A. MONTANINI, G. MOLLI & S. MELI
The presence of continental crust rocks associated with ophiolites in the External Liguride Units has been reported for a long time. In this contribution, recently collected petrological, radiometric and structural data on these rocks will be presented and their tectonic meaning discussed.
The continental crust rocks can be observed within Late Cretaceous ophiolite-bearing sedimentary melanges, considered as formed during eo-Alpine transpressive events in which the Eastern part of the Ligurian domain has been deformed. The most abundant rock types in these complexes are represented by peridotites, basalts and their original middle to Lower Cretaceous sedimentary cover. However, rocks from the upper and the lower continental crust are in some places significantly present. They are mainly represented by continental granites locally observable as pluridecametric up to kilometric exposures, in which different rock types can be recognized compositionally ranging from granites to tonalites through granodiorites. Particularly significant are the presence of basaltic dykes intruded within the granites and their primary relationships with basaltic flows testifying the outpouring of lava onto the continental crust rocks. The granites show a widespread brittle deformation part of which can be related, on the basis of mesoscopic and microstructural features, to pre-orogenic events. In one case, the continental granite slice is overlain by a sedimentary sequence (cfr. Pagani et al., 1972) consisting of "granitic" debris in transition to radiolarian cherts. Here it is possible to verify a brittle poliphasic deformation consisting in a (originally ?) low angle cataclastic fault zone cross cut by later high angle faults predating the radiolarite sedimentation.
Among the lower crustal rocks both mafic and felsic granulites are represented. Mafic rocks are dominant and include: (1) two-pyroxene granulites, (2) olivine coronitic metagabbros, (3) amphibole-rich metagabbros, (4) garnet granulites. Amphibolites formed through retrogression of granulitic rocks are quite common. The felsic granulites consist of feldspars + quartz + garnet + rutile ± pyroxenes ± biotite. The igneous protoliths of the mafic granulites are interpreted as originated in the deep crust through cumulus of plagioclase and highly aluminous pyroxenes (with minor olivine and amphibole) from slightly LREE-enriched and variably evolved basaltic magmas. Subsolidus equilibration occurred at relatively high P (7-8 kbar) and T (780-850°C). Most granulites show a retrograde metamorphism from upper-amphibolite to greenschist facies closely related to deformations progressively changing from plastic to brittle. In many rocks two distinct, superimposed, deformative events can be recognized:
(1) High-T plastic deformations (± recrystallisation) originated both in granulite- and upper amphibolite facies in discrete high-T shear zones. In the amphibolites, shearing is related to the development of greenish-brown hastingsite + ilmenite after clinopyroxene + Ti-rich pargasite + green spinel. The retrograde hastingsitic amphiboles record temperature between 600 and 700°C.
(2) Lower-T brittle deformations, related to development of retrograde assemblage starting from the amphibolite-greenschist transition. The lowest grade conditions (T < 300°C) recorded in the mafic granulites are given by the occurrence of pumpellyte in some cataclastic rocks. A minimum depth of 4-5 km for the cataclastic deformations in the granulites (as well as in the granites) is constrained by (i) absence of prehnite (Frey et al., 1991), (ii) formation of sericite instead of caolinite onto the feldspar (Evans, 1988) and (iii) occurrence of ultracataclasites.
The composition of the retrograde amphiboles indicate that shearing and cataclasis probably occurred at intermediate crustal levels. The variable extent of deformations and retrograde effects and the chemical changes related to hydration (increase of K, Rb, Ba and K/Rb ratio; loss of Ca, Na), are typical of retrograde shear zones (e.g. Beach, 1980; Sandiford, 1984). In synthesis, many lines of evidence indicate that the complex history of retrogression and deformation of the granulites is related to involvement into high- and low-T shear zones during their progressive uplift starting from depth corresponding to P = 7-8 kbar.
Sm/Nd and 40Ar/39Ar dating techniques applied to an undeformed metagabbro apparently devoid of retrograde effects help to place some constraints on the cooling history and the timing of deformation and uplift of the granulites. The obtained Sm/Nd pl-cpx-whole rock isochron gives an age of 291 ± 9 Ma (MSWD = 0.21), whereas the Ar/Ar age recorded by the Ti-rich pargasitic amphibole is considerably younger (plateau age = 228 ± 2 Ma). The Late Carboniferous age can be interpreted as the time of emplacement of the igneous protolith (perhaps slightly underestimated due to re-equilibration in granulite-facies conditions which implies slow cooling rate). Because the estimated closure temperature for argon diffusion in the amphibole range between 500 and 550°C, the amphibole age of 228 Ma record cooling and uplift probably related to extensional thinning of the crust. In addition, this age put an upper limit for the timing of high-T shearing of granulites and amphibolites (T > 600°C) and a lower limit for the cataclastic deformations (T < 450°C) observed either in granulites and granites. Similar, Permian to Triassic post-peak cooling ages have been determined in granulite- and amphibolite-facies rocks from the Ivrea zone (e.g. Brodie et al., 1989; Boriani & Villa, 1995).
Accordingly to the recent petrological and tectonic interpretations of the Northern Apennine ophiolites (Piccardo et al., 1992; Hoogerduijng Strating et al., 1993; Molli, et al., 1995) we can coherently insert our data in a rifting history with final tectonic denudation of the upper mantle occurring in pericontinental position. The remarkable association of granulites with subcontinental mantle (External Liguride ultramafics) characterized by a similar retrograde history is consistent with this hypothesis. During this history low-angle detachment faults are involved in the bulk deformation of the continental crust as well as the upper mantle. Stretched slices of continental crust were coupled with the ultramafic body as "extensional allochtons" and later intruded by basaltic dykes and covered by basaltic flows and radiolarian cherts. The presence of lower continental crust rocks in the eastern part of the Ligurian Domain suggests the role of the upper plate for the European and lower plate for the southern continental margin during the very late asymmetric stage of passive Tethyan continental rifting.