Metabasites with granulite facies relics occur as a lenticular body (2 km long and 100-150 m thick) embedded in the migmatites at Montiggiu Nieddu (Figure 3) (Ghezzo et al., 1979; Franceschelli et al., 2002). Two lithotypes have been distinguished: ultramafic amphibolites and plagioclase banded amphibolites. Ultramafic amphibolites appear as a dark-green to black, massive to slightly schistose body divided into three compositional layers: A: chlorite-rich amphibolite; B: plagioclase-rich greenish amphibolite; C: dark green garnet-rich amphibolite. These layers include 20-30 cm thick intercalations consisting almost totally (up to 90-95%) of coarse-grained amphibole. Garnet-rich nodules and veins are also frequently observed.
Plagioclase banded amphibolites consist of alternating dark green (amphibole-rich) and white (plagioclase-rich) decimetre- to decametre-thick layers. The dark green layers are made up of amphibole, plagioclase, garnet, relics of orthopyroxene and clinopyroxene, chlorite, biotite, epidote and sphene. The white layers consist of plagioclase, amphibole, garnet and rare clinopyroxene. In ultramafic amphibolites, the igneous minerals are plagioclase, olivine, clinopyroxene and orthopyroxene.
The post-igneous metamorphic evolution of Montiggiu Nieddu metabasites may be divided into three stages: a first stage (corona stage), with the appareance of clinopyroxene, orthopyroxene, spinel and garnet (Figure 11a,b); a second stage, mainly characterised by the widespread appearance of clinoamphibole, chlorite, spinel and orthoamphibole; third stage minerals, mostly replacing mafic minerals, consist of tremolite, chlorite, fayalite, talc, epidote, albite, calcite, dolomite, serpentine and probably corundum.
Figure 11. Montiggiu Nieddu
Photomicrographs showing steps in the metamorphic evolution of granulite rocks from Montiggiu Nieddu (NE Sardinia). A: corona microstructure developed around igneous olivine and plagioclase; on the lower side, igneous clinopyroxene is replaced by brown amphibole. One polar. B: igneous plagioclase (anorthite) replaced by garnet. Note the spinel growth on plagioclase (Spl1) and at the boundary between garnet and amphibole (Spl2). One polar.