Deciphering the metamorphic architecture and magmatic patterns of large hot orogens: Insights from the central Grenville Province

Abstract

Widespread exposure of the orogenic core in the Mesoproterozoic Grenville Province, provides opportunity to assess high-T processes in large hot orogens. The Manicouagan–Escoumins transect in the central Grenville cuts through mostly granulite-facies deep to shallow crustal levels of the orogenic hinterland, and of the structurally lower Parautochthonous belt, that contain aluminous rocks with evidence of anatexis at different crustal depths. Melt-related microstructures, imaged at different scales, in conjunction with P–T pseudosections, were instrumental for assessing P–T patterns. However, the data also highlight limitations relevant to the interpretation of aluminous granulites in general, such as the difficulty to obtain peak metamorphic T, and the unreliability of Ca in garnet as a P–T indicator in apatite-bearing rocks. Monazite ages show that the mid-P portion of the hinterland records the oldest and longest duration granulite-facies metamorphism, associated to crustal thickening and spreading of the orogen at 1.08–1.04 Ga. In contrast, the high-P portion records younger metamorphic ages (1.05–1.02 Ga) decreasing upwards, and linked to local thickening behind the orogenic front at that time. ~1.04 Ga ages are also reported at higher, low-P crustal levels and are consistent with younging and shorter duration of metamorphism at the structural top. Short-lived and youngest (1.0–0.98 Ga) metamorphism in the Parautochthonous belt marks the final propagation of the orogen towards its foreland. The hinterland of the central Grenville is also characterized by diverse syn-orogenic and mostly high-T magmatism suggesting that the mantle was an important heat contributor to the metamorphism. The metamorphic domains of the hinterland along the transect are bounded by long-lived deformation zones, underpinned by 1.1–1.0 Ga magmatic bodies, that likely represent reactivated crustal scars inherited from the SE Laurentian margin on which the Grenville orogen was built.