Mafic Andesite Research Articles

Volcanisme triasique calco-alcalin a shoshonitique du Nevada occidental

Volcanisme triasique calco-alcalin a shoshonitique du Nevada occidental

Field characteristics of deposits from spatter-rich pyroclastic density currents at Summer Coon volcano, Colorado

The Oligocene, deeply eroded Summer Coon composite volcano contains mafic andesite deposits that are massive to poorly bedded, have abundant flattened and deformed spatter clasts, have varying proportions of dense lithic clasts, and are supported mostly by a coarse-ash matrix. Although superficially these deposits resemble typical facies from Strombolian eruptions (emplaced ballistically, by fallout, and by rolling and local grain-avalanches down steep cone slopes), there are several lines of evidence that lead to an interpretation that the deposits were emplaced by pyroclastic density currents. These include local coarse-tail grading, deformation of spatter clasts in a down-flow direction, incorporation of matrix ash and lapilli into flattened spatter clasts, imbrication of large clasts, plastering of spatter on stoss sides of large lithic blocks and lenses of lithic-rich material on lee sides, deposition on angles less than the angle of repose, and a paucity of clast shapes associated with Strombolian mechanisms. The deposit characteristics are consistent with rapid sedimentation from a low-particle-concentration, turbulent flow onto an aggrading bed. We infer two potential mechanisms for generating these density currents: (1) explosive magma–water interaction involving lithic debris and relatively unfragmented melt; and (2) collapse of oversteepened upper cone slopes due to rapid accumulation of spatter from voluminous Strombolian eruptions.

Evolution of Lascar Volcano, Northern Chile

Lascar Volcano in the Central Andes, is an ESE–WNW elongate composite stratocone. Stage I (<43 ka) formed a mafic andesite stratocone and culminated in three coarse-grained pyroclastic flow eruptions. Stage II switched to the west and magmas evolved to silicic andesite and dacite of a lava dome complex and hot rock avalanche apron. Stage II culminated with a major explosive eruption (volume 15 km 3 ) at 26.45 ka producing the Soncor ignimbrite. No caldera formed. Laharic and alluvial fan deposits and a debris avalanche formed from 26.45 to 19.2 ka, indicating a humid climatic period. The dacitic and andesitic Western Stratocone (Stage III) developed over the Soncor vent. Stage III culminated in the 9.2 ka Tumbres eruption which formed andesitic scoria flow fans and a summit collapse crater. Stage IV activity switched to the east forming an andesite lava, three nested summit craters, which repeatedly filled with lava and collapsed. Absence of significant erosion since 9.2 ka is due to the much drier Holocene climate. Activity since 1984 displays cycles of lava dome formation in the summit crater, lava dome subsidence with crater collapse, vulcanian to plinian explosive eruptions culminating in the major explosive eruption of 19/20 April 1993. The total volume of magma erupted from Lascar over a 43 ka period is estimated at 30 to 40 km 3 .

Magmatic processes that generated the rhyolite of Glass Mountain, Medicine Lake volcano, N. California

Glass Mountain consists of a 1 km3, compositionally zoned rhyolite to dacite glass flow containing magmatic inclusions and xenoliths of underlying shallow crust. Mixing of magmas produced by fractional crystallization of andesite and crustal melting generated the rhyolite of Glass Mountain. Melting experiments were carried out on basaltic andesite and andesite magmatic inclusions at 100, 150 and 200 MPa, H2O-saturated with oxygen fugacity controlled at the nickel-nickel oxide buffer to provide evidence of the role of fractional crystallization in the origin of the rhyolite of Glass Mountain. Isotopic evidence indicates that the crustal component assimilated at Glass Mountain constitutes at least 55 to 60% of the mass of erupted rhyolite. A large volume of mafic andesite (2 to 2.5 km3) periodically replenished the magma reservoir(s) beneath Glass Mountain, underwent extensive fractional crystallization and provided the heat necessary to melt the crust. The crystalline residues of fractionation as well as residual liquids expelled from the cumulate residues are preserved as magmatic inclusions and indicate that this fractionation process occurred at two distinct depths. The presence and composition of amphibole in magmatic inclusions preserve evidence for crystallization of the andesite at pressures of at least 200 MPa (6 km depth) under near H2O-saturated conditions. Mineralogical evidence preserved in olivine-plagioclase and olivine-plagioclase-high-Ca clinopyroxene-bearing magmatic inclusions indicates that crystallization under near H2O-saturated conditions also occurred at pressures of 100 MPa (3 km depth) or less. Petrologic, isotopic and geochemical evidence indicate that the andesite underwent fractional crystallization to form the differentiated melts but had no chemical interaction with the melted crustal component. Heat released by the fractionation process was responsible for heating and melting the crust.

A petrologic re-investigation of the Adak volcanic center, central Aleutian arc, Alaska

The Adak volcanic center is located in the central part of the Aleutian arc and consists of three main volcanic vents. Andrew Bay Volcano, the oldest center, has been mostly removed by erosion. The next youngest vent, Mount Adagdak, was built in three major volcanic stages whereas Mount Moffett, the largest volcanic edifice, consists of a main cone and a parasitic cone each with several magmatic phases. Adak is unique compared to other modern Aleutian volcanic centers in that it contains two xenolith suites (Conrad and Kay, 1984; Debari et al., 1987). One suite consisting predominantly of mafic xenoliths occurs on Mount Moffett whereas an assemblage of ultramafic and mafic xenoliths is found on Mount Adagdak. Lavas erupted at Adak span the compositional range from 48.4 to 65.0 wt.% SiO2 and are characterized by significant variations in Al2O3, MgO, Sr, Ni and Cr. On Harker diagrams, this variability produces compositional trends with significant scatter. The Adak suite has total REE contents that vary from 32 to 154 ppm but do not correlate systematically with silica. (LaYb)n ratios range from 2.41 to 21.72 with the majority of lavas between 2.41 and 6.06. On process identification diagrams, the Adak suite plots as steeply sloping trends that contrast with the horizontal patterns of most other Aleutian centers. Measured isotopic ranges are large and nearly equal to those for the entire arc. Although they span similar silica ranges, subtle geochemical and isotopic differences distinguish the different volcanic vents of Adak. On Mount Moffett, a geochemically and isotopically distinct group of andesites (55.5–57.9 SiO2), the mafic andesites, occur on its NE flank. These lavas have elevated MgO, Ni and Cr but are depleted in Al2O3 relative to other Mount Moffett andesites with similar silica. They also have more heterogeneous REE abundances and isotopic ratios than most of the other andesites. Significant compositional differences exist between Adak and the other volcanic centers of the central Aleutian arc. Although these differences are characteristic of all geochemical systems, they are greatest for major and rare-earth elements and isotopic ratios. The lack of coherent relationships on major- and trace-element Harker diagrams, the isotopic variability, as well as the steeply sloping trends on REE process identification diagrams suggest that the Adak volcanic suite was not formed predominantly by closed-system crystal fractionation, but must be the product of a complex open-system process(es). The significant isotopic variability displayed by the suite suggests that contamination by an isotopically distinct contaminant must also have been an important petrologic component in the evolution of the suite. REE data are also suggestive of a role for magma mixing. Such a complex petrologic evolution is consistent with an immature lithospheric plumbing system. Based on REE systematics, the xenolith suites of Adak cannot, as previously proposed, be related to the host lavas or the rest of the Adak suite through crystal fractionation schemes. Rather they are probably accidental fragments derived from various depths along lithospheric conduits. In light of their relation to xenolith-bearing units, the mafic andesites of Adak presumably represent hybrid magmas formed during the interaction of ascending magmas with lithospheric wall rock. They are, therefore, characteristic of immature volcanic centers and unlikely to be related directly to the magmatic processes responsible for the generation of primary arc magmas. Because of the close proximity of the vents and the subtle compositional differences between their lavas, the Adak volcanic center was probably supplied by a single, deep lithospheric plumbing system that fed separate crustal magma chambers. The absence of historic volcanic activity on Adak suggests this plumbing system was abandoned before complete conduit development. This decline in magmatism may reflect a re-adjustment of volcano spacing within this part of the Aleutian arc.

The origin of Mount St. Helens andesites

Mount St. Helens volcano has intermittently produced mainly dacitic products but occasionally erupted a more diverse suite of lavas including basalts and andesites. Petrogenetic relations between these magmas provide insight into the dynamics of the subjacent magma system. Mineralogical and geochemical features of representative lavas erupted during the past 2200 years can distinguish three basaltic and three andesitic variants. The mafic lavas include: (a) transitional, olivine+plagioclase basalts with low K 2O and incompatible trace-element abundances; (b) calc-alkaline, olivine+plagioclase + clinopyroxene basalts enriched in K 2O, TiO 2, and incompatible trace elements; and (c) calc-alkaline. olivine+plagioclase basaltic andesites with incompatible trace-element contents transitional between the two basalt types. Intermediate lavas include (a) tholeiitic, two-pyroxene andesites, (b) calc-alkaline, plagioclase+two-pyroxenes±olivine±amphibole mafic andesites (56–59% wt.% SiO 2), and (c) calc-alkaline, plagioclase+two-pyroxenes+amphibole high-silica andesites (61–62 wt.% SiO 2). Eruption of these magmatic variants within the same eruptive phase implies the existence of different petrogenetic lineages, and that the plumbing system is sufficiently complex to simultaneously isolate and preserve numerous magma batches. It is unlikely that any of the andesites (or dacites) are derived by fractional crystallization of the recognized basaltic variants. Formation of the andesites simply by contamination (or assimilation-fractional crystallization) of basaltic magma is also improbable. More plausibly, the andesites represent mixing between basaltic and dacitic end-member magmas, each of which may be somewhat heterogeneous or vary in composition with time. In this model, efficient mixing must occur in some parts of the magma plumbing system, while some conduits or storage reservoirs must be effectively isolated.

The volcanic and magmatic evolution of Volcán Ollagüe, a high-K, late quaternary stratovolcano in the Andean Central Volcanic Zone

Volcán Ollagüe is a high-K, calc-alkaline composite volcano constructed upon extremely thick crust in the Andean Central Volcanic Zone. Volcanic activity commenced with the construction of an andesitic to dacitic composite cone composed of numerous lava flows and pyroclastic deposits of the Vinta Loma series and an overlying coalescing dome and coulée sequence of the Chasca Orkho series. Following cone construction, the upper western flank of Ollagüe collapsed toward the west leaving a collapse-amphitheater about 3.5 km in diameter and a debris avalanche deposit on the lower western flank of the volcano. The deposit is similar to the debris avalanche deposit produced during the May 18, 1980 eruption of Mount St. Helens, U.S.A., and was probably formed in a similar manner. It presently covers an area of 100 km 2 and extends 16 km from the summit. Subsequent to the collapse event, the upper western flank was reformed via eruption of several small andesitic lava flows from vents located near the western summit and growth of an andesitic dome within the collapse-amphitheater. Additional post-collapse activity included construction of a dacitic dome and coulée of the La Celosa series on the northwest flank. Field relations indicate that vents for the Vinta Loma and post-collapse series were located at or near the summit of the cone. The Vinta Loma series is characterized by an anhydrous, two-pyroxene assemblage. Vents for the La Celosa and Chasca Orkho series are located on the flanks and strike N55 W, radial to the volcano. The pattern of flank eruptions coincides with the distribution in the abundance of amphibole and biotite as the main mafic phenocryst phases in the rocks. A possible explanation for this coincidence is that an unexposed fracture or fault beneath the volcano served as a conduit for both magma ascent and groundwater circulation. In addition to the lava flows at Ollagüe, magmas are also present as blobs of vesiculated basaltic andesite and mafic andesite that occur as inclusions in nearly all of the lavas. All eruptive activity at Ollagüe predates the last glacial episode (∼ 11.000 a B.P.), because post-collapse lava flows are overlain by moraine and are incised by glacial valleys. Present activity is restricted to emission of a persistent, 100-m-high fumarolic steam plume from a vent located within the summit andesite dome. Sr and Nd isotope ratios for the basaltic andesite and mafic andesite inclusions and lavas suggest that they have assimilated large amounts of crust during crystal fractionation. In contrast, narrow ranges in 143Nd/ 144Nd and 87Sr/ 86Sr in the andesitic and dacitic lavas are enigmatic with respect to crustal contamination.

The 1982-83 Eruption at Galunggung Volcano, Java (Indonesia): Oxygen Isotope Geochemistry of a Chemically Zoned Magma Chamber

Mount Galunggung is a historically active volcano in southwestern Java that has erupted four times in the last two centuries. During the most recent event, which occurred during a 9–month interval in 1982– 83, some 305× 106 m3 of medium–K, calc–alkaline magma was erupted. This eruption was unusual because of its duration, the diversity of eruption dynamics and products, and the range of lava compositions produced. The composition of juvenile material changed gradually during the course of the eruption from initial plagioclase (An60–75) and two–pyrozene bearing andesites with ∼58% SiO2 to final plagioclase (An85–90), diopside, and olivine (Fo85–90) bearing primitive magnesium basalts with ∼47% SiO2 Mineralogical and compositional relationships indicate a magmatic evolution involving differentitation of high–Mg parental melt. The eruptive volumes of ∼ 35 × 106 m3 andesite, 120 × 106 m3 mafic andesite, and 150× 106 m3 basalt are consistent with the idea that the 1982– 83 eruption progressively tapped and drained a magma chamber that had become chemically stratified through extensive crystal fractionation. Separates of plagioclase and pyroxene have δ18O(‰ SMO W) ranges of + 5. 6 to + 6.0 and + 5.3 to + 5.6, respectively, with Δ18Oplag–px values of + 0.4 to + 0.6‰o, indicating internal O–isotope equiliburium at temperature of ∼1100–850° C. The magenesian basalts have magmatic 18O/ 16O ratios similar to those of mid–ocean ridge basalt, and the O–isotope ratios of compositionally evolved derivative melts show no evidence for contamination of the galunggung magmas by 18O–rich crust during differentiation. Andesites and transitional mafic and sites have a more variable O–isotope character, with laves and phenocrysts having both higher and lower δ 18O values than observed in the parental magnesium basalts. These features are interpreted to reflect intramagma chamber processes affecting the upper portions of the differentiating Galunggung magma body before the 1982–83 eruption.

Tertiary extension‐related volcanism, Old Woman Mountains area eastern Mojave Desert, California

Tertiary volcanism in the Old Woman Mountains area, eastern California overlapped temporally and spatially with lithospheric extension during the early Miocene. Field relations and restoration of movement along major faults indicate that the primary locus of magmatism probably lay along or flanked the axis of the Piute Mountains. The maximum age of volcanism is unknown but is probably about 20–21 Ma (after regional extension began). Most of the volcanism terminated after emplacement of the 18.5 Ma Peach Springs Tuff. Contrary to conventional models which suggest that extension‐related volcanism is basaltic or bimodal, the volcanic rocks of the Old Woman Mountains area represent a calc‐alkaline continuum dominated by mafic andesite; the most mafic rocks are mildly alkalic. Compositional variability within the suite is the result of mixing a mantle component with silicic crust, accompanied initially by minor olivine fractionation. Petrographic and field observations indicate that both magma mixing and assimilation of granitic crust have operated to produce the observed chemical variation. Trace elements and isotopes preclude binary mixing and suggest that mixing involved several crustal end members. This is consistent with the heterogeneous nature of both the exposed basement and a lower crustal xenolith suite sampled by a Tertiary dike. The mildly alkalic nature of the mafic lavas, the enrichment in large ion lithophiles and light rare earth elements, and the relatively radiogenic character of the most primitive uncontaminated basalt suggest a source similar to enriched mantle lithosphere. Alternatively, these geochemical and isotopic signatures could be attributed to contamination of asthenospheric basalt with mafic lower crust or low‐degree partial melts of this crust.

Geochemical and isotopic variations in a bimodal magma System: The Kaiser Spring Volcanic Field, Arizona

Volcanism in the Kaiser Spring field of west‐central Arizona postdates detachment faulting and crustal extension in the Basin and Range Province to the south. The volcanic products of this field include a diverse assemblage of rhyolitic and basaltic lavas that include topaz‐ and garnet‐bearing high‐silica rhyolites (HSR), hornblende‐phyric low‐silica rhyolites (LSR), tholeiitic basalts (TB) that vary from subalkaline to alkaline, alkali‐olivine basalts (AOB), and a quartz‐bearing mafic andesite (QMA). Extrusion of HSR lavas clearly preceded eruptions of LSR lava; however, both occurred concurrently with the effusion of basaltic lavas along the periphery of the silicic field. Compared with LSR lavas, the HSR are compositionally evolved, being strongly depleted in feldspar‐compatible elements and having a variable, but high, content of incompatible elements. Rayleigh fractionation calculations and analyses of LSR matrix glass are consistent with derivation of HSR magmas by extensive crystallization of LSR parent melts. Both LSR and HSR lavas are characterized by εNd of −6 to −12, 87Sr/86Sri of 0.7056 to 0.7086, and unradiogenic 206Pb/204Pb of 16.96 to 17.32. Differentiated mafic lavas have Mg values of 54 to 58. The AOB is isotopically similar to basalts from the San Francisco volcanic field, having εNd of +1.2, 87Sr/86Sr of 0.7042, and 206Pb/204Pb of 18.18. Two samples of TB differ markedly from the AOB, having εNd of −6.0 and −7.7, 87Sr/86Sr of 0.7073 and 0.7082, and 206Pb/204Pb of 17.87 and 18.39. With the exception of lower 206Pb/204Pb values, the TB lavas are isotopically similar to lower crustal mafic xenoliths from central Arizona. The QMA has εNd of −2.0 and 87Sr/86Sr of 0.7048, but a Pb isotopic composition that is similar to that of the rhyolites. At least three components are needed to model the combined Sr, Nd, and Pb isotopic compositions of the Kaiser Spring basalts and rhyolites. The isotopic composition of the AOB is consistent with its derivation from an OIB‐like subcontinental mantle reservoir. The rhyolites were derived primarily from a comparatively unradiogenic crustal source, perhaps coupled with subordinate mantle input. The QMA formed by mixing of this same crustal component with mantle‐derived basalt. The isotopic compositions of the TB lavas indicate that a second crustal component contributed to their geochemistry. This component may be mafic lower crust that is similar to amphibolite and eclogite xenoliths from central Arizona. The TB may represent contaminated mantle‐derived melts or melts formed by lower crustal anatexis. The Pb isotopic compositions of the HSR, LSR, TB, and QMA compare favorably with the isotopic composition of local Precambrian granitoids, and indicate that the crustal components of the Kaiser Spring lavas belong to the Mojave crustal terrane.

Summary of pre-1980 tephra-fall deposits erupted from Mount St. Helens, Washington State, USA

Mount St. Helens has been a prolific source of tephra-fall deposits for about 40 000 years. These tephra deposits (1) record numerous explosive eruptions, (2) form important regional time-stratigraphic marker beds, and (3) record repeated changes in composition within and between eruptive periods. Recognized tephra strata record more than 100 explosive eruptive events at Mount St. Helens; those tephra strata are classified as beds, layers, and sets. Tephra sets, each of which consists of a group of beds and layers, define in part the nine eruptive periods recognized at the volcano. Individual tephra sets are distinguished from stratigraphically adjacent sets by differences in composition or by evidence of clapsed time. Several tephra units from Mount St. Helens form important marker beds at distances of hundreds of kilometers downwind from the volcano. Cummingtonite phenocrysts, which are known in ejecta from only Mount St. Helens in the Pacific Northwest, characterize some marker beds and readily identify their source. The tephra sequence also records eruption of the mafic andesites that mark the appearance of the modern Mount St. Helens and numerous changes in composition among dacite, basalt, and andesite since that time.

Eruptive history of Mount Mazama and Crater Lake Caldera, Cascade Range, U.S.A.

New investigations of the geology of Crater Lake National Park necessitate a reinterpretation of the eruptive history of Mount Mazama and of the formation of Crater Lake caldera. Mount Mazama consisted of a glaciated complex of overlapping shields and stratovolcanoes, each of which was probably active for a comparatively short interval. All the Mazama magmas apparently evolved within thermally and compositionally zoned crustal magma reservoirs, which reached their maximum volume and degree of differentiation in the climactic magma chamber ∼ 7000 yr B.P. The history displayed in the caldera walls begins with construction of the andesitic Phantom Cone ∼ 400,000 yr B.P. Subsequently, at least 6 major centers erupted combinations of mafic andesite, andesite, or dacite before initiation of the Wisconsin Glaciation ∼ 75,000 yr B.P. Eruption of andesitic and dacitic lavas from 5 or more discrete centers, as well as an episode of dacitic pyroclastic activity, occurred until ∼ 50,000 yr B.P.; by that time, intermediate lava had been erupted at several short-lived vents. Concurrently, and probably during much of the Pleistocene, basaltic to mafic andesitic monogenetic vents built cinder cones and erupted local lava flows low on the flanks of Mount Mazama. Basaltic magma from one of these vents, Forgotten Crater, intercepted the margin of the zoned intermediate to silicic magmatic system and caused eruption of commingled andesitic and dacitic lava along a radial trend sometime between ∼ 22,000 and ∼ 30,000 yr B.P. Dacitic deposits between 22,000 and 50,000 yr old appear to record emplacement of domes high on the south slope. A line of silicic domes that may be between 22,000 and 30,000 yr old, northeast of and radial to the caldera, and a single dome on the north wall were probably fed by the same developing magma chamber as the dacitic lavas of the Forgotten Crater complex. The dacitic Palisade flow on the northeast wall is ∼ 25,000 yr old. These relatively silicic lavas commonly contain traces of hornblende and record early stages in the development of the climatic magma chamber. Some 15,000 to 40,000 yr were apparently needed for development of the climactic magma chamber, which had begun to leak rhyodacitic magma by 7015 ± 45 yr B.P. Four rhyodacitic lava flows and associated tephras were emplaced from an arcuate array of vents north of the summit of Mount Mazama, during a period of ∼ 200 yr before the climactic eruption. The climactic eruption began 6845 ± 50 yr B.P. with voluminous airfall deposition from a high column, perhaps because ejection of ∼ 4−12 km 3 of magma to form the lava flows and tephras depressurized the top of the system to the point where vesiculation at depth could sustain a Plinian column. Ejecta of this phase issued from a single vent north of the main Mazama edifice but within the area in which the caldera later formed. The Wineglass Welded Tuff of Williams (1942) is the proximal featheredge of thicker ash-flow deposits downslope to the north, northeast, and east of Mount Mazama and was deposited during the single-vent phase, after collapse of the high column, by ash flows that followed topographic depressions. Approximately 30 km 3 of rhyodacitic magma were expelled before collapse of the roof of the magma chamber and inception of caldera formation ended the single-vent phase. Ash flows of the ensuing ring-vent phase erupted from multiple vents as the caldera collapsed. These ash flows surmounted virtually all topographic barriers, caused significant erosion, and produced voluminous deposits zoned from rhyodacite to mafic andesite. The entire climactic eruption and caldera formation were over before the youngest rhyodacitic lava flow had cooled completely, because all the climactic deposits are cut by fumaroles that originated within the underlying lava, and part of the flow oozed down the caldera wall. A total of ∼ 51−59 km 3 of magma was ejected in the precursory and climactic eruptions, and ∼ 40−52 km 3 of Mount Mazama was lost by caldera formation. The spectacular compositional zonation shown by the climactic ejecta — rhyodacite followed by subordinate andesite and mafic andesite — reflects partial emptying of a zoned system, halted when the crystal-rich magma became too viscous for explosive fragmentation. This zonation was probably brought about by convective separation of low-density, evolved magma from underlying mafic magma. Confinement of postclimactic eruptive activity to the caldera attests to continuing existence of the Mazama magmatic system.

Content and behavior of fluorine in Japanese quaternary volcanic rocks and petrogenetic application

Fluorine contents in about 160 representative Quaternary volcanic rocks and 15 hornblende and biotite phenocrysts in a calc-alkali series in Japan have been determined by a selective ion-electrode method. Tholeiites have the lowest contents and the narrowest range (58–145 ppm), while alkali basalts have the highest contentws and the widest range (301–666 ppm), high-alumina basalts have intermediate values (188–292 ppm). F contents in basalts clearly increase from east to west across the Japanese Islands, as do alkalies, P 2O 5 REE, U, Th and H 2O. The volcanic rocks studied are divided into two groups on the basis of F: (1) witt, increasing % SiO 2 or advancing fractionation, F contents show either progressive enrichment; or (2) with increasing fractionation, F contents show rather constant values. The former is produced by fractionation of anhydrous phases from basalt to mafic andesite magmas; the tholeiite series of Nasu volcanic zone (outer zone), northeastern, Japan is a typical example. The latter group is derived through separation of amphibole-bearing phases from basaltic magmas at various depths from upper mantle (about 30 km) to upper crust; the alkali series in southwestern Japan and the calc-alkali series of Chokai volcanic zone (inner zone), northeastern Japan, are examples.

VOLCANIC ROCKS FROM KAYO-DAKE VOLCANO, AKITA PREFECTURE

Kayo-dake volcano is located in Akita Prefecture and is one of the volcanoes of Nasu Volcanic Zone. The main stage of volcanic activity was in the Early Quaternary. The volcano consists of lavas and a small amount of pyroclastic rocks. They are basalt, mafic andesite and andesite which belong to the tholeiite series and andesite of the calc-alkali rock, series. The rocks of the tholeiite series surpass those of the calc-alkali rock series in quantity. In the beginning of volcanic activity, basalt and mafic andesite of the tholeiite series were effused, and then, andesite belonging to the same rock series was erupted. After an intermission of activity, andesite of the calc-alkali rock series forming parasitic volcanoes, lava flows from a central crater, and a central cone, was effused. It may be a distinctive feature of this volcano that a comparatively large amount of andesite of the tholeiite series is present.

Eruption of Akita-Komagatake volcano

The Akita-Komagatake volcano (1637.4m above sea level), located on the border between Akita and Iwate prefectures, northeastern Japan, crupted suddenly on September 18th, 1970. The eruption consisted of small amounts of lava flow and volcanic ejecta and occurred at the summit crater on the Medake central cone inside the Akita-koma-gatake, a Glen Coe-type caldera. Petrographically, the rock consists of phenocrystic plagioclase (An94-62), olivine, hypersthene, augite and magnetite set in the groundmass of plagioclase, augite, hypersthene, pigeonite, magnetite and brown glass. There are conspicuous difference between this new lava and all of the previous lavas from this volcano. Namely, the new lava belongs to the Kuno's hypersthenic rock series (calc-alkaline series) and is intermediate andesite in composition, whereas the previous lavas belong to the pigconitic rock series (theoleiitic series) and are basalts and mafic andesites.

Petrogenesis of the lavas of the Usu volcano, Hokkaido, on the basis of some fractional crystallization process

Detailed petrographic descriptions of the lavas from Usu volcano were made already by several researchers, but there has been no satisfactory petrological consideration on the remarkable difference in chemical composition between somma and dome-lavas. The somma-lavas, forming a stratocone, are tholeiitic basalt or mafic andesite (SiO2=49.36-53.46%); on the other hand, the succeeding dome-lavas and pumice are dacite or rhyolite (SiO2=68.89-73.04%). The present paper have applied Osborn's (1959) idea, which proposes two fractional crystallization trends depend on the Po2 conditions, to interprete the genesis of the Usu lavas. If the chemical compositions of the lavas are plotted in the FeO+Fe2O3/FeO+Fe2O3+MgO-SiO2 diagram, the sommalavas lie on a steep ascending trend while the dome-lavas lie on a gentle one. With some assumptions, the idealized fractional crystallization of the parental magma can be calculated as follows; (1) The genesis of somma-lavas; Fractional crystallization on the tholeiite trend. (2) The genesis of dome-lavas; Fractional crystallization on the talc-alkali rock trend. For the dome-lavas, the trend of FeO and Fe2O3 contents could not be shown as a straight line; to maintain Po2, 0.4% oxygen must be added to the andesitic magma. After all, primary magma is solidified as crystal aggregates and residual felsic lava in proportions as shown in Figure 4.

Petrology of Nasu volcano

Nasu volcano, about 150km. north of Tokyo, is a group of large stratovolcanoes arranged in a N-S direction. The highest peak (Chausudake) is 1917m above sea level. This volcano occupies a significant position geologically, dividing the Nasu volcanic zone into the southern and northern subzones. The basement rocks are composed of Cretaceous granitic rocks, Neogene sediments, and Quaternary welded dacitic tuffs. The activity of Nasu volcano started in the latest Pleistocene age. Tremendous volumes of basalts and mafic andesites of the tholeiite series were erupted during the first stage and the main part of the volcano is composed of them. The second stage was followed successively by eruption of intermediate andesites of the calc-alkalic rock series associated with small amounts of mafic andesites of the tholeiite series, resulting in the peaks of each stratovolcano. On the surrounding part of Chausudake which was built in the latest stage, solfataric activity is still vigorous, depositing large amounts of sulfur. The chemical analyses of fifteen representative rocks of Nasu volcano, including five new analyses are given in Table 1. The compositions of both tholeiite and calc-alkalic rock series are characterized by high CaO and low alkalies. However, the difference between them is clearly shown when they are plotted on the MgO-FeO+Fe2O3-Na2O+K2O diagram, and each series of Nasu volcano is distributed within the fields of the tholeiite and calc-alkalic rock series of the Nasu northern subzone.