EVIDENCE FOR AN ENRICHED MANTLE SOURCE FOR JOTUNITE (ORTHO-PYROXENE MONZODIORITE) ASSOCIATED WITH THE ST. URBAIN ANORTHOSITE, QUEBEC

Mafic orthopyroxene monzodiorite (jotunite) lithologies are exposed in the St. Urbain plutonic suite as a marginal facies to quartz mangerit e and massif anorthosite intrusive bodies and as dikes within a variet y of host rocks. High concentrations of Ti, Fe, P, K, Ba, Nb, La, Ce, Zn, Ga, Zr and Y characterize these rocks and are distinctive of many mafic lithologies associated with anorthosite massifs worldwide. Chara cteristically low concentrations of Ni and Cr, in conjuction with low Mg numbers, have been used by previous investigators as evidence for e ither partial melting of mafic granulitic lower crust or extensive fra ctional crystallization of a mantle-derived magma. In an attempt to di stinguish between these competing models, we note that jotunite displa y many features that bear a strong resemblance to continental tholeiit ic flood basalts, including chemical signatures on normalized multi-el ement ('spider') diagrams. Ratios of incompatible trace elements and p atterns on rare earth and 'spider' diagrams collectively indicate that the jotunite rocks were derived from an enriched, rather than deplete d, mantle source. Enrichment may have occured by subduction-derived fl uids or by mixture with a plume component prior to partial melting so that isotopic and trace-element compositions are decoupled. Small amou nts of partial melting of mafic granulite has been advanced as an alte rnative model; we show, however, that the experimental data on which t his model is built are not applicable. Our preferred model begins with partial melting of a trace-element enriched mantle source that fracti onates olivine at high to moderate pressures. Increasing concentration s of P (and Ti) eventually caused a contraction of the olivine stabili ty field in favor of orthopyroxene. Fractional crystallization may yie ld the series of rocks from anorthosite, leuconorite, oxide-apatite ga bbronorite, to jotunite. Mafic magmas emplaced into continental crust are typically attributed to incipient rifting or mantle upwelling, whi ch are features common to many models for the genesis of anorthosite a nd related rocks. (C) 1998 Elsevier Science B.V.