Anatectic amphibole and restitic garnet in Variscan migmatite from NE Sardinia, Italy: insights into partial melting from mineral trace elements

摘要

We report results of a laser-ICP-MS investigation of trace element contents in the main constituent minerals of an amphibolebearing migmatite from the Variscan orogen in northeastern Sardinia. The migmatite is associated with migmatised orthogneiss and Alsilicate- bearing pelitic migmatites. The protolith of the amphibole-bearing migmatite was a mid-Ordovician igneous rock of intermediate composition characterised by a biotite + plagioclase + quartz assemblage. The migmatite consists of mesosomes and tonalitic (or, less frequently, granodioritic) leucosomes, characterised by amphibole crystals (potassian ferropargasite) up to 2 cm in size. The tonalitic leucosomes aremade up of quartz, plagioclase,±K-feldspar, biotite,±amphibole, garnet. The mesosomes are foliated rocks made up of the same minerals with different modal proportions. In leucosomes, amphibole is the most abundant mafic mineral, occurring as euhedral crystals rich in plagioclase, quartz, and small garnet inclusions. Garnet occurs as corroded and fractured grains in the matrix or within the amphibole. Zircon forms euhedral bipyramidal grains up to a few hundreds of micrometres in size. Some amphibole rims have higher REE and negative Eu anomalieswhereas cores exhibit lower REE and positive Eu anomalies. Garnet has strongly fractionated REE patterns with chondrite-normalised abundances up to 2000 for HREE. Plagioclase has flat REE patterns with pronounced positive Eu anomalies. Zircon displays fractionated REE patterns with HREE enrichment, LREE depletion, positive anomalies for Ce and negative ones for Eu.Monazite shows high REE abundances, LREE enrichment, HREE depletion and negative Eu anomalies. Garnet is mostly a restitic phase, as indicated by significant variation inHREEconcentrations between grains in themesosome, the absence of a noticeableEu anomaly, andYdepletion in the leucosomes as compared to the mesosomes. In the leucosomes and mesosomes, the cores of zoned amphibole are characterised by positive Eu anomalies: these crystallised from or in the presence of melt produced by anatexis of the original Bt + Pl + Qtz protolith. Adjacent rimswith negative Eu-anomalies developed in coexistencewith a Eu-depleted melt that had experienced plagioclase fractionation.