PRE OR SYN-GRENVILLIAN DEFORMATION OF THE PARAUTOCHTHONOUS BELT, CENTRAL GRENVILLE PROVINCE (MANICOUAGAN RESERVOIR AREA)?

Evidence for pre-Grenvillian deformation is largely preserved in the western Grenville Province, whereas earlier structures are regionally transposed in the eastern Grenville Province. New field-based structural, metamorphic and geochronological data from the Parautochthonous Belt in the Central Grenville Province constrain this contradiction.

D₁ and D₂ structures indicate reverse, top-to-the-NNE shearing with a SSE and SSW plunging lineation, respectively. D₁ is folded by D₂ and becomes completely transposed towards the overlying Allochthonous Belt. Local D₃ structures indicate oblique-normal, down-to-the-SW shearing. We constrained the age of D₂ and D₃ by combining results of laser ablation (LA-ICP-MS) and chemical abrasion isotope dilution (CA-ID-TIMS) U-Pb geochronology on zircon. Group 2 zircon in an early-D₂ leucosome yielded a weighted mean (WM) ²⁰⁶Pb/²⁰⁷Pb date of 1009 ± 18 Ma (LA-ICP-MS) and, two analyses from tips yielded a WM ²⁰⁶Pb/²³⁸U date of 1002 ± 2 Ma (CA-ID-TIMS). Group 1 zircon yielded a WM ²⁰⁶Pb/²⁰⁷Pb date of 992 ± 22 Ma (LA-ICP-MS) and, one analysis of a tip yielded a ²⁰⁶Pb/²³⁸U date of 986 ± 4 Ma (CA-ID-TIMS). A late D₂ pegmatite yielded a WM ²⁰⁶Pb/²⁰⁷Pb date of 961 ± 22 Ma (LA-ICP-MS). D₂ is thus bracketed between 986 and 961 Ma. This timing is synchronous, within error, with a syn-D₃ leucosome that yielded a WM ²⁰⁶Pb/²³⁸U date of at 993 ± 3 Ma (CA-ID-TIMS).

The age of D₁ can be inferred by linking deformation, metamorphism and geochronology. S₁ is defined by kyanite, a first generation of biotite and kyanite-bearing leucosome. The oldest 1002 Ma zircon group in the early-D₂ leucosome is depleted in HREE, suggesting that growth occurred after or synchronously with a phase that preferentially sequestered these elements, most likely garnet. These observations are most compatible with D₁ forming along a prograde path in the kyanite field, first crossing the muscovite-dehydration melting reaction to produce the kyanite-bearing leucosome, then the biotite dehydration-melting reaction to produce garnet and zircon depleted in HREE. This latter melt would then have crystallized on the retrograde path during D₂.

D₁ is therefore most likely Grenvillian, and there is no evidence for pre-Grenvillian deformation or metamorphism in this part of the Parautochthonous Belt.