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

Abstract

The silicate parageneses of variably retrogressed eclogites are extensively used by metamorphic petrologists to reconstruct the tectonometamorphic evolution of ophiolitesbearing units and high-pressure continental tectonic slices in orogenic belts from initial burial to exhumation. On the other hand, the opaque mineralogy of these rocks is generally not studied in detail although the characterization of Fe-Ti oxides and sulphides in metabasites has a great potential [Clark, 1997; Dunlop and O¨ zdemir, 1997; Frost, 1991b] to better understand the processes controlling the formation and stability of magnetic minerals (mainly magnetite and pyrrhotite) in subduction zones and collisional orogens and to improve geological interpretation of magnetic survey data. [3] The Hercynian basement of northern Sardinia provides a case study to define the relationships between metamorphic evolution and magnetic properties of eclogite and amphibolite facies metabasites in a number of structurally and petrologically well-studied outcrops within a representative crustal section of the southern European Variscan belt. [4] To characterize the magnetic properties of these rocks, we conducted a series of minero-petrographical analyses and mineral magnetic measurements on a suite of samples representative of all the main mafic/ultramafic lenses of the region. In this study, we report on new data and interpretations which are essential (1) to characterize and to verify primary and secondary oxide contributions to the overall magnetization, (2) to link the stability/instability of magnetic assemblages to specific metamorphic stages, and (3) to provide a preliminary regional-scale perspective on the level of magnetization in all the main metamorphic mafic rock units of the Hercynian orogenic belt in northern Sardinia.