Amphibolite associated with the Thetford Mines Ophiolite Complex at Belmina Ridge, Quebec

Abstract

Amphibolite in the Quebec Appalachians at Belmina Ridge (45°58′N; 71°28′W) constitutes a steeply dipping, allochthonous sheet as much as 800 m thick, between the obducted Thetford Mines Ophiolite Complex to the east and rocks of the allochthonous Caldwell Group to the west. The amphibolite, which structurally and chemically is unlike amphibolite in the adjacent Caldwell, is fresh, coarse to fine grained, and massive to layered. It is composed chiefly of edenitic to pargasitic amphibole, pyrope- and grossular-rich almandine, salite, and epidote, with small amounts of quartz, plagioclase, and accessory minerals. Metamorphism ranged from [Formula: see text] at the base of the sheet to T ~ 780 °C at the top, near the tectonic contact with rocks of the superjacent ophiolite complex, under a Barrovian facies series with Ptotal between 5 and 7 kbars (500–700 MPa).The amphibolite is interpreted to have been derived from ocean-floor basalt that was metamorphosed under a 20 km thick, westward-directed, obducted allochthon of oceanic lithosphere. With a thermal gradient of 40 °C∙km−1, the temperature at the base of the allochthon would have been 800 °C, whereas the temperature at the base of the Thetford Mines Ophiolite Complex (the uppermost third of the allochthon) would have been 312 °C or less, in agreement with the serpentinization history of the complex. The dynamic environment during obduction could continually furnish "fresh" (hot) lithosphere to override the site of metamorphism of the amphibolite. At the site, the temperature of the base of the allochthon rose progressively, and a relatively high temperature was maintained for ample time, of the order of 1 Ma, to have continuously heated the underlying rocks to raise them to a high temperature without the allochthon ever having been unrealistically hot. The allochthon was dismembered tectonically toward the close of obduction. The amphibolite was thrust westward and upward, to be emplaced at relatively little depth on rocks of the Caldwell Group, and under the ophiolite complex. The petrogenesis proposed for the amphibolite at Belmina Ridge is consistent with the apparently simple metamorphic history of the rock.