Early subduction dynamics recorded by the metamorphic sole of the Mt. Albert ophiolitic complex (Gaspé, Quebec)

Abstract : The metamorphic sole of the Mt. Albert ophiolitic complex (Gaspé peninsula, Quebec, Canada) is a sliver of Ordovician oceanic crust accreted to the base of an incomplete ophiolitic sequence, along a suture zone throughout the north-eastern Appalachians linked to the Taconian orogeny. A detailed mineralogical study of the rocks in the metamorphic section of the sole is provided in this publication: these rocks record valuable information in terms of petrological processes and conditions of accretion, with limited retrogression. The petrology of the metamorphic sole shows that it originates from ocean floor and that it is the equivalent of the Shick-Shock volcanics group, metamorphosed to granulite/higher amphibolite facies. Presence of aluminosilicate bearing metapelites allows constraining pressure conditions in a more robust way than in the case of the famous Semail ophiolite (Oman). Thermobarometric estimates for peak metamorphic conditions for the metamorphic sole of the Mont Albert ophiolitic complex indicate temperatures above 800 °C close to the contact with the overlying peridotite, decreasing to ~650 °C within <30 m, at pressure around 1 GPa. Structurally lower subunits of the sole metamorphosed in the amphibolite/higher greenschist facies retain evidence of basaltic protolith accreted to lower pressures. Evidence of partial melting in high-grade subunits, contradictory with the apparent lack of melting products, may be linked to preferential accretion of melt-poor lithologies to the overlying plate, melt-rich lithologies being too weak to allow coupling and accretion to non-serpentinised peridotite. The reconstructed petrological evolution is consistent with stepwise accretion of metamorphic soles in nascent obduction/subduction systems as proposed for the Semail example and other metamorphic soles worldwide: the subducted oceanic plate is metamorphosed up to granulite facies by heat transfer from the still hot overlying peridotite, and accreted to the base of the future ophiolite when the rheological contrast between the two is minimum. Thermobarometric estimates indicate clock-wise pressure-temperature paths. The inferred thermal evolution requires burial of the metamorphic sole below warm, young (or rejuvenated) oceanic lithosphere, consistent with subduction initiation close to the ridge followed by rapid accretion and exhumation of the metamorphic sole. In this context, exhumation along higher geothermal gradient than burial shows that cooling of the mantle wedge above the plate interface takes place slowly compared to exhumation of the metamorphic sole, even though the plate interface itself has been cooling between burial and exhumation of the rocks.
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Benoît Dubacq, Mathieu Soret, Ella Jewison, Philippe Agard. Early subduction dynamics recorded by the metamorphic sole of the Mt. Albert ophiolitic complex (Gaspé, Quebec). Lithos, Elsevier, 2019, 334-335, pp.161-179. ⟨10.1016/j.lithos.2019.03.019⟩. ⟨hal-02187161⟩

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