bluuebunny/crossref_metadata_2025_split huggingface dataset:
Blue amphiboles and their significance for the metamorphic history of the Pan‐African Gariep belt, Namibia
Authors: FRIMMEL H. E. , HARTNADY C. J. H. 
Published: May 2007

ABSTRACTThe metamorphic history of mafic exotic blocks from a tectonic melange zone within an allochthonous ophiolitic terrane (Marmora Terrane) of the Pan‐African Gariep orogenic belt in south‐western Namibia was studied, based on mineral parageneses and amphibole composition. Glaucophane described previously from these rocks could not be verified. Instead, two types of blue amphiboles were distinguished: (i) rims of (ferro‐) edenitic to pargasitic to barroisitic hornblende composition around brownish amphibole phenocrysts replacing magmatic clinopyroxene, and (ii) deep blue porphyroblasts of magnesio‐riebeckite with little ferro‐glaucophane component in a highly metasomatized albite‐rich rock. Textural and mineralogical evidence, particularly the existence of up to three different amphibole generations in metagabbro samples, supports a multiphase metamorphic history experienced by these exotic blocks. The first metamorphic event, M1, is interpreted as very low‐P hydrothermal oceanic metamorphism that affected the igneous protoliths at up to amphibolite facies temperatures. Subsequent M2 metamorphism was syntectonic and is characterized by temperatures similar to those attained during M1 but higher pressures indicating burial to 15–20 km. This event is related to a subduction process. The third metamorphic event, M3, was low grade and of regional nature. It is the only one recorded in the sedimentary envelope of the exotic blocks. The formation of magnesio‐riebeckite is considered a retrograde reaction at greenschist facies during M2. The results indicate that in the Gariep belt subduction and subsequent obduction have occurred, although blueschist facies metamorphism has not been reached.
Fluid infiltration and regional metamorphism of the Waits River Formation, north‐east Vermont, USA
Authors: LÉGER A. , FERRY J. M. 
Published: May 2007

Abstract
The Siluro‐Devonian Waits River Formation of north‐east Vermont was deformed, intruded by plutons and regionally metamorphosed during the Devonian Acadian Orogeny. Five metamorphic zones were mapped based on the mineralogy of carbonate rocks. From low to high grade, these are: (1) ankerite‐albite, (2) ankerite‐oligoclase, (3) biotite, (4) amphibole and (5) diopside zones. Pressure was near 4.5kbar and temperature varied from c. 450° C in the ankerite‐albite zone to c. 525° C in the diopside zone. Fluid composition for all metamorphic zones was estimated from mineral equilibria. Average calculated χco2[= CO2/(CO2+ H2O)] of fluid in equilibrium with the marls increases with increasing grade from 0.05 in the ankerite‐oligoclase zone, to 0.25 in the biotite zone and to 0.44 in the amphibole zone. In the diopside zone, χCO2 decreases to 0.06.Model prograde metamorphic reactions were derived from measured modes, mineral chemistry, and whole‐rock chemistry. Prograde reactions involved decarbonation with an evolved volatile mixture of χCO2 > 0.50. The χCO2 of fluid in equilibrium with rocks from all zones, however, was generally <0.40. This difference attests to the infiltration of a reactive H2O‐rich fluid during metamorphism. Metamorphosed carbonate rocks from the formation suggests that both heat flow and pervasive infiltration of a reactive H2O‐rich fluid drove mineral reactions during metamorphism. Average time‐integrated volume fluxes (cm3 fluid/cm2 rock), calculated from the standard equation for coupled fluid flow and reaction in porous media, are (1) ankerite‐oligoclase zone: c. 1 × 104; (2) biotite zone: c. 3 × 104; (3) amphibole zone: c. 10 × 104; and diopside zone: c. 60 × 104. The increase in calculated flux with increasing grade is at least in part the result of internal production of volatiles from prograde reactions in pelitic schists and metacarbonate rocks within the Waits River Formation.The mapped pattern of time‐integrated fluxes indicates that the Strafford‐Willoughby Arch and the numerous igneous intrusions in the field area focused fluid flow during metamorphism. Many rock specimens in the diopside zone experienced extreme alkali depletion and also record low χCO2. Metamorphic fluids in equilibrium with diopside zone rocks may therefore represent a mixture of acid, H2O‐rich fluids given off by the crystallizing magmas, and CO2‐H2O fluids produced by devolatilization reactions in the host marls. Higher fluxes and different fluid compositions recorded near the plutons suggest that pluton‐driven hydrothermal cells were local highs in the larger regional metamorphic hydrothermal system.
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.
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.
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.
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.
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.