Magnetic petrology of variably retrogressed eclogites and amphibolites: A case study from the Hercynian basement of northern Sardinia (Italy)

摘要

This study constitutes the first attempt to integrate rock magnetism and metamorphic petrology in the Hercynian basement of northern Sardinia. The investigation focused on the magnetic petrology of variably retrogressed eclogites and amphibolites from two suites of basic meta-igneous rocks, which occur along a major tectonic line (Posada-Asinara Line), within a medium-grade (MG) and a high-grade (HG) metamorphic complex. Consistent with petromagnetic results, HG metabasites contain variable amounts of monoclinic pyrrhotite (intergrown with rutile) and titanomagnetite (occurring as inclusions in garnet), abundant ilmenite (associated to secondary hornblende and with sphene ± low-Ti-magnetite rims), and rutile both as inclusions in ilmenite and as discrete grains. In MG metabasites, pyrrhotite is restricted to amphibolites of the Posada Valley area where it occurs as rare inclusions in garnet. All samples are characterized by variable amounts of ilmenite, rutile, and sphene which show the same microstructural features described in HG rocks. Microstructural evidence and geothermobarometric data indicate that (1) pyrrhotite and titanomagnetite likely formed prior to and remained stable during the eclogite facies metamorphic peak and (2) the growth of ilmenite and sphene can be attributed to the amphibolite facies retrogression, mainly due to model reactions such as garnet + omphacite + rutile + H2O → hornblende + plagioclase + ilmenite and amphibole + ilmenite + O2 → sphene + magnetite + quartz + H2O. The results from our combined petrological and petromagnetic study corroborate the hypothesis that significant volumes of mafic/ultramafic rocks, similar to some of the investigated outcrops, may account for the magnetic anomalies flanking the northeastern part of the Posada-Asinara Line.