bluuebunny/crossref_metadata_2025 huggingface dataset:
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.
Crust‐mantle relationships in the French Variscan chain: the example of the Southern Monts du Lyonnais unit (eastern French Massif Central)
Authors: GARDIEN V. , TEGYEY M. , LARDEAUX J. M. , MISSERI M. , DUFOUR E.
Published: May 2007

Garnet lherzolite from the Lyonnais area (eastern French Massif Central) occurs as several lenses elongated within the regional foliation of garnet‐biotite‐sillimanite gneisses. Within the peridotites a mylonitic foliation can be observed which clearly is oblique to the regional foliation of the surrounding gneisses. Petrological and thermobarometric studies emphasize a tectonometamorphic re‐equilibration for both crustal and mantle rocks characterized by a prograde metamorphic stage followed by retrograde evolution. During the burial stage, interpreted as lithospheric subduction, the peridotites underwent their mylonitic deformation, under high‐pressure conditions (23–30 kbar). In contrast, the paragneisses have suffered their deformation during the retromorphic evolution under mesozonal conditions (6–8 kbar, 700°C). Our thermobarometric investigations allow us to interpret the granulitic/ultramafic association from the Monts du Lyonnais area as a lithospheric section buried into a Palaeozoic subduction zone, laminated during continental collision and uplifted by erosion processes.
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.
High‐temperature ‘clockwise’P‐T paths and melting in the development of regional migmatites: an example from southern Brittany, France
Authors: JONES K. A. , BROWN MICHAEL
Published: May 2007

The Southern Brittany Migmatite Belt (SBMB), which evolved through the metamorphic peak between c. 400 Ma and c.. 370 Ma ago, consists of a heterogeneous suite of high‐grade gneisses and anatectic migmatites, both metatexites and diatexites. Rare garnet‐cordierite gneiss layers record evidence of an early prograde P‐T path. In these rocks, growth‐zoned garnet cores and a sequence of included mineral assemblages in garnet, from core to rim, of Qtz + Ilm + Ky, Pl + Ky + St + Rt + Bt and Pl + Sil + St + Rt + Bt constrain a prograde evolution during which the reactions Ilm + Ky + Qtz→ Aim + Rt, Ms + Chl→ St + Bt + Qtz + V and St + Qtz→ Grt + Sil + V were crossed. Parts of this prograde evolution are preserved as inclusion assemblages in garnet in all other rock types. In all rock types, garnet has reverse zoned rims, and garnet replacement by cordierite and/or biotite and plagioclase suggests the following reactions have occurred: Grt + Sil + Qtz→ Crd → Hc ± Ilm, Bt + Sil + Qtz → Crd ± Hc → Ilm → Kfs + V and (Na + Ca + K + Ti) + Grt → Bt + Pl + Qtz. Microstructural analysis of reaction textures in conjunction with a petrogenetic grid has enabled the construction of a tightly constrained ‘clockwise’P–T path for the SBMB. The high‐temperature part of the path has a steep dT/dP slope characteristic of near isothermal decompression. It is proposed that the P‐T path followed by the SBMB is the result of the inversion, by overthrusting, of a back‐arc basin and that such a tectonic setting may be applicable to other high‐temperature migmatite terranes. The near isothermal decompression is at least partly driven by the upward (diapiric) movement of the diatexite/anatectic granite core of the SBMB.
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.
Nappe structure of the Sambagawa belt
Authors: HARA I. , SHIOTA T. , HIDE K. , OKAMOTO K. , TAKEDA K. , HAYASAKA Y. , SAKURAI Y.
Published: May 2007

The thermal structure of the Sambagawa belt, which is reflected by minerals produced during the highest temperature of metamorphism, generally is characterized by the occurrence of the highest grade schists in the middle of the structural pile. The origin of this structure has been analysed in the upper horizon of the Sambagawa schist sequence in Central Shikoku as an example for the structure of the whole belt. The upper horizon of the Sambagawa schist sequence in Central Shikoku consists of three nappes, the Saruta nappe II, the Saruta nappe I and the Fuyunose nappe in descending order of structural level. The Saruta nappe II shows a downward increase of metamorphic grade from the garnet zone, through the albite‐biotite zone, to the oligoclase‐biotite zone. The Saruta nappe I consists mainly of rocks in the albite‐biotite zone and partly in the oligoclase‐biotite zone, but the direction of the increase of metamorphic grade is not clear. The Fuyunose nappe shows an upward increase in metamorphic grade which changes from the glaucophane zone to the barroisite zone. It is concluded that the Saruta nappe II and Saruta nappe I were overturned and then mechanically coupled with the Fuyunose nappe. The Sambagawa metamorphic field, which is of the highest temperature phase of metamorphism, appears to have had an inverted thermal gradient and a thermal structure comparable with that expected in the deeper parts of a subduction complex.
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.
Clasts of high‐grade Sanbagawa schist in Middle Eocene conglomerates from the Kuma Group, central Shikoku, south‐west Japan
Authors: YOKOYAMA K. , ITAYA T.
Published: May 2007

Middle Eocene conglomerates which overlie the Sanbagawa metamorphic rocks contain clasts of metamorphic rock with isotope ages of 120‐85 Ma, which fall within the age range reported from the Sanbagawa metamorphic rocks. They were derived from the chlorite to oligoclase zones of the Sanbagawa metamorphic belt. Clasts of garnet amphibolite and oligoclase‐biotite schist show a mineral assemblage similar to the highest grade Sanbagawa schists. However, the metamorphic temperatures estimated by various mineralogical thermometers show that some of the clasts were formed at higher temperatures than the in situ Sanbagawa metamorphic rocks. Such higher grade rocks were at the surface by the Middle Eocene and for the most part they have been eroded away. Cretaceous and post‐Cretaceous sediments overlie, or are in fault contact with, the Sanbagawa metamorphic rocks which suggests that rocks in the belt were uplifted and eroded from the latest Cretaceous to Middle Eocene time after strike‐slip movement along the Median Tectonic Line. Since the Middle Eocene, the belt has experienced relatively slow uplift which was locally around 2 km in central Shikoku.
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.
Alpine olivine‐ and titanian clinohumite‐bearing assemblages in the Erro‐Tobbio peridotite (Voltri Massif, NW Italy)
Authors: SCAMBELLURI M. , STRATING E. H. HOOGERDUIJN , PICCARDO G. B. , VISSERS R. L. M. , RAMPONE E.
Published: May 2007

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 Piedmont‐Ligurian basin.
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.
Metamorphism and granite genesis in the Hidaka Metamorphic Belt, Hokkaido, Japan
Authors: OSANAI Y. , KOMATSU M. , OWADA M.
Published: May 2007

The Main Zone of the Hidaka Metamorphic Belt is an uplifted crustal section of island‐arc type. The crust was formed during early Tertiary time, as a result of collision between two arc–trench systems of Cretaceous age. The crustal metamorphic sequence is divided into four metamorphic zones (I–IV), in which zone IV is in the granulite facies.

A detailed study of the evolution of the Hidaka Belt, based on a revised P–T–t analysis of the metamorphic rocks, notably a newly found staurolite‐bearing granulite, confirms a prograde isobaric heating path, after a supposed event of tectonic thickening of accretionary sedimentary and oceanic crustal rocks. During the peak metamorphic event (c. 53 Ma), the regional geothermal gradient attained 33–40° C km−1, and the highest P–T condition obtained from the lowest part of the granulite unit is 830° C, 7 kbar. In this part, XH2O of Gt–Opx–Cd gneiss is about 0.15 and that of Gt–Cd–Bt gneiss is 0.4. The P–T–XH2O condition of the granulite unit is well within a field where fluid‐present partial melting of pelitic and greywacke metamorphic rocks takes place. This is in harmony with the restitic nature of the Gt–Opx–Cd gneiss in the lowest part of the granulite unit.

The possibility that partial melting took place in the Main Zone is significant for the genesis of the peraluminous (S‐type) granitic rocks within it. The S‐type granitic rocks in this zone are Opx–Gt–Bt tonalite in the granulite zone, Gt–Cd–Bt tonalite in the amphibolite zone, and Cd–Bt–Mus tonalite in the Bt–Mus gneiss zone. The mineralogical and chemical nature of these strongly peraluminous tonalitic rocks permit them to be regarded as having been derived from S‐type granitic magma generated by crustal anatexis of pelitic metamorphic rocks in deeper crust.
Chronology of the high‐pressure metamorphism of Norwegian garnet peridotites/pyroxenites
Authors: JAMTVEIT B. , CARSWELL D. A. , MEARNS E. W.
Published: May 2007

Eclogite facies mineral assemblages are variably preserved in mafic and ultramafic rocks within the Western Gneiss Region (WGR) of Norway. Mineralogical and microstructural data indicate that some Mg–Cr‐rich, Alpine‐type peridotites have had a complex metamorphic history. The metamorphic evolution of these rocks has been described in terms of a seven‐stage evolutionary model; each stage is characterized by a specific mineral assemblage. Stages II and III both comprise garnet‐bearing mineral assemblages. Garnet‐bearing assemblages are also present in Fe–Ti‐rich peridotites which commonly occur as layers in mafic complexes. Sm–Nd isotopic results are reported for mineral and whole rock samples from both of these types of peridotites and related rocks.

The partitioning of Sm and Nd between coexisting garnet and clinopyroxene is used to assess chemical equilibrium. One sample of Mg–Cr‐type peridotite shows non‐disturbed partitioning of Sm and Nd between Stage II garnet and clinopyroxene pairs and yields a garnet–clinopyroxene–whole‐rock date of 1703 ± 29 Ma (I= 0.51069, MSWD = 0.04). This is the best estimate for the age of the Stage II high‐P assemblage. Other Stage II garnet–clinopyroxene pairs reflect later disturbance of the Sm–Nd system and yield dates in the range 1303 to 1040 Ma. These dates may not have any geological significance. Stage III garnet–clinopyroxene pairs typically have equilibrated Sm–Nd partitioning and two samples yield dates of 437 ± 58 and 511 ± 18 Ma. This suggests that equilibration of the Stage III high‐P assemblage is related to the Caledonian orogeny and is more or less contemporaneous with high‐P metamorphism of ‘country‐rock’eclogites in the surrounding gneisses. The Sm–Nd mineral data for the Fe–Ti‐rich garnet peridotites and for a superferrian eclogite, which occurs as a dyke within the Gurskebotn Mg–Cr‐type peridotite, are consistent with a Palaeozoic high‐P metamorphism.

Finally a synoptic P–T–t path is proposed for the Mg–Cr‐type peridotites which is consistent with the petrological and geochronological data.
Metamorphism in Archaean greenstone belts: calculated fluid compositions and implications for gold mineralization
Authors: POWELL R. , WILL T. M. , PHILLIPS G. N.
Published: May 2007

An inescapable consequence of the metamorphism of greenstone belt sequences is the release of a large volume of metamorphic fluid of low salinity with chemical characteristics controlled by the mineral assemblages involved in the devolatilization reactions. For mafic and ultramafic sequences, the composition of fluids released at upper greenschist to lower amphibolite facies conditions for the necessary relatively hot geotherm corresponds to those inferred for greenstone gold deposits (XCO2= 0.2–0.3). This result follows from the calculation of mineral equilibria in the model system CaO–MgO–FeO–Al2O3–SiO2–H2O–CO2, using a new, expanded, internally consistent dataset.

Greenstone metamorphism cannot have involved much crustal over‐thickening, because very shallow levels of greenstone belts are preserved. Such orogeny can be accounted for if compressive deformation of the crust is accompanied by thinning of the mantle lithosphere. In this case, the observed metamorphism, which was contemporaneous with deformation, is of the low‐P high‐T type. For this type of metamorphism, the metamorphic peak should have occurred earlier at deeper levels in the crust; i.e. the piezothermal array should be of the ‘deeper‐earlier’type. However, at shallow crustal levels, the piezothermal array is likely to have been of ‘deeper‐later’type, as a consequence of erosion. Thus, while the lower crust reached maximum temperatures, and partially melted to produce the observed granites, mid‐crustal levels were releasing fluids prograde into shallow crustal levels that were already retrograde. We propose that these fluids are responsible for the gold mineralization. Thus, the contemporaneity of igneous activity and gold mineralization is a natural consequence of the thermal evolution, and does not mean that the mineralization has to be a consequence of igneous processes.

Upward migration of metamorphic fluid, via appropriate structurally controlled pathways, will bring the fluid into contact with mineral assemblages that have equilibrated with a fluid with significantly lower XCO2. These assemblages are therefore grossly out of equilibrium with the fluid. In the case of infiltrated metabasic rocks, intense carbonation and sulphidation is predicted. If, as seems reasonable, gold is mobilized by the fluid generated by devolatilization, then the combination of processes proposed, most of which are an inevitable consequence of the metamorphism, leads to the formation of greenstone gold deposits predominantly from metamorphic fluids.
Geospeedometry of metamorphic rocks: examples in the Rantasalmi‐Sulkava and Kiuruvesi areas, eastern Finland. Biotite–garnet diffusion couples
Authors: LINDSTRÖM R. , VIITANEN M. , JUHANOJA J. , HÖLTTÄ P.
Published: May 2007

Compositional zoning in biotite–garnet pairs in metamorphic rocks from eastern Finland has been studied. The Mg profiles in the garnet side of biotite–garnet crystal pairs have been interpreted by means of Lasaga's theory (geospeedometry). However, the binary interdiffusion equations are first reformulated by starting from a ternary system and using the lattice fixed frame of reference. This frame of reference gives the fluxes directly by means of the numbers of diffusing ions, which helps to check the 1‐dimensionality of the analysis assumed in Lasaga's theory. It is also shown that the recently argued effect of the third cation Ca is negligible in our samples. We were able to investigate satisfactory profiles in three samples from different areas. The values for the cooling rate are a few degrees per million years if the diffusion data obtained by Freer are adopted. The cooling rates are in agreement with recent estimates based on the K–Ar ages on biotite in the same areas.
Initial tectonothermal evolution within the Scandinavian Caledonide Accretionary Prism: constraints from 40Ar/39Ar mineral ages within the Seve Nappe Complex, Sarek Mountains, Sweden
Authors: DALLMEYER R. D. , ANDRÉASSON P. G. , SVENNINGSEN O.
Published: May 2007

In the Caledonide orogen of northern Sweden, the Seve Nappe Complex is dominated by rift facies sedimentary and mafic rocks derived from the Late Proterozoic Baltoscandian miogeocline and offshore‐continent–Iapetus transition. Metamorphic breaks and structural inversions characterize the nappe complex. Within the Sarek Mountains, the Sarektjåkkå Nappe is composed of c. 600‐Ma‐old dolerites with subordinate screens of sedimentary rocks. These lithological elements preserve parageneses which record contact metamorphism at shallow crustal levels. The Sarektjåkkå Nappe is situated between eclogite‐bearing nappes (Mikka and Tsäkkok nappes) which underwent high‐P metamorphism at c. 500 Ma during westward subduction of the Baltoscandian margin. 40Ar/39Ar mineral ages of c. 520–500 Ma are recorded by hornblende within variably foliated amphibolite derived from mafic dyke protoliths within the Sarektjåkkå Nappe. Plateau ages of 500 Ma are displayed by muscovite within the basal thrust of the nappe and are consistent with metamorphic evidence which indicates that the nappe escaped crustal depression as a result of detachment at an early stage of subduction. Cooling ages recorded by hornblende from variably retrogressed eclogites in the entire region are in the range of c. 510–490 Ma and suggest that imbrication of the subducting miogeocline was followed by differential exhumation of the various imbricate sheets. Hornblende cooling ages of 470–460 Ma are recorded from massive dyke protoliths within the Sarektjåkkå Nappe. These are similar to ages reported from the Seve Nappe Complex in the central Scandinavian Caledonides. Probably these date imbrication and uplift related to Early Ordovician arrival of outboard terranes (e.g. island‐arc sequences represented by structurally lower horizons of the Köli Nappes).

Metamorphic contrasts and the distinct grouping of mineral cooling ages suggest that the various Seve structural units are themselves internally imbricated, and were individually tectonically uplifted through argon closure temperatures during assembly of the Seve Nappe Complex. The cooling ages of 520–500 Ma recorded within Seve terranes and along terrane boundaries of the Sarek Mountains provide evidence of significant accretionary activity in the northern Scandinavian Caledonides in the Late Cambrian–Early Ordovician.
On the formation of granulites
Authors: BOHLEN S. R.
Published: May 2007

The tectonic settings for the formation and evolution of regional granulite terranes and the lowermost continental crust can be deduced from pressure–temperature–time (P–T–time) paths and constrained by petrological and geophysical considerations.

P–T conditions deduced for regional granulites require transient, average geothermal gradients of greater than 35°C km−1, implying minimum heat flow in excess of 100 mWm−2. Such high heat flow is probably caused by magmatic heating. Tectonic settings wherein such conditions are found include convergent plate margins, continental rifts, hot spots and at the margins of large, deep‐seated batholiths. However, particular P–T–time paths do not allow specific tectonic settings to be distinguished at this time. Under different conditions, both clockwise, CW (Pmax attained before Tmax), and anticlockwise, ACW (Pmax attained slightly after Tmax), paths are possible in the same tectonic setting. Both CW and ACW end‐member paths can yield nearly isobaric cooling, IBC, paths. Such cooling paths are clearly not an artefact of thermobarometry, but can be constrained by solid–solid and devolatilization equilibria and geophysical modelling.

In terms of understanding the evolution of the deep crust, a potentially significant group of regional granulite terranes are those that show evidence for ACW‐IBC paths. Such paths are the likely result of: (i) episodic igneous activity resulting in intrusions within all levels of the crust, (ii) thickening of the crust by magmatic underplating, (iii) slow uplift as a result of the formation of a deep, garnet‐rich crustal root and (iv) excavation resulting from a later tectonic event unrelated to that resulting in the formation of the granulites. The later event might be triggered by the delamination of the garnet‐rich, lowermost crust.
Two‐stage decompression in garnet‐bearing mafic granulites from Sostrene Island, Prydz Bay, East Antarctica
Authors: THOST D. E. , HENSEN B. J. , MOTOYOSHI Y.
Published: May 2007

Mafic garnet‐bearing granulites from Sostrene Island, 150 km southwest of Davis Station on the coast of Prydz Bay, East Antarctica, exhibit two‐stage symplectic coronas on garnet, formed after peak metamorphic conditions (M1). An outer corona of Opx (Mg66) + Pl (An94–97) + minor Hbl mantles a finer‐grained inner corona of Opx (Mg67) + Pl (An95–96) + Spl (Mg36). Both symplectites contain minor ilmenite–magnetite intergrowths. The finer‐grained symplectite also occurs along a fracture cleavage in the garnet.

The outer corona originated during a second metamorphic event (M2) via the reaction Grt + Cpx (Hbl) + SiO2= Opx + Pl (1), whereas the inner corona formed later in response to decompression and minor deformation, resulting in the fracture cleavage in the garnet, according to the reaction Grt = Opx + Pl + Spl (2). The grossular content of the garent (XGrs= 0.168) is almost exactly that which is required for the stoichiometric breakdown by reaction (2) (calculated XGrs= 0.167). The mafic rocks are silica undersaturated, and the SiO2 for reaction (1) was most probably derived externally from the surrounding felsic gneisses.

Preferred P–T estimates for M1 based on garnet core (Prp40Alm42Grs17Sps1)–matrix Opx–Cpx–Hbl pairs are c. 10 kbar at 980°C. The fine‐grained symplectite formed post‐peak M2 at c. 7 kbar and 850°C. The enclosing felsic gneisses yield pressure estimates of between 5 and 7 kbar, which compare with conditions of c. 6 kbar and 775°C in the nearby Bolingen Islands. These lower P–T estimates are considered to be representative of the widespread 1100‐Ma metamorphic event recognized in outcrops along the Prydz Bay coast. The high‐P, high‐T estimates derived from the garnet relics provide evidence for an earlier, possibly Archaean, high‐grade metamorphic event.
Late Sveconorwegian (Grenville) high‐pressure granulite facies metamorphism in southwest Sweden
Authors: JOHANSSON L. , LINDH A. , MÖLLER C.
Published: May 2007

Mafic granulite, garnet amphibolite and charnockite occur in the southwest Swedish part of the Baltic Shield. This part is generally considered to be the continuation of the Grenville collisional belt in Canada. The area with granulite facies rocks, the Southwest Swedish Granulite Region (SGR), is considerably larger than previously thought. The SGR is bounded to the east and west by two major tectonic zones. The first quantitative age data and P–T determinations for the high‐grade metamorphism in the SGR are presented.

Conventional geothermobarometry was applied to mafic granulites from five localities. The estimated P–T conditions for the peak of metamorphism range from 705°C and 8.1 kbar at Hallandsås in the south, to 770°C and 10.5 kbar at Ullared in the north (medium‐ to high‐P granulite facies conditions). Sm–Nd geochronology on minerals from the mafic granulites at Hallandsås and Ullared give late Sveconorwegian (Grenville) ages of 907 ± 12 and 916 ± 11 Ma for the high‐grade metamorphism, which is considerably younger than previously thought.

Our results stress the hitherto underestimated importance of the late Sveconorwegian high‐grade metamorphism in the southwestern part of the Baltic Shield.
The nature and distribution of fluids during amphibolite facies metamorphism, Naxos (Greece)
Authors: BUICK I. S. , HOLLAND T. J. B.
Published: May 2007

On the basis of fluid inclusion evidence, pervasive influx of deep‐seated CO2‐rich fluids has been invoked to account for mid‐ to upper amphibolite facies (M2B) metamorphism on the island of Naxos (Cyclades, Greece). In this paper, mineral devolatilization and melt equilibria are used to constrain the composition of both syn‐ and post‐peak‐M2B fluids in the deepest exposed levels of the metamorphic complex. The results indicate that peak‐M2B fluids were spatially and compositionally heterogeneous throughout the high‐grade core of the complex, whereas post‐peak‐M2B fluids were generally water‐rich. The observed heterogeneities in syn‐M2B fluid composition are inconsistent with pervasive CO2‐flushing models invoked by previous workers on the basis of fluid inclusion evidence. It is likely that few CO2‐rich fluid inclusions on Naxos preserve fluids trapped under peak metamorphic conditions. It is suggested that many of these inclusions have behaved as chemically open systems during the intense deformation that accompanied the uplift of the metamorphic complex. A similar process may explain the occurrence of some CO2‐rich fluid inclusions in granulite facies rocks.
Some remarks on high‐temperature—low‐pressure metamorphism in convergent orogens
Authors: SANDIFORD M. , POWELL R.
Published: May 2007

Many high‐temperature–low‐pressure (high‐T–low‐P) metamorphic terranes show evidence for peak mineral growth during crustal thickening strain increments at pressures near the maximum attained during the heating–cooling cycle. Such terranes are not readily explained as the conductive response to crustal thickening since the resulting Moho temperatures would greatly exceed the crustal liquidus and because heating due to conductive equilibration on length scales appropriate to lithospheric‐scale strains must greatly outlast the deformation. Consequently, high‐T–low‐P metamorphism may be generated during crustal thickening only when significant heat is advected within the crust, as for example may occur during the segregation of granitic melts. We show that without the addition of asthenospheric melts and at strain rates appropriate to continental deformation the conditions required for significant lower crustal melting during deformation are only likely to be attained if heat flow into the lower crust during crustal thickening is increased substantially, for example, by removing the mantle part of the lithosphere. A simple parameterization of lithospheric deformation involving the vertical strain on the scale of the crust, c, and the lithosphere, 1 respectively, allows the potential energy of the evolving orogen to be readily evaluated. Using this parameterization we show that an important isostatic consequence of the deformation geometries capable of generating such high‐T–low‐P metamorphism during crustal thickening (with c1) is an imposed upper limit to crustal thicknesses which is much lower than for homogeneous deformations (fc= f1) for the same initial lithospheric configuration.