All issues
Successor
Volume 82, Issue 2
Displaying 1-5 of 5 articles from this issue
- |<
- <
- 1
- >
- >|
-
Shoji Arai1987 Volume 82 Issue 2 Pages 51-59
Published: February 05, 1987
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSXenoliths composed of megacrystalline pyroxenes and hornblende poikilitically enclosing various proportions of olivine are rarely found among mantle peridotites, websterites and mafic rocks in Ichinomegata and Sannomegata ejecta, northeastern Japan. These rocks from Sannomegata are distinct from those of Ichinomegata, which frequently have hornblende oikocryst. Olivine of the former is more magnesian (Fo88-85) than the latter (Fo85-8). Chromian spinel of the latter is distinctly higher in Fe3+/(Cr+Al+Fe3+) ratio than that of the former. Constituent mafic minerals are more Fe-enriched than those of the mantle peridotites and almost identical to those in websterites.
Olivine cumulates may have been formed from magmas in equilibrium with Iherzolite residue, possibly after blending more fractionated websterite-forming magmas. The so-called cumulus mantle beneath Ichinomegata is poorer in olivine than that of southwestern Japan, which may be attributed to low content of olivine component in the initial melts that had formed the cumulus mantle beneath Ichinomegata.View full abstractDownload PDF (1280K) -
Kunio Masaoka1987 Volume 82 Issue 2 Pages 60-74
Published: February 05, 1987
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSStructure of granitic rocks (Cretaceous in age) distributed in the southern part of the Ryoke belt in the Kinki district has been studied. The granitic rocks intruded through three stages. The first stage granites (gneissose tonalite to granodiorite) distributed in the northern part of the studied area have undergone both D1 and D2 deformation. The D1 deformation is characterized by overspread foliation (S1) trending at low to moderate angles to the general trend of the Ryoke belt. The D2 deformation is of the left-lateral sense and resulted in the formation of strongly mylonitized belt of several kilometers wide along the Median Tectonic Line. Upright folds of left-hand echelon are also associated with the D2 deformation. The second stage granites (massive or schistose granite to granodiorite) distributed in the southern part of the studied area have undergone minor and local de formation (D3) characterized by branching shear belts caused by shear stress of left-lateral sense. The third stage granites (massive to weakly schistose tonalite to granodiorite), occurring as a pluton between the first stage and second stage granites, have undergone minor and local deformation (D4) caused by shear stress of left-lateral sense.View full abstractDownload PDF (2773K) -
Michio Kiji1987 Volume 82 Issue 2 Pages 75-80
Published: February 05, 1987
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSUltramafic rock is found associated with the hornblende porphyrite dike (intermediate rockwith SiO2 53.39% and MgO 9.09%) which has intruded shale of late Jurassic age in Tamba Belt.It is mainly composed of euhedral to subhedral Mg-rich olivine and anhedral to subhedral brown Ca- amphibole with subordinate amounts of clinopyroxene, phlogopite, chromite, and etc.According to the classification after IUGS (1973), the modal composition indicates hornblendeperidotite class. Olivine is mostly replaced by serpentine, brucite, opaque iron oxide. Brown Ca- amphiboles have pargasitic compositions. Clinopyroxenes are diopsidic and are rich in Cr. According to the textural features, this rock can be called olivine-clinopyroxene orthocumulatepossessing well-developed interstitial amphiboles. Field and petrol ogical studies suggest that theultramafic rock has cognate relation to the hornblende porphyrite and was formed under thewater-rich condition.View full abstractDownload PDF (2015K) -
Satoshi Kanisawa, Ken Shibata1987 Volume 82 Issue 2 Pages 81-84
Published: February 05, 1987
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSThe Adachi volcano situated to the west of Sendai, Northeast Japan is assumed to be thesource of the Adachi-Medeshima pumice belonging to the Late Pleistocene. Strontium isotoperatios of four samples of the products of the Adachi volcano were determined. The determinedrocks are 1) fresh and white pumice (cummingtonite dacite), 2) cummingtonite dacite (lithicfragment), 3) leucocratic cummingtonite tonalite (lithic fragment), 4) hornblende bearing cummingtonite tonalite (lithic fragment). 87Sr/86Sr ratios of all the rocks fall within a narrow rangebetween 0.70435 and 0.70447. The results obtained in the present study support an assumption thatthese rocks are co-genetic in origin.View full abstractDownload PDF (289K) -
Shoji Arai, Natsuko Takahashi1987 Volume 82 Issue 2 Pages 85-89
Published: February 05, 1987
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSA xenolith of dunite was found in the Ichinomegata ejecta, northeastern Japan. It consists of olivine (80%), clinopyroxene (7.4%), spinel-pyroxene symplectite (5.4%), chromian spinel (4.7%), plagioclase (1.3%), amphiboles (0.6%) and orthopyroxene (0.3%). Olivine is Fo86.8 in average composition, and Cr/(Cr+Al) atomic ratio of spinel is around 0.2. Al2O3 and TiO2 contents of clinopyroxene exceed 5 wt.% and 1 wt.%, respectively. Coexistence of relatively magnesian olivine with Al-rich spinel and Al- and Ti-rich clinopyroxene indicates a cumulative origin from some low-degree partial melt (e.g. alkali basalt) of a pyrolitic material. The dunite has suffered from metasomatic alteration which led to the subsolidus crystallization of titanian pargasite and kaersutite and the entrapment of fluid inculsions into olivine and spinel. Amphiboles in this rock are far more enriched in Ti than those in any ultramafic xenoliths ever documented from Ichinomegata. The metasomatizing fluid may have been enriched in Ti and Na over K, implying the consequence of some intra-plate magmatism.View full abstractDownload PDF (1061K)
- |<
- <
- 1
- >
- >|