A HERCYNIAN SUTURE ZONE IN SARDINIA - GEOLOGICAL AND GEOCHEMICAL EVIDENCE

The Hercynian basement of Northern Sardinia consists of two main tectonometamorphic complexes: the Southern Paleozoic Low to Medium Grade Metamorphic Complex (LMGMC) and the Northern (Precambrian ?) High Grade Metamorphic Complex (HGMC). These complexes are separated by the "Posada-Asinara Line" which is a several kilometer-wide mylonitic belt consisting of micaschist, paragneiss and quartzite that cuts across all Northern Sardinia and contains scattered bodies of amphibolites with granulite and eclogite facies relics. Major element composition of amphibolites indicates they were derived from metamorphosed basalts of tholeiitic affinity. REE and the relative abundances of HFS elements strongly suggest an oceanic basaltic protolith (N- and T-type MORB). A whole-rock Sm-Nd isochron for the amphibolite protoliths yields an age of ca. 960 Ma. The Northern Sardinia basement can be interpreted as a continental collisional zone with the "Posada-Asinara Line" representing a suture zone squeezed between a crustal nappe (HGMC), which represents the overthrust continental margin, and the inner portion of the Variscan orogenic wedge, which represents the underthrust continental margin with its Paleozoic cover (LMGMC). Geological and geochemical evidence indicate that the "Posada-Asinara Line" may be considered as part of the South European Hercynian Suture Zone.