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Alan Cooper - One of the best experts on this subject based on the ideXlab platform.
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Structural evolution of the semischist–schist transition in an accretionary complex: the Otago Schist section at Lake Hāwea, New Zealand
New Zealand Journal of Geology and Geophysics, 2017Co-Authors: Alan Cooper, Richard J. NorrisAbstract:ABSTRACTPermo-Triassic, Rakaia Terrane, pelitic and psammitic schists at Lake Hāwea, Otago, show progressive metamorphism from TZII, pumpellyite–actinolite facies to TZIV, greenschist facies. The sequence shows evidence of five phases of deformation. D1 and D2 are progressive synmetamorphic phases, forming macroscopic nappe-like folds, interpreted to have formed by internal deformation within the Rakaia–Caples accretionary wedge during terrane collision. Thick S1 quartz veins are the form surface for definition of D2 folding, intensification of which progressively destroys evidence of D1 at higher textural and metamorphic grades. D3–5 are post metamorphic, less pervasive, kink folds (commonly conjugate) and brittle fault zones that predate the intrusion of lamprophyre magma and carbonatitic fluids associated with the Alpine Dike Swarm (c. 25 Ma). They relate to tectonic relaxation and exhumation of the schist in Cretaceous–Paleogene times. Fluid flow has been ubiquitous throughout the deformation, resulti...
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k ar age of a lamprophyre Dike Swarm near lake wanaka west otago south island new zealand
New Zealand Journal of Geology and Geophysics, 1996Co-Authors: C J Adams, Alan CooperAbstract:Abstract Lamprophyres, occurring as Dikes and high‐level diatremes, intrude greenschist facies Haast Schist in the southern part of the Alpine Dike Swarm, near Wanaka, western Otago. Of 12 lamprophyres dated by conventional K‐Ar techniques, 10 give whole‐rock ages ranging between 25.2 and 31.9 Ma, with kaersutites separated from four samples forming a tighter group at 22.9–27.8 Ma. A late Oligocene ‐ early Miocene age of intrusion is compatible with field evidence of Dike emplacement postdating a major postmetamorphic fold phase that, in the Lake Wakatipu area, deforms Oligocene Bobs Cove Beds along the trace of the Moonlight Fault. Two samples from a Dike in the Matukituki River area yield discordant kaersutite, K‐feldspar, ocellus, and whole‐rock ages that are older (43–126 Ma) than those from compositionally similar Dikes elsewhere in the Swarm. Similar anomalous ages have been determined previously from lamprophyres and associated intrusives of the northern part of the Swarm, at Haast River, and are a...
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K‐Ar age of a lamprophyre Dike Swarm near Lake Wanaka, west Otago, South Island, New Zealand
New Zealand Journal of Geology and Geophysics, 1996Co-Authors: C J Adams, Alan CooperAbstract:Abstract Lamprophyres, occurring as Dikes and high‐level diatremes, intrude greenschist facies Haast Schist in the southern part of the Alpine Dike Swarm, near Wanaka, western Otago. Of 12 lamprophyres dated by conventional K‐Ar techniques, 10 give whole‐rock ages ranging between 25.2 and 31.9 Ma, with kaersutites separated from four samples forming a tighter group at 22.9–27.8 Ma. A late Oligocene ‐ early Miocene age of intrusion is compatible with field evidence of Dike emplacement postdating a major postmetamorphic fold phase that, in the Lake Wakatipu area, deforms Oligocene Bobs Cove Beds along the trace of the Moonlight Fault. Two samples from a Dike in the Matukituki River area yield discordant kaersutite, K‐feldspar, ocellus, and whole‐rock ages that are older (43–126 Ma) than those from compositionally similar Dikes elsewhere in the Swarm. Similar anomalous ages have been determined previously from lamprophyres and associated intrusives of the northern part of the Swarm, at Haast River, and are a...
C J Adams - One of the best experts on this subject based on the ideXlab platform.
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k ar age of a lamprophyre Dike Swarm near lake wanaka west otago south island new zealand
New Zealand Journal of Geology and Geophysics, 1996Co-Authors: C J Adams, Alan CooperAbstract:Abstract Lamprophyres, occurring as Dikes and high‐level diatremes, intrude greenschist facies Haast Schist in the southern part of the Alpine Dike Swarm, near Wanaka, western Otago. Of 12 lamprophyres dated by conventional K‐Ar techniques, 10 give whole‐rock ages ranging between 25.2 and 31.9 Ma, with kaersutites separated from four samples forming a tighter group at 22.9–27.8 Ma. A late Oligocene ‐ early Miocene age of intrusion is compatible with field evidence of Dike emplacement postdating a major postmetamorphic fold phase that, in the Lake Wakatipu area, deforms Oligocene Bobs Cove Beds along the trace of the Moonlight Fault. Two samples from a Dike in the Matukituki River area yield discordant kaersutite, K‐feldspar, ocellus, and whole‐rock ages that are older (43–126 Ma) than those from compositionally similar Dikes elsewhere in the Swarm. Similar anomalous ages have been determined previously from lamprophyres and associated intrusives of the northern part of the Swarm, at Haast River, and are a...
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K‐Ar age of a lamprophyre Dike Swarm near Lake Wanaka, west Otago, South Island, New Zealand
New Zealand Journal of Geology and Geophysics, 1996Co-Authors: C J Adams, Alan CooperAbstract:Abstract Lamprophyres, occurring as Dikes and high‐level diatremes, intrude greenschist facies Haast Schist in the southern part of the Alpine Dike Swarm, near Wanaka, western Otago. Of 12 lamprophyres dated by conventional K‐Ar techniques, 10 give whole‐rock ages ranging between 25.2 and 31.9 Ma, with kaersutites separated from four samples forming a tighter group at 22.9–27.8 Ma. A late Oligocene ‐ early Miocene age of intrusion is compatible with field evidence of Dike emplacement postdating a major postmetamorphic fold phase that, in the Lake Wakatipu area, deforms Oligocene Bobs Cove Beds along the trace of the Moonlight Fault. Two samples from a Dike in the Matukituki River area yield discordant kaersutite, K‐feldspar, ocellus, and whole‐rock ages that are older (43–126 Ma) than those from compositionally similar Dikes elsewhere in the Swarm. Similar anomalous ages have been determined previously from lamprophyres and associated intrusives of the northern part of the Swarm, at Haast River, and are a...
Wilson Teixeira - One of the best experts on this subject based on the ideXlab platform.
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petrology and sr nd characteristics of the nova lacerda Dike Swarm sw amazonian craton new insights regarding its subcontinental mantle source and mesoproterozoic geodynamics
International Geology Review, 2012Co-Authors: Vicente Antonio Vitorio Girardi, P Correa C Da Costa, Wilson TeixeiraAbstract:The NNW-trending Nova Lacerda tholeiitic Dike Swarm in Mato Grosso State, Central Brazil, intrudes the Nova Lacerda granite (1.46 Ga) and the Jauru granite-greenstone terrain (ca. 1.79–1.77 Ga). The Swarm comprises diabases I and II and amphibolites emplaced at ca. 1.38 Ga. Geochemical data indicate that these are evolved tholeiites characterized by high LILE/HSFE and LREE/HSFE ratios. Isotopic modelling yields positive ϵNd(T) values (+0.86 to +2.65), whereas values for ϵSr(T) range from positive to negative (+1.96 to -5.56). Crustal contamination did not play a significant petrogenetic role, as indicated by a comparison of isotopic data (Sr–Nd) from both Dikes and country rocks, and by the relationship between isotopic and geochemical parameters (SiO2, K2O, Rb/Sr, and La/Yb) of the Dikes. We attribute the origin of these tholeiites to fractional crystallization of evolved melts derived from a heterogeneous mantle source. Comparison of the geochemical and isotopic data of the studied Swarm and other thole...
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magnetic fabrics and rock magnetism of proterozoic Dike Swarm from the southern sao francisco craton minas gerais state brazil
Tectonophysics, 2004Co-Authors: Maria Irene Bartolomeu Raposo, Alexandre O Chaves, Paulo Lojkaseklima, Manoel S Dagrellafilho, Wilson TeixeiraAbstract:Magnetic fabric and rock magnetism studies were performed on 32 mafic Dikes of a Proterozoic Dike Swarm from the southern Sao Francisco Craton (SFC; Minas Gerais State, SE Brazil). Magnetic anisotropies were determined by applying anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of remanent magnetization (ARM). The latter was performed imposing both anhysteretic (total (AAR) and partial (pAAR)) and isothermal remanence magnetizations (AIRM). Partial anhysteretic remanence anisotropy was performed based on remanent coercivity spectra from a pilot specimen of each site. In most sites, AMS is dominantly carried by ferromagnetic minerals, however, in some sites, the paramagnetic contribution exceeds 70% of bulk susceptibility. Rock magnetism and thin section analysis allow classifying the Dikes as non-hydrothermalized and hydrothermalized. Magnetic measurement shows that the mean magnetic susceptibility is usually lower than 5×10−3 (SI). Ti-poor titanomagnetites up to pure magnetite pseudo-single-domain (PSD) grain sizes carry the majority of magnetic fabrics for non-hydrothermalized Dikes whereas coarse to fine grained Ti-poor titanomagnetites carry the majority of magnetic fabrics for hydrothermalized Dikes. Three primary AMS fabrics are recognized which are coaxial with ARM fabric, except for two Dikes, from both non-hydrothermalized and hydrothermalized Dikes. Normal AMS fabric surprisingly is not dominant (31%). The parallelism between AMS, pAAR0–30, pAAR30–60 and pAAR60–90 fabrics in the hydrothermalized Dikes indicates that magnetic grains formed due to late-stage crystallization or to remobilization of iron oxides due to hydrothermal alteration after Dike emplacement have acquired a mimetic fabric coaxial with the primary fabric given by coarse-grained early crystallized Ti-poor titanomagnetites. This fabric is interpreted as magma flow in which the analysis of Kmax inclination permitted the inference that the Dikes were fed by horizontal or subhorizontal fluxes (Kmax<30°). Intermediate AMS fabric is the most important (41%) in the investigated Swarm. It is interpreted as due to vertical compaction of a static magma column with the minimum stress along the Dike strike. ARM determinations for these sites also remained intermediate except for two Dikes. In one of them, AIRM fabric resulted in normal AMS fabric while for the other AAR fabric resulted in inverse AMS fabric. A combination of AMS and ARM fabrics suggest that magmatic fabric for both Dikes were overprinted by some late local event, probably related to Brasiliano orogenic processes after Dike emplacement. InverseInverse AMS fabric is a minority (four Dikes). ARM determinations also remained inverse suggesting a primary origin for inverse AMS fabric.
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40ar 39ar and rb sr geochronology of the uruguayan Dike Swarm rio de la plata craton and implications for proterozoic intraplate activity in western gondwana
Precambrian Research, 1999Co-Authors: Wilson Teixeira, Paul R Renne, Jorge Bossi, Nestor Campal, Manoel Sousa Dagrella FilhoAbstract:Abstract The Uruguayan Dike Swarm intrudes country rocks of the Rio de la Plata Craton that were deformed and metamorphosed to high grade during the 2200–1900 Ma Transamazonian orogeny. Slow cooling of the continental crust following this event is suggested by the 40 Ar – 39 Ar plateau ages of hornblende (2016±11 Ma) and biotite (1817±10 Ma) from the Pintos post-tectonic granodiorite. There are seven 40 Ar – 39 Ar age determinations on mafic Dikes and felsic veins from the Swarm between 1727 and 1700 Ma. One Dike yielded concordant plateau ages for igneous hornblende (1727±10 Ma) and biotite (1725±10 Ma). These ages are taken as the best estimate for the emplacement of the Uruguayan Swarm. A Rb–Sr whole rock best fit line calculated for 15 Dikes yielded 1766±124 Ma, in agreement (within error) with the 40 Ar – 39 Ar plateau ages. The origin of the Uruguayan Swarm is tectonically related to the emplacement of the Illescas and Minas de Corrales rapakivi granites that crop out in the Nico Perez domain. The tectonic setting of these units is also compatible with bimodal magmatism and basin formation associated with extensional events in the Uruguayan and Brazilian shields. On the whole, the geologic framework suggests that the Uruguayan Dike Swarm marks a transcontinental intraplate episode within the South America continent which occurred shortly after the Transamazonian orogeny. The results of initial release portions of 40 Ar – 39 Ar spectra (Dikes and felsic veins) coupled with a Rb–Sr mineral isochron from one felsic vein are between 1370 and 1170 Ma. Eight K–Ar ages range from 1600 to 1170 Ma, suggesting variable argon loss in the systems. These isotopic disturbances were probably favored by low-grade hydrothermal overprints, exemplified by differences in the 40 Ar – 39 Ar spectra from one felsic vein (muscovite and biotite), and suggested also by the secondary mineralogy that may be present in the Dikes. These post-emplacement ages could result from Mesoproterozoic crustal shortening, recognized in the Nico Perez domain, interpreted to be tectonically associated with the Namaqua orogeny of Southern Africa. This inference is important in understanding the Late Mesoproterozoic evolution of Western Gondwana and brings potential implications for a paleogeographic reconstruction between the paleocontinent Plata and Amazonia at the time of the Grenville orogeny.
Paul R Renne - One of the best experts on this subject based on the ideXlab platform.
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40ar 39ar and rb sr geochronology of the uruguayan Dike Swarm rio de la plata craton and implications for proterozoic intraplate activity in western gondwana
Precambrian Research, 1999Co-Authors: Wilson Teixeira, Paul R Renne, Jorge Bossi, Nestor Campal, Manoel Sousa Dagrella FilhoAbstract:Abstract The Uruguayan Dike Swarm intrudes country rocks of the Rio de la Plata Craton that were deformed and metamorphosed to high grade during the 2200–1900 Ma Transamazonian orogeny. Slow cooling of the continental crust following this event is suggested by the 40 Ar – 39 Ar plateau ages of hornblende (2016±11 Ma) and biotite (1817±10 Ma) from the Pintos post-tectonic granodiorite. There are seven 40 Ar – 39 Ar age determinations on mafic Dikes and felsic veins from the Swarm between 1727 and 1700 Ma. One Dike yielded concordant plateau ages for igneous hornblende (1727±10 Ma) and biotite (1725±10 Ma). These ages are taken as the best estimate for the emplacement of the Uruguayan Swarm. A Rb–Sr whole rock best fit line calculated for 15 Dikes yielded 1766±124 Ma, in agreement (within error) with the 40 Ar – 39 Ar plateau ages. The origin of the Uruguayan Swarm is tectonically related to the emplacement of the Illescas and Minas de Corrales rapakivi granites that crop out in the Nico Perez domain. The tectonic setting of these units is also compatible with bimodal magmatism and basin formation associated with extensional events in the Uruguayan and Brazilian shields. On the whole, the geologic framework suggests that the Uruguayan Dike Swarm marks a transcontinental intraplate episode within the South America continent which occurred shortly after the Transamazonian orogeny. The results of initial release portions of 40 Ar – 39 Ar spectra (Dikes and felsic veins) coupled with a Rb–Sr mineral isochron from one felsic vein are between 1370 and 1170 Ma. Eight K–Ar ages range from 1600 to 1170 Ma, suggesting variable argon loss in the systems. These isotopic disturbances were probably favored by low-grade hydrothermal overprints, exemplified by differences in the 40 Ar – 39 Ar spectra from one felsic vein (muscovite and biotite), and suggested also by the secondary mineralogy that may be present in the Dikes. These post-emplacement ages could result from Mesoproterozoic crustal shortening, recognized in the Nico Perez domain, interpreted to be tectonically associated with the Namaqua orogeny of Southern Africa. This inference is important in understanding the Late Mesoproterozoic evolution of Western Gondwana and brings potential implications for a paleogeographic reconstruction between the paleocontinent Plata and Amazonia at the time of the Grenville orogeny.
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paleomagnetism and dating of the early cretaceous florianopolis Dike Swarm santa catarina island southern brazil
Physics of the Earth and Planetary Interiors, 1998Co-Authors: Maria Irene Bartolomeu Raposo, Marcia Ernesto, Paul R RenneAbstract:Abstract The Florianopolis (FL) basaltic Dikes are well exposed on Santa Catarina Island, close to the present southeastern border of the Parana Magmatic Province in Southern Brazil, and their intrusion was related to the tectonic and magmatic evolution of rifting in the southern Atlantic. The Dikes are vertical or subvertical, and cut the crystalline basement rocks, mainly Late Proterozoic granites. Most of the Dikes exhibit NE trends, corresponding to structural trends of the crystalline basement. A few NW-trending Dikes can also be observed, occasionally crosscutting the NE Dikes. Samples from 73 Dikes were collected for paleomagnetic work, and subjected to both alternating field and thermal demagnetization. Characteristic magnetizations of normal and reversed polarities interpreted as original thermoremanences are carried by Ti-poor magnetites showing remanent coercive forces in the range 18–45 mT. The corresponding paleomagnetic pole (FL pole) is located at 3.3°E, 89.1°S (N=65; α95=2.6°; k=47). Samples for 40 Ar/ 39 Ar dating were collected at the same paleomagnetic sites. Plagioclase samples from nine Dikes were analyzed by stepwise heating; the apparent age spectra are strongly discordant due to excess 40 Ar contamination, but five samples yielded plateau ages ranging from 119.0±0.9 Ma to 128.3±0.5 Ma. One additional sample yielded an isochron age of 121.5±0.5 Ma. The inferred ages apparently define two modes at ∼119–122 Ma and ∼126–128 Ma, although the data set is probably insufficient to determine whether Dike intrusion was continuous or episodic. The ages of 128–119 Ma are coeval with the final stages of the rifting at this latitude, suggesting that the emplacement of the FL Dikes is related to extension of the continental crust just prior to the formation of oceanic crust. By comparing the FL pole with the existing paleomagnetic poles for the NW Ponta Grossa Dike Swarm (mainly 130.5 Ma) to the north, the Parana volcanics (mainly 133–132 Ma), and the Central Alkaline Province (127–130 Ma) on the western side of the Parana Province it is concluded that the differences in pole positions reflect plate displacements, and therefore the FL pole is probably dominated by the Dikes with ages younger than 127 Ma.
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age of the ponta grossa Dike Swarm brazil and implications to parana flood volcanism
Earth and Planetary Science Letters, 1996Co-Authors: Paul R Renne, Katja Deckart, Marcia Ernesto, Gilbert Fe Raud, E. M. PiccirilloAbstract:The Ponta Grossa Dike Swarm (PGDS) occurs in a NW-trending, 200 km wide zone exposed just east of the Parana´basin in southeastern Brazil. The predominantly basaltic Dikes intrude crystalline basement, Paleozoic-Mesozoic sediments, and (rarely) flows of the Parana´-Etendeka flood volcanic province (PEP). The PGDS resembles the failed arm of a rift-rift-rift triple junction, related to the separation of South America and Africa. Detailed geochemical studies of the Dikes (including major/minor/trace element and Sr-Nd isotopic analyses) indicate that they probably represent feeders for the voluminous phase of flood volcanism, represented by relatively uncontaminated, predominantly high-TiO2 lavas of the northern PEP, where lava accumulations reach 1700 m thick.40Ar/39Ar stepwise degassing data, using both laser and radiofrequency induction furnace, on plagioclase separates from eighteen Dikes and one sill yield seventeen plateau ages: three are between 120.7 ± 1.3 Ma and 125.8 ± 0.6 Ma, and fourteen are clustered between 129.2 ± 0.4 Ma and 131.4 ± 0.5 Ma. Isochron ages are not significantly different from the plateau ages, and plateau ages are adopted in all but two cases as being the most accurate age estimates. The age-probability distribution for the dominant pulse (131.4 ± 0.4 to 129.2 ± 0.4 Ma) shows a pronounced peak at 130.5 Ma; this distribution probably reflects the magma production history in the region. The new geochronologic data are consistent with conclusions based on paleomagnetic and chemical-stratigraphic data that the PGDS is younger than the volumetrically dominant majority of volcanism in the southern PEP, which occurred at 133-132 Ma. The younger (commonly NE-trending) Dikes may reflect the initiation of full drift, which was coincident with major basin development in the adjacent continental borderland at 125-120 Ma. The PGDS may indeed represent the failed arm of a rift-rift-rift triple junction, but the triple junction did not coincide exactly in time or space with the site that would be inferred for plume impact.
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Age of the Ponta Grossa Dike Swarm (Brazil), and implications to Parana´flood volcanism
Earth and Planetary Science Letters, 1996Co-Authors: Paul R Renne, Katja Deckart, Marcia Ernesto, Gilbert Fe´raud, E. M. PiccirilloAbstract:The Ponta Grossa Dike Swarm (PGDS) occurs in a NW-trending, 200 km wide zone exposed just east of the Parana´basin in southeastern Brazil. The predominantly basaltic Dikes intrude crystalline basement, Paleozoic-Mesozoic sediments, and (rarely) flows of the Parana´-Etendeka flood volcanic province (PEP). The PGDS resembles the failed arm of a rift-rift-rift triple junction, related to the separation of South America and Africa. Detailed geochemical studies of the Dikes (including major/minor/trace element and Sr-Nd isotopic analyses) indicate that they probably represent feeders for the voluminous phase of flood volcanism, represented by relatively uncontaminated, predominantly high-TiO2 lavas of the northern PEP, where lava accumulations reach 1700 m thick.40Ar/39Ar stepwise degassing data, using both laser and radiofrequency induction furnace, on plagioclase separates from eighteen Dikes and one sill yield seventeen plateau ages: three are between 120.7 ± 1.3 Ma and 125.8 ± 0.6 Ma, and fourteen are clustered between 129.2 ± 0.4 Ma and 131.4 ± 0.5 Ma. Isochron ages are not significantly different from the plateau ages, and plateau ages are adopted in all but two cases as being the most accurate age estimates. The age-probability distribution for the dominant pulse (131.4 ± 0.4 to 129.2 ± 0.4 Ma) shows a pronounced peak at 130.5 Ma; this distribution probably reflects the magma production history in the region. The new geochronologic data are consistent with conclusions based on paleomagnetic and chemical-stratigraphic data that the PGDS is younger than the volumetrically dominant majority of volcanism in the southern PEP, which occurred at 133-132 Ma. The younger (commonly NE-trending) Dikes may reflect the initiation of full drift, which was coincident with major basin development in the adjacent continental borderland at 125-120 Ma. The PGDS may indeed represent the failed arm of a rift-rift-rift triple junction, but the triple junction did not coincide exactly in time or space with the site that would be inferred for plume impact.
Nobuo Geshi - One of the best experts on this subject based on the ideXlab platform.
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Vertical and lateral propagation of radial Dikes inferred from the flow-direction analysis of the radial Dike Swarm in Komochi Volcano, Central Japan
Journal of Volcanology and Geothermal Research, 2008Co-Authors: Nobuo GeshiAbstract:Dikes with lateral and outward intrusion directions (named L-type Dike) and Dikes with vertical and upward intrusion directions (V-type Dike) are recognized in a radial Dike Swarm of Komochi Volcano, Japan, by the combined analysis of the preferred orientations of deformed vesicles and anisotropy of magnetic susceptibility (AMS). Intrusion directions of magmas were examined for 41 Dike outcrops. Among them, 19 Dikes were classified as L-type Dike and 15 Dikes were as V-type. The L-type Dikes distribute over the area of the Dike Swarm radiating from the central conduit, which is named Daikokuiwa stock, and the V-type Dikes occur mainly in the peripheral portion of the volcano. The L-type Dikes are characterized with evolved whole-rock compositions similar to those of rocks consisting of the central conduit, whereas the V-type Dikes have less-fractionated composition and are enrich in mafic phenocryst as compared to the L-type Dikes. The outward intrusion directions of the L-type Dikes and their petrological similarity to the rock of the central conduit indicate that these Dikes intruded from the shallow part of the central conduit where the magmas underwent fractional crystallization and degassing. The petrological characteristics of the V-type Dikes suggest that the less-fractionated magma intruded directly from a magma chamber, in which mafic phenocryst crystals accumulated. The larger Dike thickness and higher magnetic foliation of the V-type Dike as compared to the L-type Dike indicate higher magmatic overpressure, which was possibly result of the direct connection to the pressurized magma chamber and vertical-growth of the Dikes with buoyant magma. Solidification of the central conduit is favorable for the accumulation of internal excess pressure in the magma chamber to break the wall rock. Fissure eruptions independent of the central conduit have occurred in many volcanoes and these fissure eruptions might be fed by the Dikes directly propagated from the deeper magma chamber.
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Structural development of Dike Swarms controlled by the change of magma supply rate: the cone sheets and parallel Dike Swarms of the Miocene Otoge igneous complex, Central Japan
Journal of Volcanology and Geothermal Research, 2004Co-Authors: Nobuo GeshiAbstract:Abstract Dike and sheet Swarms of the post-cauldron activity of the Miocene Otoge igneous complex, central Japan, show systematic structural and petrological evolution. A concentric cone sheet Swarm (Otoge cone sheets) measuring 10–15 km3 was formed at the beginning of the post-cauldron stage and was later succeeded by the formation of a parallel Dike Swarm (Shitara central Dike Swarm) measuring 3–5 km3. The Otoge cone sheets have a less fractionated character, with little compositional variations, whereas the Shitara central Dike Swarm has a fractionated character with wide compositional variations, this despite that both Swarms reflect a single fractionation trend. The compositional change from the Otoge cone sheets to the Shitara central Dike Swarm can be explained by fractional crystallization due to the decline in the supply of less fractionated hot magma into the reservoir. The structural change from the Otoge cone sheets to the Shitara central Dike Swarm reflects the change in the stress field from a local compressive field to a regional extensive one, with the decline of magma replenishment into the reservoir during the final stage of volcanic activity. The concentrated distribution of the cone sheets suggests the sill-like or flat-top shaped magma reservoir beneath the center of the Otoge cone sheets.
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Fractionation and magma mixing within intruding Dike Swarm: evidence from the Miocene Shitara-Otoge igneous complex, central Japan
Journal of Volcanology and Geothermal Research, 2000Co-Authors: Nobuo GeshiAbstract:Abstract The analysis of intrusion pattern and petrological character of the central Dike Swarm in the Miocene igneous complex of the Shitara district, central Japan clarified that magma mixing between a strongly differentiated magma and a less-differentiated magma occurred within a Dike Swarm. The Dike rocks have a wide compositional variation ranging from 5.5 to 0.7 wt.% MgO. They are divided into P1- and P2-types. The P2-type rocks provide many lines of evidence for magma mixing such as reversely zoned phenocrysts, bimodal composition distribution, and dissolution texture, whereas P1-type rocks do not. Phenocryst compositions of P2-type suggest that the magma mixing occurred between a less-fractionated phenocryst-poor magma and a strongly fractionated crystal-rich magma. Concentration ratios among incompatible elements show that the mixing end components were derived from a similar parental magma common to P1-type by fractional crystallization in a near closed system. The Dikes with evidence for the magma mixing (P2-type) are distributed only in the southern marginal part of the Dike Swarm, whereas P1-type Dikes do not show any such localization. The distribution and the intrusion direction of the Dikes indicate a nearly horizontal outward flow of magmas in the southern part of the Dike Swarm and accompanied magma mixing in the Dike during intrusion. The fractionated end component is inferred to be a product of crystal fractionation within small and ephemeral magma pockets in the Dike Swarm. Magma mixing is thought to have occurred when a newly intruded Dike ruptured the magma reservoir. The frequency of magma mixing was controlled mainly by competition between the lifetime of ephemeral magma reservoir and frequency of Dike intrusions. The condition of magma mixing was satisfied only in the southern part of the Dike Swarm affected by the preceding volcanic activities.