Ca distribution between almandine-rich garnet and plagioclase in pelitic and psammitic schists from the metamorphic basement of North-Eastern Sardinia

Abstract

Garnet and plagioclase pairs from fourteen selected samples, from garnet to sillimanite zones, collected along a NS traverse throughout the metamorphic basement of NE Sardinia, have been analyzed by microprobe.

Beyond the garnet isograd, plagioclase has albitic composition and the garnet (a Ca-rich almandine) shows Ca/ Ca+Mg+Fe ratios of about 0.35–0.30, fairly constant from core to rim.

Towards the North, still in the garnet zone, when on the large albitic core of plagioclase a thin and discontinuous oligoclasic rim (An₂₂–An₁₈) formed, we observe in the garnet edge an abrupt decrease of the Ca/Ca+Mg+Fe ratio (0.27–0.16).

In the staurolite and sillimanite zones garnet does not show significant Ca-zoning and it is characterized by low Ca content (Ca/Ca+Mg+Fe<0.1); the coexisting plagioclase has oligoclasic (An₁₆–An₂₇) composition.

The chemical data and the microstructural evidence on growth time indicate that the garnet and plagioclase had a strong mutual interference in determining the relative Ca distribution.

The most relevant reactions are discussed and, in particular, the antipathetical Ca-zoning, recorded by garnet and plagioclase in the garnet zone, is considered as the evidence of temperature increase during growth of the two minerals. It is also suggested that the sharp variation of Ca content at the garnet edge was controlled by the discontinuous nature of plagioclase solid solution in the “peristeritic” range.

The order of appearance of garnet and oligoclase in the basement of NE Sardinia is also discussed in comparison with other well known metamorphic sequence (Vermont, New Zealand and Dalradian). It is concluded that the different order of appearance is controlled other than the different nature of the calcic phases in the lower grade zones also by the $X_{CO_2 }$ in the fluid phase.