Early stage P–T metamorphic evolution of retrogressed amphibolites from NE Sardinia, Italy

Massimo Scodina

;Gabriele Cruciani;Marcello Franceschelli;Hans-Joachim Massonne

2018-01-01

---

Abstract

The Golfo Aranci area belongs to the Migmatite Complex of the Inner Zone of the Variscan Sardinian metamorphic basement. In this area, located a few kilometres north of the town of Olbia, a large lensoid amphibolite body, 2 km long in NE-SW direction and 100-150 m wide, crops out. Within this body two main lithologies can be distinguished: retrogressed amphibolites and ultramafic amphibolites. The retrogressed amphibolites are coarse-grained, dark-green rocks with a schistose to weakly massive aspect. Within these amphibolites centimetric-sized layers locally occur which are featured by millimetric porphyroblastic garnet. These layers, oriented parallel to the regional schistosity, consist of millimetric (up to 1 cm) euhedral and subhedral garnet porphyroblasts in a matrix of green amphibole, plagioclase, quartz and aggregates with a clinopyroxene + plagioclase fine-grained symplectite-type texture. Garnet porphyroblasts, which can reach 30% vol. in these layers, contain a large amount of inclusions of amphibole, plagioclase, quartz and rare clinopyroxene and show a thin coronitic rim made up of plagioclase and amphibole. Amphibole inside this rim and in the matrix can be zoned with a retrograde phase (actinolite) at its margin. Garnet porphyroblasts are almandine rich (56–59 mol%) and spessartine poor (1–7 mol%), with intermediate grossular (26–28 mol%) and pyrope (10–16 mol%) contents. From core to rim pyrope progressively increases from 10 to 16 mol%, spessartine contents decrease from 7 to 1 mol%. We calculated a P–T path applying pseudosection modelling in the NCKFMASHO+Ti+Mn system to garnet bearing layers in retrogressed amphibolites. Pseudosections related to the garnet rim were corrected for fractionation of elements in garnet. The garnet core grew at granulite facies conditions of T = 680–720°C and P = 0.7–1.0 GPa. These conditions fit the XNa ratio of 0.07 in clinopyroxene and the XCa ratio of 0.35 in plagioclase included in garnet well. The garnet rim grew at T = 680–700°C and P = 1.3–1.4 GPa. These conditions are compatible with the XCa ratio of 0.16 in plagioclase of the matrix, which likely represents the beginning of decompression after the end of garnet growth. The P–T pseudosection modelling for the retrogressed amphibolites demonstrates an anti-clockwise P–T path from the granulite- to the high-pressure granulite-facies with a pressure increase of 0.3–0.7 GPa and a slight decrease in temperature. It is likely that garnet porphyroblasts stopped their growth after reaching the peak pressure and, thus, before the decompression characterized by the growth of amphibole. We can assume that after this decompression phase, the retrogressed amphibolites likely followed the same metamorphic retrograde path as the adjacent ultramafic amphibolites during further exumation.