Magnetic Petrology of Variably Retrogressed Eclogites: a Case Study From the Hercynian Basement of Northern Sardinia (Italy)

The silicate parageneses of variably retrogressed eclogite facies metabasites are extensively used by metamorphic petrologists to reconstruct the tectono-metamorphic evolution which ophiolites-bearing units and high-pressure continental tectonic slices experienced 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 its characterization has a great potential to better understand the processes controlling the formation and stability of magnetic minerals (mainly magnetite and pyrrhotite) in collisional orogens and to improve geological interpretation of magnetic survey data. To define the relationships between metamorphic evolution and magnetic properties of eclogite and amphibolite facies metabasites we conducted a series of minero-petrographical analyses and mineral magnetic measurements in several outcrops of the Hercynian basement of Northern Sardinia. Magnetic susceptibility measurements with a hand-kappameter have been recorded for each outcrop together with structural features. The carriers of magnetization and their distribution within selected samples are described using a set of rock magnetic analyses (low-field magnetic susceptibility, natural and artificial remanences, thermomagnetic analyses, coercivity of remanence, hysteresis loops), and the petrological characterization of opaque minerals. Investigated samples include eclogites and amphibolites hosted in both kyanite-zone medium grade schists (MG) and high grade migmatitic gneisses (HG). Consistently with petromagnetic results and their variable magnetic susceptibilities (12.38-1047.7 10-8 m3/kg), HG metabasites contain variable contents 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), rutile both as inclusions in ilmenite and as discrete grains. In MG metabasites (28.71- 97.98 10-8m3/kg) pyrrhotite is restricted to amphibolites as rare inclusions in garnet whereas ilmenite, rutile and sphene 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 (680-720°C, >1.5 GPa); 2 - the growth of ilmenite and sphene can be attributed to the amphibolite facies retrogression (500-650°C, 0.3-0.6 GPa), mainly due to model reactions such as garnet + omphacite+ rutile+ H2O --> hornblende + plagioclase+ ilmenite and amphibole+ ilmenite+ O2 --> sphene+ magnetite + quartz + H2O. These preliminary interpretations are the first contribution of an ongoing broader magnetic petrology investigation in the Hercynian basement in Sardinia. Particularly, these results point to a renewed interest for a further research effort i) to verify primary and secondary oxide contributions to the overall magnetization, ii)to link the stability/instability of magnetic assemblages to specific metamorphic or late magmatic processes, and iii) to provide a regional scale perspective on the level of magnetization in the different rock units. These data, integrated with geological information, will also be used to discuss the bearing of the highly-magnetic metabasites on the geological interpretation of regional anomalies revealed by aeromagnetic surveys in the region.