High pressure-low temperature metamorphism in the Gran Paradiso basement, Western Alps

The Gran Paradiso basement complex of the French and Italian Alps is composed of metasediments, termed the gneiss minuti, and metabasic rocks, both of which are intruded by a late Hercynian granite. The Bonneval gneiss, which crops out at the western edge of the complex, is composed of highly deformed metasediments, volcanics and volcaniclastic rocks. Eclogites, now highly altered, occur in the metabasic rocks. Kyanite and blue-green amphibole are locally present in the gneiss minuti and aegirine plus riebeckite occur in the Bonneval gneiss. A moderately high pressure - low temperature metamorphic event of probable Alpine age occurred in the basement complex. This metamorphic event differs from that in the overlying Sesia unit and ophiolites of the Schistes lustrés nappe in being at lower pressures (below the ab = jd₁₀₀+ qz transition) and post-dating the major (D2_A) deformation. The origin of the metamorphism is discussed and interpreted as a probable consequence of the overlying nappe pile which was emplaced during the D2_A event. Subsequent greenschist facies metamorphism in the basement complex is a consequence of thermal relaxation during uplift.     

P–T conditions for the eclogitic re-equilibration of the metaophiolites from Val d'Ala di Lanzo (internal Piemontese zone, Western Alps)

Abstract A study has been made of the high-pressure early-Alpine re-equilibration in the eclogites and metasedimentary cover of the Val d'Ala di Lanzo ophiolite. All of the main high-pressure minerals have been analysed and their compositions used to determine re-equilibration temperatures. The minimum conditions proposed ( P = 1.3 GPa, T = 450–460°C) are also indicated by the presence of a jadeite+quartz-bearing metagranite.
The temperatures are compared with those reported for similar eclogites from the Voltri Group, the Aosta Valley and the Valais. Comparison of recalculated temperatures shows that the temperature (and probably the pressure) of the eclogitic re-equilibration increased in the Aosta Valley and the Valais, in keeping with what has been observed in the internal Penninic basement of the Gran Paradiso and Monte Rosa crystalline massifs.     

Deformation and metamorphism in the Bard area of the Sesia Lanzo Zone, Western Alps, during subduction and uplift

The structure, microstructure and petrology of a small area close to the village of Bard in Val d'Aosta (Italy) has been studied in detail. The area lies across the contact between the Gneiss Minuti (GM) and the Eclogitic Micaschist (EMS) Complexes of the Lower element of the Sesia portion of the Sesia-Lanzo Zone (Western Alps). Both complexes have undergone high-pressure metamorphism, but the metamorphic assemblages indicate a sudden increase in pressure in going across the contact from the GM to the EMS. Therefore, we interpret the contact as a thrust dividing the lower element of the Sesia into two sub-elements. This interpretation is supported by structural evidence.
The early Alpine (90-70 Ma) metamorphic history is best preserved in the EMS and is one of increasing pressure associated with thrusting. The maximum P/T recorded in the EMS is >1500 MPa (>15kbar) and 550°C and in the GM is < 1500-1300 MPa (< 15-13 kbar) and 500-550°C. We suggest that the rocks were probably in an active Benioff zone during this time.
From then on the histories of the GM and EMS are the same. Deformation continued and the thrust and thrust slices were folded during decreasing pressure. We interpret the first postthrusting deformation in terms of uplift associated with continued shortening of the crust and underplating after the Benioff zone had become inactive and a new Benioff zone had developed further to the north-west.
A still later deformation and the Lepontine metamorphism (38 Ma) are related to continued uplift. Much of this deformation is characterized by structures indicative of vertical shortening and lateral spreading as the mountains rose above the general level of the surface.     

Alpine metamorphism of pelitic schists in the Nufenen Pass area, Lepontine Alps

Abstract The effects of Tertiary Alpine metamorphism on pelitic Mesozoic cover rocks have been studied along a cross-section in the central Lepontine Alps in the Nufenen Pass area, Switzerland. Greenschist facies to amphibolite facies conditions are indicated by the formation of the index minerals chloritoid, garnet, staurolite and kyanite in pelitic rocks. Regional metamorphism reached maximum conditions during the interkinematic period between a main Alpine penetrative (D2) and a late Alpine (D3) crenulation type deformation phase or synchronous with the late Alpine deformation. Based on AFM phase relationships four different metamorphic zones can be distinguished: (1) chloritoid zone; (2) staurolite + chlorite zone; (3) staurolite + biotite zone; and, (4) kyanite zone. The isograds that separate these zones can be modelled by univariant reactions in the KFMASH system. The conditions of metamorphism calculated from geological ther-mobarometers for the maximum post-D2 por-phyroblast stage are from North to South: 500° C at 5-6 kbar and 600° C at 7-8 kbar. Detailed thermobarometry of garnet por-phyroblasts with complex textures suggests that maximum temperature was reached later than maximum pressure. Early garnet growth occurred along a prograde P-T-path, post-D2 rims grew with increasing temperature but decreasing pressure, and finally post-D3 garnet formed along a retrograde P-T-path. It may be concluded from the calculated pressure and temperature difference over a short distance (3 km) across the mapped area that the isogradic surfaces of the post-D2 metamorphism are steeply oriented. The data also suggest that isobaric and isothermal surfaces are parallel. Much of the observed metamorphic pattern can be explained as the result of a significant post-D2 differential uplift of the hot Pennine area relative to the Helvetic area along a tectonic contact zone. The closely spaced isograds (isotherms) in the North may then be interpreted as a thermal effect owing to the emplacement of the hot Pennine rocks against the Got-thard massif with its cover. Whereas, in the Pennine metasediments, post-D2 porphyroblast formation can be related to the decompression path which was steep enough for dehydration reactions to proceed. It is also remarkable that late kyanite porphyroblasts probably formed with decreasing pressure. The interpretation given here for the Nufenen Pass area may also apply to the Luk-manier Pass area where similar metamorphic patterns have been reported by Fox (1975). The formation of the ‘Northern Steep Belt’;, as denned by Milnes (1974b), and the associated late Alpine fold zones may, therefore, have significantly modified the metamorphic pattern of the Helvetic-Penninic contact zone.     

Petrological constraints on the eclogite- and blueschistfacies metamorphism and P-T-t paths in the Western Alps*

Phase relationships in the model mafic system and geothermobarometry allow discrimination of four main groups of high-P rocks in the nappes of the Western Alps: very high-P eclogite-facies (including kyanite eclogites and coesite-pyrope assemblages), eclogite-facies (paragonite-zoisite eclogites), high-T blueschist-facies (glaucophane-garnet ± lawsonite assemblages) and low-T blueschist-facies (glaucophane-lawsonite ± pumpellyite assemblages). The blueschist-facies-eclogite-facies transition is promoted chiefly by increasing T, low bulk X_Mg and relatively low μ_H2O. The variety of assemblages and the heterogeneous approach to equilibrium observed in the Alpine rocks are not only constrained by the intersection of the reaction surfaces in P-T-X space, but also by the effect and timing of the processes which control kinetics (i.e. pervasive deformation and fluid infiltration). The faster rate of dehydration reactions relative to hydration reactions along with the fact that different bulk compositions crossed the reaction curves at different temperatures (and times), all may have induced μ_H2O gradients and contributed to the heterogeneous distribution of deformation through a process of reaction-enhanced ductility. Also mass-transfer may have been an effective process in determining the type of high-P assemblage in particular rock volumes. As regards the P-T-t paths, only the post-climax histories are recorded well in the Alpine nappes. Post-eclogitic exhumation paths at decreasing temperatures characterize structurally higher nappes which were first subducted during the early-Alpine (Cretaceous) event. In contrast, more or less isothermal decompression paths characterize structurally deeper nappes formed by westward propagation of the underthrust surfaces during the early-Alpine event and the subsequent meso-Alpine (Palaeogene) collision between the ‘European’and ‘African’plates. In the Western Alps, prevalent eclogite-facies conditions were attained during the metamorphic climax of the early-Alpine subduction, while blueschist-facies recrystallization characterizes the early-Alpine exhumation of the eclogitized units and the subsequent intracontinental underthrusts linked to the meso-Alpine continental collision.     

Eclogitic metamorphism in carbonate rocks: the example of impure marbles from the Sesia-Lanzo Zone,Italian Western Alps1

The impure marbles of the internal Sesia-Lanzo Zone underwent a multi-stage metamorphic evolution of Alpine age and retain early-Alpine eclogitic assemblages, partially recrystallized under blueschist to greenschist facies conditions. These high-P assemblages consist of carbonates, phengite, quartz, omphacite, grossular-rich (locally spessartinic) garnet, zoisite and Al-rich titanite. Retrogressive stages are characterized by the growth of glaucophane, paragonite, phlogopite, tremolite and albite. Halogen-rich biotite and amphibole are also present. P-T estimates of the early-Alpine metamophism have been calculated from these unique high-P assemblages, in order to test the applicability of some calibrations to impure carbonate systems. In particular, some Gt-Cpx calibrations and the phengite geobarometer give results (T= 575 ± 45° C at 15 kbar for the eclogitic climax and T≤ 500° C at PH₂O ≤ 9 kbar for early-Alpine retrogressive stages) which are within the range obtained from the surrounding lithologies. Phase relationships in P-T-XCO₂ space indicate that mineral assemblages in the impure marbles coexisted with H₂O-rich fluids (XCO₂ <0.03) during their entire Alpine evolution.     

Eo-Alpine HP metamorphism in the Permian intrusives from the steep belt of the central Alps (Languard-Campo nappe and Tonale Series)

Abstract

Diorites and granitoids that intruded the Upper Austroalpine units of the central Alps during the Permian display map-pable tectonic imprints and metamorphic transformations that were acquired during the Alpine tectonometamorphic cycle. Superposed heterogeneous deformations interacted with metamorphic re-equilibration stages and created a range of textural types that reflect local deformation gradients: coronitic transformations textures, normally foliated S-tectonites and mylonitic foliations. The three textural types are distinguished on maps recording foliation trajectories of successive deformation phases, which are correlated to the evolution of metamorphic assemblages. Tectonic deformation of Alpine age is represented by three generations of ductile syn-metamorphic structures. The mineral assemblages stable during the first Alpine deformation phase (D1) are Amp_II + P1_II + white mica, + Zo/Czo + Grt + Qtz ± Mg-Ch1 ± Ilm in metadiorites and P1_II + white mica_I + Zo/Czo + Grt + Amp_II + Qtz ± Ilm/Ttn in metagranitoids; the successive foliations D2a and D2b are defined by greenschist facies minerals. Thermobarometric estimates allow T = 500–600 °C and P = 1.1 ± 0.2 GPa conditions to be determined during D1 and T ≤ 350 °C and P ≤ 0.5 GPa during D2. Relict igneous minerals in metadiorites allow to determine intrusive conditions of T = 879 ± 110 °C and P = 0.4–0.7 GPa. Radiometric ages and P/T ratio of Alpine P_maxT_Pmax suggest that the inferred P-T-d-t path may represent the thermal state of the initial Alpine subduction stages. © 2000 Éditions scientifiques et médicales Elsevier SAS     

Application of K-feldspar–jadeite–quartz barometry to eclogite facies metagranites and metapelites in the Sesia Lanzo Zone (Western Alps, Italy)*

The eclogite facies assemblage K-feldspar–jadeite–quartz in metagranites and metapelites from the Sesia-Lanzo Zone (Western Alps, Italy) records the equilibration pressure by dilution of the reaction jadeite+quartz=albite. The metapelites show partial transformation from a pre-Alpine assemblage of garnet (Alm₆₃Prp₂₆Grs₁₀)–K-feldspar–plagioclase–biotite±sillimanite to the Eo-Alpine high-pressure assemblage garnet (Alm₅₀Prp₁₄Grs₃₅)–jadeite (Jd_80–97Di_0–4Hd_0–8Acm_0–7)–zoisite–phengite. Plagioclase is replaced by jadeite–zoisite–kyanite–K-feldspar–quartz, and biotite is replaced by garnet–phengite or omphacite–kyanite–phengite. Equilibrium was attained only in local domains in the metapelites and therefore the K-feldspar–jadeite–quartz (KJQ) barometer was applied only to the plagioclase pseudomorphs and K-feldspar domains. The albite content of K-feldspar ranges from 4 to 11 mol% in less equilibrated assemblages from Val Savenca and from 4 to 7 mol% in the partially equilibrated samples from Monte Mucrone and the equilibrated samples from Montestrutto and Tavagnasco. Thermodynamic calculations on the stability of the assemblage K-feldspar–jadeite–quartz using available mixing data for K-feldspar and pyroxene indicate pressures of 15–21 kbar (±1.6–1.9 kbar) at 550±50 °C. This barometer yields direct pressure estimates in high-pressure rocks where pressures are seldom otherwise fixed, although it is sensitive to analytical precision and the choice of thermodynamic mixing model for K-feldspar. Moreover, the KJQ barometer is independent of the ratio PH2O/P_T. The inferred limiting a(H₂O) for the assemblage jadeite–kyanite in the metapelites from Val Savenca is low and varies from 0.2 to 0.6.     

The origin of strain patterns resulting from contemporaneous deformation and metamorphism in the Sambagawa metamorphic belt

Abstract The magnitudes of plastic strains of 104 metacherts were determined from the deformed shape of initially spherical radiolarians in the Sambagawa high- P type metamorphic belt of Western Shikoku, Japan. The strain magnitude increases with increasing metamorphic temperature from several per cent to 250%. The a₂/a₃ ratio of strain ellipsoids in the higher metamorphic grades decreases with increasing metamorphic grade while the a₁/a₂ ratio increases rapidly. The long axis of the strain ellipsoid for every grade is nearly parallel to the length of the metamorphic belt, suggesting that the flow direction of the synmetamorphic deformation was uniform along the belt. A map of strain zones within the Sambagawa high- P type metamorphic belt reveals that the metamorphic belt underwent a progressive bulk inhomogeneous shear deformation and that the high-grade zones represent a deep-seated boundary shear zone on the accretionary wedge between a subducting oceanic plate and the immobile rigid continental plate.     

Garnet–chloritoid equilibria in eclogitic pelitic rocks from the Sesia zone (Western Alps): their bearing on phase relations in high pressure metapelites

Abstract A detailed study of garnet–chloritoid micaschists fom the Sesia zone (Western Alps) is used to constrain phase relations in high pressure (HP) metapelitic rocks. In addition to quartz, phengite, paragonite and rutile, the micaschists display two distinct parageneses, namely garnet + chloritoid + chlorite and garnet + chloritoid + kyanite. Talc has never been observed. Garnet and chloritoid are more magnesian when chlorite is present instead of kyanite. The distinction of the two equilibria results from different bulk rock chemistries, not from P–T conditions or redox state. Estimated P–T conditions for the eclogitic metamorphism are 550–600°C, 15–18 kbar.
The presence of primary chlorite in association with garnet and chloritoid leads us to construct two possible AFM topologies for the Sesia metapelites. The paper describes a KFMASH multisystem for HP pelitic rocks, which extends the grid of Harte & Hudson (1979) towards higher pressures and adds the phase talc. Observed parageneses in HP metapelites are consistent with predicted phase relations. Critical associations are Gt–Ctd–Chl and Gt–Ctd–Ky at relatively low temperatures and Gl–Chl–Ky and Gt–Tc–Ky at relatively high temperatures.     

Faulted backfold versus reactivated backthrust: the role of inherited structures during late extension in the frontal Piémont nappes east of Pelvoux (Western Alps)

In the central part of the internal Western Alps, widespread multidirectional normal faulting resulted in an orogen-scale radial extension during the Neogene. We revisit the frontal Piémont units, between Doire and Ubaye, where contrasting lithologies allow analysing the interference with the N–S trending Oligocene compressive structures. A major extensional structure is the orogen-perpendicular Chenaillet graben, whose development was guided by an E–W trending transfer fault zone between the Chaberton backfold to the north and the Rochebrune backthrust to the south. The Chaberton hinge zone was passively crosscut by planar normal faults, resulting in a E–W trending step-type structure. Within the Rochebrune nappe, E–W trending listric normal faults bound tilted blocks that slipped northward along the basal backthrust surface reactivated as an extensional detachment. Gravity-driven gliding is suggested by the general northward tilting of the structure in relation with the collapse of the Chenaillet graben. The stress tensors computed from brittle deformation analysis confirm the predominance of orogen-parallel extension in the entire frontal Piémont zone. This can be compared with the nearby Briançonnnais nappe stack where the extensional reactivation of thrust surfaces locally resulted in prominent orogen-perpendicular extension. Such a contrasting situation illustrates how the main direction of the late-Alpine extension may be regionally governed by the nature and orientation of the pre-existing structures inherited from the main collision stage.     

Structure and metamorphism of the Karakorum gneisses in the Braldu-Baltoro Valley (North Pakistan)

Abstract

The Karakorum gneisses outcrop north of the complex suture separating the Indian-Pakistan plate from the Europe-Asia block; they grade to deformed earlier members of the Karakorum batholith ranging in age from Cretaceous to Miocene and are cross-cut by its later members. The main interest of the region lies in the fact that very young high-grade gneisses (Miocene), outline the southern edge of the Europe-Asia Plate. The tectonic and metamorpic evolution of the Braldu-Baltoro region is interpreted here as resulting from a poly phased history. The following structural sequence has been defined : - (1) A Dl isoclinal folding was accompanied by subparallel healed shear zones and by intense boudinage, and cross-cut by a dense net of post-Dl hetero-geneous leucogranitic veins and stocks; - (2) a major phase of EW trending recumbent folds (D2), is followed by (3) large open D3 folds generating EW trending domai structures (Dassu and Panmah domes); and (4) a late set of brittle to locally more ductile structures such as the southern thrust contact of the Karakorum gneisses with the Shyok suture zone. The sequence proposed here differs from other interpretations (Rex et al. 1988). We consider that the Dl event only may be attributed to the main India-Asia collision and that the D2-D3 events, interpreted as having occurred in a continuum, correspond to a late reactivation of the major thrusts and sutures related to continuing continental subduction.

A Dl-related intermediate pressure assemblage is preserved (Grt-St-Ky) in the upper levels of the tectonic pile; the estimated PT conditions determined are 10-4 Kb and 700°--525°C. In the core of the large D3 domes, late granoblastic recrystallization is widespread together with almost complete S1-S2 transposition, incipient melting and development of a low-pressure sillimanite-bearing assemblage where relicts of higher pressure minerals are locally preserved. Corresponding PT conditions are 650°-550°C and a lower pressure (5.5 to 2.5 Kb). As most of the observed structures at the lower levels (mineral lineations, boudinage) are clearly associated with (or reworked by) D2 and accentuated by D3 which was accompagnied by partial melting, D2 and D3 are interpreted as representing a continuum developed in the same PT field. It can be assumed also that the Baltoro granite was emplaced by the end of this combined D2-D3 event. From the Miocene ages published for the Baltoro granite (20 Ma to 8 Ma), the low-pressure evolution of the Karakorum gneisses may represent a very young high-grade assemblage. The age of Dl is less defined but at least older than 36 Ma old leucogranites.

The sharp contact along the Shyok Suture zone, interpreted as a large thrust (Main Karakorum Thrust - MKT) of this young high-grade metamorphic terrene against the older (older than 30-45 Ma from late undeformed intrusives) Kohistan-Ladakh island-arc domain, is interpreted, following Mattauer (1985), as resulting from the interaction between the still-ongoing northward movement of the Indo-Pakistan plate and an opposite southward continental subduction, seismically active, operating in Pamir.     

羌塘中部高压变质带的退变质作用及其构造侵位

羌塘中部的高压变质带主要由榴辉岩、石榴石白云母片岩和蓝片岩等组成,它们在遭受高压变质作用之后折返,构造侵位于晚古生代展金组地层中,二者以韧性变形带为接触边界.本文以高压变质带中的榴辉岩和韧性变形带为研究对象,讨论了高压变质带折返过程中的退变质作用特征及折返时代.研究表明,榴辉岩在高峰期变质作用之后的折返过程中经历了由榴辉岩相→蓝片岩相→绿帘角闪岩相的退变质作用演化过程;在高压变质带构造侵位过程形成的韧性变形带中,白云母石英片岩的白云母40Ar-39Ar坪年龄为219±2Ma.高压变质带在219Ma左右构造侵位于展金组地层中,并于214Ma之前最终抬升出露地表.     

The role of viscous heating in Barrovian metamorphism of collisional orogens: thermomechanical models and application to the Lepontine Dome in the Central Alps

Thermal models for Barrovian metamorphism driven by doubling the thickness of the radiogenic crust typically meet difficulty in accounting for the observed peak metamorphic temperature conditions. This difficulty suggests that there is an additional component in the thermal budget of many collisional orogens. Theoretical and geological considerations suggest that viscous heating is a cumulative process that may explain the heat deficit in collision orogens. The results of 2D numerical modelling of continental collision involving subduction of the lithospheric mantle demonstrate that geologically plausible stresses and strain rates may result in orogen‐scale viscous heat production of 0.1 to >1 μW m^?3, which is comparable to or even exceeds bulk radiogenic heat production within the crust. Thermally induced buoyancy is responsible for crustal upwelling in large domes with metamorphic temperatures up to 200 °C higher than regional background temperatures. Heat is mostly generated within the uppermost mantle, because of large stresses in the highly viscous rocks deforming there. This thermal energy may be transferred to the overlying crust either in the form of enhanced heat flow, or through magmatism that brings heat into the crust advectively. The amplitude of orogenic heating varies with time, with both the amplitude and time‐span depending strongly on the coupling between heat production, viscosity and collision strain rate. It is argued that geologically relevant figures are applicable to metamorphic domes such as the Lepontine Dome in the Central Alps. We conclude that deformation‐generated viscous dissipation is an important heat source during collisional orogeny and that high metamorphic temperatures as in Barrovian type metamorphism are inherent to deforming crustal regions.     

Geometry and kinematics of early Alpine nappes in a Briançonnais basement (Ambin Massif,Western Alps)

Petrological and structural observations from the Ambin pre-alpine basement dome and from its Briançonnais and Piedmont covers show an early D1 nappe-forming event overprinted by a major D2 (+?D3) ductile shearing deformation. The D1 event is characterised by garnet-blueschist facies metamorphic assemblages retrogressed to greenschist facies conditions during D2 then D3 stages near the top of the dome. North-verging D1 structures preserved in the core of the dome are consistent with alpine evolutionary models, in which exhumation of HP–LT metamorphic alpine rocks occurs initially in a north–south direction. To cite this article: J. Ganne et al., C. R. Geoscience 336 (2004).     

U–Pb zircon geochronological and petrographic constraints on late to post‐collisional Variscan magmatism and metamorphism in the Ligurian Alps,Italy

In the Ligurian Alps, the Barbassiria massif (a Variscan basement unit of the Briançonnais domain) is made up of orthogneisses derived from K‐rich rhyolite protoliths and minor rhyolite dykes. However, on account of subsequent Alpine deformation and a related blueschist facies metamorphic overprint that are pervasive within the Barbassiria Orthogneisses, little evidence of the earlier Variscan metamorphism is preserved. In this study, new U–Pb laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) dating of zircon from the Barbassiria Orthogneisses and dykes was undertaken to unravel the relationships between protolith magmatism and the Variscan metamorphic overprint. The results suggest a protolith age for the Barbassiria Orthogneisses of ~315–320 Ma (i.e., Early/Late Carboniferous), and constrain the age of a subsequent rhyolite dyke emplacement event to 260.2 ± 3.1 Ma (i.e., Late Permian). The Variscan high‐temperature (greenschist–amphibolite facies) metamorphic event that affected the Barbassiria Orthogneisses was likely associated with both tectonic burial and compression during the final stages of the Variscan collision during the Late Carboniferous period. Emplacement of late‐stage rhyolite dykes that cut the Barbassiria Orthogneisses is linked to a diffuse episode of Late Permian rhyolite volcanism that is commonly observed in the Ligurian Alps. The age of this dyke emplacement event followed a ~10–15 Ma Mid‐Permian gap in the volcano‐sedimentary cover sequence of the Ligurian Alps, and represents the post‐orogenic stage in this segment of the Variscides. Copyright © 2012 John Wiley & Sons, Ltd.     

Constraining P–T conditions during thrusting of a higher pressure unit over a lower pressure one (Gran Paradiso,Western Alps)

Identifying higher pressure units overlying lower pressure ones is a first order argument to determine the presence of large‐scale thrusting. For the first time, petrology is used to quantify the pressure difference between two stacked units in the Western Alps. In the Gran Paradiso Massif, the Money unit crops out as a tectonic window below the Gran Paradiso unit. The reconstruction of the Alpine evolution of these two units and the history of their tectonic contact has been achieved using a multidisciplinary approach that combines meso‐ and microstructural analysis and pseudosection calculations. In both units, four stages of deformation and metamorphism have been identified. Stage 1 reflects the phase of continental crust subduction and P–T conditions of ~18–20 kbar, 480–520 °C and of ~13–18 kbar, 500–530 °C have been estimated for the Gran Paradiso and the Money units respectively. This yields a maximum difference of ~20 km in the depth reached by these two units during the early Alpine history. Thrusting of the Gran Paradiso unit over the Money unit (stage 2) led to the development of the main foliation and occurred in the high‐P part of the albite stability field at P–T conditions of ~12.5–14.5 kbar and 530–560 °C, identical in both units. The thrust contact was folded during stage 3 together with the entire Money unit, and then both units were exhumed together (stage 4). During this polyphase evolution, detrital garnet has been partially dissolved, while the earliest Na‐bearing phases (glaucophane, paragonite) have been overprinted by the low‐P mineral associations. The uncertainties on derived pressures between the two units are unfortunately larger than hoped, and this is attributed to the muscovite solid‐solution model not incorporating a pyrophyllite component.     

Ductile-brittle transition in pre-Alpine amphibolite facies mylonites during evolution from water-present to water-deficient conditions (Mont Mary nappe, Italian Western Alps)

In the Austroalpine Mont Mary nappe (Italian Western Alps) discrete zones of mylonites–ultramylonites developed from coarse-grained, upper amphibolite facies metapelites of pre-Alpine age. The syn–mylonitic mineral assemblage is quartz–biotite–muscovite–plagioclase–garnet–sillimanite–ilmenite–graphite, and formed via the model hydration reaction: Grt₁+Kfs+H₂O=Bt₂+Ilm₂+Qtz+Ms± Sil .Grain-size reduction of about three orders of magnitude was accompanied by extensive recrystallization of all minerals except sillimanite, and by compositional changes of garnet and biotite. Deformation took place at temperatures of 510–580  °C under low-pressure conditions (0.25–0.45 GPa) and corresponds to the latest stages of pre-Alpine metamorphic evolution. The pre-Alpine mylonitization conditions were close to the brittle-ductile transition, as indicated by syn–mylonitic generation of pseudotachylytes and high differential stress inferred from quartz grain-size piezometry. The brittle-ductile behaviour at a relatively high temperature, and the absence of annealing textures in quartz aggregates, are suggestive of water-deficient conditions during mylonitization. These were accomplished through progressive consumption of water by syn–kinematic hydration reaction and by adsorption onto the greatly increased grain boundary area resulting from dynamic recrystallization.     

燕山西段及北京西山晚中生代逆冲构造格局及其地质意义

燕山西段及北京西山晚中生代逆冲构造集中分布于三个NE向带状区域中,三个带状区域的间隔约为60km,延伸长度自东向西依次减小,呈现出明显的逆冲构造发育的三角形区域。三角形区域的北界为“内蒙地轴”南缘断裂西段,南西界与中元古代早期古盆地构造边界一致,东南部边界则与华北克拉通基底新太古代-古元古代中部碰撞造山带的东部边界大致吻合。逆冲构造具有基底卷入的厚皮构造与盖层内部的薄皮构造共存的构造属性,上盘运动方向总体指向NW,逆冲构造变形主要发生在140~130Ma。逆冲后伸展构造变形以发育在主要逆冲构造后侧为主,并利用先存构造薄弱带。先存构造薄弱带在有利区域构造应力和其他影响因素的作用下导致的构造活化,可能是燕山板内构造变形的重要机制之一。主要逆冲变形前后均有大规模岩浆活动的构造-岩浆时空组合表明,收缩构造造成地壳加厚及由此引发的深部地壳重熔,难以作为统一的机制对这些特征进行合理阐释,需要有其他方式的深部热物质与能量的参与。北京西山霞云岭—长操、教军场—大安山以及马兰—胡林等逆冲断层,是一个统一的大规模的逆冲构造的不同组成部分,具典型、连续的断坪-断坡结构,它形成于髫髻山组(148~146Ma)之后、南窖闪长岩(128Ma)侵入之前,而不是“印支期(或更早)”,它与南大寨—八宝山逆冲构造构成北京西山晚中生代逆冲构造格局。区域性的NW-SE向收缩构造作用及南大寨—八宝山逆冲构造上覆岩席的构造加载,可能是北京西山的蓝晶石带和硬绿泥石带为代表的高压动力变质作用的基本构造原因。