Range Of Metamorphic Grade Research Articles

Zircon dissolution and growth during metamorphism

Kohn et al. 2015 (this volume) present models of zircon dissolution and growth in hydrous MORB and metapelite bulk compositions under metamorphic conditions. They evaluate the propensity metamorphic minerals in equilibrium with zircon, to assess what minerals (besides zircon) affect whole rock Zr mass balance. Results indicate that mineral reactions over a range of metamorphic grades could play an important role in the dissolution and growth of metamorphic zircon, with Zr preferentially incorporated into melt, rutile, garnet, and hornblende with increasing temperatures and decreasing pressures.

Graphite thermometry in a low-pressure contact aureole, Halifax, Nova Scotia

Abstract Intrusion of the late Devonian South Mountain Batholith, southern Nova Scotia, produced a low-pressure contact metamorphic aureole in its metasedimentary host rocks. The effects of contact metamorphism are particularly well developed in pelitic rocks of the Halifax Group on the eastern margin of the batholith. Contact metamorphic isograds and mineral assemblages suggest low-pressure metamorphism, with P–T conditions at the contact estimated at 2.5–3.0 kbar and ca. 650 °C. In this study, Raman spectroscopy of carbonaceous material (RSCM) was used to obtain temperatures from graphite, which is common throughout the contact aureole. Temperature estimates range from ca. 360 °C just outside the cordierite-in isograd to ca. 640 °C in the sillimanite–K-feldspar zone near the contact, the latter consistent with the temperature estimated from the corresponding silicate mineral assemblage. Three different RSCM calibrations produced very similar results except at the high-temperature end of the observed range. A thermal profile constructed from the RSCM data was used to constrain a 2D numerical model for post-intrusion conductive cooling of the batholith along its eastern margin. Comparison of RSCM vs model thermal profiles suggest that observed differences between the thermal structure of the inner and outer aureole were controlled by the subsurface geometry of the pluton contact. The model predicts that peak temperatures in country rocks within 1 km of the contact were reached within 50 ka of intrusion, but that the outer part of the aureole took 250–500 ka to reach peak temperatures. The results confirm the utility of RSCM thermometry for acquiring temperature data over a range of metamorphic grades.

Devolatilization history and trace element mobility in deeply subducted sedimentary rocks: Evidence from Western Alps HP/UHP suites

Metapelitic rocks of the Schistes Lustrés in the Cottian Alps, Italy (peak metamorphic conditions of 350–500°C, 1.2–2.0GPa) and at the UHP Lago di Cignana locality (Valtournenche, Italy; ~550°C, 2.5–3.0GPa) preserve records of prograde devolatilization in their mineral modes and chemistry, contents of volatiles and fluid-mobile elements (elements relatively mobile in aqueous fluids), and B and N isotope compositions. This suite allows study of prograde devolatilization history, across a wide range in metamorphic grade, in metasedimentary rocks that experienced high-P/T prograde paths similar to those experienced in most modern subduction zones.Across grade, whole-rock samples are in general uniform in their concentrations of relatively fluid-mobile elements N, B, Li, Cs, Ba, and Rb, normalized to the concentrations of the less mobile K2O and Al2O3, showing only hints of loss in several of the highest-grade samples. With increasing grade, ion microprobe analyses of phengites show subtle decrease in B concentration, uniformity in Ba and Cs concentrations, and increase in Li concentrations, the latter likely due to release from chlorite during its breakdown. In one Cignana sample, phengite inclusions in garnets are enriched in B relative to matrix phengite, consistent with either whole-rock B loss after garnet growth or, more likely, closed-system behavior and partitioning of B into paragonite or tourmaline stabilized after garnet growth. In samples with both paragonite and phengite, paragonite shows relative enrichment in B and Sr, and phengite is enriched in Cs, Ba, and presumably also N and Rb (the latter showing strong whole-rock correlations with K2O).Whole-rock δ15N shows a hint of shift to higher values in the highest grade rocks (Cignana) and, accordingly, calculated prograde dehydration histories for appropriate bulk compositions, using the Perple-X database, indicate that significant (~20%) dehydration would for some rocks occur over the temperature interval of 450 to 550°C, largely related to the breakdown of chlorite (and to a lesser extent carpholite). Small amounts of loss of N into these fluids could have resulted in minor shift in δ15N, with decrease in whole-rock N concentration masked by heterogeneity inherent with the sedimentary protoliths. Partitioning of Cs and Li (possibly also Rb and Ba) from white micas into H2O-rich fluids largely produced by chlorite breakdown could similarly have produced the subtle decreases in the concentrations in these elements noted in several high-grade samples. Neoblastic tourmaline in higher-grade rocks likely sequestered some fraction of the B lost from micas, resulting in a lack of obvious whole-rock B loss to accompany the up-grade trend of decreasing B concentrations in phengite. This tourmaline shows core-to-rim decrease in δ11B consistent with growth during small amounts of progressive B loss from phengites.Taken together, the whole-rock and SIMS data presented here, and the whole-rock dataset of Busigny et al. (2003), demonstrate impressive retention, during prograde forearc devolatilization, of elements thought to be relatively fluid-mobile (particularly H, N, B, Li, Ba, and Cs). Retention of these elements in metasedimentary rocks subducted to depths overlapping those beneath arc volcanic fronts (~90km estimated for subsolidus, peak Cignana metamorphism) implies their availability for transfer into arc source regions, in aqueous fluids or silicate melts, or into the mantle to depths beyond subarc regions.

Variation of illite/muscovite 40Ar/39Ar age spectra during progressive low-grade metamorphism: an example from the US Cordillera

40Ar/39Ar step-heating data were collected from micron to submicron grain-sizes of correlative illite- and muscovite-rich Cambrian pelitic rocks from the western United States that range in metamorphic grade from the shallow diagenetic zone (zeolite facies) to the epizone (greenschist facies). With increasing metamorphic grade, maximum ages from 40Ar/39Ar release spectra decrease, as do total gas ages and retention ages. Previous studies have explained similar results as arising dominantly or entirely from the dissolution of detrital muscovite and precipitation/recrystallization of neo-formed illite. While recognizing the importance of these processes in evaluating our results, we suggest that the inverse correlation between apparent age and metamorphic grade is controlled, primarily, by thermally activated volume diffusion, analogous to the decrease in apparent ages with depth observed for many thermochronometers in borehole experiments. Our results suggest that complete resetting of the illite/muscovite Ar thermochronometer occurs between the high anchizone and epizone, or at roughly 300 °C. This empirical result is in agreement with previous calculations based on muscovite diffusion parameters, which indicate that muscovite grains with radii of 0.05–2 μm should have closure temperatures between 250 and 350 °C. At high anchizone conditions, we observe a reversal in the age/grain-size relationship (the finest grain-size produces the oldest apparent age), which may mark the stage in prograde subgreenschist facies metamorphism of pelitic rocks at which neo-formed illite/muscovite crystallites typically surpass the size of detrital muscovite grains. It is also approximately the stage at which neo-formed illite/muscovite crystallites develop sufficient Ar retentivity to produce geologically meaningful 40Ar/39Ar ages. Results from our sampling transect of Cambrian strata establish a framework for interpreting illite/muscovite 40Ar/39Ar age spectra at different stages of low-grade metamorphism and also illuminate the transformation of illite to muscovite. At Frenchman Mtn., NV, where the Cambrian Bright Angel Formation is at zeolite facies conditions, illite/muscovite 40Ar/39Ar data suggest a detrital muscovite component with an apparent age ≥967 Ma. The correlative Carrara Fm. is at anchizone conditions in the Panamint and Resting Spring Ranges of eastern California, and in these locations, illite/muscovite 40Ar/39Ar data suggest an early Permian episode of subgreenschist facies metamorphism. The same type of data from equivalent strata at epizone conditions (greenschist facies) in the footwall of the Bullfrog/Fluorspar Canyon detachment in southern Nevada reveals a period of slow-to-moderate Late Cretaceous cooling.

Controls on porphyroblast size along a regional metamorphic field gradient

Garnet-bearing schists from the Waterville Formation of south-central Maine provide an opportunity to examine the factors governing porphyroblast size over a range of metamorphic grade. Three-dimensional sizes and locations for all garnet porphyroblasts were determined for three samples along the metamorphic field gradient spanning lowest garnet through sillimanite grade, using high-resolution X-ray computed tomography. Comparison of crystal size distributions to previous data sets obtained by stereological methods for the same samples reveals significant differences in mode, mean, and shape of the distributions. Quantitative textural analysis shows that the garnets in each rock crystallized in a diffusion-controlled nucleation and growth regime. In contrast to the typical observation of a correlation between porphyroblast size and position along a metamorphic field gradient, porphyroblast size of the lowest-grade specimen is intermediate between the high- and middle-grade specimens’ sizes. Mean porphyroblast size does not correlate with peak temperatures from garnet-biotite Fe-Mg exchange thermometry, nor is post-crystallization annealing (Ostwald Ripening) required to produce the observed textures, as was previously proposed for these rocks. Robust pseudosection calculations fail to reproduce the observed garnet core compositions for two specimens, suggesting that these calc-pelites experienced metasomatism. For each of these two specimens, Monte Carlo calculations suggest potential pre-metasomatism bulk compositions that replicate garnet core compositions. Pseudosection analyses allow the estimation of the critical temperatures for garnet growth: ∼481, ∼477, and ∼485°C for the lowest-garnet-zone, middle-garnet-zone, and sillimanite-zone specimens, respectively. Porphyroblast size appears to be determined in this case by a combination of the heating rate during garnet crystallization, the critical temperature for the garnet-forming reaction and the kinetics of nucleation. Numerical simulations of thermally accelerated, diffusion-controlled nucleation, and growth for the three samples closely match measured crystal size distributions. These observations and simulations suggest that previous hypotheses linking the garnet size primarily to the temperature at the onset of porphyroblast nucleation can only partially explain the observed textures. Also important in determining porphyroblast size are the heating rate and the distribution of favorable nucleation sites.

Primary alteration and geochemical dispersion haloes of Archaean orogenic gold deposits in the Yilgarn Craton: the pre-weathering scenario

Widespread orogenic gold deposits are hosted by practically all rock types over the range of metamorphic grades in the Archaean Yilgarn Craton, Western Australia. They are normally located in second- or third-order structures, most commonly near crustal- to regional-scale deformation zones, in brittle, brittle–ductile and ductile deformational environments. Despite this extreme variability, the alteration and geochemical features of these deposits are strikingly consistent. Each gold deposit is enveloped by an alteration-zoning sequence. In greenschist facies, the alteration zones are, from distal to proximal and according to the diagnostic minerals: calcite–chlorite, calcite–ankerite and sericite. In amphibolite facies, two zones are normally detected, a distal biotite and a proximal calc-silicate zone with no intermediate zone. The ores are enriched in Ag, As, Au, Ba, CO 2 , K, Rb, S, Sb, Si, Te and W, and depleted in Na and Y relative to host-rocks. Base metals, Fe, Mg and Mn are rarely mobile in these settings. From the available, albeit limited, database there is little correlation between rock type or metamorphic grade and the elements enriched in the gold deposit. All enriched components may form primary dispersion haloes around the deposits. Parameters that most consistently define dispersion haloes for those deposits studied in the Yilgarn Craton are As, Au, Sb, Te, W, and the carbonation and sericitization indices. In nearly all cases, at least one of the parameters defines an anomaly significantly more extensive than that defined by Au alone. In some cases, Se or Y anomalies also extend beyond the Au anomaly. Importantly, there is also at least one pathfinder element whose primary dispersion extends beyond the alteration halo based on mineral assemblages, whereas the alteration–index anomalies normally do not extend beyond these limits. The primary geochemical dispersion halo may extend laterally for >200 m, and along strike for a few kilometres, from the gold orebody. A single geochemical parameter rarely defines an extensive and consistent vector towards gold ore. However, useful directional information can be achieved when trends defined by several geochemical parameters are combined.

Trace element partitioning between coexisting biotite and muscovite from metamorphic rocks, Western Labrador: structural, compositional and thermal controls

Coexisting biotite and muscovite in ten metapelitic and quartzofeldspathic rocks from western Labrador have been analyzed by electron microprobe for major and minor elements and by a laser ablation microprobe coupled to ICP-MS (LAM-ICP-MS) for selected trace elements—Li, Sc, V, Cr, Mn, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Cs, Ba, REE, Hf and Ta. The samples have experienced a single prograde Grenvillian metamorphism ranging from 490 to 680°C and from 7 to 12 kbar. The trace element compositions of coexisting micas in the metamorphic rocks are used to assess the effects of crystal structure, major element composition and temperature on the partitioning of each element between biotite and muscovite. Overall, trace element distributions are systematic across the range of metamorphic grade and bulk composition, suggesting that chemical equilibrium was approached. Most distribution coefficients (biotite/muscovite) show good agreement with published data. However, distribution coefficients for Co and Sr are significantly different from previous determinations, probably because of contamination associated with older data obtained by bulk analysis techniques. The sequence of distribution coefficients is governed mainly by the ionic radii and charges of substituting cations compared to the optimum ionic radius of each crystallographic site in the micas. In particular, distribution coefficients exhibit the sequence Cr 3+ (0.615 Å) > V 3+ (0.64 Å) > Sc 3+ (0.745 Å) in VI-sites, and Ba 2+ (1.61 Å) > Sr 2+ (1.44 Å) and Cs + (1.88 Å) > K + (1.64 Å) > Rb + (1.72 Å) > Na + (1.39 Å) in XII-sites. The distributions of Li, Sc, Sr and Ba appear to be thermally sensitive but are also controlled by major element compositions of micas. V and Zr partitioning is dependent on T and may be used to cross-check thermometry calculations where the latter suffer from retrograde re-equilibration and/or high concentrations of Fe 3+. The ranges and dependence of distribution coefficients on major element compositions provide important constraints on the values that can be used in geochemical modeling.

MINERAL EQUILIBRIA IN QUARTZ LEUCOAMPHIBOLITES (QUARTZ - GARNET - PLAGIOCLASE - HORNBLENDE CALC-SILICATES) FROM SOUTHEASTERN BRITISH COLUMBIA, CANADA

Pods and lenses of leucoamphibolite a few meters in maximum dimension contain quartz (Qtz) ‐ plagioclase (Pl) ‐ hornblende (Hbl) ‐ garnet (Grt) ‐ epidote (Ep), with or without biotite (Bt). They occur within Late Precambrian clastic metasedimentary rocks of the Esplanade Range of southeastern British Columbia, and range in metamorphic grade from upper biotite zone to kyanite (Ky) zone (defined in pelitic rocks). Mineral equilibria using the database of Berman provide estimates of metamorphic pressure (P), temperature (T), and fluid compositions. These estimates can be compared to those from coexisting pelitic rocks to provide a check on consistency. Grt‐Bt Mg‐Fe exchange geothermometry suggests temperatures ranging from 530 to 580°C at 7 kbar. The lack of kyanite in leucoamphibolites suggests a maximum P of about 7‐8 kbar. Pl‐Qtz‐Ep‐Grt assemblages without calcite suggest H2O-rich fluid compositions [generally X(H2O) > 0.8]. For a(H2O) = 1.0, Grt‐Ep‐Pl‐Qtz‐ H2O equilibria produce calculated temperatures near 565 °C at 7 kbar. Although there is considerable overlap, estimates of P and T from the leucoamphibolites do not agree very well with estimates from nearby pelites. On the basis of natural occurrences, ot her investigators have suggested a miscibility gap and near-constant activity of anorthite over the range An 40‐90. Electron-microprobe spot analyses in our study indicate that plagioclase compositions near ~An 40 and ~An90 occur within single grains and within single samples. Other samples have compositions concentrated near either An 40 or An90 or compositions with An≤40. These results are consistent with petrological studies reported by other researchers.

Geochronology and isotopic character of ultrahigh‐pressure metamorphism with implications for collision of the Sino‐Korean and Yangtze cratons, central China

The collision zone between the Sino‐Korean and Yangtze cratons in central China is marked by subduction‐related metamorphic rocks composed of crustal protoliths. These range in metamorphic grade from ultrahigh‐pressure eclogite to low‐greenschist facies. The paragenesis of the ultrahigh‐pressure rocks (coesite and diamond‐bearing) indicates subduction to depths greater than ∼120 km as a result of continental collision. Despite extreme metamorphic conditions, zircons have not undergone Pb loss but rather illustrate two periods of growth: crystallization between 700 and 800 Ma and ultrahigh‐pressure metamorphism at 218 ± 2.5 Ma. Previous studies have predicted a time‐transgressive collision, but age data presented in this study indicate that the collision was approximately coeval along the length of the suture. U‐Pb zircon ages of these ultrahigh‐pressure metamorphic rocks are 218.4 ± 2.5 Ma and 217.1 ± 8.7 Ma for the Dabie Mountains and Shandong peninsula, respectively. The similar ages imply that the Tan‐Lu fault, which offsets the ultrahigh‐pressure metamorphic rocks, is a secondary feature not related to collision or subduction. Whole rock isotopic data show that both Rb‐Sr and U‐Pb were open systems during metamorphism but that Sm‐Nd probably remained closed. Nd initial isotopes display crustal signatures, indicating that the crust subducted as a coherent slab rather than becoming intercalated with mantle material at depth. Sm‐Nd data in conjunction with U‐Pb zircon dating indicate that the ultrahigh‐pressure rocks are not typical Yangtze craton basement (2.9 Ga) but that they originally crystallized in a rift environment between ∼700 and 800 Ma. Discrepancies between 40Ar/39Ar plateaus, Sm‐Nd isochrons, and U‐Pb zircon ages render calculation of a P‐T‐t path unrealistic.

Cambrian microbial and silica gel textures in silica iron exhalites from the Mount Windsor volcanic belt, Australia; their petrography, chemistry, and origin

Stratiform quartz-hematite or magnetite lenses (ironstones) occur sporadically throughout the Mount Windsor volcanic belt at three main stratigraphic positions, including along strike from and immediately stratigraphically above the Thalanga massive sulfide deposit. The area is ideal for examination of the chemical, textural, and mineralogical changes which occur in silica iron exhalites around ancient hydrothermal submarine vent sites. Major and trace element chemical data for the ironstones indicate they are essentially composed of SiO 2 and Fe 2 O 3 (generally >99 wt %), with very low concentrations of other oxides (e.g., Al 2 O 3 , generally <0.5 wt %) and trace elements.Across the length of the Mount Windsor volcanic belt, most of the ironstones have experienced substantial recrystallization at metamorphic grades from greenschist to amphibolite facies. However, a number of samples from the eastern part of the belt are less recrystallized (prehnite-pumpellyite facies) and retain abundant evidence of microbial fossils and the inorganic maturation of amorphous phases. At least three distinct microbial morphologies are recognized; (1) approximately 1-mu m-avg-diam filaments with septa, (2) approximately 3-mu m-avg-diam filaments with no septa, and (3) 20- to 30-mu m-avg-diam filaments with no reconnecting branches. The range in metamorphic grade along the Mount Windsor volcanic belt not only permits a rare insight into the primary textures of ancient exhalites but also a useful view of the textural and mineralogical changes which accompany the progressive increase in temperature up to amphibolite facies conditions.Comparisons with similar modern deposits and experimentally produced synthetic textures suggest that the ironstones formed from the crystallization of silica iron oxyhydroxide gels. These gels formed as a result of mixing between hydrothermal fluid and seawater and conductive cooling of the product. The rare earth element geochemistry of ironstones along strike from Thalanga is compatible with migration of a hydrothermal fluid away from the vent center, followed by its accumulation and cooling in topographic depressions on the sea floor. Microaerophilic chemolithotrophic bacteria and/or fungi probably catalyzed the oxidation of Fe (super +2) to Fe (super +3) . Several metamorphosed ironstones also contain pyrite with delta 34 S values approximately 50 per mil lighter than Cambrian sulfate, suggesting the activity ofsulfate-reducing bacteria at some point during their formation. The temperature of the original hydrothermal fluid was probably between 10 degrees and 40 degrees C, originating either as the in situ product of weak hydrothermal activity in which inorganic sulfate reduction did not occur or as the cooled remnant of a higher temperature fluid which had already deposited sulfides. The inferred mode of deposition for the siliceous ironstones (as a gel) is different from that of many modern iron oxyhydroxide precipitates, which form by settling of particles to the sea floor from hydrothermal plumes. On the flanks of the Thalanga massive sulfide deposit some iron silica exhalites have been encompassed by massive sulfide suggesting that the early low-temperature Fe-Si precipitates were subsequently overprinted by hotter fluids. This provides good evidence for waxing and waning in the life of the vent system.

The Wayah granulite-facies metamorphic core, southwestern North Carolina: High-grade culmination of Taconic metamorphism in the southern Blue Ridge

Research Article| November 01, 1989 The Wayah granulite-facies metamorphic core, southwestern North Carolina: High-grade culmination of Taconic metamorphism in the southern Blue Ridge JAMES O. ECKERT, JR.; JAMES O. ECKERT, JR. 1Department of Geology, Texas A&M University, College Station, Texas 77843-3115 Search for other works by this author on: GSW Google Scholar ROBERT D. HATCHER, JR.; ROBERT D. HATCHER, JR. 2Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996-1410 Search for other works by this author on: GSW Google Scholar DAVID W. MOHR DAVID W. MOHR 3Code 2110, Naval Coastal Systems Center, Building 110, Room 3C27, Panama City, Florida 32401-5000 Search for other works by this author on: GSW Google Scholar Author and Article Information JAMES O. ECKERT, JR. 1Department of Geology, Texas A&M University, College Station, Texas 77843-3115 ROBERT D. HATCHER, JR. 2Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996-1410 DAVID W. MOHR 3Code 2110, Naval Coastal Systems Center, Building 110, Room 3C27, Panama City, Florida 32401-5000 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1989) 101 (11): 1434–1447. https://doi.org/10.1130/0016-7606(1989)101<1434:TWGFMC>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation JAMES O. ECKERT, ROBERT D. HATCHER, DAVID W. MOHR; The Wayah granulite-facies metamorphic core, southwestern North Carolina: High-grade culmination of Taconic metamorphism in the southern Blue Ridge. GSA Bulletin 1989;; 101 (11): 1434–1447. doi: https://doi.org/10.1130/0016-7606(1989)101<1434:TWGFMC>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Wayah granulite-facies metamorphic core, in the eastern Blue Ridge, southwestern North Carolina, contains the metamafic assemblage hornblende-orthopyroxene-clinopyroxene which defines the hornblende (lower) granulite facies. The metamorphic progression from the kyanite-almandine zone into the Wayah granulite core is continuous across the Hayesville fault, a major premetamorphic-peak tectonostratigraphic boundary. This metamorphic transition includes a distinct muscovite-absent (+alkali feldspar) second-sillimanite zone. Retrogression is limited. This entire range in metamorphic grade apparently was produced during a single, prograde, Paleozoic (likely Taconic) metamorphic event.The Hayesville fault crosses the study area mostly within the first-sillimanite zone, northwest of the Wayah granulite core. Both supracrustal sequences juxtaposed along this fault consist predominantly of metasedimentary rocks probably deposited during the Late Proterozoic. Although paragneiss pre-dominates, the southeastern terrane is distinguished by the presence of metamorphosed felsic, mafic, and ultramafic igneous rocks. No evidence supports the presence of Grenville basement within the study area.The second of four deformational events, synchronous with the metamorphic peak, created northeast-southwest- to east-west-trending, upright isoclinal folds which dominate the outcrop pattern. Macroscopic F2 folds deformed the Hayesville fault.Application of mineralogic thermometry and barometry yields conditions which range from 585 °C, 5.5 kb at the kyanite-sillimanite isograd to 842 °C, 9.8 kb in the hornblende granulite facies. Macroscopic distributions of migmatite and perthite, published experimental data, and barometric calculations suggest that T = ∼669 °C, P = ∼6.52 kb, and X(H2O) = ∼0.62 at the second-sillimanite isograd.Results indicate that the Wayah granulite-facies core constitutes the high-grade culmination of Paleozoic metamorphism in the southern Appalachian Blue Ridge. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The hydroxyl content of staurolite

The composition of forty-six staurolite samples spanning a range in metamorphic grade has been determined by electron microprobe analysis and an improved micro-analytical technique for water determination. The hydroxyl content per unit cell calculated on the basis for fortyeight oxygen and hydroxyl atoms ranges from 3.15 to 3.57 at Black Mountain, western New Hampshire, 2.45 to 2.94 at Jamaica, Vermont, 1.79 to 3.01 in western Maine, and 1.96 to 3.32 in northwestern Connecticut. The OH-content of staurolite is controlled by homogeneous reactions (cation-hydroxyl substitutions) and heterogeneous reactions (redox and dehydration equilibria). Closely-spaced samples in all four areas show differences in OH-content due to variations in $$\mu _{O_2 } $$ between adjacent beds. A decrease in the OH-content of staurolite was observed only in the highest grade rocks.

Archean banded iron-formations, Yilgarn Block, Western Australia

Banded iron-formation is a widely distributed although relatively minor lithology in the Archean Yilgarn Block, Western Australia, occurring in supracrustal sequences composed mainly of metamorphosed volcanogenic rocks in the northern and eastern parts of the Yilgarn Block and also in highly deformed and metamorphosed predominantly metasedimentary sequences in the gneissic terrain in the southwest. In the central part of the Yilgarn Block structural mapping and stratigraphic reconstruction indicates that individual banded iron-formation units extend over an area of at least 40 X 60 km and could extend over a much larger area. Most banded iron-formation units appear to be 5 to 50 m thick and units of 100 to 150 m are uncommon. Three scales of layering occur in iron-formation units: (a) interlayering of banded iron-formation and Fe shale. Fe shales range in thickness from several millimeters to 5 to 6 m; (b) mesobanding is the most prominent structure in the banded iron-formations; (c) millimeter scale laminations, including microbanding, occur in both banded iron-formation and Fe shales. Other structures in the banded iron-formation include pods, macules, and small spheroidal nodules. Despite modification by metamorphic and deformation processes which, among other effects, have caused thinning of mesobands, all the structures can readily be equated with those in the Hamersley Basin iron-formations. There is a wide range in metamorphic grade. At the lowest grades banded iron-formation contains quartz, magnetite, greenalite, stilpnomelane, minnesotaite, siderite, and ankerite. Fe shales contain chamosite, stilpnomelane, siderite, and pyrite. The highest grades are characterized by quartz-magnetite-clinopyroxene-orthopyroxene in iron-formation and hornblende-almandine + or - biotite assemblages in Fe shale. Mineral assemblages are essentially identical to those in many metamorphosed Proterozoic iron-formations.The bulk chemical compositions of the Yilgarn Block iron-formations are very similar to other Precambrian iron-formations from very different stratigraphic settings. Fe shales have highly variable compositions characterized by a marked positive Al 2 O 3 -TiO 2 correlation and high Cr contents.The similarity in the bulk chemical composition of iron-formations in the Yilgarn Block and other Precambrian sequences implies that deposition of these iron-formations occurred in very similar chemical environments. It is suggested that although Fe shales may be related in some way to volcanic activity, the main constituents of the banded iron-formations are not directly derived from volcanic sources. Banded iron-formations appear to have been precipitated from ocean waters which, relative to younger oceans, contained higher concentrations of Fe and Si in solution.

Northwest Margin of California Continental Borderland: Marine Geology and Tectonic Evolution

The northwest margin of the California continental borderland consists mainly of two northwest-trending pre-Neogene lithologic belts blanketed by Miocene and younger strata. These belts, which are lithologically and structurally correlated with the Franciscan Complex and Great Valley sequence of northern California, are interpreted to represent facies corresponding to the subduction complex and forearc-basin deposits of a late Mesozoic and Paleogene continental-margin arc-trench system. The outer belt, which forms the acoustic basement underlying the continental slope and Patton Ridge, is characterized by: (1) moderately high compressional velocities (5.1 to 6.2 km/sec), (2) discordant and discontinuous seismic reflectors, and (3) nonfossiliferous and diverse rock types t at range in metamorphic grade from zeolite-bearing arkosic sandstone to blueschist. Landward, the inner Great Valley sequence belt underlies Santa Rosa-Cortes Ridge and Santa Cruz and San Nicolas Basins; in contrast to the outer belt, this belt is characterized by: (1) intermediate compressional velocities (4.5 to 4.6 km/sec), (2) concordant and relatively continuous reflectors, and (3) a thick turbidite sequence of relatively undeformed and unmetamorphosed fossiliferous Cretaceous and Paleogene strata. The inner belt overlies a 6.6-km/sec layer that is probably oceanic crust, a relation consistent with the hypothesis that trapped old oceanic crust underlies the forearc basins along parts of some convergent margins. Well-developed wrench-fault structures in overlying Miocene strata record a transition from Paleogene subduction to Neogene transform faulting. Related events include widespread andesitic to dacitic volcanism and concurrent uplift and erosion of Miocene and older rocks. The proximity of these Miocene silicic volcanic rocks to the paleotrench (< 20 km) is best explained as resulting from passage of the southward-migrating Rivera ridge-fault-trench triple junction along the margin. On the basis of the age of the adjacent sea floor, passage of this triple junction occurred about 17 m.y. ago, a date that corresponds closely to the inception of volcanism and uplift in the northwest margin of the borderland. A widespread unconformity that separates highly deformed lower and lower middle iocene strata from relatively undeformed younger strata is assumed to mark the subsequent onset of wrench faulting. The timing of volcanism and uplift, and of the slightly younger wrenching event, closely matches the plate-tectonic model of Atwater and Molnar if the northwest margin of the borderland is restored to an earlier tectonic setting off Baja California, approximately 260 km southeast of its present position.

Geometry and pressure-temperature significance of the kyanite-sillimanite isograd in the Mica Creek area, British Columbia

Late Precambrian and Lower Paleozoic metapelitic rocks near Mica Creek, British Columbia, range in metamorphic grade from biotite zone to sillimanite zone. The kyanite-sillimanite isograd was established after phase 2 folding but was deformed by phase 3 folding. Topographic relief of about 2 km, combined with phase 3 folding, permits reasonably precise determination of the geometry of the isogradic surfaces. The effects of phase 3 folding have been accounted for and the isobaric surfaces at the time of metamorphism are inferred to have been dipping gently. Using the kyanite-sillimanite experimental phase diagram, intersection of the isobars and the isograd permits estimates of differences in temperature and pressure along a cross section. These estimates are about 400 bars and 20° C. Mineral geothermometry (garnet-biotite) and geobarometry (garnetplagioclase-Al 2SiO5-quartz) does not provide a fine enough resolution to detect these predicted differences.

Partitioning of Sodium between Coexisting K-Feldspar and Plagioclase from Some Metamorphic Rocks

The partitioning of sodium between pairs of coexisting potassium and plagioclase feldspars extracted from metamorphic rocks is examined to test its suitability for use as a geothermometer. Rocks from eight different areas were studied in detail, most areas including two zones of metamorphism; in this way a range of metamorphic grade from middle amphibolite facies to various grades of granulite facies was investigated. Data from the metamorphic areas indicate that the distribution of sodium between coexisting feldspar phases cannot give any indications of relative metamorphic grades. Some factors are suggested to account for the different behavior of sodium partitioning between natural and experimental systems. The results for sodium distribution contrast with the temperature-dependent distribution of trace quantities of strontium between the same feldspar pairs.

Partition of Strontium between Coexisting K-Feldspar and Plagioclase in Some Metamorphic Rocks

Trace quantities of strontium in forty-four pairs of coexisting natural potassium and plagioclase feldspars extracted from metamorphic rocks were analyzed using an X-ray spectrographic technique. The main objective of this work has been to evaluate the usefulness of strontium distribution coefficients as indicators of metamorphic grade. Rocks from six different areas were studied in detail, most areas including two zones of metamorphism; in this way a range of metamorphic grade from middle amphibolite facies to various grades of granulite facies was investigated. Strontium partitioning between coexisting feldspar phases in isofacial rocks can be described by the Nernst distribution law and is of the type (mol per cent Sr-feldspar in K-feldspar)/(mol per cent Sr-feldspar in plagioclase) = constant. Data from most areas show systematic and significant changes in strontium partitioning with gross changes in metamorphic grade. With increasing grade, strontium is preferentially taken into the K-feldspar phase....

Distribution of Fe, Mg, and Mn between Garnet and Biotite in Natural Mineral Assemblages

The distribution coefficient, $K_{D} \equiv (Mg/Fe)_{garnet}/(Mg/Fe)_{biotite}$, has been calculated for seventy garnet-biotite pairs from samples of known mineral assemblage and from a range of metamorphic grade. This distribution coefficient depends upon the metamorphic grade, but it is also dependent upon the Mn content of garnet and upon the detailed mineral assemblage. For a single assemblage from a given metamorphic grade the distribution coefficient is reduced by about 0.007 per 1 atom per cent of $(Mn/Mn + Mg + Fe)_{garnet}$. Values of $K_{D}$ for isogradic assemblages differ by about 0.045. The distribution coefficient for different metamorphic grades is compared by projection to an arbitrary zero value of $(Mn/Mn + Mg + Fe)_{garnet}$. $K_{D}$ for garnet-zone rocks is about 0.2, for staurolite-zone rocks about 0.215, for kyanite-zone rocks about 0.23, and for sillimanite-K-feldspar-zone rocks about 0.3-0.37.