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Ghostlike boninitic magmatism in the Cretaceous southern Neo-Tethys. New evidence from the Zagros ophiolites (Iran)
No full textOphiolites cropping out along the Main Zagros Thrust Zone (MZTZ) represent portions of the southern Neo-Tethys
Ocean, which existed between the Arabian and Eurasian continental margins. Several authors suggested that an intraoceanic supra-subduction zone setting (SSZ) developed in this ocean during the Late Cretaceous. However, in contrast to many other ophiolites (e.g., Albanide-Hellenide), volcanic rocks, which typically characterize SSZ settings (i.e., island arc tholeiites and boninites) are lacking. Nonetheless, the volumetrically most abundant rock-type in the MZTZ
ophiolites consists of very depleted mantle harzburgites, which have chemical features that are typical for residual mantle after boninitic-type melts extraction. Therefore, though boninitic lava flows are lacking in the MZTZ ophiolites, the occurrence of boninitic magmatism at a regional scale can be envisaged. In this contribution, we review the available data on the Kermanshah and Sarve-Abad ophiolites (SW Iran) in search for evidence for the existence of boninitic magmatism in the southern Neo-Tethys.
The Kermanshah ophiolites include SSZ sequences largely consisting of depleted mantle harzburgites, which display a significant depletion in incompatible elements and rare earth element (REE), coupled with a marked LREE
enrichment with respect to MREE. REE modeling shows that they may represent a residual mantle after 25 – 30% removal of boninitic-type melts. The mineral chemistry of Cr-spinels also supports this conclusion.
The Sarve-Abad ophiolites include cumulitic lherzolites bearing minor dunite and chromitite lenses in places. The
crystallization order in ultramafic cumulates is: olivine ± Cr spinel + clinopyroxene ± orthopyroxene, which is typical
for boninitic melts. The mineral chemistry of Cr-spinel, pyroxenes, and olivine is compatible with a genesis from a
boninitic-type melt. Indeed, the calculated TiO2 and Al2O3 compositions and Mg# in the parental melt that was in
equilibrium with these minerals are consistent with boninitic-type compositions. Whole-rock geochemistry show low
incompatible element content and a general enrichment in Th with respect to Ta and Nb. Chondrite-normalized REE
patterns are consistent with boninitic-type parental melts. REE petrogenetic modeling indicates that the Sarve-Abad ultramafic cumulates may have formed by small degrees (5-15%) of fractional crystallization from typical boninitic melts.
In conclusion, several lines of evidence indicate that episodes of boninitic magmatism occurred within the southern
Neo-Tethys Ocean during the Late Cretaceous. Some hypotheses explaining the lacking of boninitic lavas or dykes in the MZTZ ophiolites can be postulated (e.g., short lived intra-oceanic subduction, transition from intra-oceanic to continental subduction). However, further investigations should be made for testing their tectono-magmatic and geodynamic consistence
Petrology and geochemistry of mafic magmatic rocks from the Sarve-Abad ophiolites (Kurdistan region, Iran): Evidence for interaction between MORB-type asthenosphere and OIB-type components in the southern Neo-Tethys Ocean
No full textThe Sarve-Abad (Sawlava) ophiolites crop out in the Main Zagros Thrust Zone and represent remnants of the Mesozoic southern Neo-Tethys Ocean that was located between the Arabian shield and Sanandaj-Sirjan continental block. They consist of several incomplete ophiolitic sequences including gabbroic bodies, a dyke complex, and pillow lava sequences. These rocks generally range from sub-alkaline to transitional character. Mineral chemistry and whole-rock geochemistry indicate that they have compositions akin to enriched-type mid-ocean ridge basalts (E-MORB) and plume-type MORB (P-MORB). Nonetheless, the different depletion degrees in heavy rare earth elements (HREE), which can be observed in both E-MORB like and P-MORB like rocks enable two main basic chemical types of rocks to be distinguished as Type - I and Type - II. Type - I rocks are strongly depleted in HREE (YbN 9.0). Petrogenetic modeling shows that Type - I rocks originated from 7-16% polybaric partial melting of a MORB-type mantle source, which was significantly enriched by plume-type components. These rocks resulted from the mixing of variable fractions of melts generated in garnet-facies and the spinel-facies mantle. In contrast, Type - II rocks originated from 5-8% partial melting in the spinel-facies of a MORB-type source, which was moderately enriched by plume-type components. A possible tectono-magmatic model for the generation of the southern Neo-Tethys oceanic crust implies that the continental rift and subsequent oceanic spreading were associated with uprising of MORB-type asthenospheric mantle featuring plume-type component influences decreasing from deep to shallow mantle levels. These deep plume-type components were most likely inherited from Carboniferous mantle plume activity that was associated with the opening of Paleo-Tethys in the same area
Two contrasting modes of continental break-up associated with the formation of the Paleo- and Neo-Tethys in Iran: Implications for petrological and geodynamic evolution at a regional scale
No full textPetrology of mantle peridotites and intrusive mafic rocks from the kermanshah ophiolitic complex (zagros belt, Iran): Implications for the geodynamic evolution of the neo-tethyan oceanic branch between Arabia and Iran
No full textThe Kermanshah ophiolitic complex consists of a mélange formation, which includes dismembered ophiolitic sequences. These ophiolites are located along the Main Zagros Reverse Fault, which marks the ophiolitic suture zone between the Zagros belt and the Sanandaj-Sirjan zone. They represent the Neo-Tethyan oceanic lithosphere, which originally existed between the Arabian (to the south) and Eurasian (to the north) continental margins. The Kermanshah ophiolites were emplaced onto platform carbonate rocks, which represented the northeastern Arabian margin.
The Kermanshah ophiolitic complex is composed of various partial sequences, which are represented by: (1) mantle tectonites consisting of depleted lherzolites and both clinopyroxene- (cpx-) rich and cpx-free harzburgites; (2) a troctolite – cumulate gabbro – isotropic gabbro sequence mainly showing pegmatoid texture; (3) a wehrlite – cumulate gabbro – isotropic gabbro sequence showing foliated texture; (4) a dyke complex; (5) very scarce pillow basalts. Mantle tectonites are volumetrically predominant and tectonically overlay the gabbroic sequences. A number of conclusions may be drawn based on petrographic observations, mineral chemistry, whole-rock chemistry, and rare earth elements (REE) modelling carried out on both mantle tectonites and intrusive rock associations. (1) The Foliated Gabbro Unit has an N-MORB chemical signature and represents a portion of oceanic crust generated in a mid-ocean ridge setting from an N-MORB-type sub-oceanic mantle. (2) The Pegmatoid Gabbro Unit displays an E-MORB signature and represents a portion of oceanic crust most likely generated from a sub-oceanic mantle source enriched in light REE (LREE). A comparison with the well-studied Oman ophiolites suggests that this sequence may have formed during the early stage of oceanic spreading. (3) The depleted lherzolites present mild depletions in heavy REE (HREE) and variable depletion in LREE. REE modelling shows that they may represent a residual mantle after 15 – 20% removal of N-MORB melts. Some lherzolites show a moderate enrichment in La and Ce with respect to Sm, suggesting that this residual MORB mantle was subsequently trapped in a supra-subduction zone (SSZ) mantle wedge and enriched in LREE by subduction-derived fluids. (4) The depleted harzburgites present a significant depletion in incompatible elements and REE, coupled with a marked LREE enrichment with respect to medium REE. REE modelling shows that they may represent a residual mantle after 25 – 30% removal of boninitic-type melts in an intra-oceanic arc setting
Geochemistry and petrology of the Kermanshah ophiolites (Iran): Implication for the interaction between passive rifting, oceanic accretion, and OIB-type components in the Southern Neo-Tethys Ocean
No full textThe Mesozoic southern branch of the Neo-Tethys Ocean was located between the Arabian shield and the Sanandaj-Sirjan continental block of Iran. The Kermanshah ophiolitic complex crops out in the Main Zagros Thrust Zone in Iran and consists of several rock associations, which record the geodynamic evolution of this ocean. In particular, the ophiolitic rocks that record the early stage of this oceanic evolution are represented by mantle metalherzolites, metagabbros and crosscutting metabasaltic dykes, as well as by basaltic pillow lavas and dykes. Mineral chemistry and whole-rock geochemistry of the metalherzolites show that they represent portions of exhumed sub-continental mantle. Four groups of rock can be recognized within the metagabbro and volcanic / subvolcanic sequences, based on many geochemical indicators and Sr - Nd isotopic data. They are: (1) metagabbros and metadykes with a N-MORB signature; (2) alkaline basalts and trachybasalts with ocean-island basalt (OIB) affinity; (3) rocks showing plume-type MORB (P-MORB) affinity; and (4) rocks showing enriched-type MORB (E-MORB) affinity. Metagabbros and metadykes show either low or high Sm/Yb ratios, which point to a genesis from partial melting of a pure depleted MORB mantle (DMM) source or a DMM source bearing garnet-pyroxenite relics, respectively.
All volcanic and subvolcanic rock-types have variable, but generally high La/Yb, Sm/Yb, Th/Yb, Ta/Yb and Zr/Y ratios, and low Zr/Nb ratios. These features coupled with relatively low 143Nd/144Nd ratios and high 87Sr/86Sr indicate that the alkaline rocks were generated from an enriched, OIB-type (plume-type?) mantle source, whereas P- and E-MORB rocks can be interpreted as the product of variable interaction between depleted MORB-type asthenosphere and OIB-type material.
We propose a tectono-magmatic model that can explain the different rock associations cropping out in the Kermanshah ophiolites involving a type of rifted margin that is intermediate between the amagmatic Iberia/Ligurian Tethys type and the magmatic, plume-influenced East Greenland type. This model presupposes that the Sanandaj-Sirjan block was rifted from the northern margin of Arabia through passive extension, which led to the exhumation of the sub-continental mantle now represented by the metalherzolites. Meanwhile, the asthenosphere upwelling associated with the partial melting of a heterogeneous mantle source locally bearing garnet-pyroxenite relics allowed the formation of both low and high Sm/Yb N-MORB-type metagabbros and metadykes at the continent-ocean transition zone. This model is similar to the model proposed for the Ligurian Tethys. But, in contrast with the Ligurian Tethys model, extension was also associated with volcanism characterized by a marked influence of OIB-type components linked to the uprising of MORB-type, depleted asthenospheric mantle. If so, the Kermanshah ophiolites represent a particular type of continental margin ophiolite, comprising a combination of Continental Margin- and Plume-type ophiolites
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Full text linkWe 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
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