Geochemistry of metabasites from NE Sardinia, Italy: nature of the protoliths, magmatic trend, and geotectonic setting

Summary The metabasites of Montiggiu Nieddu occur as a lenticular body in the Hercynian migmatite complex of NE Sardinia and consist of two major lithological associations: ultramafic amphibolites and plagioclase-banded amphibolites, which are genetically related by processes of cumulate differentiation of an original tholeiitic magma. The ultramafic amphibolites consist of relics of igneous phases (anorthite, olivine, orthopyroxene, and clinopyroxene) and metamorphic minerals (orthopyroxene, clinopyroxene, plagioclase, garnet, chlorite, and amphibole) in variable proportions. Based on the distribution of the relics of igneous minerals three main compositional layers (A, B, C) have been identified in the ultramafic amphibolites. The plagioclase-banded amphibolites consist of dark-green and white bands made up of plagioclase, amphibole, garnet, and rare pyroxene in variable proportions. No relics of magmatic minerals were observed. The trace element patterns of the Montiggiu Nieddu metabasites are characterised by selective enrichment of incompatible elements of low ionic potential, such as Sr, Rb, Ba, and Th, and low abundance of K, Cr, Ni, and elements of high ionic potential (from Ta to Yb), which define a relatively flat trend similar to MORB composition. Ti, Y, and heavy rare earth elements are all positively correlated with Zr. This correlation suggests that no significant transport of these elements took place during metamorphism. The metabasites of Montiggiu Nieddu are characterised by low Ti, Ti/V, and Ti/Zr content, low rare earth and high-field strength element concentrations, and moderate selective enrichment in large-ion lithophile elements. They also show geochemical features similar to those of supra subduction zone tholeiites. The CaO-MgO diagram displays a segmented trend, which provides powerful evidence in favour of crystal-liquid separation during magmatic evolution. Received December 4, 2000; revised version accepted January 29, 2001