The Supracrustal Rocks of the Escoumins Area: a Well-Preserved Pinwarian Arc-Backarc Sequence in the Central Grenville, Quebec.

English Abstract:

Well-preserved amphibolite-facies supracrustal rocks (Saint-Siméon Group) with primary textures is discontinuously exposed for over 100 km in the Escoumins-Petit Saguenay area of the central Grenville Province, and consists of metasedimentary (quartzite, meta-arkose, conglomerate, calc-silicate, greywacke, paragneiss and scarce marble) and metavolcanic rocks (mainly mafic to intermediate with few felsic). The latter are interpreted to represent flows and volcaniclastic deposits (hyaloclastite, breccia and different varieties of tuff) interbedded with sedimentary rocks, and suggests the Saint-Siméon Group is a volcano-sedimentary belt. The supracrustal rocks are located in synforms separated by antiforms of migmatite and orthogneiss with calc-alkaline continental arc signature (Tadoussac Group) in an overall domes and basins geometry. The geochemistry of the volcanic rocks from the Saint-Siméon Group range from those with E-type MORB-like REE patterns with negative Nb-Ta anomalies and enrichment in Th typical of back-arc basin basalts to those with more evolved arc signature and fractionation of LREE from HREE. A bomb-bearing dacitic tuff yielded a TIMS U-Pb zircon crystallisation age of 1493 ±3 Ma (Pinwarian) with a 1000 ±3 Ma metamorphic overgrowth. U-Pb analysis on titanite gave a similar age of 1000 Ma with a second cluster at 985 Ma. All metamorphic ages correspond to the Rigolet phase of the Grenvillian orogeny. These preliminary data add new insight on the makeup of the SE Laurentian margin and indicate the presence of a significant Pinwarian arc-back arc sequence in the southcentral Grenville Province.