A. MONTANINI, L. VERNIA & S. MELI
Mafic granulites from Northern Apennines occur as blocks in sedimentary melanges of Cretaceous age, together with huge olistoliths of ophiolitic rocks and rare amphibolites and gneisses. They are the unique occurrence of deep continental basement rocks in the Northern Apennines. The associated ultramafic rocks (i.e. the External Ligurides peridotites) have been recently interpreted as subcontinental mantle emplaced at shallow depth during the incipient ocean formation (Piccardo et al., 1992).
Deformed rocks with mylonitic, blastomylonitic and "mortar" textures are frequently observed. The undeformed samples range from xenoblastic to granoblastic poligonal. Granulitic assemblages consist of plagioclase (labradorite), clinopyroxene and variable amounts of orthopyroxene, red-brown amphibole (Ti-rich pargasite), garnet, green spinel, magnetite, ilmenite, biotite, apatite, zircon, sphene. Some olivine-bearing samples show orthopyroxene-spinel symplectites at the contact between olivine and plagioclase. Rare garnet occurs in olivine-free samples as rims of spinel or as reaction product between clinopyroxene and spinel.
Geothermometric estimates (780-850°C) based on the clinopyroxene-orthopyroxene equilibria are in agreement with granulite-facies conditions. A rough pressure estimate (7-10 kbar) can be obtained considering the olivine-out and the garnet-in curves for natural and synthetic basaltic systems.
Major and trace element composition of the mafic granulites reflects cumulus processes involving plagioclase and pyroxenes. The igneous protoliths could have been derived from the underplating of subalkaline (tholeiitic) magmas at the base of the crust. Subsequent cooling and partial hydration have yielded the observed corona reactions and the red-brown pargasite.
However, most samples have undergone more or less extensive retrogression strictly related to mylonitization. Green, blue-green and pale-green amphiboles with compositions ranging from Ti-poor horneblende to actinolite partially replace clinopyroxene and pargasite. Clinopyroxene may be also replaced by epidote + chlorite+ sphene (together with green hornblende). Orthopyroxene is often converted to chlorite (± talc), whereas plagioclase may be sericitized.
The extent of this retrogression is extremely variable: some samples show only negligible amounts of acicular green amphibole locally rimming clinopyroxene, whereas other rocks are almost completely converted into a low-temperature assemblage consisting of sericite + actinolite + chlorite ± epidote + Fe-Ti oxides. The retrometamorphic processes affecting the granulitic assemblages have not been isochemical, causing mobilization of several major and trace elements (Na, K, Ca, Rb, Sr).
It is proposed that these mafic granulites could belong to the deep crust uplifted and sheared during the rifting which led to the opening of the Ligurian Tethys, according to the mechanism of simple shear proposed by Lemoine et al. (1987).