Geochronology and geochemistry of multiple generations of monazite from the Wepawaug Schist,Connecticut, USA: implications for monazite stability in metamorphic rocks

 U-Pb isotope analyses, rare earth and trace element analyses, and petrographic observations are presented for monazites from the Wepawaug Schist in southern Connecticut, USA. Two samples of kyanite zone schist were collected less than a meter apart. Each sample contains a different variety of monazite with distinct morphology, chemistry, and Pb isotopic composition. One sample preserves a largely amphibolite facies mineralogy, including kyanite, staurolite, garnet, biotite, and chlorite, with little textural evidence of later shearing. Monazites from this sample are xenoblastic with about 1 wt% ThO₂, 0.3 wt% CaO, and a more LREE enriched pattern than monazites from the second sample. These xenoblastic monazites preserve textural evidence of a retrograde reaction to apatite which involves chlorite, indicating that these monazites became unstable during retrograde chloritization of biotite. These monazites give strongly discordant U-Pb ages which fit a chord with an upper intercept age of 411±18 Ma, interpreted as the minimum growth age of these xenoblastic monazites, perhaps during amphibolite facies metamorphism. The second sample contains S-C banding, evidence of dynamic recrystallization, and abundant retrograde chlorite. This sample contains idioblastic monazites with about 3 wt% ThO₂, 0.8 wt% CaO, and with less fractionated REE patterns. These monazites give close to concordant U-Pb ages with a mean ²⁰⁷Pb*/²⁰⁶Pb* age of 388 ± 2 Ma. This age is interpreted as probably representing the time of monazite growth during retrogression of the sample from an amphibolite to greenschist facies mineralogy. Received: 26 June 1995/Accepted: 25 May 1996     

Rotated staurolite porphyroblasts in the Littleton Schist at Bolton, Connecticut, USA

Staurolite porphyroblasts, 1.5–8cm in length and 0.3–2cm in width, in the Littleton Schist at Bolton, Connecticut, contain curved quartz inclusion trails which document synkinematic rotations of at least 135°. The orientations of long axes of these staurolite crystals define a weak preferred orientation in a plane approximately parallel to the external foliation. Serial sections of four differently orientated crystals and U-stage measurements of the orientations of their inclusion trails demonstrate that the inflection hinge line and the statistical 'symmetry axis' characterizing the foliation within a porphyroblast are unrelated to the orientations of external crenulations and are, in all cases, parallel to the long axis of the porphyroblast. The cumulative rotation reflected in the curvature of the inclusion trails is a maximum in a c -axis section through the initial core of a crystal. The amount of rotation about the c -axis decreases linearly along the length of the crystal away from the nucleation site.
The sense and amount of rotation recorded by a porphyroblast is related to its orientation. A tightly constrained transition from clockwise to anticlockwise rotation defines a slip direction that coincides with the preferred orientation of the staurolite c -axes. The total rotation reflected by the inclusion trails increases as a function of the angle between the c -axes of the staurolite crystals and the slip direction.
Initially random staurolite porphyroblasts rotated during growth, as a consequence of laminar shear in the surrounding viscous matrix. This interpretation is quantitatively consistent with: the staurolite preferred orientation; its coincidence with the apparent slip direction; the correlation between both the sense and the amount of rotation and the orientation of the long axis of the porphyroblast; and the twisted conical shape of the family of surfaces defined by the inclusion trails.     

Synchronous peak Barrovian metamorphism driven by syn-orogenic magmatism and fluid flow in southern Connecticut, USA

Recent work in Barrovian metamorphic terranes has found that rocks experience peak metamorphic temperatures across several grades at similar times. This result is inconsistent with most geodynamic models of crustal over‐thickening and conductive heating, wherein rocks which reach different metamorphic grades generally reach peak temperatures at different times. Instead, the presence of additional sources of heat and/or focusing mechanisms for heat transport, such as magmatic intrusions and/or advection by metamorphic fluids, may have contributed to the contemporaneous development of several different metamorphic zones. Here, we test the hypothesis of temporally focussed heating for the Wepawaug Schist, a Barrovian terrane in Connecticut, USA, using Sm–Nd ages of prograde garnet growth and U–Pb zircon crystallization ages of associated igneous rocks. Peak temperature in the biotite–garnet zone was dated (via Sm–Nd on garnet) at 378.9 ± 1.6 Ma (2σ), whereas peak temperature in the highest grade staurolite–kyanite zone was dated (via Sm–Nd on garnet rims) at 379.9 ± 6.8 Ma (2σ). These garnet ages suggest that peak metamorphism was pene‐contemporaneous (within error) across these metamorphic grades. Ion microprobe U–Pb ages for zircon from igneous rocks hosted by the metapelites also indicate a period of syn‐metamorphic peak igneous activity at 380.6 ± 4.7 Ma (2σ), indistinguishable from the peak ages recorded by garnet. A 388.6 ± 2.1 Ma (2σ) garnet core age from the staurolite–kyanite zone indicates an earlier episode of growth (coincident with ages from texturally early zircon and a previously published monazite age) along the prograde regional metamorphic T–t path. The timing of peak metamorphism and igneous activity, as well as the occurrence of extensive syn‐metamorphic quartz vein systems and pegmatites, best supports the hypothesis that advective heating driven by magmas and fluids focussed major mineral growth into two distinct episodes: the first at c. 389 Ma, and the second, corresponding to the regionally synchronous peak metamorphism, at c. 380 Ma.     

Amphibole compositions and metamorphic history of the Rand Schist and the greenschist unit of the Catalina Schist,Southern California

The Catalina Schist and Rand Schist are two high P/T terranes in southern California. The Catalina Schist is correlated with the Franciscan Complex and occurs in the continental borderland. It consists of a blueschist-facies melange tectonically overlain by a greenschist unit, which, in turn, is overthrust by an amphibolite unit. The greenschist unit itself is inversely zoned from epidote-amphibolite fades at the top through greenschist facies in the center to transitional blueschist-greenschist facies at the base. The Rand Schist is part of the eugeoclinal Pelona-Orocopia Schist terrane, which lies interior to the present continental margin, structurally beneath Precambrian to Mesozoic sialic basement. The Rand Schist is inversely zoned from epidote-amphibolite facies to transitional blueschist-greenschist facies, similar to the greenschist unit of the Catalina Schist.Two trends in amphibole composition, one from actinolite to hornblende in greenschists and epidote amphibolites (calcic series) and the other from actinolite through winchite to crossite in glaucophanic greenschists (sodic-calcic series), are present in both the Rand Schist and the greenschist unit of the Catalina Schist. The transition from actinolite to hornblende in the calcic series is defined by increases in tschermakite, edenite, and glaucophane substitution. Amphiboles of the sodic-calcic series differ mainly in the degree of glaucophanic substitution. The similarity of amphibole trends in the two terranes indicates that they were metamorphosed at approximately the same pressures and temperatures, and is evidence that the Rand Schist originated in a subduction zone, despite its present intracontinental setting.Most glaucophanic greenschists in the Rand and Catalina Schists contain both a sodic and a calcic member of the sodic-calcic series. Textural relations indicate that calcic members generally developed after the sodic ones. This implies that sodic amphibole formerly may have been present in many of the structurally higher greenschists and epidote amphibolites. Preservation of the inverted zonations, as well as microstructural evidence for the syntectonic development of calcic and sodic-calcic amphiboles, suggest that glaucophanic greenschists, greenschists, and epidote amphibolites all formed during underthrusting (subduction). This contrasts with many orogenic belts, where replacement of blueschists by greenschists to amphibolites is attributed to thermal reequilibration during erosional unroofing.     

The clustered nucleation and growth processes of garnet in regional metamorphic rocks from north-west Connecticut,USA

Serial sectioning and imaging with a flatbed scanner yielded the three-dimensional size and spatial distribution of garnet porphyroblasts in two garnet schists and one staurolite-bearing schist from the Everett Formation, north-west Connecticut. The dominant garnet-producing reaction in all samples was chlorite+quartz=garnet+H₂O. The appearance of staurolite, and additional garnet growth in the staurolite-bearing sample, was due to the reaction chloritoid=garnet+staurolite+chlorite. Statistical measures of garnet spatial distributions, using the pair correlation function (PCF), indicate that garnet crystals are weakly to strongly clustered at length scales between 2 and 10 mm. Such clustered nucleation may reflect minor bulk compositional variations. Covariance measures between garnet size and nearest-neighbour distance, using the mark covariance function (MCF), suggest a very weak correlation between crystal size and nearest-neighbour distance for length scales of 2 mm or less. These statistical data suggest that if diffusional gradients were present around growing garnet crystals, they did not influence nucleation and growth patterns at length scales greater than c. 2 mm. Compositional maps, through the garnet centres, show that the smaller crystals have lower Mn core compositions relative to larger crystals, consistent with progressive nucleation during pro-grade metamorphism. Radius-rate plots calculated from compositional X-ray maps show similar growth rates for garnet crystals of different size, consistent with an interface-controlled growth model for garnet. The presence of minor diffusional gradients around growing garnet cannot be entirely dismissed, but the lack of observable reaction rims, the clustered spatial distribution and the radius-rate data are most consistent with an interface-controlled garnet growth model.     

Deep crustal growth of quartz, kyanite and garnet into large-aperture, fluid-filled fractures, north-eastern Connecticut, USA

Abstract A petrographic and petrological analysis of exceptionally well-preserved hydrothermal veins from the Merrimack synclinorium, north-eastern Connecticut, has been carried out in order to place new field-based constraints on fracture aperture dimensions and porosity in the lower continental crust. The veins preserve substantial open space today in outcrop, and contain mineral assemblages including subhedral to euhedral crystals of quartz, kyanite and almandine-rich garnet. Textural evidence indicates unequivocally that the vein minerals grew into macroscopic (mm- to cm-scale) open space between the vein walls. The veins are interpreted to have been large-aperture fractures along which significant advective fluid infiltration and chemical reaction occurred. The porosity of the rock mass due to open space between fracture walls today is c . 0.3%, but it could have been as large as several percent when the flow system was active. Quantitative thermobarometry results from vein mineral assemblages indicate that the fractures formed at pressures and corresponding crustal depths of c. 0.8 GPa and c. 30km, and temperatures of 550–600° C. The depth of fracture formation corresponds to published estimates of the maximum burial depth of the Merrimack synclinorium during the Acadian orogeny. The formation of large-aperture fractures could increase significantly the transient permeability of the deep crust, and therefore influence metamorphic heat and mass transfer.     

Evolution of metamorphic fluid recorded in granulite facies metacarbonate rocks from the middle segment of the Mogok metamorphic belt in central Myanmar

The Mogok metamorphic belt of Palaeogene age, which records subduction‐ and collision‐related events between the Indian and Eurasian plates, lies along the western margin of the Shan plateau in central Myanmar and continues northwards to the eastern Himalayan syntaxis. Reaction textures of clinohumite‐ and scapolite‐bearing assemblages in Mogok granulite facies metacarbonate rocks provide insights into the drastic change in fluid composition during exhumation of the collision zone. Characteristic high‐grade assemblages of marble and calcsilicate rock are clinohumite+forsterite+spinel+phlogopite+pargasite/edenite+calcite+dolomite, and scapolite+diopside+anorthite+quartz+calcite respectively. Calculated petrogenetic grids in CaO–MgO–Al₂O₃–SiO₂–H₂O–CO₂ and subsets of this system were employed to deduce the pressure–temperature–fluid evolution of the clinohumite‐ and scapolite‐bearing assemblages. These assemblages suggest higher temperature (>780–810°C) and [=CO₂/(CO₂+H₂O) >0.17–0.60] values in the metamorphic fluid for the peak granulite facies stage, assuming a pressure of 0.8 GPa. Calcite grains commonly show exsolution textures with dolomite particles, and their reintegrated compositions yield temperatures of 720–880°C. Retrograde reactions are mainly characterized by a reaction zone consisting of a dolomite layer and a symplectitic aggregate of tremolite and dolomite grown between clinohumite and calcite in marble, and a replacement texture of scapolite by clinozoisite in calcsilicate rock. These textures indicate that the retrograde reactions developed under lower temperature (<620°C) and (<0.08–0.16) conditions, assuming a pressure of 0.5 GPa. The metacarbonate rocks share metamorphic temperatures similar to the Mogok paragneiss at the peak granulite facies stage. The values of the metacarbonate rock at peak metamorphic stage are, however, distinctly higher than those previously deduced from carbonate mineral‐free paragneiss. Primary clinohumite, phlogopite and pargasite/edenite in marble have F‐rich compositions, and scapolite in calcsilicate rock contains Cl, suggesting a contrast in the halogen compositions of the metamorphic fluids between these two lithologies. The metamorphic fluid compositions were probably buffered within each lithology, and the effective migration of metamorphic fluid, which would have extensively changed the fluid compositions, did not occur during the prograde granulite facies stage throughout the Mogok metamorphic belt. The lower conditions of the Mogok metacarbonate rocks during the retrograde stage distinctly contrast with higher conditions recorded in metacarbonate rocks from other metamorphic belts of granulite facies. The characteristic low conditions were probably due to far‐ranging infiltration of H₂O‐dominant fluid throughout the middle segment of the Mogok metamorphic belt under low‐amphibolite facies conditions during the exhumation and hydration stage.     

Fluid evolution during metamorphism of the Otago Schist, New Zealand: (I) Evidence from fluid inclusions

Fluid inclusion salinities from quartz veins in the Otago Schist, New Zealand, range from 1.0 to 7.3 wt% NaCl eq. in the Torlesse terrane, and from 0.4 to 3.1 wt% NaCl eq. in the Caples terrane. Homogenization temperatures from these inclusions range from 124 to 350  °C, with modal values for individual samples ranging from 163 to 229  °C, but coexisting, low-salinity inclusions exhibiting metastable ice melting show a narrower range of T  _h from 86 to 170  °C with modes from 116 to 141  °C. These data have been used in conjunction with chlorite chemistry to suggest trapping conditions of ≈350–400  °C and 4.1–6.0  kbar for inclusions showing metastable melting from lower greenschist facies rocks, with the densities of many other inclusions reset at lower pressures during exhumation of the schist. The fluid inclusion salinities and Br/Cl ratios from veins from the Torlesse terrane are comparable to those of modern sea-water, and this suggests direct derivation of the vein fluid from the original sedimentary pore fluid. Some modification of the fluid may have taken place as a result of interaction with halogen-bearing minerals and dehydration and hydration reactions. The salinity of fluids in the Caples terrane is uniformly lower than that of modern sea-water, and this is interpreted as a result of the dilution of the pore fluid by dehydration of clays and zeolites. The contrast between the two terranes may be a result of the original sedimentary provenance, as the Torlesse terrane consists mainly of quartzofeldspathic sediments, whilst the Caples terrane consists of andesitic volcanogenic sediments and metabasites which are more prone to hydration during diagenesis, and hence may provide more fluid via dehydration at higher grades.     

A record of Lateglacial and early Holocene environmental and ecological change from southwestern Connecticut,USA

Analyses of a sediment core from Highstead Swamp in southwestern Connecticut, USA, reveal Lateglacial and early Holocene ecological and hydrological changes. Lateglacial pollen assemblages are dominated by Picea and Pinus subg. Pinus, and the onset of the Younger Dryas (YD) cold interval is evidenced by higher abundance of Abies and Alnus viridis subsp. crispa. As climate warmed at the end of the YD, Picea and Abies declined and Pinus strobus became the dominant upland tree species. A shift from lacustrine sediment to organic peat at the YD–Holocene boundary suggests that the lake that existed in the basin during the Lateglacial interval developed into a swamp in response to reduced effective moisture. A change in wetland vegetation from Myrica gale to Alnus incana subsp. rugosa and Sphagnum is consistent with this interpretation of environmental changes at the beginning of the Holocene. Copyright © 2009 John Wiley & Sons, Ltd.     

Mineralogic variations across a thick amphibolite,Bighorn Mountains,Wyoming, USA

Studies of serial suites of samples collected along six sections measured across the width of an amphibolite body, some 750 meters thick, reveal (1) the presence of a hornblende concentration near the presumed base of the body, (2) an upward sodium-enrichment trend reflected by plagioclase, and (3) an upward iron-enrichment trend reflected by variation in 100Mg/Mg+Fe²+Fe³+Mn values for hornblende. Consistent with field evidence, the data indicate formation of the amphibolite by metamorphism of a tabular mafic igneous body. The mineralogic variations are suggestive of those expected during the differentiation of a mafic sill in which the trends are controlled by early crystallization and subsequent gravitational accumulation of pyroxene. In addition to indicating igneous parentage, these variations establish the stratigraphic sequence.     

Behaviour of rigid objects during deformation and metamorphism: a test using schists from the Bolton syncline, Connecticut, USA

The behaviour of spherical versus highly ellipsoidal rigid objects in folded rocks relative to one another or the Earth’s surface is of particular significance for metamorphic and structural geologists. Two common porphyroblastic minerals, garnet and staurolite, approximate spherical and highly ellipsoidal shapes respectively. The motion of both phases is analysed using the axes of inflexion or intersection of one or more foliations preserved as inclusion trails within them (we call these axes FIAs, for foliation inflexion/intersection axes). For staurolite, this motion can also be compared with the distribution of the long axes of the crystals. Schists from the regionally shallowly plunging Bolton syncline commonly contain garnet and staurolite porphyroblasts, whose FIAs have been measured in the same sample. Garnet porphyroblasts pre-date this fold as they have inclusion trails truncated by all matrix foliations that trend parallel to the strike of the axial plane. However, they have remarkably consistent FIA trends from limb to limb. The FIAs trend 175° and lie 25°NNW from the 020° strike of the axial trace of the Bolton syncline. The plunge of these FIAs was determined for six samples and all lie within 30° of the horizontal. Eleven of these samples also contain staurolite porphyroblasts, which grew before, during and after formation of the Bolton syncline as they contain inclusion trails continuous with matrix foliations that strike parallel to the axial trace of this fold. The staurolite FIAs have an average trend of 035°, 15°NE from the 020° strike of the axial plane of this fold. The total amount of inclusion trail curvature in staurolite porphyroblasts, about the axis of relative rotation between staurolite and the matrix (i.e. the FIA), is greater than the angular spread of garnet FIAs. Although staurolite porphyroblasts have ellipsoidal shapes, their long axes exhibit no tendency to be preferentially aligned with respect to the main matrix foliation or to the trend of their FIA. This indicates that the axis of relative rotation, between porphyroblast and matrix (the FIA), was not parallel to the long axis of the crystals. It also suggests that the porphyroblasts were not preferentially rotated towards a single stretch direction during progressive deformation. Five overprinting crenulation cleavages are preserved in the matrix of rocks from the Bolton syncline and many of these result from deformation events that post-date development of this fold. Staurolite porphyroblast growth occurred during the development of all of these deformations, most of which produced foliations. Staurolite has overgrown, and preserved as helicitic inclusions, crenulated and crenulation cleavages; i.e. some inclusion trail curvature pre-dates porphyroblast growth. The deformations accompanying staurolite growth involved reversals in shear sense and changing kinematic reference frames. These relationships cannot all be explained by current models of rotation of either, or both, the garnet and staurolite porphyroblasts. In contrast, we suggest that the relationships are consistent with models of deformation paths that involve non-rotation of porphyroblasts relative to some external reference frame. Further, we suggest there is no difference in the behaviour of spherical or ellipsoidal rigid objects during ductile deformation, and that neither garnet nor staurolite have rotated in schists from the Bolton syncline during the multiple deformation events that include and post-date the development of this fold.     

A two-stage fluid history for the Orocopia Schist and associated rocks related to flat subduction and exhumation,southeastern California

ABSTRACT

Stable isotopes combined with pre-existing ⁴⁰Ar/³⁹Ar thermochronology at the Gavilan Hills and Orocopia Mountains in southeastern California record two stages of fluid–rock interaction: (1) Stage 1 is related to prograde metamorphism as Orocopia Schist was accreted to the base of the crust during late Cretaceous–early Cenozoic Laramide flat subduction. (2) Stage 2 affected the Orocopia Schist and is related to middle Cenozoic exhumation along detachment faults. There is no local evidence that schist-derived fluids infiltrated structurally overlying continental rocks. Mineral δ¹⁸O values from Orocopia Schist in the lower plate of the Chocolate Mountains fault and Gatuna normal fault in the Gavilan Hills are in equilibrium at 490–580°C with metamorphic water (δ¹⁸O = 7–11‰). Phengite and biotite δD values from the Orocopia Schist and upper plate suggest metamorphic fluids (δD ~ –40‰). In contrast, final exhumation of the schist along the Orocopia Mountains detachment fault (OMDF) in the Orocopia Mountains was associated with alteration of prograde biotite and amphibole to chlorite (T ~ 350–400°C) and the influx of meteoric-hydrothermal fluids at 24–20 Ma. Phengites from a thin mylonite zone at the top of the Orocopia Schist and alteration chlorites have the lowest fluid δD values, suggesting that these faults were an enhanced zone of meteoric fluid (δD < –70‰) circulation. Variable δD values in Orocopia Schist from structurally lower chlorite and biotite zones indicate a lesser degree of interaction with meteoric-hydrothermal fluids. High fluid δ¹⁸O values (6–12‰) indicate low water–rock ratios for the OMDF. A steep thermal gradient developed across the OMDF at the onset of middle Cenozoic slip likely drove a more vigorous hydrothermal system within the Orocopia Mountains relative to the equivalent age Gatuna fault in the Gavilan Hills.     

Tectonic Setting of the Kadiri Schist Belt, Andhra Pradesh, India

Plate tectonic activity has played a critical role in the development of petrotectonic associations in the Kadiri schist belt. The calc alkaline association of basalt, andesite, dacite and rhyolite(BADR) is the signature volcanic rock suite of the convergent margin. The N-S belt has gone below the unconformity plane of Cuddapah sediments. In the northern part geochemical and structural attributes of the Kadiri greenstone belt is studied along with microscopic observations of selected samples. Harker diagram plots of major elements generally indicate a liquid line of descent from a common source, such that BADR rocks are derived from a common parent magma of basaltic to andesitic composition. These calc-alkaline volcanic rocks are formed at convergent margins where more silicic rocks represent more highly fractionated melt. All the litho-units of this greenstone belt indicate crush and strain effects. The stretched pebbles in the deformed volcanic matrix with tectonite development along with associated greenschist facies metamorphism, alteration and hydration is remarkable. Flow foliation plane with N-S strike and very low angle(5° to 10°) easterly dip and N-S axial planar schistosity formed due to later phase isoclinal folding can be clearly identified in the field. Basic intrusives are quite common in the surrounding area. All the observations including the field setting and geochemistry clearly demonstrate ocean-continent subduction as the tectonic environment of the study area.     

Hydrothermal origin of mafic layers in alpine-type peridotites: Evidence from the seiad ultramafic complex,California, USA

Cretaceous alkaline intrusive rocks present in the Bitterfontein area comprise a suite having compositions ranging from olivine melilitite to syenite. These rocks which include some of lamprophyric affinity form part of the widespread dominantly alkaline suite of intrusives occurring in a broad zone parallel to the south western African coastal regions. This paper presents the petrology of the Bitterfontein alkaline rocks together with mineral and rock analyses. The chemistry of the rocks shows the suite to define a SiO₂ enrichment trend linking the compositions of olivine melilitite to oversaturated basic rocks. This trend is compared with trends defined by essentially tholeiitic volcanics and kimberlite-olivine melilitite compositions and a genetic relationship with the latter is apparent. Polybaric crystallisation of a larnite normative liquid in a crustal environment coupled with fractionation especially of phlogopite is important in the generation of members of the Bitterfontein suite. Phlogopite fractionation represents a deviation by the Bitterfontein suite from the evolutionary trend predicted by experimental work for low pressure anhydrous compositions in the Fo-La-Ne-SiO₂ system.     

Hornblende geothermometry of amphibolite layers of the Popple Hill gneiss, north-west Adirondack Lowlands, New York, USA

The exchange reaction tschermakite+2 diopside+2 quartz=tremolite+2 anorthite (HPCQ), in combination with the thermodynamic database TWQ (version 1.02, Berman, 1991), has been evaluated for its usefulness as a geothermometer. This reaction, which is both water conserving (independent of water fugacity) and which does not require the presence of garnet, is well suited for studying pyroxene‐bearing amphibolites. As an application of this geothermometer, we have re‐examined the amphibolites occurring in the Popple Hill gneiss of the Adirondack Lowlands of New York, USA, to better understand the magnitude of temperature variation preserved in the amphibolites themselves in this classic locality. At an assumed constant pressure of 7 kbar, the temperatures range from 619 to 682 °C from Edwards to Pierrepont and are uncorrelated with either distance along the strike of the region or with modal mineralogical variations. Hornblende exhibits a narrow compositional range suggesting that there has been little or no thermal gradient along the strike of the Lowlands. Temperatures recorded just north of Colton are, however, distinctly higher (694–758 °C). Although it is likely that the Popple Hill gneiss amphibolites experienced some effects of progressive metamorphism, particularly in the vicinity of Colton, the variations in modal mineralogy are most likely the result of such factors as local variations in the bulk chemistry of the protolith and in the fugacity of H₂O due to infiltration of diluting species (e.g. CO₂, CH₄), rather than a regional temperature variation. Temperatures recorded by the HPCQ geothermometer reported here are similar in magnitude and geographic trend to those reported for graphite–calcite carbon‐isotope thermometry by Kitchen & Valley (1995), suggesting that peak metamorphism in the Adirondack Lowlands involved laterally extensive and fairly uniform isotherms.     

Fluid evolution during metamorphism of the Otago Schist, New Zealand: (II) Influence of detrital apatite on fluid salinity

Apatite occurs in the zeolite to greenschist facies metamorphic rocks of the Otago Schist, South Island, New Zealand, as both a groundmass constituent and as a hydrothermal phase hosted in metamorphic quartz veins. Groundmass apatite from low-grade rocks, ranging from the zeolite facies to the pumpellyite–actinolite zone, has chloride contents ranging from 0–1.4 wt%, and fluoride contents ranging from 2.2–4.2 wt%, whilst groundmass apatite from the greenschist facies (chlorite to biotite zone) is virtually pure fluorapatite. Vein apatite from all grades is also fluorapatite with little or no chloride. This difference in composition is interpreted as resulting from the preservation of the primary magmatic compositions of detrital Cl-apatite grains, out of equilibrium with the metamorphic fluid, at low grades, whilst higher-grade groundmass apatite and neoformed apatite in quartz veins have compositions in equilibrium with an aqueous metamorphic fluid. The presence of detrital Cl-bearing apatite during the early stages of metamorphism may constitute a significant reservoir of Cl, given the low porosities of compacted sediments undergoing prograde metamorphism. Calculations indicate that the release of Cl from detrital apatite in the Otago Schist, as a result of re-equilibration of apatite with the pore fluid, may have had a significant effect on the salinity of the metamorphic fluid.     

Formation and composition of cemented layers in low-sulphide mine tailings, Laver, northern Sweden

Cemented layers (hardpans) are common in carbonate or sulphide-rich mine tailings and where pyrrhotite is the predominating Fe-sulphide. Laver, northern Sweden, is an abandoned Cu-mine where the tailings have low pyrrhotite content, almost no pyrite and no carbonates. Two cemented layers at different locations in the Laver tailings impoundment were investigated, with the aim to determine their effects on metal mobility. The cementing agents were mainly jarosite and Fe-oxyhydroxides in the layer formed where the tailings have a barren surface, whereas only Fe-oxyhydroxides were identified below grass-covered tailings surface. Arsenic was enriched in both layers which also exhibit high concentrations of Mo, V, Hg and Pb compared to unoxidised tailings. Sequential extraction indicates that these metals and As were mainly retained with crystalline Fe-oxides, and therefore potentially will be remobilised if the oxic conditions become more reducing, for instance as a result of remediation of the tailings impoundment.An erratum to this article can be found at     

Structurally controlled fluid flow during contact metamorphism in the Ritter Range pendant, California, USA

The mineralogy and O-isotope geochemistry of siliceous limestones from the Ritter Range pendant constrain the geometry and amount of fluid flow during contact metamorphism associated with emplacement of a pluton of the Sierra Nevada Batholith. Wollastonite (Wo) replaces calcite (Cal) + quartz (Qtz) on a layer-by-layer basis in homoclinal beds that strike NW and dip almost vertically. At the peak of metamorphism (P≈ 1500 bars, T≈ 600 °C) fluid in equilibrium with Cal, Qtz, and Wo has composition XCO₂=0.28, requiring that the Wo-forming reaction was driven by infiltration of reactive H₂O-rich fluid. The spatial distribution of Wo and Cal + Qtz records that peak metamorphic fluid flow was layer-parallel, upward. Bounds on the prograde time-integrated fluid flux associated with formation of Wo are set by: (1) the overlap in O-isotope composition between Wo-bearing and Wo-free rocks (>245 mol fluid/cm² rock); (2) the amount of fluid that would drive the Wo-reaction front upward to the present level of exposure from a point at depth where Cal, Qtz, and Wo would be in equilibrium with pure CO₂ (<1615 mol/cm²). Back-reaction of Wo to Cal + Qtz records an additional time-integrated retrograde fluid flux of ≈ 200–1000 mol/cm². The direction and amount of flow inferred from mineralogical and isotopic data agree with the results of the hydrologic model for metamorphic fluid flow in the area of Hanson et al. (1993). Fingers of Wo-bearing rock that extend farthest from the fluid source along contacts between limestone and more siliceous rocks point to strong control of flow geometry at the 0.1–100 m scale exerted by premetamorphic structures. Studies that neglect structural control at this scale may fail to predict correctly fundamental aspects of contact metamorphic fluid flow. Received: 27 January 1997 / Accepted: 2 October 1997     

Compositional appraisal and quality implication of Emiworo metacarbonate deposit in Central Nigeria

Metacarbonate deposit occurring in Emiworo area, northcentral Nigeria are low grade marble associated with Proterozoic basement complex rocks. Integrated inorganic geochemical, physical and thermal characteristics of the samples were investigated with the aim of revealing their economic viability. The phase characteristics of the metacarbonate show that dolomite is the dominant phase with calcite and phlogopite as minor phases, while silicates, graphites and pyrites constitute the accessories. Reactivity of the quicklime measured after calcination at different temperatures revealed that only the quicklime produced in 1 hour at 950°C and 1000°C was found to be reactive. Physical properties and comparative data show that the Emiworo marble displays lower water absorption and higher compressive strength, loss on ignition and bulk specific gravity, compared to other metacarbonate rocks occurring in other parts of Nigeria and elsewhere. These attributes makes the deposit suitable for many conventional industrial applications.     

广西六景泥盆系弗拉斯阶-法门阶界线层牙形石生物地层及碳同位素组成

广西六景泥盆系剖面是中国泥盆系标准剖面之一,通过对其弗拉斯阶-法门阶(F-F)界线附近地层进行详细的牙形石生物地层研究,自下而上识别出3个牙形石带:晚rhenana带、linguiformis带和triangularis带。F-F界线位于融县组下部(第7号层与第8号层之间),在谷闭组顶界之上3.32,m处。碳同位素的分析结果表明,F-F之交δ13C具有显著正偏移,增幅为2.0‰,与湖南老江冲、广西垌村和杨堤以及欧洲、美洲、非洲和澳洲等地的F-F界线附近的碳同位素记录一致,且具有相近的变化幅度。δ13C正异常与F-F界线上的生物灭绝有关,由于食微生物的高等生物灭绝,微生物大量繁盛,诱导海水缺氧,导致海洋有机碳埋藏速率增加,从而形成δ13C的正偏。F-F界线层发育一套以碎屑灰岩为特色的事件沉积,该事件沉积在广西乃至全球具有等时性,可能与小行星碰撞地球引起的全球性海啸有关。