ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1991.tb00499.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1991.tb00500.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Phase relationships in the model mafic system and geothermobarometry allow discrimination of four main groups of high‐Procks in the nappes of the Western Alps: very high‐Peclogite‐facies (including kyanite eclogites and coesite‐pyrope assemblages), eclogite‐facies (paragonite‐zoisite eclogites), high‐Tblueschist‐facies (glaucophane‐garnet ± lawsonite assemblages) and low‐Tblueschist‐facies (glaucophane‐lawsonite ± pumpellyite assemblages). The blueschist‐facies‐eclogite‐facies transition is promoted chiefly by increasingT, low bulkXMgand relatively low μH2O. 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 inP‐T‐Xspace, 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 μH2Ogradients 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‐Passemblage in particular rock volumes.As regards theP‐T‐tpaths, 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 intr

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1991.tb00501.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Petrographical and mineral chemical data are given for the eclogites which occur in the garnet‐kyanite micaschists of the Penninic Dora‐Maira Massif between Brossasco, Isasca and Martiniana (Italian Western Alps) and for a sodic whiteschist associated with the pyrope‐coesite whiteschists of Martiniana. The Brossasco‐Isasca (BI) eclogites are fine grained, foliated and often mica‐rich rocks with a strong preferred orientation of omphacite crystals and white micas. Porphyroblasts of hornblende are common in some varieties, whilst zoisite and kyanite occur occasionally in pale green varieties associated with leucocratic layers with quartz, jadeite and garnet. These features differentiate the BI eclogites from the eclogites that occur in other continental units of the Western Alps, which all belong to type C. Garnet, sodic pyroxene and glaucophane are the major minerals in the sodic whiteschist.Sodic pyroxene in the eclogites is an omphacite often close to Jd50Di50, with very little acmite and virtually no AlIV, and impure jadeite in the leucocratic layers and in the sodic whiteschist. Garnet is almandine with 20–30 mol. % for each of the pyrope and grossular components in the eclogites and a pyrope‐rich variety in the sodic whiteschist. White mica is a variably substituted phengite, and paragonite apparently only occurs as a replacement product of kyanite. Amphibole is hornblende in the eclogites, but the most magnesian glaucophane yet described in the sodic whiteschist. Quartz pseudomorphs of coesite were found occasionally in a few pyroxenes and garnets.TheP‐Tconditions during the VHP event are constrained in the eclogites by reactions which define a field ranging from 27–28 kbar to 35 kbar and from 680 to 750° C. These temperatures are consistent with the results of garnet‐pyroxene and garnet‐phengite geothermometry which suggest that the eclogites may have equilibrated at around 700° C. In the sodic whiteschist pressures ranging from 29 to 35 kbar can be deduced from the stability of the jadeite‐pyrope garnet‐glaucophane compatibility. As in the eclogites water activity must have been low. Such conditions are close to theP‐Tvalues estimated for the early Alpine recrystallization of the pyrope‐coesite rock and, like petrographical and mineralogical features, set aside the BI eclogites from the other eclogites of the Western Alps, instead indicating a close similarity to some of the eclogite bodies occurring in the Adula nappe of the Central Alps. An important corollary is that glaucophane stability, at least in Na‐ and Mg‐rich compositions and under very high pressures, may extend up to 700° C, in agreement with the HT stability lim

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1991.tb00502.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

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‐Pgranulite 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‐Pand low‐Tassemblages.In an effort to quantify this evolution, independent well‐calibrated thermobarometers were applied to basic and acid rocks. For the granulite event,P‐Testimates are 7–9 kbar and 700–800° C, and for subsequent retrograde evolution,P‐Twas 4–5 kbar and 600° C. For the eo‐Alpine eclogite metamorphism, pressure and temperature conditions were 14–16 kbar and 550° C.The inferredP‐Tpath 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 o

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1991.tb00503.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

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‐Passemblages 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‐Testimates of the early‐Alpine metamophism have been calculated from these unique high‐Passemblages, 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 andT≤ 500° C atPH2O ≤ 9 kbar for early‐Alpine retrogressive stages) which are within the range obtained from the surrounding lithologies.Phase relationships inP‐T‐XCO2space indicate that mineral assemblages in the impure marbles coexisted with H2O‐rich fluids (XCO2<0.03)

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1991.tb00504.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

The microstructures in the Erro‐Tobbio peridotite indicate several stages of recrystallization of olivine + titanian clinohumite‐bearing assemblages. The development of these assemblages is closely associated with serpentinite mylonites, in which they occur in shear bands and foliations and are inferred to have grown synkinematically, in veins, and as post‐kinematic radial aggregates. In the peridotite wall‐rock adjacent to these mylonites, the same assemblages have recrystallized statically at the expense of original olivine and pyroxenes, mesh‐textured chrysolite and antigorite veins. In addition, the olivine‐bearing assemblage occurs in widespread vein systems. The brittle deformation of the peridotite resulting in the development of these vein systems is closely related to ductile deformation of metagabbroic dykes in the peridotite. Although early metasomatism resulted in extensive rodingitization of the gabbros, some dykes show an eclogitic assemblage of Na‐clinopyroxene + garnet + chloritoid + chlorite ± talc. These observations, the microstructures and the mineral chemistry all suggest that the assemblages in the ultramafic rocks and metagabbros developed during a prograde evolution towards high pressures (>13–16 kbar, 450–550° C), and during subsequent decompression. This metamorphic evolution is considered to be related to Late Cretaceous intraoceanic subduction in the Alps‐Apennine system and closure of the Pi

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1991.tb00505.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

The retrogradeP‐Ttrajectory of the eclogitic Fe‐Ti‐gabbros from the Ligurian Alps is constrained by the appearance of mineral parageneses post‐dating the Na‐clinopyroxene + garnet eclogitic assemblage and indicating the following sequence of metamorphic events: (1) amphibolitic stage— edenite/katophorite + plagioclase (An33–43) + oxides in symplectitic aggregates; (2) glaucophanic stage— a porphyroblastic glaucophanic amphibole has overgrown the symplectite, winchite also occurs as thin rims around glaucophane and both amphiboles are, sometimes, armoured by atoll garnets; (3) albite‐amphibolite stage—barroisite/katophorite + albite + epidote + oxides ± chlorite overprint the glaucophanic stage minerals; (4) greenschist stage—represented by actinolite + albite + epidote + oxide paragenesis.The metamorphic evolution is complex and the decompression path, on aP–Tdiagram, is significantly different from those defined in the literature for the Voltri eclogites. The main features inferred from theP–Tpath are the following: (1) the pressure climax does not match the thermal climax, the maximum temperature conditions are in fact achieved during the early stage of uplift; (2) a decrease in temperature, suggested by the appearance of glaucophane after the amphibolitic symplectite; (3) successive uplift, probably accompanied by an increase in temperature. The complexity of theP‐Tpath drawn for the Voltri eclogites can be explained with a mechanism of successive underthrusts propagating from the innermost to the outermost

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1991.tb00506.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1991.tb00507.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY