Genesis and transformations of monazite, florencite and rhabdophane during medium grade metamorphism: examples from the Sopron Hills, Eastern Alps

Abstract

Electron microprobe studies on the age, mineral chemistry and alteration on accessory LREE-phosphate minerals have been carried out in medium-grade metamorphic rocks of the Sopron Hills belonging to the Lower Austroalpine tectonic unit. Monazite (and xenotime) is relatively common, whereas rhabdophane and florencite are restricted to certain rock types. A first generation of monazite was formed in mica schists during the pre-Alpine, Hercynian metamorphism at 575–700 °C and 1.8–3.8 kbar as evidenced by P–T data from the literature, their mineral paragenetic and textural characteristics and supported by Th–U–total Pb ages of ca. 300 Ma. In orthogneisses, monazite is rare and of igneous origin. Kyanite quartzites and leucophyllites that were formed by Mg metasomatism contain inherited monazite from the precursor rocks. A new generation of monazite was also formed during the Alpine metamorphism at ≤550 °C, 13 kbar according to the literature data, giving ages around 75 Ma. Pronounced negative Eu anomalies were found in the igneous monazites (Eu/Eu*<0.35), while most of the metamorphic monazites have moderately negative Eu anomalies (Eu/Eu*>0.4). Small differences have been observed in Y and HREE contents, whereas the LREE sections of the rare-earth element (REE) patterns nearly coincide. Th and Ca enter the monazite structure at the expense of REE, nearly according to the brabantitic replacement 2REE 3+↔Th 4++Ca 2+. In some mica schists, monazite is altered to rhabdophane. Rhabdophane, distinguished from monazite by quantitative electron microprobe analysis by low-oxide total, is found in many mica schists and orthogneisses. It forms fine-grained aggregates, often attached to apatite or monazite. It usually has higher Y and Ca contents and a less pronounced negative Eu anomaly than that of coexisting monazite. It may have been formed either by crystallization from REE-containing hydrous solutions or from monazite reacting with Y–Ca-containing solutions. Florencite appears only in some leuchtenbergite-bearing leucophyllites, kyanite quartzites and REE-rich clasts. It is often idioblastic and may be grown on apatite or monazite. It is chemically close to its ideal composition, but Ca, Sr and Th may replace REE in minor amounts. In some grains, ThO 2 may reach 10 wt.%. The data indicate that the charge balance is maintained by different mechanisms in low- and high-thorian florencite. No Y or HREE (above Gd) could be measured in florencite. No fractionation was observed between coexisting monazite and florencite; however, monazite inclusions in florencite are depleted in La–Ce and enriched in HREE.