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Volume 79, Issue 5
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mode of occurrence and petrographyKatsuya Ogiso1984 Volume 79 Issue 5 Pages 187-199
Published: May 05, 1984
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSThe basic metamorphic rocks of the Miho area are divided into meta-diabase and gabbroic rocks. The meta-diabase, finer-grained basic rocks characterized by granoblastic texture, is widely distributed as small masses sporadically scattered in this area, and the gabbroic rocks are exposed in limited zone around Mts. Suisho-zan and Jo-yama. The gabbroic rocks can be classified petrographically into ten rock-types, cortlandite, allivalite, anorthositic gabbro, gabbroic anothosite, olivine bearing hornblende eucrite, hornblende eucrite, hornblende gabbro, quartz gabbro, olivine bearing pyroxene dolerite, and pyroxene dolerite. The last two rock-types are chilled facies of the gabbroic rocks. In the gabbroic rocks layered structure is recognized in the field. From the field and petrographical observations and the chemical characters of the constituent minerals, it is concluded that these gabbroic rocks have been formed through crystallization differentiation of basic magma and emplaced as a layered intrusion.
Petrological characteristics of the cortlandites scattered throughout the Ryoke metamorphic belt were reviewed. There is some local variations and a difference between the cortlandites from the Chubu (especially in the north) and ones from the Kinki-Setouchi districts. The cortlandites from the Chubu district contain clinopyroxene and chromium-bearing spinel, but the cortlandites from the Kinki-Setouchi districts do not contain these minerals. Mg/(Fe+Mg) ratio of the constituent minerals and the An content in plagioclase of the cortlandites from the Chubu district are higher than those of Kinki-Setouchi districts.View full abstractDownload PDF (1025K) -
with special reference to opaque mineralsMunetomo Nedachi, Hitoshi Onuki, Takeyoshi Yoshida, Michio Tagiri1984 Volume 79 Issue 5 Pages 200-213
Published: May 05, 1984
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSAn ultramafic-mafic plutonic complex at Mt. izuishi can be divided into the following three emplaced units.
Unit I: Dunite, wehrlite and olivine clinopyroxenite (serpentinized).
Unit II: Hornblende gabbro and clinopyroxene hornblende gabbro with or without orthopyroxene.
Unit III: Hornblende gabbro.
Unit IV: Biotite-hornblende diorite.
Some small-scale iron ore deposits are embedded mainly in Unit I. Bulk chemical analyses are presented for twenty-one rocks and two iron ores (magnetitites).
All the constituents of the complex have been thermally metamorphosed owing to the emplacement of Cretaceous granitic rocks, and therefore their original petrographic chracteristics are not well preserved. However, a most plausible explanation of the origin of the com-plex is a hypothesis of successive injections differentiating from a basaltic magma.
Microprobe data are presented for olivines and oxide minerals in uitramafic rocks and ironores, and for oxide and sulfide minerals in mafic rocks. At the contact with the granitic mass the equilibrium temperature of contact metamorphism, based on the olivine-spinel geothermometer, can be assumed to be 650-700°C. At a distance from the contact, the compositions of coexisting magnetite and ilmentie in representative mafic rocks indicate consistently temperature and oxygen fugacity of about 550°C and 10-20 bars. The magnetite+ilmenite+pyrite+pyrrhotite and magnetite+ilmenite+pyrite assemblages in mafic rocks also give approximate values of fo2=10-19 and fs2=10-3 bars at 550°C for metamorphic conditions.View full abstractDownload PDF (1775K) -
TAKASHI YAMAGUCHI1984 Volume 79 Issue 5 Pages 214-232
Published: May 05, 1984
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSA liquid line of descent is well demonstrated in the Hotaka volcano. Mineral assemblage and chemical composition of phenocrysts change systematically with the change of the chemical composition of the groundmass (magmatic liquid). Crystallization sequence with increasing SiO2 content of the groundmass is as follows. Olivine, hypersthene, augite, plagioclase and magnetite coexisting in the liquid whose SiO2 content ranges from 53.5 to 57 wt%. In liquids with more than 57 wt% SiO2 content, olivine disappears. Augite, hypersthene, magnetite and plagioclase continuously crystallize until the SiO2 content of liquid attains at least 70 wt%.
The amount of the crystallized minerals during fractional crystallization is estimated to be 46 wt% (plagioclase, 22; hypersthene, 6; augite, 11; magnetite, 7), while the SiO2 content of liquid changes from 57 to 70 wt%. Plagioclase accumulation during fractional crystallization is inferred from the difference between the estimated ratio of plagioclase to other phenocrysts and the observed ratios in the modal compositions of phenocrysts.
The assemblage of groundmass pyroxenes changes with increasing SiO2 content of the groundmass in the following order: augite+pigeonite→pigeonite→pigeonite+hypersthene→hypersthene. This change reflects the decrease of the normative diopside content of the liquid. The chemical and mineralogical features of fractional crystallization is close to that of other island arc tholeiites in Japan, though the normative diopside content of the rocks of the Hotaka volcano in slightly lower than that of the others and groundmass hypersthene occurs in the rocks with more than 65 wt% SiO2.View full abstractDownload PDF (739K)
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