1,721,283 research outputs found
The East Variscan Shear Zone:geochronological constraints from the Capo Ferro area (NE Sardinia, Italy)
No full textThe exhumation of the Variscan High Grade Metamorphic Complex of Sardinia has been driven by a wide dextral shearing which reached middle crustal levels at 325 ±1.3 Ma and caused partial melting. The migmatization process has been followed by the emplacement of syn-tectonic granites dated at 316 ±2 Ma, 318 ±3 Ma and 317 ±2 Ma which produced contact metamorphism at 315 ±1.3 Ma in mylonitic Silurian orthogneiss. The NW-SE dextral shear zone of the Capo Ferro area (NE Sardinia) shows remarkable petro-tectonic analogies and can thus be related to the East Variscan Shear Zone, a major intra-continental shear zone which was active in the southern European Variscides from 325 Ma to 315 Ma and that is precisely dated with the U-Pb monazite system. The emplacement of migmatitic gneisses along large shear zones within this time interval is not restricted to Sardinia, but has also been observed in southern Corsica, in the French Massif Central, in the Western Alps and in central Spain. This pattern of shear zones is related to a complex geodynamic scenario, linked to the oblique convergence between Gondwana and Laurussia plates during the Middle-Upper Carboniferous
Provenance of the Surveyor Fan and Precursor Sediments in the Gulf of Alaska—Implications of a Combined U-Pb, (U-Th)/He, Hf, and Rare Earth Element Study of Detrital Zircons
No full textZircon U-Pb geochronology and heavy mineral composition of the Camana Formation, southern Peru: Constraints on sediment provenance and uplift of the Coastal and Western Cordilleras
No full textIn the forearc of the Central Andes of southern Peru, the Cenozoic Camana Basin (16 degrees 25'S to 17 degrees 15'S) forms a similar to NW-SE elongated depression filled with coarse-grained deltaic and fluvial deposits. These deposits are termed Camana Formation. We have applied for the first time, advanced multi-method analytical techniques to sediments of the Camana Formation in order to define precise sedimentation ages, unravel sediment provenance, and to explain its tectono-sedimentary evolution. Zircon U-Pb geochronology and multiple geological evidences suggest that the Camana Formation ranges in-age from Late Oligocene to Late Miocene, and may even extend into the Pliocene. We propose a provenance model for the Camana Formation based on U-Pb geochronology, heavy mineral analysis, and single-grain mineral chemistry by LA-ICP-MS. This model suggests that sediments of the lower part of the Camana Formation derive from rocks forming the Coastal Cordillera (i.e. the Arequipa Massif and the San Nicolas Batholith) and the widespread ignimbrites of the similar to 24-10-Ma Huaylillas volcanic arc. In contrast, sediments of the upper part of the Camana Formation derive predominantly from rocks forming the Western Cordillera (i.e. the Arequipa Massif, the Tacaza Group, and the Coastal Batholith) and products of the similar to 10-3 Ma Lower Barroso volcanic arc). Accordingly, we infer that uplift of the Coastal Cordillera has strongly influenced deposition of the Camana Formation since Late Oligocene. A marked shift in provenance within the Camana Formation at around Middle to Late Miocene time (14-12 Ma) suggests drastic uplift of the Western Cordillera at that time. This uplift has triggered increased relief and erosion in the Western Cordillera, and subsequent deposition of fluvial conglomerates in the Camana Basin. (C) 2015 Elsevier Ltd. All rights reserved.Deutscher Akademischer Austauschdienst (DAAD) [416/PKZ A/09/98944
Zirconium and hafnium in meteorites
No full textThe ratio of the two refractory trace elements zirconium (Zr) and hafnium (Hf) in meteorites has been proposed to be uniform. The most precise value available is 34.3 +/- 0.2 (1 sigma). It was obtained by isotope dilution ICP-MS applied to 15 chondrites, most of which were carbonaceous chondrites, and six achondrites. We reinvestigated the case and determined Zr/Hf ratios of a broad spectrum of meteoritic samples via laser ablation ICP-MS. Our sample suite comprised 29 chondrites and five achondrites. The main objective of the study was two-fold: we intended to verify the accuracy and precision of a relatively fast and inexpensive sample preparation method combined with expeditious laser ablation ICP-MS techniques. Furthermore, we were looking into the possibility of systematic fine-scale Zr/Hf variations among bulk meteoritic matter of different classes. The applied fusion technique together with laser ablation ICP-MS turned out to be well suited to determine relative refractory trace element abundances. Absolute Zr/Hf ratios yield uncertainties of approximately 4% (1 sigma). As opposed to the most recent findings, we observed variable Zr/Hf ratios in different meteorites ranging from approximately 28 to approximately 38. Our value for Orgueil (CI1) is 34.0 +/- 0.3 (1 sigma). Including literature data, we propose a solar system value of 34.1 +/- 0.3. Our data also suggest that H chondrites tend to exhibit higher Zr/Hf ratios (average of 35.6 +/- 0.5 [1 sigma]) while EL6 chondrites rather show low values (average of 30.8 +/- 0.6 [1 sigma]). In addition to examining Zr/Hf ratios, we also explored the content of refractory major elements in different meteorite groups. Here, we found that EL6 chondrites often show very low Ca/Al ratios. The CI1 value for CaO/Al(2)O(3) is 0.804. EL6 chondrites, however, display ratios as low as approximately 0.3. While the variation in Zr/Hf can be explained by fractional condensation processes in the early solar nebula, the observed low Ca/Al ratios in EL6 chondrites are probably attributable to deficits in oldhamite (CaS).[PA909/2-1]; [PA909/2-2
Sedimentology, geochronology and provenance of the late Permian and Triassic Mitu Group in Peru—The evolution of continental facies along a transform margin
No full textAbstract
In the late Permian and Triassic, the continental Mitu Group formed in extensional basins along the length of the Cordillera Oriental and Altiplano of present‐day Peru. Given the presence of coeval arc systems only in northern Chile and southern Ecuador but not in Peru the tectonic setting of the Mitu basin has been interpreted variably as orthogonal continental rift, sinistral transtensional rift, aulacogen and back‐arc basin. The Mitu Group comprises continental mass flow and alluvial fan, fluvial, aeolian and minor lacustrian facies and hosts thick piles of subalkaline and alkaline intermediate and felsic ignimbrites and mafic lavas. The age of the Mitu Group had originally been established as ranging from the Late Permian to the late Triassic on the basis of structural considerations and scarce biostratigraphic data. Recently, U–Pb zircon ages from ignimbrites and sedimentary rocks have been taken to constrain the Mitu Group to the Middle and Late Triassic. We performed a sedimentological, heavy mineral, and zircon geochronological and Lu‐Hf isotope study of the Mitu Group in 14 sections mainly in southern and central Peru, and one section in northern Peru. Ten new U–Pb concordia ages on ignimbrites intercalated in the Mitu Group successions offer a new robust stratigraphic framework and constrain the stratigraphy of the Mitu Group between 260 and 205 Ma. In combination with maximum likelihood ages of deposition derived from detrital zircon, U–Pb geochronology places the deposition of the Mitu Group between ca. 270 and 194 Ma (lower Guadalupian into the Sinemurian). Detrital zircon U–Pb age distributions and heavy mineral assemblages reflect a strongly recycled Precambrian Amazonian and Palaeozoic proto‐Andean provenance. The Palaeozoic detrital age patterns are highly variable, and temporally and spatially random. A local provenance can generally not be identified. εHf(
t
) values in zircon obtained from ignimbrites and sedimentary rocks indicate variable degrees of crustal recycling. In the course of the Palaeozoic, εHf(
t
) values become on average progressively less negative, with a large proportion particularly of Mitu age zircons' εHf(
t
) values encompassing less evolved and moderately juvenile compositions. Along strike of the basin stratigraphic thicknesses, and rates and times of accumulation vary strongly with larger thicknesses and rates being registered in southern Peru. This suggests that the Mitu basin had been divided into a number of subbasins with individual histories of subsidence, accumulation, and volcanism. Absent a magmatic arc, late Permian–Triassic Peru evolved in a sinistral plate tectonic and regional framework expressed particularly in the sinistral Late Gondwanide orogeny predating the Mitu Group. We interpret that a sinistral transform fault linked the subduction zones in southern Ecuador and northern Chile and that the Mitu Group basin has formed by sinistral transtensional and transpressional movements along a related transcurrent fault inboard of the transform margin. In the broader framework of the accretionary Terra Australis orogen along the western Gondwana margin, this interpretation fits its evolution in the Andean segment in an internally consistent way.Deutsche Forschungsgemeinschaft https://doi.org/10.13039/50110000165
Tectonic setting and geochronology of the Cadomian (Ediacaran-Cambrian) magmatism in Central Iran, Kuh-e-Sarhangi region (NW Lut Block)
Full text linkThe structure, age and petrogenesis of plutonic basement rocks from the Kuh-e-Sarhangi region, located in the Kashmar-Kerman tectonic zone of Central Iran are described. These intrusive rocks consist of a sub-alkaline, dominantly high-K calc-alkaline acidic suite, characterised by high SiO2 (72.60-77.17wt%), and alkalis (up to 8wt%) and low Mg# (07-35). They are also enriched in Th, U and light rare earth elements, and depleted in Nb, Ta, and Ti, thus showing a geochemical fingerprint compatible with arc magmatism. The U-Pb zircon geochronology constrains magma crystallisation and emplacement during Ediacaran-Cambrian (ca. 575-535Ma) times, under a tectonic regime dominated by transpressional tectonics as constrained by analysis of field structures. Combined inverse and forward modelling thermobarometry indicates pluton emplacement occurred in a thickened crustal environment, suggesting the Kuh-e-Sarhangi magmatic belt exposes the exhumed roots of a volcanic arc. This magmatism is interpreted as part of the Cadomian, subduction-related magmatism and framed within an oblique convergence scenario during formation of an Andean-type active margin all along the northern (proto-Tethyan) margin of the Gondwana Supercontinent
Accurate correction for the matrix interference on laser ablation MC-ICPMS boron isotope measurements in CaCO3 and silicate matrices
No full textKnowledge of the boron isotopic composition of natural samples has found wide ranging application in both low and high temperature geochemistry. More recently, the development of boron isotope measurements using highly spatially-resolved analytical techniques is of interest as it is increasingly recognised that many materials are heterogeneous with respect to their boron isotopic composition, and moreover, that this heterogeneity yields valuable information about the environment of formation and/or mechanisms of crystallisation. Here, we build on a recently proposed methodology (Standish et al. [2019] Rapid Commun. Mass Spectrom.33:959) which enables precise and accurate δ11B measurement via LA-MC-ICPMS by accounting for a scattered Ca interference primarily on 10B. We propose minor modifications to this method via the use of 1013 Ω preamplifiers on the Faraday cup of the detector, more precise measurement of the Ca interference, and improved modelling of the shape of this interference correction. This yields single laser spot 2SE precision of ∼0.5‰ with a 70 μm beam (∼7 pg B), ∼1.4‰ with a 40 μm beam (∼2 pg B), and a long-term (1.5 year) intermediate precision in a marble standard with 15 μg g−1 [B] of <0.9‰ (2SD). Thus, spatially-resolved information comparable to that achievable via SIMS is possible. Moreover, we show theoretically and empirically that the inaccuracy predominantly resulting from a scattered Ca interference on 10B is also an issue for non-CaCO3 matrices, despite their typically lower [Ca]. Encouragingly, building multi-standard calibration lines to correct for this interference is also a way forward for silicate glasses, and we demonstrate accurate and precise (<0.5‰ 2SE) measurement of a basaltic glass with 3 μg g−1 [B] using a 74 μm diameter laser beam (<1 pg B). This paves the way forward for accurate and precise spatially-resolved δ11B measurement of a diverse range of sample matrices using laser ablation as a sample introduction system for MC-ICPMS instruments that are characterised by a scattered Ca interference in the region of m/z 10–11
Early Cretaceous migmatitic mafic granulites from the Sabzevar range (NE Iran): Implications for the closure of the Mesozoic peri-Tethyan oceans in central Iran
No full textThe ophiolitic mélange of the Sabzevar Range (northern Iran) is a remnant of the Mesozoic oceanic basins on the northern margin of the Neotethys that were consumed during the Arabia-Eurasia convergence history. Occurrence of km-scale, dismembered mafic HP granulitic slices is reported in this study. Granulites record an episode of amphibole-dehydratation melting and felsic (tonalite/throndhjemite) melt segregation at c. 1.1 GPa and 800 °C. In situ U(-Th)-Pb geochronology of zircon and titanite grains hosted in melt segregations points to an Early Cretaceous (Albian) age for the metamorphic climax. Results of this study (i) impose reconsideration of the current palaeotectonic models of the Neothetyan convergent margin during the Early Cretaceous and (ii) argue that punctuated events of subduction of short-lived back-arc oceanic basins accompanied the long-lasting history of the Neotethyan subduction in the region. © 2009 Blackwell Publishing Ltd
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