Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

The Thetford Mines Ophiolite (Canadian Appalachians) is a complete ophiolitic sequence consisting of a thick mantle section (5 km), a plutonic crust and a volcano-sedimentary cover dominated by boninitic lavas. This ophiolite is interpreted as a fragment of a fore-arc oceanic lithosphere. It hosts 50 chromitite occurrences and one third of them are of podiform-type and are hosted in a dunitic envelop within the mantle tectonites.

This study presents the concentrations of a complete suite of minor and trace elements (Sc, Ti, V, Mn, Co, Ni, Zn, and Ga) in chromites from podiform chromitites of the Thetford Mines Ophiolite, and from the associated boninite lavas, using laser ablation inductively coupled to a plasmaquadrupole mass spectrometer.

The aim of this study is to use trace elements to constrain the parental melt from which the podiform chromitites have crystallized. The chromites from Thetford Mines Ophiolite podiform chromitite deposits are Cr-rich (Cr# = 100xCr/(Cr+Al) = 69 to 84), Fe-rich (Fe2+# = 100xFe2+/(Fe2++Mg) = 32 to 46), Mn-rich (1081 to 1962 ppm), Co-rich (166 to 386 ppm), Sc-rich (5.5 to 11.3 ppm), and they are Ti-poor (TiO2 = 0.06 to 0.18 wt%), Ga-poor (11.3 to 26.5 ppm), and Ni-poor (441 to 875 ppm) compared to MORB chromite (Cr# = 35; Fe2+# = 23; Mn = 881 ppm; Co = 165 ppm; Sc = 5.3 ppm; TiO2 = 0.22 wt%; Ga = 52.5 ppm; Ni = 1766 ppm). The chromites from Thetford Mines Ophiolite podiform chromitite deposits are similar to chromite in boninites from around the world and to the chromites in the boninite lavas of the Thetford Mines Ophiolite. In addition, the Al/Ti ratios of the chromites have been used to determine the nature of the melts from which podiform chromitites in the Thetford Mines Ophiolite have crystallized. These have been found to closely resemble the boninites capping the crust of the ophiolite.