bluuebunny/crossref_metadata_2025_split huggingface dataset:
Eclogites and polyphase P–T cycling in the Caledonian Uppermost Allochthon in Troms, northern Norway
Authors: KROGH E. J. , ANDRESEN A. , BRYHNI I. , BROKS T. M. , KRISTENSEN S. E. 
Published: May 2007

Eclogites in the Tromsø area, northern Norway, are intimately associated with meta‐supracrustals within the Uppermost Allochthon of the Scandinavian Caledonides (the Tromsø Nappe Complex). The whole sequence, which includes pelitic to semipelitic schists and gneisses, marbles and calc‐silicate rocks, quartzofeldspathic gneisses, metabasites and ultramafites, has undergone three main deformational/metamorphic events (D1/M1, D2/M2 and D3/M3). Detailed structural, microtextural and mineral chemical studies have made it possible to construct separate P–T paths for these three events.Chemically zoned late syn‐ to post‐D1 garnets with inclusions of Bt, Pl and Qtz in Ky‐bearing metapelites indicate a prograde evolution from 636°C, 12.48 kbar to c. 720°C, 14–15 kbar. This latter result is in agreement with Grt–Cpx geothermometry and Grt–Cpx–Pl–Qtz geobarometry on eclogites and trondhjemitic to dioritic gneisses. Maximum pressures at c. 675°C probably reached 17–18 kbar based on Cpx–Pl–Qtz inclusions in eclogitic garnets, and Grt–Ky–Pl–Qtz and Jd–Ab–Qtz in trondhjemitic gneisses. Post‐D1/pre‐D2 decompressional breakdown of the high‐P assemblages indicates a substantial drop in pressure at this stage. Inclusions and chemical zoning in syn‐ to post‐D2 garnets from metapelites record a second episode of prograde metamorphism, from 552°C, 7.95 kbar, passing through a maximum pressure of 10.64 kbar at 644°C, with final equilibration at c. 665°C, 9–10 kbar. The corresponding apparently co‐facial paragenesis Grt + Cpx + Pl + Qtz in metabasites yields c. 635°C, 8–10 kbar. In the metapelites post‐D3, Grt in apparent equilibrium with Bt, Phe and Pl yield c. 630°C, 9 kbar. The D1/M1 and D2/M2 episodes are exclusively recorded in the Tromsø Nappe Complex and must thus pre‐date the emplacement of this allochthonous unit on top of the underlying Lyngen Nappe, while the D3/M3 episode is common for the two units.A previously published Sm–Nd mineral isochron (Grt–Cpx–Am) on a partly retrograded and recrystallized ecologite of 598 ± 107 Ma represents either the timing of formation of the eclogites or the post‐eclogite/pre‐D2 decompression stage, while a Rb–Sr whole rock isochron of an apparently post‐D1/pre‐D2 granite of 433 ± 11 Ma is consistent with a K–Ar age of post‐D1/pre‐D2 amphiboles from a retrograded eclogite of 437 ± 16 Ma which most likely record cooling below the 475–500°C isotherm after the M3 metamorphism.
The illite ‘crystallinity’technique: a critical appraisal of its precision
Authors: ROBINSON D. , WARR L. N. , BEVINS R. E. 
Published: May 2007

Analysis of the precision of the illite ‘crystallinity’technique shows that machine errors are <5%, while intra‐ and inter‐sample errors are variable but are up to 12% and 14%, respectively (1σ). Consideration of this error analysis shows that the isocryst approach, which involves close contouring (e.g. 0.03 Δ2°) of illite ‘crystallinity’data, has a very low degree of confidence (<0.5) and thus is not regarded as statistically valid. If contouring is to be undertaken with a high degree of confidence (>0.8) it is necessary that contours should be at intervals of 0.1 ΔΘ2°, which is equivalent to subdivision of the anchizone into upper and lower units. Where previous interpretations have relied upon an isocryst method of contouring at less than 0.1 ΔΘ2° the conclusions must be regarded as unsubstantiated.Centrifuge separation of clay fractions (based on a Stokes’law application) gives separations in which a significant, but variable, percentage of grains have long axes greater than the size calculated. For the typical <2‐μm fraction utilized, some 20% of grains lie in the 2–4‐μm range, although the proportion is not believed to have a significant effect upon ‘crystallinity’values. The formula is applicable for grain‐sizes down to 0.5 μm. Illite ‘crystallinity’values on samples prepared by an ultrasonic disaggregation method show a small increase on those prepared by ball mill crushing. The differences are minimal at the epi/anchizone level but increase to some 10% at the anchizone/diagenetic level. The effect on grade determinations is again thought to be minimal and indicates that concern over unsuitability of the ultrasonic disaggregation method is unfounded.
Metamorphic and structural evolution of the Early Proterozoic Puolankajärvi Formation, Finland – I. Structural and textural relations
Authors: LAAJOKI K. , TUISKU P. 
Published: May 2007

The amphibolite facies Puolankajärvi Formation (PjF) occupies the western margin of the Early Proterozoic Kainuu Schist Belt (KSB) of northern Finland. The lower and middle parts of the PjF consist of turbiditic psammites and pelites and tempestitic semipelites. This report concentrates on the pelitic lithologies which include quartz–two‐mica–plagioclase schists with variable amounts of garnet, staurolite, andalusite and biotite porphyroblasts as well as sillimanite and cordierite segregations.The KSB forms a major north–south‐trending synclinorium between two Archaean blocks. It contains both autochthonous and allochthonous units and is cut by faults and shear zones. The PjF lies on the western side of the KSB and is probably allochthonous. The formation has undergone six major deformation phases (D1, D2, D3a, D3b, D4 and D5). During D3a‐D5 the maximum principal stress (σ1) changed in a clockwise direction from south‐west to north‐east. Between D2 and D3 the intermediate principal stress (σ2) changed from horizontal to vertical and the interval between D2 and D3 marks a transition from thrust to strike‐slip tectonics.Relict structures in the porphyroblasts indicate the following mineral growth–deformation evolution in the PjF. (1) Throughout the PjF there was a successive crystallization of garnet (syn‐D1), poryphyroblastic biotite (inter‐D3/4) and staurolite (inter‐D3/4) during the pre‐D4 stage. (2) A syn‐D4‐inter‐D4/5 crystallization of kyanite, sillimanite (fibrolite), porphyroblastic tourmaline, magnetite, rutile, cordierite and muscovite–biotite–plagioclase pseudomorphs after staurolite was most localized at and near D4 shear zones. (3) A syn‐ to post‐D5 generation of andalusite, ilmenohematite and sheet silicates after staurolite and after cordierite occurred near D5 faults.The evolution outlined here permits the relative dating of the PjF parageneses, which is used in the second part of the study (Tuisku & Laajoki, 1990), and, together with the knowledge of the pressure–temperature conditions during various growth events, makes it possible to compile pressure–temperature–deformation paths for the PjF.
Pumpellyites and coexisting minerals in different low‐grade metamorphic facies of Liguria, Italy
Authors: LUCCHETTI G. , CABELLA R. , CORTESOGNO L. 
Published: May 2007

Pumpellyite from four‐phase assemblages (pumpellyite + epidote + prehnite + chlorite; pumpellyite + epidote + actinolite + chlorite; pumpellyite + epidote + Na‐amphibole + chlorite, together with common excess phases), considered to be low variance in a CaO‐(MgO + FeO)‐Al2O3‐Fe2O3(+Na2O + SiO2+ H2O) system, have been examined in areas which underwent metamorphism in the prehnite‐pumpellyite, pumpellyite‐actinolite and low‐temperature blueschist facies respectively. The analysed mineral assemblages are compared for nearly constant (basaltic) chemical composition at varying metamorphic grade and for varying chemical composition (basic, intermediate, acidic) at constant metamorphic conditions (low‐temperature blueschist facies).In the studied mineral assemblages, coexisting phases approached near chemical equilibrium. At constant (basaltic) bulk rock composition the MgO content of pumpellyite increases, and the XFe3+of both pumpellyite and epidote decreases with increasing metamorphic grade, the Fe3+being preferentially concentrated in epidote. Both pumpellyite and epidote compositions vary with the bulk rock composition at isofacial conditions; pumpellyite becomes progressively enriched in Fe and depleted in Mg from basic to intermediate and acidic bulk rock compositions. The compositional comparison of pumpellyites from high‐variance (1–3 phases) assemblages in various bulk rock compositions (basic, intermediate, acidic rocks, greywackes, gabbros) shows that the compositional fields of both pumpellyite and epidote are wide and variable, broadly overlapping the compositional effects observed at varying metamorphic grade in low‐variance assemblages.The intrinsic stability of both Fe‐ and Al‐rich pumpellyites extends across the complete range of the considered metamorphic conditions. Element partitioning between coexisting phases is the main control on the mineral composition at differentP‐Tconditions.
Two‐stage decompression in orthopyroxene–sillimanite granulites from Forefinger Point, Enderby Land, Antarctica: implications for the evolution of the Archaean Napier Complex
Authors: HARLEY S. L. , HENSEN B. J. , SHERATON J. W. 
Published: May 2007

Magnesian metapelites of probable Archaean age from Forefinger Point, SW Enderby Land, East Antarctica, contain very‐high‐temperature granulite facies mineral assemblages, which include orthopyroxene (8–9.5 wt% Al2O3)–sillimanite ± garnet ± quartz ± K‐feldspar, that formed at 10 ± 1.5 kbar and 950 ± 50°C. These assemblages are overprinted by symplectite and corona reaction textures involving sapphirine, orthopyroxene (6–7 wt% Al2O3), cordierite and sometimes spinel at the expense of porphyroblastic garnet or earlier orthopyroxene–sillimanite. These textures mainly pre‐date the development of coarse biotite at the expense of initial mesoperthite, and the subsequent formation of orthopyroxene (4–6 wt% Al2O3)–cordierite–plagioclase rinds on late biotite.The early reaction textures indicate a period of near‐isothermal decompression at temperatures above 900°C. Decompression from 10 ± 1.5 kbar to 7–8 kbar was succeeded by biotite formation at significantly lower temperatures (800–850°C) and further decompression to 4.5 ± 1 kbar at 700–800°C.The later parts of this P–T evolution can be ascribed to the overprinting and reworking of the Forefinger Point granulites by the Late‐Proterozoic (c. 1000 Ma) Rayner Complex metamorphism, but the age and timing of the early high‐temperature decompression is not known. It is speculated that this initial decompression is of Archaean age and therefore records thinning of the crust of the Napier Complex following crustal thickening by tectonic or magmatic mechanisms and preceding the generally wellpreserved post‐deformational near‐isobaric cooling history of this terrain.
Identification of the scales of differential element mobility in a ductile fault zone
Authors: DIPPLE G. M. , WINTSCH R. P. , ANDREWS M. S. 
Published: May 2007

Two fundamentally different scales of element mobility have been identified in the upper‐amphibolite‐facies Hunts Brook fault zone of south‐central Connecticut, USA. Field relations, mineral chemistry, and 80 bulk rock analyses provide overwhelming evidence that the blastomylonitic schists and gneisses of the fault zone were derived from the enclosing granodioritic Rope Ferry orthogneiss. Two‐sample mass balance calculations between the Rope Ferry Gneiss and fault rocks strongly suggest that the Rope Ferry Gneiss was segregated into biotite‐rich schists and quartz–plagioclase‐rich gneisses through the centimetre‐scale transfer of Si, Al, Ca, Na, Ba, and Zr from schists to gneisses. Three‐ and four‐sample mass balance calculations provide convincing evidence that Al, Ca, Na, and Ba were lost and Si and K gained by the entire fault zone. Both scales of metasomatism have been identified at two localities separated by 20 km, implying that the metasomatic processes were pervasive throughout the fault zone.From the mass balance calculations, an apparent disparity in K mobility arises. K appears to be gained by the entire fault zone, but is immobile during the centimetre‐scale segregation. This is because mass balance calculations can only detect differential element mobility. Differential element mobility requires that the mechanism of metasomatism produces chemical potential gradients. Therefore, the identification of differentially mobile elements provides insight into the mechanisms of metasomatism. The immobility of K during segregation of schists and gneisses implies that the segregation was not achieved through the extraction of partial melt. However, the element mobility is reconcilable with deformation‐driven metamorphic differentiation. The gains of Si and K and losses of Ca, Na, Al, and Ba for the entire fault zone are also inconsistent with the intrusion or extraction of partial melt, but may reflect the infiltration of a metasomatizing aqueous fluid.
The origin of olivine‐plagioclase coronas in metagabbros from the Adirondack Mountains, New York
Authors: JOHNSON C. D. , CARLSON W. D. 
Published: May 2007

Olivine‐plagioclase coronas in metagabbros from the Adirondack Mountains, New York (USA) are spatially well‐organized reaction textures consisting most commonly of sequential layers of orthopyroxene, clinopyroxene, plagioclase, and garnet; the textures are characteristic of diffusion‐controlled reaction kinetics. Although similar coronas have been interpreted by previous workers in terms of an isochemical steady‐state diffusion model, petrographical relations and material‐balance calculations establish that coronas in the Adirondack metagabbros cannot be treated as isochemical and do not form in a single‐stage steady‐state process; instead they evolve through time in a complex open‐system reaction.In this study, the isochemical diffusion model is modified to account for elemental fluxes across the outer boundaries of the coronal reaction band, thereby approximating the open‐system behaviour of the coronas. The sequence and relative proportions of product minerals calculated by the open‐system steady‐state model correspond closely to those observed in coronas of the Adirondacks, over a wide range of values for the relative diffusivities of chemical components involved in the reaction, regardless of the particular method used to determine material balance in the reaction texture.Despite this correspondence, petrographical evidence for successive replacement of coronal product layers reveals that the Adirondack coronas evolved through one or more transient states, rather than forming in a single‐stage steady‐state process. There is no evidence that the successive replacement of coronal product layers resulted from changes in pressure or temperature, but there is petrographical evidence that these changes resulted from modification of the composition of reactant plagioclase as the corona‐forming reaction proceeded. This is confirmed by the fact that the evolution of the coronas over time can be replicated with the open‐system diffusion model by simulating the effect of the gradual exhaustion of plagioclase as a source of the Ca and Si components required for reaction. These simulations suggest that successive stages in the evolution of the coronas are characterized by these product sequences: (i) orthopyroxene‐clinopyroxene‐plagioclase‐garnet; (ii) orthopyroxene‐clinopyroxene‐garnet; and (iii) orthopyroxene‐garnet. All of these stages, and the transitions between them, are observed petrographically. Coronas in Adirondack metagabbros appear, therefore, to have originated in a complex, open‐system, diffusion‐controlled reaction in which the product assemblages changed as the reaction progressed.
Chronology of Sanbagawa metamorphism
Authors: ISOZAKI Y. , ITAYA T. 
Published: May 2007

By collating age data based on the fossil age of the protoliths, radiometric dating of the metamorphic minerals, and sedimentary records of erosion at the earth's surface, the history of the Sanbagawa metamorphism can be summarized as follows. (1) The pre‐metamorphic sedimentary rocks (Carboniferous‐Jurassic + Early Cretaceous?) became mixed and formed a thickened packet in the vicinity of an ancient trench through a variety of subduction‐related tectono‐sedimentary processes, probably in Early Cretaceous time (c., 130‐120 Ma). (2) The subducted protoliths underwent progressive metamorphism reaching a maximum depth of c. 30 km in late Early Cretaceous time (c. 116 ± 10 Ma). (3) The high‐P/T metamorphic rocks began to rise toward the surface (during the interval 110‐50 Ma) with minimum estimates for the average cooling rate around 9‐12°C/Ma and an average uplift rate around 0.4‐0.5 mm/year. (4) Finally, at some stage after reaching the erosional surface, the high‐P/T metamorphic rocks were covered unconformably by the middle Eocene (c. 50‐42 Ma) Kuma Group.On the basis of the present chronological summary of the Sanbagawa metamorphism, the areal extent of the Sanbagawa metamorphism is also discussed with respect to the weakly metamorphosed subduction‐accretion complex of the next tectonic belt to the south, the Northern Chichibu belt.
The transition from brittle to ductile deformation in the Sambagawa metamorphic belt, Japan
Authors: TORIUMI M. 
Published: May 2007

The dominant deformation mechanism during the Sambagawa metamorphism changes from brittle to ductile with increasing metamorphic temperature. The magnitude of plastic strains inferred from the shapes of deformed radiolaria in metachert increases sharply across the boundary between the epidote‐pumpellyite‐actinolite zone and the epidote‐actinolite zone. The synmetamorphic crack density of metachert is an indicator of the contemporaneous brittle strain of rocks, and it decreases sharply as the grade reaches the epidote‐actinolite zone. Hence, the ratio of the ductile strain to the brittle strain of metachert decreases rapidly across the transition to the epidote‐actinolite zone of the Sambagawa metamorphic belt.The sharp change of the ductile strain magnitude also takes place at the epidote‐actinolite grade in the Shimanto metamorphic belt of Japan, an example of the intermediate pressure facies series of metamorphism. It is concluded that the transition from brittle to ductile deformation takes place at about 300‐400°C. and is independent of pressure of metamorphism.
Petrology of very‐high‐pressure eclogitic rocks from the Brossasco‐Isasca Complex, Dora‐Maira Massif, Italian Western Alps
Authors: KIENAST J. R. , LOMBARDO B. , BIINO C. , PINARDON J. L. 
Published: May 2007

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.The P‐T conditions 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 the P‐T values 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 limit suggested by experimental studies.
Eclogitic metamorphism in carbonate rocks: the example of impure marbles from the Sesia‐Lanzo Zone, Italian Western Alps1
Authors: CASTELLI D. 
Published: May 2007

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) during their entire Alpine evolution.
Retrograde P‐T‐t path for the Voltri Massif eclogites (Ligurian Alps, Italy): some tectonic implications
Authors: MESSIGA B. , SCAMBELLURI M. 
Published: May 2007

The retrograde P‐T trajectory 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 a P–T diagram, is significantly different from those defined in the literature for the Voltri eclogites. The main features inferred from the P–T path 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 the P‐T path drawn for the Voltri eclogites can be explained with a mechanism of successive underthrusts propagating from the innermost to the outermost sector of the Ligurian Alps.