Spinel-peridotites of the Frido Unit Ophiolites (Southern Apennine-Italy): Evidence for oceanic evolution

摘要

The ophiolitic rocks of the Frido Unit include serpentinites derived from a lherzolitic and subordinately harzburgitic mantle, as suggested by microstructural and petrographical features. The serpentinites are englobed in tectonic slices where they are associated with metadolerite dykes and medium to high-grade metamorphic rocks such as amphibolite, gneiss, granofels as well as gabbro and basalt with pillow structures. The matrix of tectonic slices is mainly composed of calcschists and phyllites. The serpentinites of the Frido Unit show pseudomorphic and vein textures. Primary mantle minerals are represented by olivine, clinopyroxene, orthopyroxene and orthopyroxene exolutions, and spinel and are identifiable by of the occurrence of either fresh minerals or pseudomorphs maintaining the mineral shape. Pseudomorphic minerals are serpentine, magnetite, chlorite, and amphibole. Olivine is replaced by serpentine forming a mesh texture; orthopyroxene is mostly altered to bastite; in some cases it shows exsolution lamellae of clinopyroxene and kink bands; fresh orthopyroxene is preserved as exsolution inside clinopyroxene porphyroclasts. Clinopyroxene is armoured by amphibole rim. Spinel shows a holly-leaf habit and is often armoured by a corona of Crchlorite. The core of the analyzed spinel has a Cr-Al spinel composition corresponding to chromite (Al _2O _3=29-31 wt%; Cr _2O _3=28-37 wt%), whereas the rim has a Fe-Cr spinel composition corresponding to ferritchromite (Al _2O _3=1-2% wt; Cr _2O _3=28-30 wt%). The Cr-Al spinel/ferritchromite ratio may be various in different spinel porphyroclasts. Magnetite replaces spinel or occurs within the mesh textured serpentine. The metamorphic assemblages in the Frido Unit serpentinites allow to infer qualitatively the physical conditions operating during serpentinization. The metamorphic mineral assemblages are typical of the greenschistamphibolite transition and greenschist facies conditions, as suggested by the occurrence of tremolite/actinolite replacing clinopyroxene and of Cr-chlorite and ferritchromite after Alrich chromite. These minerals were produced by interactions between hydrothermal fluids and mantle peridotites. Serpentinites are cut by veins filled with mineralogical assemblages typical of the prehnite-pumpellyite facies, likely related to later orogenic Apennine evolution, as observed in the surrounding rocks of the same tectonic unit. Instead, early orogenic LT blueschist facies minerals recorded in the nearby metadolerite dykes were not observed in the serpentinites.