Sign In. Advanced Search. Skip Nav Destination Article Navigation. Close mobile search navigation Article navigation. Volume 86, Number Previous Article Next Article. Article Navigation. Research Article October 01, Google Scholar. GSA Bulletin 86 10 : — Article history first online:.
Abstract The intimate association of basalt, andesite, dacite, and rhyolite within a volcanic center suggests that these rocks are genetically related. This content is PDF only. Please click on the PDF icon to access. In contrast, aphyric and phenocryst-poor rocks contain sieve-textured plagioclase Fig. Clinopyroxene phenocrysts, relatively minor in volume, seem to be incompatible with fresh hornblendes, but they appear where hornblendes are completely pseudomorphed by opacite Table 2.
Sample d is exceptional, but clinopyroxenes in this sample show marginal resorption. Quartz phenocrysts are almost invariably resorbed. Moreover, quartz in d, d, d, d, d and d is jacketed by overgrowths of clinopyroxene. Significantly, quartz is commonly associated with opacitized hornblende, but quartz and fresh hornblende are never observed to coexist Table 2. Some Daisen rocks contain two coexisting pyroxenes the open squares in Fig.
Both core and rim compositions of individual phenocrysts are plotted in each sample. What causes the wide range of pyroxene temperatures and how are they observed in individual samples? Figure 5 shows back-scattered electron images BEI and distribution maps of Ca and Mg contents of orthopyroxene phenocrysts in samples d and d Chemical profiles of these orthopyroxenes are presented in Fig. Moreover, many orthopyroxene phenocrysts show reversibly zoned patterns in terms of Mg number Figs 5a—c and 6a, b.
The zoning patterns of Wo and Mg number, however, do not necessarily correlate with each other. Interestingly, however, these magnesian cores also have lower Ca contents Fig.
Back-scattered electron images, Ca- and Mg-content maps of orthopyroxene phenocrysts in Daisen dacites of Fig. Low-Ca cores have diffuse contacts with surrounding high-Ca rims, and in orthopyroxene 28 a small cavity lies between core and rim. The phenocryst cores were inherited from the calc-alkaline protolith. Reversed zoning patterns are often observed in d, and less magnesian and Ca-poor cores are surrounded by magnesian and Ca-rich rinds.
Cores have lower Ca content than rinds, but there are both reversed and normal zoning patterns in terms of Mg content. In both cases, cores have distinct compositions and diffuse contacts with the surrounding rinds. Wo content increases even within rinds having constant magnesia values. Rim compositions are similar from phenocryst to phenocryst in individual samples Fig. A model where contaminated basalt is derived from andesite is unlikely on both thermal and chemical grounds.
Possible theoretical assimilation of crust by Daisen magmas is shown in Fig. The curved lines show trends of bulk assimilation of representative basement granite d, Table 2. Most Daisen basalts are primitive; they contain only olivine phenocrysts and show no evidence of crustal assimilation Tamura et al. In addition to the unrealistic mixing ratios, the Sr and Nd isotopic variation of andesites and dacites does not follow the mixing line.
Similar results are obtained by considering other granitoid rocks in SW Japan Kagami et al. In addition, contamination of mantle-derived magma with granulites at the base of the crust is also unlikely. Thus, isotopic variability within Daisen volcano is probably mantle-derived, reflecting either isotopic variability within the magma source region associated with a single mantle diapir, or a series of diapirs with some interaction between them.
These ratios do not vary with SiO 2 , and no systematic differences exist between aphyric andesites and phenocryst-rich dacites. The numbers indicate the percentage of the assimilated granite, described in the text to be unrealistic.
Plots of selected major and trace elements vs SiO 2 in lavas from Daisen volcano were shown in figs 3 and 4 of Tamura et al. The volcanic association is clearly bimodal.
Mid-ocean ridge basalt MORB -normalized plots of trace element data for Daisen andesite and dacite lavas are shown in Fig.
All lavas show subduction-zone affinities, but the andesites and dacites show a stronger depletion in Nb and a larger enrichment in Pb than the basalts Fig. La contents are similar among the basalts, andesites and dacites, but the andesites and dacites have systematically lower values of more compatible elements Sm, Eu, Ti, Y, Yb and Lu than the basalts Fig. These lavas have trace element signatures of a typical arc volcano.
MORB-normalized plot of trace element data for all Daisen lavas. Ranges of aphyric andesites, phenocryst-poor andesites and phenocryst-rich andesites—dacites overlap, but the values for basalt diverge with decreasing incompatibility. Thus, their primary magmas were probably produced at the same general depth.
Moreover, the trace element features of the andesites and dacites are consistent with hornblende fractionation at a shallower level. Segregation of partial melt from restite crystals would produce a magma of rhyolitic composition. Hannah et al. The aphyric andesites and phenocryst-rich dacites at Daisen volcano may represent another case relating to the genesis of calc-alkaline andesites and dacites at arc volcanoes in general.
These mechanisms, however, do not provide an adequate explanation for the positive correlation between SiO 2 and modal phenocryst content in the Daisen lavas Fig.
Given that aphyric andesite magmas are the first to be generated, crystallization of these magmas and the concurrent removal of phenocryst phases may result in phenocryst-rich dacites. This scenario is consistent neither with the petrographic observations nor mineral chemistry at Daisen volcano Figs 3 — 6. Partly melted phenocrysts of plagioclase, resorbed crystals of quartz, opacitized hornblende, and high contents of Ca in orthopyroxene rims Figs 4 — 6 suggest heating of the magmas, but are not consistent with fractional crystallization accompanying temperature drop.
Breakdown of hornblende can also be triggered by decompression, but at Daisen volcano the differences of phenocryst assemblages between fresh hornblende-bearing rocks and opacite-bearing rocks Table 2 indicate that heating, accompanied by dehydration, caused the breakdown of hornblende to opacite. The volcanic products of Daisen volcano are clearly bimodal, and the production of andesites and dacites through crystal fractionation from basalts cannot be supported by major element chemical variation trends and trace element data Figs 9 and We suggest that a two-stage process, involving mid-crustal solidification of calc-alkaline magmas followed by their recurring partial melting, generated the magmatic trends and phenocryst zoning patterns observed at Daisen volcano.
Figure 13 shows our model for the evolution of mantle-derived basalt and magnesian andesite in higher-level magma chambers beneath Daisen volcano. In Fig. These bodies would have extensively evolved from primary magnesian andesite magmas through fractionation involving hornblende.
Finally, in Fig. The sequence of lavas produced shows the reverse of normal fractionation anti-fractionation , in the sense that the process progresses from dacitic partial melts to andesitic complete melts, and is accompanied by a temperature rise. It is possible that the protolith was already hot and partially molten.
In this case, solidified andesite magma bodies could have retained a high temperature before subjacent basalt magmas were emplaced. The voluminous dacitic partial melts of Daisen volcano may be predicted from their model. However, there are two factors in Daisen volcano that may have promoted the production of complete andesite melts. First, the country rock of Daisen volcano may act as a refractory insulating container.
Isotopic evidence from Daisen volcano Fig. Therefore, heating would have been localized, so that a large amount of heat could have been transferred into a relatively small andesite body. Second, because a region of partially molten magma provides an effective density barrier, basalt sills may be repeatedly intruded into the same region during an episode of andesite remelting Fig.
Both refractory insulating wall rocks and repeated injections of basalt sills over a short time scale could be necessary conditions to produce sufficient, localized heat for the generation of andesitic complete melt. Moreover, the Daisen andesites and dacites have the same trace element signatures as the associated basalts Tamura et al. The absence of phenocrysts in the aphyric magmas suggests they were ultimately superheated in the near-surface environment.
We suggest a two-stage process, involving mid-crustal solidification of bodies of this calc-alkaline magma, followed by varying degrees of partial melting from body to body, to produce the magmatic trends and phenocryst zoning patterns observed in the andesites and dacites. The heat required for this melting was probably supplied by the intermittent rise of subjacent basaltic magma. All field studies and some of the analytical work by Y.
Tamura were carried out under the guidance of I. Kagami and K. Shuto, Niigata University, are thanked for their help with isotope analyses. We particularly thank R. Fiske, Smithsonian Institution, and M. Ishiwatari, H. Ishida and M. Asada of Okayama University made the many thin sections used in this study. We thank T. Green and R. Magmas and magmatic rocks: an introduction to igneous petrology. Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
Foto Plagiolcase, Biotite and clinopyroxene in a dacite from Bulgaria. Plagiolcase, Biotite and clinopyroxene in a dacite from Bulgaria. Plagiolcase, Biotite and clinopyroxene in a Dacite from Bulgaria. Plagiolcase and Biotite in a Dacite from Bulgaria. Plagiolcase and Biotite with plagioclase inclusions in a Dacite from Bulgaria. Plagiolcase and clinopyroxene in a Dacite from Bulgaria. Plagiolcase and Hornblende in a Dacite from Bulgaria.
Plagiolcase quartz rounded and Hornblende in a Dacite with felsic groundmass from Bulgaria. Plagiolcase in a Dacite with felsic groundmass from Bulgaria. Plagiolcase and Hornblende in a Dacite with felsic groundmass from Bulgaria. Feldspars altered by sericite in a hydrothermal altered Dacite.
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