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Voluminous mafic dacitic to rhyodacitic ignimbrite fields linked to giant calderas are distinctive features of the late Miocene to Quaternary magmatic record of the central Andean Altiplano–Puna plateau. The magmas erupted through a compressionally thickened crust over a generally shallow, but variably dipping subducting Nazca plate. New analyses including whole rock major and trace element concentrations (~ 175), 143Nd/144Nd (22) and 87Sr/86Sr (23) ratios and δ18O analyses on quartz phenocrysts (18) coupled with data from the literature provide a regional and temporal perspective on the chemical variability of Puna ignimbrites between 22°S and 27°S latitude. Ranges of Pb, Sr, Nd and δ18O isotopes, variability in Al/(K + Na + Ca), Na/K and trace element ratios, chemical parallels with Paleozoic magmatic rocks and published experimental constraints link crustal contributions in the ignimbrites to a more pelitic crust in the northern Puna, a more igneous gneiss-like crust in the southern Puna and a more amphibolite-like crust near the arc. Melting in the deep crust by injection of mantle-derived melts followed by magma rise, accumulation and evolution at depths near 25–20 km is supported by heavy REE evidence for deep crustal garnet-bearing residues, negative Eu anomalies superimposed on steep REE patterns, calculated bulk Sr distribution coefficients and Puna seismic images. Temporal trends towards less evolved isotopic ratios, metaluminous compositions, flatter REE patterns and less HFSE depletion in northern Puna ignimbrites suggest an evolving crustal magma source as mafic melts continued to enter the crust. Assimilation-fractional crystallization models for Sr and modeling of δ18O data are consistent with the large ignimbrites (> 500 km3 DRE) forming as near 50–50 hybrids of enriched mantle-wedge derived (87Sr/86Sr ~ 0.7055; 143Nd/144Nd ~ 0.5126) basaltic melts and spatially variable lower to mid-crustal melts with 87Sr/86Sr ratios from 0.715 to 0.745 at 300 to 125 ppm Sr and δ18O from ~ + 12‰ to ~ + 15‰. Given a 1:1 mantle to crustal ratio, a 3:1 to 5:1 plutonic/volcanic ratio and an ignimbrite volume near 11,000 km3, the mantle magma production rate to produce the ignimbrites is a distinctly non-flare-up-like rate of less than 20 km3/km/Ma when averaged across the Puna over 7 Ma.

article i nfo Taipei City, with a population of around 8 million, as well as two nuclear power plants is located in close proximity to the Quaternary, dominantly andesitic Tatun Volcanic Group (TVG) of Northern Taiwan. We have investigated the stratigraphy of the youngest volcaniclastic deposits, as well as the morphology of lava flows and domes of the TVG in order to reconstruct the character and timing of the most recent eruptions and related hazardous events in the area. Our data indicate that recent eruptions of the group were dominated by long-term, voluminous extrusions of crystal-rich, very viscous lavas. These eruptions formed closely spaced monogenetic domes and lava flows. Based on morphological parameters of the lava flows (thicknesses 80-150 m, lengths up to 5.6 km, and volumes up to 0.6 km 3

The Kamchatka Peninsula in far eastern Russia represents the most volcanically active arc in the world in terms of magma production and the number of explosive eruptions. We investigate large-scale silicic volcanism in the past several million years and present new geochronologic results from major ignimbrite sheets exposed in Kamchatka. These ignimbrites are found in the vicinity of morphologically-preserved rims of partially eroded source calderas with diameters from ∼ 2 to ∼ 30 km and with estimated volumes of eruptions ranging from 10 to several hundred cubic kilometers of magma. We also identify and date two of the largest ignimbrites: Golygin Ignimbrite in southern Kamchatka (0.45 Ma), and Karymshina River Ignimbrites (1.78 Ma) in south-central Kamchatka. We present whole-rock geochemical analyses that can be used to correlate ignimbrites laterally. These large-volume ignimbrites sample a significant proportion of remelted Kamchatkan crust as constrained by the oxygen isotopes. Oxygen isotope analyses of minerals and matrix span a 3‰ range with a significant proportion of moderately low-δ18O values. This suggests that the source for these ignimbrites involved a hydrothermally-altered shallow crust, while participation of the Cretaceous siliceous basement is also evidenced by moderately elevated δ18O and Sr isotopes and xenocryst contamination in two volcanoes. The majority of dates obtained for caldera-forming eruptions coincide with glacial stages in accordance with the sediment record in the NW Pacific, suggesting an increase in explosive volcanic activity since the onset of the last glaciation 2.6 Ma. Rapid changes in ice volume during glacial times and the resulting fluctuation of glacial loading/unloading could have caused volatile saturation in shallow magma chambers and, in combination with availability of low-δ18O glacial meltwaters, increased the proportion of explosive vs effusive eruptions. The presented results provide new constraints on Pliocene–Pleistocene volcanic activity in Kamchatka, and thus constrain an important component of the Pacific Ring of Fire.

The Mesozoic geology of SE China is characterized by intensive and widespread magmatism. However, the tectonic regime that accounted for the Mesozoic magmatism has been an issue with little consensus. A comprehensive study of Ar-40 -Ar-39 dating, geochemistry and Sr-Nd isotopes has been conducted on basalts from southern Hunan and syenite intrusions from eastern Guangxi. Three episodes of Jurassic magmatism, i.e. alkaline basalts of c. 175 Ma in age, syenitic intrusions of c. 160 Ma and high-Mg basalts of c. 150 Ma, are identified. The older, c. 175 Ma alkaline basalts are characterized by low Sr (I-Sr = 0.7035-0.7040) and high Nd (epsilon(Nd)(T) = 5 to 6) isotopic compositions and OIB-like trace-element patterns (e.g. Nb/La > 1). In contrast, the younger, c. 150 Ma high-Mg basalts have high Sr (I-Sr c. 0.7054) and low Nd (epsilon(Nd)(T) c. -2) isotopic compositions and incompatible trace-element patterns of arc affinity. The c. 160 Ma syenitic intrusions display a relatively large range of Sr and Nd isotopic compositions (I-Sr = 0.7032-0.7082, epsilon(Nd)(T) = 5.5 to -4.1), with the Qinghu syenites having the lowest I-Sr, highest epsilon(Nd)(T) and OIB-type incompatible trace-element patterns analogous to the c. 175 Ma alkaline basalts. Such a secular variation in rock types and geochemical and isotopic characteristics reveals changes in melt segregation depth and mantle sources, which are inferred to have resulted from the post-Indosinian orogenic lithosphere extension and thinning. The c. 175 Ma alkaline basalts are suggested to have formed by small degrees of decompression melting of the astheno-sphere or an enriched lithospheric mantle source accreted by asthenosphere-derived melts during the initial extension. The c. 160 Ma syenitic and c. 150 Ma high-Mg basaltic rocks mainly originated from the enriched lithospheric mantle that melted owing to a raised geotherm caused by lithosphere thinning. This interpretation is at odds with the active continental margin related to the subduction of palaeo-Pacific plate, but consistent with continental rifting and extension for the Mesozoic of SE China.

The Northern Taiwan Volcanic Zone (NTVZ) is a Late Pliocene–Quaternary volcanic field that occurred as a result of extensional collapse of the northern Taiwan mountain belt. We report here mineral compositions, major and trace element and Sr/Nd isotope data of high-Mg basaltic andesites from the Mienhuayu, a volcanic islet formed at ∼2.6 Ma in the central part of the NTVZ. The rocks are hypocrystalline, showing porphyritic texture with Mg-rich olivine (Fo≈81–80), bronzite (En≈82–79) and plagioclase (An≈66–58) as major phenocryst phases. They have uniform whole-rock compositions, marked by high magnesium (MgO≈5.9–8.1 wt.%, Mg value≈0.6) relative to accompanying silica contents (SiO2≈52.8–54.5 wt.%). The high-Mg basaltic andesites contain the highest TiO2(∼1.5 wt.%) and lowest K2O (∼0.4 wt.%) among the NTVZ volcanic rocks. In the incompatible element variation diagram, these Mienhuayu magmas exhibit mild enrichments in large ion lithophile (LILE) and light rare earth elements (LREE), coupled with an apparent Pb-positive spike. They do not display depletions in high field strength elements (HFSE), a feature observed universally in the other NTVZ volcanics. The high-Mg basaltic andesites have rather unradiogenic Nd (eNd≈+5.1–7.2) but apparently elevated Sr (87Sr/86Sr≈0.70435–0.70543; leached values) isotope ratios. Their overall geochemical and isotopic characteristics are similar to mid-Miocene (∼13 Ma) high-Mg andesites from the Iriomote-jima, southern Ryukyus, Japan. Despite these magmas have lower LILE and LREE enrichments and Pb positive spike, their “intraplate-type” incompatible element variation patterns are comparable to those of extension-induced Miocene intraplate basalts emplaced in the Taiwan–Fujian region. Therefore, we interpret the Mienhuayu magmas as silica-saturated melts derived from decompression melting of the ascended asthenosphere that had been subtly affected by the adjacent Ryukyu subduction zone processes. This interpretation is consistent with the notion that in the northern Taiwan mountain belt post-orogenic lithospheric extension started in Plio–Pleistocene time.

Taiwan is an active mountain belt formed by oblique collision between the Luzon are and the Asian continent. Regardless of the ongoing collision in central and southern Taiwan, a post-collisional extension regime has developed since the Plio-Pleistocene in the northern part of this orogen, and led to generation of the Northern Taiwan Volcanic Zone. Emplaced at similar to0.2 Ma in the southwest of the Volcanic Zone, lavas from the Tsaolingshan volcano are highly magnesian (MgO approximate to 15 wt.%) and potassic (K2O approximate to 5 wt.%, K2O/Na2O approximate to 1.6-3.0). Whereas these basic rocks (SiO2 approximate to 48 wt.%) have relatively low Al2O3 approximate to 12 wt.%, total Fe2O3 approximate to 7.5 wt.% and CaO approximate to 7.2 wt.%, they are extremely enriched in large ion lithophile elements (LILE, e.g. Cs, Rb, Ba, Th and U). The Rb and Cs abundances, > 1000 and 120 ppm, respectively, are among the highest known from terrestrial rocks. In addition, these rocks are enriched in light rare earth elements (LREE), depleted in high Field strength elements (HFSE), and display a positive Pb spike in the primitive mantle-normalized variation diagram. Their REE distribution patterns mark with slight Eu negative anomalies (Eu/Eu * approximate to 0.90 - 0.84), and Sr and Nd isotope ratios are uniform (Sr-87/Sr-86 approximate to 0.70540-0.70551; Nd-143/(144) Nd approximate to 0.51268-0.51259). Olivine, the major phenocryst phase, shows high Fo contents (90.4 +/- 1.8; 1 sigma deviation), which are in agreement with the whole rock Mg-values (83 - 80). Spinel inclusions in olivine are characterized by high Cr/Cr+Al ratios (0.94-0.82) and have compositions similar to those from boninites that originate from highly refractory peridotites. Such petrochemical characteristics are comparable to the Group I ultrapotassic rocks defined by Foley et al. [Earth-Sci. Rev. 24 (1987) 81], such as orogenic lamproites from central Italy, Span and Tibet, We therefore suggest that the Tsaolingshan lavas resulted from a phlogopite-bearing harzburgitic source in the lithospheric mantle that underwent a recent metasomatism by the nearby Ryukyu subduction zone processes. The lavas exhibit unique incompatible trace element ratios, with Rb/Cs approximate to 8, Ba/Rb approximate to 1, Ce/Pb approximate to 2, Th/U approximate to 1 and Nb/ U approximate to 0.8, which are significantly lower than the continental crust values and those of most mantle-derived magmas. Nonmagmatic enrichment in the mantle source is therefore required. Based on published experimental data, two subduction-related metasomatic components, i.e., slab-released hydrous fluid and subducted sediment, are proposed, and the former is considered to be more pervasive for causing the extraordinary trace element ratios observed. Our observations lend support to the notion that dehydration from subducting slabs at convergent margins, as a continuing process through geologic time, can account for the fractionation of these elemental pairs between the Earth's crust and mantle. (C) 2001 Elsevier Science B.V. All rights reserved.

Volcanic glasses with minor mafic mineral fragments, such as biotite and hornblende, found in deep-sea sediments of the South China Sea Basin (SCSB) have been clearly identified as eruptive products of the Youngest Toba Tuff (YTT), northern Sumatra, Indonesia. The tephra layer occurs between marine oxygen isotopic event 5.1 (79.3 ka) and event 4.22 (64.1 ka), with an interpolated age of 74.0 ka, which is in good consistence with previous radiometric dating (73‐75 ka) and ice-core dating (71 ^ 5 ka) of the YTT. The tephra consists predominantly of bubble-wall shards with minor elongated vesicles of pumice fragments. Geochemical characteristics of the tephra, such as high total alkali content, high 87 Sr/ 86 Sr ratio and uniformity of their compositions, all suggest that the recovered tephra is of the Youngest Toba Tuff. This finding supports an extended dispersal of coarse (.63 mm) glass shards over 1500 km northeast of the Toba caldera, a direction opposite to what previously conceived. While providing a better documentation of the distribution extent of the Toba ash, this report points to the need to reestimating the eruptive volume of the YTT and re-evaluating its environmental impact. q 2000 Elsevier Science B.V. All rights reserved.

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

Samples of minerals from the Manaslu leucogranite in the central Himalaya were microsampled and measured for RbSr mineral ages. The ages obtained are used in conjunction with models for RbSr closure temperatures, to reconstruct the thermochronological history of the Manaslu granite. The RbSr results indicate a rapid change in temperature between 21 Ma and 17 Ma of 60–70°C/million yr, which, in turn, indicates unroofing at 2 mm/yr. The temperature-time curve inferred from RbSr data is in excellent agreement with existing ArAr data on the Manaslu granite and its thermal aureole. The results confirm that parts of the Himalaya, in addition to the Gangdese belt, were eroded rapidly in the 20-17 Ma time period, and this erosion could account for the observed rapid increase in seawater 87 Sr 86 Sr . A tourmaline-whole rock age of 29.5 Ma was obtained on a sample from the Chokkang Arm, but may be unreliable because of post-crystallization alteration of feldspar. Microsampling approaches are demonstrated that allow high precision isotopic data to be obtained on minerals sampled at scales of 100 μm or less. Microanalysis, combined with models for Sr redistribution, can be a useful thermochronological tool that can augment the 40Ar39Ar approach.

SILICA-rich lavas, erupted at island-arc or continental volcanoes, are often produced by a complex process involving the assimilation of crust into a crystallizing, mantle-derived basaltic magma1. The different strontium, neodymium and oxygen isotopic compositions of mantle-derived magmas and continental crust provide a powerful method for tracing the different contributions to continental silicic magmas, and for understanding the parameters controlling the composition and volume of erupted magma1–4. In the large rhyolite eruptive centres of the western United States, the largest-volume, explosive rhyolite eruptions have more mantle-like Nd isotope ratios than other silicic lavas from the same centre2–4, a relationship that has been interpreted as reflecting increased influx of mantle-derived basaltic magma to a crustal magma chamber before large-volume eruptions1. Here we report isotope data for lavas from Unzen volcano, which suggest a similar relationship: the Nd isotope composition is more mantle-like in three larger-volume dacite eruptions (>0.1 km3) than in one small-volume (0.02 km3) eruption. We accordingly suggest that, in small-volume systems like Unzen, where the timescales for magma-chamber evolution are of the order of decades, isotope data such as those presented here might be used in volcanic hazard evaluation.

The Tatun volcanic area in the northern tip of Taiwan is composed of Pleistocene andesite and andesitic tuff. Its high relief causes extraordinarily heavy rainfall (> 3000 mm yr−1), especially during the winter monsoon season. Bauxite deposits were formed on the windward northern slopes. Soil samples from two profiles in the bauxite deposits were studied by chemical analysis, X-ray diffraction, differential thermal analysis and hydrogen and oxygen isotopic analyses. The soil profiles can be divided into two zones by chemical characteristics. In the upper zone (zone A), the chemical composition of the soil samples undergoes little change except depletion of Na and Ca due to leaching. The major minerals are quartz, chlorite and illite. In the lower zone (zone B), all major elements suffer rapid dissolution except Al2O3. Gibbsite grows drastically at the expense of clays, while quartz diminishes sharply. Fe2O3 and TiO2 are enriched at the boundary between the two zones due to coprecipitation of Fe and Ti as hematite and goethite, probably as a result of increasing oxygen fugacity. Mobilization of Fe in zone B is attributed to its low Eh. Thus zone A is inferred as a permeable layer and zone B as an aquifer. The weathering rate in this area is estimated to be 12 cm ka−1. The most thoroughly leached soil samples contain up to 80% gibbsite. The high gibbsite contents and the analyses of coexisting waters allow hydrogen and oxygen isotopic fractionation factors between gibbsite and water to be calculated as 0.992 ± 0.002 and 1.016 ± 0.001, respectively.