The Barrovian metamorphism of the Lepontine dome is manifested by isograds that cross‐cut tectonic nappe contacts, which is commonly interpreted as metamorphism that occurred after nappe emplacement. ...However, the pervasive mineral and stretching lineation in amphibolite facies, associated with top‐to‐foreland shearing, suggests that peak Barrovian conditions are coeval with nappe‐overthrusting. Here, we combine mapping and U‐Pb zircon dating to better constrain the relation between metamorphism and overthrusting. Metamorphic zircon rims show two age populations at 31–33 and 22–24 Ma. The younger population is locally observed in post‐foliation dikes (and associated metasomatism) likely sourced from deep‐migmatites exhuming along the Alpine backstop. The older population occurs regionally and is found in syn‐kinematic migmatites which occur along a crustal‐scale shear zone. Below this shear zone, magmatic and detrital zircon cores suggest that the Cima Lunga unit, previously interpreted as a tectonic mélange with Mesozoic fragments, was a pre‐Variscan metasedimentary sequence intruded by Permian granitic sills, now orthogneisses. This unit was strongly sheared along the top of the Simano nappe during overthrusting of a rock pile here‐termed Maggia‐Adula nappe. This large‐scale nappe emplacement imprinted the regional lineation and peak temperatures until 31–33 Ma. Péclet (1–10) and Brinkman (0.002–1.8) numbers, estimated for the overthrusting, suggest an advection‐dominated heat transfer caused by rock exhumation, with some diffusion (conduction) during nappe emplacement. Diffusion contributed to Barrovian isograds discordant to the thrust. Shear heating was important if stress times shearing rate >∼5·10−6 W·m−3 within the nappe. The thermal evolution after overthrusting was spatially heterogeneous until ca. 22 Ma.
Plain Language Summary
The Lepontine area constitutes the core of the Central European Alps. It has a dome structure and it is internally formed by rock units which register pressure and temperature conditions typical of collisional orogens. The temperatures recorded by minerals are high and the origin of the heat that affected the Lepontine units is still unclear. In this study, we implemented different branches of geology to reveal the age of the Alpine events which juxtaposed the Lepontine units, their provenance and evolution. We performed extensive geological mapping to define the lithologies and structures of rocks. From 13 samples, we extracted 1158 zircon crystals that we analyzed and dated. Fieldwork permitted us to discover new rock units and better characterize the transition between the large‐scale units constituting the Lepontine dome. We propose a geodynamic scenario involving a major large‐scale Alpine unit. The emplacement of this unit generated the main heating event at 31–33 Ma, which is widespread and resulted in peak temperature conditions. The thermal evolution after this event was regionally complex and spatially heterogeneous. Locally in the south we document magmatic/fluid injections at 22–24 Ma, which were sourced from still‐hot regions in the roots of the orogen.
Key Points
Geological mapping reveals widespread syn‐tectonic migmatitic rocks along a main crustal‐scale shear zone inside the Lepontine dome
Alpine nappe emplacement is regionally recorded at 31–33 Ma, locally overprinted in the south by magmatic/fluid pulses at 22–24 Ma
Mapping and detrital zircon crystals suggest a structurally coherent Cima Lunga unit of pre‐Variscan age, as part of the Simano nappe
The Adamello intrusive suite is a composite batholith in Northern Italy, with an estimated 2000 km3 volume, assembled incrementally over a time span of 10 to 12 million years. The history of ...crystallization has been studied in detail through laser ablation ICP-MS and SIMS U–Pb geochronology of zircon, which records prolonged crystallization of each of the different intrusive units at mid-crustal levels between 43·47 and 33·16 Ma. The magmas were episodically extracted from this storage area and ascended to the final intrusion level at ∼6 km paleo-depth. Each batch of melt cooled very rapidly down to the ambient temperature of 250°C, evidenced by distinct cooling paths recorded by amphibole, biotite and K-feldspar 40Ar/39Ar dates. The magma source area was moving from SW to NE with time, causing increasing thermal maturity in the mid-crustal reservoir. The resulting temporal trend of higher degrees of crustal assimiliation in the course of the evolution of the magmatic system can be traced through Hf and O isotopes in zircon. Rough estimates of magma emplacement rates (‘magma flux’) yield very low values in the range of 10-4 km3/yr, typical of mid-to-upper crustal plutons and increase with time. Although we cannot discern a decrease of magma flux from our own data, we anticipate that a dramatic decrease of magma flux between 33 and 31 Ma along the northern contact lead to cessation of magma emplacement.
The Val Fredda igneous complex in the southern Adamello batholith (N. Italy) consists of dioritic to gabbroic sills and dykes that were injected at 6–10 km depth into partly crystallized tonalites ...and granodiorites. High-precision U–Pb age determinations by chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA–ID–TIMS) show very similar dispersion of zircon U–Pb dates over 90–200 ka and identical age distributions with a dominant mode at 42.5 Ma for six samples ranging in composition from gabbro to granodiorite. The co-variation of the probability density curves of zircon dates from mafic and felsic units suggests that they shared a common thermal history with periods of enhanced and reduced zircon growth, reflecting lowered and increased magma temperatures, respectively. However, trace element compositions, Ti-in-zircon temperatures and Hf isotopic compositions of zircon from mafic lithologies are distinctly different from those in felsic zircon and suggest their crystallization occurred in isotopically and chemically diverse magma batches. These magma batches formed in the lower crust from mingling and mixing of residual melts (derived from fractional crystallization of mainly amphibole from basaltic melt) with crustal partial melts at high temperatures above zircon saturation. Zircons crystallized during incipient cooling of these magmas and were entrained into the ascending melts, which were emplaced and rapidly solidified in the upper crust. The reported age dispersions imply that fractional crystallization and hybridization in the lower-to-middle crust, ascent into the upper crust and solidification did not last for more than 200 ka. The small magma volumes and flux also preclude significant zircon crystallization at the upper crustal emplacement level.
The results of the first direct experiments on the passage of pulses of ultra-wideband radiation of subnanosecond duration in the Earth’s atmosphere at a distance of more than 10 km are presented. In ...contrast to the work calculated, the preservation of the amplitude–time shape of the pulses in the process of increasing the distance is shown. The establishment of this fact is of decisive importance in the practical application of ultra-wideband pulses in new technological developments.
The procedure for the numerical study of the stress-strain state and the stability of the mesh structures of polymer composite materials under intensive force and thermal effects is considered. A ...simplified mathematical model of the thermomechanical behavior of the mesh structure is proposed, taking into account the reversible and irreversible changes in the physicome-chanical properties of the material during heating. The design process of computational algorithms is described.
Basanites of the Chyulu Hills (Kenya Rift) contain mafic Mg–Al and Ca–Al granulite xenoliths. Their protoliths are interpreted as troctolitic cumulates; however, the original mineral assemblages were ...almost completely transformed by subsolidus reactions. Mg–Al granulites contain the minerals spinel, sapphirine, sillimanite, plagioclase, corundum, clinopyroxene, orthopyroxene and garnet, whereas Ca–Al granulites are characterized by hibonite, spinel, sapphirine, mullite, sillimanite, plagioclase, quartz, clinopyroxene, corundum, and garnet. In the Mg–Al granulites, the first generation of orthopyroxene and some spinel may be of igneous origin. In the Ca–Al granulites, hibonite (and possibly some spinel) are the earliest, possibly igneous, minerals in the crystallization sequence. Most pyroxene, spinel and corundum in Mg–Al and Ca–Al granulites formed by subsolidus reactions. The qualitative P–T path derived from metamorphic reactions corresponds to subsolidus cooling, probably accompanied, or followed by, compression. Final equilibration was achieved at T ≈ 600–740°C and P <8 kbar, in the stability field of sillimanite. The early coexistence of corundum and pyroxenes (± spinel), as well as the association of sillimanite and sapphirine with clinopyroxene and the presence of hibonite, makes both types of granulite rare. The Ca–Al hibonite-bearing granulites are unique. Both types enlarge the spectrum of known Ca–Al–Mg-rich granulites worldwide.
Stable isotope and trace element compositions of igneous amphiboles from different tectonic settings (ocean island basalts, intraplate alkaline basalts, subduction-related andesitic complexes) were ...compiled to help understand the role of fluids and melts in subduction-related mantle metasomatism and to evaluate the use of selected trace element ratios (Pb/Pb*(N)=Pb/(√(Ce·Pr)) and Ba/Nb(N), normalized to primitive mantle) to help detect possible metasomatism. Comparisons of stable H and O isotope compositions and trace element ratios of amphiboles from ocean island basalts (Canary Islands), intraplate basalts, and subduction-related calc-alkaline andesitic series (Carpathian–Pannonian Region, CPR) indicate systematic distributions in δD–δ18O–Pb/Pb*(N)–Ba/Nb(N) diagrams that are related to metasomatic processes in the mantle and the migration of fluids and melts derived from subducted crustal slabs. In order to interpret these data for the amphiboles from the CPR, ophiolites of the Penninic and the Meliata–Vardar complexes as potential sources of subducted crustal melts and fluids in the mantle of the Carpathian–Pannonian Region were also analyzed. On the basis of published fluid/rock partition coefficients the compositions of fluids emanating from subducted ophiolites were calculated. The calculated fluid compositions—especially for blueschists of the Meliata complex—fit the amphibole trends, indicating that such fluids could have been responsible for the mantle metasomatism beneath the CPR.
► Amphibole compositions reflect effects of fluids and melts in mantle metasomatism. ► Both fluid and melt induced metasomatism result in high Pb/Pb* and Ba/Nb ratios. ► Elevated oxygen isotope compositions would preferentially indicate melt admixing. ► Sediment-derived melt infiltration can be detected by coupled 18O and Pb enrichments. ► Ophiolites provided a source of fluids in the Carpathian–Pannonian Region mantle.
PDF
