1,721,011 research outputs found
Mantle sources, magma genesis, and mantle residua in intra-oceanic arcs, as deduced from the Jurassic-Cretaceous Tethyan ophiolites
No full textGeochemistry and petrogenesis of mantle peridotites from the Nehbandan Ophiolitic complex, Eastern Iran
No full textOphiolitic exposures from the East and Northeast of Nehbandan. Eastern Iran, are remnants of a Cretaceous oceanic basin between Lui and Afghan continental blocks. Based on petrographic studies, they contain harzburgite. clinopyroxene-harzburgite and Iherzolite equilibrated in spinel peridotite fades. Chemistry of clinopyroxene in Iherzolite is characterized by low Na20 and TiO2, contents suggesting sub-oceanic origin. Spinel is a ubiquities accessory phase yielding chromium number [Cr# = l00*Cr/(Cr+Al)] in the range of 31-37 and 15-21 for harzburgite and Iherzolite. respectively. Melting degrees on the basis of spinel chemistry is 5 to 8% for Iherzolite and 12.5 to 14.2% for harzburgite. Cr# of magmalic spinel is 50-51 which correlate with Al-rich type of podiform chromitites. Whole-rock chemistry shows that most of the peridotitic samples exhibit evident U-shaped primitive mantle-normalized rare earth element patterns. In addition, in multi-element diagrams, relative to neighboring ra..
Petrological and geochemical constraints on the origin of the Nehbandan ophiolitic complex (eastern Iran): Implication for the evolution of the Iranian sector of the Neo-Tethys
No full textSubduction-unrelated magmatism of southern periphery of Paleotethys: constraints from Late Paleozoic magmatism from the south of Masuleh, western Alborz
No full textThe Alborz Mountains in north of Iran correlates with the Paleotethyan Suture so preserves valuable clues for geodynamic clarifications and paleotectonic reconstructions. During life span of Paleotethys from Early Paleozoic to Late Triassic, major parts of Alborz appear as a continental margin in southern border of the oceanic basin. To test paleotectonic setting of Alborz during Late Paleozoic and its passive or active condition, geochemical data of magmatic rocks can provide useful clues. The Masuleh area (western Alborz) involves important exposures of Late Paleozoic volcanic associations. These volcanics are poorly studied and understood, so we intend to present new geochemical data about them. Field studies characterize various lithological units in tectonic and stratigraphic contact with volcanic units including Late Paleozoic low-grade metamorphics (slate-phyllite) and calcareous units of upper Devonian, Carboniferous to Permian. The whole rock geochemical data has been obtained by XRF and ICP-MS at Ferrara University, Italy. The volcanics mainly comprise basaltic to trachy-basaltic compositions. They show moderate to high alteration reflected in their LOI content (2.2 to 7.8 wt.%). Thus, for major element we used recalculated anhydrous values. The SiO2 abundances of 45.3 to 50.7 wt.% display basic nature of the studied rocks. Other major element components such as TiO2, Al2O3, CaO and MgO are in the ranges of 1-4.4, 13.7-18.3, 5-10.7 and 2.5-14.9 wt.%, respectively. Moreover, Mg# [MgO*100/(MgO+FeO*)] varies from 19-71. Wide range of major element variations likely corresponds to different modal mineralogy and also various extent of melt evolution and fractionation processes. Total alkali element abundance (Na2O+K2O) displays elevated values (1.95-7.9 wt.%) reflecting alkaline composition of the samples. Compatible elements such as Ni (2.2-213.7 ppm) and Cr (17-739 ppm) indicate highly varied amounts, as well, suggesting nearly primitive to extremely fractionated nature. In La/Sm vs. La plot, the compositional trend is consistent with fractional crystallization process. Chondrite-normalized REE patterns and primitive mantle -normalized spider diagrams are characterized by similar patterns suggesting genetic relationships of different samples. The spider diagrams display humped-shaped patterns in which the LILEs (Rb, Ba, Sr and K) and HFSEs (e.g. Th, Ta, Nb, Zr and REEs) show enrichment with increasing incompatibility and a slightly negative Nb anomaly. These patterns are consistent with typical intraplate alkaline magmatism (OIBs). REE patterns are characterized by pronounced negative slope reflecting high LREE/HREE enrichment ((La/Yb) N= 5-17). Moreover, La and Yb represent enrichment of 47-248 and 6.5-22 times chondrite abundances, respectively. Immobile trace elements (e.g. La, Y, Zr and Nb) discrimination diagrams suggest subduction unrelated within-plate mantle origin similar to OIB source. Furthermore, the mantle source nature and partial melting degrees are inferred from modeling based on incompatible element ratios (Sm/Yb vs. La/Yb plot) suggesting that parental melt derived from garnet-bearing lherzolite and partial melting of <15%. Finally, we conclude that the area located to the southern margin of Paleotethys during Late Paleozoic was a passive margin (Gondwanian affinity) and the magmatic activity was related to thermal perturbation of mantle via hot spot/plume effects in an extensional tectonic regime
Chemical constraints on the genesis and geodynamic setting of Late Cretaceous magmatism in northeast of Tehran, Central Alborz
No full textThe Alborz range is part of Alpine- Himalayan orogenic system which preserved important magmatic features related to the evolution of Gondwana-Eurasia junction realm. Several well-developed sub-marine Cretaceous volcanic units are exposed in Central to Northern Alborz e.g. in Marzan-Abad and Lahijan areas. The volcanics display mainly basaltic composition. Study of chemical signatures of these rocks is a key issue to constrain the tectonomagmatic environment of melt formation and paleogeographic reconstructions. In this research we aim to study the volcanic outcrops located to the northeast of Tehran (south of Amol), Central Alborz. Stratigraphic relationships show that the volcanics are interlayered with marly limestone, limestone and marls of Late Cretaceous. Here, we present new geochemical data on these rocks to identify the origin and condition of magma genesis which helps to infer paleotectonic setting of Alborz magmatism during Late Cretaceous. The major and trace element contents of whole rock samples have been obtained by XRF and ICP-MS at Ferrara University, Italy. Using recalculated anhydrous major elements, the samples display wide range of variation in SiO2 (42-68 wt.%), TiO2 (0.45- 4.7 wt.%), Al2O3 (12.6-18.8 wt.%), Mg# (17.8-49) and Na2O + K2O (2.9-11.7 wt.%), likely reflecting various mineralogy and different degrees of fractionation. Based on Nb/Y (1.8-3.4) and Zr/Ti (0.01-0.29) ratios the rocks plot within alkali basalt, trachy-andesite and trachyte fields. High Zr (83-738 ppm) and Nb (43-111 ppm) contents is also consistent with alkaline nature of the samples. Other incompatible element values shown by normalized multi-element patterns are enriched as well, which is similar to typical within-plate alkaline basalts, such as ocean island basalts (OIB). Furthermore, no negative Nb and Ta relative depletion can be observed in these patterns. In chondrite- normalized REE patterns, the rocks display elevated LREE enrichment with respect to HREE, exemplified by high (La/Yb)N ratio (15-33). Overall, the REE enrichment of the rock suites is 155- 375 and 8.5- 17 times chondrite value for La and Yb, respectively. Using trace element abundances such as Zr, Y, Nb and Ti contents and also the ratios of Zr/P2O5, Nb/Y, Zr/Y, Ti/Y, Th/Yb and Ta/Yb, the basaltic rocks plot in the fields of alkaline and subduction-unrelated OIB-like magmas. Moreover, theoretical modeling using REE ratios e.g. Sm/Yb and La/Yb displays that the parental melt originated from partial melting of deep garnet-bearing enriched mantle with low (<15%) partial melting. Therefore, we conclude that the tectonic environment of melt formation is unrelated to subduction process and mostly corresponds to within-plate magmatism in an extensional setting. This conditions is more probably consistent with hot spot/mantle plume activity and continental rift-related events
Association of MORB and SSZ ophiolites along the shear zone between Coloured Mélange and Bajgan Complexes (North Maran, Iran): Evidence from the Sorkhband area
No full textOne of the largest worldwide accretionary wedges is exposed in the Makran region (SE Iran). The backstop of this accretionary wedge consists of an imbricate stack of continental and oceanic units, referred as North Makran domain. This domain is characterized by a km-thick shear zone, along which the metamorphic Bajgan Complex is thrust onto the Coloured Mélange Complex. Along this shear zone two slices of ophiolites have been identified in the Sorkhband area. The upper tectonic slice consists of gabbros, whereas the lower one consists of mantle peridotites associated with dunites and chromitite ore deposits. Petrography and geochemistry of gabbros clearly indicate an N-MORB-type affinity, suggesting that they were generated at mid-ocean ridge setting. In contrast, mantle peridotites consist of harzburgites and depleted harzburgites, both showing geochemical features suggesting their genesis in a SSZ setting. The new data presented in this paper indicate that the slices of ophiolites from Sorkhband area derived from two different oceanic domains representing two different geodynamic settings. This occurrence provides new evidence that the boundary between the Coloured Mélange and the Bajgan Complexes represents a first-order tectonic structure that played an important role in the geodynamic evolution of the Makran area
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
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