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Forthcoming papers in the Journal of Metamorphic Geology (2005)

Oxford, UK : Blackwell Science Inc

Journal of metamorphic geology 23 (2005), S. 0

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ISSN: 1525-1314

Quelle: Blackwell Publishing Journal Backfiles 1879-2005

Thema: Geologie und Paläontologie

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1111/j.1525-1314.2005.00570.x

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Phase relationships in grunerite–garnet-bearing amphibolites in the system CFMASH, with applications to metamorphic rocks from the Central Zone of the Limpopo Belt, South Africa (2005)

ZEH, A. ; HOLLAND, T. J. B. ; KLEMD, R.

Oxford, UK : Blackwell Science Inc

Journal of metamorphic geology 23 (2005), S. 0

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ISSN: 1525-1314

Quelle: Blackwell Publishing Journal Backfiles 1879-2005

Thema: Geologie und Paläontologie

Notizen: A petrogenetic grid in the model system CaO–FeO–MgO–Al2O3–SiO2–H2O is presented, illustrating the phase relationships among the minerals grunerite, hornblende, garnet, clinopyroxene, chlorite, olivine, anorthite, zoisite and aluminosilicates, with quartz and H2O in excess. The grid was calculated with the computer software thermocalc, using an upgraded version of the internally consistent thermodynamic dataset HP98 and non-ideal mixing activity models for all solid solutions. From this grid, quantitative phase diagrams (P–T pseudosections) are derived and employed to infer a P–T path for grunerite–garnet-bearing amphibolites from the Endora Klippe, part of the Venetia Klippen Complex within the Central Zone of the Limpopo Belt. Agreement between calculated and observed mineral assemblages and garnet zonation indicates that this part of the Central Zone underwent a prograde temperature and pressure increase from c. 540 °C/4.5 kbar to 650 °C/6.5 kbar, followed by a post-peak metamorphic pressure decrease. The inferred P–T path supports a geotectonic model suggesting that the area surrounding the Venetia kimberlite pipes represents the amphibolite-facies roof zone of migmatitic gneisses and granulites that occur widely within the Central Zone. In addition, the P–T path conforms to an interpretation that the Proterozoic evolution of the Central Zone was controlled by horizontal tectonics, causing stacking and differential heating at c. 2.0 Ga.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1111/j.1525-1314.2005.00554.x

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Experimental deformation of partially melted granite revisited: implications for the continental crust (2005)

ROSENBERG, C. L. ; HANDY, M. R.

Oxford, UK : Blackwell Science Inc

Journal of metamorphic geology 23 (2005), S. 0

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ISSN: 1525-1314

Quelle: Blackwell Publishing Journal Backfiles 1879-2005

Thema: Geologie und Paläontologie

Notizen: A review and reinterpretation of previous experimental data on the deformation of partially melted crustal rocks reveals that the relationship of aggregate strength to melt fraction is non-linear, even if plotted on a linear ordinate and abscissa. At melt fractions, Φ 〈 0.07, the dependence of aggregate strength on Φ is significantly greater than at Φ 〉 0.07. This melt fraction (Φ = 0.07) marks the transition from a significant increase in the proportion of melt-bearing grain boundaries up to this point to a minor increase thereafter. Therefore, we suggest that it is the increase of melt-interconnectivity that causes the dramatic strength drop between the solidus and a melt fraction of 0.07. We term this drop the ‘melt connectivity transition’ (MCT). A second, less-pronounced strength drop occurs at higher melt fractions and corresponds to the breakdown of the solid (crystal) framework. This is the ‘solid-to-liquid transition’ (SLT), corresponding to the well known ‘rheologically critical melt percentage’. Although the strength drop at the SLT is about four orders of magnitude, the absolute value of this drop is small compared with the absolute strength of the unmelted aggregate, rendering the SLT invisible in a linear aggregate strength v. melt-fraction diagram. On the other hand, the more important MCT has been overlooked in previous work because experimental data usually are plotted in logarithmic strength v. melt-fraction diagrams, obscuring large strength drops at high absolute strength values. We propose that crustal-scale localization of deformation effectively coincides with the onset of melting, pre-empting attainment of the SLT in most geological settings. The SLT may be restricted to controlling flow localization within magmatic bodies, especially where melt accumulates.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1111/j.1525-1314.2005.00555.x

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Melting and melt segregation in the aureole of the Glenmore Plug, Ardnamurchan (2005)

HOLNESS, M. B. ; DANE, K. ; SIDES, R. ; [weitere]

Oxford, UK : Blackwell Science Inc

Journal of metamorphic geology 23 (2005), S. 0

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ISSN: 1525-1314

Quelle: Blackwell Publishing Journal Backfiles 1879-2005

Thema: Geologie und Paläontologie

Notizen: Contact metamorphism caused by the Glenmore plug in Ardnamurchan, a magma conduit active for 1 month, resulted in partial melting, with melt now preserved as glass. The pristine nature of much of the aureole provides a natural laboratory in which to investigate the distribution of melt. A simple thermal model, based on the first appearance of melt on quartz–feldspar grain boundaries, the first appearance of quartz paramorphs after tridymite and a plausible magma intrusion temperature, provides a time-scale for melting. The onset of melting on quartz–feldspar grain boundaries was initially rapid, with an almost constant further increase in melt rim thickness at an average rate of 0.5–1.0 × 10−9 cm s−1. This rate was most probably controlled by the distribution of limited amounts of H2O on the grain boundaries and in the melt rims.The melt in the inner parts of the aureole formed an interconnected grain-boundary scale network, and there is evidence for only limited melt movement and segregation. Layer-parallel segregations and cross-cutting veins occur within 0.6 m of the contact, where the melt volume exceeded 40%. The coincidence of the first appearance of these signs of the segregation of melt in parts of the aureole that attained the temperature at which melting in the Qtz–Ab–Or system could occur, suggests that internally generated overpressure consequent to fluid-absent melting was instrumental in the onset of melt movement.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1111/j.1525-1314.2005.00560.x

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Forthcoming papers in the Journal of Metamorphic Geology (2005)

Oxford, UK : Blackwell Science Inc

Journal of metamorphic geology 23 (2005), S. 0

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ISSN: 1525-1314

Quelle: Blackwell Publishing Journal Backfiles 1879-2005

Thema: Geologie und Paläontologie

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1111/j.1525-1314.2005.00567.x

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Discrete ultrahigh-pressure domains in the Western Gneiss Region, Norway: implications for formation and exhumation (2005)

ROOT, D. B. ; HACKER, B. R. ; GANS, P. B. ; [weitere]

Oxford, UK : Blackwell Science Inc

Journal of metamorphic geology 23 (2005), S. 0

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ISSN: 1525-1314

Quelle: Blackwell Publishing Journal Backfiles 1879-2005

Thema: Geologie und Paläontologie

Notizen: New eclogite localities and new 40Ar/39Ar ages within the Western Gneiss Region of Norway define three discrete ultrahigh-pressure (UHP) domains that are separated by distinctly lower pressure, eclogite facies rocks. The sizes of the UHP domains range from c. 2500 to 100 km2; if the UHP culminations are part of a continuous sheet at depth, the Western Gneiss Region UHP terrane has minimum dimensions of c. 165 × 50 × 5 km. 40Ar/39Ar mica and K-feldspar ages show that this outcrop pattern is the result of gentle regional-scale folding younger than 380 Ma, and possibly 335 Ma. The UHP and intervening high-pressure (HP) domains are composed of eclogite-bearing orthogneiss basement overlain by eclogite-bearing allochthons. The allochthons are dominated by garnet amphibolite and pelitic schist with minor quartzite, carbonate, calc-silicate, peridotite, and eclogite. Sm/Nd core and rim ages of 992 and 894 Ma from a 15-cm garnet indicate local preservation of Precambrian metamorphism within the allochthons. Metapelites within the allochthons indicate near-isothermal decompression following (U)HP metamorphism: they record upper amphibolite facies recrystallization at 12–17 kbar and c. 750 °C during exhumation from mantle depths, followed by a low-pressure sillimanite + cordierite overprint at c. 5 kbar and c. 750 °C. New 40Ar/39Ar hornblende ages of 402 Ma document that this decompression from eclogite-facies conditions at 410–405 Ma to mid-crustal depths occurred in a few million years. The short timescale and consistently high temperatures imply adiabatic exhumation of a UHP body with minimum dimensions of 20–30 km. 40Ar/39Ar muscovite ages of 397–380 Ma show that this extreme heat advection was followed by rapid cooling (c. 30 °C Myr−1), perhaps because of continued tectonic unroofing.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1111/j.1525-1314.2005.00561.x

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Fluid evolution and thermal structure in the rapidly exhuming gneiss complex of Namche Barwa–Gyala Peri, eastern Himalayan syntaxis (2005)

CRAW, D. ; KOONS, P. O. ; ZEITLER, P. K. ; [weitere]

Oxford, UK : Blackwell Science Inc

Journal of metamorphic geology 23 (2005), S. 0

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ISSN: 1525-1314

Quelle: Blackwell Publishing Journal Backfiles 1879-2005

Thema: Geologie und Paläontologie

Notizen: High-grade gneisses (amphibolite–granulite facies) of the Namche Barwa and Gyala Peri massifs, in the eastern Himalayan syntaxis, have been unroofed from metamorphic depths in the late Tertiary–Recent. Rapid exhumation (2–5 mm year−1) has resulted in a pronounced shallow conductive thermal anomaly beneath the massifs and the intervening Tsangpo gorge. The position of the 300 °C isotherm has been estimated from fluid inclusions using CO2–H2O immiscibility phase equilibria to be between 2.5 and 6.2 km depth below surface. Hence, the near-surface average thermal gradient exceeds 50 °C km−1 beneath valleys, although the thermal gradient is relatively lower beneath the high mountains. The original metamorphic fluid in the gneisses was 〉90% CO2. This fluid was displaced by incursion of brines from overlying marine sedimentary rocks that have since been largely removed by erosion. Brines can exceed 60 wt% dissolved salts, and include Ca, Na, K and Fe chlorides. These brines were remobilized during the earliest stages of uplift at 〉500 °C. During exhumation, incursion of abundant topography-driven surface waters resulted in widespread fracture-controlled hydrothermal activity and brine dilution down to the brittle–ductile transition. Boiling water was particularly common at shallow levels (〈2.5 km) beneath the Yarlung Tsangpo valley, and numerous hot springs occur at the surface in this valley. Dry steam is not a major feature of the hydrothermal system in the eastern syntaxis (in contrast to the western syntaxis at Nanga Parbat), but some dry steam fluids may have developed locally.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1111/j.1525-1314.2005.00612.x

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A Model to Estimate Potential Submersed Aquatic Vegetation Habitat Based on Studies in Lake Pontchartrain, Louisiana (2005)

Cho, Hyun J. ; Poirrier, Michael A.

Oxford, UK; Malden, USA : Blackwell Science Inc

Restoration ecology 13 (2005), S. 0

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ISSN: 1526-100X

Quelle: Blackwell Publishing Journal Backfiles 1879-2005

Thema: Biologie

Notizen: The depth distribution of submersed aquatic vegetation (SAV) was studied in Lake Pontchartrain, Louisiana, to develop a model to predict changes in SAV abundance from changes in environmental quality. We conducted annual line-intercept surveys from 1997 through 2001 and monitored monthly photosynthetically active radiation at four sites with different shoreface slopes. The following relationships between SAV distribution and environmental factors were used as model parameters: (1) water clarity controls SAV colonization depth; (2) fluctuation in annual mean water level and wave mixing determines SAV minimum colonization depth; and (3) site differences in SAV areal coverage under the comparable water quality conditions are due to shoreface slope differences. These parameters expressed as mathematical components of the model are as follows: mean water clarity determines SAV colonization depth (Zmax= 2.3/Kd); mean water level and wave mixing controls SAV minimum depth (Zmin= 0.3 m); and shoreface slope angle (θ) determines the distance from Zmin to Zmax. The equation developed for the potential SAV habitat (PSAV) model is PSAV = (2.3 − 0.3 ×Kd)/(sinθ×Kd). The model was validated by comparing empirical values from the dataset to values predicted by the model. Although the model was developed to predict the PSAV in Lake Pontchartrain, it can be applied to other coastal habitats if local SAV light requirements are substituted for Lake Pontchartrain values. This model is a useful tool in selecting potential restoration sites and in predicting the extent of SAV habitat gain after restoration.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1111/j.1526-100X.2005.00080.x

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Assessing Simple Versus Complex Restoration Strategies for Industrially Disturbed Forests (2005)

Rayfield, Bronwyn ; Anand, Madhur ; Laurence, Sophie

Oxford, UK; Malden, USA : Blackwell Science Inc

Restoration ecology 13 (2005), S. 0

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ISSN: 1526-100X

Quelle: Blackwell Publishing Journal Backfiles 1879-2005

Thema: Biologie

Notizen: Assessing the community-level consequences of ecological restoration treatments is essential to guide future restoration efforts. We compared the vegetation composition and species richness of restored sites that received a range of restoration treatments and those of unrestored sites that experienced varying levels of disturbance. Our study was conducted in the industrially degraded landscape surrounding Sudbury, Ontario, Canada. The Great Lakes–St. Lawrence Forest once present in this area was degraded through logging, mining, and smelting activities beginning in the late 1800s until restoration of the most visibly degraded areas began in 1974. Restoration treatments ranged from simple abiotic enhancements to complex, multistage revegetation treatments using native and non-native species, which included fertilizing, spreading of ground dolomitic limestone, understory seeding, and tree planting. Canonical correspondence analysis was used to determine which restoration treatments explained differences in the community structure among sites. We found that native understory vascular species richness was similar in restored sites that received more complex restoration treatments and unrestored sites that were mildly disturbed; however, the role of planted trees and non-native species in the restored communities remains unclear. Understory vascular seeding played a key role in determining community composition of vascular understory and overstory communities, but the time since restoration commenced was a more important factor for nonvascular communities because they received no direct biotic enhancements. The use of non-native species in the vascular seed mix seems to be slowly encouraging the colonization of native species, but non-natives continue to dominate restored sites 25 years after restoration began.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1111/j.1526-100X.2005.00082.x

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Restoration of Native Perennials in a California Annual Grassland after Prescribed Spring Burning and Solarization (2005)

Moyes, Andrew B. ; Witter, Martha S. ; Gamon, John A.

Oxford, UK; Malden, USA : Blackwell Science Inc

Restoration ecology 13 (2005), S. 0

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ISSN: 1526-100X

Quelle: Blackwell Publishing Journal Backfiles 1879-2005

Thema: Biologie

Notizen: Grasslands dominated by exotic annual grasses have replaced native perennial vegetation types in vast areas of California. Prescribed spring fires can cause a temporary replacement of exotic annual grasses by native and non-native forbs, but generally do not lead to recovery of native perennials, especially where these have been entirely displaced for many years. Successful reintroduction of perennial species after fire depends on establishment in the postfire environment. We studied the effects of vegetation changes after an April fire on competition for soil moisture, a key factor in exotic annual grass dominance. As an alternative to fire, solarization effectively kills seeds of most plant species but with a high labor investment per area. We compared the burn to solarization in a study of establishment and growth of seeds and transplants of the native perennial grass Purple needlegrass (Nassella pulchra) and coastal sage species California sagebrush (Artemisia californica). After the fire, initial seed bank and seedling densities and regular percent cover and soil moisture (0–20 cm) data were collected in burned and unburned areas. Burned areas had 96% fewer viable seeds of the dominant annual grass, Ripgut brome (Bromus diandrus), leading to replacement by forbs from the seed bank, especially non-native Black mustard (Brassica nigra). In the early growing season, B. diandrus dominating unburned areas consistently depleted soil moisture to a greater extent between rains than forbs in burned areas. However, B. diandrus senesced early, leaving more moisture available in unburned areas after late-season rains. Nassella pulchra and A. californica established better on plots treated with fire and/or solarization than on untreated plots. We conclude that both spring burns and solarization can produce conditions where native perennials can establish in annual grasslands. However, the relative contribution of these treatments to restoration appears to depend on the native species being reintroduced, and the long-term success of these initial restoration experiments remains to be determined.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1111/j.1526-100X.2005.00084.x

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