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Petrogenesis of the Cenozoic magmatism in north-western Iran
Get PDFA petrological study has been conducted on three volcanic districts (Tafresh, Nowbaran and Bijar-Qorveh) of NW Iran with the purpose to give a contribution to the scientific community about the tectono-magmatic framework of the Arabia-Eurasia collision zone. These volcanoes belong to the so-called Urumieh-Dokthar Magmatic Arc (UDMA) running from NW to SE along the western margin of Iran, whose volcanism is related to the NE-ward Neotethys Ocean subduction beneath the Iranian plate since Early Cretaceous time, evolved into Arabia-Iran continental collision during early Cenozoic.
Tafresh investigated rocks mainly range from basaltic andesites to rhyolites and are probably linked to closed-system magma evolutionary processes. Such processes involved fractionation of i) mainly ferromagnesian minerals and plagioclase, followed by ii) removal of plagioclase and lesser amphibole (with minor clinopyroxene) and finally iii) lesser alkali feldspar and minor amphibole in the most evolved terms. LILE-enriched and HFSE-depleted geochemical signature (likely originated from a hydrous primitive melt equilibrated with a spinel-bearing peridotite source) inferred the typical subduction-related trend emplaced in a subduction-related setting. Strongly evolved rocks are supposed to be derived from crustal anatexis of a mixed meta-sedimentary source. One sample shows distinctive adakitic signature (high La/Yb and Sr/Y ratios, low Yb and HFSE contents) which are interpreted as the product of the melting of a meta-mafic source rock (i.e., subducted oceanic slabs) with residual garnet and amphibole.
Differently, Nowbaran Quaternary melanephelinites result by far the most peculiar igneous rocks of the entire Bitlis-Zagros Orogen. The absence of feldspars and melilite is coupled with extremely low SiO2 content and very high CaO and Al2O3 abundances, leading to ultracalcic compositions. Moreover, these rocks show high Mg# and very high Ni and Cr values, which likely suggest a primitive character of these melts. Isotopic ratios and primitive-mantle normalized pattern indicate hybrid sources, as their trend exhibits subduction-related imprinting mixed with HiMU-OIB features. Such uncommon compositions are thought to be unlikely derived from a classic four-phase (i.e. C-H-free) peridotitic mantle or from digestion of carbonatic compositions. More likely, they are generated from carbonated apatite-hornblendite-rich metasomes which are considered as the products of interaction between peridotitic matrix and partial melts derived from arc cumulates (formed by crystallization of hydrous and CO2-bearing magmas generated during previous subduction-related arc).
On the other hand, two main volcanic cycles have been recognized in Bijar-Qorveh area. Upper Miocene (~9.2-8.3 Ma) compositions mostly range from trachy-andesites to trachytes (with minor rhyolitic terms) whereas Pleistocene (~1.3-0.5 Ma) samples are mostly represented by trachybasalts and tephrites, with lesser alkali basalts. Major and trace elements of Miocene rocks (i.e., Dehgolan and Qorveh) likely suggest fractional crystallization of ferromagnesian minerals then followed by removal of plagioclase and amphibole. These rocks exhibit high-K calcalkaline affinity, as also shown by primitive mantle-normalized patterns characterized by strong LILE and LREE enrichments, typical of subduction-related magmas. Only three Dehgolan samples show by far higher K2O-TiO2-P2O5 and Rb contents, which are thought to be related to a Ti-phlogopite-rich source rock. Quaternary samples (i.e., Qezelke Kand, Bijar and Takab) consist of alkaline series showing mainly sodic to strictly potassic (i.e. Takab) affinity. They show high Mg#, Ni and Cr content which probably is indicative that primary magma was not affected by extensive fractionation. Moreover, isotopic ratios and incompatible multielemental patterns show mixed features of both intraplate-like and subduction-related end members. Takab rocks always cluster distinctly, showing higher K2O and Rb abundances which could be linked to a phlogopite-rich mantle source. Noteworthy, Qezelke Kand outcrops are characterized by the association of basic products with four high silica rocks which have been referred to as adakites, likely deriving from the melting of subducted basalts in eclogite facies. Bijar-Qorveh Miocene high-K volcanic products represent the final magmatic phase of the calcalkaline volcanism occurring in Central Iran during Tertiary period, whereas Quaternary basic magmatism is to be considered as the expression of regional tensional tectonics or related to variations in the source mineralogy during melting processes
On the geodynamics of the Aegean rift
No full textThe Aegean rift is considered to be either a classic backarc basin, or the result of the westward escape of Anatolia, or the effect of a gravitational collapse of an over-thickened lithosphere. Here these models are questioned. We alternatively present a number of geodynamic and magmatic constraints suggesting a simple model for the genesis of the extension as being related to the differential advancement of the upper lithosphere over a heterogeneous lower African plate. The Greek microplate overrides the Ionian oceanic segment of the African plate faster than the Anatolian microplate over the thicker Levantine more continental segment. This setting is evidenced by GPS-velocity gradient in the hangingwall of the Hellenic-Cyprus subduction system and requires a zone of rifting splitting the hangingwall into two microplates. This mechanism is unrelated to the replacement of retreated slab by the asthenosphere as typically occurs in the backarc of west-directed subduction zones. The supposed greater dehydration of the Ionian segment of the slab is providing a larger amount of fluids into the low velocity channel at the top of the asthenosphere, allowing a faster decoupling between the Greek microplate and the underlying mantle with respect to the Anatolian microplate. Slab ruptures associated with the differential retreat controlled by the inherited lithospheric heterogeneities in the lower plate and the proposed upwelling of the mantle suggested by global circulation models would explain the occurrence and coexistence of slab-related and slab-unrelated magmatism. (C) 2009 Elsevier B.V. All rights reserved
Neogene ultrabasic volcanic rocks in central Urumieh-Dohktar Magmatic Arc (NW Iran): melilitites and nephelinites in subduction setting
No full textAbout 180 km SW Tehran (central-northern Iran) young (likely <1 Myr old) and small volume lava flows are emitted
from volcanic chains close to Nowbaran town. These volcanoes belong to the so-called Urumieh-Dokthar Magmatic
Arc (UDMA), running from NW to SE along the western margin of Iran. The volcanism of the UDMA is related to the
NE-ward Neotethys Ocean subduction beneath the Iranian plate since Early Cretaceous time, evolved into Arabia-Iran
continental collision during early Cenozoic. The lava flows consist of strongly ultrabasic magmas, represented by
melilitites and nephelinites. Clinopyroxene is the most abundant microphenocryst, followed by olivine, calcite and
biotite. Diopside is always twinned or shows normal zoning, and olivine is Mg-rich, with Fo content ~86. Primary
calcite largely occurs as large plagues (often including opaque minerals and diopside); Mg-rich (Mg# ~0.87) and Tirich
(TiO2 up to ~7.3 wt%) phlogopite-eastonite is also frequent. Cancrinite-sodalite group minerals are the main
groundmass foids, together with nepheline and minor Ti-magnetite.
The investigated rocks show anomalous mineral paragenesis and whole-rock chemical compositions compared to the
other Cenozoic volcanic rocks occurring in Iran. SiO2 content is extremely low (down to 36.0 wt%), with CaO reaching
contents as high as ~19.2 wt%, coupled with generally high MgO (~9.1-13.9 wt%). Alkalis range between ~2.2 and 6.2
wt%, with Na2O/K2O varying from ~0.9 to 8.5. Primitive mantle-normalized patterns show marked troughs at K, Rb
and Pb and enrichment in Nb and Ta, resembling typical HIMU-OIB compositions.
The presence of these compositions imply a carbonatitic component in the mantle source, which is uncommon in
subduction-related settings, and certainly unique within the entire Cenozoic volcanic rocks of UDMA. Experimental
petrology studies in the last decades have demonstrated that carbonated peridotite is able to produce melts characterized
by extremely low content of silica but high amount of CaO and MgO (e.g. Presnall and Gudfinnsson, 2005; Hammouda
and Keshav, 2015). It is therefore necessary to understand these peculiar rock compositions in order to constrain their
mantle source, the role of carbonates and their origin in a subduction-related setting
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Strontium stratigraphy of the upper Miocene Lithothamnion Limestone in the Majella Mountain, central Italy, and its palaeoenvironmental implications
Get PDFThe 87Sr/86Sr isotope ratio has been widely used as a physical tool to date and correlate carbonate successions due to the long Sr residence time in comparison with the ocean mixing time. If this method works on oceanic successions, marginal basins may show different Sr isotope records in comparison with the coeval ocean one due to sea-level variations, continental run-off and restricted water exchanges. In this work, we present the 87Sr/86Sr isotope record of the upper Miocene carbonate ramp of the Lithothamnion Limestone (Majella Mountain, central Apennines), as an example of the onset of restricted water exchanges between a marginal basin and the ocean water masses. The overall latemost Tortonian–early Messinian Sr isotope record of the Lithothamnion Limestone fits below the global reference line. This deviation has been interpreted as due to the strong control that freshwater input and enhanced continental run-off, linked to the migration of the Apennine accretionary wedge and foredeep system, have had on the central Adriatic water chemistry. These results imply that an accurate oceanographic and geodynamic framework along with diagenetic overprint investigation has to be taken into consideration prior to apply SIS on carbonate successions on marginal basins, even when facies analyses indicate fully marine conditions. This seems to be the case for the upper Miocene Central Mediterranean carbonate successions, but may have more general validity and be extended to other recent or past marginal basins
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