Behaviour of rare-earth elements in highly evolved granitic systems: Evidence from Proterozoic molybdenite mineralized aplites and associated granites in northern Sweden

Abstract

Field relations, mineralogy, major- and trace-element contents (including REE analyses of whole-rock samples and minerals) of three Proterozoic granites and their associated molybdenite mineralized aplites have been studied at Allebuoda, Munka and Kåtaberget in northern Sweden. The granites crystallized from melts that were not saturated with water. The mineralized potassic aplites formed by quenching of residual melts caused by sudden pressure drop, H 2O saturation and vapour escape during tectonic rupturing. Leucogranites with higher Na 2O K 2O ratios from Allebuoda and Munka crystallized during H 2O-saturated equilibrium conditions in which the exsolved vapour could continuously migrate away. The pressure was probably ∼ 3 kbar at Munka, and somewhat lower at Allebuoda. The granites have REE patterns characterized by LREE enrichments and negative Eu anomalies. In comparison, the potassic aplites and the more sodic leucogranites are depleted in LREE, enriched in HREE and have larger negative Eu anomalies. Allanite and monazite are the most important REE carriers in the granites. These minerals are strongly enriched in LREE, whereas fluorite and xenotime, which are more abundant in the aplites, are most enriched in HREE. Due to the strong control of accessory minerals on the REE balance, REE are of limited use in petrogenetic modelling of highly evolved granitic systems.