Post-collisional tectonics in the Northern Cottian Alps (Italian Western Alps)

Field mapping and structural analysis have allowed us to characterise the fault geometry and the post-metamorphic tectonics of an area located in the Northern Cottian Alps (inner Western Alps). Two main faulting stages were distinguished here. The first (Oligocene?-Early Miocene) is related to the development of an E–W-striking left-normal shear zone. This shear zone is interpreted as an antithetical of two regional, N–S right-lateral structures: the Col del Lis-Trana Deformation Zone (LTZ) and the Colle delle Finestre Deformation Zone (CFZ). The second faulting stage (post-Early Miocene) is related mainly to the development of N–S normal faults, coeval with the extensional reactivation of the LTZ and the CFZ. We discuss this kinematic evolution in the framework of the geodynamic evolution of the Western Alps.     

From granulites to eclogites in the Sesia zone (Italian Western Alps): a record of the opening and closure of the Piedmont ocean

The Sesia zone (Italian Western Alps) offers one of the best preserved examples of pre-Alpine basement reactivated, under eclogite facies conditions, during the Alpine orogenesis. A detailed mineralogical study of eclogitized acid and basic granulites, and related amphibolites, is presented. In these rare weak to undeformed rocks microstructural investigations allow three main metamorphic stages to be distinguished.
(a) A medium- to low- P granulite stage giving rise to the development of orthopyroxene + garnet + plagioclase + brown amphibole + ilmenite ± biotite in basic granulites and garnet + K-feldspar + plagioclase + cordierite + sillimanite + biotite + ilmenite in acid granulites.
(b) A post-granulite re-equilibration, associated with the development of shear zones, producing discrete amphibolitization of the basic granulites and widespread development of biotite + sillimanite + cordierite + spinel in the acid rocks.
(c) An eo-Alpine eclogite stage giving rise to the crystallization of high- P and low- T assemblages.
In an effort to quantify this evolution, independent well-calibrated thermobarometers were applied to basic and acid rocks. For the granulite event, P-T estimates are 7–9 kbar and 700–800° C, and for subsequent retrograde evolution, P-T was 4–5 kbar and 600° C. For the eo-Alpine eclogite metamorphism, pressure and temperature conditions were 14–16 kbar and 550° C.
The inferred P-T path is consistent with an uplift of continental crust produced by crustal thinning prior to the subduction of the continental rocks. In the light of the available geochronological constraints we propose to relate the pre-Alpine granulite and post-granulite retrograde evolution to the Permo-Jurassic extensional regime. The complex granulite-eclogite transition is thus regarded as a record of the opening and of the closure of the Piedmont ocean.     

Glaucophanites and eclogites from Val Chiusella,Sesia-Lanzo zone (Italian Alps)

In the metabasites of Val Chiusella, metamorphic assemblages are present, corresponding to the glaucophane schist facies, i.e. garnet glaucophanites to omphacite-garnet glaucophanites, as well as to the eclogite facies, i.e., glaucophane eclogites, eclogites, and omphacite felses. Both groups of assemblages are divided by the critical reaction 1 zoisite +1 glaucophane 1.2 omphacite+0.8 garnet+0.7 paragonite +1.4 quartz+0.8 H₂O. From textural evidence it is clear that in the investigated area this reaction proceeded to the right according to a prograde metamorphism. Correspondingly, K ^garn-cpx _D(Fe/Mg) values of coexisting garnet-omphacite pairs in the glaucophane schist facies assemblages are higher than in the eclogite facies assemblages and reflect a temperature increase from about 450 ° C to about 550 ° C at minimum water vapour pressures of 12 to 16 kb.     

Reaction Textures and Metamorphic Evolution of Sapphirine-bearing Granulites from the Gruf Complex, Italian Central Alps

Mineral chemistries and textures are described from a suiteof sapphirine-bearing granulites from the Gruf Complex of theItalian Central Alps. The granulites contain combinations ofgarnet, orthopyroxene, sapphirine, sillimanite, cordierite,biotite, quartz, spinel, corundum, staurolite, plagioclase,K-feldspar, ilmenite and rutile, in assemblages with low (usuallynegative) variance. They are outstanding in that they preservea textural and chemical record of a protracted metamorphic evolution. Reaction textures are common and include: (i) pseudomorphs (e.g.of sillimanite after kyanite); (ii) relatively coarse-grainedmonomineralic reaction rims (e.g. of cordierite between sapphirineand quartz); (iii) fine-grained symplectitic coronas (e.g. oforthopyroxene + sapphirine round garnet); (iv) inclusions, ingarnet cores, of minerals (e.g. staurolite) not found elsewherein the rocks. Detailed microprobe study has revealed large chemical variationswithin each phase. Different textural types of each phase havedifferent compositions, and strong zoning is preserved in garnet(Mg/(Mg + Fe) from 0.30 to 0.61) and coarse sapphirine. Inclusionpopulations in garnet correlate with host composition. The textural and chemical features are interpreted in termsof successive equilibrium assemblages and reactions. Metamorphicconditions operative at each stage in the evolution are calculatedusing published geothermometers and geobarometers as well asthermodynamically calibrated MAS and FASH equilibria. The resultsare used to construct a P—T-time path for the sapphirine-granulites,which can be summarized as follows: (i) Increasing T at high P (>7 kb). Partial melting. (ii) A maximum T of 830 ?C attained at 10 kb. (iii) Almost isothermal decompression, reaching 750 ?C at 5kb, under conditions of low µ_H2O. (iv) Further cooling, and decompression. Localized hydration.Rocks exposed. The P—T-time path is interpreted as the product of a singlemetamorphic cycle (the tertiary ‘Lepontine’ event)and is extrapolated to the Gruf Complex as a whole. When combinedwith published geochronological data, the results indicate anaverage uplift rate in excess of 2 mm/yr for the Gruf Complexbetween 38 and 30 Ma ago. An in situ partial melting origin for the sapphirine-granulitesis favoured. Extraction of an iron-rich granitic liquid froma normal pelitic palaeosome could generate a refractory residuewith the required Mg, Al-rich composition. The change in bulksolid composition during partial melting is thought to accountfor the extraordinarity strong zoning in the garnets.     

Geotectonic significance of the metabasites of the Kinzigitic Series,Ivrea-Verbano zone (Western Italian Alps)

Summary Geochemical investigations have been carried out on the metabasites of the Ivrea-Verbano Zone (NW-Italy). The Ivrea-Verbano Zone is commonly considered to be a section through the lower continental crust. It is constituted by a steeply dipping sequence of metamorphic rocks (Kinzigitic Series), intruded by a mafic-ultramafic complex.The metabasites of the Kinzigitic Series have a meta-igneous origin. Their protoliths show tholeiitic affinity in the NW and central part of the zone and alkalic affinity in the SE. This fact together with the rock associations suggests that a variation of the paleogeographic environment from NW to SE occurs in the Ivrea-Verbano Zone.

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Geotektonische Bedeutung der Metabasite in den Kinzigiten der Ivrea-Verbano Zone (Italienische West-Alpen)

Zusammenfassung Die Ivrea-Verbano-Zone wird im allgerneinen der unteren kontinentalen Kruste zugeordnet. Es handelt sich dabei um eine sehr steil stehende metamorphe Serie (Kinzigit-Serie), die von elnem mafischen-ultramafischen Komplex intrudiert wurde.Die geochemischen Untersuchungen der in der Kinzigit-Serie vorkommenden Metabasite zeigen einen tholeiitischen Chemismus im nordwestlichen und im zentralen Bereich und einen alkalinen im südöstlichen Bereich.Die chemische Entwicklung dieser Magmatite im Zusammenhang mit den unterschiedlichen Nebengesteinen weisen auf eine deutliche Änderung des paläogeographischen Milieus in der Ivrea-Verbano-Zone von Nordwesten nach Südosten hin.

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With 8 Figures     

Geochemistry of micas from the Finero spinel-lherzolite,Italian Alps

The Finero lherzolite is distinct amongst the tectonically emplaced slices of mantle in the Ivrea Zone (Italian Alps) for its abundant coarse phlogopite. An average composition (SiO₂ 39.9, TiO₂ 0.97, Al₂O₃ 16.0, Cr₂O₃ 1.16, FeO 2.73, MgO 24.5, NiO 0.16, BaO 0.31, Na₂O 0.58, K₂O 8.7, Rb₂O 0.056, Cl 0.03, F 0.10 wt.%) is similar in Fe, Cr, Ni, Ba and F/Cl to primary-textured micas from coarse garnet-lherzolite xenoliths from S. Africa, but is higher in Ti, Na, Rb, and Al, and lower in halogens. The distinct values of Ti and Fe for five specimens of Finero peridotites demonstrate local spatial variation. The overall ranges of TiO₂ (0.5–1.7) and FeO (2.3–3.6) fall within the range for secondary-textured micas in peridotite xenoliths from S. Africa. The Finero micas are lower in both K/Rb and K/Ba than the primary and secondary micas from S. Africa, and their mean values of K/Rb (110–220) and K/Ba (15–39) are lower than for almost all bulk rocks, but fit well with the ranges of 109–180 and 12–49 for the high-K lavas of the Roman region.Although all evidence is indicative rather than conclusive, the chemical properties of the Finero micas are consistent with introduction of an alkaline phase into peridotite during or before emplacement of the Finero complex from the upper mantle into the crust, and the coarse, partly-deformed textures can be explained by incomplete metamorphic equilibration during prolonged deformation. The alkaline phase is tentatively attributed to the uppermost mantle.     

Slope stabilization in difficult conditions: the case study of a debris slide in NW Italian Alps

A case study of a debris slide (estimated volume of about 35,000 m³) is described in this paper. This slide occurred in April 2009 in the North Western Italian Alps (Aosta valley) and damaged the SR25 road along the Valgrisenche valley. Ground investigations started with severe safety and logistic issues being posed. Given the need to open as soon as possible the road, the design of the landslide stabilization works was carried out using a “design as you go” approach. The stabilization measures were conceived to be flexible in order to allow for changes and integration during construction, in line with the progressive refinement of the geological–geotechnical slope model being developed. Back analysis by means of the limit equilibrium method (LEM) and the finite element method (FEM) was used. Groundwater level rise following heavy rainfall and spring snow melting was found to be the main cause of the debris slide. The stabilization works were designed by using both the LEM and FEM methods. The stability conditions of the engineered slope were assessed based on the available performance monitoring data.     

Lithostratigraphic setting and P-T metamorphic evolution for the Dora Maira Massif along the Piedmont Zone boundary (middle Susa Valley,NW Alps)

A new structural and petrological study, associated with a detailed geological mapping, allowed to better understand the tectonic relationships between the Dora Maira Massif and the Piedmont Zone in the middle Susa Valley. In the study area, a thick sequence of calcschists, previously attributed to the stratigraphic cover of the Dora Maira, is interpreted as belonging to the Piedmont Zone. Four deformation phases were identified: D₁ represents the eclogite facies stage, D₂ developed under greenschist facies conditions, transposed the early foliation and was responsible for the development of the regional schistosity S₂. D₃ is characterized by close to open folds with N dipping axial surfaces and finally, D₄ developed macro-scale folds with E-dipping axial planes. Structural analyses allow to infer a relative timing for the tectonic contact between the two nappes which were coupled after the eclogite facies metamorphism but before the development of the S₂ foliation under greenschist facies conditions. Petrographic investigation into metapelite samples permitted to identify two main metamorphic assemblages within the Dora Maira polymetamorphic basement: M₁ (Phe + Pg + Cld + Grt + ChlI + Qtz + Rt) assemblage defines the S₁ relict foliation, while M₂ (Ms + Pg + Ab + ChlII + Qtz + Ilm ± Bt) assemblage is related to the regional foliation S₂. PERPLE_X pseudosection modelling allowed to reconstruct a P-T path for a garnet–chloritoid-bearing micaschist from the northern part of the Dora Maira Massif, which reached eclogite facies conditions at 18–20 kbar and 515–525°C (M₁/S₁ event) and then was exhumed during increasing T (10–11 kbar and 555–565°C). The M₂ assemblage defining the S₂ regional foliation developed at P < 7 kbar and T < 575°C. According to the structural evolution, the tectonic coupling between Dora Maira and Piedmont Zone took place during exhumation along the subduction channel.     

Zircon inheritance in an igneous rock suite from the southern Adamello batholith (Italian Alps)

Zircons from a suite of basic to acidic calcalkaline igneous rocks from the southern Adamello batholith (S AB), Southern Alps, N Italy, display complex U–Pb isotopic patterns which are mainly due to the presence of variable amounts of isotopically heterogeneous, inherited radiogenic Pb, and to minor postmagmatic loss of Pb. Inherited Pb is mainly composed of 1) a 1100 Ma Pb component located in zircons devoid of visible cores and 2) a 450 Ma component associated with conspicuous bubble-rich turbid cores. In concordia representation the data points conform to lower intercept ages of 40 Ma. A linear fit of three samples devoid of visible cores from the granodiorite defines an intrusion age of 39.3 Ma. U–Pb systematics of zircon (in particular U content) and crystal morphology are clearly related. Zircons of type G1, which form relatively late in the zircon crystallization sequence, consistently show the highest U contents in each zircon population. These late zircons, however, are not devoid of inherited radiogenic lead. In a population from a granodiorite, cores are randomly distributed throughout the morphological spectrum. The presence of old inherited zircon components in all investigated samples furnishes proof for involvement of crustal material in the genesis of the S AB rocks. Samples characterized by crustal _Nd and _Sr values usually show enhanced zircon inheritance. Inheritance varies with differentiation and reaches a maximum for intermediate to acidic members; these compositions show the highest Zr saturation temperatures calculated for the rock spectrum studied. Textural relations between zircon and major phases indicate that the magmas of the leucocratic rocks were saturated with Zr at an early stage of crystallization. On the other hand, Zr solubilities and textural relations consistently show, that melts of basic to intermediate rocks were not saturated with Zr. Extension of the Zr solubility model to mineral/melt mixtures of tonalitic bulk composition demonstrates that Zr solubility in the residual melt is drastically reduced by crystallization of plagioclase and amphibole. Survival of xenocrystic zircons in the mafic to intermediate rocks of the S AB can best be explained in terms of dissolution kinetics. Since temperature and H₂O content of these Zr-undersaturated melts were favourable for relatively rapid zircon dissolution, inherited zircons (in particular trace-element rich unstable cores) cannot have been exposed to such conditions over extended time periods. Therefore, the tonalitic or more basic magmas of the S AB cannot have been derived from crustal sources by slow processes such as burial metamorphism.     

Petrological evolution of the Middle Triassic Predazzo Intrusive Complex,Italian Alps

The Predazzo Intrusive Complex (PIC), a Ladinian plutonic body located in the Southern Alps (NE Italy), is made up of a 4.5 km³ gabbroic to syenitic and syenogranitic intrusion, basaltic to latitic volcanic products (about 6 km³ in volume) and by an extended dike swarm intruding both intrusive and volcanic rocks. An extensive field survey of the complex, followed by detailed petrographic and geochemical analyses, allowed the identification of three different magmatic units: a Shoshonitic Silica Saturated Unit (SS), 3.1 km³ in volume, a Shoshonitic Silica Undersaturated Unit (SU), 0.3 km³ in volume, and a Granitic Unit (GU), 1.1 km³ in volume. K-affinity, marked Nb and Ti negative anomalies and a strong Pb enrichment are distinctive markers for all PIC lithotypes. A general HFSE (Th, U, Pb), LREE (La, Ce, Pr, Nd) and Na enrichment characterizes the SU suite with respect to the SS series. Mass balance calculations, based on major and trace element whole rock and mineral compositions, have been used to simulate the fractionation process of SS and SU suites, showing (i) the complexity of the evolutionary stages of the PIC and (ii) the analogy between the calculated subtracted solid assemblages and the natural cumulitic lithotypes outcropping in the area. The field relationships between the various portions of the intrusive complex, the volcanic products and the dike swarm define the temporal evolution of the PIC, in which the SS magma batch was followed by the GU and later on by the SU intrusion. The presence, in both eastern and western portions of the complex, of a transitional magmatic contact between the intrusive rocks of the SS suite and the volcanics is not in favour of the hypothesis of a caldera collapse to explain the ring-like shape of the PIC.     

New Constraints on the P-T Evolution of the Alpe Arami Garnet Peridotite Body (Central Alps, Switzerland)

The metamorphic evolution of the garnet peridotite body of AlpeArami, Central Alps, is a matter of current controversy. Inthis paper, the inter- and intragrain distribution of majorand trace elements obtained by electron and ion probe microanalysesis used to better constrain the P–T evolution of thisperidotite. Using the compositions of homogeneous porphyroclastcores, peak metamorphic conditions of 1180 ± 40°Cand 5·9 ± 0·3 GPa are estimated, basedon consistent results from the application of several independentthermometers (Fe–Mg exchange between garnet, pyroxenesand olivine, Ni exchange between garnet and olivine, Co andNi exchange between orthopyroxene and clinopyroxene), the Al-in-orthopyroxenebarometer and the Ca–Cr systematics of garnet. Orthopyroxeneand clinopyroxene porphyroclasts are, however, not in equilibriumwith respect to some elements with low diffusivities, such asCa, Ti, Cr, V and Sc. This disequilibrium appears to be themain cause for the lower P–T values suggested by someof the previous workers. On the other hand, there is no evidencefor an ultradeep (>200 km) origin of the Alpe Arami bodyas postulated recently. Chemical zonation profiles across mineralgrains suggest that during retrograde evolution a near-isothermaldecompression was followed by accelerated cooling. KEY WORDS: Alpe Arami; Central Alps; garnet peridotite; ultrahigh-pressure metamorphism; geothermobarometry; secondary ion mass spectrometry (SIMS)     

Brittle–ductile–brittle deformation during cooling of tonalite (Adamello, Southern Italian Alps)

Late- to post-magmatic deformation in slightly diachronous contiguous intrusions of the north-western Adamello batholith (Southern Alps, Italy) is recorded as, from oldest to youngest: (i) joints, (ii) solid-state ductile shear zones, (iii) faults associated with epidote-K-feldspar veins and (iv) zeolite veins and faults. Structures (ii) to (iv) are localized on the pervasive precursory network of joints (i), which developed during the earliest stages of pluton cooling. High temperature ( 500 °C), ductile overprinting of joints produced lineations, defined by aligned biotite and hornblende, on the joint surfaces and highly localized mylonites. The main phase of faulting, producing cataclasites and pseudotachylytes, occurred at  250 °C and was associated with extensive fluid infiltration. Cataclasites and pseudotachylytes are clustered along different E–W-striking dextral strike-slip fault zones correlated with the activity of the Tonale fault, a major tectonic structure that bounds the Adamello batholith to the north. Ductile deformation and cataclastic/veining episodes occurred at P = 0.25–0.3 GPa during rapid cooling of the batholith to the ambient temperatures ( 250 °C) that preceded the exhumation of the batholith. Timing of the sequence of deformation can be constrained by ³⁹Ar–⁴⁰Ar ages of  30 Ma on pseudotachylytes and various existing mineral ages. In the whole composite Adamello batholith, multiple magma pulses were intruded over the time span 42–30 Ma and each intrusive body shows the same ductile-to-brittle structural sequence localized on the early joint sets. This deformation sequence of the Adamello might be typical of intrusions undergoing cooling at depths close to the brittle–ductile transition.     

边坡稳定性温度效应数值模拟研究

气候变化引起环境温度改变,岩土体的性质随之产生相应变化。文章分析了温度对边坡稳定性产生的影响,于不同 温度状况下开展粘性土直剪试验,探究粘聚力及内摩擦角随温度变化的特征。进一步利用有限元软件ANSYS 结合实测地温 数据模拟边坡温度场演变过程,基于试验结果将温度场推导出粘聚力场,利用瑞典条分法结合粘聚力分布特征求得边坡安 全系数。通过一系列室内试验发现,下蜀土粘聚力受温度影响变化规律较为明显;土体含水量和密度不同使土体随温度升 高相应的产生热软化或热固结现象。通过ANSYS 模拟南京城郊边坡夏季高温期间温度场的演变,推算出了边坡安全系数的 变化规律。结果表明,对于高含水量的土体,在夏季高温环境下边坡稳定性降低,而城区温度更高相应边坡的安全系数通 常更低,高温对边坡稳定性具有负面作用。     

Local shear zone pattern and bulk deformation in the Gran Paradiso metagranite (NW Italian Alps)

The Gran Paradiso nappe of the northwestern Alps mostly consists of augen gneisses derived from the Alpine deformation of Permian granitoids. The regional foliation of the augen gneisses developed at lower amphibolite facies conditions and is associated with a top-to-west sense of shear. The granitoid protolith is preserved in the kilometre-scale low-strain domain of the Piantonetto Valley and mainly consists of a porphyritic metagranite including joints, leucocratic dykes and biotite-rich schlieren. In this low-strain domain, the Alpine deformation is mainly localized in discrete ductile shear zones within weakly foliated metagranite. The shear zones mostly dip towards S–SE in a shallow (shear zones ₁) to steep inclination (shear zones ₂). The shear zones show typical features that can be explained by reactivation of pre-existing joints and planar compositional heterogeneities. Palaeostress and strain analysis indicate that shear zones and the metagranite foliation both formed in the presence of a strong component of flattening. The kinematics of individual shear zones depends on the orientation of the original heterogeneities (acting as nucleation planes) and by partitioning of strain components at the kilometre-scale with concentration of the flattening component to the Piantonetto low-strain domain. The strain geometry and the kinematics of individual shear zones within Piantonetto are not directly connected to the top-to-west sense of tectonic transport observed elsewhere in the Gran Paradiso nappe. However, the bulk stress ellipsoid reconstructed for the incipient shear zone network within very weakly deformed granites is oriented consistently with the bulk direction of tectonic transport within the Gran Paradiso massif. We conclude that the shear zone network of the Piantonetto Valley is representative of the incipient stages of ductile deformation of a granite nappe. Even if its architecture is determined by the arrangement of pre-existing structural and compositional heterogeneities, aspects of the large-scale bulk strain can be derived from this local shear zone pattern.     

Pyroxenes and olivines as indicators of the petrological evolution of the Ivrea-Verbano basic formation (Italian Western Alps)

Summary Microprobe analyses of olivines, orthopyroxenes and clinopyroxenes of the Ivrea-Verbano basic formation reveal that in the Balmuccia Periodotites (regarded as residual mantle) the phases are more magnesian than in the overlaying ultramafic-mafic series (interpreted as a cumulitic complex formed by fractionation of mantle-derived magma).Pyroxenes underwent sub-solidus re-equilibration. Whole phase (host + exsolved lamellae) compositions give a temperature in the range 946°C–1236°C, compatible with the igneous crystallisation. The compositions of the host phases give temperatures in the range 712°C to 919°C, indicating that the samples did not attain the same degree of equilibrium during the sub-solidus unmixing. The sub-solidus unmixing of the pyroxenes may have occurred either during the superimposed granulite-facies metamorphism or during the slow cooling of the complex intruded into deep crustal levels.

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Pyroxene und Olivine als Indikatoren der petrologischen Entwicklung der basischen Formation von Ivrea-Verbano (Italienische Westalpen)

Zusammenfassung Mikrosondenanalysen von Olivinen, Orthopyroxenen und Klinopyroxenen der basischen Formation von Ivrea-Verbano zeigen, daß in den Balmuccia Peridotiten (die als residuales Mantelmaterial angesehen werden) die Phasen Mg-reicher sind als in den überlagernden ultramafischen bis mafischen Serien (die als Kumulationskomplex gedeutet werden, der durch Fraktionierung eines aus dem Mantel stammenden Magmas gebildet wurde).Die Pyroxene wurden unter Subsolidus-Bedingungen reequilibriert. Die Zusammensetzungen der Gesamtphasen (Wirt + Entmischungslamellen) geben Temperaturen im Bereich von 946–1236°C, was mit einer magmatischen Kristallisation verträglich ist. Die Zusammensetzungen der Wirt-Phasen geben Temperaturen im Bereich von 712–919°C an, was anzeigt, daß die Proben während der Subsolidus-Entmischung nicht denselben Grad der Equilibirierung erreichten. Die Subsolidus-Entmischung der Pyroxene kann entweder während der überlagerten Metamorphose in Granulitfazies oder während der langsamen Abkühlung des in tiefe Krustenschichten intrudierten Komplexes geschehen sein.

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With 4 Figures     

Deformation and metamorphism at the South-margin of the Alps,East of Bellinzona,Switzerland

Deformation and metamorphism of rock series at the South-margin of the Alps, E of Bellinzona, are studied. Magmatic relics and metamorphic Minerals and rock types of the Tonalite and of the Augengneiss of Melirolo are examined. These bodies are regarded as of magmatic origin. Zoning of plagioclase is ascribed to rise of melts and cooling under loss of volatiles. Subvolcanic dykes branch off the Augengneiss into the Tonale Series. The magmatics are late-alpine and intruded into the Southerly cover plate on top of the subduction zone. The cover plate was still cold and unaffected by alpine deformation during this intrusion. Later deformation and heating of cover plate and magmatics is late-alpine. The Tonalite-deformation was quantified by a study of xenolith-deformation. The Tonalite was flattened to 1/20 of its original width. We extrapolate this flattening to the Southerly steep zone, now 5 km broad. We conclude that this zone was 100 km wide befor late-alpine flattening. This deformation occurred during the higher T part of lepontine heat-rise. A maximum T of ca 580 C and pH₂0 5–6 kbar is reached after the main deformation. Considerable stretching upward buried the niveau of intrusion, formerly shallow, under 15–20 km of rocks. The Tonale Series suffered 3-fold rotational refolding under subsiding T after the main deformation. An early-alpine subduction eclogite was found imbricated into the Tonale Series. It still contains parts of the subduction paragenesis preserved or pseudomorphosed and relics of subduction preferred orientation. Under late-alpine deformation it is changed into amphibolite. This proves continuation of subduction from Val Sesia to our area and further E. The magmatics are interpreted as subduction-magmatites. They cannot be derived as partial melts from the subducted slice which underlies the Tonale Series, from which the eclogite was imbricated into the Tonale Series during late-alpine movements.Voll has found a more Northerly subduction zone (Adula Nappe, Misox Zone). This may have reached our area in sufficient depth to produce partial melts before late-alpine flattening of the marginal zone. Last movements at the Insubric Line formed blastomylonites at 300 C indicating vertical movements. In our area lepontine garnet-staurolite-kyanite-schists abut directly against the Insubric line and the Seealpen. Lower T areas towards the S are cut off by the Insubric Line.

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Zusammenfassung Die Arbeit studiert und korreliert Deformation und Metamorphose der Gesteinsserien am Alpen-S-Rand E von Bellinzona. Magmatische und metamorphe Minerale von Tonalit und Augengneis von Melirolo werden erfa\t, ferner Gesteinstypen ausgegliedert. Beide Körper werden als magmatisch angesehen. Zonierung im Plagioldas wird Aufstieg von Schmelzen und Abkühlung unter Entgasung zugeschrieben. Vom Augengneis nach S abzweigende Subvulkanit-GÄnge wurden gefunden. Die Magmatite sind jungalpin in den alpin noch nicht deformierten, kalten Teil der südlichen Deckelplatte über der Subduktionszone eingedrungen. Ebenfalls noch jungalpin werden dann Magmatite und Deckelplatte deformiert und geheizt. Quantifizierung der Tonalit-Deformation mittels Schollen ergibt PlÄttung auf im Mittel 1/20 der Ausgangsbreite. Diese PlÄttung wird auf den heute 5 km breiten, steilen Alpen-S-Rand extrapoliert, damit für diesen eine Breite von 100 km vor der jungen Einengung ermittelt. Die Deformation fÄllt in den Hoch-T-Teil des aufsteigenden Metamorphose-Astes der Lepontin-Heizung. Das T-Maximum bei ca. 580 C, 5–6 kbar, überlebt die Deformation. Hochdehnung hat das erst seichte Inlrusionsniveau betrÄchtlich belastet. 3fache Wiederfaltung in der Tonale-Serie erfolgt rotational, meist S-vergent, bei fallender T. In der Tonale-Serie wurde ein eoalpiner Subduktions-Eklogit gefunden, der eoalpine Paragenesen und Regelung erhalten hat, jungalpin deformiert und in Amphibolit umgewandelt wurde. Er beweist: Subduktion hÄlt vom Val Sesia über unser Gebiet nach E an. Wir fassen die Magmatite als Subduktions-Magmatite auf. Sie können nicht der schon obduziert seicht unter der Tonale-Serie liegenden Subduktions-Serie entstammen, aus der der Eklogit jungalpin in die Tonale-Serie eingeschuppt ist. Im N fandVoll eine zweite, subduzierte Serie: Aduladecke + Misoxer Zone. Es scheint möglich, da\ diese vor der jungen Einengung des S-Randes unter unserem Gebiet die nötige Tiefe für Teilschmelzbildung erreichten. Letzte Bewegungen direkt an der Insubrischen Linie erzeugen bei ca. 300 C Blastomylonite, deuten auf Vertikalbewegung. Das Areal der Lepontin-Metamorphose grenzt mit Granat-Staurolith-Disthen-Glimmerschiefern direkt an Insubrische Linie und Seealpen. Einst nach S zu OberflÄchen-T vermittelnde Teile sind durch die Insubrische Linie weggeschnitten.

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Résumé Les auteurs étudient et mettent en corrélation la déformation et le métamorphisme des séries de roches au bord sud des Alpes, à l'E. de Bellinzona. Les minéraux magmatiques et métamorphiques de la tonalité et du gneiss oeillé de Melirolo sont examinés, ainsi que les types de roche. Les deux massifs sont considérés comme magmatiques. La zonation du plagioclase est attribuée à la montée du magma et à son refoidissement avec dégazéification. On a trouvé des filons de roche subvolcanique émanant du gneiss oeillé. Les magmatites, d'âge alpin tardif, ont été mises en place dans une partie froide, encore exempté de toute déformation alpine, de la fraction méridionale de la plaque de couverture au-dessus de la zone de subduction. La magmatites et la plaque de couverture ont été ensuite déformées par le mouvement alpin tardif et soumises à un échauffement. Le degré de déformation de la tonalité, évalué à partir des enclaves, donne un applatissement de l'ordre de 1/20 de l'épaisseur originelle. Cet applatissement est extrapolé à la bordure méridionale des Alpes, aujourd'hui redressée et large de 5 km., ce qui conduit pour cette dernière à une largeur de 100 km. avant son amincissement récent. La déformation s'est produite lors de la phase à haute température de la phase ascendante du métamorphisme de réchauffement lépontien. La température maximum d'environ 580 C, sous 5 à 6 kbar, s'est prolongée après la déformation. Une forte extension a considérablement affecté ce niveau d'intrusion, alors à faible profondeur. Il s'en est suivi, dans la série du Tonale, une plissement 3 fois répété, rotationel, le plus souvent déversé vers le sud, par température descendante. Dans la série de Tonale, fut découverte une eclogite de subduction éoalpine, qui est marquée par une paragenèse et un réglage éoalpins, et qui déformée par les mouvements alpins récents, a été transformée en amphibolite. Ceci montre que la subduction s'étend du Val Sesia vers l'E. par delà notre région. Nous considérons les magmatites comme étant de subduction. Elles ne peuvent pas dériver de la série en subduction se trouvant à faible profondeur sous la série du Tonale, déjà soumise à une obduction, et qui a incorporé l'éclogite dans la série du Tonale lors des mouvements alpins récents. Au nord,Voll a trouvé une deuxième série marquée par la subduction: la nappe de l'Adula + la zone de Misox. Il semble possible que celles-ci aient atteint, sous notre région, les températures nécessaires à une fusion partielle, et ce, avant l'amincissement récent de la bordure méridionale. Les derniers mouvements survenus directement le long de la ligne insubrique ont produit des blastomylonites à une température de quelque 300 C, en impliquant des mouvements verticaux. L'aire du métamorphisme lépontien, à schistes à grenat, staurotide et disthène, touche directement la ligne insubrique et les Seealpes. Les parties formant la transition aux températures de surface ont été supprimées par le jeu de la ligne insubrique.

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List of symbols D Diameter of a sphere of equal volume to the strain ellipsoid - 1,2,3 Natural Strain - s Strain-Magnitude - v Strain-Symmetry - r Radius - s₁, str₁ First cleavage and first stretching lineation of late-alpine age - B₁ Axis of folds round str₁, folding s₁ - s_2–4, b_2–4 Cleavage planes and axes of 2nd-4th folds - T Temperature (C) - X, Y, Z Principal axes of the Strain Ellipsoid     

Contrasting thermomechanical evolutions in the Southalpine metamorphic basement of the Orobic Alps (Central Alps, Italy)

In the Southern Alps a progressive metamorphic zonation, with an increase in the geothermal gradient from NE to SW, has been widely proposed. However, recent investigations have shown that the greenschist metamorphic imprint of the low-grade zone corresponds to a metamorphic retrogression following amphibolite facies conditions. On the other hand, in the medium-grade zone, a later low-pressure, high-temperature (LPHT) metamorphic event has also been proposed. In an attempt to resolve these different interpretations, new petrological and partly new structural data have been obtained for two sectors of the Orobic Alps, traditionally attributed to different metamorphic zones. Thermobarometric determinations, supported by microstructural analysis, indicate the following different pressure-retrograde paths in each sector: (1) in the Val Vedello basement (VVB) rocks, a first metamorphic imprint characterized by P = 7–9 kbar and T = 570–610°C was followed by a greenschist retrogression ( P ≤ 4 kbar and T ≤ 500° C); (2) in the Lario basement (LB) rocks, the first detectable metamorphic stage, characterized by mineral assemblages indicating P = 7–9 kbar and T = 550–630° C, was followed by a LPHT event, synkinematic with F2 extensional deformation. A greenschist retrogression marks the final uplift of these rocks.
Reinterpretation of the available geochronological data indicates a diachronism for the two thermomechanical evolutions. In the light of these data, we interpret the retrograde P–T–t path of the VVB rocks as a pre-Permian post-thickening uplift and the retrograde P–T–t evolution of the LB rocks as a Permo-Mesozoic uplift related to the extensional tectonic regime of the Tethyan rifting.     

Fluid-controlled crustal metasomatism within a high-pressure subducted mélange (Mt. Hochwart, Eastern Italian Alps)

The Nonsberg–Ultental Region of northern Italy contains a Palaeozoic mélange that was partially subducted during the Variscan orogeny. This mélange is constituted mainly by metapelites characterized by shale-type REE-patterns, displaying partial melting which began under high-pressure conditions. The resulting migmatites enclose minor slivers of mantle-wedge peridotites that have been incorporated into the mélange during subduction. Peridotites display important large ion lithophile elements (LILE) enrichment consequent to amphibole recrystallization contemporaneously with metapelite migmatization at P ≈ 2.7 GPa and T ≈ 850 °C in the garnet–peridotite field. Crustal and mantle (ultramafic) rocks of the mélange display the same Sm–Nd ages of about 330 ± 6 Ma, which dates both the metamorphic peak and the migmatization event. The zircon U–Pb age of the metasomatic amphibolitic contact between garnet peridotite and migmatite is identical (333.3 ± 2.4 Ma) within analytical errors. Therefore, metasomatism, migmatization and peak metamorphism are constrained to the same event. The presence of Cl-rich apatite and ferrokinoshitalite in the contact amphibolite, together with the trace-element patterns of peridotites, suggest that metasomatism was driven by Cl- and LILE-rich fluids derived from ocean water transported into the subduction zone by sediments and crustal rocks. These fluids interacted with the crust, prompting partial melting under water oversaturated conditions and partitioning LILE from the crust itself. Peridotites, which were well below their wet solidus temperature, could not melt but they recrystallized in the crustal mélange under garnet-facies conditions. Crustal fluids caused extensive hydration and LILE-enrichment in peridotites and severe Sm–Nd isotope disequilibrium between minerals, especially in the recrystallized peridotites. The proposed scenario suggests massive entrapment of crustal aqueous fluids at high-pressure conditions within subduction zones.