REDISTRIBUTION OF THE RARE-EARTH ELEMENTS AMONG COEXISTING MINERALS IN METAMAFIC ROCKS ACROSS THE EPIDOTE-OUT ISOGRAD: AN EXAMPLE FROM THE ST. ANTHONY COMPLEX, NORTHERN NEWFOUNDLAND, CANADA

Abstract

Integrated results of in situ major-element and rare-earth-element analyses of coexisting Ca amphibole, plagioclase, epidote and titanite have been made from a suite of metamafic rocks from the St. Anthony Complex, Newfoundland, across the epidote-out reaction isograd to examine the redistribution of the REE as a consequence of the breakdown of epidote. Important major-element variations include a decrease in X Cum Amp and an increase in X An Pl across the reaction isograd, and an increase in X Czo Ep within the epidote-amphibolite facies. These major-element changes correlate with variations in ∑REE and the LREE:HREE ratio in several of the phases, and also with systematic changes in D *REE partitioning across the epidote-out isograd. Onuma-type diagrams for the partitioning of the REE between coexisting minerals confirm that the REE substitute at single elastic sites in titanite, epidote and plagioclase, whereas in Ca amphibole they occupy two sites, M 4 and M 4′, with the size of the M 4′ peak being positively correlated with X Cum Amp in Ca amphibole. A REE mass balance across the epidote-out isograd yields very good matches between the measured and reconstructed REE abundances in all cases, indicating that metamorphism was essentially isochemical with respect to the rare-earth elements, and that all the hosts for these elements were analyzed. The following major-element mass-balanced reaction, determined by the method of singular value decomposition from mineral compositions in representative samples of epidote amphibolite and plagioclase amphibolite from this ophiolitic complex, provides a realistic representation of the epidote-out isograd in the study area: 2.94 CaAmp1 + 5.00 Pl1 + 1.00 Ep + 5.21Ttn1 = 3.14 CaAmp2+ 5.65 Pl2 + 5.29 Ttn2. Integration of the calculated stoichiometric coefficients of this reaction with the measured REE contents in each phase provides a good match for the MREE and HREE abundances, but a poorer match for the LREE, which show a deficiency on the product side. However, the results are compatible with the measured increases in LREE in Ca amphibole and plagioclase on the product side of the reaction, as a result of the breakdown of epidote and a reduction in LREE abundance in titanite. We highlight the role of epidote and titanite as the principal carriers of the REE in epidote amphibolite, and that of Ca amphibole as the dominant carrier in plagioclase amphibolite.