Garnet-rich veins in ultramafic amphibolites from NE Sardinia, Italy

Massimo Scodina

;Gabriele Cruciani;Marcello Franceschelli;Hans-Joachim Massonne

2018-01-01

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Abstract

In the Golfo Aranci area, located a few kilometres north of the town of Olbia, a large amphibolite lens crops out in which an ultramafic body approximately 100 m long and 50 m thick occurs. These ultramafic amphibolites are characterized by a massive to weakly-schistose structure and a medium grain size. Within this ultramafic body three main compositional layers can be distinguished. The uppermost layer (about 25 m thick) shows a dark green to black colour and is featured by a high amount of millimetric to centimetric garnet crystals, which can be cumulated in ellipsoidal nodules (up to 15 cm in diameter) or centimetric thick veins. These veins are characterized by a well-defined structure from the margin to the centre: the boundary between the vein and the hosting amphibolites is defined by an irregular symplectitic microstructure of spinel and amphibole, up to 2 mm thick. The inner margin domain of the vein consists of unzoned garnet (up to 3 mm in diameter) which contains inclusions of amphibole, spinel, chlorite and corundum. Garnet-rich veins locally show a central white area in which zoned garnet occurs in a matrix. This Matrix consists of two epidote species: one is represented by casually oriented elongated euhedral crystals (7.0–7.6 wt% Fe2O3) and the other by anhedral crystals with lower contents of iron (0.3–0.9 wt% Fe2O3). Within the matrix patches with sub-rounded regular forms can be also found. These patches, that likely represent a preexisting mineral according to their shape, are made of epidote, spinel, corundum and margarite. Garnet porphyroblasts in the centre of the vein are euhedral to subhedral and show a noticeable compositional zoning and contain epidote inclusions and small chlorite veins. From core to rim four zoning stages can be defined: core, mantle, inner rim and outer rim. The garnet core is almandine rich (49 mol%), with intermediate grossular (22 mol%) and pyrope (27 mol%) contents. Towards the mantle the grossular content (52 mol%) increases and the pyrope (6 mol%) and almandine (41 mol%) contents decrease. The garnet inner rim is grossular (42 mol%) and almandine (36 mol%) rich. The pyrope content in the entire rim is 22 mol%. Almandine and grossular contents in the garnet outer rim are 40 and 37 mol%, respectively). Spessartine contents remain constantly low at 1-2 mol% throughout the whole garnet. Preliminary thermodynamic modelling with pseudosections allowed us to reconstruct the P–T path segment recorded by garnet growth. P–T pseudosections were calculated in the NCKFMASHO+Ti+Mn system using the vein core as bulk-rock composition. The P–T conditions (around T = 600°C and P = 1.5 GPa) for the garnet core were obtained by using the host rock as bulk rock composition, due to the fact that mol% values for Ca, Fe and Mg in the garnet are over or under the corresponding range obtained by calculations with the vein core as bulk composition. This could mean that the garnet core grew before the vein formation. Therefore core-to-mantle P–T conditions deserve further insights and should be considered with caution. The P–T trajectory based on the compositional change of garnet from the mantle to the rim is similar to other metabasic rocks from north Sardinia: after the peak pressure the rock experienced a strong pressure decrease and a moderate temperature increase to granulite-facies conditions The rim conditions (T = 640–680°C and P = 0.8–0.9 GPa) point to a subsequent slight P–T decrease towards the amphibolite facies, in which the exhumation of the rock continued during the Variscan orogeny.