Internal Liguride Units Research Articles

LITHOSTRATIGRAPHY AND DEFORMATION HISTORY OF THE LAGO NERO UNIT (CHENAILLET MASSIF, WESTERN ALPS)

In the Western Alps, the Schistes Lustres includes ophiolitic units that, even if affected by HP metamorphism, are relatively poorly deformed. Among them, the Lago Nero unit displays a thick and well-developed sedimentary cover, that allows a comparison with the analogue successions of the Northern Apennines. This sedimentary cover includes hemipelagic deposits, represented by Radiolarite Fm., Metalimestone and Replatte Fm., showing a transition to turbiditic (Gondran Flysch) and mass-gravity deposits (Rocher Renard complex). This succession, characterized by a coarsening upward trend, records the approaching of an oceanic floor section to the accretionary wedge. In this picture, the Rocher Renard complex can be regarded as a formation related to a frontal erosion of the accretionary wedge, whereas the Gondran Flysch is interpreted as a turbidite deposits supplied by the continental margin of the Europe plate. The following deformation history includes four deformation phases. The first phase D1 deformations, characterized by HP metamorphism (lawsonite±Na-amphibole±phengite), were achieved during the coherent underplating in the alpine subduction zone. The D2, D3 and D4 phases was developed under retrograde metamorphic conditions, from grren-schist to very-low grade, during the progressive exhumation of the Lago Nero unit. All the collected data suggest that the succession of the Lago Nero unit can be correlated with those of the Internal Liguride units where the same sequence of hemipelagic to turbiditic and mass-gravity deposits has been recognized.

THE RECORD OF FRONTAL TECTONIC EROSION IN A FOSSIL ACCRETIONARY WEDGE: EVIDENCES FROM THE BOCCO SHALE (INTERNAL LIGURIDE OPHIOLITIC SEQUENCE, NORTHERN APENNINE, ITALY)

In the Northern Apennines, the Internal Liguride units are characterized by an ophiolite sequence that represents the stratigraphic base of a Late Jurassic-Early Paleocene sedimentary cover. The Bocco Shale represents the youngest deposit recognized in the sedimentary cover of the ophiolite sequence, sedimented just before the inception of subduction-related deformation history. The Bocco Shale was unconformably deposited over the underlying formations, from the Gottero Sandstone to the Palombini Shale. The deposits belonging to the Bocco Shale can be subdivided into two different groups of deep sea sediments. The first group is represented by slide, debris flow and high density turbidity current-derived deposits, whereas the second group consists thin bedded turbidites. Facies analysis and provenance studies indicate, for the former group, small and scarcely evoluted flows that rework an oceanic lithosphere and its sedimentary cover. We interpret the Bocco Shale as an ancient example of a deposit related to the frontal tectonic erosion of the accretionary wedge slope. The frontal tectonic erosion resulted in a large removal of material from the accretionary wedge front reworked as debris flows and slide deposits sedimented on the lower plate above the trench deposits. These deposits may have been successively deformed and metamorphosed during the following accretion processes. The frontal tectonic erosion was probably connected with subduction of oceanic crust characterized by positive topographic relief. The frontal tectonic erosion can be envisaged as a common process during the convergence-related evolution of the Ligure-Piemontese oceanic basin in the Late Cretaceous- Early Tertiary time span. This interpretation can be also suggested for the analogues deposits from oceanic units of Western Alps and Corsica.

Petrography and geochemistry of Western Tethys Mesozoic sedimentary covers (Alpine Corsica and Northern Apennines): a valuable tool in constraining sediment provenance and margin configuration

In the Western Tethys, the rifting processes related to the opening of the Ligure-Piemontese oceanic basin led to conjugate passive continental margins characterized by an asymmetry in their structures, lithologic composition and tectonic evolution.The asymmetry of margin configuration implicates differences between the two conjugate margins that represented the source areas of Mesozoic sedimentary cover of the Ligure-Piemontese oceanic and ocean-continent transitional domains. Up to the Late Cretaceous basin inversion, these sediments were mainly supplied by the two passive continental margins.In order to highlight the differences between the two source areas, we studied pre-late Cretaceous sedimentary units from the Balagne Nappe (Alpine Corsica) and Internal Liguride units (Northern Apennines) as representative of the Ligure-Piemontese oceanic basin located close to the Europe margin, and from the External Liguride units (Northern Apennines), that are interpreted as derived from the domain that joined the oceanic area to the Adria continental margin.This work integrates data from rudites, arenites and pelites of the studied units. Petrographic data on rudites and arenites indicates some differences in the source area composition. Particularly, only the External Liguride units contain rock fragments of medium to high grade metamorphic rocks and large chromiferous spinel fragments up to mm size. Major (XRF) and trace element (ICP-MS) data on pelite fraction show a first-order overall chemical homogeneity for all the units, with no contribution from a dissecting volcanic arc. However, significant compositional differences exist: the most striking is represented by the contribution of mafic/ultramafic rocks (e.g. high Mg#, Cr, Cr/V ratio) found only in sediments supplied from the Adria margin.Overall, petrographic and geochemical data provide matching evidence for a different composition of the two source areas, which were characterized by the occurrence of upper crustal rocks in the Europe margin and by a complete crustal and upper mantle section in the Adria margin. This configuration is probably inherited from the asymmetric opening mechanisms of the Western Tethys.

New standardized illite crystallinity data from low- to very-low grade metamorphic rocks (Northern Apennines, Italy)

The present study is aimed at testing the effectiveness of the Warr & Rice (1994) Crystallinity Index Standard (CIS) scale as a potential indicator of metamorphic grade. The test was carried out on rock samples belonging to well-defined metamorphic facies charaeterized by known metamorphic P-T conditions. The selected rocks come from various tectonic units of the Northern Apennines and had been the object of earlier studies on both their metamorphic conditions and illite crystallinity. All the IC data collected on the rocks of these units had been previously standardized with respect to the standard rocks slab series (32, 34 and 35) provided by Kubler. Some units belong to the Internal Liguride Domain (ILU), while others come from the Tuscan Domain (TDU). The Internal Liguride Units yielded average IC values standardized to the CIS scale of 0.29 (± 0.04) °Δ2𝛉 in the epidote blueschist facies (Cravasco/Voltaggio U.), 0.34 (± 0.06) °Δ2𝛉 in the pumpellyite-actinolite facies (Mt. Figogna U.), and 0.53 (± 0.07) °Δ2𝛉 in the prehnite-pumpellyite facies (Bracco/Val Graveglia U.). In the TDU average CIS scale-standardized IC values vary from 0.27 ± 0.03 to 0.30 ± 0.03 °Δ2𝛉 in rocks belonging to the metamorphic zone defined by Qz + Py association, from 0.33 ± 0.03 to 0.37 ± 0.03 °Δ2𝛉 in rocks of the Qz + Py + Ka zone, and from 0.40 ± 0.02 to 0.42 ± 0.03 °Δ2𝛉 in the rocks of the Qz + Ka zone. Although the standardized IC values may be considered consistent with the overall metamorphic conditions estimated in both the ILU and TDU, in general, they appear too high when compared with the temperatures estimated in the TDU by the calcite-dolomite geothermometer, which yielded maximum temperatures of 370–380°C in the Qz + Py zone, 300–312°C in the Qz + Py + Ka zone, and 260–280°C in the Qz + Ka zone. IC values previously normalized to Kubler9s standards (about 0.04 °Δ2𝛉 lower) also conform to the metamorphic facies conditions in all rocks selected from both the ILU and TDU. Although IC data and temperatures are only roughly related, and much caution must therefore be exercised in correlating these parameters, in general, Kubler9s standardized IC data from TDU also seems to exhibit good overall agreement with the thermal conditions.

Debris flow and slide deposits at the top of the Internal Liguride ophiolitic sequence, Northern Apennines, Italy: A record of frontal tectonic erosion in a fossil accretionary wedge

Abstract In the Northern Apennines, the Internal Liguride units are characterized by an ophiolite sequence that represents the stratigraphic base of a late Jurassic–early Paleocene sedimentary cover. The Bocco Shale represents the youngest deposit recognized in the sedimentary cover of the ophiolite and can be subdivided into two different groups of deep sea sediments. The first group is represented by slide, debris flow and high density turbidity current‐derived deposits, whereas the second group consists of thin‐bedded turbidites. Facies analysis and provenance studies indicate, for the former group, small and scarcely evoluted flows that rework an oceanic lithosphere and its sedimentary cover. We interpret the Bocco Shale as an ancient example of a deposit related to the frontal tectonic erosion of the accretionary wedge slope. The frontal tectonic erosion resulted in a large removal of materials, from the accretionary wedge front, that was reworked as debris flows and slide deposits sedimented on the lower plate above the trench deposits. The frontal tectonic erosion was probably connected with subduction of oceanic crust characterized by positive topographic relief. This interpretation can be also applied for the origin of analogous deposits of Western Alps and Corsica.

TRACE ELEMENT AND ND-SR ISOTOPE GEOCHEMISTRY OF THE GABBROIC ROCKS FROM THE NORTHERN APENNINE OPHIOLITES: GEOCHRONOLOGY AND RELATIONS WITH ASSOCIATED PERIDOTITES

The Northern Apennine ophiolites crop out in different stratigraphic settings. The petrological and geochemical features of the mantle-gabbro association are well known for the ophiolites of the Internal Liguride Units (Rampone et al., 1998). The mantle ultramafics are mainly clinopyroxene- poor plagioclase-bearing lherzolites, residual after the extraction in Permian times of MORB-type liquids, which were intruded in Middle Jurassic times by N-MORB-type gabbros. The mantle ultramafics of the ophiolites from the External Liguride Units dominantly consist of rather fertile plagioclase-bearing lherzolites with pyroxenite bands (Rampone et al., 1995), and they are considered as slices of subcontinental mantle, probably accreted to the lithosphere in the Proterozoic. Little is known about the rare gabbroic rocks from the External Liguride ophiolites, as well as about the mantle ultramafics from the ophiolites which crop out in Southern Tuscany. In this work, we have carried out traceelement and Nd-Sr isotopic investigations of i) the gabbroperidotite association from Southern Tuscany ophiolites (Lanciaia Unit), and ii) the gabbroic rocks from the External Liguride ophiolites. This has allowed us to depict a complete overview of the gabbro-peridotite association of the Northern Apennine ophiolites. The gabbroic rocks from the different Units contain clinopyroxenes with similar trace element compositions, that yield N-MORB equilibrium liquids. Moreover, clinopyroxenes show marked trace element zonings, similarly to what was observed for the gabbroic rocks from ODP Leg 153 in the MARK area (Ross and Elthon, 1997), most likely related to the entrapment of interstitial liquid (Tribuzio et al., 1999). The gabbroic rocks considered in this study give Sm/Nd mineral isochron ages ranging from 179 to 170 Ma, interpreted to date the primary magmatic crystallization. These ages are significantly older than those recently found (163 ± 2 to 166 ± 2 Ma) on the basis of U/Pb and Ar/Ar geochronological determinations on zircons and amphiboles for the gabbroic rocks of the Western Alps ophiolites (Bill et al., 1997; Rubatto et al., 1998). 87Sr/86Sr of clinopyroxenes and initial eNd (8.5 to 8.9) are consistent with a formation from N-MORB. The gabbroic rocks from Internal Liguride ophiolites and the basalts (Rampone et al., 1998) from both Internal and External Liguride ophiolites show closely similar initial eNd. The incompatible trace element compositions of basalts from the different Units resemble those calculated for the parental liquids of the gabbroic rocks. The studied peridotites from Southern Tuscany ophiolites are porphyroclastic spinel-lherzolites. These rocks have a residual geochemical fingerprint, as shown by the marked depletion in incompatible trace elements of clinopyroxenes, similarly to what was observed for the Internal Liguride lherzolites. The clinopyroxenes from the southern Tuscany lherzolites indeed have slightly lower Sm/Nd values and slightly higher Sr contents than those from the Internal Liguride lherzolites. Major element mineral compositions of Southern Tuscany lherzolites yield relatively low-temperature estimates for the spinel-facies equilibration (ca. 1000 °C), similarly to those found for the External Liguride lherzolites. On the other hand, the latter rocks cleary differ from the Southern Tuscany lherzolites in the occurrence of accessory Ti-rich amphibole and in the fact that clinopyroxenes have markedly higher concentrations of incompatible trace elements. Nd and Sr isotopic compositions of clinopyroxenes from the Southern Tuscany lherzolites calculated at 170 Ma are respectively higher (eNd ca. 11.0) and lower than those of associated gabbros. Therefore, there is not a genetic meltresidue relationship for this gabbro-peridotite association, as inferred by Rampone et al. (1998) for the Internal Liguride ophiolites. The clinopyroxenes from the External Liguride lherzolites are characterized by markedly heterogeneous Nd and Sr isotopic compositions, coupled with a relatively constant Sm/Nd value (Rampone et al., 1995). Similar geochemical relations have been also observed for the Lanzo lherzolitic massif in the Western Alps (Bodinier et al., 1991). In the External Liguride Units, the clinopyroxenes from the Monte Nero lherzolite body, which is locally intruded by gabbroic to basaltic dykes, display Nd and Sr isotopic compositions that are consistent with those of analyzed gabbroic rocks. As a whole, the ophiolites of the Northern Apennine/Western Alps system are characterized by a homogeneous composition of the gabbroic complex and by a marked heterogeneity of the associated mantle section. The compositional spread of the mantle lherzolites can be related to two major events: 1) partial melting at 300-270 Ma, in relation to the post-Variscan extensional regime; 2) partial re-equilibration with N-MORB liquids derived from deeper astenospheric sources at 180-160 Ma.

Compositional variation in K-white micas and chlorites coexisting in Al- saturated metapelites under late diagenetic to low-grade metamorphic conditions (Internal Liguride Units, Northern Apennines, Italy)

The chemical composition of coexisting illites and chlorites was investigated in Al-saturated metapelites of very-low- to low-metamorphic grade (temperature 160-350 degrees C; pressure: 2-7 kbar). The main chemical characteristics of illites consist of a deficiency in interlayer cations, an important celadonitic substitution and a slight deviation of the octahedral sheet from dioctahedral to trioctahedral character. Interlayer cation deficiency appears to be controlled by K XII +Al IV --> SiIV+[] XII substitution and by the entry of some (Mg+Fe) VI into the octahedral layer in excess of a full dioctahedral occupancy. The first substitution represents the main mechanism of interlayer cation deficiency control: it appears to be mostly dependent on metamorphic temperature. The small amounts of (Mg+Fe) VI producing deviation from a perfect dioctahedral occupancy only balance about 0.2 vacancies in the interlayer sites: this mechanism appears more dependent on bulk rock chemistry than on metamorphic conditions. Most of the Mg and Fe cations present in the octahedral layer result from celadonitic substitution, which is essentially related to the metamorphic pressure. Use of Sigma (Mg+Fe) as a measure of the celadonite content is shown to be much more reliable than Sigma Si, which is also influenced by interlayer occupancy. The main chemical characteristics of chlorites are: a) an ubiquitous octahedral cation deficiency, which appears to decrease systematically with increasing metamorphic grade; this trend is coupled with an increase of (Mg+Fe) VI and a decrease of Al VI ; b) a conspicuous substitution of Al IV for Si, which is seen to increase systematically with increasing metamorphic grade; c) a largely constant value of Fe/(Mg+Fe) ratio, which seems to be independent of the metamorphic grade. The octahedral cation deficiency is dependent on rock composition, as well as on temperature and pressure conditions. Thus it can not be proposed as a reliable geothermometer. On the contrary, the Si IV content can provide a reliable geothermometer at constant host-rock composition.

Isotopic contrasts within the Internal Liguride ophiolite (N. Italy): the lack of a genetic mantle–crust link

It is widely accepted that oceanic lithosphere is generated by partial melting of fertile mantle peridotites producing basaltic melts and residual peridotites. This theory implies a cogenetic relationship between peridotites and associated crustal rocks, but the actual existence of such a genetic link has been tested in very few places. In this paper, we use Sr and Nd isotopes to test the relationship between mafic and ultramafic mantle rocks from a MORB-type ophiolite in the Internal Liguride Units of the Northern Apennines. This ophiolite is a remnant of the oceanic lithosphere of the Jurassic Ligurian Tethys, and consists of depleted mantle peridotites intruded by a gabbroic complex and covered by pillow lavas and ophiolitic breccias. Whole rocks and mineral separates from the gabbroic rocks yield a Sm–Nd isochron with an age of 164±14 Ma. The whole rock data for pillow lavas are also consistent with this isochron, yielding an initial value of ε Nd (164)=8.6±0.3 (1σ). The mantle peridotites, by contrast, have ε Nd(164) values ranging from 11.9 to 14.8, indicating an extreme depletion unlike that seen in modern oceanic mafic and ultramafic rocks. These results demonstrate that some ophiolites consist of mantle and crustal sections that are not genetically linked by a simple melt–residue relationship, and consequently do not represent mature oceanic lithosphere. Similar evidence has previously been available only from the Xigaze and Trinity ophiolites. The Internal Liguride peridotites yield Sm–Nd model ages of about 270 Ma, assuming a normally depleted mantle source with ε Nd = 9, indicating a Permian time of `extra' depletion. The ophiolite thus consists of mantle peridotites which were depleted during Permian time, and were later intruded and covered by MORB-type magmas during Jurassic time. On a regional scale, this interpretation is consistent with widespread evidence that extensional processes leading to asthenospheric upwelling and magma production were active during the Permian.

Metamorphic grade in metapelites of the Internal Liguride Units (Northern Apennines, Italy)

Metamorphic grade in metapelites of the Internal Liguride Units (Northern Apennines, Italy)

The deformation history of an accreted ophiolite sequence: the Internal Liguride units (Northern apennines, Italy)

The Mt. Gottero, Bracco/Val Graveglia and Colli/Tavarone units from the Northern Apennines, Italy, are remnants of the Ligure-Piemontese oceanic lithosphere that were involved in the tectonic events related to convergence between the European and Adria plates. These events included late Paleocene – middle Eocene subduction followed by late Eocene continental collision.These units, referred to as Internal Liguride units, consist of Jurassic ophiolites covered by a thick sedimentary sequence, which includes late Jurassic-Santonian pelagic deposits (chert, Calpionella limestone and Palombini shale) and Campanian-Early Paleocene trench deposits (Val Lavagna shale and Mt. Gottero sandstone) that are overlain by Early Paleocene lower slope deposits (Bocco shale). This sequence is interpreted as recording trenchward motion of the oceanic lithosphere.The structural analysis of the Internal Liguride units reveals a pre-Oligocene deformation history, which includes the following stages:– stage 1: pre-folding, bedding-parallel veins developed during burial and dewatering of a thick sedimentary sequence;– stage 2: D1 phase distinguished by non cylindrical, isoclinal folds with approximately similar geometries. The associated slaty cleavage is characterized by a mineral assemblage related to P/T metamorphic climax of 0.3–0.4 GPa and 180°-300°C. The asymmetric recrystallized tails around detrital minerals and framboidal pyrites indicate a non-coaxial deformation history characterized by top-to-west sense of shear. Cataclastic shear zones developed in the latest stage of D1;– stage 3: D2 phase marked by overturned, parallel folds associated with a crenulation cleavage. The following D3 phase is characterized by gentle folds with a steep axial plane.The sedimentary records and the structural features support the interpretation of the Internal Liguride units as slices of oceanic lithosphere deformed in a subduction setting. Their deformation history probably developed during underthrusting, cohereht underplating and later exhumation in an accretionary wedge related to an east-dipping, low-rate subduction zone.