Magnesian andesite and basalt from Shodo-Shima Island, southwest Japan, and their bearing on the genesis of calc-alkaline andesites
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Research subject. Andesitic complexes of the Tondono caldera and Lokon-Empung volcano locate din the northeastern part of the Sulawesi Island (Indonesia). Aim. To determine the petrogenesis conditions of andesites in the northeast Sulawesi Island based on detailed studies of volcanic rocks of the Tondono caldera and Lokon-Empung volcano. Materials and methods. We studied volcanic rock samples collected by I.Yu. Safonova. To determine the petrogenesis conditions of andesites, conventional petrochemical, geochemical and mineralogical methods were used. Melt inclusions were also studied using a MIRA 3 LMU scanning microscope equipped with Aztec Energy XMax 80 system of microanalysis, and a Horiba LabRam HR800 Raman spectroscope. The PT-parameters of crystallization were estimated from the data on the composition of melt inclusions using approaches reported by K.D. Putirka, F. Yavuz and D.K. Yıldırım. Results. The Sulawesi andesites represent tholeiitic and calc-alkaline island-arc magmas. Pyroxenes phenocrysts crystallized from melts that evolved with accumulation of alkalis and silica. Plagioclase phenocrysts crystallized from the felsic magmas, which are characterized by a decreasing role of alkalis. The composition of volcanic glass of the mesostasis suggests participation of felsic melts with a very high content of alkalis. The compositions of minerals and glasses in inclusions and in the mesostasis allowed us to estimate PT-parameters of the petrogenesis of the andesites. The phenocrysts of pyroxene crystallized in two intermediate magma chambers at depths of 27.6–14.6 and 11.3–7.2 km and temperatures ranging from 1150 to 970ºС. The phenocrysts of plagioclase crystallized at 930–910 and 900–890ºС. The microcrystals (laths) of plagioclase in the mesostasis crystallized at lower temperatures of 875–865 and 840–810ºС. Conclusions. The andesites of the Sulawesi Island were derived from tholeiitic to calc-alkaline melts compositionally similar boninites. The compositions of the glasses in melt inclusions and mesostasis showed three types of compositionally different parental magmas, which produced the Sulawesi andesites. The phenocrysts of pyroxenes crystallized from these melts in two magma chambers at depths of 27.6 to 7.2 km and at temperatures of 1150 to 970ºС. The phenocrysts and laths of plagioclase crystallized at lower temperatures of 930 to 810ºС.
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Petrography and bulk rock chemistry of volcanic rocks and plutonic xenoliths from HigashiIzu monogenetic volcano group (HIMVG), the Izu Peninsula, central Japan, revealed a bimodal volcanic activity-high alumina basalts to calc-alkaline andesites (SiO2=48-59 wt%) vs. calcalkaline dacites (SiO2=68-73 wt%)-. The samples include 7 rock types based on bulk compositions and phenocrysts assemblages; ol + pl ± aug basalts, ol + pl ± aug andesites, ol + pl ± aug + hyp andesites and pl + ho + hyp ± (ol + aug) dacites. Two types of basalts and andesites with different modal compositions of xenocrystic minerals (corroded quartz and sodic resorbed plagioclase) are found in HIMVG, i.e. X-poor type (less than 1 vol% xenocrysts) and X-rich type (more than 1 vol% xenocrysts). The X-poor type includes mostly of basalts and shows iron enrichment on the SiO2 vs. FeO*/MgO diagram. On the other hands, the X-rich type with more than 1 vol% xenocrysts includes mostly andesites and does not show iron enrichment. In the X-poor type, modal composition of phenocrysts of olivine and chromian spinel, and bulk MgO, Ni, and Cr contents decrease abruptly with an increasing SiO2, whereas in the X-rich type, contents of these elements are variable and decrease moderately. Partialy fused granitic xenoliths from Omuroyama scoria cone contain quartz, partialy resorbed plagioclase, and glass. Bulk rock composition of these xenoliths are similar to that of dacites. In the HIMVG area, a concentric zonal pattern of rock types and phenocryst assemblages are observed. The basalts erupted at outer zone, while andesites and dacites, at inner zone. The volcanic products had been basaltic and andesitic untill the eruption of Kawagodaira, after which they became andesitic and dacitic.
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Petrographical and geochemical characteristics of calc-alkaline andesites on Shodo-Shima Island, SW Japan, having bulk compositions largely identical to the continental crust, are presented. The following petrographic observations suggest a role for magma mixing in producing such andesite magmas: (1) two types of olivine phenocrysts and spinel inclusions, one with compositions identical to those in high-Mg andesites and the other identical to those in basalts, are recognized in terms of Ni–Mg and Cr–Al–Fe3+ relations, respectively; (2) the presence of orthopyroxene phenocrysts with mg-number >90 suggests the contribution of an orthopyroxene-bearing high-Mg andesite magma to production of calc-alkaline andesites; (3) reversely zoned pyroxene phenocrysts may not be in equilibrium with Mg-rich olivine, suggesting the involvement of a differentiated andesite magma as an endmember component; (4) the presence of very Fe-rich orthopyroxene phenocrysts indicates the association of an orthopyroxene-bearing rhyolitic magma. Contributions from the above at least five endmember magmas to the calc-alkaline andesite genesis can also provide a reasonable explanation of the Pb–Sr–Nd isotope compositions of such andesites.
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Abstract Titiraupenga is an early Pleistocene volcano sited approximately 20 km northwest of Lake Taupo, North Island, New Zealand, at the junction of the Taupo Volcanic Zone and a line of andesite volcanoes extending from Ohakune in the south to Waiheke Island in the north. Exposures on the volcano are restricted to a central plug (type 1 andesite) and rare exposures of flows on the lower slopes (type .2 andesite). Both type 1 and type 2 andesites are porphyritic with phenocrysts of orthopyroxene, augite, plagioclase, and scarce olivine in a groundmass of plagioclase, pyroxene, and iron OXIde. Pigeonite occurs only in the groundmass and as reaction rims to orthopyroxene phenocrysts in type 1 andesites. Chemically, most samples are low-Si andesites, and variation is probably due to varying degrees of pyroxene accumulation within the magma reservoir. Type 2 andesites are inhomogeneous and probably result from mixing of magmas from different parts of the reservoir. Aphyric lenses (type 3 andesite) within type 2 andesites are thought to represent a wholly liquid portion of the reservoir which was incorporated in the type 2 magma during eruption. Slight differences in isotopic composition of type 3 magma may indicate some crustal contamination.
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Within the Zitácuaro–Valle de Bravo (ZVB) region of the central Mexican Volcanic Belt (MVB), three lava series have erupted during the Quaternary: (1) high-K2O basaltic andesites and andesites; (2) medium-K2O basaltic andesites, andesites and dacites; (3) high-TiO2 basalts and basaltic andesites. The dominant feature of the first two groups is the lack of plagioclase accompanying the various ferromagnesian phenocrysts (olivine, orthopyroxene, augite, and hornblende) in all but the dacites. This absence of plagioclase in the phenocryst assemblages of the high-K2O and medium-K2O intermediate lavas is significant because it indicates high water contents during the stage of phenocryst equilibration. In contrast, the high-TiO2 group is characterized by phenocrysts of plagioclase and olivine. The spatial distribution of these three lava series is systematic. The southern section of the ZVB transect, 280–330 km from the Middle America Trench (MAT), is characterized by high-K2O melts that are relatively enriched in fluid-mobile elements and have the highest 87Sr/86Sr ratios. Medium-K2O basaltic andesite and andesite lavas are present throughout the transect, but those closest to the MAT are MgO-rich (3·5–9·4 wt %) and have phenocryst assemblages indicative of high magmatic water contents (3·5–6·5 wt % water) and relatively low temperatures (950–1000°C). In marked contrast, the northern section of the ZVB transect (380–480 km from the MAT) has high-TiO2, high field strength element (HFSE)-enriched magmas that have comparatively dry (< 1·5 wt % magmatic water) and hot (1100–1200°C) phenocryst equilibration conditions. The central section of the ZVB transect (330–380 km from the MAT) is a transition zone and produces moderately light rare earth element (LREE) and large ion lithophile element (LILE)-enriched, medium-K2O lavas with phenocryst assemblages indicative of intermediate (1·5–3·5 wt %) water contents and temperatures. The high-K2O series compositions are the most enriched in LILE and LREE, with a narrow range of radiogenic 87Sr/86Sr from 0·704245 to 0·704507, 143Nd/144Nd values ranging from 0·512857 to 0·512927 (εNd = 4·27–5·63), and 208Pb/204Pb values from 38·248 to 38·442, 207Pb/204Pb values from 15·563 to 15·585, and 206Pb/204Pb values from 18·598 to 18·688. The medium-K2O series compositions are only moderately enriched in the LILE and LREE, with a broader range of 87Sr/86Sr, but similar 143Nd/144Nd and 208Pb/204Pb values to those of the high-K2O series. In contrast, the high-TiO2 series compositions have little enrichment in LILE or LREE and instead are enriched in the HFSE and heavy rare earth elements (HREE). The high-TiO2 lavas are isotopically distinct in their lower and narrower range of 143Nd/144Nd. The isotopic variations are believed to reflect the upper mantle magma source regions as the low content of phenocrysts in most lavas precludes significant upper crustal assimilation or magma mixing, other than that represented by the presence of quartz xenocrysts (< 2 vol. %) with rhyolitic glass inclusions, which are found in many of these lavas. The systematic spatial variation in composition of the three lava series is a reflection of the underlying subduction-modified mantle and its evolution.
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