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  • 1
    ISSN: 1432-0819
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract The present paper reports the results of a detailed stratigraphical, petrological and geochemical investigation on the island of Stromboli, Aeolian arc, Southern Tyrrhenian sea. Major and trace element data determined on a large quantity of samples from well-established stratigraphic positions indicate that the magmatological evolution of the island through time was more complex than previously known. The activity of the exposed part of Stromboli, which occurred over a time span of about 100 000 years, started with the emission of high-K calc-alkaline (HKCA) volcanics, which were covered by calc-alkaline (CA), shoshonitic (SHO), high-K calc-alkaline (HKCA) and potassic (KS) products. The most recent activity consists of HKCA lavas and the present-day SHO-basaltic volcanics emitted by mildly explosive “strombolian” activity. Most of the products are lavas, with minor amounts of pyroclastic rocks emplaced mainly during the early stages of activity. The transition from the SHO to the KS cycle was associated with the collapse of the upper part of the volcanic apparatus; the transition from KS to the present-day SHO activity has been found to have occurred at the time of the sliding of the western portion of the volcano that generated the “Sciara del Fuoco” depression. The rock series cropping out at Stromboli show variable enrichment in potassium, incompatible trace elements and radiogenic Sr which increase from CA through HKCA, and SHO up to KS rocks. Major, trace element and Sr-isotopic data agree in indicating that the HKCA and SHO series evolved by crystal/liquid fractionation starting from different parental liquids, whereas crustal assimilation appears to have been the leading process during the evolution of KS volcanics. Mixing processes also played a role although they can be well documented only when they occurred between magmas with different isotopic and geochemical characteristics. Geochemical modelling based on trace element and isotopic data indicates that the mafic magmas of the different volcanic series may be generated by melting of an upper mantle heterogeneously enriched in incompatible elements and radiogenic Sr by addition, via subduction, of different amounts of crustal material. Geochemical data, however, are also in agreement with the alternative hypothesis that the most mafic magmas of the different series have been generated by combined processes of fractional crystallization, assimilation and mixing of a CA magma in a deep-sited magma chamber; the mafic magmas formed by these complex processes were successively emplaced in a shallow reservoir where they evolved by simple fractional crystallization (HKCA and SHO series) and by assimilation of crustal material (KS). The occurrence of changes in the geochemical signatures of the magmas at the time of the structural modification of the volcano is believed to favour the hypothesis that the variable composition observed in the volcanic rocks of Stromboli is the result of processes occurring within the volcanic system.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1365-3121
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Boron isotope data are presented for Cenozoic Western Anatolia rocks, which define two main associations: (i) calc-alkaline, shoshonitic and ultra-potassic rocks (Early to Middle Miocene); and (ii) Late Miocene–Quaternary intraplate alkali basalts. Boron data, together with Sr–Nd isotope and other trace elements, are consistent with a progressive dehydration of the slab, producing fluid phases gradually depleted in B (and 11B). These fluids were added to the supraslab mantle, triggering a partial melting that gave rise to orogenic magmatism. The stretching and tearing of the slab caused by the faster convergence of Greece over Africa with respect to Anatolia facilitated an interaction of the upwelling subslab asthenosphere with residual slab-fluids during the Late Miocene followed by production of typical intraplate magmas during the Pleistocene–Holocene, whose relatively high δ11B (approximately −2‰) is considered representative of the local asthenosphere not affected by subduction contamination.
    Type of Medium: Electronic Resource
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  • 3
    Publication Date: 2007-10-08
    Description: Analogue models of polyphase deformation involving crustal differences in strength, thickness and density give insights into lateral and vertical strain propagation during Late Cretaceous shortening and Early Tertiary left-lateral shearing related to the early development of the North America-Caribbean plate boundary in southern Mexico. Analogue models reproduce a two-phase deformation characterized by a first stage of compression orthogonal to the plate boundary, simulating deformation induced by the Laramide orogeny, followed by a later stage of left-lateral transpression associated with the transfer of the Chortis block from the North American to the Caribbean plate during the early stage of development of the new plate boundary in Early Tertiary times. Based on detailed structural observations in the Guerrero-Morelos platform and the western part of the Mixteco terrane of southern Mexico, we document that a transpressive regime affected continental red bed sequences of Early Paleocene to Late Eocene, and rotated and refolded Laramide structures during this second phase. Our model ends before the transtensional regime that affected the region, which is marked by a volcanic episode of Late Eocene-Oligocene. This change in the deformation regime records the passage of the NW tip of the Chortis block (North America-Cocos-Caribbean triple junction), when subduction replaced transform faulting along the southern Mexico margin. The models focus on the structures formed around the flanks of a thicker/more rigid crustal block that simulates the rock assemblages of the Palaeozoic orogens of southern Mexico (Mixteco-Oaxaca-Juarez block, MOJB). The comparison of the mechanism of deformation of three different analogue models with the natural prototype explains most of the structures observed around the MOJB. Counterclockwise vertical-axis rotations of pre-existing structures in the western flank of the MOJB observed in the Guerrero-Morelos platform are consistent with the modelled structures. Vertical movements of the modelled MOJB induced by the transpressive regime can explain the Papalutla thrust and the basement upheaval and gravitational sliding of the cover in the Tentzo Ranges observed at the western and northern margins of the MOJB, respectively. The growth and propagation of thrusting controlled by the geometry of the block along the eastern margin also correlates with the Vista Hermosa fault. The propagation of strain to the north increases with higher contrast in strength of the thick block with respect to the adjacent modelled crust. Analogue modelling failed to reproduce all the structural details of southern Mexico and, specifically, the structures observed inside the MOJB. The latter, however, are controlled by pre-existing discontinuities, which are not simulated in the model. As a whole, the results demonstrate that crustal heterogeneity in a developing left-lateral plate boundary zone produces a stronger vertical coupling between ductile and brittle crust and a widening of the deformation zone along the margin of the North America plate in southern Mexico.
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  • 4
    Publication Date: 2009-12-28
    Description: We present analogue models that illustrate the tectonic evolution of the continental margin of southwestern Mexico and the Early Cenozoic deformation of the Xolapa complex. Together with geological data they suggest that oblique convergence caused distributed deformation and mountain building near the present-day margin of southern Mexico in a general left-lateral transpressional regime. A similar deformation is also observed north of the Xolapa complex in Maastrichtian to Paleocene sedimentary and volcanic rock units. Since post-Oligocene exhumation of middle crust does not significantly affect Late Eocene to Oligocene volcanic rocks, we infer that the evolution of the transform margin led to the formation of discrete boundaries that eventually decoupled exhumed mid-lower crust from the onshore upper-crust sequences since the Late Eocene.
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  • 5
    ISSN: 1436-5073
    Keywords: external PIXE ; EMP ; volcanic rocks ; partition coefficients
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The major and trace element compositions of several minerals and their surrounding groundmass in the volcanic rocks from the post-caldera Nea-Kameni island of the Santorini volcanic complex, Aegean Sea, have been determined, in-situ with particle-induced X-ray emission (PIXE) and wavelength dispersive X-ray analysis with an electron microprobe (EMP). The lavas are typically calc-alkaline dacites. All samples are porphyritic with a phenocryst mineralogy dominated by plagioclase, augite, hyperstene and Ti-magnetite. The phenocrysts range in size from 200 μm to 2.5 mm. The PIXE and EMP analyses were done on sections polished with diamond paste. They were sufficiently thick (≈ 100 μm) to stop the 2.7 MeV protons used for the analysis and yet thin enough for individual minerals to be seen with transmitted light. The specific minerals and groundmass areas to be analyzed had been selected and marked through conventional microscopic examination prior to analysis. Solid/liquid partition coefficients, which depend much less significantly on the method for determining the concentrations, were calculated from the element abundances in phenocrysts and corresponding groundmass.
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  • 6
    Publication Date: 1989-07-01
    Description: The present paper reports the results of a detailed stratigraphical, petrological and geochemical investigation on the island of Stromboli, Aeolian arc, Southern Tyrrhenian sea. Major and trace element data determined on a large quantity of samples from well-established stratigraphic positions indicate that the magmatological evolution of the island through time was more complex than previously known. The activity of the exposed part of Stromboli, which occurred over a time span of about 100 000 years, started with the emission of high-K calc-alkaline (HKCA) volcanics, which were covered by calc-alkaline (CA), shoshonitic (SHO), high-K calc-alkaline (HKCA) and potassic (KS) products. The most recent activity consists of HKCA lavas and the present-day SHO-basaltic volcanics emitted by mildly explosive “strombolian” activity. Most of the products are lavas, with minor amounts of pyroclastic rocks emplaced mainly during the early stages of activity. The transition from the SHO to the KS cycle was associated with the collapse of the upper part of the volcanic apparatus; the transition from KS to the present-day SHO activity has been found to have occurred at the time of the sliding of the western portion of the volcano that generated the “Sciara del Fuoco” depression. The rock series cropping out at Stromboli show variable enrichment in potassium, incompatible trace elements and radiogenic Sr which increase from CA through HKCA, and SHO up to KS rocks. Major, trace element and Sr-isotopic data agree in indicating that the HKCA and SHO series evolved by crystal/liquid fractionation starting from different parental liquids, whereas crustal assimilation appears to have been the leading process during the evolution of KS volcanics. Mixing processes also played a role although they can be well documented only when they occurred between magmas with different isotopic and geochemical characteristics. Geochemical modelling based on trace element and isotopic data indicates that the mafic magmas of the different volcanic series may be generated by melting of an upper mantle heterogeneously enriched in incompatible elements and radiogenic Sr by addition, via subduction, of different amounts of crustal material. Geochemical data, however, are also in agreement with the alternative hypothesis that the most mafic magmas of the different series have been generated by combined processes of fractional crystallization, assimilation and mixing of a CA magma in a deep-sited magma chamber; the mafic magmas formed by these complex processes were successively emplaced in a shallow reservoir where they evolved by simple fractional crystallization (HKCA and SHO series) and by assimilation of crustal material (KS). The occurrence of changes in the geochemical signatures of the magmas at the time of the structural modification of the volcano is believed to favour the hypothesis that the variable composition observed in the volcanic rocks of Stromboli is the result of processes occurring within the volcanic system. ©1989 Springer-Verlag
    Print ISSN: 0258-8900
    Electronic ISSN: 1432-0819
    Topics: Geosciences
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  • 7
    Publication Date: 2008-09-01
    Description: The potassic (K) to ultrapotassic (UK) volcanic rocks cropping out in the Vardar Zone of Macedonia and southern Serbia span in age from Late Miocene (6.57 Ма) to Pleistocene (1.47 Ма). The main identified outcrops are in the Kumanovo, Sveti Nikole, Shtip and Demir Kapia areas; the southernmost occurrences of these volcanic rocks are located in the large Kozuf Massif (Voras Massif in Greece) at the Macedonia–Greek border. Three distinct groups may be distinguished. The first group has a shoshonitic affinity and occurs in the Kozuf Massif (LMg-K group); it includes shoshonites to rare rhyolites, with latites and trachytes being the most widespread products. The second group consists of potassic rocks (HMg-K group, K2O/Na2O between 1.0 and 1.8) occurring in both southern Serbia (Cer and Slavujevci) and Macedonia (Djuristhe, near Sveti Nikole). The third group, present only in Macedonia, consists of ultrapotassic rocks (UK group, K2O/Na2O 〉1.8, Mg# 〉71) classified as UK shoshonites, UK latites and UK phonotephrites; overall, they show a “Roman Province type” affinity (Group III of Foley, Venturelli, Green, Toscani, Earth Sci Rev 24:81–134, 1987). Geochemically, the studied rocks exhibit strong enrichment in LILE, Th and Pb, as well as relative depletion in Ta–Nb and Hf; such signatures are typical of magmas generated in convergent geotectonic settings. In the HMg-K and UK rocks, Sr and Nd isotopic ratios vary from 0.70768 to 0.71040, and 0.51243 to 0.512149, respectively. The rocks of the LMg-K group show relatively limited Sr and Nd isotope variations (0.7087–0.7093 and 0.51233–0.51229), which correlate with a decrease in MgO and increase in SiO2 contents. The geochemical features of the LMg-K volcanic rocks indicate that their evolution was mainly driven by fractional crystallization coupled with contamination by feldspar-rich crustal materials. In contrast, the HMg-K and UK rocks have not been significantly modified by crustal contamination, and their geochemical features are considered to reflect lithospheric mantle heterogeneity acquired during the subduction of the Western Vardar Ocean and the Apulian plate. The metasomatizing agent was apparently more enriched in Zr, Th, Ta and Ce than in fluid-mobile elements, such as Pb and Cs, suggesting that it was characterized by a high melt/fluid ratio. The potassic and ultrapotassic magmatic activity developed in response to the Pliocene–Pleistocene extension in the Vardar Zone, in turn related to the opposite propagation of extension in the Aegean and Pannonian basins (respectively SW and NE). ©2008 Springer-Verlag
    Print ISSN: 0930-0708
    Electronic ISSN: 1438-1168
    Topics: Geosciences
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  • 8
    Publication Date: 2020-02-01
    Description: The Elazig and Tunceli provinces in eastern Anatolia host a complex succession of Miocene-Pleistocene effusive and explosive volcanic rocks, divided into four distinct volcanic phases. The most abundant and widespread products are the calcalkaline Mazgirt volcanic rocks, characterized by wide Sr isotope variations (87Sr/86Sr ~0.7054-0.7077) and narrower 143Nd/Nd (~0.51246-0.51260) and Pb isotopes (e.g., 206Pb/204Pb ~18.89-19.13). New 40Ar-39Ar ages indicate that Mazgirt volcanic activity occurred between ~16.3 and 15.1 Ma. The other three volcanic phases are represented by the Tunceli mildly alkaline basaltic lavas (~11.4-11.0 Ma), and the Pliocene Karakoçan (~4.1 Ma) and Pleistocene Elazığ (~1.9-1.6 Ma) Na-alkali basaltic lavas with clear OIB-like geochemical signature. Mazgirt volcanics can be subdivided on the base of mode of emplacement into lava flows and lava domes units characterized by petrographic, chemical and isotopic and differences: lava flows are calcalkaline, whereas lava domes mostly belong to a high-K calcalkaline series and are, on average, more LREE- and 87Sr-enriched. Lava domes are more porphyritic, with a phenocryst assemblage dominated by amphibole, whereas plagioclase and clinopyroxene are the most abundant phenocryst phases in lava flows, pointing out that evolution of dome magmas occurred in conditions of slightly higher pressure, favouring the crystallization of hydrous phases. The Karabakır Formation, previously reported as late Miocene-Pliocene, encloses Mazgirt volcanics and is capped by Tunceli basalts. These new age data constrain the Karabakır Formation emplacement from early to late Miocene. The evolution of this igneous activity mirrors the geodynamic framework of the region: the early-middle Miocene Mazgirt volcanics represent arc volcanism related to Eurasia-Arabia convergence. The late Miocene Tunceli basalts postdate the onset of post-collisional tectonics in Eastern Anatolia, whereas the Karakoçan and Elazığ volcanic rocks were emplaced after the initiation of strike-slip motion on the North Anatolian and East Anatolian Fault systems.
    Print ISSN: 2038-1719
    Electronic ISSN: 2038-1727
    Topics: Geosciences
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  • 9
  • 10
    Publication Date: 1992-12-15
    Print ISSN: 0935-1221
    Electronic ISSN: 1617-4011
    Topics: Geosciences
    Published by Schweizerbart
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