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Volume 81, Issue 9
Displaying 1-5 of 5 articles from this issue
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WATCHARAPORN KEANKEO, TSUYOSHI KITAMURA, KATSIJTOSHI TANIDA1986 Volume 81 Issue 9 Pages 341-347
Published: September 05, 1986
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSTephroite and knebelite samples from the Hijikuzu mine were chemically analyzed by an electron microprobe analysis (EPMA) and examined by X-ray diffraction, optical and DTA methods. The minerals occur as aggregates of small prismatic and/or granular crystals in the bedded manganese ore deposits of the Hijikuzu mine in the highly metamorphosed Jurassic quartzite caused by the Miyako granitic mass. EPMA leads to formulas (Mn1.97Mg0.03Ca0.02)2.02Si0.99O4.00 for tephroite and (Mn1.34Fe2+0.61Mg0.07)2.02Si0.99O4.00 for knebelite.View full abstractDownload PDF (360K) -
ICHIRO SUNAGAWA, AYAKO SUGIBUCHI1986 Volume 81 Issue 9 Pages 348-358
Published: September 05, 1986
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSThrough a comparison of etch figure morphologies observed on natural { 10_??_1} surfaces of high quartz phenocrysts occurring in a dacitic tuff and those produced by laboratorical etching, growth and post growth histories of the crystals were analysed. It was demonstrated that the crystals grew by the spiral growth mechanism, and then experienced a very short dissolution period by disequilibrium above the phase transition temperature, 573°C, and possibly a weak plastic deformation. This was followed by a rapid quenching, due to adiabatic expansion of the magma, which resulted in crack formation and phase transition. Through phase transition from high to low quartz, Dauphine twins were formed, associated by the multiplication of dislocations due to the attendant volume change.View full abstractDownload PDF (5613K) -
Toshihiko Hayashi1986 Volume 81 Issue 9 Pages 359-369
Published: September 05, 1986
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSGeological and petrographical studies were carried out on the rocks from Early Cretaceous granitoids (112-122Ma) in the Tanohata area, northeastern Japan. It was revealed that the granitic complex called the Tanohata pluton consists of six granitic intrusives, which are different in lithology and intruded at the different stages. The largest body is a concentric texturally and compositionally zoned pluton. This body is definede as the Tanohata aoned pluton. Other five smaller bodies are distrbuted aroud the zoned pluton.
The granitoids that make up the Tanohata zoned pluton are progressively more felsic inward; from biotite hornblede quartz diorite_??_tonalite (A-2 type) at the margin to biotite leucogranodiorite (A-7 type) in the center through biotite hornblende tonalite_??_granodiorite (A-3, 4 type), hornblende biotite granodiorite (A-5 type) and biotite granodiorite (A-6 type). The mineral assemblages of these rock types are as follows: pl+opx+cpx+ho+bio+qz+K-f (A-2 type), pl+cpx+ho+bio+qz+K-f (A-3 type), pl+ho+bio+qz+K-f (A-4, 5 type), pl±ho+bio+qz+K-f (A-6 type), and pl+bio+qz+K-f (A-7 type, pl+opx+cpx+ho+bio) is foud with the A-2 and A-3 types. The zoned pluton is generally fine-grained and well-foliated at the margin and becomes progressively coarse-grained and poorly-foliated towards center. Mafic minerals decrease and quartz and K-feldspar slightly increase from the margin to the center (from 27-21 to 7-4 in the color index). The anorthite content of plagioclase core decrease from 60-52 at the margin to 35-28 in the center.
Continuous geological and petrographical changes of these rocks (at least the A-2_??_A-7 types) suggest that this zoned pluton formed by differentiation of a single magma, and that the crystallization sequences of principal minerals is plagioclase, orthopyroxene, clinopyroxene, hornblende, biotite, quartz, and K-feldspar.View full abstractDownload PDF (998K) -
Trace elements and petrogenesisShintaro Hayashi1986 Volume 81 Issue 9 Pages 370-383
Published: September 05, 1986
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSTwenty-seven representative volcanic rocks from Chokai volcano, Northeast Japan, were analysed for Co, Sc, Sr, Ce, Y, Zr, Nb, Rb, Cs, and Ba to discuss the origin of the rocks in combination with the previously reported geochemical and mineralogical data. The major and trace element variations are indicative of a considerable difference in the origin between olivinefree rock (OLF) suite and olivine-bearing rock (OLB) suite.
The OLF sequence was formed by crystallization differentiation from the associated olivinefree mafic andesite. The most evolved andesite of the OLF suite can be produced after approximately 31% fractionation of the phenocryst minerals from the parental mafic andesite magma. Their mineralogical features are consistent with this model.
The OLB suite has two sets of mineral group as apparent phenocrysts but neither can be in equilibrium with another. They make straight variation trend in MgO-K2O diagram; crystallization differentiation cannot account for. An internal magma mixing model was proposed to solve all the conflicts. Basaltic melts mixed with felsic andesitic magma and a series of the OLB suite was produced. Before the mixing, the basaltic melts contained phenocrysts of Mg-rich olivine, augite, spinel and Ca-rich plagioclase, while less Mg-rich augite, hypersthene, and Na-rich plagioclase were crystallizing with magnetite and ilmenite in the felsic ones. Consequently, the mixtures included the disequilibrated sets of the mineral group. All the elemental variation trends should make simple lines between the end members. Mineralogical and geochemical consanguinitie indicate that the felsic end members were the differentiation products of the OLF suite. The basaltic end component cannot be the parental magma of OLF suite, because the Zr/Rb ratio of the latter is larger than that of the former.View full abstractDownload PDF (973K) -
Teiko Ichinose, Kenji Shuto, Ryuichi Yashima1986 Volume 81 Issue 9 Pages 384-391
Published: September 05, 1986
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSUltramafic and mafic inclusions, including intermediate ones, up to 3 cm in size found in high magnesian tholeiitic basalts are divided into the following three groups based on the mineral assembalge and texture: 1) poikilitic peridotite and pyroxenite, 2) pyroxenite including webster-ite, clinopyroxenite and orthopyroxenite, and 3) gabbro and diorite. The 1st group consists of euhedral olivine chadacrysts with chrome spinel inclusions, and clinopyroxene and/or orthopyrox-ene oikocrysts. The 2nd one shows equigranular in texture, but the last one does poikilitic and equigranular. Most of them contain interstitial patches composed of plagioclase, magnetite, clinopyroxene and glass. Petrographic features suggest that these inclusions are cumulates derived from the host magma.View full abstractDownload PDF (2089K)
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