The Evolution of Granitic Magmas: Evidence From Their Accessory Phases

J. A. Jennings Department of Earth Sciences, University of Keele, Staffordshire, ST5 5BG, UK

G. Rowbotham Department of Earth Sciences, University of Keele, Staffordshire, ST5 5BG, UK

It is generally agreed that zoning in minerals and the partitioning of trace elements between mineral phases are important parameters when considering sequences of crystallization and the maintenance of equilibrium between minerals and melt.

The petrography and accessory mineral chemistry of three Hercynian granites and a granodiorite from within the Aber Ildut massif, N.W.France have been studied by electron microprobe. All the granites within the plutons contain zircon, apatite, monazite and ilmenite. The granodiorite contains the additional REE-bearing phases, allanite and titanite.

Monazite is euhedral and varies from colourless to yellow and possesses a high birefringence, which reflects variations in Th content. Pleochroic haloes are apparent and confirm significant radioactivity. Allanite is located within crystals of alkali feldspar and apatite. In feldspar grains it is euhedral, but where it is located within apatite it is anhedral. In both cases it shows an irregular distribution of interference colours.

Core and rim analyses of monazite grains from the two rock types show chemical zoning with respect to LREE and Th; in both the granite and granodiorite the rims have lower concentrations of LREE than the core with a corresponding increase in Th content. This may be the result of rapid cooling, where these elements have insufficient time to equilibrate. Y and U show little variation across the grains.

LREE concentrations in co-existing allanite are approximately half those in monazite and whilst this may be a function of respective partition coefficients, its irregular optical properties and absence of concentric chemical zoning could support an interpretation that this is a secondary phase.

Whole rock chondrite-normalized REE profiles show that all the individual plutons possess similar patterns and concentrations and a comparison with similar normalized mineral spectra show that both monazite and allanite exert a significant influence on the LREE fraction. Application of monazite/melt and allanite/melt partition coefficients in the granodiorite show that 20% of the bulk-rock LREE are concentrated in monazite and the remaining 80% in allanite. Similar calculations for the granites show that the LREE are totally contained within monazite.

The co-existence of monazite and allanite is uncommon. It is considered on this occasion that allanite acts as a substitute for monazite in its role as an alternative repository for the REE and Th and that this is is facilitated by conditions of lower temperature and pressure and therefore is considered to be secondary.

It is noted that Sm/Nd ratios in monazite within particular plutons show variation; this indicates that these elements may have undergone fractionation, with Nd being incorporated in the monazite lattice in preference to Sm. It is suggested that where the principal repository for the LREE is located in one particular phase then this will affect whole rock ratios with a consequential effect on the Sm-Nd systematics which rely on the coherent behaviour of Sm and Nd during mobilisation.

Optically zircon shows concentric zoning. Concentrations of U,Th,and Y are substantially lower than in co-existing monazite and P and Si display sympathetic behaviour with P substituting for Si. The concentrations of Y and Ti fluctuate from core to rim where an increase in the Y content is balanced by a concomitant decrease in Ti content, possibly brought about by steric constraints. Both of these elements are of similar ionic radius and compete for the same lattice site. Y and P show a positive correlation demonstrating that the coupled substitution:- ZrVIII SiIV ¤ REEVIII PIV ( zircon ¤ isostructural xenotime) is significant in zircon from these plutons.

Calculations of crystallization temperatures of all the granitoids in the Aber Ildut massif, made on the basis of zirconium solubility experiments, give consistent results of around 750 °C.


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