Geochemical, geochronological and isotopic constraints on the origin of members of the allochthonous Shawanaga and basal Parry Sound domains, Central Gneiss Belt, Grenville Province, Ontario

Abstract

The Sand Bay and Lighthouse (Lower- and Upper-) and Armer Bay gneiss associations comprise metasupracrustal rocks within, respectively, the Shawanaga and basal Parry Sound domains – elements of deep-crustal thrust sheets of the Central Gneiss Belt of the Grenville Province in Ontario. New and published geochemical, isotope and detrital zircon data for metavolcanic and metasedimentary rocks of these gneiss associations suggest origin in one or more late Mesoproterozoic Laurentian margin back-arcs.Mean ɛNd values for the Sand Bay, Lower- and Upper Lighthouse gneiss associations and the Armer Bay metavolcanic amphibolites are, with the exception of Sand Bay, close to DM values for their estimated time of eruption (+2.8, +5.6, +5.0 and +4.2 respectively). Taken in conjunction with extant and new geochemical analyses and field relations these data are compatible with protoliths of all the units originating in rifted arcs or back-arcs deposited on thin (Sand Bay, Armer Bay) to vanishingly thin (Lighthouse) pre-existing crust. Detrital zircon data indicate the Sand Bay units are younger than ca. 1260Ma. The Lower Lighthouse unit formed after ca. 1234Ma and metasediments close to the base of the Upper Lighthouse gneiss association formed at or after ca. 1350Ma, whereas deposition age at the top of the unit is estimated at less than ca. 1249Ma. A metasediment layer within the Armer Bay amphibolites has detrital ages clustered at 1251Ma, similar to the Upper Lighthouse metasediments. A quartz rich metasediment from the top of Armer Bay gneiss association is estimated to have a maximum age of deposition of ca. 1322Ma. The units represent one or more rifting events of the Andean-type Laurentian margin in a post 1.35Ga episode(s) distinct from that documented in the Muskoka-Ojibway arc at 1.45Ga. As such they may be the tectonically reworked equivalents of rifted arcs and back-arcs related to Parry Sound domain arc(s) and related to magmatic units of similar age in the Composite Arc Belt.