Bronzite Andesite Research Articles

Eocene volcanism during the incipient stage of Izu–Ogasawara Arc: Geology and petrology of the Mukojima Island Group, the Ogasawara Islands

AbstractThe Ogasawara Islands mainly comprise Eocene volcanic strata formed when the Izu–Ogasawara–Mariana Arc began. We present the first detailed volcanic geology, petrography and geochemistry of the Mukojima Island Group, northernmost of the Ogasawara Islands, and show that the volcanic stratigraphy consists of arc tholeiitic rocks, ultra‐depleted boninite‐series rocks, and less‐depleted boninitic andesites, which are correlatable to the Maruberiwan, Asahiyama and Mikazukiyama Formations on the Chichijima Island Group to the south. On Chichijima, a short hiatus is identified between the Maruberiwan (boninite, bronzite andesite, and dacite) and Asahiyama Formation (quartz dacite and rhyolite). In contrast, these lithologies are interbedded on Nakodojima of the Mukojima Island Group. The stratigraphically lower portion of Mukojima is mainly composed of pillow lava, which is overlain by reworked volcaniclastic rocks in the middle, whereas the upper portion is dominated by pyroclastic rocks. This suggests that volcanic activity now preserved in the Mukojima Island Group records growth of one or more volcanoes, beginning with quiet extrusion of lava under relatively deep water followed by volcaniclastic deposition. These then changed into moderately explosive eruptions that took place in shallow water or above sea level. This is consistent with the uplift of the entire Ogasawara Ridge during the Eocene. Boninites from the Mukojima Island Group are divided into three types on the basis of geochemistry. Type 1 boninites have high SiO2 (>57.0 wt.%) and Zr/Ti (>0.022) and are the most abundant type in both Mukojima and Chichijima Island Groups. Type 2 boninites have low SiO2 (<57.1 wt.%) and Zr/Ti (<0.014). Type 3 boninites have 57.6–60.7 wt.% SiO2 and are characterized by high CaO/Al2O3 (0.9–1.1). Both type 2 and 3 boninites are common on Mukojima but are rare in the Chichijima Island Group.

Origin of Late Oligocene to Middle Miocene Adakitic Andesites, High Magnesian Andesites and Basalts from the Back-arc Margin of the SW and NE Japan Arcs

Late Oligocene to Middle Miocene adakitic andesites are found in the southern part of Okushiri Island, the northern Noto Peninsula and in the Toyama region in the present-day back-arc margin of the SW and NE Japan arcs. On Okushiri Island, adakitic andesite is accompanied by moderately alkaline basalt, whereas on the Noto Peninsula, adakitic andesite has been erupted along with high magnesian andesite (HMA), bronzite andesite and tholeiitic basalt. Adakitic andesites from all three locations are characterized by high Sr/Y and low Y, and have higher MgO contents than adakitic melts generated by experimental melting of metabasalt and amphibolite. They also have higher Ni and Cr contents than either Archaean tonalite–trondhjemite–granodiorite (TTG) suites or Early Cretaceous adakitic granites, which have been attributed to partial melting of subducted oceanic crust. The Noto Peninsula adakitic andesite has Sr and Nd isotopic compositions identical to normal mid-ocean ridge basalt (N-MORB), whereas the Okushiri Island and Toyama adakitic andesites are more isotopically primitive than N-MORB. The Noto Peninsula primary adakitic melt was derived from subducted oceanic N-MORB crust, whereas the Okushiri Island and Toyama primary adakites are interpreted as melts of subducted N-MORB and sediment that have subsequently interacted with the overlying mantle wedge peridotite. To explain the comagmatism of adakite, HMA and basalt, the following model is proposed. A hydrated adakitic diapir ascends from the subducting slab and is heated because it enters the overlying hot mantle wedge. The subsequent establishment of thermal and H2O gradients in the adakitic diapir and surrounding mantle wedge peridotite results in concurrent generation of adakitic andesite magma in the inner adakitic diapir region (low temperature and high H2O content), HMA and bronzite andesite magmas in the intermediate peridotite region (intermediate temperature and H2O content), and tholeiitic basalt magma in the outer peridotite region (high temperature and lower H2O content). Comagmatic adakite and mildly alkaline basalt are found in cooler and wetter adakitic diapirs and hotter and drier peridotite regions respectively. The most likely tectono-magmatic situation for the genesis of adakitic magmas in this example of a cool subduction zone involves upwelling of hot asthenosphere into the subcontinental lithosphere beneath the back-arc side of the NE Japan arc and northern end of the SW Japan arc, during the period spanning the pre-Japan Sea opening to syn-opening stages. The unusually high temperature conditions established in the mantle wedge owing to upwelling of hot asthenosphere caused partial melting of the relatively cool subducting Pacific plate, resulting in the generation of adakitic magmas.

Rainfall-induced landslides in the residual soil of andesitic terrain, western Japan

Rainfall triggered landslides are frequent problems in the residual soil of andesitic terrain in western Japan. Characteristics of residual soils over bronzite andesite, procedure of in situ permeability measurement, matric suction and soil moisture content change and stability analyses considering unsaturated-saturated soils as integral system are presented in this paper. The paper highlights two landslides of small andesitic hillock of western Japan and describes modelling of rainwater seepage, slope stability analysis and contributing parameters for landsliding in andesitic terrain. For both landslides, results of geomorphological and geotechnical analyses were used as a direct input to the numerical modelling. For transient conditions, a finite element analysis was used to model the fluctuations in pore water pressure during the rainfall, with the computed hourly rainfall rate as the surface boundary condition. This was then followed by the slope stability analysis using the temporal pore water pressure distributions derived from the seepage analysis. Obtained trend for the factor of safety indicates that the most critical time step for failure was a few hours following the antecedent moisture content of previously day peak rainfall. Time of failure estimated by modelling has shown good match with time declared by eyewitnesses.

Petrology and Geochemistry of Boninite-Series Volcanic Rocks, Chichi-Jima, Bonin Islands, Japan

An Eocene submarine boninite-series volcanic center is exposed on the island of Chichi-jima, Bonin Islands, Japan. Five rock types—boninite, bronzite andesite, dacite, quartz dacite, and rhyolite—were distinguished within the boninite volcanic sequence on the basis of petrography and geochemistry. Boninites contain high Mg, Ni, and Cr contents indicative of primitive melts, but have high Si contents relative to other mantle-derived magmas. All boninite-series rocks contain very low incompatible-element concentrations, and concentrations of high-field-strength elements in primitive boninites are less than half of those found in depleted mid-ocean ridge basalts. Abundances of large-ion lithophile elements are relatively high in boninite-series rocks, similar to the enrichments observed in many island-arc lavas. Trends for both major- and trace-element data suggest that the more evolved members of the boninite magma series were derived primarily through high-level fractional crystallization of boninite. Textural features, such as resorption and glomeroporphyrocrysts, and reverse chemical zonations suggest that magma mixing contributed to the development of the quartz dacites.

Peridotite xenoliths from Grenada, Lesser Antilles Island Arc

Ultramafic xenoliths comprising harzburgite, lherzolite (reacted harzburgite) and spinel-rich dunite, occur in alkali olivine basalts (M series) of Grenada in the Lesser Antilles island arc. Textures are protogranular, porphyroclastic and granular; the latter are restricted to dunites and areas of the harzburgites/lherzolites where interaction with host magma has occurred. Primary mineralogy comprises olivine, orthopyroxene, clinopyroxene, and spinel. Harzburgites are residual from a fractional partial melting event totaling ~≤22%. Infiltration of harzburgite by (and reaction with) basalt has produced: a wehrlite, with partial dissolution of primary spinel, an increase in the oxygen fugacity (ƒO2) from primary values 1–2 log ƒO2 units above the fayalite-magnetite-quartz (FMQ) buffer, to 2–2.5 log units above the buffer; reaction of orthopyroxene to form patches of intergrown olivine and clinopyroxene, and bronzite andesite glass (60 wt%, SiO2 18–20 wt% Al2O3 and 3–4 wt% Na2O) with flat to light rare earth element-depleted, chondrite-normalized abundances. Refertilisation of the mantle by reacting melts, producing a clinopyroxene-rich lithology, may form a source of ankaramitic (high-Ca) arc basalts.

Petrogenesis of the tholeiitic basalt, calc-alkaline basaltic andesite and high magnesian andesite lava succession of the Oligo-Miocene Anamizu Formation in northeastern Noto Peninsula, central Japan.

The Oligo-Miocene Anamizu Formation in the Ushitsu-Matsunami area consists of a lower volcano-sedimentary member and an upper volcanic member. The upper member is essentially composed of three lava series; tholeiite (basalt and basaltic andesite), calc-alkaline (basaltic andesite) and high-magnesian andesite (bronzite andesite), in ascending order. The tholeiitic basalt is further divided into K-poor and K-rich (K-feldespar bearing) types. The tholeiitic basaltic andesite has higher FeO*/MgO (∼3) than the others (1-1.5). The calc-alkaline basaltic andesite has higher Cr and Ni than tholeiitic basaltic andesite. Bronzite andesite contains higher MgO, Cr and Ni than common calc-alkaline andesite of the same SiO2 content (∼60 wt%), but lower than those of boninite and sanukite. All the rock series are depleted in HFSE (Nb and Ti) in comparison with N-MORB and OIB, suggesting typical subduction-related arc magmas. High Zr/Y ratios of the tholeiitic basalt resemble those of active continental margin magmas rather than island-arc magmas. The HFSE, Ni and Cr compositions indicate a progressive depletion or increasing degree of partial melting of the mantle wedge source in the order tholeiite basalt (basaltic andesite)→calc-alkaline basaltic andesite→bronzite andesite. Spinel in the three series shows different trends: Cr-poor (Cr# 0.49 to 0.54) in the tholeiitic basalt, fairly Cr-rich (Cr# 0.61) in the calc-alkaline basaltic andesite, and Cr-rich (Cr# 0.73) in the high-magnesian andesite. These trends indicate different mantle restites; lherzolite for the tholeiitic basalt and harzburgite for the calc-alkaline basaltic andesite and high-magnesian andesite, and increasing degree of depletion in the order as above. On the contrary, LILE and LREE exhibit a gradual enrichment of the source in the same order. Corresponding decrease of TiO2/K2O suggests that the enrichment has been due to addition of fluids derived from the descending slab. These data indicate that the K-poor tholeiitic basalt magma has been formed by partial melting of the lherzolitic upper mantle wedge under almost anhydrous conditions; whereas the K-rich tholeiitic basalt magma may have been produced by partial melting of the metasomatized lherzolitic mantle source under slightly hydrous conditions. The calc-alkaline and bronzite andesite magmas have been produced by partial melting of the hydrous, metasomatized, harzburgitic mantle wedge. The stratigraphy of the lava succession in the studied area (tholeiite basalt-basaltic andesite→calc-alkaline basaltic andesite→bronzite andesite) indicates that depletion and hydration (metasomatism) of the mantle source have progressed simultaneously.

瀬戸内海西部，芸予諸島の後期中新世火山岩類（芸予火山岩類）及び前‐中期中新世火山岩類（瀬戸内火山岩類）について

Miocene volcanic rocks were discovered in the Geiyo Islands, western Setouchi region of southwest Japan. These occur as dikes or volcanic necks with marginal pyroclastic rocks. The volcanic rocks are composed of high-alkali tholeiitic olivine basalt, olivine andesite and bronzite andesite, in which the olivine andesite from Oge-shima chemically corresponds to high magnesian andesite. Whole rock K-Ar age determinations were carried out on olivine basalts, olivine andesites and bronzite andesites. An olivine andesite from Osakishimo-shima and two bronzite andesites from Okamura-shima yielded late Miocene ages of 8.2±0.7 Ma, 7.8±0.4 Ma and 8.0±0.4 Ma, respectively. It follows that subalkaline volcanism distinctively occurred at late Miocene age in the western Setouchi region. Accordingly, we give a name of “Geiyo volcanic rocks” to these volcanic rocks, which include late Miocene high-alkali tholeiitic olivine basalt reported from the adjacent area. On the other hand, olivine basalts from Okubi-shima and Ko-oge-shima give middle Miocene ages of 12.8±0.6 Ma and 15.4±0.8 Ma, respectively. In addition, an olivine andesite from Oge-shima has been dated as 17.4±0.9 Ma, indicating early Miocene age. On the basis of the radiometric ages, petrography and whole rock chemistry, these volcanic rocks are considered to be a member of the Setouchi volcanic rocks.

Water content and hydrogen isotopic composition of glassy bronzite andesite (sanukite) and glassy rhyolite from Goshikidai and Kanayama, northeast Skikoku, Japan.

Water content and hydrogen isotopic composition of glassy bronzite andesite (sanukite) and glassy rhyolite from Goshikidai and Kanayama, northeast Skikoku, Japan.

Effects of admixtures and alkali compounds on alkali aggregate reaction

Tests of mortar bar expansion are reported with bronzite andesite reactive aggregate found in Japan. The effects of chemical admixtures, sodium alkalis and of fly ash and slag on expansion are presented. Chemical admixtures appeared to reduce expansion whereas alkali compounds caused large expansion with sodium salts more aggressive than potassium salts. Fly ash and slag were found effective in controlling expansion.

Concentration mechanisms of iron oxides and alumina in deep weathering crusts (Goshikidai, Kagawa, Western Japan)

Concentration of Fe-oxides and alumina in weathering processes are main geological reactions for lateritization and bauxitization, respectively. In western Japan, red-coloured soil formed by weathering processes developed in many places. This soil is composed of hydrous Fe-oxide minerals, hydrous alumina minerals and other minerals. It was formed in the upper part of deep weathering crust by weathering processes under some kind of sub-tropical climate, probably in the Pliocene. One of these occurrences is observed in the upper part of Goshikidai and Konodai, west part of Takamatsu city, northeast Shikoku Island, west Japan. A deep weathering crust is distributed on wide hilly plains ranging from 250 to 400 m a.m.s.l. in the northwestern region of Takamatsu city. Original rock of the weathering crust is bronzite andesite and glassy bronzite andesite, so-called ‘sanukite’. The andesites had been weathered under some special climate, and the geological age of the weathering is the same as above. The mineral assemblage and formation mechanism are similar to those of laterite and bauxite. The weathering crust developing in this region are subdivided into the three following zones: (1) A zone, composed of hydrous Fe-oxides and metahalloysite with small amounts of gibbsite and it is associated with white veins of metahalloysite; (2) B zone, composed of hydrous Fe-oxides and metahalloysite (some material is associated with α-cristobalite); and C zone, composed of metahalloysite or halloysite and α-cristobalite with relict crystals of feldspar and quartz, and some material is associated with montmorillonite. Chemical analyses of the materials of the three zones show the formation mechanisms of the weathering crust.

Magma mixing in boninite sequence of Chichijima, Bonin Islands

Boninite from Chichijima can be divided into two major types: (A) porphyritic type (phenocryst mode exceeds 1%) and (B) aphyric type (phenocryst mode is less than 1%). Both types are further subdivided by means of microphenocryst assemblages into five subtypes: (I) Olivine (Ol) + Clinoenstatite (Cen) + Bronzite (Brz); (II) Ol + Brz; (III) Brz; (IV) Brz + Pigeonite (Pig) + Augite (Aug); (V) Cen + Brz. There is a systematic correlation between these rock types and the bulk composition of aphyric boninites. The most magnesian boninites are of type I, and the least magnesian ones are of types III and V. Type II is slightly higher in CaO than type I at a given FeO ∗/MgO ratio. Type IV is distinct from the others for its high CaO and low incompatible element contents at a given FeO ∗/MgO ratio. Type IV cannot be derived from the others by crystallization differentiation. Compositional data for the other types of boninite define rectilinear compositional variations which lie on a tie-line connecting Fe-rich bronzite and primitive boninite. Microphenocrysts in equilibrium with boninite magma are more magnesian, so it would appear that the Fe-rich bronzite was in equilibrium not with boninite magma but with some evolved descendant (e.g. bronzite andesite magma). The variations of aphyric boninites are then best explained by a combination of mixing of primitive boninite and evolved descendants together with equilibrium crystallization processes. In order to account for this combination of processes, a density-stratified, open-system magma chamber of the type proposed for oceanic ridge systems seems appropriate. The upper part of the magma chamber is composed of differentiated (bronzite andesite) magma and the lower part consists of hot, dense and primitive boninite magma. Crystallization and sporadic overturn and mixing give rise to the spectrum of magma types erupted at the surface.

13J. K-Ar ages of bronzite andesites from Choshi and their tectonic significance.(Abstracts of Papers Presented at the Spring Meeting of the Society)

13J. K-Ar ages of bronzite andesites from Choshi and their tectonic significance.(Abstracts of Papers Presented at the Spring Meeting of the Society)

22P. Genesis of orthopyroxene phenocryst in the bronzite andesite of Goshikidai, Kagawa Prefecture.(Abstracts of Papers Presented at the Fall Meeting of the Society)

22P. Genesis of orthopyroxene phenocryst in the bronzite andesite of Goshikidai, Kagawa Prefecture.(Abstracts of Papers Presented at the Fall Meeting of the Society)

南関東第三紀火山岩類の化学組成とストロンチウム同位体比

The major elements and strontium isotopic ratios for nine Tertiary volcanic rocks from southern Kanto, central Japan have been determined. Four of them are bronzite andesites from Choshi, eastern end of Boso Peninsula, and the others are high-alkali tholeiite basalts from Kamogawa (three), South Boso Peninsula, and from Yokosuka (one), Miura Peninsula, and from Oiso (one), Kanagawa. All are submarine volcanic products and intruded into Miocene to Oligocene strata. The major elements of the bronzite andesites seem quite similar to those of the olivine andesites, one group of the contemporaneous volcanic rocks in Setouchi volcanic provinece. The initial ratios of 87Sr/86Sr in the Choshi bronizite andesites are for 0.70399-0.70417. In contrast, those in the olivine andesites in the Setouchi province are as high as 0.70494-0.70710. These data are indicative of upper mantle heterogenity for Sr isotopes beneath the Japan arc in Tertiary. The high-alkali tholeiite basalts have Sr isotopic ratios ranging from 0.7036 to 0.7039. Although some basalts from Kamogawa have the high initial ratios (0.70633-0.70576), this is probably due to the alteration by rock-sea water interaction.

Estimated primary composition of Matsuyama bronzite andesite and its H2O-solubility(Abstracts of Papers Submitted for the Meeting of the Society)

Estimated primary composition of Matsuyama bronzite andesite and its H₂O-solubility(Abstracts of Papers Submitted for the Meeting of the Society)

Igneous Geology of the Tokatoka District, Northland

Abstract All the igneous rocks and breccia bodies described belong to the Manukau Breccia of the Waitemata Group (Otaian), and they form small bosses, dikes, and sills intruded along north-north-west and east-north-east trends; no surface flows were found. The rocks have been divided into the following eight types: olivine nephelinite, olivine basalt, pyroxene basalt, bronzite andesite, augite andesite, augite-hypersthene andesite, hornblende andesite, and dacite. The mineralogy and petrography of each rock type is described, illustrating a mineralogical gradation from olivine basalt to dacite through part of a differentiation series; an unusual feature of the series is the high calcium content of the plagioclase and pyroxene mineral groups.

GEOLOGY OF THE MAYAGÜEZ AREA, PUERTO RICO

The Mayaguez area forms the southwestern corner of Puerto Rico, west of 67° W. and south of 18° 15′ N. One-third of the 640 square kms is covered by thick alluvium. Unconformities separate a basal complex, two sequences of highly folded igneous and sedimentary rocks, and a younger sequence of gently dipping sedimentary rock. The basal Bermeja complex contains serpentinite, silicified porphyritic volcanic rock with some sedimentary rock, and minor spilite, amphibolitized spilite, and amphibolite. It is exposed chiefly in some anticlinal cores in southwestern Puerto Rico. Limestone, mudstone, andesite, and basalt form the older folded sequence. The Rio Loco formation, bronzite andesite porphyry in part with pillow structures, was extruded perhaps in the Cenomanian. The Mayaguez group includes most of the rocks in southwestern Puerto Rico: the Yauco mudstone, Parguera limestone, Brujo limestone, Melones limestone, Maricao basalt, Sabana Grande andesite, and El Rayo volcanic rocks. The maximum possible age range is Turonian to Maestrichtian. The group ranges in thickness from about 800 m in the south to 3800 m in the north, and it varies in lithology from limestone in the south to mudstone and volcanic rock in the north, indicating a volcanic center to the north during that time. The second folded sequence contains andesitic volcanic rock, bedded tuff, and massive limestone. The San German formation (Maestrichtian) includes andesite, the Cabo Rojo agglomerate member, and the Cotui limestone member. The Jicara formation, massive limestone and bedded tuff, is Paleocene; there is one exposure of an unnamed ? Eocene marl. Post-Eocene limestone and conglomerate are also exposed in the area. The structure of the basement complex is obscured by its massiveness and by the cover of younger rocks. Two major deformations have affected the rocks of southwestern Puerto Rico since Cenomanian to Santonian time. In the Maestrichtian, the first of these formed folds with a N. 60° W. trend, asymmetric or overturned to the south. Near the south coast the folding of thin Mayaguez group rocks was probably influenced by trends in the Bermeja complex which caused deviations in the regional trends and also some overturning to the north. The San German formation, deposited unconformably on the eroded surface of the folded Mayaguez group, contains large allochthonous blocks of older and contemporaneous rocks. These blocks, up to 2 km by 1 km in exposure, were deposited by slumping or sliding due to gravity within and at the base and top of the San German formation near Lajas and San German. Most rocks in the blocks are extremely contorted and contain deformed Foraminifera. The San German and Jicara formations and perhaps the ?Eocene marls were deformed into gentle open folds trending east in the area covered by this report. Oligocene, Miocene, and younger sedimentary rocks have been tilted and uplifted. Large east-west left-lateral transcurrent faults cross the area, offsetting and offset by two sets of transverse faults (N. 45° E., N. 20° W.): most faults are probably Maestrichtian to Oligocene, although minor faulting has continued to the present. Dikes and sills of quartz diorite porphyry and mica-quartz dacite porphyry intrude the ?Maestrichtian San German formation and older units. A diorite plug cuts the Bermeja complex, and a granodiorite plug intrudes the Mayaguez group.