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 crusta 1 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 crusta 1 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 AI-Ti-phlogopite. A marked redistribution ofHREE-Y -Zr between garnet and xenotime (where present) and zircon, but also ofLREE between feldspars (K-feldspar and plagioclase) and monazite, is suggested.

Genesis of the Hengling magmatic belt in the North China Craton: Implications for Paleoproterozoic tectonics

“…In contrast, high field-strength elements (HFSE) and rare earth elements (REE) are relatively immobile during metamorphism, and in general tend to reflect igneous processes (Middelburg et al, 1988). However, during granulite facies metamorphism, Y, Zr, Th, U and REEs may also be modified (e.g., Bea and Montero, 1999;Villaseca et al, 2003). In this study, fresh samples were mostly selected from the margins of dykes/sills, and some from the centers; one sample was from a pegmatite vein in the Longwangtang sill (sample 07DX11), and another was from a pyroxenite enclave in the same pegmatite vein in this sill (sample 07DX10) (all indicated in Table 1).…”

Zr–LREE rich minerals in residual peraluminous granulites, another factor in the origin of low Zr–LREE granitic melts?

“…7). The high modal proportions of feldspar in SCS granulite xenoliths together with the scarcity of monazite (Villaseca et al, 2003), suggests that feldspars control virtually the entire LREE-Eu budget.…”

“…It has been observed that the modal amount of zircon, monazite and apatite decreases dramatically with increasing metamorphic grade (Reid, 1990;Bea and Montero, 1999) and other accessory minerals (e.g. xenotime) are completely con sumed before extreme granulite conditions are reached (Villaseca et al, 2003). Nevertheless, SCS granulite xenoliths show Zr and REE contents mostly in the range of 155 to 280 and 120 to 225 ppm, respectively, similar to peraluminous metamorphic rocks from outcropping migmatite terranes.…”

“…Bea et aI., 1994;Fraser et aI., 1997) and lower crustal xenoliths (Reid, 1990;Villaseca et aI., 2003). Moreover, there has been recent discussion on the apparent existence of low-temperature granitic melts, as indicated by their Zr (and LREE) contents (Miller et aI., 2003;Chappell et aI., 2004), based on the dependence of zircon (and monazite) solubility on tem-and monazite (Villaseca et aL, 2003). Most of these granulites also have unzoned almandine-pyrope garnets (AI114s-so-Pyr4s-ss) (Villaseca et aI., 1999).…”

In-situ trace element analysis of clinopyroxene on thin section by using LA-ICP-MS

“…Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has become one of the most useful in-situ analytical tools for geological materials over the past few years, even though it has a relatively lower sensitivity than secondary ion mass spectrometry (SIMS) because of elemental fractionations (Longerich et al, 1996;Norman et al, 1996;Chen et al, 1997;Eggins et al, 1998;Ridley, 2000;Becker and Tenzler, 2001;Villaseca et al, 2003;Zack et al, 2002;Cox and Witon, 2003;Kabashima et al, 2003;Cocherie and Robert, 2008;Frei and Gerdes, 2009). The advantages of the LA-ICP-MS technique are fast in-situ analysis, low sample consumption, and lack of polyatomic interference species (Rusoo et al, 2002;Ishida et al, 2004).…”