Fabrizio Innocenti

Geochemical and petrological evidence of the subduction of delaminated Adriatic continental lithosphere in the genesis of the Neogene-Quaternary magmatism of central Italy

Abstract Serri, G., Innocenti, F. and Manetti, P., 1993. Geochemical and petrological evidence of the subduction of delaminated Adriatic continental lithosphere in the genesis of the Neogene-Quaternary magmatism of central Italy. In: M.J.R. Wortel, U. Hansen and R. Sabadini (Editors), Relationships between Mantle Processes and Geological Processes at or near The Earths Surface. Tectonophysics, 223: 117–147. The Neogene-Quaternary magmatism of the northern Apenninic arc took place in four phases separated in space and time which become progressively younger from west to east: Phase I, ~ 14 Ma; Phase II, 7.3-6.0 Ma; Phase III, 5.1-2.2 Ma; Phase IV, 1.3-0.1 Ma. This magmatism is the result of the activation of three physically separate sources: (1) the Adriatic continental crust, extracted from the mantle in the late Proterozoic; (2) a strongly refractory, recently K-enriched harzburgitic mantle located in the mechanical boundary layer (MBL) of the lithosphere; and (3) a recently metasomatized, cpx-rich mantle, compositionally variable from Iherzolite to wehrlite-clinopyroxenite, interpreted as an ephemerally K-enriched asthenosphere. The Adriatic continental crust is the dominant source of the acid plutonic and volcanic rocks of the Tuscan region. The acid magmatism is mostly found inside an ellipsoidal area (about 150 × 300 km) centred on Giglio Island, here defined as the Tuscan Crustal Dome. Within this area, mantle-derived magmas unaffected by important crustal contamination processes and mixing with crustal anatectic melts have so far not been found. Pure crustal magmas are rare but are represented, for example by some of the San Vincenzo and Roccastrada rhyolites. Virtually all the Tuscan acid centres show evidence of mixing with potassic mantle-derived magmas. Major and trace elements, as well as 87 Sr 86 Sr and 143 Nd 144 Nd data, on primitive rocks (Mg# > 65) reveal two groups of mantle-derived magmas. These define two distinct mantle enrichment trends, both essentially due to the additions of K-rich components which metasomatized separate, compositionally diverse, upper mantle sectors. In both cases the most remarkable mineralogical effect of these enrichment processes is the production of variable amount of phlogopite through reaction between fluids and/or melts with the mantle. The rocks of group I (ol-hy and Q-normative, lamproites, ultrapotassic high-Mg latites, ultrapotassic shoshonites and shoshonites: saturated trend) are considered to be derived by partial melting at low pressure ( 87 Sr 86 Sr (> 0.717) , Ce/Sr (> 0.3) and K 2 O Na 2 O (> 6–7) , and low ratios of 143 Nd 144 Nd (~ 0.5121-0.5120) and Ba/La ( 87 Sr 86 Sr (> 0.712) and K 2 O Na 2 O (> 8–9) values, and low 143 Nd 144 Nd ( , Ba/La ( 0.10) ratios. These constraints do not allow to exclude a complete identity between the K-rich components which metasomatized the mantle sources of the saturated and undersaturated trend magmas. The geochemical and isotopic features of the components that metasomatized the mantle sources of the northern Apenninic arc magmatism can be explained by a geodynamic process which causes a large amount of crustal materials to be incorporated within the upper mantle. We propose that the delamination and subduction of the Adriatic continental lithosphere related to the still ongoing northern Apennine continental collision provide a viable mechanism to explain the genesis and eastward discontinuous migration of the magmatism in central Italy. The subduction of delaminated lithospheric mantle with lower crustal slivers would have exposed uppermost mantle (Adriatic MBL) and crustal units previously imbricated in the Apennine chain to the heating advected by the upwelling of a recently and ephemerally K-enriched asthenospheric mantle wedge and by the underplating of magmas derived from it. We consider that the diapiric uprising of a hot, crustally contaminated asthenosphere occurs in the wake left above the sinking of the Adriatic delaminated/subducting continental lithosphere. The delamination/subduction process of the Adriatic lithosphere has probably started in the Early-Middle Miocene, but earlier than 15-14 Ma ago, as indicated by the age and petrologic characteristics of the first magmatic episode (Sisco lamproite) of the northern Apennine orogenesis.

Tertiary to Quaternary evolution of volcanism in the Aegean region

Summary Widespread orogenic volcanic activity has continued in the Aegean area from the Oligocene to present. Two main phases of activity are recognized. One developed in the North Aegean area from Oligocene to Middle Miocene times and a second started in the Pliocene, building the active South Aegean volcanic arc. Between these two phases, Upper Miocene to Quaternary volcanism of variable petrogenetic affinity occurred to a limited extent, essentially on the margins of the Aegean microplate. The products erupted during the Oligo-Miocene phase consist mainly of calc-alkaline and shoshonitic intermediate lavas and pyroclastics with minor acidic and basic rock types. The volcanic activity started in the northernmost part of the North Aegean area with mostly calc-alkaline intermediate and acidic volcanics. The volcanism shifted successively southwards becoming progressively enriched in potassium. This evolution is interpreted as being related to an increase in the dip of the Benioff zone under the Eurasian plate, resulting from a reduction in the plate convergence rate after continental collision. The volcanic products of the active south Aegean arc are mainly andesites with minor basalts and rhyolites which display the chemical character typical of calc-alkaline series erupted on thin continental margins. The South Aegean arc is believed to be the surface expression of active subduction of the African plate. Scattered Upper Miocene to Quaternary activity is interpreted as occurring in zones of tensional strain along the borders of the Aegean microplate.

Evolution of Eolian arc volcanism (Southern Tyrrhenian Sea)

Abstract The Eolian arc is located at the boundary between the converging African and European plates. Its volcanism is characterized by a marked evolution in a restricted time (less than 1 my). A progressive transition is observed from typical calc-alkaline series toward shoshonitic rocks produced by past and present activity (Vulcano and Stromboli). By comparison with circum-Pacific island arcs, the evolution of Eolian volcanism can be related to a rapid deepening of the Benioff zone. The occurrence of shoshonitic rocks and the continental nature of the crust on both sides of the plate boundary suggest that the Eolian arc is in a senile stage of evolution. Petrology and Sr isotopic data indicate a probable mantle source for Eolian volcanism.

Analogue modelling of continental extension: a review focused on the relations between the patterns of deformation and the presence of magma

Abstract Continental extension may occur in two main different modes, narrow and wide rifting, which mainly differ in the width of the deformed region. A third mechanism, the core complex, has been considered either a distinct mode of extension or a local anomaly within wide rifts. In terms of causative processes, continental rifting may be explained by both active or passive mechanisms, which also differ in the volume of magmatic products and in the rheological properties and stratification of the extending lithosphere. Both numerical and analogue models have investigated the main parameters controlling the extension of a rheologically layered lithosphere. In particular, analogue models have highlighted that the style of deformation is mainly controlled by the competition between the total resistance of the lithosphere and the gravitational forces; this competition, in turn, is mainly controlled by boundary conditions, such as the applied strain rate and the rheological characteristics of the extending lithosphere. Magmatic bodies eventually present within the continental lithosphere may significantly affect the process of extension. Both the thermal and mechanical effects related to the presence of magma strongly weaken the lithosphere and localise strain; this effect may have important implications for the mode of continental extension. At a crustal scale, magmatic intrusions may affect significantly the local fault pattern also favouring the development of core complex structures. Results of analogue models, performed taking into account the presence of an initially underplated magma and reproducing various continental extensional settings, suggest a close interaction between deformation and magma emplacement during extension. Particularly, magmatic underplating influences deformation localising strain in correspondence to the low-viscosity body, while on the other hand, rift kinematics and associated deformation has a major control on the pattern of magma emplacement. In particular: (1) During orthogonal rifting, magma is passively squeezed from an axial position towards the footwall of the major boundary faults; emplacement occurs in a lateral position in correspondence to lower crust domes. This process accounts for the close association between magmatism and the development of core complex structures, as well as for the occurrence of off-axis volcanoes in continental rifts. (2) During oblique rifting, deformation causes magma to emplace within the main rift depression, giving rise to intrusions with oblique and en echelon patterns. In nature, these patterns are found in continental rifts and also in some oceanic ridges. (3) Polyphase first orthogonal–second oblique rifting models suggest lateral squeezing and off-axis emplacement in the first phase and oblique en echelon intrusions in the successive oblique rifting phase. This evolution matches the magmatic and tectonic history of the Main Ethiopian Rift. (4) Development of transfer zones between offset rift segments has a great influence on both magma migration and deformation. Particularly, magma accumulates in correspondence to the transfer zone, with a main flow pattern that is perpendicular to the extension direction. This pattern may explain the concentration of magmatism at transfer zones in continental rifts. Overall, analysis of centrifuge models and their comparison with nature suggest that deformation and magma emplacement in the continental crust are intimately related, and their interactions constitute a key factor in deciphering the evolution of both continental and oceanic rifts.

The campanian ignimbrite: a major prehistoric eruption in the Neapolitan area (Italy)

A geological, chemical and petrographical study of the Campanian ignimbrite, a pyroclastic flow deposit erupted about 30,000 years ago on the Neapolitan area (Italy), is reported. The ignimbrite covered an area of at least 7,000 km2; it consists of a single flow unit, and the lateral variations in both pumice and lithic fragments indicate that the source was located in the Phlegraean Fields area.Textural features, areal distribution and its morphological constraints suggests that the eruption was of the type of highly expanded low-temperature pyroclastic cloud. The original composition was strongly modified by post-depositional chemical changes involving most of the major and trace elements. No primary differences in the composition of the magma have been recognized. The Campanian ignimbrite is a nearly saturated potassic trachyte, similar to many other trachytes of the Quaternary volcanic province of Campania. Its chemistry indicates an affinity with the so-called «low-K association» of the Roman volcanic province.

Phlegraean Fields 1982–1984: Brief chronicle of a volcano emergency in a densely populated area

The essential features of the ongoing potential pre-eruptive crisis at the Phlegraean Fields begun in August 1982 are summarized and the main problems faced by scientists responsible of volcanic hazards evaluation in such a densely populated area are discussed.

The Phlegraean Fields: magma evolution within a shallow chamber

Abstract A systematic petrological and chemical study of the volcanic products of the Phlegraean Fields has been accomplished based on the new stratigraphy described by Rosi et al. (this volume). The majority of Phlegraean rocks belong to the “potassic” series of the Roman province. The compositional spectrum ranges from trachybasalts to latites, trachytes, alkali trachytes, and peralkaline phonolitic trachytes. Trachybasalts are extremely rare and there is a sharp compositional gap between them and the latites. The series between latites and the trachytic varieties is complete. Trachytic rocks are much more voluminous than latites. The order of appearance of the main solid phases is: olivine, clinopyroxene, plagioclase, alkali feldspar, biotite, and oxides. Mineral compositions obtained by microprobe analyases are compatible with the evolution of the rock chemistry. However, primitive compositions of plagioclase (An 80 ) and clinopyroxene (diopside) persist in the cores of phenocrysts, even at the trachytic and alkali trachytic stage. Fractional crystallization within a shallow magma chamber has been the dominant process for the generation of Phlegraean rock series. The volume of the magma chamber is estimated to have been at least 240 km 3 at the moment of the eruption of the Campanian Ignimbrite, nearly 35,000 yr. ago. This event was followed by a large caldera collapse. The depth of the chamber cannot be precisely evaluated. However, its top must have been very shallow, probably at 4–5 km, as suggested by contact metamorphic rocks obtained from deep geothermal wells within the caldera. Volcanological and petrological data favor a model of upward migration of lighter liquids produced mostly by fractionation along the cool walls of the chamber, the deeper part of which is occupied by a convecting trachybasaltic magma. Progressive migration of eruptive vents toward the caldera center and the contemporaneous strong reduction in the volume of the erupted products, suggest that the chamber behaved as a closed system. The volume of magma was progressively reduced by both cooling and extraction to the surface.

Effects of magma storage and ascent on the kinetics of crystal growth

The size distributions of crystals of olivine, plagioclase and oxides of the 1991/93 eruption at Mt. Etna (Italy) are analyzed. The simultaneous collection of this information for different minerals gives precious insight into the cooling history of lavas. Three distinct episodes are detectable: a storage of the magma in a deep reservoir, characterized by nearly constant and low nucleation and growth rates (near to equilibrium); an ascent phase, with an ever increasing nucleation rate related to volatile exsolution; and finally a quenching phase. In addition to geochemical and geophysical evidence, the similarity of the crystal size distributions of the present eruption with those of previous ones of this century makes it possible to exclude that crystal size distributions of Etnean lavas are due to mixing of different populations. This strongly suggests that the main features of the volcano feeding system have not changed despite observed variations in the magma output rates.

Geochemical characteristics of potassic volcanics from Mts. Ernici (Southern Latium, Italy)

Major elements, trace elements and 87Sr/86Sr data are reported for the Quaternary potassic alkaline rocks from the Mts. Ernici volcanic area (Southern Latium — Italy). These rocks are represented by primitive types which display high Mgv, low D.I., variable degrees of silica undersaturation and different K2O contents which allowed the distinction of a potassium series (KS) and a high potassium series (HKS). All the analyzed samples have high LIL element contents and high 87Sr/86Sr which ranges between 0.707–0.711. They also have fractionated REE patterns. The KS rocks have lower LIL element concentrations and 87Sr/86Sr ratios than the HKS rocks with a large compositional gap between the two series. Minor but still significant isotopic and trace element variations are also observed within both KS and HKS. The genesis cannot be completly explained either by crystal liquid fractionation, mixing or assimilation processes or by different degrees of equilibrium partial melting from a homogeneous source, thus indicating that both the KS and HKS consist of several geochemically and isotopically distinct magma types. The data suggest that the KS and HKS magmas originated by low degrees of melting of a garnet peridotite mantle heterogeneously enriched in LIL elements and radiogenic strontium, possibly accompanied by disquilibrium melting of some accessory phases. The occurrence of a geochemical anomaly within the mantle is believed to be due to fluid metasomatism probably generated by dehydration of a lithospheric slab subducted during the Late Tertiary development of the Apennine Chain.

The magmatic evolution of the late Miocene laccolith–pluton–dyke granitic complex of Elba Island, Italy

Since late Miocene time, post-collisional extension of the internal parts of the Apennine orogenic belt has led to the opening of the Tyrrhenian basin. Extensive, mainly acidic peraluminous magmatism affected the Tuscan Archipelago and the Italian mainland during this time, building up the Tuscan Magmatic Province as the fold belt was progressively thinned, heated and intruded by mafic magmas. An intrusive complex was progressively built on western Elba Island by emplacement, within a stack of nappes, of multiple, shallow-level porphyritic laccoliths, a major pluton, and a final dyke swarm, all within the span from about 8 to 6.8 Ma. New geochemical and Sr–Nd isotopic investigations constrain the compositions of materials involved in the genesis of the magmas of Elba Island compared to the whole Tuscan Magmatic Province. Several distinct magma sources, in both the crust and mantle, have been identified as contributing to the Elba magmatism as it evolved from crust-, to hybrid-, to mantle-dominated. However, a restricted number of components, geochemically similar to mafic K-andesites of the Island of Capraia and crustal melts like the Cotoncello dyke at Elba, are sufficient to account for the generation by melt hybridization of the most voluminous magmas ( c. e Nd (t) −8.5, 87 Sr/ 86 Sr 0.715). Unusual magmas were emplaced at the beginning and end of the igneous activity, without contributing to the generation of these hybrid magmas. These are represented by early peraluminous melts of a different crustal origin (e Nd (t) between −9.5 and −10.0, 87 Sr/ 86 Sr variable between 0.7115 and 0.7146), and late mantle-derived magma strongly enriched in incompatible elements (e Nd (t) = −7.0, 87 Sr/ 86 Sr = 0.7114) with geochemical–isotopic characteristics intermediate between contemporaneous Capraia K-andesites and later lamproites from the Tuscan Magmatic Province. Magmas not involved in the generation of the main hybrid products are not volumetrically significant, but their occurrence emphasizes the highly variable nature of crust and mantle sources that can be activated in a short time span during post-collisional magmatism.