A geochronological study was performed on zircon grains from a middle‐lower crustal shear zone exposed in the northern sector of the Ivrea‐Verbano Zone (Southern Alps, Italy) for the first time. The ...shear zone developed at the boundary between mafic rocks of the External Gabbro unit and ultramafic rocks of the Amphibole Peridotite unit. It is ~10–20 m wide, can be followed along a NE strike for several kilometers, and consists of an anastomosing network of mylonites and ultramylonites. Zircon grains were studied in thin sections and as separates from three representative outcrops along the shear zone. Zircon grains are more abundant in the shear zone compared to wall rocks and are generally equant, rounded to subrounded with dimensions up to 500 μm. U‐Pb data are mainly discordant, and the apparent 206Pb/238U dates show a large variation from Permian to Jurassic. Isotopic data, combined with microstructural, morphological, and internal features of zircon, reveal an inherited age component and suggest partial zircon recrystallization under high‐temperature conditions during Late Triassic to Early Jurassic. High‐temperature deformation in the shear zone, at lower crustal levels, was coeval with amphibolite to greenschist facies mylonitic deformation at upper crustal levels and is inferred to be related to Mesozoic rifting processes at the Adriatic margin.
Key Points
Lower crustal high‐temperature deformation is Late Triassic to Early Jurassic
Large‐scale shear zones at different crustal levels were coeval
Zircon U‐Pb dating of mylonitic mafic rocks provides the first evidence of lower crustal deformation associated with Mesozoic rifting
New insights into the origin of high-Mg andesites are inferred from the mineral chemistry and U-Pb geochronology of Tertiary amphibole-rich ultramafic intrusive rocks (hornblendites) and included ...clinopyroxene-bearing dunitic clots from the southern Adamello batholith (Central Alps). The hornblendites consist mostly of amphibole grains with brown cores (Ti-pargasite) that grade through brownish-green (Mg-hornblende) to light green (edenite) rims. Brown amphibole contains olivine (Fo = 85-87 mol %) and clinopyroxene inclusions with irregular boundaries indicating disequilibrium with the host amphibole. The ultramafic clots are interpreted to represent fragments of older cumulates dismembered by the injection of the amphibole-forming melts, thereby providing evidence for a melt-rock reaction process. Amphibole from the hornblendites shows a marked trace element zoning. From the brown core outward to the brownish-green portion of a single crystal, a significant enrichment is observed in light rare earth elements, Th and U, coupled with a decrease in Ti and heavy rare earth elements. The melt in equilibrium with the brownish-green amphibole has an adakitic trace element signature (e.g. high La sub(N)/Yb sub(N) and Sr/Y). Based on amphibole/liquid partition coefficients, a fractional crystallization process driven by amphibole could explain most of these chemical variations. However, the outward increase of highly compatible elements in amphibole (e.g. Mg, Ni, Co, and Zn) argues against closed-system fractional crystallization. The assimilation of olivine is considered the most efficient mechanism to supply or buffer the highly compatible elements in the evolving system during amphibole crystallization. In situ U-Pb zircon geochronology of hornblendites and associated amphibole gabbros reveals the occurrence of inherited cores, thereby providing evidence for assimilation of crustal material. We propose that a differentiation process controlled by amphibole crystallization and assimilation of slightly older ultramafic cumulates may produce melts rich in SiO sub(2) and MgO with adakitic trace element signatures.
Titanite/liquid partition coefficients for most of the trace elements relevant in petrogenetic studies are provided for titanite-saturated liquids equilibrated at 1.5 GPa and 850 °C starting from ...lamproitic compositions. The high compatibility observed for REE, HFSE, Sr, V and Sc, and the strong incompatibility observed for actinides, large ion lithophile and light elements are discussed in terms of available crystal-chemical mechanisms for incorporation and crystal-structure control.
The exchange vectors Na
1Ca
−1 and Al
1Ti
−1 allow local charge balance to be achieved after incorporation of REE
3+ and R
5+ at the Ca and Ti site, respectively. The significant amounts of H measured are also relevant in this regard the exchange mechanism being (OH)
1O
−1 at the O1 site. The incorporation of U
4+ and Th
4+ at the Ca site is scarce, and is likely balanced by that of Mg
2+ at the Ti site; both these substitutions are responsible for strong structural strain.
Titanite can thus be considered an important repository for REE and HFSE in metamorphic and igneous rocks, and its role must be accounted for when modelling trace-element residence during metamorphic reactions and late magmatic crystallisation history. Due to the measured differences in compatibility, titanite crystallisation increases the values of Nb/Ta and LREE/HREE ratios in residual liquids. The similar compatibility of U and Pb makes titanite suitable for U–Pb geochronology of igneous rocks only after common Pb correction. Finally, this study confirms that the titanite end member is not suitable for radioactive waste disposal due to the discussed crystal-chemical constraints, and that titanite-based waste forms should contain high amounts of Na
+ and Mg
2+.
We present a major, trace and rare earth element characterization of single glass shards from fifteen tephra layers found in the astronomically-tuned KC01B deep-sea core (Ionian Sea, Central ...Mediterranean-36°15.25′N, 17°34.44′E, 3642 m water depth). KC01B is considered a reference core for the Mediterranean area since it provides a new reliable astronomical tuned timescale for the last 1.112 My (Lourens, 2004). The studied deposits punctuate the marine record in a time span ranging from ca 16 ka to ca 191 ka B.P. encompassing the climatic zones Y, X, W and V. They are related to volcanic activity of the Campania Plain, Aeolian Islands, Mt. Etna and Pantelleria Island. Along with major main markers of the Central Mediterranean area such as Y-1, Y-5, and X-6, astronomically dated at 16.7 ka, 39.1 ka, and 110.5 ka, respectively, the succession contains a number of well preserved deposits which can represent useful inter-archive instruments of correlation. The Campania Plain, in particular, sourced at least nine compositionally homogeneous tephra layers prior to the Campanian Ignimbrite event. The data obtained in this work provide a new detailed analytical reference database for Ionian Sea tephrochronology and for proximal-distal correlation studies. They may help to unravel from the marine record the history of poorly known or unknown explosive activity on land since the middle Pleistocene along with chemical composition, size and dispersal of the products.
•A tephrostratigraphic record from the Ionian Sea extending back to ca 200 ka.•Major and trace element dataset of fifteen tephra layers astronomically dated.•Major and potential inter-archive tephra markers.•Insights into the volcanism of Campania Plain, Mt. Etna, Aeolian Arc and Pantelleria.
Histiotus is a Neotropical genus of bat that currently includes 11 species. The systematics of Histiotus has been the focus of several studies over the last decades. However, no broad systematic ...revision has been made, and taxonomic issues such as synonymies, use of subspecies, and specimens that do not fit the description of valid species still persist, as pointed out by several authors. Histiotus alienus was described in 1916 and is known only by the holotype. Here we present a second record of H. alienus and an amended diagnosis of this species. We use qualitative, quantitative, and morphometric analyses based on data from 184 specimens of Histiotus and almost all valid species. Our amended diagnosis establishes the taxonomic limits of H. alienus, as well as a comprehensive comparison with congeners. We also explore new diagnostic characters for H. alienus and provide a few notes on the natural history of this species. Our results highlight skull similarities among Histiotus species and reinforce the usefulness of external morphology for their correct identification. Despite our new insights into the taxonomy of the genus, several taxonomic issues remain, and a comprehensive revision of the genus is needed.
One method to determine the presence of non-flying small mammals is the use of owl pellets or regurgitates. Tyto furcata is an owl species found in many types of environment, varying from untouchable ...forests to urban areas. The species has typical nocturnal habits and a varied diet of mammals (including rodents, marsupials, bats), amphibians, reptiles, and even other birds and has been shown to move on average 1 km (rarely 2 km) from the nesting site to foraging areas. Here, Pontes et al assess the rodent diversity in Reserva Natural da Guaricica.
We document for the first time the role played by natural solidification paths on the partitioning of rare earth elements (REE) between clinopyroxene and melt. To do this, we investigated the ...compositional variation of clinopyroxenes formed under increasing cooling rate conditions from core to rim of a dike at Mt. Etna volcano. As the rate of cooling increases, clinopyroxenes are progressively depleted in Si + Ca + Mg counter-balanced by enrichments in Al + Na + Ti. Consequently, the concentration of REE in clinopyroxene increases due to an increased ease of locally balancing the excess charge at the M2 site as the number of surrounding tetrahedral aluminium atoms increases. Since Al
iv
in clinopyroxene is a charge-balancing cation for REE, the partition coefficients (D
REE
) measured at the dike chilled margin are distinctly higher than those from the dike interior. We conclude that, in naturally solidifying magmas, kinetically controlled cation substitution reactions can be treated in terms of the energetics of the various charge-imbalanced configurations. This finding is corroborated by the near-parabolic dependence of D
REE
on cation radius due to charge-balance mechanisms described by the lattice strain model.
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