition and the appearance of cordierite during melt crystallization at lower-P.

In these rocks monazite shows complex internal textures consistent with corrosion and overgrowths, suggestive of partial dissolution in melt followed by new growth during melt crystallisation (or later fluid infiltration). In situ U–Pb dating of monazite by LA–ICP–MS, yielded: (a) a range of ‘main’ Grenvillian metamorphic ages (1070–1030 Ma) best recorded by BSE-dark cores, with a maximum concentration at ca. 1060 Ma; these ages likely represent episodic melt crystallisation related to the granulite facies metamorphism, and (b) late-Grenvillian ages at 1000–960 Ma, mostly restricted to BSE-bright rims; these ages are related to late fluid infiltration, coeval with ultra-potassic magmatism in the same area. However, no clear link was found between the microstructural setting of monazite and their U–Pb ages, likely because high-T metamorphism obliterated earlier prograde microstructures.

Growth of monazite over a wide time span during the culmination of the Grenvillian orogeny together with the strong T gradient inferred by the P–T pseudosections, is consistent with protracted lateral transport of ductile crust beneath a plateau, in accordance with predictions of thermo-mechanical modeling of large hot orogens.