Rare-earth element geochemistry of alnoitic Cretaceous rocks and ultramafic xenoliths from Île Bizard (Québec, Canada)

Abstract

The Île Bizard intrusion, located ∼ 10 km west of Montréal, is a Cretaceous alnoitic intrusion containing numerous ultramafic xenoliths. Four samples of the alnoite matrix and ten ultramafic xenoliths (lherzolite, spinel-bearing lherzolite, harzburgite, spinel-bearing harzburgite, websterite, garnet-spinel websterite) were analysed for major and rare-earth elements (REE). The alnoite samples are strongly LREE-enriched, with chondrite-normalized [ La Yb ] CN = 135−590 . Their REE abundances are similar to those of a melt produced by a small degree of partial melting of a LREE-enriched garnet lherzolite mantle source. All Île Bizard ultramafic xenoliths have [ La Yb ] CN > 1 , and some harzburgite xenoliths have U-shaped REE patterns. Websterite xenoliths are in equilibrium with their alnoite host with respect to REE. Geochemical data indicate that harzburgite xenoliths are residues of partial melting from the upper mantle.