In situ LA–ICP–MS dating of monazite from aluminous gneisses: insights on the tectono-metamorphic history of a granulite-facies domain in the central Grenville Province

Abstract

In situ U–Pb dating of monazite from granulite-facies anatectic aluminous gneisses of the hinterland of the Grenville Province (Manicouagan area) is used to constrain the age of metamorphic events. Matrix grains in these rocks show complex internal textures consistent with extensive corrosion and overgrowths which are attributed to partial dissolution of earlier monazite in anatectic melt followed by new growth during melt crystallization or subsequent fluid infiltration. The new monazite data show the following: (i) inherited “pre-Grevillian” ages up to ca. 1400 Ma in some rocks; (ii) “main Grenvillian” ages in the general range of ca. 1070–1020 Ma, with a variable spread in individual samples and a general cluster at 1070–1050 Ma; and (iii) “late Grenvillian” ages at ca. 1010–990 Ma, mostly restricted to backscatter electron (BSE)-bright rims of matrix grains. The wide age range of the main Grenvillian metamorphism suggests episodic growth of monazite over a wide time span, consistent with protracted residence of the host rocks under high-temperature conditions. The clusters in the age distribution likely represent major episodes of melt crystallization in the respective rocks, following the granulite-facies metamorphism. In contrast, the growth of the late Grenvillian monazite at ca. 1000 Ma is attributed to late fluid infiltration of the host rocks under greenschist-facies conditions, coeval with ultrapotassic magmatism. It is the first report of a late Grenvillian metamorphic overprint on granulite-facies mineral assemblages in the hinterland and is consistent with the model of extensional collapse of the orogen.