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Shigenori Maruyama - One of the best experts on this subject based on the ideXlab platform.
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intermediate p t type Regional Metamorphism of the isua supracrustal belt southern west greenland the oldest pacific type orogenic belt
Tectonophysics, 2015Co-Authors: Tatsuyuki Arai, Soichi Omori, Tsuyoshi Komiya, Shigenori MaruyamaAbstract:Abstract The 3.7–3.8 Ga Isua Supracrustal Belt (ISB), southwest Greenland, might be the oldest accretionary complex on Earth. Regional Metamorphism of the ISB has a potential to constrain the tectonothermal history of the Earth during the Eoarchean. Chemical and modal analyses of metabasite in the study area (i.e., the northeast part of the ISB) show that the metamorphic grade increases from greenschist facies in the northern part of the study area to amphibolite facies in the southern part. To determine the precise metamorphic P–T ranges, isochemical phase diagrams of minerals of metabasite were made using Perple_X. A synthesis of the estimated metamorphic P–T ranges of the ISB indicates that both the metamorphic pressure and temperature increase systematically to the south in the study area from 3 kbar and 380 °C to 6 kbar and 560 °C. The monotonous metamorphic P–T change suggests that the northeast part of the ISB preserves Regional Metamorphism resulting from the subduction of an accretionary complex although the ISB experienced metamorphic overprints during the Neoarchean. Both the presence of the Regional Metamorphism and an accretionary complex having originating at subduction zone suggest that the ISB may be the oldest Pacific-type orogenic belt. The progressive Metamorphism can be considered as a record of intermediate-P/T type geothermal gradient at the subduction zone in the Eoarchean. Intermediate-P/T type geothermal gradient is typical at the current zones of subducting young oceanic crust, such as in the case of the Philippine Sea Plate in the southwest part of Japan. Considering the fact that almost all Metamorphisms in the Archean are greenschist–amphibolite facies, the intermediate-P/T type geothermal gradient at the ISB might have been worldwide in the Archean. This would indicate that the subduction of young micro-plates was common because of the vigorous convection of hot mantle in the Archean.
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pressure temperature conditions of ongoing Regional Metamorphism beneath the japanese islands
Gondwana Research, 2009Co-Authors: Soichi Omori, Shigenori Maruyama, Saeko Kita, M SantoshAbstract:Abstract We evaluate the pressure–temperature (P–T) conditions of ongoing Regional Metamorphism at the top of the oceanic crust of the subducted Pacific and Philippine Sea plates through a combination of phase diagrams and hypocenter distribution and based on the dehydration-induced earthquake hypothesis. The brute-force method was employed to find the best match thermal structure to link the hypocenter distribution and dehydration. The estimated thermal structure varies far from the values obtained from numerical simulation. Our estimates are consistent with the qualitative physical prediction for the variation of temperature in different subduction zones and provide quantitative constraints for the models. In northeastern Japan, the P–T path for the top of the oceanic crust turns to the high-T side at a depth of around 90 km. The depth corresponds to the location of the volcanic front and an active convection of the wedge mantle below this depth is suggested. Our computations also reveal the effect of an exceptional scenario beneath the Kanto region. The temperature in the Kanto region, where the cold lid of the Philippine Sea plate prevents heating by the return-flow of mantle wedge above, is much lower than that of northeastern Japan. The subduction of younger Philippine Sea plate leads to a higher-temperature in the oceanic crust. In the central Shikoku region, the thermal structure exhibits high-T/P nature. Heating by shear deformation can explain the high-T/P path in the depth range from 20 to 35 km. The Kyushu area shows moderate type T/P path reaching up to eclogite facies conditions. In the Kii and central Shikoku region, the thermal structure exhibits high-T/P nature. However, the absolute values for the areas seem to have problem in physical context. Our results has risen the significance of sediment subduction in the southwest Japan and requirement for further improvements in this technique including the aspect of variation of the bulk composition of the subducted material.
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a revolutionary new interpretation of a Regional Metamorphism its exhumation and consequent mountain building
Journal of Geography, 2004Co-Authors: Shigenori Maruyama, Hideki Masago, Ikuo Katayama, Yasuyuki Iwase, Mitsuhiro ToriumiAbstract:The discovery of ultrahigh-pressure rocks from collision-type orogenic belts has revolu-tionized the classic interpretation of (1) progressive and retrogressive Metamorphism recorded on surface exposures of Regional metamorphic belts, (2) geochronology of that Metamorphism, (3) origin of metamorphic textures, (4) P-T-t path, (5) metamorphic facies series, (6) exhumation model, and (7) role of fluids during Regional Metamorphism. Based mainly on our recent studies of the Kokchetav, Dabie Shan, Indonesia, and the Franciscan and Sanbagawa belts, we point out or predict the following seven revolutionary paradigm shifts.So-called mineral isograds defined on the maps of Regional metamorphic belts were a misunderstanding of the progressive dehydration reaction during subduction, because extensive late-stage hydration has mostly obliterated the progressive minerals in pelitic-psammitic, and metabasic rocks. Progressively zoned garnet has survived as the sole progressive mineral that was unstable with the majority of matrix-forming minerals. The classic Barrovian isograds should be carefully re-examined.The well-documented SHRIMP chronology of spot-dating zoned zircons with index minerals from low-P in the core, through HP-UHP in the mantle to low-P on the rim clearly shows that the slow exhumation speed of 23-40 m.y. from mantle depth to mid-crustal level was followed by mountain building with doming at latest stage. Extensive hydration of the UHP-HP unit occurred due to fluid infiltration underneath, when the UHP-HP unit intruded a low-grade to un-metamorphosed unit at a mid-crustal level.Most deformation textures such as mineral lineations, porphyroblasts, pull-apart or boudinaged amphiboles, formed during extensive hydration at the late-stage, hence do not indicate a progressive stress regime.The P-T-t path determined by thermobarometry using mineral inclusions in garnet and forward-modeling of garnet zoning, independent of the matrix minerals, indicates an anticlockwise path in the P-T space, and it follows an identical P-T change in the metamorphic facies series. This is consistent with the numerically calculated geotherm along the WadatiBenioff plane.Collision-type orogenic belts have long been regarded as being characterized by the intermediate-type metamorphic facies series. The kyanite-sillimanite-type is an apparent type facies series formed by late-stage extensive hydration. In contrast, the original high-P to ultrahigh-P type facies series with an anticlockwise kink-point at around 10 kb is a progressive type.A collision-type Regional metamorphic belt crops out as a very thin unit sandwiched between overlying and underlying low-P or weakly metamorphosed units. The metamorphic belt has an aspect ratio (thickness vs width) of 1 : 100, and it extends for several hundreds to a thousand km. It resembles a thin mylonitic intrusion from the mantle extending a depth of from 100-200 km into the crustal rock unit. The underlying unit is thermally metamorphosed in the andalusite-sillimanite type facies series.The major reason for the misunderstanding of the progressive Metamorphism in collisiontype orogenic belts is the underestimation of the role of fluids derived from the underlying the low-grade metamorphic unit, when juxtaposed at a mid-crustal level. The circulation of fluids at a plate boundary is more important than a P-T change.
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archean Regional Metamorphism of the isua supracrustal belt southern west greenland implications for a driving force for archean plate tectonics
International Geology Review, 2000Co-Authors: Mamoru Hayashi, Tsuyoshi Komiya, Yasuo Nakamura, Shigenori MaruyamaAbstract:The Isua supracrustal belt (∼3.8 Ga) constitutes the oldest accretionary complex in the world. Petrochemical and geothermobarometric studies of more than 1500 rock samples of the Isua belt have enabled us to estimate the extent of Regional Metamorphism, the petrotectonic environment, and the subduction-zone geothermal gradient in the Archean. The following line of evidence indicates progressive, prograde Metamorphism from greenschist (Zone A) through albite-epidote-amphibolite (Zone B) to amphibolite facies (Zones C and D) in the northeastern part of the Isua supracrustal belt: (1) the systematic change of mineral paragenesis in metabasites and metapelites; (2) progressive change of the composition of major metamorphic minerals, including plagioclase, amphibole, chlorite, epidote, and garnet; (3) normal zoning of amphibole and garnet; and (4) the absence of relict minerals of high-grade amphibolitic Metamorphism even in the lowest metamorphic zone. Metabasites of the Isua belt vary extremely in Mg#, causi...
Jay J Ague - One of the best experts on this subject based on the ideXlab platform.
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degassing of organic carbon during Regional Metamorphism of pelites wepawaug schist connecticut usa
Chemical Geology, 2018Co-Authors: Shuang Zhang, Jay J Ague, Alberto Vitale BrovaroneAbstract:Abstract A comprehensive understanding of the degassing systematics of organic carbon (OC) during Regional Metamorphism is necessary to evaluate the role that Metamorphism plays in the global carbon cycle. In this study, weight percentages and δ 13 C values of OC were measured in 70 samples of metapelites from the Wepawaug Schist, Connecticut, where classic Barrovian Metamorphism occurred and graphitic OC is widespread. Relative to low-grade chlorite + biotite zone rocks, our mass balance analysis shows that OC in the metapelites underwent progressive loss from −14% (−0.06 g OC per 100 g rock) in the garnet zone, through −21% (−0.09 g/100 g) in the staurolite zone, to −26% (−0.11 g/100 g) in the kyanite zone. The average δ 13 C values in different metamorphic zones (ranging from −14.74‰ to −16.24‰) are all much higher than normal organic material in marine sediments, and increase slightly from the chlorite + biotite zone to the garnet zone and decrease slightly at higher metamorphic grades. Organic carbon degassing in the form of CH 4 during the late stage of diagenesis or in the earliest stages of Metamorphism could produce this significant 13 C enrichment. Under the assumption that the 13 C enrichment is caused by graphite degassing during the lowest-grade Metamorphism (chlorite zone or lower), the degassing profile of OC during the Regional Metamorphism is reconstructed by combining the δ 13 C and OC mass change data. The computed results indicate that graphitic OC in the Wepawaug Schist probably underwent considerable loss at lowest-grade metamorphic conditions, ranging from ~−40% to ~−90% (or from −0.23 g OC per 100 g rock to −2.8 g OC per 100 g rock), and remained relatively inert at higher grades. Based on the mass balance analysis, δ 13 C systematics, and exploratory modeling results, this study argues that the lowest-grade or pre-metamorphic stages would be the more efficient OC liberators, and that the degassing potential of OC in the major stages of Barrovian Metamorphism appears to be much more restricted. Additional independent studies are required to decipher the early degassing of OC after the deposition of organic matter, which could in turn help better constrain the degassing of OC during Regional Metamorphism.
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focused pulses of Regional Metamorphism
Geochimica et Cosmochimica Acta, 2007Co-Authors: Ethan F. Baxter, Jay J Ague, Penelope J LancasterAbstract:Growing evidence is emerging to support the idea that Metamorphism, even in a Regional context, may be punctuated – or dominated – by relatively short pulses of heating, fluid flow, and/or mineral growth. Here, we describe data from two Barrovian metamorphic terranes which test this idea. In the Barrovian zones of Scotland, garnet Sm/Nd geochronology from the garnet and sillimanite zones yield the same peak metamorphic ages [1] (~465 Ma). The age is similar to the age of crystallization of large igneous bodies in the area. The contemporaneity of peak ages is explained by an efficient, advective component of heating, perhaps mediated by synchronous fluid flow [2]. The duration of this regionwide pulse of peak Metamorphism is constrained by new Srin-apatite diffusion modeling. Apatite grains have detrital cores and metamorphic overgrowths and are included within porphyroblasts (e.g. garnet, staurolite). Modeling of intragrain diffusion of Sr constrains the duration of peak Metamorphism to <250 kyr for garnet through staurolite zone samples. Garnet multi-component diffusion modeling from the sillimanite zone corroborates this brief pulse duration. The Wepawaug Schist of Connecticut USA, also yields contemporaneous peak-T garnet Sm/Nd ages from different grades across the terrane (~380 Ma). This age matches a population of texturally young zircons associated with igneous intrusions in these rocks. Garnet cores from the kyanite zone, which have growth textures indicative of extremely rapid growth [3], have been dated by Sm/Nd at 388.6 Ma. This age is matched by another population of zircons also associated with igneous intrusions. This earlier prograde growth event may be related to another pulse of metamorphic growth, brought on by magmatic fluid and heat. Brief pulses of metamorphic heating and mineral reactions, perhaps catalyzed by the introduction of fluids, may be superimposed on Regional scale conductive heating at tectonic rates. Such short pulses could help explain the discrepancy between rapid lab-based reaction kinetics and much slower time-integrated field-based reaction kinetics [4].
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release of co2 from carbonate rocks during Regional Metamorphism of lithologically heterogeneous crust
Geology, 2000Co-Authors: Jay J AgueAbstract:Prograde Regional Metamorphism drives CO 2 from carbonate rock to crustal fluids that ascend and ultimately interact with the atmosphere and oceans. The observed loss of CO 2 from metamorphic belts remains problematic, however, because the cooling that accompanies fluid ascent favors reactions that add CO 2 to metacarbonate rock by removing CO 2 from fluids. A new two-dimensional model of coupled mass transfer, chemical reaction, and heat transport was developed to assess how rock devolatilization proceeds along the upward escape paths of crustal fluids during prograde Metamorphism. The model is based on upper greenschist to lower amphibolite facies growth of amphibole in metacarbonate layers and garnet and biotite in intercalated metapelite layers of the Wepawaug Schist, Connecticut (Acadian orogeny). The modeling indicates that during heating, CO 2 concentrations were larger in metacarbonate layers than in adjacent metapelite layers because amphibole growth in metacarbonates produced CO 2 , whereas garnet and biotite growth in metapelites produced H 2 O. The resulting cross-layer concentration gradients drove H 2 O into the metacarbonate layers and CO 2 out by diffusion and the transverse component of mechanical dispersion. Such cross-layer mass transfer can continually force rock decarbonation while fluids ascend, dominating the effects of cooling, unless fluid fluxes are large and prograde heating rates are small. Consequently, prograde Metamorphism of carbonate-bearing sedimentary sequences containing significant amounts of pelitic rock will release CO 2 to Regionally migrating fluids in a wide range of orogenic settings, regardless of whether flow is in a direction of increasing or decreasing temperature. Regional CO 2 release can be driven by outcrop-scale processes of volatile exchange between contrasting lithologies.
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evidence for major mass transfer and volume strain during Regional Metamorphism of pelites
Geology, 1991Co-Authors: Jay J AgueAbstract:Systematic examination of published sedimentary and metamorphosed pelite analyses has revealed evidence of significant mass transfer and volume strain during Regional Metamorphism. Statistical analysis of the data shows that Barrovian zone Metamorphism of pelitic schist generally causes increases in the whole-rock concentrations of the low-solubility elements Ti and All The observed increases in Ti and Al contents as functions of metamorphic grade are almost certainly due to residual enrichment caused by the removal of other more soluble species. Application of mass-balance principles to the petite compositional trends indicates that the average pelite may lose as much as 30% of its mass and volume during progressive Metamorphism from subgreenschist to amphibolite facies conditions. The bulk of the lost mass is silica, not volatiles. In addition, other elements, particularly Ca, Na, and K, appear to be highly mobile in deep-crustal Metamorphism. Contrary to conventional interpretations, it is concluded that the Regional Metamorphism of pelites is not an isochemical process.
John M Ferry - One of the best experts on this subject based on the ideXlab platform.
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fluids in the crust during Regional Metamorphism forty years in the waterville limestone
American Mineralogist, 2016Co-Authors: John M FerryAbstract:![Figure][1] Research over the last four decades on carbonate rocks of the Waterville limestone, Maine, U.S.A., has contributed to the development of both concepts and methodologies for understanding fluid-rock interaction during Regional Metamorphism, including: (1) buffering of fluid composition by mineral reactions, (2) infiltration of carbonate rocks by aqueous fluids, (3) petrologic fluid-rock ratios, (4) infiltration-driven Metamorphism, (5) one- and two-dimensional continuum models for coupled fluid flow and mineral reaction, (6) time-integrated fluid fluxes, and (7) channelized, horizontal fluid flow in the direction of increasing temperature within chemically isolated layers. Disagreement between the last concept and both hydrodynamic models for metamorphic fluid flow and empirical evidence for homogenization of fluid composition at a scale much larger than layer thickness motivated development of the latest models for coupled fluid flow and mineral reaction in the Waterville limestone. The new models consider a flow medium composed of layers that differ in the initial amounts and compositions of minerals, both horizontal flow in the direction of increasing temperature and vertical flow in the direction of decreasing pressure and temperature, significant but imperfect homogenization of fluid composition across layering by CO2-H2O interdiffusion, and infiltration by fluids that are spatially variable in composition on the kilometer scale with CO2 content increasing with increasing grade of Metamorphism. The new models reproduce measured progress of the biotite-forming reaction in the Waterville limestone over a range of spatial scales spanning six orders of magnitude, from differences in reaction progress of up to a factor of ~100 between adjacent centimeter-thick layers to the coexistence of mineral reactants and products over a distance of ~13 km parallel to the metamorphic field gradient. Results imply that the present spatial distributions of reaction progress represent a steady state achieved when rocks closely approached equilibrium with the infiltrating fluid during Metamorphism. The new models resolve what for many years appeared to be fundamental discrepancies among petrologic data for Regionally metamorphosed carbonate rocks, hydrodynamic models of Regional Metamorphism, and the length scales of mass transport of volatiles across layering by diffusion. [1]: pending:yes
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re evaluation of infiltration driven Regional Metamorphism in northern new england new transport models with solid solution and cross layer equilibration of fluid composition
Journal of Petrology, 2013Co-Authors: John M Ferry, Nathan W Winslow, Sarah C PennistondorlandAbstract:The spatial distribution of reactants and products of infiltrationdriven decarbonation reactions can be a record of the geometry and amount of reactive fluid flow during Regional Metamorphism. In the past, these distributions have been interpreted assuming (1) minerals are fixed in composition and (2) single layers are chemically isolated. Because neither assumption is normally valid, new transport models were developed that specifically predict the spatial distribution of reactants and products of the well-studied biotite-forming reaction in marls from northern New England.The models consider (1) isothermal and isobaric (iso-P^T) flow, (2) horizontal flow in the direction of increasing T (up-T flow), and (3) vertical, upward flow in the direction of decreasing P andT (down-P^T flow). All models assume a medium composed of many thin layers that differ in the amounts and compositions of mineral solid solutions prior to reaction, that fluid flow is parallel to layering, and that fluid composition is homogenized across layering over a distance much greater than layer thickness. All models reproduce (1) the amounts of mineral reactants and products at outcrops in Maine and Vermont where there are extensive data, and (2) Regional-scale observations of mineral reactants only at low grades and a region several kilometers wide at higher grade where mineral reactants and products coexist. Models of up-T flow and down-P^T flow are preferred because they additionally predict spatially widespread complete reaction at the highest grades. Results show that when reactants and products are solid solutions, down-P^T flow predicted by simple hydrodynamic models of Regional metamorphic fluid flow is fully consistent with observed widespread spatial distributions of mineral reactants and products in metamorphic terrains. Results further demonstrate that layer-scale variations in reaction progress are better explained in terms of layer-by-layer variations in initial mineral abundances and compositions coupled with homogenization of fluid composition across layers than by channelized fluid flow focused into layers with elevated reaction progress. All models predict minimum time-integrated fluid fluxes of 10mol fluid cm 2 rock, 1^2 orders of magnitude less than what has been estimated assuming minerals are fixed in composition.
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development of spatial variations in reaction progress during Regional Metamorphism of micaceous carbonate rocks northern new england
American Journal of Science, 2006Co-Authors: Sarah C Pennistondorland, John M FerryAbstract:Progress (ξ) of the infiltration-driven reaction, muscovite + ankerite + quartz + rutile + H 2 O = biotite + calcite + plagioclase + CO 2 , which occurred during Barrovian and Buchan Regional Metamorphism in east-central Vermont and south-central Maine, can vary by a factor of ten or more at all spatial scales down to that of adjacent lithologic layers < 1 cm thick. Values of proxies for the activity of CO 2 and δ 18 O fluid , K s (6) ≡ [(a phl )(a an )(a cal ) 2 ]/[(a ms )(a dol ) 3 (a gtz ) 2 ] and δ 18 O Cal , are uniform within error of measurement over distances up to ≈1 m across layering. The conventional explanation of cm- to dm-scale variations in ξ in terms of layer-parallel channeled fluid flow cannot explain the uniformity in the proxies. Observed cm- to dm-scale variations in ξ are better explained by (a) mineral reactants and products that are solid solutions, (b) layer-by-layer variations in the amounts and compositions of minerals prior to reaction, and (c) uniformity of K s (6) on a spatial scale larger than the scale of variations in ξ during subsequent infiltration and reaction. The m-scale uniformity in K s (6) and δ 18 O Cal is interpreted as homogenization of a CO2 and δ 18 O fluid caused by the combined effects of intergranular diffusion and hydrodynamic dispersion. Reaction progress therefore was driven by the interplay of layer-parallel advection of chemically-reactive H 2 O-rich fluid at decameter and larger scales and cross-layer transport of CO 2 and H 2 O by diffusion/dispersion at scales of;as! m and less. Regardless of whether mineral reactants and products are solid solutions, the geochemical tracer considered, or the mechanism of fluid-rock reaction, the geometry of fluid flow can never be determined at a scale smaller than the one over which the concentration of the tracer is homogenized in the fluid within error of measurement by diffusion and dispersion.
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three dimensional geometry of metamorphic fluid flow during barrovian Regional Metamorphism from an inversion of combined petrologic and stable isotopic data
Geology, 2002Co-Authors: Boswell A Wing, John M FerryAbstract:Inverse calculations reveal the three-dimensional geometry of time-integrated fluid flux over a 120 km2 area during peak Barrovian Regional Metamorphism in southeastern Vermont. Prograde changes in whole-rock CO2, 18O, and 13C and calculated fluid compositions at the peak of Metamorphism were inverted assuming tracer mass balance to obtain the time-integrated fluid flux in three dimensions. Peak metamorphic fluid flow was spatially nonuniform with flux magnitudes ranging from ∼0 to 3·105 mol fluid/cm2 rock and flux directions ranging from vertical (upward and downward) to horizontal. Averaged over the entire study area, the magnitude of the time-integrated metamorphic fluid flux vector is ∼3.4·104 mol fluid/cm2 rock. The average flux vector trends 45° to the southwest and points upward at 36° from the present horizontal, parallel to formation boundaries on a Regional scale. Fluids in the terrain carried ∼3·103 mol CO2/cm2 rock toward Earth's surface during the peak of Metamorphism. Results suggest that local cross-layer transport processes are secondary to terrain-scale metamorphic fluid flow in driving prograde decarbonation reactions. Regional structure exerts a first-order control on the gross geometry of peak metamorphic fluid flow.
Tsuyoshi Komiya - One of the best experts on this subject based on the ideXlab platform.
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intermediate p t type Regional Metamorphism of the isua supracrustal belt southern west greenland the oldest pacific type orogenic belt
Tectonophysics, 2015Co-Authors: Tatsuyuki Arai, Soichi Omori, Tsuyoshi Komiya, Shigenori MaruyamaAbstract:Abstract The 3.7–3.8 Ga Isua Supracrustal Belt (ISB), southwest Greenland, might be the oldest accretionary complex on Earth. Regional Metamorphism of the ISB has a potential to constrain the tectonothermal history of the Earth during the Eoarchean. Chemical and modal analyses of metabasite in the study area (i.e., the northeast part of the ISB) show that the metamorphic grade increases from greenschist facies in the northern part of the study area to amphibolite facies in the southern part. To determine the precise metamorphic P–T ranges, isochemical phase diagrams of minerals of metabasite were made using Perple_X. A synthesis of the estimated metamorphic P–T ranges of the ISB indicates that both the metamorphic pressure and temperature increase systematically to the south in the study area from 3 kbar and 380 °C to 6 kbar and 560 °C. The monotonous metamorphic P–T change suggests that the northeast part of the ISB preserves Regional Metamorphism resulting from the subduction of an accretionary complex although the ISB experienced metamorphic overprints during the Neoarchean. Both the presence of the Regional Metamorphism and an accretionary complex having originating at subduction zone suggest that the ISB may be the oldest Pacific-type orogenic belt. The progressive Metamorphism can be considered as a record of intermediate-P/T type geothermal gradient at the subduction zone in the Eoarchean. Intermediate-P/T type geothermal gradient is typical at the current zones of subducting young oceanic crust, such as in the case of the Philippine Sea Plate in the southwest part of Japan. Considering the fact that almost all Metamorphisms in the Archean are greenschist–amphibolite facies, the intermediate-P/T type geothermal gradient at the ISB might have been worldwide in the Archean. This would indicate that the subduction of young micro-plates was common because of the vigorous convection of hot mantle in the Archean.
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archean Regional Metamorphism of the isua supracrustal belt southern west greenland implications for a driving force for archean plate tectonics
International Geology Review, 2000Co-Authors: Mamoru Hayashi, Tsuyoshi Komiya, Yasuo Nakamura, Shigenori MaruyamaAbstract:The Isua supracrustal belt (∼3.8 Ga) constitutes the oldest accretionary complex in the world. Petrochemical and geothermobarometric studies of more than 1500 rock samples of the Isua belt have enabled us to estimate the extent of Regional Metamorphism, the petrotectonic environment, and the subduction-zone geothermal gradient in the Archean. The following line of evidence indicates progressive, prograde Metamorphism from greenschist (Zone A) through albite-epidote-amphibolite (Zone B) to amphibolite facies (Zones C and D) in the northeastern part of the Isua supracrustal belt: (1) the systematic change of mineral paragenesis in metabasites and metapelites; (2) progressive change of the composition of major metamorphic minerals, including plagioclase, amphibole, chlorite, epidote, and garnet; (3) normal zoning of amphibole and garnet; and (4) the absence of relict minerals of high-grade amphibolitic Metamorphism even in the lowest metamorphic zone. Metabasites of the Isua belt vary extremely in Mg#, causi...
Johannes Hammerli - One of the best experts on this subject based on the ideXlab platform.
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Zn and Pb mobility during Metamorphism of sedimentary rocks and potential implications for some base metal deposits
Mineralium Deposita, 2015Co-Authors: Johannes Hammerli, Nicholas H.s. Oliver, Carl Spandler, Paolo Sossi, Gregory M DippleAbstract:Comprehension of the genesis of Pb-Zn ore systems is currently limited by a poor understanding of where these metals are sourced from. Our study of metal mobility during Regional Metamorphism in the Mt. Lofty Ranges, South Australia, demonstrates that in staurolite-absent siliciclastic metasedimentary rocks, biotite contains >80 % of the bulk rock Zn, as well as a considerable proportion of the total Pb. Fluid flow through these metasedimentary rocks led to a continuous depletion of Pb and Zn on a mineral and bulk rock scale during prograde Regional Metamorphism. We calculate that ∼80 % of the bulk rock Zn and ∼50 % of the bulk rock Pb were mobilised, mainly through reactions involving biotite. These reactions led to a calculated Pb and Zn “loss” of ∼2.7 and 27 Mt, respectively, in the high-grade metamorphic zone. Halogen contents of apatite and biotite and bulk rock Zn isotope data provide evidence that Cl-rich metamorphic fluids were important for metal transport. Hence, fluid flow accompanying prograde Metamorphism of typical sedimentary rocks can mobilise base metals to the degree required to potentially supply significant Pb-Zn ore systems.
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cl br of scapolite as a fluid tracer in the earth s crust insights into fluid sources in the mary kathleen fold belt mt isa inlier australia
Journal of Metamorphic Geology, 2014Co-Authors: Johannes Hammerli, Nicholas H.s. Oliver, Carl Spandler, Brian RuskAbstract:A combination of analytical methods, including trace element analysis of Br in scapolite by LA-ICP- MS, was employed to unravel the fluid–rock interaction history of the Mary Kathleen Fold Belt of northern Australia. Halogen ratios in the metamorphic and hydrothermally derived scapolite from a range of rock-types record interaction between the host rocks and magmatic-hydrothermal fluids derived from granite plutons and Regional Metamorphism. The results show that halite-dissolution supplied at best only minor chlorine to fluids in the Fold Belt. Chlorine/bromine ratios in metamorphic scapolite indicate that fluids were dominantly derived from basinal brines formed from sub-aerial evaporation of seawater beyond the point of halite saturation. This bittern fluid infiltrated the under- lying sedimentary sequences prior to Regional Metamorphism. Zoned scapolite in a major late meta- morphic mineralized shear-zone records three discrete pulses of magmatic and metamorphic fluid, and it is suggested that fluid mixing may have assisted mineralization along and around this shear zone. As a crucial prerequisite for halogen fluid tracer studies using scapolite, we find in our samples that Cl and Br do not fractionate when incorporated in scapolite. Furthermore, unaltered rims of heavily retrogressed scapolite show indistinguishable Cl/Br signatures compared with fresh grains from the same sample indicating retrograde Metamorphism did not significantly affect Cl and Br signatures in scapolite group minerals.
