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.     

Alkali mapping of the japanese quaternary volcanic rocks

Of ca. 200 Quaternary volcanic centers in the Japanese islands, ca. 120 are characterized by K₂O and Na₂O concentrations of their ejecta normalized to SiO₂ = 60% K₂O values give a geographically well-defined trend of regularly increasing away from the volcanic front. Na₂O values show no geographical trend. Relationships between the SiO₂-normalized K₂O and the depth of the deep seismic zone (K-h relation) are different for different arcs. The Southwest Japan arc shows distinctly higher K₂O and less regular geographic trend than the others probably due to crustal contamination. In the normal segments of the Northeastern Japan belt, K₂O increases ca. 2.5 to 3.0% per 100 km depth of the seismic zone. Groups of volcanic centers near the junction between the Northeast Honshu and Izu-Mariana arcs give characteristically lower K₂O while Rishiri volcano far behind the arc junction between the Kurile and Honshu arcs also gives a low K₂O value.     

Major element frequency distribution of the Japanese Quaternary volcanic rocks

Published chemical analyses of the Japanese Quaternary volcanic rocks are being collected to build a comprehensive data file. About 1600 selected major element analyses which include all 13 components of the bulk chemistry of the lavas and pyroclastics are considered here. Care was taken to correct misprints in the published analyses through correspondence with the original authors or analysts. Analyses are weighted against the volume of the volcano on which rock the analyses are made. Total volume of the volcanic material thus treated in this paper amounts to 3900 km³ or about 80% of the total Quaternary volcanic materials. The volume-weighted histograms for the most components show a unimodal profile when the large-scale caldera-forming pyroclastic flow and fall deposits are disregarded. SiO₂ histogram shows a broad primary mode around 57 %. For many components, the large-scale pyroclastic deposits form a secondary mode at the positions corresponding to a felsic composition. This strongly suggests that the mode of generation and ascent of such calderaforming felsic magma is different from that of the ordinary cone- and dome-building basalt-andesite-dacite series of magmas. Number-of-analyses histograms for the same set of combinations reveals that number frequencies show quite similar profiles as those for the volume-weighted histograms for the Japanese Quaternary volcanics. The profiles of the histograms for the Northeast Japan arc is very much similar as those of the whole Japanese islands while those for Izu-Mariana and Southwest Japan arcs are different. The differences are due to the different proportions of the rocks belonging to tholeiitic, calc-alkalic and alkali rock series in each arc. The whole Japan histograms may be considered as a good representative of the chemical characteristics of the average Japanese Quaternary (less than the last half million years) volcanic rocks.     

江西盛源盆地橄榄玄粗岩系列火山岩长石矿物特征及成因信息

江西盛源盆地橄榄玄粗岩系列火山岩中的长石矿物特征为：斜长石斑晶具有反环带结构和交代净边结构,基质中存在大量的斜长石微晶,且在电子探针下研究发现基质中斜长石微晶具有钾长石环边的矿物学特点,为此类火山岩归属于橄榄玄粗岩系列火山岩提供了矿物学证据。通过对长石矿物组合特征进行研究,探讨该地区橄榄玄粗岩系列火山岩的成因以及成岩时的物理化学条件等方面的信息。     

On the behaviour of fluorine in volcanic processes

Volatile compunds play a very important role in both the evolution of magmas and the eruptive processes. Despite great interest in the knowledge of volatile distribution in magma chambers prior to eruptions, direct evidence of this kind is very difficult to obtain because a major quantity of gaseous species is released to the atmosphere during volcanic phenomena.Good estimates of volatile contents in magmas have been obtained by their distribution in coexisting mineral phases and microprobe analysis of glass inclusions; however, a sufficient set of data is not yet available to provide direct evidence of volatile concentrations in magma chambers before eruptive processes.Owing to their volatility, water, hydrogen, carbon dioxide, sulphur and chlorine compounds are largely concentrated in the explosive cloud. On the other hand, molecular species of fluorine, which are more soluble than water in magmatic melts, strongly partition into this latter phase rather than into the fluid. As a consequence, fluorine compounds are normally present in small concentrations in fumarolic gases and are not expected to degas appreciably from quenched volcanic products.With reference to the influence of weathering processes, recent research has shown that unaltered volcanic glasses have lost but a minor quantity of fluorine as a result of secondary reactions. Because of this, analytical data for fluorine in fresh igneous rocks would not differ significantly from the actual values pertaining to the magmatic stage.The distribution of fluorine in samples from Italy (Vesuvius, Vulcano, Lipari, Roccamonfina, Phlegraean Fields) and Greece (Santorini) appears to be correlated with the concentration of potassium, which is in accordance with theoretical assumptions and analytical results in other areas.According to experimental data, the presence of fluorine in magmas of silicate composition considerably enhances the solubility of water.Higher concentrations of water would mean a higher potential explosivity, and the possibility that the observed concentrations of fluorine can serve as a measure of different degrees of stored energy is considered.Contribution of C.N.R. Centro di Studio per la Mineralogia e la Geochimica dei Sedimenti.     

An outline of the geology and geochemistry,and the possible petrogenetic evolution of the volcanic rocks of the Tonga-Kermadec-New Zealand island arc

The Pleistocene-Recent volcanism of this arc extends nearly linearly NNE from northern New Zealand for some 2800 km. Along its western margin lies an active marginal basin (Lau Basin and Havre Trough) which has its southern termination in the Taupo volcanic zone (TVZ, New Zealand). The New Zealand arc segment is developed within a continental crust, whereas the Tonga-Kermadec segments are developed on a ridge system within the oceanic basin. Submarine morphology suggests that the Kermadec volcanoes represent a less advanced stage of evolution relative to those of Tonga.Magmas erupted within the TVZ are dominantly rhyolitic (≈16,000 km³) with subordinate andesites and rare high-alumina tholeiites and dacites. The Kermadec Islands are dominated by tholeiites and basaltic andesites, with subordinate andesites and dacites. The Tongan Islands are dominated by basaltic andesites, with locally developed andesites and dacites. These Tonga-Kermadec lavas are characterised by subcalcic groundmass clinopyroxenes, whereas the younger group of TVZ andesites contain groundmass hypersthene and augite.Geochemically, the TVZ andesites are systematically enriched (relative to those of Tonga-Kermadec) in “incompatible” elements (e.g. K, Rb, Cs, Ba, light REE, U, Th, Zr, Pb), are less Fe-enriched, and contain more radiogenic Sr and Pb (excepting certain ²⁰⁷Pb/²⁰⁴Pb compositions). The evidence points to crustal equilibration of the TVZ andesites prior to eruption.A complete overlap of major and trace element chemistry (including TiO₂) is observed between the Kermadec-TVZ tholeiites and basaltic andesites, and the ocean floor tholeiites of the Lau Basin. Compared to the Tongan lavas, those of the Kermadecs exhibit a greater degree of chemical variability, also reflected in the greater heterogeneity in their Pb isotopic compositions. Moreover, many of the Tonga-Kermadec basaltic andesites exhibit more depleted “incompatible” trace element abundances than the Kermadec and TVZ tholeiites.The “primary” magmas of this arc are interpreted to be of basaltic andesite type, derived from Benioff zone melting (essentially anhydrous), but extensively modified by low-pressure crystal fractionation processes. The Kermadec tholeiites are explained as products of relatively shallow upper mantle partial fusion induced during the earlier stages of diapiric rise of Benioff zone-derived magmas, which are sufficiently hot to intersect the peridotite solidus. This should result in the production and intermixing of a series of magmas extending from olivine tholeiite to basaltic andesite composition. The voluminous rhyolites of TVZ are interpreted as the products of crustal fusion involving Mesozoic sediments.     

Types of quaternary volcanic activity in Northeastern Japan

The northeastern Japan forms a typical arcuate structure with a remarkable zonal arrangement of many geologic features, including the distribution of Quaternary volcanoes. Thus two distinct zones of volcanoes are noted here: i. e., Nasu zone on the east and Tyokai zone on the west. Some of the volcanoes of Nasu zone are characterized by the presence of pumice flows or pumice falls, sometimes of very large scales. These pumice flows belong to the calc-alkali rock series. While pumice flows or falls are rare in the Tyokai zone, where they are present though on small scale. Migration of the center of activity is noted along linear fissures, running either from east to west, or north to south at some volcanoes of Nasu zone, and consequently large swarms of volcanoes are common in this zone. While central eruption with definite center is typical of the Tyokai zone. From the petrographical and petrochemical study on the lavas and pyroclastics the original magma of the Nasu zone is estimated to be tholeiitic, and that of the Tyokai zone high-alumina basaltic. Abundant calc-alkali rocks are derived from these parental magmas. Thus the difference in the volcanic activity should be ascribed to the different nature of the parental magmas or the magmas derived from them.     

Dispersion of uranium in accessory apatite in crystalline rocks and its possible petrogenetic meaning

The coefficient of variation for grain-by-grain fission track uranium analysis of apatites from igneous rocks seems to reflect the temperature of crystallization and the cooling rate. For metamorphic rocks the coefficient represents a complex record of the homogeneity of the source and of metamorphic neocrystallization. As a test case 41 West Carpathian rocks have been examined and the coefficients of variation for U in apatites found to be: granitic rocks 0.30–0.79, paragneisses 0.35–0.95, migmatites 0.55–0.87, and volcanic rocks 0.30–0.40. Most of the frequency distributions are lognormal, though for some cases a normal distribution gives a better fit, and some are incompatible with either of the two distributions.     

The origin ofBe in island-arc volcanic rocks

The presence of cosmogenic¹⁰Be (t_1/2 = 1.5 × 10⁶ y) in island-arc volcanic rocks has been interpreted as indicating that sediments are subducted to the depths of island-arc magma genesis [1,2]. We have measured¹⁰Be and⁹Be (the stable common isotope) in phenocryst and groundmass fractions separated from four Aleutian volcanic rocks.¹⁰Be and⁹Be covary linearly in three of the rocks proving that the¹⁰Be was incorporated prior to the formation of phenocrysts. In the other rock¹⁰Be and⁹Be do not covary; the lack of covariation indicates either that the rock incorporated¹⁰Be during weathering or that⁹Be-rich xenocrysts were incorporated by mechanical mixing. Our results strongly support the hypothesis of Brown et al. [1] and Tera et al. [2] that sediments containing¹⁰Be are an integral part of island-arc magmatic systems. As we cannot identify the depth at which the¹⁰Be was incorporated we still cannot completely rule out the possibility of assimilation of sediments or altered crust near the Earth's surface as a source of¹⁰Be.     

Pleistocene volcanic rocks from the Fly-Highlands province of western Papua New Guinea: A note on new Sr and Nd isotopic data and their petrogenetic implications

RB-Sr and Sm-Nd isotopic and trace-element-abundance values have been determined for 15 mafic and intermediate rocks from six Pleistocene volcanic centres of the Fly-Highlands province. ⁸⁷Sr/⁸⁶Sr and _{N d} values range from 0.70362 to 0.70540, and +1.9 to +5.9, respectively. These new data can be accounted for by contamination of mantle-derived magmas by the continental crust through which the magmas have risen. They do not, however, preclude derivation of some of the Sr and Nd from subducted crust, nor are they inconsistent with Sr and Nd enrichments having taken place by means of mantle metasomatic events. Nevertheless, there is no Benioff zone beneath the Fly-Highlands province (although there is geological evidence for Cretaceous subduction). A preferred interpretation is that uncontaminated, mantle-derived magmas are related to the Pliocene crustal uplift that caused the development of the highlands and which formed in response to a mid-Tertiary continent/island-arc collision.     

Palaeomagnetism of upper cretaceous volcanic rocks in Sicily

From Upper Cretaceous volcanic rocks of Southeast Sicily 107 cores from 19 sites were collected giving a mean palaeomagnetic pole position at 62°N, 223°E, A₉₅ = 5.4° after AF-cleaning. This pole agrees with the Upper Cretaceous pole of Northern Africa indicating that no large post-Cretaceous relative motion has occurred between Africa and Sicily.     

Peralkaline acid volcanic rocks of oceanic islands

Occurrences of peralkaline acid volcanic rocks on oceanic islands are reviewed. Peralkaline differentiates are usually associated with mildly alkaline or transitional basalts and often with related sodic intermediate rocks. A compositional gap between basaltic and salic rocks is not invariably present. Although a comenditic end member is more usual in the oceanic suites, pantellerites are particularly well developed on Socorro Island and also on Gran Canaria where they form extensive ignimbrite sheets. There may be a genetic distinction between peralkaline rocks of islands which lie near the crests of oceanic rises and those which are built on broad submarine plateaux.     

Palaeomagnetism of Cretaceous and Jurassic volcanic rocks in west Sicily

The mean palaeomagnetic pole position obtained from Upper Cretaceous rocks in west Sicily is at 21°N, 100°E (A₉₅ = 15°), and at 38°N, 67°E (A₉₅ = 31°) obtained from Middle Jurassic rocks. These pole positions are completely different from comparable pole positions for southeast Sicily and Africa and imply a clockwise rotation of west Sicily since the Upper Cretaceous of about 90° relative to southeast Sicily and Africa and also a clockwise rotation of about 60° relative to “stable” Europe. The sense of rotation of west Sicily is opposite to any known rotation of other crustal blocks in the central Mediterranean.     

The quaternary volcanic rocks of the Geghama highland, Lesser Caucasus, Armenia: Geochronology, isotopic Sr-Nd characteristics, and origin

This paper reports on isotope-geochronologic, petrologic, and isotope-geochemical (Sr-Nd) studies of Quaternary magmatism in the Geghama neovolcanic area, Armenia, Lesser Caucasus. According to these studies, the period of youngest volcanic activity in the region lasted about 700 000 years, from 700 ka to a few tens of thousands of years ago. We found the time limits of five discrete phases of Quaternary volcanism: I (about 700 ka), II (550?480 ka), III (190?150 ka), IV (110?70 ka), and V (later than 50 ka). These phases seem to have lasted a few tens of thousands of years and were separated by quiescent periods of comparable durations. The petrologic and isotope-geochemical characteristics of Geghama effusive rocks show that they belong to the bimodal association rhyolite-trachyandesite and basaltic trachyandesite; this association was largely generated by fractional crystallization of primary basite melts, with the assimilation of crustal material by deep magmas being much less important. The isotopic parameters of the volcanic rocks studied here (0.70410–0.70437 for ⁸⁷Sr/⁸⁶Sr and +3.3 to +4.0 for ?_Nd) are practically identical for intermediate to basic and acid varieties in this association and are similar to those for young basites in the other areas of the Lesser Caucasus; this circumstance indicates a common origin for all Quaternary magmatic formations in the region. The petrogenesis of these varieties probably involved a single mantle source of the OIB type with certain regional peculiarities in the composition of the melts it generated. An analysis of the locations of Quaternary volcanoes in central Armenia (Geghama and Aragats areas) and in the Kars plateau, which is adjacent to it in the west, provided evidence of an eastward lateral migration of youngest magmatic activity in the region over time. The latest episodes of this migration took place in the eastern Geghama area, which must be the first to produce eruptions in the future.     

Uranium in volcanic rocks from the central Andes

The distribution of U in volcanic rocks from two transects across the Central Andes (latitudes 27°–28° S and 16°–18° S, respectively) differs from that of K. For a given SiO₂, content of the rocks, K systematically increases with the distance from the trench, while the highest U abundances are found in the rocks overlying the thickest segments of the continental crust, which are situated in the middle parts of the transects. It is suggested that this variation of U reflects crustal contamination.