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1.
Calc-alkaline intermediate rocks are spatially and temporally associated with high-Mg andesites (HMAs, Mg#>60) in Middle Miocene Setouchi volcanic belt. The calc-alkaline rocks are characterized by higher Mg# (strongly calc-alkaline trend) than ordinary calc-alkaline rocks at equivalent silica contents. Phenocrysts in the intermediate rocks have petrographical features such as: (1) coexisting reversely and normally zoned orthopyroxene phenocrysts in single rock; (2) sieve type plagioclase in which cores are mantled by higher An%, melt inclusion-rich zone; and (3) reversely zoned amphibole phenocrysts with opacite cores. In addition, mingling textures and magmatic inclusions were observed in some rocks. These petrographic features and the mineral chemistry indicate that magma mixing was the most important process in producing the strongly calc-alkaline rocks. The core composition of normally zoned orthopyroxene phenocrysts and the mantle composition of reversely zoned orthopyroxene phenocrysts have relatively high Mg# (85–90) in maximum. Although basaltic and high-Mg andesitic magmas are candidate as possible mafic end-member magmas, basaltic magma is excluded in terms of phenocryst assemblage and bulk composition. HMA magmas are suitable mafic end-member magmas that precipitated high Mg# (90) orthopyroxene, whereas andesitic to dacitic magma are suitable felsic end-members. In contrast, it is difficult to produce the strongly calc-alkaline trend through fractional crystallization from a HMA magma, because it would require removal of plagioclase together with mafic minerals from the early stage of crystallization, whereas the precipitation of plagiolase is suppressed due to the high water content of HMA magmas. These results imply that Archean Mg#-rich TTGs (>45–55), which are an analog of the strongly calc-alkaline rocks in terms of chemistry and magma genesis, can be derived from magma mixing in which a HMA magma is the mafic end-member magma, rather than by fractional crystallization from a HMA magma.  相似文献   

2.
The Bishop Tuff, a well known Quaternary high-silica rhyolite in east-central California, is widely considered the type example of a vertically and monotonically zoned pyroclastic deposit that represents zoning in the source magma reservoir, inverted during the process of pyroclastic emplacement. However, the deposit of plinian pumice, which forms the base of the Bishop Tuff and represents the initial 10% or so of all magma erupted during the event that produced the Bishop Tuff, contains features at odds with monotonie zoning for the reservoir. Relative to overlying ignimbrite, the plinian deposit contains a reversal in trace-element zoning. Moreover, the 87Sr/86Sr is significantly higher than that in overlying ignimbrite (about 0.7084 vs 0.7064), and melt inclusions trapped in quartz phenocrysts exhibit notable variability of trace-element concentrations, even within a single host crystal (e.g., U: 10.77 to 8.91 ppm).These data have been previously interpreted as due to processes of chemical fractionation and evolution operating within a magma system closed to chemical interactions with its roof rocks. For example, the reversal in trace-element zoning has been explained by the first-erupted magma being erupted from somewhat below the top of a monotonically zoned reservoir. However, we submit that the reversed zoning and other above-noted features can be explained equally well as consequences of minor assimilation of roof rocks into a magma reservoir that was erupted from the top down.The basal part of the Bishop Tuff exhibits extreme concentrations and depletions of trace elements, relative to the average composition of crustal rocks. For example, the upward decrease of Sr in the Bishop magma reservoir (downward decrease in the ignimbrite) results in concentrations as low as 2–4 ppm. Because of the attendant ‘chemical leverage’, assimilation of < 1 wt.% of Sierra Nevada batholith rocks typical of the area could readily reverse an ‘uncontaminated’ Sr (and other trace elements) trend of zoning and could also substantially raise 87Sr/86Sr. Small-scale trace-element variability in the uppermost part of the Bishop magma reservoir, as recorded by the above-mentioned melt inclusions, may simply reflect melt heterogeneity produced by the process of assimilation.  相似文献   

3.
The common occurrence of compositionally and mineralogically zoned ash flow sheets, such as those of the Timber Mountain Group, provides evidence that the source magma bodies were chemically and thermally zoned. The Rainier Mesa and Ammonia Tanks tuffs of the Timber Mountain Group are both large volume (1200 and 900 km3, respectively) chemically zoned (57–78 wt.% SiO2) ash flow sheets. Evidence of distinct magma batches in the Timber Mountain system are based on: (1) major- and trace-element variations of whole pumice fragments; (2) major-element variations in phenocrysts; (3) major-element variations in glass matrix; and (4) emplacement temperatures calculated from Fe-Ti oxides and feldspars. There are three distinct groups of pumice fragments in the Rainier Mesa Tuff: a low-silica group and two high-silica groups (a low-Th and a high-Th group). These groups cannot be related by crystal fractionation. The low-silica portion of the Rainier Mesa Tuff is distinct from the low-silica portion of the overlying Ammonia Tanks Tuff, even though the age difference is less than 200,000 years. Three distinct groups occur in the Ammonia Tanks Tuff: a low-silica, intermediate-silica and a high-silica group. Part of the high-silica group may be due to mixing of the two high-silica Rainier Mesa groups. The intermediate-silica group may be due to mixing of the low- and high-silica Ammonia Tanks groups. Three distinct emplacement temperatures occur in the Rainier Mesa Tuff (869, 804, 723 °C) that correspond to the low-silica, high-Th and low-Th magma batches, respectively. These temperature differences could not have been maintained for any length of time in the magma chamber (cf. Turner, J.S., Campbell, I.H., 1986. Convection and mixing in magma chambers. Earth-Sci. Rev. 23, 255–352; Martin, D., Griffiths, R.W., Campbell, I.H., 1987. Compositional and thermal convection in magma chambers. Contrib. Mineral. Petrol. 96, 465–475) and therefore eruption must have occurred soon after emplacement of the magma batches into the chamber. Emplacement temperatures of the pumice fragments from the Ammonia Tanks Tuff show a continuous gradient of temperatures with composition. This continuous temperature gradient is consistent with the model of storage of magma batches in the Ammonia Tanks group that have undergone both thermal and chemical diffusion.  相似文献   

4.
The 29.5 Ma Wah Wah Springs Formation which erupted from the Indian Peak Caldera has an estimated volume of > 3900 km3 making it one of the largest ignimbrites on earth. The magma was calc-alkaline, dacitic (68 wt. % SiO2) and phenocryst-rich (38 vol.%). Phenocrysts include plagioclase (An 47), magnesio-hornblende, Mg-biotite, quartz, Fe-Ti oxides, diopsidic-augite, and rare Ca-poor pyroxene, in order of decreasing abundance. Apatite, zircon and pyrrhotite occurs as inclusions within phenocrysts. Atmospheric glass losses (1040 km3) account for bulk-rock compositions that have SiO2 contents ranging from 63 to 67 wt.%. Glass compositions are high-silica rhyolite.Phenocrysts equilibrated at temperatures ranging from about 790 to 850°C and oxygen fugacities approximately 2.6 log units above the QFM buffer. Confining pressure estimates using the aluminum-in-hornblende geobarometer calibrated for calc-alkaline volcanic rocks suggest a mean pressure of 230±50 MPa corresponding to 7.5±1.5 km depth. These estimates are consistent with caldera formation accompanying emplacement.Crystal compositions for phenocrysts and mineral inclusions within phenocrysts are remarkably homogeneous throughout the outflow tuff, although minor zoning does occur. Given the dacitic composition of the magma, the weakly zoned phenocryst population cannot be modeled to produce the observed high-silica glass (melt) indicating open-system behavior for the magma. The high-silica rhyolite glass is interpreted to be an artifact of efficient magma mixing accompanying addition of highly evolved magma, or melt to intermediate composition magma. Mixing was followed by magma hybridization. Additional support for this hybridization model includes: (1) physically and chemically distinct populations of augite; (2) minor but unbiquitous resorbed plagioclase, biotite and hornblende phenocrysts; and (3) reverse zoning in some of the plagioclase euhedra within pumice lapilli.  相似文献   

5.
The Mt Somers Volcanics are part of a suite of mid-Cretaceous (89 ± 2 Ma) intermediate to silicic volcanics, erupted onto an eroded surface of Torlesse sediments. Rock types vary from basaltic andesite to high-silica rhyolite. Andesites are medium- to high-K with phenocrysts of plagioclase, orthopyroxene and pigeonite. Dacites are peraluminous and commonly contain granulite facies xenoliths and garnet xenocrysts. Equilibrium mineral assemblages indicate metamorphic pressures of close to 6 kbar at 800°C. Rhyolites are peraluminous with phenocrysts of quartz, sanidine, plagioclase, biotite, garnet and orthopyroxene. The ferromagnesian phases show textural evidence of magmatic crystallization and are chemically distinct from xenocryst phases in dacites. Equilibrium assemblages indicate that early magmatic crystallization occurred at close to 7 kbar (20 km depth) at above 850°C, with melt-water contents of less than 3.5%. Major-element contents, trace-element contents and an initial 87Sr/86Sr ratio of 0.7085 indicate that the rhyolites formed by partial melting of dominantly quartzo-feldspathic Torlesse sediments, leaving a granulite-facies residue. The chemical variation displayed by the rhyolites is best explained by fractional crystallization of the observed high-pressure phenocryst assemblage. Most elements show a compositional gap between rhyolite and dacite. The major-element, trace-element and Sr isotope compositions of the intermediate lavas are best explained by assimilation of lower crustal material combined with fractional crystallization in mantle-derived tholeiitic magmas. Magmatism was the result of heat and magma flux from the mantle, during the change from compressive to extensional tectonics after the culmination of the Rangitata Orogeny.  相似文献   

6.
The magma generation at Unzen volcano may be considered as the product of crustal material mixed with mantle magma accompanied by fractional crystallization (AFC). The magma in the Unzen volcano is estimated to consist of about 50–80% of residual magma (F) and about 30–70% assimilated crustal material (A) relative to the original magma. Concerning the 1991–1995 eruption, it is estimated that the magma formed as the result of mixing of about 50–60% crustal material and about 55–65% of residual magma. An alternative magma eruption model for the 1991–1995 eruption is proposed here. In the early stage, the isotopic characteristics of 1991 eruption are defined by AFC process in the deeper magma chamber. Later, the magma ascended through the conduit and quiescently stayed for a long time in a shallow reservoir before eruption. The minerals continuously crystallized as phenocrysts especially at the chilled top and outer margin in the shallow chamber. The crystallized phenocryst mush was reworked into the central part of the magma chamber by means of magma convection and rapid magma ascent. Therefore, the reaction between phenocrysts and melt occurs only in internal chemical disequilibrium in the magma chamber. In contrast, the isotopic compositions of the original magma shall be little influenced by the above processes throughout its eruptive history. The 1991–1995 eruptive rocks of the Unzen volcano show their characteristics in Sr and Nd isotopic values independent of their two previous eruptions. However, the isotopic values of early eruptive product could represent the original magma value. This result also supports the previous work of Chen et al. (1993) [Chen, C.H., DePaolo, D.J., Nakada, S., Shieh, Y.N., 1993. Relationship between eruption volume and neodymium isotopic composition at Unzen volcano. Nature 362, 831–834], that suggested the Nd of early or precursory eruptive products could be a qualitative indicator of the maximum size of a continuing or impending eruption.  相似文献   

7.
Detailed petrographic analysis of calcalkaline volcanic rocks of Shirouma-Oike volcano, Japan, reveals that the complex phenocryst assemblage (Ol+Cpx+Opx+Hb+Bt+Qz+Pl+Mt+Hm) in the younger group volcanic rocks can be divided into two groups, a high temperature group (Ol+Cpx±An-rich Pl) and a low temperature group (Op+Hb+Bt+Qz±Ab-rich Pl+Mt+Hm). Compositional zonation of the phenocrystic minerals, normal zoning in olivine and clinopyroxene, and reverse zoning in orthopyroxene and plagioclase, indicate that these two groups of phenocrysts precipitated from two different magmas which mixed before the eruption. The low temperature magma is a stagnant magma in a shallow magma chamber, to which high temperature basaltic magma is intermittently supplied. Magma mixing is also indicated in olivine-bearing two pyroxene andesite of the older group volcanic rocks, by the coexistence of normally zoned Mg-rich clinopyroxene phenocrysts and reversely zoned Fe-rich clinopyroxene phenocrysts, and by reverse zoning in orthopyroxene phenocrysts. It is concluded that magma mixing is an important process responsible for the generation of the disequilibrium features in calc-alkaline volcanic rocks.  相似文献   

8.
Phenocrysts in volcanic rocks are commonly used to deduce crystallization processes in magma chambers. A fundamental assumption is that the phenocrysts crystallized in the magma chambers at isobaric and nearly equilibrium conditions, on the basis of their large sizes. However, this assumption is not always true as demonstrated here for a porphyritic alkali basalt (Kutsugata lava) from Rishiri Volcano, northern Japan. All phenocryst phases in the Kutsugata lava, plagioclase, olivine, and augite, have macroscopically homogeneous distribution of textures showing features characteristic of rapid growth throughout the crystals. Rarely, a core region with distinct composition is present in all phenocryst phases. Phenocrysts, excluding this core, are occasionally in direct contact with each other, forming crystal aggregates. The equilibrium liquidus temperature of plagioclase, the dominant phase (35 vol%) in the Kutsugata lava, can never exceed the estimated magmatic temperature, unless the liquidus temperature increases significantly due to vesiculation of the magma during ascent. This suggests that most phenocrysts in the Kutsugata lava were formed by decompression of the magma during ascent in a conduit, rather than by cooling during residence in a magma reservoir. In the magma chamber before eruption, probably located at depth of more than 7 km, only cores of the phenocrysts were present and the magma was nearly aphyric (<5 vol% crystals), though the observed rock is highly porphyritic with up to 40 vol% crystals. The Kutsugata magma is inferred to have been rich in dissolved H2O (>4 wt.%) in the magma chamber, and liquidus temperatures of phenocryst phases were significantly suppressed. Large undercooling caused by decompression and degassing of the magma was the driving force for significant crystallization during ascent because of the increase in liquidus temperature due to vapor exsolution. Low ascent rate of the Kutsugata magma, which is suggested by pahoehoe lava morphology and no association of pyroclastics, gave sufficient time for crystallization. Furthermore, the large degree of superheating of plagioclase in the magma chamber caused plagioclase crystallization with low population density and large crystal size, which characterizes the porphyritic nature of the Kutsugata lava. Alkali basalt is likely to satisfy these conditions and similar phenomena are suggested to occur in other volcanic systems.  相似文献   

9.
Apoyo caldera, near Granada, Nicaragua, was formed by two phases of collapse following explosive eruptions of dacite pumice about 23,000 yr B.P. The caldera sits atop an older volcanic center consisting of lava flows, domes, and ignimbrite (ash-flow tuff). The earliest lavas erupted were compositionally homogeneous basalt flows, which were later intruded by small andesite and dacite flows along a well defined set of N—S-trending regional faults. Collapse of the roof of the magma chamber occurred along near-vertical ring faults during two widely separated eruptions. Field evidence suggests that the climactic eruption sequence opened with a powerful plinian blast, followed by eruption column collapse, which generated a complex sequence of pyroclastic surge and ignimbrite deposits and initiated caldera collapse. A period of quiescence was marked by the eruption of scoria-bearing tuff from the nearby Masaya caldera and the development of a soil horizon. Violent plinian eruptions then resumed from a vent located within the caldera. A second phase of caldera collapse followed, accompanied by the effusion of late-stage andesitic lavas, indicating the presence of an underlying zoned magma chamber. Detailed isopach and isopleth maps of the plinian deposits indicate moderate to great column heights and muzzle velocities compared to other eruptions of similar volume. Mapping of the Apoyo airfall and ignimbrite deposits gives a volume of 17.2 km3 within the 1-mm isopach. Crystal concentration studies show that the true erupted volume was 30.5 km3 (10.7 km3 Dense Rock Equivalent), approximately the volume necessary to fill the caldera. A vent area located in the northeast quadrant of the present caldera lake is deduced for all the silicic pyroclastic eruptions. This vent area is controlled by N—S-trending precaldera faults related to left-lateral motion along the adjacent volcanic segment break. Fractional crystallization of calc-alkaline basaltic magma was the primary differentiation process which led to the intermediate to silicic products erupted at Apoyo. Prior to caldera collapse, highly atypical tholeiitic magmas resembling low-K, high-Ca oceanic ridge basalts were erupted along tension faults peripheral to the magma chamber. The injection of tholeiitic magmas may have contributed to the paroxysmal caldera-forming eruptions.  相似文献   

10.
We describe a magma mingling episode from Ruapehu volcano between two andesite magmas, one very much minor in volume relative to the other. The event acted to trigger eruption of the andesitic Pourahu pyroclastic flow which is preserved in a thick sequence of tephras and laharic deposits in the southeastern ring plain of the volcano. The predominant andesite is pale brown coloured and porphyritic containing phenocrysts of plagioclase-clinopyroxene-orthopyroxene-Fe-Ti oxides. Rare clasts of a darker andesite are different texturally, less vesicular, and contain distinctive microphenocrysts of plagioclase and quench olivine. Equally rare clasts, of streaky pumice consisting of interbanded ‘dark’ and ‘light’ andesite attest to mingling between these two andesite components.Chemical analyses of discrete clasts demonstrate that the Pourahu pyroclastic flow andesites span much of the compositional spectrum of Ruapehu andesites. This observation demonstrates heterogeneity in the products of a relatively small eruption. The darker clast analyses and those from associated distal fall deposits lie within the fields defined by the dominant light coloured clasts. Phenocryst and microphenocryst geothermometry suggest slightly higher temperatures in the dark component. However, glasses from groundmass and phenocryst inclusions in the same specimen may differ considerably, leading us to conclude that many phenocrysts are in fact xenocrystic and were incorporated in the melts as they migrated towards the surface.We prefer a model in which a small volume of hot andesite magma injects a vent-feeding magma chamber, triggering vesiculation and eruption. We infer that the process of magma withdrawal extended downward into the magma body causing the dark component to intermingle with the lighter (dominant) component, ‘sucking’ more dark magma into the chamber. Our observations are entirely consistent with the existence of a plexus of small, possibly interlinked magma chambers beneath Ruapehu.  相似文献   

11.
We consider the origin of rhyolites associated with tholeiitic basalt in bimodal provinces, as exemplified by the Rattlesnake Tuff of the High Lava Plains of eastern Oregon, in comparison to rhyolites associated with calcalkaline suites in light of recent models of extraction of rhyolite from crystal mush (Hildreth, J Volcanol Geotherm Res, 136:169–198, 2004; Bachmann and Bergantz, J Petrol, 45:1565–1582, 2004). The High Lava Plains encompass a strongly bimodal, tholeiite-rhyolite suite, spatially and compositionally related to the Snake River Plain and Yellowstone Plateau. In our assessment we draw the distinction between fractionation dominated processes to make rhyolites from rhyolites and processes required to make the parental rhyolite melt. New isotopic data and compositional zoning profiles in phenocrysts confirm that crystal fractionation dominated the generation of progressively more evolved, discrete rhyolites in the zoned Rattlesnake Tuff and are consistent with an origin of the least evolved high-silica rhyolites by partial melting of a mafic crust. While the most evolved rhyolites are compositionally virtually indistinguishable from those of calcalkaline suites, the parental rhyolites from bimodal suites are more Fe-rich than their calcalkaline counterparts. Oxygen isotope thermometry yields pre-eruptive temperatures of 860°C, in keeping with 800–880°C zircon saturation temperatures. High magmatic temperatures are common among rhyolites of bimodal suites, distinguishing them from cooler rhyolites of calcalkaline suites. Extraction of interstitial melt from a granodioritic mush cannot produce compositions of the Rattlesnake Tuff on the basis of major and trace element arguments (especially Fe, Ba, Sr, and Eu) and on the basis of temperature considerations. Chemically viable parental crystal mushes are syenite and alkali (A-type) granites for the production of all more evolved Rattlesnake Tuff rhyolites; ferro-dacitic mush is required for production of the least-evolved, parental Rattlesnake Tuff rhyolite. Paucity of such ferro-dacitic compositions in tholeiitic bimodal suites, especially compared to the abundance of dacitic (granodioritic) compositions in calcalkaline suites, argues against the mush extraction model for the parental rhyolite. Furthermore, rhyolites of bimodal suites lack associated voluminous eruptions of crystal-rich ignimbrite that might represent a parental mush, as exemplified by the “monotonous intermediate” Fish Canyon Tuff in calcalkaline suites. We conclude that extensive fractionation is common among rhyolites and may obscure their ancestry. Fe-rich parental rhyolites common in bimodal tholeiitic suites, as represented by Rattlesnake Tuff, may often be the result of partial melting of mafic to intermediate crust, in contrast to calcalkaline high-silica rhyolites that are related to voluminous suites of intermediate intrusive rocks where the pre-plutonic mush-extraction model works better. This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic rhyolites.  相似文献   

12.
The strongly peralkaline Green Tuff, Pantelleria, is an example of a thin, densely welded air-fall tuff which mantles an area of at least 85 km2. Offshore the tuff is correlated with the Y-6 ash layer in the central Mediterranean Sea, and the total volume of the eruption is estimated at 7 km3 D.R.E. New petrological data suggests that the tuff was erupted from a zoned magma chamber containing a cooler, more fractionated upper zone relative to be bulk of the magma. Analysis of the distribution of accessory lithic fragments in terms of existing models of eruption dynamics indicates emplacement by a plinian-type eruption. It is shown that, due to the low viscosity of pantelleritic ejecta, dense welding can occur at moderate tephra accumulation rates and a rate of the order of 1 cm/minute is suggested for the Green Tuff; this yields an estimate for the eruption duration of rather less than one day. It is predicted that welded tuff should be formed during large plinian eruptions of pantelleritic magma, and therefore that welded airfall tuffs should be common in areas of peralkaline volcanism.  相似文献   

13.
The Pagosa Peak Dacite is an unusual pyroclastic deposit that immediately predated eruption of the enormous Fish Canyon Tuff (5000 km3) from the La Garita caldera at 28 Ma. The Pagosa Peak Dacite is thick (to 1 km), voluminous (>200 km3), and has a high aspect ratio (1:50) similar to those of silicic lava flows. It contains a high proportion (40–60%) of juvenile clasts (to 3–4 m) emplaced as viscous magma that was less vesiculated than typical pumice. Accidental lithic fragments are absent above the basal 5–10% of the unit. Thick densely welded proximal deposits flowed rheomorphically due to gravitational spreading, despite the very high viscosity of the crystal-rich magma, resulting in a macroscopic appearance similar to flow-layered silicic lava. Although it is a separate depositional unit, the Pagosa Peak Dacite is indistinguishable from the overlying Fish Canyon Tuff in bulk-rock chemistry, phenocryst compositions, and 40Ar/39Ar age.The unusual characteristics of this deposit are interpreted as consequences of eruption by low-column pyroclastic fountaining and lateral transport as dense, poorly inflated pyroclastic flows. The inferred eruptive style may be in part related to synchronous disruption of the southern margin of the Fish Canyon magma chamber by block faulting. The Pagosa Peak eruptive sources are apparently buried in the southern La Garita caldera, where northerly extensions of observed syneruptive faults served as fissure vents. Cumulative vent cross-sections were large, leading to relatively low emission velocities for a given discharge rate. Many successive pyroclastic flows accumulated sufficiently rapidly to weld densely as a cooling unit up to 1000 m thick and to retain heat adequately to permit rheomorphic flow. Explosive potential of the magma may have been reduced by degassing during ascent through fissure conduits, leading to fracture-dominated magma fragmentation at low vesicularity. Subsequent collapse of the 75×35 km2 La Garita caldera and eruption of the Fish Canyon Tuff were probably triggered by destabilization of the chamber roof as magma was withdrawn during the Pagosa Peak eruption.  相似文献   

14.
 The Cerro Chascon-Runtu Jarita Complex is a group of ten Late Pleistocene (∼85 ka) lava domes located in the Andean Central Volcanic Zone of Bolivia. These domes display considerable macroscopic and microscopic evidence of magma mixing. Two groups of domes are defined chemically and geographically. A northern group, the Chascon, consists of four lava bodies of dominantly rhyodacite composition. These bodies contain 43–48% phenocrysts of plagioclase, quartz, sanidine, biotite, and amphibole in a microlite-poor, rhyolitic glass. Rare mafic enclaves and selvages are present. Mineral equilibria yield temperatures from 640 to 750  °C and log ƒO2 of –16. Geochemical data indicate that the pre-eruption magma chamber was zoned from a dominant volume of 68% to minor amounts of 76% SiO2. This zonation is best explained by fractional crystallization and some mixing between rhyodacite and more evolved compositions. The mafic enclaves represent magma that intruded but did not chemically interact much with the evolved magmas. A southern group, the Runtu Jarita, is a linear chain of six small domes (<1 km3 total volume) that probably is the surface expression of a dike. The five most northerly domes are composites of dacitic and rhyolitic compositions. The southernmost dome is dominantly rhyolite with rare mafic enclaves. The composite domes have lower flanks of porphyritic dacite with ∼35 vol.% phenocrysts of plagioclase, orthopyroxene, and hornblende in a microlite-rich, rhyodacitic glass. Sieve-textured plagioclase, mixed populations of disequilibrium plagioclase compositions, xenocrystic quartz, and sanidine with ternary composition reaction rims indicate that the dacite is a hybrid. The central cores of the composite domes are rhyolitic and contain up to 48 vol.% phenocrysts of plagioclase, quartz, sanidine, biotite, and amphibole. This is separated from the dacitic flanks by a banded zone of mingled lava. Macroscopic, microscopic, and petrologic evidence suggest scavenging of phenocrysts from the silicic lava. Mineral equilibria yield temperatures of 625–727  °C and log ƒO2 of –16 for the rhyolite and 926–1000  °C and log ƒO2 of –9.5 for the dacite. The rhyolite is zoned from 73 to 76% SiO2, and fractionation within the rhyolite composition produced this variation. Most of the 63–73% SiO2 compositional range of the lava in this group is the result of mixing between the hybrid dacite and the rhyolite. Eruption of both groups of lavas apparently was triggered by mafic recharge. A paucity of explosive activity suggests that volatile and thermal exchanges between reservoir and recharge magmas were less important than volume increase and the lubricating effects of recharge by mafic magmas. For the Runtu Jarita group, the eruption is best explained by intrusion of a dike of dacite into a chamber of crystal-rich rhyolite close to its solidus. The rhyolite was encapsulated and transported to the surface by the less-viscous dacite magma, which also acted as a lubricant. Simultaneous effusion of the lavas produced the composite domes, and their zonation reflects the subsurface zonation. The role of recharge by hotter, more fluid mafic magma appears to be critical to the eruption of some highly viscous silicic magmas. Received: 23 August 1998 / Accepted: 10 March 1999  相似文献   

15.
The Monte Guardia rhyolitic eruption (~22 ka, Lipari, Aeolian Islands, Italy) produced a sequence of pyroclastic deposits followed by the emplacement of lava domes. The total volume of dense magma erupted was nearly 0.5 km3. The juvenile clasts in the pyroclastic deposits display a variety of magma mixing evidence (mafic magmatic enclaves, streaky pumices, mineral disequilibria and heterogeneous glass composition). Petrographic, mineralogical and geochemical investigations and melt inclusion studies were carried out on the juvenile clasts in order to reconstruct the mixing process and to assess the pre-eruptive chemico-physical magmatic conditions. The results suggest that the different mingling and mixing textures were generated during a single mixing event between a latitic and a rhyolitic end member. A denser, mixed magma was first erupted, followed by a larger volume of an unmixed, lighter rhyolitic one. This compositional sequence is the reverse of what would be expected from the tapping of a zoned magma chamber. The Monte Guardia rhyolitic magma, stored below 200 MPa, was volatile-rich and fluid-saturated, or very close to this, despite its relatively low explosivity. In contrast to previous interpretations, there exists the possibility that the rhyolite could rise and erupt without the trigger of a mafic input. The entire data collected are compatible with two possible mechanisms that would generate a reversely zoned sequence: (1) the occurrence of thermal instabilities in a density stratified, salic to mafic magma chamber and (2) the intrusion of rising rhyolite into a shallower mafic sill/dike.  相似文献   

16.
Thirty-six basalt samples from near East Pacific Rise 13°N are analyzed for major and trace elements. Different types of zoned plagioclase phenocrysts in basalts are also backscatter imaged, and major element profiles scanned and analyzed for microprobe. Basalts dredged from a restricted area have evolved to different extents (MgO=9.38wt%—6.76wt%). High MgO basalts are modeled for crystalliza-tion to MgO of about 7wt%, and resulted in the Ni contents (≈28 ppm) that are generally lower than that in observed basalts (>60 ppm). It suggests that low MgO basalts may have experienced more intensive magma mixing. High MgO (9.38wt%) basalt is modeled for self-"mixing-crystallization", and the high Ni contents in low MgO basalts can be generated in small scale and periodical self-mixing of new magma (high MgO). "Mixing-crystallization" processes that low MgO magmas experienced accord with recent 226Ra/230Th disequilibria studies for magma residence time, in which low MgO magmas have experi-enced more circles of "mixing-crystallization" in relatively longer residence time. Magma mixing is not homogeneous in magma chamber, however, low MgO magmas are closer to stable composition pro-duced by periodical "mixing-crystallization", which is also an important reason for magma diversity in East Pacific Rise. Zoned plagioclase phenocrysts can be divided into two types: with and without high An# cores, both of which have multiple reversed An# zones, suggesting periodical mixing of their host magmas. Cores of zoned plagioclase in low MgO (7.45wt%) basalt differ significantly with their mantle in An#, but are similar in An# with microlite cores (products of equilibrium crystallization) in high MgO (9.38wt%) basalt, which further shows that plagioclase phenocryst cores in low MgO basalts may have formed in their parental magmas before entering into the magma chamber.  相似文献   

17.
Physical and chemical analyses of distal tephra from the 1912 eruption of Novarupta, Alaska, show considerable variations in glass and mineral compositions. A combination of a 150°C range in temperature deduced from iron-titanium oxide geothermometry, and curved patterns in bivariant element plots of glass compositions indicate that a chamber of compositionally zoned magma existed prior to the eruption. Magma-mixing cannot explain these features. The magma chamber may have resembled the model recently proposed by McBirney (1980). A highly silicic, quartz-phyric magma with mean phenocryst compositions of An25 plagioclase, Fs42 orthopyroxene, at a temperature of 880°C and a water pressure of 1.4 kbar, was located above a more mafic, hotter magma, bearing phenocrysts of An45 plagioclase and Fs35, orthopyroxene.Our results on distal tephras compare favorably with those from a recently completed study at source by Hildreth (1983), suggesting that useful petrologic information about distant volcanoes can be obtained from both types of deposits. Compositionally heterogeneous abyssal tephra layers are common in the Gulf of Alaska. Eruptions from chambers of zoned magma may account for many of these layers.  相似文献   

18.
Pargasite commonly occurs in the dacitic groundmass of the 1991–1995 eruption products of Unzen volcano. We described the occurrence and chemical compositions of amphibole in the dacite, and also carried out melting experiments to determine the low-pressure stability limit of amphibole in the dacite. The 1991–1995 ejecta of the Unzen volcano show petrographic evidence of magma mixing, such as reverse compositional zoning of plagioclase and amphibole phenocrysts, and we used a groundmass separate as a starting material for the experiments. Reversed experiments show that the maximum temperature for the crystallization of amphibole is 930°C at 196 MPa, 900°C at 98 MPa, and 820°C at 49 MPa. Compared with the experimental results on the Mount St. Helens dacite, present experiments on the Unzen dacitic groundmass show that amphibole is stable to pressures ca. 50 MPa lower at 850°C. Available Fe–Ti oxide thermometry indicates the crystallization temperature of the groundmass of the Unzen dacite to be 880±30°C, suggesting that the groundmass pargasite crystallized at >70 MPa, corresponding to a depth of more than 3 km in the conduit. The chlorine content of the groundmass pargasite is much lower than that of phenocrystic magnesiohornblende in the 1991–1995 dacite of Unzen volcano, indicating that vesiculation/degassing of magma took place before the crystallization of the groundmass pargasite. The present study shows that the magma was water oversaturated and that the degassing of magma along with magma mixing caused crystallization of the groundmass amphibole at depths of more than 3 km in the conduit.  相似文献   

19.
Rhyolites occur as a subordinate component of the basalt-dominated Eastern Snake River Plain volcanic field. The basalt-dominated volcanic field spatially overlaps and post-dates voluminous late Miocene to Pliocene rhyolites of the Yellowstone–Snake River Plain hotspot track. In some areas the basalt lavas are intruded, interlayered or overlain by ~15 km3 of cryptodomes, domes and flows of high-silica rhyolite. These post-hotspot rhyolites have distinctive A-type geochemical signatures including high whole-rock FeOtot/(FeOtot+MgO), high Rb/Sr, low Sr (0.5–10 ppm) and are either aphyric, or contain an anhydrous phenocryst assemblage of sodic sanidine ± plagioclase + quartz > fayalite + ferroaugite > magnetite > ilmenite + accessory zircon + apatite + chevkinite. Nd- and Sr-isotopic compositions overlap with coeval olivine tholeiites (ɛNd = −4 to −6; 87Sr/86Sri = 0.7080–0.7102) and contrast markedly with isotopically evolved Archean country rocks. In at least two cases, the rhyolite lavas occur as cogenetic parts of compositionally zoned (~55–75% SiO2) shield volcanoes. Both consist dominantly of intermediate composition lavas and have cumulative volumes of several 10’s of km3 each. They exhibit two distinct, systematic and continuous types of compositional trends: (1) At Cedar Butte (0.4 Ma) the volcanic rocks are characterized by prominent curvilinear patterns of whole-rock chemical covariation. Whole-rock compositions correlate systematically with changes in phenocryst compositions and assemblages. (2) At Unnamed Butte (1.4 Ma) the lavas are dominated by linear patterns of whole-rock chemical covariation, disequilibrium phenocryst assemblages, and magmatic enclaves. Intermediate compositions in this group resulted from variable amounts of mixing and hybridization of olivine tholeiite and rhyolite parent magmas. Interestingly, models of rhyolite genesis that involve large degrees of melting of Archean crust or previously consolidated mafic or silicic Tertiary intrusions do not produce observed ranges of Nd- and Sr-isotopes, extreme depletions in Sr-concentration, and cogenetic spectra of intermediate rock compositions for both groups. Instead, least-squares mass-balance, energy-constrained assimilation and fractional crystallization modeling, and mineral thermobarometry can explain rhyolite production by 77% low-pressure fractional crystallization of a basaltic trachyandesite parent magma (~55% SiO2), accompanied by minor (0.03–7%) assimilation of Archean upper crust. We present a physical model that links the rhyolites and parental intermediate magmas to primitive olivine tholeiite by fractional crystallization. Assimilation, recharge, mixing and fractional melting occur to limited degrees, but are not essential parts of the rhyolite formation process. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic rhyolites.  相似文献   

20.
Large continental silicic magma systems commonly produce voluminous ignimbrites and associated caldera collapse events. Less conspicuous and relatively poorly documented are cases in which silicic magma chambers of similar size to those associated with caldera-forming events produce dominantly effusive eruptions of small-volume rhyolite domes and flows. The Bearhead Rhyolite and associated Peralta Tuff Member in the Jemez volcanic field, New Mexico, represent small-volume eruptions from a large silicic magma system in which no caldera-forming event occurred, and thus may have implications for the genesis and eruption of large volumes of silicic magma and the long-term evolution of continental silicic magma systems.40Ar/39Ar dating reveals that most units mapped as Bearhead Rhyolite and Peralta Tuff (the Main Group) were erupted during an ∼540 ka interval between 7.06 and 6.52 Ma. These rocks define a chemically coherent group of high-silica rhyolites that can be related by simple fractional crystallization models. Preceding the Main Group, minor amounts of unrelated trachydacite and low silica rhyolite were erupted at ∼11–9 and ∼8 Ma, respectively, whereas subsequent to the Main Group minor amounts of unrelated rhyolites were erupted at ∼6.1 and ∼1.5 Ma.The chemical coherency, apparent fractional crystallization-derived geochemical trends, large areal distribution of rhyolite domes (∼200 km2), and presence of a major hydrothermal system support the hypothesis that Main Group magmas were derived from a single, large, shallow magma chamber. The ∼540 ka eruptive interval demands input of heat into the system by replenishment with silicic melts, or basaltic underplating to maintain the Bearhead Rhyolite magma chamber.Although the volatile content of Main Group magmas was within the range of rhyolites from major caldera-forming eruptions such as the Bandelier and Bishop Tuffs, eruptions were smaller volume and dominantly effusive. Bearhead Rhyolite domes occur at the intersection of faults, and are cut by faults, suggesting that the magma chamber was structurally vented preventing volatiles from accumulating to levels high enough to trigger a caldera-forming eruption.  相似文献   

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