Abstract

The Lower Ordovician Tourelle Formation crops out on the Gaspé Peninsula in the allochthonous external domain of the Quebec Appalachians. Coarse sandstone beds dominate this submarine fan deposit, and contain immature detritus (volcanic fragments, radiolarian chert, amphiboles, pyroxenes, chromite) shed from initial uplifts within the Taconic orogenic belt. Pre-Tourelle sediments were eroded directly from a Lower Paleozoic Grenville-type craton and its fringing carbonate bank. Recycling of these sediments provided the bulk of the detritus found in the Tourelle sandstones.Volcanic rock fragments comprise 4% of framework grains. They are intermediate to acid in average composition, contain primary biotite, and record prehnite–pumpellyite grade metamorphism in the source area. This volcanic source also contributed fresh angular plagioclase (albite) to the Tourelle sand fraction. The volcanic source was probably an ensialic arc associated with the closing of the Iapetus Ocean, and is tentatively identified with the Cambrian to Middle Ordovician Tetagouche volcanics of northern New Brunswick.In addition to rounded, recycled grains of zircon, tourmaline, garnet, and sphene, the Tourelle heavy mineral suite contains prismatic hornblende, hypersthene, and clinopyroxene. These minerals may have been derived from arc lavas, their intrusive equivalents, or from an obducted ophiolite sheet. Ophiolite obduction and erosion is preferred because the graphic heavy mineral population contains 18% chromite, or 8.2 × 10−4 g Cr/cm3 sandstone. This is 1/15 the chromite concentration of peridotite, an essential component of any ophiolite suite.Provenance criteria for these syn-orogenic sandstones require a plate tectonic model involving southeastward subduction beneath a block of continental crust, and emplacement of ophiolite. Previous models for the Quebec Appalachians are unable to account for detrital composition of Tourelle sands and must undergo major modification.This paper reveals that sedimentary petrography is essential to satisfactory unravelling of orogenic history. Neglect of sediment composition may invalidate tectonic reconstructions based solely on stratigraphic and structural considerations.

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