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REE-bearing minerals in the albitites of central Sardinia, Italy

Published online by Cambridge University Press:  05 July 2018

G. Carcangiu ,
M. Palomba  and
M. Tamanini
G. Carcangiu
Affiliation:
Centro Studi Geominerari e Mineralurgici del C.N.R., Fac. di Ingegneria, Università. di Cagliari, 09123 Cagliari, Italy
M. Palomba
Affiliation:
Centro Studi Geominerari e Mineralurgici del C.N.R., Fac. di Ingegneria, Università. di Cagliari, 09123 Cagliari, Italy
M. Tamanini
Affiliation:
Centro Studi Geominerari e Mineralurgici del C.N.R., Fac. di Ingegneria, Università. di Cagliari, 09123 Cagliari, Italy
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Abstract

Recent studies on albitite rocks located in the granodiorite complex of Central Sardinia have revealed that epidote has a widespread occurrence as a light rare-earth element (LREE)-bearing accessory common phase. Titanite has been recorded as a heavy rare earth element (HREE)-bearing mineral. The Hercynian granodiorite complex of Central Sardinia is composed chiefly of quartz, Ca-plagioclase, K-feldspar and biotite and of a wide variety of secondary assemblages, mainly allanite, titanite and zircon. Albitic plagioclase and quartz are the main mineral components of the albitites. Additional minerals include, besides allanite and epidote, a more calcic-plagioclase (oligoclase), K-feldspar, chlorite, titanite and more rarely muscovite. The mineral assemblages and REE-bearing minerals of albitites were analysed by wavelength dispersive spectrometry (WDS). Chemical data suggest that there is a near complete solid-solution between epidote and allanite whereas little variations in HREE of titanites were detected. In epidote-group minerals a pronounced zoning in REE was observed while titanite was recorded unzoned. Textural relations were studied by SEM to distinguish primary from secondary epidotes. Chemical criteria to recognize magmatic from alteration epidotes were also applied. The alteration epidotes mainly occur and generally originate from plagioclase alteration and from leaching of magmatic allanite. Comparison of textures using both the SEM technique and EPMA data showed that the characteristic ‘patchy zoning’, observed in epidotes, corresponds with different amounts of REE in these minerals.

The schematic model proposed for the epidote-forming reactions during the metasomatic processes that affected the granodiorites involves: (i) the instability of the anorthitic component of plagioclase; (ii) the simultaneous formation of albite; (iii) the leaching of the magmatic allanite with a redistribution of REE in the epidotes of the albitites.

Keywords

rare earth elementsalbitite rocksSardiniaepidoteallanitetitanite
Type
Mineralogy
Information
Mineralogical Magazine , Volume 61 , Issue 405 , April 1997 , pp. 271 - 283
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1997

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