REE FRACTIONATION BY ACCESSORY MINERALS IN EPIDOTE-BEARING

DAHLQUIST, J.A.

MINERALOGICAL MAGAZINE

The Mineralogical Society

Lugar: London; Año: 2001 vol. 65 p. 463 - 475

0026-461X

A study of the distribution of REE in epidote-bearing metaluminous granitoids from Sierra de Chepes, Sierras Pampeanas, Argentina, reveals that a large proportion of the REE reside in the accessory minerals (allanite, epidote, titanite, apatite and zircon), and therefore these minerals control the behaviour of REE in granitic magmas. Well-developed chemical zonation in titanite indicates that the REE content decreases in the melt during crystallization of this mineral. The textural and chemical characteristics of euhedral epidote suggest a magmatic origin, and in that case it may have played an important role in the fractionation of the REE. The amount of silica and any other geochemical parameter indicative of fractionation progress in the dominant granodioritic-tonalitic facies (gtf) do not correlate with observed variations in the REE patterns. When many accessory minerals are involved, as in the gtf, the differentiated melts (e.g. aplites) are REE poor. Thus, the presence/absence of accessory minerals in granitoids can be indicative of the generation of differentiated melt enriched or poor in REE and other trace elements. This may have an economic significance, as it may allow us to predict the probable geochemistry of the differentiated melts (i.e. those that tend to develop mineralization) from the textural analysis of the ‘regional’ granitic rock. Finally, the type and abundance of accessory minerals in the granitic suite can also help us to define the geotectonic environment where magmas were generated.REE in epidote-bearing metaluminous granitoids from Sierra de Chepes, Sierras Pampeanas, Argentina, reveals that a large proportion of the REE reside in the accessory minerals (allanite, epidote, titanite, apatite and zircon), and therefore these minerals control the behaviour of REE in granitic magmas. Well-developed chemical zonation in titanite indicates that the REE content decreases in the melt during crystallization of this mineral. The textural and chemical characteristics of euhedral epidote suggest a magmatic origin, and in that case it may have played an important role in the fractionation of the REE. The amount of silica and any other geochemical parameter indicative of fractionation progress in the dominant granodioritic-tonalitic facies (gtf) do not correlate with observed variations in the REE patterns. When many accessory minerals are involved, as in the gtf, the differentiated melts (e.g. aplites) are REE poor. Thus, the presence/absence of accessory minerals in granitoids can be indicative of the generation of differentiated melt enriched or poor in REE and other trace elements. This may have an economic significance, as it may allow us to predict the probable geochemistry of the differentiated melts (i.e. those that tend to develop mineralization) from the textural analysis of the ‘regional’ granitic rock. Finally, the type and abundance of accessory minerals in the granitic suite can also help us to define the geotectonic environment where magmas were generated.