SYNOROGENIC METAMORPHIC AND MAGMATIC PATTERNS IN THE CENTRAL GRENVILLE PROVINCE (QUEBEC)

The Grenville Province is characterized by widespread exposure of high-grade metamorphic rocks, and diverse, locally abundant syn-orogenic magmatism in the hinterland. Along the Manicouagan-Escoumins transect (MET; central Grenville), the hinterland is mostly made of formerly deep to shallow, granulite-facies crustal levels (HPh, MPh, LPh) organized during the Ottawan orogenic phase, and lies structurally above high to mid-P granulite-facies parts of the Parautochthonous belt that were metamorphosed during the Rigolet phase. The MPh is limited to the south by a splay of the St Fulgence deformation zone (SFDZ) against 1.1 Ga orthogneisses, and anorthosite, and 1.0 Ga granitioids suites that disrupt the framework of the pre-Grenvillian crust. However, further south, there is a well-defined, low-P amphibolite facies domain metamorphosed during the Ottawan phase.

Anatectic aluminous rocks from contrasting crustal levels record biotite dehydration melting and partial melt loss at different P-ranges, and distinctive P–T patterns: 13–14 kbar, 850°C with moderate-gradient P–T paths in the high-P portion of the Parautochthonous belt; 15–16 kbar, 875°C with steeper P–T paths in the HPh; 8–9 kbar, ~850°C and moderate P–T paths in the MPh; and 4–5 kbar at ~700 in the LPh. Monazite U–Pb ages in the HPh young from 1.05 to 1.02 Ga towards the structural top, and they cover a wide range at the sample scale (1.08–1.05 to 1.02 Ga) in the MPh, where in addition, Rigolet ages (1.0–0.98 Ga) are recorded by a texturally distinct generation of monazite.

Ottawan magmatisn is mostly in the ~1.07 – 1.05 Ga range including I-type granitoids scattered throughout the MET, with the largest bodies aligned SSE of the SFDZ. Rigolet-age magmatism (~1.0 to 9.8 Ga) is represented by mafic sills and swarms of ultrapotassic dykes (0.98 Ga) and felsic pegmatite (1.00–0.99 Ga) in MPh, and leucogranite in the amphibolite-grade LPh. In addition, 1.015 Ga granite in the HPh, and 1.017 – 1.015 Ga mangerite in the MPh fall in the time interval between the two orogenic phases. The large majority of the plutonic units represent high-T magmas, mantle-derived, or involving melting of intermediate crust. It is suggested that in the central Grenville the base of the crust remained hot throughout the orogeny and that mantle-derived heat was important during metamorphism.