Residence and redistribution of REE, Y, Zr, Th and U during granulite-facies metamorphism: behaviour of accessory and major phases in peraluminous granulites of central Spain

Abstract

Accessory minerals are thought to play a key role in controlling the behaviour of certain trace elements such as REE, Y, Zr,
Th and U during crustal melting processes under high-grade metamorphic conditions. Although this is probably the case at
middle crustal levels, when a comparison is made with granulite-facies lower crustal levels, differences are seen in trace element
behaviour between accessory minerals and some major phases. Such a comparison can be made in Central Spain where two
granulite-facies terranes have equilibrated under slightly different metamorphic conditions and where lower crustal xenoliths are
also found. Differences in texture and chemical composition between accessory phases found in leucosomes and leucogranites
and those of melanosomes and protholiths indicate that most of the accessory minerals in melt-rich migmatites are newly
crystallized. This implies that an important redistribution of trace elements occurs during the early stages of granulite-facies
metamorphism. In addition, the textural position of the accessory minerals with respect to the major phases is crucial in the
redistribution of trace elements when melting proceeds via biotite dehydration melting reactions. In granulitic xenoliths from
lower crustal levels, the situation seems to be different, as major minerals show high concentration of certain trace elements, the
distribution of which is thus controlled by reactions involving final consumption of Al-Ti-phlogopite. A marked redistribution
of HREE–Y–Zr between garnet and xenotime (where present) and zircon, but also of LREE between feldspars (K-feldspar and
plagioclase) and monazite, is suggested.