In the Internal Western Alps, the Gran Paradiso massif is an eclogitic gneiss dome of Alpine age. It is largely composed of porphyritic metagranites which have long been interpreted as Variscan intrusives in a pre-Variscan basement. Strong deformation and metamorphism affected the entire massif during the Alpine orogeny. The present study confirms earlier, preliminary results that suggested a mid-Permian age for some of the granitic protoliths, thus postdating the last Variscan orogenic granitoids. We also extend that result to the bulk of the massif, from the top contact near Bonneval (Arc valley, France) to the structurally deepest part in Noasca (Orco valley, Italy). Six new U– Pb age determinations were carried out on zircons (IDTIMS and SHRIMP) from four porphyritic orthogneisses and two samples of host rocks with magmatic affinities. All ages are Permian, ranging from 264 to 277 Ma. The oldest ages pertain to the Noasca granite, in the core of the dome, and to a massive gneiss (Bivacco Carpano) that belongs to the main pre-granite metasedimentary and metavolcanic pile. Similar ages were obtained for granites and their host rocks, suggesting that a large part of the Gran Paradiso dome consists of volcanics, volcano-clastic rocks and granites of Permian age, rather than earlier Variscan assemblages. From their dominantly Permian protoliths, Gran Paradiso and other Piemonte massifs (Monte Rosa and Dora Maira), which are part of the Internal Crystalline Massifs, are very different in paleogeographic significance from the External Crystalline Massifs, which represent the European basement, where Permian magmatism is sparse. The Piemonte basement massifs may represent the easternmost edge of a Brianconnais – Grand-Saint-Bernard allochthonous terrane with more affinity to the Apulian basement than to the European basement.
We combined structural analysis, thermobarometry and oxygen isotope geochemistry to constrain the evolution of kyanite and/or andalusite-bearing quartz veins from the amphibolite facies metapelites of the Simano nappe, in the Central Alps of Switzerland. The Simano nappe records a complex polyphase tectonic evolution associated with nappe stacking during Tertiary Alpine collision (D1). The second regional deformation phase (132) is responsible for the main penetrative schistosity and mineral lineation, and formed during top-to-the-north thrusting. During the next stage of deformation (D3) the aluminosilicate-bearing veins formed by crystallization in tension gashes, in tectonic shadows of boudins, as well as along shear bands associated with top-to-the-north shearing. D2 and D3 are coeval with the Early Miocene metamorphic peak, characterised by kyanite + staurolite + garnet + biotite assemblages in metapelites. The peak pressure (P) and temperature (T) conditions recorded are constrained by multiple-equilibrium thermobarometry at 630 +/- 20 degrees C and 8.5 +/- 1 kbar (similar to 27 km depth), which is in agreement with oxygen isotope thermometry indicating isotopic equilibration of quartz-kyanite pairs at 670 +/- 50 degrees C. Quartz-kyanite pairs from the aluminosilicate-bearing quartz veins yield equilibration temperatures of 645 +/- 20 degrees C, confirming that the veins formed under conditions near metamorphic peak. Quartz and kyanite from veins and the surrounding metapelites have comparable isotopic compositions. Local intergranular diffusion in the border of the veins controls the mass-transfer and the growth of the product assemblage, inducing local mobilization of SiO2 and Al2O3. Andalusite is absent from the host rocks, but it is common in quartz veins, where it often pseudomorphs kyanite. For andalusite to be stable at T-max, the pressure in the veins must have been substantially lower than lithostatic. An alternative explanation consistent with structural observations would be inheritance by andalusite of the kyanite isotopic signature during polymorphic transformation after the metamorphic peak.
This study investigates the Tertiary migmatite belt of the Central Alps of Switzerland and Italy. Regional field relations are presented and, based on structural, textural and petrologic arguments, the spatial and age distribution of the Alpine migmatites are discussed. Alpine migmatites are almost entirely confined to the Southern Steep Belt (SSB), the regional-scale, transpressional shear-zone at the southern margin of the Central Alps. Migmatites surfacing in more northerly parts of the Lepontine area are derived from pre-Alpine, probably Variscan or older periods of partial melting, connected to the intrusion of bodies of granitic to quartz-dioritic composition, as well as mafic and granitic dykes. Except for the Bergell and Novate intrusives, Tertiary igneous activity in the Central Alps is limited to in-situ migmatisation and to the intrusion of aplitic and pegmatitic dykes and smaller (<50 m) granitoid bodies. Two processes contributed to the origin of this migmatite belt: (1) In the course of regional Barrovian metamorphism, water-assisted partial melting of granitoid rocks was induced in a large part of the southern Lepontine area, commonly leading to a maximum of 10-25 vol % total leucosome. (2) In pelitic rocks of the southeastern Lepontine area, a smaller leucosome-fraction is found, which is essentially a result of muscovite dehydration melting. Pressure-temperature conditions of partial melting estimated for amphibole-bearing leucosomes are 0.6-0.8 GPa and 700 +/- 50 degrees C, indicating mid-crustal partial melting. Thermally retentive chronometers set fairly tight limits for this event at 25-30 Ma. The spatial relationship between migmatites and SSB, as well as the styles of variable deformation in the leucosomes, indicate that partial melting and deformation were coupled processes. Observations suggest that the focused deformation in the SSB led to episodic injection of hydrous fluids, which in turn triggered water-assisted partial melting and associated strain partitioning into the "weak" partially molten rocks. Processes of partial melting in the migmatite belt appear to be continuous in time and space with the regional thermal history that produced upper amphibolite facies metamorphism without partial melting in adjacent areas to the north, i.e. outside the SSB.
The crystalline basement of the southern Calabrian-Peloritani nappe pile edifice (Aspromonte Massif and Peloritani Mountains) consists of several tectonic slices, which were stacked during the Miocene Apennine thrust sheet emplacement (Fig. 1). The two lowermost tectonic slices of this nappe-edifice, here called respectively Aspromonte-Peloritani Unit and Samo-Africo Complex, tectonically overlap along a thick Late Oligocene mylonitic shear zone, which involves both remnants of the Hercynian chain and Alpine metamorphic rocks (Figs. 1-2). A structural and petrological study was employed to reconstruct both the entire P-T evolution of the two units and the kinematics of orogenic transport, which characterise the Late Oligocene mylonitic stage. P-T estimates were made using an integrated approach derived from conventional thermobarometry, computation of metamorphic equilibria in the NaCaKFMASH system, and an analysis of the deformation behaviour of quartz, feldspar and garnet. The shear event marks the beginning of the joint structural and metamorphic history. Prior to this event, the two units had undergone very different tectonometamorphic evolutions. The upper unit (Aspromonte-Peloritani Unit) underwent pre-mylonitic HT-LP Variscan polyphase retrograde metamorphism (peak conditions 0.39-0.5 GPa; 650 degrees-675 degrees C). By contrast, the deeper Samo-Africo Complex underwent pre-mylonitic LT-HP metamorphism (peak conditions 0.98-1.2 GPa; 480 degrees-560 degrees C), linked to crustal thickening of an Early Alpine age. Syn-mylonitic P-T estimates confirm the beginning of the joint tectonometamorphic history, evolving from 500 degrees to 350 degrees C with P ranging from 0.775 to 0.325 GPa. This event, linked to Late Oligocene Alpine shearing, developed during the uplift and exhumation of the crystalline basement rocks along a deep-seated compressional shear zone. The above results lead us to suggest a new geodynamic scenario for the southern sector of the Calabrian Peloritani Orogen: Following subduction of the Aspromonte-Peloritani Unit and the Samo-Africo Complex these two units were extruded along the collisional suture. In this tectonic reconstruction, the Samo-Africo Complex is interpreted as a post-Variscan sedimentary succession involved in a complete Alpine orogenic cycle, consisting of an Early Alpine crustal thickening stage and a Late Alpine Africa-verging compressional shear stage.
Calc-schists of the Mesozoic metasedimentary units in the Central Alps, between the Simplon fault, the Bergell intrusion, the Gotthard massif and the Insubric line, are investigated to assess their metamorphic evolution and to gain a better understanding of scapolite stability and its phase relations. These rocks have only experienced the Mesoalpine metamorphic event. The abundance of silicate minerals in the calc-schists results in a series of characteristic mineral assemblages with increasing metamorphic grade. In the outer part of the Central Alps at lower amphibolite facies conditions, mineral assemblages containing clinozoisite and plagioclase are diagnostic. In areas of intermediate metamorphic grade, clinozoisite and plagioclase are replaced by scapolite-bearing assemblages and K-feldspar begins to form. In the upper amphibolite facies region around Bellinzona clinopyroxene is observed as an additional phase within the scapolite-bearing assemblages. The evolution of mineral assemblages observed from the outer to the central part of the Central Alps can also be identified within individual higher grade rocks. Phase diagram calculations show a good agreement with the observed mineral assemblages and their evolution, helping to validate the new thermodynamic model for scapolite that was used. They allow estimation of the temperature of peak metamorphism. However, the thermodynamic modelling leads to the conclusion that such carbonate-bearing rocks are not well suited to constrain the pressure of metamorphism.
Mafic rocks containing eclogite relics are fairly widespread in the crystalline nappe stack of the Swiss Central Alps. This study addresses the spatial distribution of eclogite relics in the Central Alps, their field relations, structural and petrological characteristics, and their PTt-history. Implications for the assembly of the nappe stack are explored.The majority of eclogite-facies relics is confined to a single super-unit of tectonic melange, interpreted as a tectonic accretion channel (TAC). Numerous mafic high-pressure (HP) lenses have been discovered through systematic fieldwork in the TAC units of the Central Alps, an up-to-date inventory of which is presented. Systematic documentation of select samples with HP imprint yields clockwise PT-paths. Prograde phase relations are seldom preserved, except in the chemical zoning of garnet porphyroblasts. However, when present, relic assemblages indicate HP-LT conditions indicative of a subduction setting. Maximum recorded pressures are substantially different from one location to the next (1.9 to 3.3 GPa). Depending on the degree of rehydration, reaction sequences are derived from observed relics, local replacement relations and assemblages. Quantitative constraints on the detailed PT-path are extracted by combining isochemical phase diagrams and TWQ-thermobarometry with petrographic information. HP lenses from different locations display substantially different paths, both within and between different melange zones of the TAC. PT-conditions reflecting the late-Alpine Barrovian overprint of mafic HP lenses are in agreement with the coherent regional pattern derived from metasediments, i.e., maximum temperatures (similar to 600 degrees C in the central Lepontine belt, 700-750 degrees C in the southern parts) were reached at pressures between 0.75 and 0.55 GPa.Four samples have been dated by Lu-Hf isotopic analysis of garnet, clinopyroxene, matrix phases and whole-rock powders. The age span covers a range from >70 to similar to 36 Ma, much larger than previously documented for Alpine HP rocks from the Central Alps. Petrological data of the samples and their Lu-Hf isotopic system indicate a protracted HP history for at least some of the sub-units of the TAC, with garnet growth under eclogite-facies conditions starting before 70 Ma in some parts of the TAC, and continuing as late as 36 Ma in others.These data have implications for the dynamics of melange formation within the TAC, with internal fragmentation and mixing, and pronounced mobility of the tectonic zones, probably during the early, subductional stages and again during the post-collisional extrusion along the plate boundary. After 32 Ma, when the Barrovian overprint reached its maximum temperature, the TAC appears to have been exhumed as part of the then-coherent crystalline nappe stack.
Ordovician magmatism can be observed in the whole Veporic unit. New single and multi-grain zircon U–Pb dating reveal an intensive magmatic activity between 470–460 Ma and a minor magmatic phase around 440 Ma in the Northern Veporic Unit. Carboniferous ages of SHRIMP and single grain TIMS U–Pb dating in this unit document intense metamorphism and magmatism during Variscan orogeny (350–330 Ma). Previously published multi-grain U–Pb data in the same area have been interpreted to show Cambrian precursor ages. A new interpretation of this multi-grain data is suggested, revealing an Ordovician (464 ± 23) precursor age and a Carboniferous metamorphic overprint. Together with previously published data on the Southern Veporic Unit and the data presented in this study, a common Ordovician precursor age for the whole Veporic unit is recognized. The isotopic signature for the Pre-Variscan basement in the Veporic unit is distinct from published data for the Tatric unit (Gneisses: Sr/Srini,460 between 0.712 and 0.715; Ndmeas,460 between –8 and –10. Amphibolites: Sr/Srini,460 between 0.708 and 0.710: Ndini,460 between 4.5 and 5.6). Only the Kralova Hola granites with a Variscan magmatic intrusion age between 330–360 Ma reveal distinct signatures and show similarities with diorites and granites from the High Tatra Mountains with Sr/Srini,330 of 0.704 and 0.706 and Ndini,330 of –0.5 and –1.7.
This work characterizes the typomorphic features, i.e. crystal chemical and spectroscopic parameters, of different amazonite generations from Precambrian granitic pegmatite (Western Keivy, Kola Peninsula, Russia). The crystals of various generations and colors (blue, greenish-blue and green) were investigated using X-ray fluorescence and X-ray diffraction, UV-VIS-NIR, infrared spectrometry, electronic paramagnetic resonance (EPR), UV-fluorescence, heating and irradiation experiments. From the peripheral zones to the central part of the pegmatite, amazonite changes its composition systematically in terms of some major (e.g., K2O) and trace components (e.g., Rb, Cs, Pb, H2O) can be seen to increase in the amazonite samples. The later generations of amazonite contain the maximum concentrations of U (23 ppm) and Th (67 ppm). The amazonites are characterized by a high degree of Al/Si order. All the parameters obtained from X-ray diffraction (Delta r, Delta t > 0.9) as well as optical microscopy (2V > 80 degrees) show that the studied amazonites belonged to the maximum ordered microcline. The nature of the amazonite color is explained by the presence of hole-electron centers of Pb and Al and structural centers of Fe-IV(3+) that cause the corresponding absorption bands of 380 nm (Al-O--Al; Fe3+), 625 nm (Pb+) and 740 nm (O--Pb) in the UV-VIS-NIR spectra. The EPR spectra show that the signals of Pb centers disappear at the same time as the absorption of characteristic bands after heating: hole Pb centers at approximately 280 degrees C, and electron Pb centers at 450-500 degrees C. The colors of all the treated samples were intensified by irradiation because of the formation of Pb and Al hole centers. After X-ray irradiation the intensities of the bands at 740 nm and 380 nm increased. The most typical features of the amazonitic K-feldspar in the Precambrian pegmatite are seen in the UV-VIS-NIR and EPR spectra and are related to the form of electron-hole paramagnetic centers. The crystal chemical and spectrometric parameters of amazonite may be important for the exploration of diverse deposits of rare metals and REE mineralizations and their economic evaluation.
22 new apatite fission track (Fr) ages from the Adula nappe are used to gain insight into the late-stage thermal and exhumation history of this tectonic unit and the adjacent Tambo nappe. Apatite FT ages vary from middle Miocene to Pliocene, with generally long track lengths and slightly skewed but otherwise narrow track length distributions. The FT data do not show any horizontal gradient in exhumation, which would be compatible to a N-S tilting of the nappe such as may have been expected from the strong PT gradient found in the pre-Mesoalpine eclogite occurrences or from the Mesoalpine metamorphic gradient. Furthermore, no E-W tilting has been detected, such as could have been assumed as a result of late-stage updoming of the Lepontine dome to the west. Only the southernmost samples, close to the Insubric line, show relatively old ages (8-11 Ma), and track length data indicate continued fast exhumation close to the Insubric line up to that time. The area of youngest FT ages coincides with the highest pressure values as derived from Mesoalpine mineral assemblages, suggesting that the middle Adula nappe has undergone slightly increased exhumation and by this process has modified the present-day pressure contours of the eastern Lepontine dome. Apatite FT ages along the sediment zone between Adula and overlying Tambo nappe (Misox zone) contrast with ages found at the Splugen sediment zone between Tambo and Suretta nappe further E. They are interpreted as the result of late-stage normal faulting between Adula and Tambo nappe with local heating of the Tambo nappe basis. A comparison between valley bottom samples in the Adula nappe reveals that the sampled valleys show distinct age patterns which illustrate the recent relief evolution and suggest that the Alpine water divide has moved to the N during the Miocene. Accordingly, the valley formation and its influence on the uppermost crustal isothermal planes have visibly influenced the apatite age pattern and provide an explanation for part of the scatter in apatite FT ages observed within the Adula nappe.
This paper accompanies the new map sheet (1:100'000) of the Swiss-Italian Central Alps, introducing the tectonic units in condensed form. The map sheet "Sopra Ceneri" of this core part of the Alps integrates a wealth of classical field studies from the Lepontine Alps with numerous detailed investigations made over the past decades. In the present notes the main tectonic units distinguished on the map are individually characterized, and the dominant rock types surfacing are described. All of the main units that make up the crystalline nappe stack of the Central Alps are comprised in this inventory. Their polyphase deformation is reviewed, and the metamorphic structure exposed in this classic orogen is presented. Particular emphasis is placed upon recent work on the exposed nappe- and plate-boundaries, inasmuch as these results bear directly on the map. The recognition of several tectonic melange units, their internal make-up and their role within the orogen are key elements of this study. Implications on the tectono-metamorphic evolution of the Central Alps are outlined, and some of the main controversies and remaining questions are spelled out. The map and this condensed text thus assemble an Up-to-date introduction to the geology of the Central Alps.
