The composite Poshteh Pluton, at the northeastern margin of the Central Iranian Microplate near Taybad in eastern Iran, is positioned at a critical tectonic junction, south of the inferred main ...Paleotethys suture and along the major regional Doruneh Fault system. It consists of two distinct intrusions. Quartz monzonite is dated in this study by zircon U–Pb ID-TIMS to 215.8 ± 0.5 Ma, an age that coincides with the time of closure of the Paleotethys during the late collisional stages of the Eo-Cimmerian Orogeny. It is geochemically very similar to coeval plutons present along and north of the Paleotethys suture, where they intruded Carboniferous-Permian arc sequences, ophiolites and flysch. The Poshteh quartz monzonite is located south of the suture in a position similar to the Anarak and related complexes further west, which previously have been interpreted as reflecting Mesozoic and Cenozoic disruption of the Eo-Cimmerian Orogen by extensional and transtensional processes. The Triassic quartz monzonite was subsequently invaded by granite at 41.23 ± 0.31 Ma. The emplacement was in part structurally controlled by the Doruneh Fault system and associated to hydrothermal alteration and Fe mineralization. The granite is thus a coeval member of a widespread late Eocene to Oligocene plutonic suite in the region, and likely the result of delamination and melting of the subcontinental lithosphere.
Extensive carbonation of peridotite results in listvenite, a rock composed of magnesite and quartz. At Gråberget, Røros, SE-Norway, a variably serpentinized peridotite body, surrounded by the Røros ...schists, a former abyssal sediment displays all stages of transformation of peridotite to quartzite. In this paper we record the sequence of steps in this process by combining the observation of mineral assemblages, textural relationships and geochemistry, and variations in Pb isotopic compositions. Initial serpentinization, a stage that also involved an enrichment in fluid-mobile elements (Pb, Sb and As), was followed by carbonation through CO
2
fluids that formed soapstone, and eventually listvenite. The listvenite grades by decreasing amounts of carbonates into fuchsite bearing quartzite. The carbonates dissolved during supergene alteration and formed pores coated with oxides of Fe, Mn and Ni resulting in a brown rock color. The quartzite displays porous stylolites enriched in Pb, As and Sb and fuchsite with porous chromite grains as the only relicts of the original mineralogy in the peridotite. The dissolution of the carbonate occurred at oxidizing conditions at temperatures below 150 °C, where the solubility of magnesite is higher than that of quartz. Formation of quartzite from peridotite is supported by low REE contents and lack of zircons in the two rock types. The transformation involved enrichment of Pb, coupled with the elimination of Mg and enrichment of Si. This chemical fractionation and selective transfer of elements to the continents is an important mechanism and needs to be taken into account in models of continental evolution.
The end of the Permian period is marked by global warming and the biggest known mass extinction on Earth. The crisis is commonly attributed to the formation of the Siberian Traps Large Igneous ...Province although the causal mechanisms remain disputed. We show that heating of Tunguska Basin sediments by the ascending magma played a key role in triggering the crisis. Our conclusions are based on extensive field work in Siberia in 2004 and 2006. Heating of organic-rich shale and petroleum bearing evaporites around sill intrusions led to greenhouse gas and halocarbon generation in sufficient volumes to cause global warming and atmospheric ozone depletion. Basin scale gas production potential estimates show that metamorphism of organic matter and petroleum could have generated >
100,000 Gt CO
2. The gases were released to the end-Permian atmosphere partly through spectacular pipe structures with kilometre-sized craters. Dating of a sill intrusion by the U–Pb method shows that the gas release occurred at 252.0
±
0.4 million years ago, overlapping in time with the end-Permian global warming and mass extinction. Heating experiments to 275 °C on petroleum-bearing rock salt from Siberia suggests that methyl chloride and methyl bromide were significant components of the erupted gases. The results indicate that global warming and ozone depletion were the two main drivers for the end-Permian environmental crisis. We demonstrate that the composition of the heated sedimentary rocks below the flood basalts is the most important factor in controlling whether a Large Igneous Provinces causes an environmental crisis or not. We propose that a similar mechanism could have been responsible for the Triassic-Jurassic (~
200 Ma) global warming and mass extinction, based on the presence of thick sill intrusions in the evaporite deposits of the Amazon Basin in Brazil.
The Sanandaj-Sirjan metamorphic-magmatic Zone (SaSZ) represents the Mesozoic active continental margin of the Neotethys Ocean between the Arabian plate and the Iranian plateau. The zone evolved at ...the margin of ancient Ediacaran basement, as documented in this paper with an age of 557 Ma for a trondhjemite. The Mesozoic activity of the SaSZ was most intense with sedimentation and magmatism in the Jurassic between 170 and 150 Ma, but this period was then followed by a 20 million years gap with almost no magmatic activity, which has been attributed to flat slab tectonics. The present paper is concentrated on the geochemistry, and zircon CA-ID-TIMS geochronology, of a bimodal, but mainly basaltic, volcanic and subordinate plutonic succession that represents the resumption of magmatism in a deep and gradually shallowing basin in the northern SaSZ between 130 and 110 Ma. The mafic to intermediate volcanic rocks are predominantly calc-alkaline and have εNd values of +0.3 to +3.9. The felsic volcanic rocks are dominantly high-K calc-alkaline with εNd values of −0.5 to +1.9. A suite of Cretaceous trondhjemites yields εNd values of −4.6 to −4.9. Together, the geochemical, lithological and geochronological indications suggest that the thick Barremian-Aptian bimodal magmatic suite represents magmatism in a continental arc to back-arc setting, controlled by extension and probably related to foundering of the flat slab and slab roll-back. These events initiated melting of the mantle in the northern SaSZ, heralding a period of extension and the subsequent development of ophiolites all along the Neothetyan suture.
•The northern Sanandaj-Sirjan Zone is dominated by a Cretaceous supracrustal sequence.•The sequence is bimodal, mainly basaltic, locally cut by trondhjemite, and 130–110 Ma.•Trondhjemite representing the Ediacaran basement formed at 557 Ma.•Lithologies, geochemistry, and Nd and Sr isotopes point to an extensional setting.•A back-arc at an active continental margin and slab roll-back is proposed.
► We report ID-TIMS U–Pb zircon ages for plutons in the Sanandaj–Sirjan Zone of Iran. ► Mid Jurassic activity was prominent in the Boroujerd, Astaneh and Alvand complexes. ► Other magmatic episodes ...are recorded at various sites at 150, 110, 60 and 35
Ma. ► The extended activity reflects subduction of the Arabian plate below central Iran.
The Sanandaj–Sirjan Zone (SSZ), which runs parallel to the Zagros fold and thrust belt of Iran, underwent a multistage evolution starting with Neotethys initiation, its subsequent subduction below the Iranian continental crust, and eventual closure during convergence of Arabia towards central Iran. Plutonic complexes are well developed in the northern part of the SSZ and we have dated a number of them by ID-TIMS U–Pb on zircon. The new data record the following events: a Mid Jurassic period that formed the Boroujerd Plutonic Complex (169
Ma), the Astaneh Pluton (168
Ma) and the Alvand Pluton (165
Ma); Late Jurassic emplacement of the Gorveh Pluton (157–149
Ma); Mid Cretaceous (109
Ma) formation of a I-type phase in the Hasan Salary Pluton near Saqqez, followed by Early Paleocene (60
Ma) intrusion of A-type granite in the same pluton; and the youngest intrusive event recorded so far in the SSZ with the intrusion of granite in the Gosheh–Tavandasht Complex near Boroujerd at 34.9
Ma. These different events reflect specific stages of subduction-related magmatism prior to the eventual Miocene collision between the two continental blocks.
An igneous zircon reference material (OG1) was characterised for U‐Pb isotopes by ID‐TIMS, and utilised to evaluate SIMS (SHRIMP) instrumental mass fractionation (IMF) of radiogenic Pb isotopes ...(207Pb*/206Pb*). The TIMS 207Pb*/206Pb* reference value for OG1 was 0.29907 ± 0.00011 (95% confidence limit), 3465.4 ± 0.6 Ma. The high 207Pb* (∼ 30 μg g−1), negligible common Pb, and isotopic homogeneity permitted precise (± 1–2‰) 207Pb*/206Pb* measurements within the analytical sessions. External reproducibility of mean 207Pb*/206Pb* ratios between sessions was demonstrated for one instrument, yielding a mean IMF of +0.87 ± 0.49‰. The mean 207Pb*/206Pb* ratios between instruments were dispersed beyond uncertainties, with session IMF values from +3.6 ± 1.7‰ to −2.4 ± 1.3‰, and a grand mean IMF value (twenty‐six sessions) of +0.70 ± 0.52‰, indicating a tendency towards elevated 207Pb*/206Pb*. The specific causes of variability in IMF are unclear, but generally reflect subtle differences in analytical conditions. The common practice in SIMS of assuming that IMF for Pb+ is insignificant could result in systematic age biases and underestimated uncertainties, of critical importance for precise correlation of Precambrian events. Nevertheless, a zircon RM such as OG1 can be readily incorporated into routine dating to improve 207Pb*/206Pb* accuracy and external reproducibility.
Le zircon de référence (OG1) a été caractérisé pour les isotopes U‐Pb par spectrométrie de masse et dilution isotopique (ID‐TIMS), et utilisé pour évaluer le fractionnement de masse instrumental (IMF) d'un spectromètre de masse à ion secondaire (SIMS) de type SHRIMP, affectant la mesure du rapport des isotopes radiogéniques de Pb (207Pb*/206Pb*). La valeur de référence 207Pb*/206Pb* mesurée au TIMS du rapport pour OG1 était 0.29907 ± 0.00011 (intervalle de confiance à 95%), 3465.4 ± 0.6 Ma. La concentration élevée en 207Pb* (∼ 30 μg g−1), négligeable en Pb commun et la grande homogénéité isotopique de ce zircon ont permis des mesures précises (± 1–2‰) du 207Pb*/206Pb* durant les sessions analytiques. La reproductibilité externe sur les moyennes de 207Pb*/206Pb* obtenues durant différentes sessions a été testée pour un instrument, conduisant à un IMF moyen de +0.87 ± 0.49‰. Les valeurs moyennes de 207Pb*/206Pb* obtenues sur différents instruments étaient dispersées plus largement que leurs incertitudes, avec des valeurs d'IMF variant de +3.6 ± 1.7‰à−2.4 ± 1.3‰, et une moyenne des moyennes IMF (sur vingt six sessions) de +0.70 ± 0.52‰, montrant une tendance vers une surestimation des 207Pb*/206Pb*. Les causes particulières de variation de l'IMF d'un instrument à un autre ne sont pas claires, mais doivent refléter des variations subtiles des conditions analytiques. La pratique courante en SIMS qui repose sur le fait que l'IMF pour Pb+ est négligeable, peut donc induire dans des biais systématiques des âges obtenus et une sous estimation des incertitudes, tout ceci ayant une importance critique quand il s'agit de corréler des événements d'âge précambrien. Néanmoins, il reste qu'un matériau de référence tel que le zircon OG1 peut être utilement incorporé dans tout protocole de routine de datation afin d'améliorer la justesse et la reproductibilité externe sur la mesure de 207Pb*/206Pb*.
Large quantities of monazite from different pegmatite bodies of the Itambe pegmatite district were investigated to assess their suitability as U–Pb and Sm–Nd isotope reference materials for LA-ICP-MS ...and to track the origin of a piece of the Moacyr monazite (termed here Itambé), a widely used reference material for LA-ICP-MS U–Pb geochronology. Monazite from the largest pegmatite bodies in the district (the Bananeira, Coqueiro and Paraíso pegmatites) are Ce-monazite, with negligible amounts of the huttonite and brabantite components. They are homogeneous in major and trace elements, which makes them potential candidates as compositional reference materials. U–Pb LA-ICP-MS and ID-TIMS analyses yielded identical ages within error. Although the ID-TIMS ages (507.7±1.3 (207Pb⁎/235U) and 513.6±1.2Ma (206Pb⁎/238U)) were reversely discordant, spot ages determined by LA-ICP-MS geochronology were concordant at ca 508Ma. The Bananeiro monazite was assessed as a LA-ICP-MS U–Pb primary reference material against other known reference materials (treated as unknowns). This approach successfully reproduced the previously published ages of the reference materials. MREE/HREE fractionation (ie, (La/Gd)N and (Gd/Lu)N values), Eu/Eu⁎ and the chondrite-normalized REE patterns suggest that the “Itambé” monazite aliquot is very similar to that from the Coqueiro pegmatite. This similarity is likewise apparent in their Sm–Nd isotope compositions. Moreover, the εNdi values of the “Itambé” monazite fragment (εNdi=−4.2) and those from all the major pegmatites in the district, are distinct from other reference materials (eg, Managountry; εNdi=−22.3) as well as gem-quality monazite from c. 490–520Ma pegmatites from the Araçuaí Orogen, further to the south. The εNdi can provide a further distinction for tracing Brazillian gem-quality monazite reference materials.
•We propose the Bananeira monazite as a primary reference material for U–Pb LA-ICP-MS analyses.•The widely used “Moacyr” monazite (here termed Itambé) is from the Coqueiro pegmatite, from the Itambé district.•The Itambé monazite is a promising Sm–Nd reference material for LA-ICP-MS.•The Coqueiro monazite is a potential reference material for trace elements analyses for LA-Q-ICP-MS.•The εNd can provide a further distinction for tracing reference materials, apart from the U–Pb age.
Studies of mineral equilibria in metamorphic rocks have given valuable insights into the tectonic processes operating at convergent plate margins during an orogeny. Geodynamic models simulating ...orogenesis and crustal thickening have been constrained by temperature and pressure estimates inferred from the mineral assemblages of the various lithologies involved along with age constrains from increasingly precise geochronological techniques. During such studies it is assumed that the pressure experienced by a given rock is uniquely related to its depth of burial. This assumption has been challenged by recent studies of high pressure (HP) and ultrahigh pressure (UHP) rocks. Here, we describe an example of Caledonian HP metamorphism from the Bergen Arcs in western Norway, and show that the associated formation of Caledonian eclogites at the expense of Proterozoic granulites was related to local pressure perturbations rather than burial, and that the HP metamorphism resulted from fluid-induced weakening of an initially dry and highly stressed lower crust when thrust upon the hyperextended margin of the Baltic shield.
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