Residence and partitioning of REE and selected trace elements in amphibolite-facies metabasites : an example from the St. Anthony complex, northern Newfoundland

• Main Author: Mulrooney, Daniel Joseph
• Format: Thesis
• Language: English
• Published: Memorial University of Newfoundland 2004
• Subjects: St. Anthony; Newfoundland
• Online Access: https://research.library.mun.ca/6786/; https://research.library.mun.ca/6786/1/DanielJosephMulrooney.pdf; https://research.library.mun.ca/6786/3/DanielJosephMulrooney.pdf

Metabasites from the metamorphic sole of the Hare Bay Allochthon, northern Newfoundland, exhibit a narrow metamorphic field gradient from greenschist- through epidote-amphibolite to amphibolite-facies. The metamorphic facies are defined from the distribution of the following mineral assemblages (all with quartz and titanite): chlorite-epidote-plagioclase-Ca amphibole (greenschist), Ca amphibole-plagioclase-epidote (epidote amphibolite), and Ca amphibole-plagioclase (plagioclase amphibolite). The assemblage Ca amphibole-plagioclase-garnet-biotite-apatite (biotite amphibolite) equilibrated in a K₂O-enriched shear-zone. Representative samples from each metamorphic facies were were analyzed for rare earth elements (REE) and a suite of trace elements (Sc, V, Cr, Ni, Cu, Zn, Sr, Rb, Y, Zr, Nb) in bulk rocks and the major minerals Ca amphibole, plagioclase, epidote, titanite and garnet, by solution ICP-MS and laser-ablation microprobe (LAM)-ICP-MS respectively. Major-element concentrations of bulk rocks and minerals were determined by XRF and electron probe microanalysis (EPMA) respectively. -- On the basis of bulk-rock, major- and trace-element data and chondrite-normalized REE data, two suites of metabasites were distinguished: (i) gamet-absent greenschist and amphibolite characterized by relatively low SiO₂ and A1₂O₃ and Y+MREE-enrichment; and (ii) garnet-bearing amphibolite with relatively low FeOtotal + MgO, high P₂O₅ and Zr+LREE enrichment. The bulk P₂O₅ and LREE enrichment in garnet-bearing rocks is inferred to be sequestered in unanalysed fine-grained apatite. -- Ca amphibole, plagioclase, epidote, garnet and titanite exhibit systematic distributions of major- and most trace-elements, including REE in epidote amphibolite, plagioclase amphibolite and biotite amphibolite assemblages, indicating a close approach to equilibrium. However, variations in the distributions of a few trace elements (e.g., Rb and Nb between certain phases) suggest that equilibrium may not have been achieved for all elements at a ...