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C Verdel - One of the best experts on this subject based on the ideXlab platform.
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variation of illite muscovite 40 ar 39 ar age spectra during progressive Low Grade Metamorphism an example from the us cordillera
Contributions to Mineralogy and Petrology, 2012Co-Authors: C Verdel, Ben A Van Der Pluijm, Nathan A NiemiAbstract: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.
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variations in the illite to muscovite transition related to metamorphic conditions and detrital muscovite content insight from the paleozoic passive margin of the southwestern united states
The Journal of Geology, 2011Co-Authors: C Verdel, Nathan A Niemi, Ben A Van Der PluijmAbstract:X-ray diffraction (XRD) analysis of the illite to muscovite transition in pelitic rocks has been widely used in regional studies of Low-Grade Metamorphism. Variations in detrital muscovite content of sediments are also measurable with XRD and may, in fact, be difficult to distinguish from metamorphic gradients. We utilize field transects to isolate detrital muscovite versus metamorphic reaction components of illite-muscovite variations in Paleozoic rocks of the western United States. One transect focuses on a single stratigraphic interval (Middle Cambrian pelites) and extends from Death Valley in the west to the Grand Canyon in the east. Detrital muscovite content is roughly constant along this 500-km-long transect, so illite-muscovite variations are ascribed to differences in Low-Grade Metamorphism. In terms of both illite crystallinity and polytype composition, there is an overall increase in metamorphic Grade to the west along the transect, from the diagenetic metapelitic zone at the Grand Canyon to epizone-Low anchizone conditions near Death Valley. Most of the regional variation in illite parameters occurs between Frenchman Mountain and the southern Nopah Range, corresponding to the transition from cratonic to miogeoclinal facies and also to the leading edge of the Sevier thrust belt. In contrast to this "horizontal" transect that highlights metamorphic reactions, a vertical transect through the Paleozoic stratigraphy of the Grand Canyon targets temporal variations in the flux of detrital muscovite. In the Grand Canyon, illite crystallinity reaches a maximum at the base of the Pennsylvanian Supai Group before decreasing upsection. Deposition of the Supai Group was coeval with formation of basementcored uplifts during the Ancestral Rocky Mountains orogeny, and we suggest that the upsection change in illite composition at the Grand Canyon reflects input of detrital muscovite eroded from these uplifts
Nathan A Niemi - One of the best experts on this subject based on the ideXlab platform.
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variation of illite muscovite 40 ar 39 ar age spectra during progressive Low Grade Metamorphism an example from the us cordillera
Contributions to Mineralogy and Petrology, 2012Co-Authors: C Verdel, Ben A Van Der Pluijm, Nathan A NiemiAbstract: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.
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variations in the illite to muscovite transition related to metamorphic conditions and detrital muscovite content insight from the paleozoic passive margin of the southwestern united states
The Journal of Geology, 2011Co-Authors: Charles Verdel, Nathan A Niemi, Ben A Van Der PluijmAbstract:AbstractX-ray diffraction (XRD) analysis of the illite to muscovite transition in pelitic rocks has been widely used in regional studies of Low-Grade Metamorphism. Variations in detrital muscovite content of sediments are also measurable with XRD and may, in fact, be difficult to distinguish from metamorphic gradients. We utilize field transects to isolate detrital muscovite versus metamorphic reaction components of illite-muscovite variations in Paleozoic rocks of the western United States. One transect focuses on a single stratigraphic interval (Middle Cambrian pelites) and extends from Death Valley in the west to the Grand Canyon in the east. Detrital muscovite content is roughly constant along this 500-km-long transect, so illite-muscovite variations are ascribed to differences in Low-Grade Metamorphism. In terms of both illite crystallinity and polytype composition, there is an overall increase in metamorphic Grade to the west along the transect, from the diagenetic metapelitic zone at the Grand Cany...
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variations in the illite to muscovite transition related to metamorphic conditions and detrital muscovite content insight from the paleozoic passive margin of the southwestern united states
The Journal of Geology, 2011Co-Authors: C Verdel, Nathan A Niemi, Ben A Van Der PluijmAbstract:X-ray diffraction (XRD) analysis of the illite to muscovite transition in pelitic rocks has been widely used in regional studies of Low-Grade Metamorphism. Variations in detrital muscovite content of sediments are also measurable with XRD and may, in fact, be difficult to distinguish from metamorphic gradients. We utilize field transects to isolate detrital muscovite versus metamorphic reaction components of illite-muscovite variations in Paleozoic rocks of the western United States. One transect focuses on a single stratigraphic interval (Middle Cambrian pelites) and extends from Death Valley in the west to the Grand Canyon in the east. Detrital muscovite content is roughly constant along this 500-km-long transect, so illite-muscovite variations are ascribed to differences in Low-Grade Metamorphism. In terms of both illite crystallinity and polytype composition, there is an overall increase in metamorphic Grade to the west along the transect, from the diagenetic metapelitic zone at the Grand Canyon to epizone-Low anchizone conditions near Death Valley. Most of the regional variation in illite parameters occurs between Frenchman Mountain and the southern Nopah Range, corresponding to the transition from cratonic to miogeoclinal facies and also to the leading edge of the Sevier thrust belt. In contrast to this "horizontal" transect that highlights metamorphic reactions, a vertical transect through the Paleozoic stratigraphy of the Grand Canyon targets temporal variations in the flux of detrital muscovite. In the Grand Canyon, illite crystallinity reaches a maximum at the base of the Pennsylvanian Supai Group before decreasing upsection. Deposition of the Supai Group was coeval with formation of basementcored uplifts during the Ancestral Rocky Mountains orogeny, and we suggest that the upsection change in illite composition at the Grand Canyon reflects input of detrital muscovite eroded from these uplifts
M. Frey - One of the best experts on this subject based on the ideXlab platform.
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clay diagenesis and Low Grade Metamorphism of tithonian and berriasian sediments in the cameros basin spain
Clay Minerals, 2001Co-Authors: J F Barrenechea, M. Frey, Jacinto Alonsoazcarate, M. Rodas, J. R. MasAbstract:The clay mineral assemblages of the Tithonian and Berriasian sediments (Tera and Oncala Groups) in the eastern part of the Cameros basin are investigated at seven localities. The Lowest-Grade assemblage, located on the southern border of the basin, contains calcite + quartz + hematite + kaolinite + mixed-layer illite-smectite (R = 1, 6585% illite layers) + discrete illite (IC = 0.50.65D82y). Systematic increases in the illite and chlorite crystallinities suggest increasing metamorphic Grade from the northwest part of the basin to the southeast. This trend does not folLow the pattern previously described for the overlying late Berriasian–early Aptian sediments (Urbio´n and Enciso Groups), which exhibit a higher metamorphic Grade. This may result from local variations in sedimentary facies, as well as the circulation of hot migratory fluids. Tertiary compression occurring long after the main metamorphic event is considered to be responsible for the enhanced illite and chlorite crystallinities measured in the SE extreme of the basin.
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very Low Grade Metamorphism of the taveyanne formation of western switzerland
Contributions to Mineralogy and Petrology, 1997Co-Authors: David Schmidt, J. Mullis, Susanne T Schmidt, Rafael Ferreiro Mahlmann, M. FreyAbstract:The andesitic early Oligocene Taveyanne metagreywacke of the Helvetic nappes of western Switzerland shows an increase of metamorphic Grade from zeolite facies through Lower greenschist facies. Electron microprobe analysis, fluid inclusion thermometry, stable isotope analysis, coal rank, illite and chlorite crystallinity and thermodynamic calculations were carried out to determine metamorphic conditions. Evaluation of all techniques used in this study suggest that only combinations of different parameters yield reliable information to constrain very Low-Grade metamorphic conditions. Electron microprobe analyses are presented for actinolite, chlorite, epidote, phengite, laumontite, prehnite, pumpellyite, and titanite. With increasing metamorphic Grade, chlorite is enriched in tetrahedral Al, pumpellyite becomes poorer in Fetot and more homogeneous in chemical composition, and titanite tends to incorporate Ti at the expense of Al and Fe3+. Metamorphic P-T conditions were determined by a combination of fluid inclusion microthermobarometry, stable isotope thermometry on quartz-calcite veins, chlorite “geothermometry” and thermodynamic calculations. Peak temperatures range from 210–250 °C for zeolite facies to 270–300 °C for prehnite-pumpellyite facies to 300–360 °C for pumpellyite-actinolite facies. An evaluation of 289 chlorite analyses indicates that the tetrahedral Al content is negatively correlated with the saponite component. Temperatures derived from chlorite “geothermometry” match maximum temperature conditions mentioned above. Illite crystallinity data for shales and slates intercalated with the Taveyanne metagreywacke indicate that the diagenetic zone correlates with the zeolite facies, the upper anchizone with the prehnite-pumpellyite facies, and the Lower epizone with the pumpellyite-actinolite facies. A comparison of coal rank and illite crystallinity data (n=12,r=0.91) yielded R max values of 2.9 and 5.5% for the Lower and upper boundary of the anchizone, respectively.
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Very Low-Grade Metamorphism of the Taveyanne greywacke, Glarus Alps, Switzerland
Journal of Metamorphic Geology, 1994Co-Authors: M. Rahn, J. Mullis, K. Erdelbrock, M. FreyAbstract:Detailed textural and chemical data for mineral assemblages on a regional scale are presented for the metaandesitic Eocene-Oligocene Taveyanne greywacke of the Glarus Alps, Eastern Switzerland. Presented data indicate an increase of metamorphic Grade from zeolite facies to prehnite-pumpellyite and pumpellyite-actinolite facies. Low-Grade outcrops contain laumontite, minor corrensite and pumpellyite (assemblage type 1), whereas outcrops of higher metamorphic Grade contain prehnite and two populations of pumpellyite (type 2), prehnite—pumpellyite-(Al)—white mica (type 3), a single outcrop shows pumpellyite-actinolite facies (type 4). From the zeolite to prehnite-pumpellyite/pumpellyite-actinolite facies there are indications for an increase of the chemical equilibrium domain size for the critical paragenesis from a single detrital grain ≤1 mm) in type 1, to a few millimetres in type 2, and to a whole thin section in type 3. Metamorphic P-T conditions were determined by a combination of chlorite thermometry, fluid inclusion and vitrinite reflectance data. Peak temperatures range from 170-190d C for zeolite facies to 270-310d C for prehnite-pumpellyite and pumpellyite-actinolite facies. For the higher temperature range, pressures of 2-3 kbar are derived indicating a geothermal gradient of 24-32d C km-1. The well-constrained temperature estimations derived for the assemblages provide a useful test of the different empirical calibrations of chlorite thermometers recently proposed. The best correspondence to the temperatures determined here is for the Cathelineau calibration. In addition, in the Lower Grade samples differences in textures and calculated temperatures provide a mean to distinguish between detrital and newly formed chlorites.
Ben A Van Der Pluijm - One of the best experts on this subject based on the ideXlab platform.
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variation of illite muscovite 40 ar 39 ar age spectra during progressive Low Grade Metamorphism an example from the us cordillera
Contributions to Mineralogy and Petrology, 2012Co-Authors: C Verdel, Ben A Van Der Pluijm, Nathan A NiemiAbstract: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.
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variations in the illite to muscovite transition related to metamorphic conditions and detrital muscovite content insight from the paleozoic passive margin of the southwestern united states
The Journal of Geology, 2011Co-Authors: C Verdel, Nathan A Niemi, Ben A Van Der PluijmAbstract:X-ray diffraction (XRD) analysis of the illite to muscovite transition in pelitic rocks has been widely used in regional studies of Low-Grade Metamorphism. Variations in detrital muscovite content of sediments are also measurable with XRD and may, in fact, be difficult to distinguish from metamorphic gradients. We utilize field transects to isolate detrital muscovite versus metamorphic reaction components of illite-muscovite variations in Paleozoic rocks of the western United States. One transect focuses on a single stratigraphic interval (Middle Cambrian pelites) and extends from Death Valley in the west to the Grand Canyon in the east. Detrital muscovite content is roughly constant along this 500-km-long transect, so illite-muscovite variations are ascribed to differences in Low-Grade Metamorphism. In terms of both illite crystallinity and polytype composition, there is an overall increase in metamorphic Grade to the west along the transect, from the diagenetic metapelitic zone at the Grand Canyon to epizone-Low anchizone conditions near Death Valley. Most of the regional variation in illite parameters occurs between Frenchman Mountain and the southern Nopah Range, corresponding to the transition from cratonic to miogeoclinal facies and also to the leading edge of the Sevier thrust belt. In contrast to this "horizontal" transect that highlights metamorphic reactions, a vertical transect through the Paleozoic stratigraphy of the Grand Canyon targets temporal variations in the flux of detrital muscovite. In the Grand Canyon, illite crystallinity reaches a maximum at the base of the Pennsylvanian Supai Group before decreasing upsection. Deposition of the Supai Group was coeval with formation of basementcored uplifts during the Ancestral Rocky Mountains orogeny, and we suggest that the upsection change in illite composition at the Grand Canyon reflects input of detrital muscovite eroded from these uplifts
Diego Morata - One of the best experts on this subject based on the ideXlab platform.
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nature and p t t constraints of very Low Grade Metamorphism in the triassic jurassic basins coastal range central chile
Andean Geology, 2005Co-Authors: Mauricio Belmar, Diego MorataAbstract:RESUMEN:Las secuencias volcano sedimentarias del Triasico-Jurasico en la Cordillera de la Costa de Chile central (34°45'/ 35° 20'S) desarrolladas en las cuencas de Vichuquen-Tilicura y Hualane-Gualleco evidencian asociaciones metamorficas indicativas de condiciones de muy bajo grado. Las metapelitas del Triasico y Jurasico Inferior alcanzaron condiciones de anquizona (datos de IC, bo, reflectividad de vitrinita e inclusiones fluidas: P »1.3, T»190) a los 181-184 Ma, durante un contexto geodinamico extensional caracterizado por un gradiente geotermico de »35°Ckm-1. Sin embargo, en secuencias de rocas volcanicas maficas, estratigraficamente mas altas, de edad del Jurasico Medio a Superior, la presencia de clorita (Xc =0.98-0.99), tres tipos de epidota (XFe3+ = 41.14±0.90, 30.08±1.53 y 18.05±1.75), titanita, pumpellyita rica en hierro y pumpellyita pobre en hierro e hidrogranate (+ albita y cuarzo) es indicativa de un ajuste en el equilibrio metamorfico en las condiciones de la facies prehnita pumpellyita (P »3 kbar, T »300°C) y bajo variaciones importantes en la fugacidad de oxigeno. Se propone un cambio en el contexto geodinamico desde un ambiente extensional (en el que se alcanzaria la maxima subsidencia y se desarrollaria el metamorfismo de tipo 'burial' en las secuencias metapeliticas del Triasico- Jurasico Inferior) a un ambiente contraccional, en el que la superposicion local de las secuencias del Triasico y Jurasico Inferior sobre las del Jurasico Medio-Superior permitiria alcanzar las condiciones P-T calculadas y, de este modo, justificar las diferencias encontradas entre las condiciones metamorficas de las metapelitas del Triasico-Jurasico inferior y las de las metabasitas del Jurasico Medio-Superior.
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40ar 39ar dating of volcanism and subsequent very Low Grade Metamorphism in a subsiding basin example of the cretaceous lava series from central chile
Chemical Geology, 2005Co-Authors: Francisco Fuentes, Gilbert Feraud, Luis Aguirre, Diego MorataAbstract:Abstract 40Ar/39Ar geochronological method is applied to date both emplacement and subsequent very Low-Grade Metamorphism of a thick lava series, the Veta Negra Formation from the Cordon de Chacana area, in the Coastal Range of central Chile. On the same lava series, and even on the same rock sample, it was possible to measure apparently valid ages of both the emplacement of lava fLows (by dating transparent plagioclase) and the very Low-Grade metamorphic event (by dating sericite and/or adularia). Sericite was dated on step heated single grains of strongly altered plagioclases, the radiogenic argon from remaining fresh plagioclase appearing as negligible on most part of the age spectrum. The concordance between one plateau age of 118.7±0.6 Ma displayed by fresh plagioclase and a previously measured plateau age on plagioclase from one lava fLow from a region due west from Santiago [Bustamante Hill, Aguirre, L., Feraud, G., Morata, D., Vergara, M., Robinson, D., 1999. Time interval between volcanism and burial Metamorphism and rate of basin subsidence in a Cretaceous Andean extensional setting. Tectonophysics 313, 433–447] may indicate that the volcanic activity of the Veta Negra Formation was synchronous along the 80-km long latitudinal segment studied. Ages of 96.8±0.2 and 97.0±1.6 Ma displayed by adularia and sericite (respectively) are slightly older than ages of 93–94 Ma given by adularia from the Bustamante Hill. These data alLow to define a period of about 22 Ma between volcanism and Metamorphism. Because of the contemporaneity of some metamorphic minerals and Cretaceous plutons of the region, the very Low-Grade metamorphic event may be the result of the additional effect of burial and a regional thermal event. Nevertheless, a significant effect of burial (relatively to the contribution of high thermal gradient event(s)) seems revealed by an apparent age gradient on very Low-Grade minerals with the stratigraphic depth.
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geodynamic implications of the regional very Low Grade Metamorphism in the Lower cretaceous of the coastal range in central chile
2005Co-Authors: Diego Morata, Mauricio Belmar, Luis Aguirre, Gilbert Feraud, Ciencias FisicasAbstract:Introduction Magmatic and metamorphic features of the Lower Cretaceous volcanic formations of central Chile strongly contrast with those found in younger units in the same segment. Sorne km thick of bi-modal K-rich calcalkaline volcanism with widespread basalts and basal tic andesites morphologically emplaced as extensive flood basalts (trapps) under an extensional regime, absence of volcanic centres, mantle-rich plutonism, and regional nondeformational very Low-Grade Metamorphism of the burial type under moderate vertical thermal gradients, characterize most of the evolution of this segment of the Pacifie margin. ln sharp contrast with this, starting from the 'rnid-Cretaceous,' a dramatic change in scenary is recorded. Magmatic foci moved toward the east along several basins where central volcanism predominated with products close to a tholeiitic composition giving rise to large scale hydrothermalism. ln this paper we will focuse on the nature of the Metamorphism which affects the Early Cretaceous formations in the Coastal Range of central Chile (=32° 35' 33° 10'S, fig. 1). These formations are known as the La Prado, Veta Negra and Las Chi/cas (fig. 1). Our purpose is to put in evidence the genesis of this Metamorphism, its relation with the rather coeval plutonic magmatism in the region and with the geodynamic 'rnid-Cretaceous" event that closed the Early Cretaceous evolutionary process.
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significance of k ar dating of very Low Grade Metamorphism in triassic jurassic pelites from the coastal range of central chile
Clay Minerals, 2004Co-Authors: Mauricio Belmar, Diego Morata, Francisco Munizaga, Perez C De Arce, Salvador Morales, F J CarrilloAbstract:K-Ar isotopic dating of very Low-Grade Metamorphism affecting Triassic-Jurassic rocks in the Coastal Range of central Chile was carried out on whole rocks and their <2 μm size fractions. In the study area, a regional-burial Low-Grade Metamorphism at anchizone conditions (T ≤190°C) and Low-pressure conditions (P ≥1.3 kbar) has been described. The highest observed temperatures are related to a contact Metamorphism produced by nearby Jurassic intrusions, with a P-T estimate at the immediate contact zone of ∼650-690°C and 4 kbar. The whole-rock K-Ar age of 174±5 Ma is interpreted as belonging to the contact Metamorphism due to the intrusion of Jurassic plutons (165±5 Ma to 175±5 Ma). A time-interval of ∼20 Ma between the diagenesis (206 Ma) and the anchizonal very Low-Grade Metamorphism (181-184 Ma) is obtained, and a rate of subsidence of ∼120 m/Ma is proposed for these Triassic-Jurassic basins. A thermal influence on the burial, very Low-Grade, regional Metamorphism is invoked.
