Using Trace Elements in Chromites to Constrain the Origin of Podiform Chromitites in the Thetford Mines Ophiolite, Quebec, Canada

摘要

The Thetford Mines Ophiolite is interpreted as a fragment of a fore-arc oceanic lithosphere. This complete ophiolitic sequence consists of a thick mantle sequence, a plutonic crustal component, and a volcano-sedimentary sequence dominated by boninitic lavas. One third of its 50 chromitite occurrences are podiform 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 (Canadian Appalachians) and from the associated boninite lavas, using laser ablation inductively coupled to a plas-maquadrupole mass spectrometer. The chromites from Thetford Mines Ophiolite podiform chromitite deposits are Cr rich (Cr# = 100 deg Cr/(Cr + Al) = 69-84), Ti poor (TiO_2 = 0.06-0.18 wt %), Ga poor (13-26.5 ppm), Ni poor (441-875 ppm) and are similar to chromite in boninites from around the world and to the chromites in the boninite lavas of the Thetford Mines Ophiolite. 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 resemble closely the boninites capping the crust of the ophiolite. The empirical partition coefficients for minor and trace elements (Sc, Ti, V, Mn, Co, Ni, Zn, and Ga) in chromite calculated from the chromite and their host lava (boninite and MORB) compositions are similar to experimentally determined values.