Gianfranco Principi

Interaction between Mid‐Ocean Ridge and Subduction Magmatism in Albanian Ophiolites

Albanian ophiolites are represented by two different coeval belts, each displaying well‐exposed, complete ophiolitic sequences that originated in the same oceanic basin and each showing distinct geochemical characteristics. The eastern belt is characterized by suprasubduction zone (SSZ) ophiolitic sequences, including island arc tholeiitic and boninitic volcanic series. The western belt, although composed mainly of mid‐ocean ridge‐type (MOR‐type) ophiolites with high‐Ti geochemical affinity, also exhibits alternating sequences showing distinct geochemical affinities referable to MOR‐ and SSZ‐type volcanics. These volcanics can be geochemically subdivided into four groups: (1) group 1 basalts show high field strength element (HFSE) and rare earth element (REE) concentrations similar to those of ocean‐floor basalts; (2) group 2 basalts, basaltic andesites, dacites, and rhyolites, characterized by HFSE and light REE depletion similar to those in many low‐Ti volcanics from SSZ settings; (3) group 3 basalts exhibit geochemical features intermediate between groups 1 and 2 but also bear SSZ features, being characterized by HFSE depletion with respect to the N‐MORBs; (4) group 4 boninitic dikes display very low‐Ti contents and typically depleted, \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Ophiolites, Ligurides and the tectonic evolution from spreading to convergence of a Mesozoic Western Tethys segment

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Late Cretaceous flysch deposits of the Northern Apennines, Italy: age of inception of orogenesis-controlled sedimentation

\end{document} ‐shaped REE patterns. These different magmatic groups are interpreted as having originated from fractional crystallization from different primary basalts that were generated, in turn, from partial melting of mantle sources progressively depleted by previous melt extractions. Consequently, group 1 basalts may derive from partial melting of a fertile MORB source, while group 3 basalts may derive from 10% partial melting of a mantle that previously experienced MORB extraction. Finally, the group 2 basalts and group 4 boninites may be derived from about 10% partial melting of a mantle peridotite previously depleted by primary melt extraction of group 1 and group 3 primary melts. To explain the coexistence of these geochemically different magma groups, we present a model based on the complexity of the magmatic processes that may take place during the initiation of subduction in proximity to an active MOR. This model implies that the initiation of subduction processes close to such a ridge leads to contemporaneous eruptions in a fore‐arc setting of MORBs (group 1) generated from the extinguishing MOR and the initiation of group 3 basalts generated in the SSZ mantle wedge from a moderately depleted mantle source. The development of the subduction in a young, hot lithosphere caused the generation of island arc tholeiitic basalts (group 2) and boninites (group 4) from strongly depleted mantle peridotites in the early stages of subduction, soon after the generation of group 1 and group 3 basaltic rocks.

Late Triassic, Early and Middle Jurassic Radiolaria from ferromanganese-chert ‘nodules’ (Angelokastron, Argolis, Greece): evidence for prolonged radiolarite sedimentation in the Maliac-Vardar Ocean

All the geological constraints for an exhaustive reconstruction of the Triassic to Tertiary tectonic history of the southern Dinaric-Hellenic belt can be found in Albania and Greece. This article aims to schematically reconstruct this long tectonic evolution primarily based on a detailed analysis of the tectonic setting, the stratigraphy, the geochemistry, and the age of the ophiolites. In contrast to what was previously reported in the literature, we propose a new subdivision on a regional scale of the ophiolite complexes cropping out in Albania and Greece. This new subdivision includes six types of ophiolite occurrences, each corresponding to different tectonic units derived from a single obducted sheet. These units are represented by: (1) sub-ophiolite mélange, (2) Triassic ocean-floor ophiolites, (3) metamorphic soles, (4) Jurassic fore-arc ophiolites, (5) Jurassic intra-oceanic-arc ophiolites, and (6) Jurassic back-arc basin ophiolites. The overall features of these ophiolites are coherent with the existence of a single, though composite, oceanic basin located east of the Adria/Pelagonian continental margin. This oceanic basin was originated during the Middle Triassic and was subsequently (Early Jurassic) affected by an east-dipping intra-oceanic subduction. This subduction was responsible for the birth of intra-oceanic-arc and back-arc oceanic basins separated by a continental volcanic arc during the Early to Middle Jurassic. From the uppermost Middle Jurassic to the Early Cretaceous, an obduction developed, during which the ophiolites were thrust westwards firstly onto the neighboring oceanic lithosphere and then onto the Adria margin.

THE GEOLOGY OF THE ZLATIBOR-MALJEN AREA (WESTERN SERBIA): A GEOTRAVERSE ACROSS THE OPHIOLITES OF THE DINARIC-HELLENIC COLLISIONAL BELT

The Ligurian formations, a stack of sediments deposited over an oceanic (containing ophiolites) basement in the Ligurian—Piedmont oceanic basin, provide unique records for reconstructing the opening, evolution and closure of the portion of Western Tethys that separated the European-Iberia plates to the NW from the Africa-Adria plates to the SE (Abbate et al., 1970, 1980, 1986; Principi and Treves, 1984). They are present all along the Alps and the Apennines down to Calabria, except for central Italy. This chapter deals with the Northern Apennines, where these successions are well exposed and studied.

Deformation pattern in the underthrust carbonate-rich sequence of the Sibillini Thrust (central Italy): insights for shear zone evolution in modern subduction complexes

The South Apuseni Mountains are located in the inner zone of the Carpathian belt. This area is characterized by a complex assemblage of nappes, in which Jurassic igneous associations are well represented. New geological and geochemical data on these igneous associations document the occurrence of Middle Jurassic ophiolites overlain by Late Jurassic calc-alkaline volcanic rocks. The ophiolite sequence is characterized by: (1) an intrusive section mainly represented by small gabbroic bodies showing both layered and isotropic textures, as well as scarce ultramafic cumulates, melagabbros, gabbronorites, and ferrogabbros; (2) a basaltic sheeted dike complex; (3) a volcanic sequence including massive and pillow-lava basalts and rare pillow breccias; and (4) very rare Callovian-Oxfordian radiolarian cherts. Although chemically variable, gabbros show a clear high-Ti magmatic affinity. Basaltic rocks display N-MORB-normalized incompatible-element patterns consistent with the compositions of present-day mid-ocean ridge basalts. Their high-Ti magmatic affinity is indicated by the chondrite-normalized REE patterns, which are rather flat or slightly enriched in light REE. All these features suggest that ophiolites of the South Apuseni Mountains were generated in a mid-ocean ridge setting. Evidence of ductile deformation and synkinematic metamorphism are lacking, suggesting that ophiolites were dismembered in multiple slices at shallow structural levels during the orogenic phases. Calc-alkaline rocks are represented by massive lava flows including basalts, basaltic andesites, andesites, dacites, and rhyolites, as well as by some granitoid complexes intruded into the ophiolitic sequence. Volcanics show highly porphyritic textures, with clinopyroxene, orthopyroxene, hornblende, and plagioclase phenocrysts, and incompatible-element compositions characterized by Ta-Nb, P, and Ti depletion, as well as by Rb-Ba-Th and La-Ce enrichment. These rocks also show marked light REE enrichments, commonly interpreted to be a consequence of mantle source enrichment by subduction-derived components. Ophiolites represent the remnants of an oceanic basin, whereas calc-alkaline rocks represent a magmatic island-arc setting developed over the previously formed oceanic lithosphere. The geological and petrological data suggest that the South Apuseni Mountains ophiolites can be correlated with the mid-ocean ridge-type ophiolites of the Vardar zone. They are interpreted to have assumed their present location in the inner zone of the Carpathian belt during the tectonic escape of Adria-related microplates during the Late Paleogene-Early Neogene.

The Ligurian Units of Western Tuscany (Northern Apennines): insight on the influence of pre-existing weakness zones during ocean closure

Abstract Calcareous nannofossils have proved to be very effective in determining the age of Cretaceous flysch sequences of the Northern Apennines. Here, we focus on the beginning of flysch sedimentation, which replaced previous pelagic deposition during the Late Cretaceous convergence stage. In all the examined sequences an early to late Campanian age has been determined for the bases of the flysch formations, implying an essentially synchronous start of orogenesis-controlled sedimentation within the Ligurian Apenninic (Ligurian) Ocean Basin. Data obtained from the siliciclastic turbidite sequence (Vat Lavagna/Mt. Gottero Sandstones) overlying Jurassic ophiolites and sedimentary cover, indicate continuous sedimentation from earliest Campanian to early Paleocene. Thus, either a forearc or ‘dormant trench’ tectonic setting seems to be required for this sequence. Regarding the calcareous Helminthoid Flysch, a minor diachrony in the basal ages suggest a general younging from south to north. The onset ages of flysch sedimentation range from early Campanian (Southern Tuscany Flysch), to early-middle Campanian (Mt. Caio/Ottone Flysch), to latest Campanian (S. Remo, Mt. Antola, Mt. Cassio, Mt. Caio/Orocco Flysch). This diachrony could be the consequence of Late Cretaceous-early Tertiary transcurrent tectonics in the Apenninic Basin and/or of multiple source areas (from both the European/Iberian and Adriatic margins). These tectonic implications are in agreement with the kinematic evidence of a transpressional regime along the Iberian/Adriatic plate boundary (Apenninic sector) during the Late Cretaceous.

The Jurassic–Early Cretaceous basalt–chert association in the ophiolites of the Ankara Mélange, east of Ankara, Turkey: age and geochemistry

In the Argolis, the Basal Sequence, constituting the eastern Pelagonian margin which bordered the Maliac-Vardar oceanic domain, includes shallow-water carbonates of Late Triassic-Early Jurassic, condensed pelagic limestones of Early-Middle Jurassic, radiolarian cherts of late Middle-Late Jurassic age and siliceous mudstones and sandstones rich in ophiolite fragments. Up-section, coarse breccias, including clasts of boninites derived from the ophiolite obducted onto the Pelagonian margin in Late Jurassic times crop out. Near Angelokastron a small quarry exposes pervasively sheared dark reddish-brown, radiolarian-bearing cherty shales with disrupted fragments of chert and chert nodules impregnated by ferro-manganese oxides. These shales occur in the footwall of a thrust bringing them into contact with the Pantokrator Limestone of the Basal Sequence. We collected more than 30 samples of the chert fragments and the shaly matrix. Thirteen nodules and one matrix sample yielded determinable radiolarians. Low to non-detectable concentrations of trace metals such as Co, Cr, Cu, Ni, Zn, and Pb indicate a hydrothermal origin of the ferro-manganese mineralization. The radiolarian taxa found indicate four age groups for the nodules that are embedded in the siliceous shale matrix that yielded a Middle Jurassic age (middle Bathonian). The first group includes a nodule of Late Triassic age (late Norian to Rhaetian); the second group nodules of Early Jurassic age (late early to late Pliensbachian and probably middle-late Toarcian); the third group nodules of early Middle Jurassic age (Aalenian–Bajocian); the last group finally includes nodules of late Middle Jurassic age (Bajocian–Bathonian). The presence of Upper Triassic to Middle Jurassic Mn-impregnated chert nodules in a Middle Jurassic matrix indicates a deep oceanic environment of deposition outside the Pelagonian realm (easternmost Adria Plate), which at that time was a shallow-water carbonate platform with a thin pelagic limestone cover. The chert nodules are with all certainty derived from the oceanic Maliac-Vardar domain and were, together with their host formation, tectonically emplaced onto the Pelagonian margin. We speculate that these nodules, more lithified than their matrix, were exhumed on the slope of an intra-oceanic accretionary wedge and were redeposited in the Middle Jurassic siliceous mudstones on the floor of the subducting Maliac-Vardar Ocean.