202 research outputs found
Reply to: Comment to: "Volcanic activity from the Neogene to the present evolution of the Western Mediterranean area. A review" by M. Lustrino
No full textWhat 'anorogenic' igneous rocks can tell us about the chemical composition of the upper mantle: case studies from the circum-Mediterranean area
No full textThe composition of the upper mantle bounded by the Canaries, Eastern Anatolia, Libya and Poland is indirectly investigated by means of the chemical composition of igneous rocks with 'anorogenic' geochemical characteristics emplaced during the Cenozoic. The relatively homogeneous composition of these products in terms of incompatible trace-element content and Sr-Nd-Pb isotopic composition is unexpected, considering the variable lithospheric structure of this large area and the different tectono-thermal histories of the various districts. In order to reconcile the geochemical characteristics with a statistical sampling model, it would be necessary to propose volumes of the enriched regions much lower than the sampling volumes for each volcano (that is, less than 10 cubic metres), or alternatively, efficient magma blending from larger areas. The data are consistent with a relatively well-stirred and mixed sub-lithospheric upper mantle, in the solid state, which is also hard to understand. This contrasts with the situation under oceans where magma blending from diverse sources and sampling theory can explain the compositional statistics
A tentative recostruction of the Phanerozoic geodynamic evolution of circum-Italian realm. The processes that changed the subcontinental European mantle
No full textPhanerozoic Geodynamic Evolution of the Circum-Italian Realm
No full textThe Phanerozoic geodynamic evolution of Europe is reviewed for the purpose of identifying its bearing on the petrogenesis of the Cenozoic European Volcanic Province. Several events capable of modifying the chemistry and mineralogy of the mantle, such as subduction of oceanic crust, continent-continent collision, and ocean formation are emphasized. The area now occupied by the Mediterranean Sea and, in general, all of Europe, underwent a complex geodynamic evolution, involving large relative crustal movements. The Paleozoic to Recent evolution of the circum-Mediterranean Sea area can be summarized as follows: (1) extension during the Precambrian (presence of ~3000 to 4000 km wide oceanic crust between Laurussia (consisting of the Laurentian and Baltica-Fennoscandian cratons) and Gondwana (South America, Africa, Australia, India, and Antarctica); (2) collisional movements with the formation of "Andean-type" margins during the Late Precambrian to Middle Paleozoic, followed by "Himalayan-type" margins during the Carboniferous (Hercynian orogeny sensu stricto); (3) change of plate movements and development of tensional (transtensive) stresses at the end of the Paleozoic, as indicated by the formation of the North Atlantic-Tethys rift system, with the Cretaceous formation of the Ligurian-Piedmontese and the Mesogean Ocean; (4) the Alpine orogeny, with a two-stage compressive cycle—(a) Eoalpine (Paleogene closure of the Ligurian-Piedmontese Ocean; formation of the Betic Cordillera, western-northern Alps, and Carpatho-Balkan Arc), with Europe-verging thrusts; and (b) Neoalpine (Neogene-Pleistocene formation of the Apennine, Maghrebide, Dinaride, and Hellenide chains, plus the backthrusted southern Alps, all with African vergence; opening of two diachronous backarc basins—the Ligurian-Provençal Basin and the Tyrrhenian Sea—in the western Mediterranean). Hercynian-age modifications (the most important of which are subduction-related) led to almost unique isotopic ratios, such as low 143Nd/144Nd, 206Pb/204Pb, 3He/4He, and slightly radiogenic 87Sr/86Sr ratios. During the Cenozoic and Quaternary, widespread magmatic activity developed throughout Europe. These products, mainly represented by mildly to strongly alkaline rocks with sodic affinity and tholeiitic mafic rocks (basanite, alkali basalts, tholeiitic basalts), show quite uniform geochemical and isotopic compositions typical of a within-plate tectonic setting. Moreover, subductionrelated magmatism (mainly represented by low-to high-K calc-alkaline and shoshonitic series + ultrapotassic rocks such as lamproites) developed in response to the subduction systems of the Alpine orogeny. With respect to the circum-Italian realm, the igneous rocks emplaced during the last 30 Ma are essentially related to the Alpine orogeny. This activity is represented by rocks of extremely variable composition (alkaline—both sodic and potassic to ultrapotassic—and subalkaline [tholeiitic and calc-alkaline]) and probably carbonatitic
Volcanic activity from the neogene to the present evolution of the Western Mediterranean area. a review
No full textThe Neogene to Present geodynamic and magmatological evolution of the western Mediterranean area may be summarized as follows: 1) Paleocene to Present volcanic activity in the Massif Central, France, related to the presence of a mantle plume in the Alpine foreland; 2) roughly continuous W-NW subduction of the Adria Plate from the Oligocene under the southern European margin; 3) development of a subduction-related back-arc basin during the Oligocene (Ligurian-Provençal Basin); 4) split of the Sardinia-Corsica Block from the Provençal basement; 5) collapse of the Betic orogen, with rapid exhumation of deep crustal and mantle rocks and development of volcanic activity in SE Spain (~30-2 Ma); 6) subduction-related magmatism in Sardinia (28-15 Ma) and eastern Spain (24-15 Ma); 7) a mid-Miocene 'leap' in the subduction system, from the Ligurian-Provençal Basin to the Tyrrhenian Sea, with a shift from Hercynian to Alpine terrane overthrusts; 8) opening of the Tyrrhenian Sea as a back-arc basin; 9) Neogene-Quaternary eastward-moving distensive and compressive tectonic waves and coeval magmatism along the western and southern margins of the Italian peninsula (Tuscan, Roman and Campanian Provinces and Aeolian Islands); 10) volcanic activity in the Betic foreland (Calatrava Province, ~9-1 Ma); 11) Plio-Pleistocene development of rift systems and coeval magmatism in Sardinia, northern and eastern Sicily (Mt. Etna and Hyblean Mts.) and the Strait of Sicily. Intense volcanic activity accompanied the evolution of the last 30 Ma in the western and central Mediterranean, with a wide range of magmatic products which may be grouped into: a) oceanic floor basalts (Ligurian-Provençal Basin and Tyrrhenian bathyal plain with Magnaghi, Vavilov and Marsili seamounts), whose compositions vary from N-MORB to E-MORB and pure low-K calc-alkaline basalts; b) subalkaline series with both tholeiitic and calc-alkaline affinities; c) alkaline products with extreme compositions ranging from mildly alkaline types with sodic affinity to strongly SiO2-undersaturated with both sodic and potassic to ultrapotassic affinity, possibly including also carbonatitic lithotypes. The Sr-Nd-Pb isotopic compositions of these products comprise virtually all the most common worldwide reservoirs and testify to the extremely heterogeneous compositions of the mantle sources of this sector of European lithosphere/ asthenosphere system
Debated topics of modern igneous petrology
No full textProgress made thanks to the great amount of high-quality geochemical and isotopic data gathered over the last twenty years has allowed important insights into igneous petrology (e.g., Continental Flood Basalt petrogenesis, mantle source characterization, geophysical models of mantle plume systems, primary melt compositions, isotopic systematics of crustal and mantle domains, etc.). This large dataset has also been used to relate the compositional characteristics of igneous rocks with specific tectonic settings and to infer the geodynamic processes involved. However, inferring a tectonic setting mainly on the basis of geochemical constraints may fail, because partial melts with substantial compositional differences can originate from the same source, and the same melts may have been generated in different tectonic settings . Moreover, geochemical characterization of the main mantle components is still hotly debated, even in terms of concepts such as asthenosphere and lithosphere. Asthenospheric mantle is quite often believed to be a geochemically homogeneous convecting domain, whereas the lithospheric one is thought to be a variably enriched, heterogeneous, non-convecting reservoir, capable of retaining geochemical and isotopic gradients for periods of time exceeding 2 Ga. These assumptions are clearly over-simplifications, particularly when relationships between physical and geochemical mantle characteristics are not properly constrained. What emerges from recent literature is the «misuse» of petrological concepts tending towards the most convenient explanations and «effectively shutting out the entire creative thought process of the human mind» (Sheth, 1 999). More appropriate use of petrological data is necessary to stimulate true scientific growth, especially as regards our knowledge of mantle-crust dynamics
Volcanic activity in the western Mediterranean during the last 30 Ma
No full textA strong volcanic activity accompany the last 30 Ma evolution of the western and central Mediterranean sea with wide range of compositions which can be grouped in: a) oceanic floor products (Ligurian-Provencal basin and Tyrrhenian sea bathial plain with Magnaghi, Vavilov and Marsili seamounts), whose compositions vary from N-MORB to E-MORB and pure low-K calcalkaline basalts; b) subalkaline series of both tholeiitic (OIB-like) and calcalkaline affinity; c) alkaline products with the most extreme compositional variability ranging from mildly alkaline types with sodic affinity to strongly SiO2-undersaturated with both sodic and potassic to ultrapotassic affinity, exotic compositions such as lamproites and kamafugites, and also quartz-saturated peralkaline trachytes and rhyolites. The Sr-Nd-Pb isotopic compositions of these products comprise virtually all the most common worldwide reservoirs and testify the extreme heterogeneous compositions of the mantle sources of this sector of European lithosphere/asthenosphere system
Comment on "high-pressure melting experiments on garnet clinopyroxenite and the alkalic to tholeiitic transition in ocean-island basalts" by Keshav et al. [Earth Planet. Sci. Lett. 223 (2004) 365-379]
No full text[No abstract available
How the delamination of lower crust can influence basaltic magmatism
No full textThe Earth’s lithosphere can focus basaltic magmatism along pre-existing weakness zones or discontinuities. However, apart from the influence on the geochemistry of magmas emplaced in subduction tectonic settings (mantle wedge metasomatism related to dehydration of the subducting plates) the role of lithosphere as a magma source for intra-plate (both oceanic and continental), continental margin, and mid-ocean ridge magmatism is not yet fully understood. In many cases intra-plate magmatism has been explained with the existence of deep thermal anomalies (mantle plumes) whose origin has been placed near the upper–lower mantle transition zone (660 km discontinuity) or even deeper, near the mantle–core boundary (~2900 km). Also in many continental flood basalt provinces (mostly initiated at craton margins) an active role for mantle plumes has been invoked to explain the high melt productivity. In these cases, no active role for melt production has been attributed to the lithospheric mantle. Potential contaminations of asthenospheric or even deeper mantle melts are often considered the only influence of the lithosphere (both crust and mantle) in basalt petrogenesis. In other cases, an active role of the lithospheric mantle has been proposed: the thermal anomalies related to the presence of mantle plumes would trigger partial melting in the lithospheric mantle. At present there is no unequivocal geochemical tracer that reflects the relative role of lithosphere and upper/lower mantle as magma sources. In this paper another role of the lithosphere is proposed. The new model presented here is based on the role of lower crustal and lithospheric mantle recycling by delamination and detachment. This process can explain at least some geochemical peculiarities of basaltic rocks found in large and small volume igneous provinces, as well as in mid-ocean ridge basalts. Metamorphic reactions occurring in the lower continental crust as a consequence of continent–continent can lead to a density increase (up to 3.8 g/cm3) with the appearance of garnet in the metamorphic assemblage (basaltYamphiboliteYgarnet clinopyroxenite/eclogite) leading to gravitative instability of the overthickened lithospheric keel (lower crust+lithospheric mantle). This may detach from the uppermost lithosphere and sink into the upper mantle. Accordingly, metasomatic reactions between SiO2-rich lower crust partial melts and the uprising asthenospheric mantle (replacing the volume formerly occupied by the sunken lithospheric mantle and the lower crust) lead to formation of orthopyroxene-rich layers with strong crustal signatures. Such metasomatized mantle volumes may remain untapped also for several Ma before being reactivated by geological processes. Partial melts of such sources would bear strong lower crustal signatures giving rise to Enriched Mantle type 1 (EMI)-like basaltic magmatism. Basaltic magmatism with such a geochemical signature is relatively scarce but in some cases (e.g., Indian Ocean) it can be a geographically widespread and long-lasting phenomenon
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