Potassium variation across the New Britain volcanic arc

Late Cainozoic volcanoes of the New Britain island arc overlie an inclined Benioff zone that extends to a depth of at least 580 km. The rocks are tholeiitic basalt, andesite, dacite, and rhyolite. Unlike many other examples of island arcs described in the literature, K₂O contents in rocks with the same SiO₂ content do not increase progressively as depth,h, to the New Britain Benioff zone increases. The most complex relationships between K₂O, SiO₂, andh are shown by volcanoes overlying the deeper part of the Benioff zone. In these, the K₂O contents of rocks containing more than about 60% SiO₂,decrease as depth to the Benioff zone increases. The New Britain volcanic arc provides a striking exception to the generalisation thatK-h relationships are essentially similar in all island arcs.     

The magmatic evolution of the Seiland province,and Caledonian plate boundaries in northern Norway

The synorogenic intrusive activity of the eastern part of the Seiland province evolved from tholeiitic basalt low in K and Ti, through high-K calc-alkaline magmas and possible transitional basalt, to alkaline olivine basalt and picrite; finally, highly differentiated alkaline magmas and carbonatites were emplaced. For intrusive rocks with equivalent SiO₂ contents, K₂O, K₂O + Na₂O and K₂O/Na₂O increased with time, and the degree of iron enrichment in basaltic suites dimished. The western part of the province shows no equivalent evolution, tholeiitic magmas being emplaced at the same time as calc-alkaline magmas to the east.The magmatism is believed to have stemmed from a diapiric complex established in the mantle above a Benioff zone dipping to the east beneath the deforming Andean-type margin of the Baltic plate. Tectonic shortening of the continental edge and rearward movement of the underthrust plate relative to the asthenosphere resulted in migration of the plate junction, steepening of the seismic zone, and increasing depth to the magmagenetic region.     

On the lateral variations in chemical composition and volume of quaternary volcanic rocks across Japanese arcs

The Fe/Mg+Fe) ratios (X_Fe) of the Quaternary basalts (SiO₂ < 53 wt.%) in the Japanese arcs were examined. The X_XFe of relatively magnesian basalts decreases from the volcanic front toward the Japan Sea across the arcs. Based on the partition coefficient of Mg-Fe²⁺ between olivine and liquid, it is suggested that all the basalts near the volcanic front, which are mostly tholeiitic basalts, are significantly fractionated, whereas many basalts near the Japan Sea, which are mostly alkali basalts, are little fractionated. The K₂ O content in the primary basalt magmas increases toward the Japan Sea. Combining the X_Fe and K₂ O data, it is suggested that relatively large amounts of tholeiitic magmas are produced near the volcanic front, but they fractionate during their ascent, whereas smaller amounts of alkali basalt magmas are formed near the Japan Sea, but they can ascend with less fractionation. The density of primary tholeiite magma is significantly larger than that of primary alkali basalt magmas. It is most likely that primary tholeiite magmas cannot ascend beyond the upper crust and would fractionate to produce less dense tholeiitic magmas near the volcanic front, whereas primary alkali basalt magmas can ascend through the upper crust without fractionation, as far as buoyancy is the principal ascending force. In the Japanese arcs, the stress field may be less compressional near the Japan Sea than near the volcanic front, so that magmas can ascend more rapidly in the latter region than in the former. These two factors may be responsible for the above mentioned chemical variations of basalt magmas across the arcs. The variation in volume of the Quaternary volcanic rocks across the arcs can be explained by the presence of a melt-rich zone above but nearly parallel to the subducted slab.     

The Clarno Formation of central Oregon,U.S.A. — Volcanism on a thin continental margin

The Clarno Formation (mostly Eocene) of central Oregon, U.S.A., was formed as North America moved westward over subducting Pacific Ocean crust. The Clarno is a volcanic and volcanogenic assemblage whose flow rocks show: a calc-alkaline pattern on a Harker diagram, K₂O-SiO₂ diagram, alkali-SiO₂ diagram, and AFM diagram; and a pattern transitional between calc-alkaline and tholeiitic on a SiO₂-FeO^*/MgO diagram. Its basalts are chemically similar to those of intra-oceanic island arcs (e.g., K₂O of 0.30%), but subaerial deposition of the entire formation plus differentiation to rocks of high SiO₂ and alkali contents indicate that the Clarno was formed on a continental margin. Comparison of the Clarno with other Pacific-margin volcanic suites indicates that the Clarno was formed on thin (20–30 km) continental crust overlying a subduction zone of about 120 km depth.     

Relation of lava compositions to volcano size and structure in El Salvador

Most of the lavas at the nine volcanic centers along the volcanic front of El Salvador are basalts, basaltic andesites and andesites. The compositional variation within and among these centers can be explained by fractionation processes within the crust. Cognate gabbroic inclusions found in the lavas have appropriate mineralogy (plagioclase, olivine, magnetite and augite) to be cumulates formed by fractional crystallization. Two main variation trends occur, depending on the proportion of plagioclase removal. The more common, or normal, trend has a high (> 55%) proportion of plagioclase being removed. A less common, Al-rich, trend has a low (40%) proportion of plagioclase being removed. The Al-rich trend is found only at volcanoes that lack large negative Bouguer gravity anomalies. These volcanoes are unlikely to have large shallow magma chambers and fractionation probably occurs deeper in the crust where plagioclase removal is inhibited.The incompatible element (Na₂O, K₂O, Rb, Ba) contents of lavas vary systematically with the volume of the volcanic centers. At the same level of SiO₂, large volcanic centers have higher incompatible element contents than small volcanic centers. This suggests that open system fractionation in a periodically refilled chamber is the controlling factor. The large difference in Ba contents of lavas between eastern (low) and western (high) El Salvador suggests a difference in the mantle source region.     

Major-element petrochemistry of some extrusive rocks from the volcanically active Mariana Islands

Volcanic rocks from six of the currently or recently active volcances of the Mariana Island are show little variation in major element abundances. SiO₂ content averages 51.5 wt.%. The flows are high in Al₂O (mean 17.7 wt.%) and Fe oxides (mean 10.1 wt.% calculated as FeO only), and moderate in MgO content (mean 4.7 wt.%), Na₂O (mean 2.7 wt.%), and K₂O (mean 0.7 wt.%). Only the rocks from Farallon de Pajaros, the northernmost of the Mariana Islands, deviate slightly from the average of the analyses. Three analyses from this island are slightly higher in SiO₂ (about 54 wt.%) and Al₂O₃, and are lower in total Fe oxides and MgO. According to preferred classification, the lavas of the Mariana Islands can be termed mela-andesites, high-alumina basalts, or calc-alkaline (orogenic) basalts. The K₂O values (mean 0.7 wt.%) obtained from lavas of the Mariana Islands are significantly higher than the K₂O values (about 0.33 wt.%) from volcanics of the Izu chain to the north. Inasmuch as the substantial scatter in location of earthquake foci beneath both arcs prevents accurate delineation of the upper boundary of the Benioff zone, it presently cannot be determined whether this discrepancy in K₂O values reflects a difference in depth from the volcanic are to the dipping seismic zone or relates to other phenomena. The older volcanic islands within the Mariana-Bonin island chain apparently defined an island arc system during Eocene to Miocene time. This indicates that the present plane of convergence between the Pacific plate and the Philippine Sea plate has defined the convergence between these plates since Eocene time.     

Volcanism and structure of southwestern Bolivia

Cenozoic volcanic activity started in southwestern Bolivia during the Oligocene. From the Early Miocene up to the Holocene, volcanism produced about 8000 km³ of lavas and pyroclastic rocks. Five major cycles could be distinguished.North—south-trending fissures in the eastern part of southwestern Bolivia were sources of Miocene ignimbrites. The eruptive centers of Pliocene ignimbrites are scattered throughout the investigated area, but are concentrated in the western part. During Pleistocene times small ignimbrite flows were formed by collapse of composite volcanoes.The oldest lavas (Oligocene) are alkaline and differ considerably from younger rocks, which are rhyodacites and dacites with only a small limited range of compositions. Based on the K₂O/Na₂O ratios and the mineralogical composition, the lavas can be subdivided into a calc-alkaline and a high-K calc-alkaline (shoshonitic) association. Hornblende is absent from lavas with high K₂O/Na₂O ratios.In southwestern Bolivia a westward migration of volcanic activity is apparent. The potassium content of the lavas decreases from the Miocene to the Holocene, whereas the sodium content increases. The potassium content cannot, therefore, be correlated with the depth of the Benioff zone. It is suggested, that the lavas and ignimbrites were formed by partial melting of material from different crustal levels.     

Lateral variations of H2O contents in quaternary magmas of Northeastern Japan

Volcanoes of the East Japan volcanic arc are divided into two groups on the basis of their phenocryst assemblages; volcanoes with lavas or pyroclastic rocks containing quartz phenocrysts and no hornblende phenocrysts (type A), and those with rocks containing hornblende phenocrysts and no quartz phenocrysts (type B). Type A volcanoes occur only in the narrow region along the volcanic front, whereas type B volcanoes are distributed in the area closer to the Sea of Japan.Recent experimental studies on calc-alkaline andesite-dacite under H₂O-saturated and -undersaturated conditions indicate that the liquidus temperature (maximum thermal stability limit) of quartz decreases drastically with increasing H₂O content in magma, whereas the liquidus temperatures of hornblende and biotite are relatively constant with variations in the H₂O content and bulk chemical composition of the magma.It is suggested from the lateral variation of mafic phenocryst assemblages [1] and from the above result that the temperature of the parental magmas of these volcanoes increases, and their H₂O contents decrease, towards the volcanic front in the East Japan volcanic arc.Such lateral variations in the H₂O contents of magmas under the East Japan volcanic arc are in agreement with those of other incompatible elements (K, Rb, REE, etc.). If H₂O-undersaturated partial melting of upper mantle peridotite can be represented by the univariant line (olivine, Ca-rich clinopyroxene, orthopyroxene and liquid coexist) in the system H₂OMg₂SiO₄z.sbnd;CaMgSi₂O₆z.sbnd;SiO₂, the decrease of H₂O content in the magma suggests that the melting temperature of the peridotitic mantle may gradually increase, and so the degree of partial melting may increase, towards the volcanic front. The lateral variation of other incompatible elements can also be explained by this model.     

A worldwide comparison of alkali olivine basalts and their differentiation trends

Worldwide alkali olivine basalts (AOB) and their differentiation series have been subdivided into continental, oceanic, or island-arc assemblages according to the inferred crustal environment at their time and place of eruption. No systematic differences have been found in major element composition of the AOB's from these three different environments. As plotted on (Na₂O + K₂O) vs. SiO₂ and AMF diagrams, AOB differentiation trends also show no differences between environments. Thus, AOB appears to be a primary magma generated at sufficient depth in the mantle that its major element content is unaffected by chemical or thermal differences between mantle regions underlying continents, ocean basins, or island arcs. The major element chemistry of AOB is also apparently unaffected by passage through different types of crust.     

Compositions of deep-sea ash layers derived from north pacific volcanic arcs: variations in time and space

Glass separates from 115 ash layers derived from the Kamchatkan (DSDP Site 192; 34 layers), the eastern Aleutian (DSDP Site 183; 56 layers) and the Alaska Peninsula (DSDP Site 178; 25 layers) volcanic arcs have been analyzed for up to 28 elements. In addition, the abundance and diversity of associated mafic phenocrysts have been evaluated. The resulting data set has made possible an evaluation of the late Miocene to Recent changes in composition of ashes derived from North Pacific volcanic arcs and of the factors controlling the evolution of highly siliceous magmas.We find no evidence for a general transition from arc tholeiite to calc-alkalic magma parentage of ashes derived from the volcanic arcs during the last 10 m.y., but instead find 0.1- to 0.5-m.y. intervals during which particular types of volcanism are prevalent. Most convincing is the transition from arc tholeiite to calc-alkalic for ashes derived from Kamchatka during the last 0.8 m.y., a change believed to be associated with a landward shift in the site of magma generation. Considered together, ashes derived from North Pacific volcanic arcs have been becoming more siliceous during the last 1.5 m.y. and may be associated with accelerated subduction during the same time interval.Hydrous phenocrysts (e.g., biotite) are typically associated with low-silica deep-sea ashes, but not with terrestrial volcanic rocks of comparable silica contents, suggesting the important role of water in the evolution of siliceous magma. REE patterns and relative abundances of mafic phenocrysts demonstrate the importance of fractional crystallization in controlling the evolution of highly siliceous arc magmas. REE increase with increasing silica, but become less concentrated in ashes with SiO₂ > 64%. Eu anomalies increase throughout the SiO₂ range. Initial fractionation is dominated by clinopyroxene and plagioclase with amphibole strongly influencing fractionation above 64% SiO₂.     

Discovery of double-peaking potassic volcanic rocks in Langshan Group of the Tanyaokou hydrothermal-sedimentary deposit,Inner Mongolia,and its indicating significance

Since the mid-1980s,Tanyaokou large Zn-Cu-Fe sulfides deposit,located at the southwest end of Langshan-Zhaertaishan-Bayan Obo Mesoproterozoic metallogenic belt in the west section of the northern margin of the North China Platform[1?9](Fig.1),has been confirmed to be submarine volcanic exhalative-sedimentary metamorphosed deposit hosted in the miogeosynclinal mud-carbonaceous formation of the Langshan Group(LG)[1],or submarine volcanic exha-lative-deposition-altered deposit[2]or stratabo…     

Dehydration of clastic sediments in subduction zones: Theoretical study using thermodynamic data of minerals

Pseudosections for two sediments and one basalt calculated in the system K₂O–Na₂O–CaO–MgO–FeO–Fe₂O₃–Al₂O₃–TiO₂–SiO₂–H₂O for the P–T range 10 to 35 kbar, 300 to 900°C give useful insights into the amount of H₂O released from oceanic crust in subduction zones. In cold subduction zones (20 kbar–300°C to 35 kbar–500°C) hydrous minerals storing 3 to 4 wt% H₂O are still present in metasediments at depths of 120 km. In the same environment, metabasite releases 1 wt% H₂O in the depth range 100 to 120 km, but 4.5 wt% H₂O is transported to greater depths. In hot subduction zones (300°C hotter than the cold subduction zone at 100 km depth), dehydration events of metasediments in the depth range 50 to 80 km correspond to the breakdown of chlorite and paragonite. In the calculations no further water is released at greater depths because the modal content of phengite, the only hydrous mineral phase at these depths, remains almost constant. For the same P–T path, metabasite shows continuous dehydration between 40 and 80 km releasing almost 3 wt% H₂O. At 120 km depth less than 0.4 wt% of H₂O remains. In an average modern subduction zone (～6°C/km) most dehydration of sediments occurs at depths of 70 to 100 km and that of basalts at depths of 80 to 120 km. Only 1.3 wt% H₂O in metasediments and 1.6 wt% H₂O in metabasalt has the potential to be subducted to depths greater than 120 km. The dehydration behavior of sediments concurs with the generally held idea that subduction zone fluids are most effectively transported to great depths by cold subduction. In hot subduction zones, such as those characteristic of early Earth, most H₂O carried by oceanic crust is liberated at depths less than 120 km and, thus, would not contribute to island‐arc magmatism.     

Temporal variation in the lavas of Mayon volcano, Philippines

Chemical and petrographic analyses of 51 sequential lava flows from the central vent of Mayon volcano show cyclical variation. In the two most recent cycles, from 1800 to 1876 and from 1881 to the present, one to three basaltic flows are followed by six to ten andesitic flows. Modal and whole-rock chemical parameters show the most regular cyclical variation; calculated groundmass chemical parameters vary less regularly. There is also a long-term trend, over approximately 1700 years of exposed section, toward more basic compositions.The cyclical variation in modes and the chemical composition of the lavas apparently results from periodic influxes of basaltic magma from depth into a shallow magma system. Fractional crystallization of olivine, augite, hypersthene, calcic plagioclase, magnetite and pargasitic hornblende produces successively more andesitic lavas until the next influx of basaltic magma. Differentiation in a deep zone of magma generation is not excluded by the data, but is more likely responsible for the overall change toward more basic compositions than for the cyclical variation.Three points in a cycle — the beginning of basaltic lavas, the beginning of andesitic lavas and a leveling-off of SiO₂, K₂ O and K₂O/Na₂O values — correspond roughly to the beginning of frequent effusive eruptions (with or without an early Plinian eruption), frequent weak to moderately explosive (Strombolian) eruptions, and less frequent explosive (Vulcanian) eruptions, respectively. Recognition of the current stage in a cycle can give a qualitative indication of the nature of forthcoming eruptions. Changes in several specific parameters may precede basaltic lavas and allow early detection of basaltic influxes. These include minima in the glass inclusion/plagioclase phenocryst and phenocryst/groundmass ratios, vesicularity and groundmass TiO₂, a decrease in hypersthene phenocrysts, and constant values for the whole-rock K₂O/Na₂O ratio. The Mayon area is densely populated, making prediction of eruption type important for safety and land-use planning.     

Calc-alkaline and shoshonitic lavas from five Andean volcanoes (between latitudes 21° 45′ and 24°30′S) and the distribution of the Plio-Quaternary volcanism of the south-central and southern Andes

The Plio-Quaternary volcanic rocks of the south-central Andes (southward from latitude 18°S) contain two associations: calc-alkaline and shoshonitic which coincide with seismic belts as geographically distinct zones aligned parallel to the oceanic trench. There is a continuous gradation from calc-alkaline to shoshonitic associations. The shoshonitic association appears to the north of latitude 26°S; southwards, the calc-alkaline association directly abuts against the continental (Argentinian) alkaline association.Thirty-one lavas from the Plio-Quaternary calc-alkaline Socompa, Lascar, Sairecabur and Tocorpuri and shoshonitic Sierra de Lipez volcanoes were studied. The lavas are porphyric with abundant glass. The distribution and the nature of the phenocrysts vary according to the chemistry of the calc-alkaline lavas. Petrographic evidence for crystal fractionation has been observed. Occasional phenocrysts of alkali feldspars occur in the shoshonitic lavas. The K₂O and SiO₂ contents increase from calc-alkaline to shoshonitic lavas with distance away from the oceanic trench. In lavas from Socompa, Lascar, Sairecabur and Tocorpuri calc-alkaline volcanoes, K₂O, Li and Rb increase and K/Rb and Sr decrease with increasing SiO₂; Ba increases with decreasing Sr, probably as a result of plagioclase fractionation. In lavas from Sierra de Lípez shoshonitic volcano, SiO₂ is high, K₂O is high and rather constant and Li, Rb, Ba and Sr increase with increasing SiO₂. Bolivian shoshonitic lavas appear to be genetically related to the calc-alkaline suite.The calc-alkaline lavas may be derived by crystal fractionation from a parental magma of andesitic nature that originated in or above the subjacent Benioff zone.     

Nd and Sr isotopic study of volcanic rocks from Japan

Two older granitic rocks and some selected Quaternary volcanic rocks from the Japanese Islands were analyzed in a reconnaissance study for the purpose of examining the relationships between Nd and Sr isotopic abundances and the megatectonic structure around the Japanese Islands. Model ages of ～0.9 AE were determined by the Nd and Sr methods on a Paleozoic gneiss which confirms that a relatively ancient acidic basement exists in the Japanese Islands. The Nd and Sr isotopic data show that the Cretaceous granodiorite is the result of partial melting of older crust.The Nd of tholeiitic rocks from the Izu arc gives ε_Nd ranging from 8.3 to 9.3 and with the corresponding ε_Sr from ?14.5 to ?18.5. These results are identical to those found for the Mariana arc. These values are distinctly lower than typical MORB by around 1～2 εu. This difference in ε_Nd between arcs and MORB is attributed to the contribution of oceanic sediments to the partial melts produced during subduction of oceanic crust. The Hakone volcano is clearly confirmed as belonging to an oceanic source by Nd isotopic results.ε_Sr-ε_Nd values of the volcanics from a section along the Fossa Magna show a clear indication that they are a blend of oceanic mantle material and continental crustal material. The crustal component clearly increases in going from south to north. Volcanics across the Northeast Japan arc also show a distinct correlation of ε_Sr-ε_Nd related to the position relative to the active subduction zone but with the opposite trend. These relationships of the present isotopic pattern and the zonal arrangement relative to the subduction zone suggest the former existence of a local spreading center in the Japan Sea.In general there appear to be regular isotopic relationships between the Izu-Mariana oceanic island arc and the continental island arc of Japan which indicates that partially melted or assimilated older continental basement is admixed with young rising oceanic arc magmas.     

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, P₂O₅ REE, U, Th and H₂O.The volcanic rocks studied are divided into two groups on the basis of F: (1) witt, increasing % SiO₂ 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.     

Geochemical evidence for sundering of the West Mariana arc in miocene ash from the Parece Vela Basin

Glass and mineral fragments from discrete volcanic ash layers were sampled from DSDP/IPOD Site 450 in the Parece Vela Basin, Philippine Sea and analyzed by electron microprobe. The ashes are interpreted as eruptive products of the adjacent West Mariana arc system between 25 and 14 Ma B.P., and have compositions between basaltic andesite and rhyolite, and rarely, boninite. ‘Continuous’ chemical trends appear to reflect mixing of mafic and silicic magmas. ‘Discontinuous’ trends between these end-members are relatively few, and are consistent with ‘liquid lines’ produced by fractional crystallization. Andesitic tephra become progressively richer in MgO and CaO through the middle Miocene, while boninite appears towards the end of the sequence, between 14 and 15 Ma B.P. Coeval rhyolitic glasses become richer in K₂O and Na₂O, with maximum concentrations at about 15 Ma B.P. Chronologic changes in fractionation type and composition of parent magmas are interpreted to reflect the subaerial volcanic evolution of the West Mariana arc. The appearance of boninite is believed to signal early stages of arc sundering, and corresponds temporally with regional uplift of the sea floor above the carbonate compensation depth, precursor to a new pulse of back-arc spreading.     

Major and trace element provenance signatures in stream sediments from the Kando River, San'in district, southwest Japan

Abstract Basement rocks in the catchment of the Kando River in southwest Japan can be divided into two main groups. Paleogene to Cretaceous felsic granitoids and volcanic rocks dominate in the upstream section, and more mafic, mostly Miocene volcanic and volcaniclastic rocks occur in the downstream reaches. Geochemically distinctive Mount Sambe adakitic volcanic products also crop out in the west. X‐ray fluorescence analyses of major elements and 14 trace elements were made of two size fractions (<180 and 180–2000 µm) from 86 stream sediments collected within the catchment, to examine contrasts in composition between the fractions as a result of sorting and varying source lithotype. The <180 µm fractions are depleted in SiO₂ and enriched in most other major and trace elements relative to the 180–2000 µm fractions. Na₂O, K₂O, Ba, Rb and Sr are either depleted relative to the 180–2000 µm fractions, or show little contrast in abundance. Sediments from granitoid‐dominated catchments are distinguished by greater K₂O, Th, Rb, Ba and Nb than those derived from the Miocene volcanic rocks. Granitoid‐derived <180 µm fractions are also enriched in Zr, Ce and Y. Sediments derived from the Miocene volcanic rocks generally contain greater TiO₂, Fe₂O₃*, Sc, V, MgO and P₂O₅, reflecting their more mafic source. Sediments containing Sambe volcanic rocks in their source are marked by higher Sr, CaO, Na₂O and lower Y, reflecting an adakitic signature that persists into the lower main channel, where compositions become less variable as the bedload is homogenized. Normalization against source averages shows that compositions of the 180–2000 µm fractions are less fractionated from their parents than are the <180 µm fractions, which are enriched for some elements. Contrast between the size fractions is greatest for the granitoid‐derived sediments. Weathering indices of the sediments are relatively low, indicating source weathering is moderate, and typical of temperate climates. Some zircon concentration has occurred in granitoid‐derived <180 µm fractions relative to 180–2000 µm counterparts, but Th/Sc and Zr/Sc ratios overall closely reflect both provenance and homogenization in the lower reaches.     

Geology and geochemistry of lavas at Nekoma volcano: Implications for origin of Quaternary low-K andesite in the north-eastern Honshu arc, Japan

Abstract Nekoma volcano forms part of the arc axis volcanic array of the North-eastern Honshu arc, Japan, which is commonly characterized by medium-K lava suites. However, Nekoma is exceptional because many of its lavas are low-K. This anomaly has been a matter of debate. Nekoma was active from 1.1 to 0.35 Ma. The volcano consists of thick andesite flows and domes associated with block and ash flow deposits produced during lava dome formation. A horseshoe-shaped collapse caldera was formed at the summit and small lava domes extruded into the caldera. Stratigraphy, published K–Ar ages, and tephrochronology define three stages of volcanic activity, about 1.1 Ma (Stage 1), 0.8–0.6 Ma (Stage 2) and 0.45–0.35 Ma (Stage 3; post caldera stage). Low-K andesites occur in all stages. Extremely low-K andesite was also associated in Stage 2 and medium-K andesite was dominant in Stage 3. In general, lavas changed from low-K to medium-K after caldera formation. Geochemical study of the Nekoma lavas shows that both low-K and medium-K lavas are isotopically similar and were derived from a common source. Adatara and Azuma volcanoes, which lie close to Nekoma, also have both low-K and medium-K andesites. However, Sr isotope ratios or temporal-spatial variations in K-level lava classification vary between the three centers. Comparisons of K suites and Sr isotope ratios with frontal arc volcanoes in North-east–Honshu suggest source heterogeneity existed in both medium- and low-K suites. The K contents of lavas and their Sr isotopes are not simply related. This requires re-examination of models for chemical variation of andesites in arcs.     

Criticism of generalised models for the magmatic evolution of arc-trench systems

Recent geological and petrological results from the Lesser Antilles island arc and Papua New Guinea, and from other regions of arc-trench-type volcanism, provide notable exceptions to the spatial, volumetric, and temporal relationships claimed for generalised arc models. For example, many alkalic and shoshonitic associations do not appear to be developed over the deepest parts of downgoing slabs, and there are now several well-documented exceptions to the K₂O/SiO₂/depth-to-Benioff-zone relationship. Moreover, the temporal sequence of early tholeiitic → middle calcalkalic → late shoshonitic/alkalic is not well substantiated, although shoshonitic rocks do appear to be developed most commonly in regions with a long history of plate interactions. Exceptions to the generalised arc model are symptomatic of the need to look for the unique features of individual island arcs, rather than just similarities between different ones, so that the major factors controlling arc evolution may be determined.