bluuebunny/crossref_metadata_2025 huggingface dataset:
Low‐thermal‐gradient Hercynian metamorphism in the eastern Pyrenees
Authors: AYORA C. , LlESA M. , DELGADO J.
Published: May 2007

Abstract
Fluids with compositions in the system CO2‐H2O‐NaCl were trapped in quartz veins enclosed in low‐grade metamorphic rocks (chlorite zone) on the southern flank of the Canigó Massif, eastern Pyrenees. The veins, which also contain arsenopyrite crystals, were formed contemporaneously with the main Hercynian foliation and metamorphism. Volumetric properties of the fluid and the results of arsenopyrite geothermometry suggest P‐T trapping conditions of 4.6–6 kbar and 450–530° C. This implies that an episode of metamorphism with an average geothermal gradient of 25° C km−1 occurred during the main deformation event. This episode preceded the low‐P/high‐T metamorphism described around domes and to date considered as characteristic of the Hercynian orogeny in the Pyrenees.
Fluid infiltration during contact metamorphism of interbedded marble and calc‐silicate hornfels, Twin Lakes area, central Sierra Nevada, California
Authors: DAVIS S. R. , FERRY J. M.
Published: May 2007

Abstract
In the Twin Lakes area, central Sierra Nevada, California, most contact metamorphosed marbles contain calcite + dolomite + forsterite ± diopside ± phlogopite ± tremolite, and most calc‐silicate hornfelses contain calcite + diopside + wollastonite + quartz ± anorthite ± K‐feldspar ± grossular ± titanite. Mineral‐fluid equilibria involving calcite + dolomite + tremolite + diopside + forsterite in two marble samples and wollastonite + anorthite + quartz + grossular in three hornfels samples record P± 3 kbar and T± 630° C. Various isobaric univariant assemblages record CO2‐H2O fluid compositions of χCO2= 0.61–0.74 in the marbles and χCO2= 0.11 in the hornfelses. Assuming a siliceous dolomitic limestone protolith consisting of dolomite + quartz ° Calcite ± K‐feldspar ± muscovite ± rutile, all plausible prograde reaction pathways were deduced for marble and hornfels on isobaric T‐XCO2 diagrams in the model system K2O‐CaO‐MgO‐Al2O3‐SiO2‐H2O‐CO2. Progress of the prograde reactions was estimated from measured modes and mass‐balance calculations. Time‐integrated fluxes of reactive fluid which infiltrated samples were computed for a temperature gradient of 150 °C/km along the fluid flow path, calculated fluid compositions, and estimated reaction progress using the mass‐continuity equation. Marbles and hornfelses record values in the range 0.1–3.6 × 104 cm3/cm2 and 4.8–12.9 × 104 cm3/cm2, respectively. For an estimated duration of metamorphism of 105 years, average in situ metamorphic rock permeabilities, calculated from Darcy's Law, are 0.1–8 × 10−6 D in the marbles and 10–27 × 10−6 D in the hornfelses. Reactive metamorphic fluids flowed up‐temperature, and were preferentially channellized in hornfelses relative to the marbles. These results appear to give a general characterization of hydrothermal activity during contact metamorphism of small pendants and screens (dimensions ± 1 km or less) associated with emplacement of the Sierra Nevada batholith.
Contrasting mineral parageneses in high‐temperature calc‐silicate granulites: examples from the Eastern Ghats, India
Authors: DASGUPTA S.
Published: May 2007

AbstractThree types of mineral associations are described from calc‐silicate granulites from the Eastern Ghats, India, where geothermobarometry in associated rocks suggests extremely highP–Tconditions of metamorphism (c. 9 ± 1 kbar, 950° C). These mineral associations are: (i) calcite + quartz + scapolite + plagioclase, (ii) calcite + scapolite + wollastonite + porphyroblastic garnet + coronal garnet and (iii) calcite + quartz + wollastonite + scapolite + porphyroblastic garnet + coronal garnet, all coexisting with K‐feldspar, titanite and clinopyroxene. The first two associations evolved through nearly isobaric cooling retrograde paths, whereas the third evolved through a nearly isothermal decompression path followed by an isobaric cooling retrograde path. Textural and compositional characteristics suggest the following mineral reactions in the calc‐silicate granulites: calcite + quartz = wollastonite + CO2, calcite + plagioclase = scapolite, calcite + scapolite + wollastonite = porphyroblastic garnet ± quartz + CO2, CaTs + wollastonite = coronal garnet (association ii) and wollastonite + scapolite = coronal garnet (association iii) + quartz + CO2. Andradite content in garnet was buffered by the redox equilibria wollastonite + hedenbergite + O2= andradite + quartz (association iii) and wollastonite + andradite + CaTs + scapolite = hedenbergite + calcite + grossular + O2(association ii). The contrasting mineral parageneses have been ascribed to interplay of variables such asXCO2,fO2,fHClin the fluid, bulk Na content and the nature of the retrogradeP–T–XCO2paths through which the rocks evolved.
Partially retrograded eclogites of the Münchberg Massif, Germany: records of a multi‐stage Variscan uplift history in the Bohemian Massif
Authors: O'BRIEN P. J.
Published: May 2007

Abstract
Eclogites with a wide range in bulk composition are present in the Münchberg Massif, part of the Variscan basement of the Bohemian Massif in north‐east Bavaria. New analyses of the primary phases garnet, omphacite, phengite and amphibole, as well as the secondary phases clinopyroxene II, various amphiboles, biotite/phlogopite, plagioclase, margarite, paragonite, prehnite and pumpellyite, reveal a complex uplift history. New discoveries were made of samples with very jadeite‐rich primary omphacite as well as a secondary omphacite in a symplectite with albite. Various geothermobarometric techniques, together with thermodynamic databases (incorporating separately determined activity–composition values) and experimental data have clustered the minimum conditions for the primary assemblages to the P–T range 650 ± 60° C, 14.3 ± 1 kbar. However, jadeite (in omphacite)–kyanite–paragonite (in phengite) and zoisite–grossular (in garnet)–kyanite–quartz relationships suggests pressures of 25–28 kbar at the same temperatures. The fact that the secondary omphacite–plagioclase assemblage yields pressures within a few hundred bars of the minimum pressures for the plagioclase‐free assemblages strongly suggests that the minimum values are serious underestimates.

Zoning, inclusion suites and breakdown reactions of primary phases, in addition to new minerals formed during uplift, define a polyphase metamorphic evolution which, from geochronological evidence, occurred solely within the Variscan cycle. The complex breakdown in other Bohemian Massif eclogites and the distinct variation in their temperatures during uplift suggest a multi‐stage thrusting model for the regional evolution of the eclogites. Such an evolution has significance with respect to incorporation of mantle slices into crustal sequences and fluid derivation from successively subducted units, possibly driving the breakdown reactions.
Generation of pseudotachylite under granulite facies conditions, and its preservation during cooling
Authors: CLARKE G. L. , NORMAN A. R.
Published: May 2007

Abstract
Discontinuous ultramylonite zones cut Proterozoic granulite facies gneisses in MacRobertson Land, east Antarctica, and preserve evidence of ductile non‐coaxial flow and reverse sense of shear. Cross‐cutting relationships indicate that ultramylonite deformation involved overthrusting to the east, but progressively rotated to involve overthrusting to the north; rotation of the principal compressive stress axes is inferred. Extensive pseudotachylite developed during ultramylonitization, the history of individual ultramylonite zones having involved a single episode of pseudotachylite generation. Neoblastic sillimanite indicates ultramylonitization occurred at >520° C. On the basis of inferred recrystallized granulite facies mineral assemblages ultramylonitization occurred at >700° C, and ≤7.3 ± 0.5 kbar, at aH2O± 0.3 and low aCO2. Comparison of these values with those suggested by metamorphic assemblages in rocks unaffected by mylonitization indicates that the Rayner Complex experienced a late increase in pressure of 1–2 kbar during ultramylonitization. The P‐T‐aH2O conditions of the ultramylonite zones are inferred to have been close to the solidus for minimum melting, pseudotachylite generation having involved a limited pressure drop during brittle fracturing at high strain rates. Most of the pseudotachylite veins are undeformed; the mechanism(s) of fracturing and melting must have caused strain hardening in rocks surrounding the ultramylonite, further strain having been mostly accommodated by a new or subsidiary shear zone. Renewed stress at reduced strain rates, or renewed stress in zones in which the proportion of pseudotachylite was significantly higher, could have led to the rare occurrences of deformed pseudotachylite. The preservation of fine‐grained pseudotachylite is dependent on it remaining dry.
Dehydration veins in diagenetic and very‐low‐grade metamorphic rocks: features of the crustal seismogenic zone and their significance to mineral facies
Authors: COOMBS D. S.
Published: May 2007

Abstract
Declining temperatures during decay of a hydrothermal system, or during uplift and erosion, tend to result in veins involving progressive hydration reactions, e.g. veins with laumontite cutting prehnitepumpellyite facies rocks, and stilbite veins cutting laumontite veins.

In contrast, examples are described of analcime replacement of heulandite along fractures in heulanditized vitric tuff, of replacement of analcime by albite along fractures in quartz‐analcime rock, of joint‐controlled replacement of heulandite in tuff by laumontite + quartz + (Na, K)‐feldspars, of replacement of laumontite by prehnite + quartz along fractures in alumontitized vitric tuff, and of laumontitebearing feldspathic sandstones cut by vein assemblages of quartz and prehnite ° Calcite. The vein mineral assemblage, sometimes with pumpellyite and/or epidote in the prehnite‐bearing veins, tends to spread as a zone of dehydration into the adjacent country rock. Except perhaps for albite replacement of analcime, and for laumontite replacement of heulandite, these open‐system reactions involve cation activity ratios in the fluid. All involve dehydration. They are favoured by an increase in temperature, and except under certain situations where P‐T equilibrium curves have negative slopes, are favoured by a fall in PH2O. Evidence indicates that in at least some cases the triggering mechanism was a drop in PH2O; this may be a widespread phenomenon associated with brittle fracture in the seismogenic upper crust. This may cause fluid pressure to drop from values approaching lithostatic to nearer hydrostatic, and equilibrium may be displaced to yield a less hydrous assemblage that appears as a dehydration vein and vein verge. The dehydration vein assemblage may be diagnostic of a higher grade mineral facies and adds to the mineral complexity attributable to varying permeabilities and fluid pressures in upper crustal strata. Mineral facies are likely to be more uniformly distributed in higher grade rocks from below the brittle‐ductile transition zone.

Reactions involving complex solid solutions are inappropriate as facies boundaries.
Diagenesis and very low‐grade metamorphism of volcaniclastic sandstones from contrasting geodynamic environments, North Island, New Zealand: the Murihiku and Waipapa terranes
Authors: BLACK P. M. , CLARK A. S. B. , HAWKE A. A.
Published: May 2007

Abstract
The Mesozoic Murihiku and Waipapa terranes are two accretionary wedges of linked forearc and trench sediments, respectively, that were juxtaposed in the early Cretaceous.

Late Triassic to late Jurassic Murihiku terrane volcaniclastic sediments are folded into a regional syncline and have been diagenetically altered. There is a general relationship between zeolite occurrence, clay mineralogy, vitrinite reflectance and stratigraphic position. Youngest Jurassic sediments contain heulandite, analcime and stilbite, whereas late Triassic to mid‐Jurassic sediments have laumontite and heulandite (in detail the zeolite distribution is complicated). Tuffaceous horizons on the eastern limb of the syncline are calcitized rather than zeolitized. Post‐diagenetic fractures associated with uplift are laumontite‐filled. The inferred geothermal gradient is c. 15° C km−1.

The Waipapa terrane is an accretionary complex dominated by imbricated terrigenous sediments of Triassic and Jurassic age with enclosed Permian to Jurassic pelagic sediments and basalts. Late Jurassic sediments are massive volaniclastic sandstones. The sediments are non‐foliated, and metamorphic minerals in the massive sandstones have crystallized in specific domains. The observed metamorphic succession of prehnite‐pumpellyite and pumpellyite‐actinolite facies assemblages was overprinted in the imbricated rocks during a thermal event that was late in the deformation sequence and broadly coincident with hydraulic fracturing and veining.

The metamorphic successions in the two terranes and their relationships to structural features are in excellent accord with accretionary complex models.
Compositional variations of Cretaceous pumpellyites along the western margin of South America and their relation to an extensional geodynamic setting
Authors: AGUIRRE L.
Published: May 2007

Abstract
Pumpellyites in pervasively altered basic to intermediate rocks from three Cretaceous Andean volcanic sequences, the Colombian Diabasic Group, the Ecuadorian‐Peruvian Celica Formation/Casma Group and the Chilean Ocoite Group, were studied to test their compositional behaviour in relation to changes in geodynamic setting. They occur mostly in assemblages of the prehnite‐pumpellyite facies filling amygdules and inside plagioclase phenocrysts.

(a) Pumpellyites from the three geodynamic settings define three distinct compositional fields in AFM space: (i) those in the Diabasic Group plot closer to the Fe corner; (ii) those in the Ocoite Group plot closer to the Al corner; (iii) those in the Celica Formation/Casma group plot between, and partly overlapping, fields (i) and (ii).

(b) Pumpellyites in group (i) formed in T‐MORB like, K‐poor tholeiites (high Fe/Al), affected by ocean‐floor metamorphism in an oceanic back‐arc basin. Pumpellyites in group (ii) generated in K‐rich, calc‐alkaline (low Fe/Al) to shoshonitic metabasites affected by burial metamorphism in an ensialic, aborted, marginal basin with moderate attenuation of the continental crust. Pumpellyites in group (iii) formed in basic and intermediate, calc‐alkaline to tholeiitic rocks, metamorphosed in ensialic marginal basins with various degrees of continental crust thinning, from splitting to moderate attenuation. A correlation between pumpellyite and host‐rock composition is suggested by these characteristics.

(c) Pumpellyites in prehnite‐pumpellyite facies assemblages of some of the units studied plot inside higher (and lower) grade reference fields (AFM space) corresponding to different geodynamic settings elsewhere. This anomaly is attributed to the changing characteristics of the marginal basins at the South American margin and emphasizes the need to compare equal facies referred to equal settings.
Geothermal‐type alteration in a burial metamorphosed volcanic pile, central Chile
Authors: VERGARA M. , LEVI B. , VILLARROEL R.
Published: May 2007

Abstract
The Upper Cretaceous and Tertiary volcanic sequences in the Chilean Andes are affected by burial metamorphism. For example, in central Chile (between 32°30’and 35°S), the Abanico Formation, a folded upper Cretaceous to Palaeogene unit composed of basic lavas, tuffs and ash flows of intermediate composition and volcaniclastic sandstones, is characterized by heulandite‐ to laumontite‐bearing zeolite facies assemblages in its upper part passing with depth to prehnite–pumpellyite facies assemblages.

However, at c. 33°30'S in the Abanico Hill area, located just east of a graben at Santiago (the longitudinal Central Valley), the alteration pattern is unrelated to stratigraphic depth. It is characterized by a lateral increase in grade defined by assemblages with yugawaralite (reported for the first time in the Andes) laumontite, then wairakite ± epidote, and finally with abundant epidote successively closer to the graben boundary. This pattern was formed in a palaeogeothermal system with a high‐T and a very low‐P gradient. Geothermal alteration has also been inferred from the border zone of a Neogene caldera in the Abanico Hill area, and from the western fault boundary of the graben.

These expressions of geothermal alteration, together with the occurrence of caldera structures, the huge volume of ignimbrites and numerous epithermal precious metal deposits of late Cretaceous and Tertiary age in central and northern Chile, suggest that fossil geothermal systems of this age are probably common features in the Chilean Andes.
Mg–Fe carpholites from aluminous schists in the Diahot region and implications for preservation of high‐pressure/low‐temperature schists, northern New Caledonia
Authors: BLACK P. M. , MAURIZOT P. , GHENT E. D. , STOUT M. Z.
Published: May 2007

Abstract
Mg–Fe carpholite is widespread in the Diahot region of New Caledonia in highly aluminous schists and as veins in what was originally a clay‐rich hydrothermal alteration envelope about massive suphide deposits. These carpholites have Fe/(Fe + Mg) ratios of 0.03–0.65 and no significant Mn component. Mg‐carpholite + quartz occur in assemblages with chlorite or pyrophyllite, pyrophyllite + kaolinite and pyrophyllite + diaspore. Temperatures of 230–320° C and minimum pressures of 7 kbar are indicated for the Mg–Fe carpholite‐bearing rocks. The regional distribution of aragonite and Mg–Fe carpholite parallel to a major zone of dominantly transcurrent movement and oblique to the trend of the subduction complex indicates the high‐P/low‐T schists owe their rapid uplift and preservation to the vertical component of the transcurrent faulting.
P–T–t paths and tectonic history of an early Precambrian granulite facies terrane, Jining district, south‐east Inner Mongolia, China
Authors: LIANGZHAO LU , SHIQIN JIN
Published: May 2007

Abstract
The widespread khondalite series of south‐east Inner Mongolia consists largely of biotite–sillimanite–garnet gneiss and quartzo‐feldspathic gneiss with some marble and mafic granulite layers. It has experienced two metamorphic events at c. 2500 and 1900–2000 Ma.

A pre‐peak stage of the first metamorphism at T= 600–700°C and P > 6–7 kbar is recognized by the relict amphibolite facies assemblage Ky–Grt–Bt–Pl–Qtz and ‘protected’inclusions of biotite, hornblende, sodic plagioclase and quartz in garnet or orthopyroxene. The peak stage, with T=c. 800 ± 50°C and P 8–10 kbar, is characterized by the widespread granulite facies assemblages Sil–Grt–Bt–Kfs–Pl–Qtz in gneiss and Opx–Cpx–Pl ± Hbl ± Grt in granulite. The P–T–t path suggests that the supracrustal sequence was buried in the lower crust by tectonic thickening during D1–D2.

The beginning of the second metamorphism is characterized by further temperature rise to 700°C or more at lower pressure. This stage is manifested by the appearance of cordierite after garnet, fibrolite (Sil2) after biotite in gneiss and transformation of Hbl1 into Opx2 and Cpx2 in granulite. Coronas of symplectitic Opx2 + Pl2 surrounding Grt1 and Cpx1 in mafic granulite are interpreted as products of near‐isothermal decompression. The P–T–t path may be related tectonically to waning extension of the crust by the end of the early Proterozoic.
Two types of Precambrian high‐grade metamorphism, Inner Mongolia, China
Authors: XISHAN LIU , WEI JIN , SHUXUN LI , XUECHUN XU
Published: May 2007

AbstractArchaean and Proterozoic granulite facies complexes of Inner Mongolia differ in lithological association, tectonic style, mineral assemblage and metamorphicP–Tpath. A nearly isobaric cooling path for Archaean high‐grade metamorphic rocks is suggested by reaction textures and geothermobarometry. Early Proterozoic metamorphic rocks show nearly isothermal decompression. Archaean metamorphism may have been caused by magmatic accretion, whereas early Proterozoic metamorphism suggests a major continental thickening event followed by exhumation.
Ultra‐high‐pressure metamorphism of carbonate rocks in the Dabie Mountains, central China
Authors: WANG XIAOMIN , LIOU J. G.
Published: May 2007

Abstract
Widespread ultra‐high‐P assemblages including coesite, quartz pseudomorphs after coesite, aragonite, and calcite pseudomorphs after aragonite in marble, gneiss and phengite schist are present in the Dabie Mountains eclogite terrane. These assemblages indicate that the ultra‐high‐P metamorphic event occurred on a regional scale during Triassic collision between the Sino‐Korean and Yangtze cratons. Marble in the Dabie Mountains is interlayered with coesite‐bearing eclogite and gneiss and as blocks of various size within gneiss. Discontinuous boudins of eclogite occur within marble layers. Marble contains an ultra‐high‐P assemblage of calcite/aragonite, dolomite, clinopyroxene, garnet, phengite, epidote, rutile and quartz/coesite. Coesite, quartz pseudomorphs after coesite, aragonite and calcite pseudomorphs after aragonite occur as fine‐grained inclusions in garnet and omphacite. Phengites contain about 3.6 Si atoms per formula unit (based on 11 oxygens). Similar to the coesite‐bearing eclogite, marble exhibits retrograde recrystallization under amphibolite–greenschist facies conditions generated during uplift of the ultra‐high‐P metamorphic terrane. Retrograde minerals are fine grained and replace coarse‐grained peak metamorphic phases. The most typical replacements are: symplectic pargasitic hornblende + epidote after garnet, diopside + plagioclase (An18) after omphacite, and fibrous phlogopite after phengite. Ferroan pargasite + plagioclase, and actinolite formed along grain boundaries between garnet and calcite, and calcite and quartz, respectively.

The estimated peak P–T conditions for marble are comparable to those for eclogite: garnet–clinopyroxene geothermometry yields temperatures of 630–760°C; the garnet–phengite thermometer gives somewhat lower temperatures. The minimum pressure of peak metamorphism is 27 kbar based on the occurrence of coesite. Such estimates of ultra‐high‐P conditions are consistent with the coexistence of grossular‐rich garnet + rutile, and the high jadeite content of omphacite in marble. The fluid for the peak metamorphism was calculated to have a very low XCO2 (<0.03). The P–T conditions for retrograde metamorphism were estimated to be 475–550°C at <7 kbar.
Granulites and garnet–cordierite gneisses from Dronning Maud Land, Antarctica
Authors: BUCHER‐NURMINEN K. , OHTA Y.
Published: May 2007

Abstract
The central sector of Mühlig‐Hofmannfjellet (3°E/71°S) in western Dronning Maud Land (East Antarctic shield) is dominated by large intrusive bodies of predominantly orthopyroxene‐bearing quartz syenites (charnockites). Metasedimentary rocks are rare; however, two distinct areas with banded gneiss–marble–quartzite sequences of sedimentary origin were found during the Norwegian Antarctic Research Expedition NARE 1989/90. Cordierite‐bearing metapelitic gneisses from two different localities contain the characteristic mineral assemblage: cordierite + garnet + biotite + K‐feldspar + plagioclase + quartz ± sillimanite ± spinel. Thermobarometry indicates equilibration conditions of about 650°C and 4 kbar. Associated orthopyroxene–garnet granulites, on the other hand, revealed pressures of about 8 kbar and temperatures of 750°C. The earlier granulite facies metamorphism is not well preserved in the cordierite gneisses as a result of excess K‐feldspar combined with interaction with an H2O‐rich fluid phase, probably released by the cooling intrusives. These two features allowed the original high‐grade K‐feldspar + garnet assemblages to recrystallize as cordierite–biotite–sillimanite gneisses, completely re‐equilibrating them. Phase relationships indicate that the younger metamorphic event occurred in the presence of a fluid phase that varied in composition between the lithologies.
The metamorphic signature of contemporaneous extension and shortening in the central Himalayan orogen: data from the Nyalam transect, southern Tibet
Authors: HODGES K. V. , BURCHFIEL B. C. , ROYDEN L. H. , CHEN Z. , LIU Y.
Published: May 2007

Abstract
Geological relationships and geochronological data suggest that in Miocene time the metamorphic core of the central Himalayan orogen was a wedge‐shaped body bounded below by the N‐dipping Main Central thrust system and above the N‐dipping South Tibetan detachment system. We infer that synchronous movement on these fault systems expelled the metamorphic core southward toward the Indian foreland, thereby moderating the extreme topographic gradient at the southern margin of the Tibetan Plateau. Reaction textures, thermobarometric data and thermodynamic modelling of pelitic schists and gneisses from the Nyalam transect in southern Tibet (28°N, 86°E) imply that gravitational collapse of the orogen produced a complex thermal structure in the metamorphic core. Amphibolite facies metamorphism and anatexis at temperatures of 950 K and depths of at least 30 km accompanied the early stages of displacement on the Main Central thrust system. Our findings suggest that the late metamorphic history of these rocks was characterized by high‐T decompression associated with roughly 15 km of unroofing by movement on the South Tibetan detachment system. In the middle of the metamorphic core, roughly 7–8 km below the basal detachment of the South Tibetan system, the decompression was essentially isothermal. Near the base of the metamorphic core, roughly 4–6 km above the Main Central thrust, the decompression was accompanied by about 150 K of cooling. We attribute the disparity between the P–T paths of these two structural levels to cooling of the lower part of the metamorphic core as a consequence of continued (and probably accelerated) underthrusting of cooler rocks in the footwall of the Main Central thrust at the same time as movement on the South Tibetan detachment system.
Petrological evolution of amphibolite shear zones, Cheyenne Belt, south‐eastern Wyoming, USA
Authors: NYMAN M. W. , TRACY R. J.
Published: May 2007

Abstract
Metre‐scale amphibolite boudins in the Cheyenne Belt of south‐eastern Wyoming are cut and deformed by shear zones which preserve a full strain transition across 7 cm, from relatively undeformed amphibolite with a relict igneous texture to mylonitic amphibolite with an L‐S tectonic fabric. The strain transition is marked by the progressive rotation of amphibole + plagioclase aggregates into parallelism with the shear‐zone boundary. An increase in strain magnitude is indicated by development of the tectonic fabric and progressive reduction of amphibole and plagioclase grain size as a result of cataclasis. Bulk chemistry of five samples across a single strain transition shows no significant or systematic variation in major element chemistry except for a minor loss of SiO2, which indicates that the shear zone was a system essentially closed to non‐volatile components during metamorphism and deformation. Amphibolites throughout the shear zone consist of amphibole and plagioclase with only minor amounts of quartz, chlorite, epidote, titanite and ilmenite. Within the relatively undeformed amphibolite, amphibole and plagioclase have wide compositional ranges in single thin sections. Amphibole compositions vary from actinolitic hornblende to magnesio‐hornblende with increases in Al, Fe, Na and K contents and decreases in Si and Mg that can be modelled as progress along tschermakite, edenite and FeMg‐1 exchange vectors from tremolite. Plagioclase ranges from An60 in cores to An30 within grain‐boundary domains. With increasing strain magnitude, local variation of amphibole composition decreases as amphibole becomes predominantly magnesio‐hornblende. Plagioclase composition range also decreases, although grain‐boundary domains still have higher albite content. These petrological data indicate that shear‐zone metamorphism was controlled by the magnitude of strain during synmetamorphic deformation. SEM and microprobe imaging indicate that chemical reactions occurred by a dissolution and reprecipitation process during or after cataclastic deformation. This suggests that grain‐boundary formation was an important process in the petrological evolution of the shear zone, possibly by providing zones for fluid ingress to facilitate metamorphic reactions. These results highlight the necessity for conducting detailed microstructural evaluation of rocks in order to interpret petrological, isotopic and geochronological data.
Arrested charnockite formation at Kottavattam, southern India
Authors: RAITH M. , SRIKANTAPPA C.
Published: May 2007

AbstractAt Kottavattam, southern Kerala (India), late Proterozoic homogeneous leptynitic garnet–biotite gneisses of granitic composition have been transformed on a decimetric scale into coarse‐grained massive charnockitesensu strictoalong a set of conjugate fractures transecting the gneissic foliation. Charnockitization post‐dates the polyphase deformation, regional high‐grade metamorphism and anatexis, and evidently occurred at a late stage of the Pan‐African tectonothermal history. Geothermobarometric and fluid inclusion data document textural and chemical equilibration of the gneiss and charnockite assemblages at similarPlith–Tconditions (650–700°C, 5–6 kbar) in the presence of carbonic fluids internally buffered by reaction with graphite and opaque mineral phases (XCO2= 0.7–0.6;XH2O= 0.2–0.3;XN2= 0.1; logfO2= ‐17.5).

Mineralogical zonation indicates that charnockitization of the leptynitic gneiss involved first the breakdown of biotite and oxidation of graphite in narrow, outward‐migrating transition zones adjacent to the gneiss, followed by the breakdown of garnet and the neoblastesis of hypersthene in the central charnockite zone. Compared to the host gneiss, the charnockite shows higher concentrations of K, Na, Sr, Ba and Zn and lower concentrations of Mg, Fe, Ti, V, Y, Zr and the HREE, with a complementary pattern in the narrow transition zones of biotite breakdown. ThePlith–T–XH2Odata and chemical zonation patterns indicate charnockitization through subsolidus‐dehydration reaction in an open system. Subsequent residence of the carbonic fluids in the charnockite resulted in low‐grade alteration causing modification of the syn‐charnockitic elemental distribution patterns and the properties of entrapped fluids. We favour an internally controlled process of arrested charnockitization in which, during near‐isothermal uplift, the release of carbonic fluids from decrepitating inclusions in the host gneiss into simultaneously developing fracture zones led to a change in the fluid regime from ‘fluid‐absent’in the gneiss to ‘fluid‐present’in the fracture zones and to the development of an initial fluid‐pressure gradient, triggering the dehydration reaction.
Carbon‐isotope constraints on fluid advection during contrasting examples of incipient charnockite formation
Authors: HARRIS N. B. W. , JACKSON D. H. , MATTEY D. P. , SANTOSH M. , BARTLETT J.
Published: May 2007

AbstractIncipient charnockite formation within amphibolite facies gneisses is observed in South India and Sri Lanka both as isolated sheets, associated with brittle fracture, and as patches forming interconnected networks. For each mode of formation, closely spaced drilled samples across charnockite/gneiss boundaries have been obtained and δ13C and CO2abundances determined from fluid inclusions by stepped‐heating mass spectrometry.

Isolated sheets of charnockite (c.50 mm wide) within biotite–garnet gneiss at Kalanjur (Kerala, South India) have developed on either side of a fracture zone. Phase equilibria indicate low‐pressure charnockite formation at pressures of 3.4 ± 1.0 kbar and temperatures of about 700°C (forXH2O= 0.2). Fluid inclusions from the charnockite are characterized by δ13C values of −8% and from the gneiss, 2 m from the charnockite, by values of −15%. The large CO2abundances and relatively heavy carbon‐isotope signature of the charnockite can be traced into the gneiss over a distance of at least 280 mm from the centre of the charnockite, whereas the reaction front has moved only 30 mm. This suggests that fluid advection has driven the carbon‐isotope front through the rock more rapidly than the reaction front. The carbon‐front/reaction‐front separation at Kalanjur is significantly larger than the value determined from a graphite‐bearing incipient charnockite nearby, consistent with the predictions of one‐dimensional advection models.

Incipient charnockites from Kurunegala (Sri Lanka) have developed as a patchy network within hornblende–biotite gneiss. CO2abundances rise to a peak near one limb of the charnockite, and isotopic values vary from δ13C ofc.−5.5% in the gneiss to −9.5% in the charnockite. The shift to lighter values in the charnockite can be ascribed to the formation of a CO2‐saturated partial melt in response to influx of an isotopically light carbonic fluid.

Thus, incipient charnockites from the high‐grade terranes of South India and Sri Lanka reflect a range of mechanisms. At shallower structural levels non‐pervasive CO2influxed along zones of brittle fracture, possibly associated with the intrusion of charnockitic dykes. At deeper levels,in situmelting occurred under conditions of ductile deformation, leading to the development of patchy charnockites.
Crustal trajectory of sapphirine‐bearing granulites from Ganguvarpatti, South India: evidence for an isothermal decompression path
Authors: MOHAN A. , WINDLEY B. F.
Published: May 2007

AbstractGanguvarpatti is part of a Precambrian terrane characterized by granulite facies rocks, including charnockites, mafic granulites, sapphirine‐bearing granulites, leptynites and gneisses. A sequence of reactions deduced from the multiphase reaction textures provide information on the metamorphic history of this area, as they formed in response to decompression during uplift. Geothermobarometry and constraints from reaction textures define a segment of aP–Tpath traversed by the granulites of Ganguvarpatti. Near‐peak metamorphic conditions ofc.800°C and 8 kbar were succeeded by a symplectitic stage at a significantly lower pressure (c.700°C and 4.5 kbar), documenting a nearly isothermal decompressionP–Tpath and rapid uplift (c.12 km) followed by cooling. The presence of many fluid inclusions of extremely low density in the charnockites is consistent with a nearly isothermal uplift path. Attainment of a maximum pressure ofc.8 kbar indicatesc.27 km depth of burial during metamorphism. This would imply a total crustal thickness ofc.65–70 km at 2.6–2.5 Ga. Such a profound crustal thickness and a clockwise decompressiveP–Tpath is interpreted as a consequence of tectonic thickening of crust, accomplished by collision tectonics of the southern granulite terrane against the Dharwar craton along the Palghat–Cauvery shear zone via northward subduction.
Age of younger tonalitic magmatism and granulitic metamorphism in the South Indian transition zone (Krishnagiri area); comparison with older Peninsular gneisses from the Gorur–Hassan area
Authors: PEUCAT J. J. , MAHABALESWAR B. , JAYANANDA M.
Published: May 2007

AbstractA major episode of continental crust formation, associated with granulite facies metamorphism, occurred at 2.55–2.51 Ga and was related to accretional processes of juvenile crust. Dating of tonalitic–trondhjemitic, granitic gneisses and charnockites from the Krishnagiri area of South India indicates that magmatic protoliths are 2550–2530 ± 5 Ma, as shown by both U–Pb and207Pb/206Pb single zircon methods. Monazite ages indicate high temperatures of cooling corresponding to conditions close to granulite facies metamorphism at 2510 ± 10 Ma. These data provide precise time constraints and Sr–Nd isotopes confirm the existence of late tonalitic–granodioritic juvenile gneisses at 2550 Ma. Pb single zircon ages from the older Peninsular gneisses (Gorur–Hassan area) are in agreement with some previous Sr ages and range between 3200 ± 20 and 3328 ± 10 Ma. These gneisses were derived from a 3.3–3.5‐Ga mantle source as indicated from Nd isotopes. They did not participate significantly in the genesis of the 2.55‐Ga juvenile magmas. All these data, together with previous work, suggest that the 2.51‐Ga granulite facies metamorphism occurred near the contact of the ancient Peninsular gneisses and the 2.55–2.52‐Ga ‘juvenile’tonalitic–trondhjemitic terranes during synaccretional processes (subduction, mantle plume?). Rb–Sr biotite ages between 2060 and 2340 Ma indicate late cooling probably related to the dextral major east–west shearing which displaced the 2.5‐Ga juvenile terranes toward the west.
Structural relations of charnockites of the Archaean Dharwar craton, southern India
Authors: NAHA K. , SRNIVASAN R. , JAYARAM S.
Published: May 2007

AbstractTwo varieties of charnockites are recognized in the Dharwar craton of southern India. The style and sequence of structures in one charnockite variety, and related intermediate to basic granulites, are similar to those in the supracrustal rocks of the Dharwar Supergroup and the adjacent Peninsular Gneiss. This style has isoclinal folds with long limbs and sharp hinges with an axial planar fabric in some instances. Additional evidence of flattening is provided by pinch‐and‐swell and boudinage structures, with basic granulites forming boudins in the more ductile charnockites/enderbites in the limbs of isoclinal folds. These folds are involved in near‐coaxial upright folding resulting in the bending of the axial planes of the isoclinal folds and the associated boudins. All these structures are overprinted by non‐coaxial upright folds with axial planes striking nearly N–S. The map pattern of charnockites suggests that this sequence of structures is present not only on a mesoscopic scale, but also on a macroscopic scale. Charnockites of this variety provide, in some instances, evidence of having been migmatized to give rise to hornblende–biotite gneiss and biotite gneiss, which form a part of the Peninsular Gneiss terrane.

The second variety comprises charnockitesensu strictowith an entirely different structural style. This type occurs in the tensional domains of the hinge zones of the later buckle folds, in the necks of foliation boudinage, in shear zones and in release joints parallel to the axial planes of the later folds in the Peninsular Gneiss. Because the non‐coaxial later folds are associated with a strain pattern different from, and later than, that of the isoclinal folds of the first generation, it follows that charnockites of the Dharwar craton have evolved in at least two distinct phases, separate both in time and in process.
Petrology of lawsonite‐, pumpellyite‐ and sodic amphibole‐bearing metabasites from north‐east Oman
Authors: EL‐SHAZLY ALEY EL‐DIN K.
Published: May 2007

The assemblages chlorite‐pumpellyite‐lawsonite‐albite‐quartz, chlorite‐lawsonite‐quartz‐epidote and chlorite‐epidote‐albite‐quartz occur in metabasaltic blocks and veins in a metamorphosed tectonic mélange in the structurally highest unit of the autochthonous and parautochthonous section underlying the Semail ophiolite in Saih Hatat, north‐east Oman. The pre‐Permian basement of this section contains mafic units characterized by the assemblage crossite‐epidote‐chlorite‐quartz‐albite /el actinolite. These assemblages indicate a down‐section increase in metamorphic grade from ‘lawsonite‐albite facies' conditions in the mélange to ‘epidote‐blueschist’ facies conditions in the basement.

Application of empirically and experimentally based thermobarometers as well as petrogenetic grids calculated for a model basaltic system indicates that the P‐T conditions of metamorphism ranged from 3 to 6 kbar and 250 to 300d̀ C for the mélange and P > 6.8 kbar, T > 310d̀ C for the basement units. Textural relations interpreted in the context of petrogenetic grids indicate that these units followed clockwise P‐T paths of evolution. The estimated P‐T conditions and down‐section increase in metamorphic grade in central, western and northern Saih Hatat are consistent with the hypothesis relating metamorphism to the Late Cretaceous tectonic loading of the continental margin by an ophiolite slab < 18 km in thickness. These results contrast with field and petrological observations documented for blueschists and eclogites exposed along the eastern coast of Saih Hatat which may have formed at an earlier stage in response to an Early Cretaceous collisional event.
Controls on the mechanisms of fluid infiltration and front advection during regional metamorphism: a stable isotope and textural study of retrograde Dalradian rocks of the SW Scottish Highlands
Authors: FEIN J. B. , GRAHAM C. M. , HOLNESS M. B. , FALLICK A. E. , SKELTON A. D. L.
Published: May 2007

Vein‐controlled retrograde infiltration of H2O‐CO2 fluids into Dalradian epidote amphibolite facies rocks of the SW Scottish Highlands under greenschist facies conditions resulted in alteration of calcite‐rich marble bands to dolomite and spatially associated 18O enrichment of about 10%. on a scale of metres. Fluid inclusion data indicate that the retrograde fluid was an H2O‐salt mixture with a low CO2 content, and that the temperature of the fluid was about 400d̀ C. Detailed petrographic and textural (backscattered electron imaging) studies at one garnet‐grade locality show that advection of fluid into marbles proceeded by a calcite‐calcite grain edge flow mechanism, while alteration of non‐carbonate wall‐rock is associated with veinlets and microcracks.

Stable isotopic analysis of carbonates from marble bands provides evidence for advection of isotopic fronts through carbonate wall‐rocks perpendicular to dolomite veins, and fluid fluxes in the range 2.4–28.6 m3/m2 have been computed from measured advection distances. Coincidence of isotope and reaction fronts is considered to result from reaction‐enhanced kinetics of isotope exchange at the reaction front. Front advection distances are related to the proportion of calcite to quartz in each marble band, with the largest advection distance occurring in nearly pure calcite matrix. This relationship indicates that fluid flow in carbonates is only possible along fluid‐calcite‐calcite grain edges. However, experimental constraints on dihedral angles in calcite‐fluid systems require that pervasive infiltration occurred in response to calcite dissolution initiated at calcite‐calcite grain junctions rather than to an open calcite pore geometry.

The regional extent of the retrograde infiltration event has been documented from the high δ18O of dolomite‐ankerite carbonates from veins and host‐rocks over an area of least 50 × 50 km in the SW Scottish Highlands. Isotopically exotic 18O‐rich retrograde fluids have moved rapidly upwards through the crust, inducing isotopic exchange and mineral reaction in wall‐rocks only where lithology, pore geometry or mineral solubilities, pressure and temperature have been appropriate for pervasive infiltration to occur.
Contrasting retrograde oxygen isotope exchange behaviour and implications: examples from the Langtang Valley, Nepal
Authors: MASSEY J. A. , HARMON R. S. , HARRIS N. B. W.
Published: May 2007

Quantitative modelling of oxygen exchange by diffusion during slow cooling has been compared to the observed oxygen isotope distributions from high‐grade metamorphic and granitic rocks of the High Himalayan Crystallines, Langtang Valley, central Nepal, in order to investigate the effect of retrograde diffusional exchange on the preservation of high‐temperature, oxygen isotope systematics.

Modelled fractionations, using water‐present diffusion data reported in the literature, predict quartz‐mica fractionations to be much larger than those at peak metamorphic and igneous conditions due to low closure temperatures for micas. Quartz‐feldspar fractionations may be less than those at peak conditions, and in some samples may even be slightly negative.

The observed oxygen isotope fractionations in the metamorphic rocks are small and largely appear to record equilibrations close to peak conditions determined by other methods. Hence these rocks clearly do not conform to predictions of fluid‐present diffusional retrograde exchange. It is suggested that their retrograde history was therefore within an anhydrous closed system in which diffusion was slow and hence mineral closure temperatures were high. The granitic rocks record rather larger quartz‐biotite fractionations, approaching those predicted by the diffusion modelling. However, quartz‐feldspar fractionations are large and hence, although significant retrograde exchange has clearly occurred, simple diffusion alone is not sufficient to explain the observed data and open‐system exchange may be required. The presence of fluids during the retrograde history of this part of the section is supported by petrographic evidence.

The different retrograde oxygen exchange histories recorded between the regional metamorphic and magmatic regimes of the Langtang section would appear to support the importance of water on the kinetics of such exchange, and suggests that in its absence, diffusional exchange may become insignificant, allowing oxygen isotope thermometry to record meaningful high‐temperature data.
Stable isotopic signatures of superposed fluid events in granulite facies marbles of the Rauer Group, East Antarctica
Authors: BUICK I. S. , HARLEY S. L. , CARTWRIGHT I. , MATTEY D.
Published: May 2007

The role of volatiles in the stabilization of the lower (granulite facies) crust is contentious. Opposing models invoke infiltration of CO2‐rich fluids or generally vapour‐absent conditions during granulite facies metamorphism. Stable isotope and petrological studies of granulite facies metacarbonates can provide constraints on these models. In this study data are presented from metre‐scale forsteritic marble boudins within Archaean intermediate to felsic orthogneisses from the Rauer Group, East Antarctica.

Forsteritic marble layers and associated calcsilicates preserve a range of 13C‐ and 18O‐depleted calcite isotope values (δ13C= ‐9.9 to ‐3.0% PDB, δ18O = 4.0 to 12.1% SMOW). A coupled trend of 13C and 18O depletion (∼2%, ∼5%, respectively) from core to rim across one marble layer is inconsistent with pervasive CO2 infiltration during granulite facies metamorphism, but does indicate localized fluid‐rock interaction. At another locality, more pervasive fluid infiltration has resulted in calcite having uniformly low, carbonatite‐like δ18O and δ13C values. A favoured mechanism for the low δ18O and δ13C values of the marbles is infiltration by fluids that were derived from, or equilibrated with, a magmatic source. It is likely that this fluid‐rock interaction occurred prior to high‐grade metamorphism; other fluid‐rock histories are not, however, ruled out by the available data. Coupled trends of 13C and 18O depletion are modified to even lower values by the superposed development of small‐scale metasomatic reaction zones between marbles and internally folded mafic (?) interlayers. The timing of development of these layers is uncertain, but may be related to Archaean high‐temperature (>1000d̀C) granulite facies metamorphism.