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Volume 3 (1969 - 1970･･･
- Issue 4 Pages 187-
- Issue 2-3 Pages 87-
- Issue 1 Pages 1-
Volume 3, Issue 1
Displaying 1-6 of 6 articles from this issue
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John A. Cooper, John R. Richards1969 Volume 3 Issue 1 Pages 1-14
Published: 1969
Released on J-STAGE: April 08, 2008
JOURNAL FREE ACCESSIsotopic measurements have been made on lead extracted from samples of andesite, propylitized andesite and rhyolite, and from galenas present in quartz-calcite veins occurring in the Miocene Hauraki Goldfield region, New Zealand. Measurement bias due to isotopic fractionation has been removed by the double-spike technique. All samples show varying degrees of J-type anomaly. Although there is some overlap, the galena samples generally appear more anomalous than the andesites; lead in the veins is isotopically different from lead in adjacent andesite. There is no evidence of contamination by ancient, low radiogenic rock systems, or of isotopic fractionation during vein formation. Although the vein galenas may be slightly enriched in preferentially-extracted radiogenic lead, the results are thought to represent isotopic inhomogeneity in the sources. With one exception the vein galena, rhyolite, propylitized andesite, and hydrothermally altered rocks form a linearly-distributed set on the 208Pb/204Pb - 206Pb/204Pb diagram, which appears to be distinct from the unaltered andesite. A similar but less distinct trend is present on the 207Pb/204Pb - 206Pb/204Pb diagram. It is suggested that the vein galenas and rhyolites, together with the propylitization and hydrothermal fluids, could all have originated from the one bulk source-material, the thick Mesozoic greywacke-argillite sequence, and that the andesites have come from another, necessarily deeper source. Propylitization processes appear to have introduced the hydrothermal-rhyolite-galena-type lead into the altered andesites. A criticism is made of the use of lead isotope results from hydrothermal material as input data for the calculation of single-stage model growth-curves.View full abstractDownload PDF (6736K) -
Carl E. Hedge, Roy J. Knight1969 Volume 3 Issue 1 Pages 15-24
Published: 1969
Released on J-STAGE: April 08, 2008
JOURNAL FREE ACCESSIsotopic compositions of lead and strontium and concentrations of lead, uranium, thorium, rubidium, and strontium were measured in a suite of volcanic rocks, ranging from basalt to rhyodacite in composition, and in granite and gabbro xenoliths from a traverse across northern Honshu. The observed 238U/204Pb (μ) ratio ranges from 2.4 in tholeiitic basalt at the east end of the traverse to 11.6 in alkalic basalt from the west end. The isotopic composition is slightly less radiogenic to the west. The 87Sr/86Sr ratios of most of the samples fall within the range observed in oceanic basalts, but the granite xenolith and the rhyodacite are slightly more radiogenic.View full abstractDownload PDF (4281K) -
Philip O. Banks, Nobumichi Shimizu1969 Volume 3 Issue 1 Pages 25-34
Published: 1969
Released on J-STAGE: April 08, 2008
JOURNAL FREE ACCESSU-Pb isotopic analyses of zircons from two granodiorites of the Abukuma Plateau indicate a period of plutonic activity in that area approximately 110m.y. ago. Zircons from two granodiorites of the Shidara area have anomalously high 207Pb/206Pb ratios, resulting from older radiogenic lead acquired during generation or emplacement of the magmas. The inherited component has a minimum age of about 600 m. y. and probably is considerably older. Two periods of emplacement are suggested for the Shidara granodiorites, one at 70-75 m.y. and the other at perhaps 80-90 m.y.View full abstractDownload PDF (4610K) -
Naoko Ueno, Minoru Ozima, Akira Ono1969 Volume 3 Issue 1 Pages 35-44
Published: 1969
Released on J-STAGE: April 08, 2008
JOURNAL FREE ACCESSRb-Sr and K-Ar age determinations were made on the metasediments from Shidara and Takato regions in the Ryoke metamorphic belt, Central Japan. Both total rock isochron age and total rock-biotite isochron ages by Rb-Sr method and mineral age by K-Ar method fall in a range from 60 to 70 m.y. These are similar to the ages which have been obtained for the Ryoke granites, and are considered to indicate the time of the contact metamorphism due to the intrusion of the granites. The age of the regional metamorphism is not yet obtained, but would be obtained by further Rb-Sr total rock analyses of low grade metasediments.View full abstractDownload PDF (3928K) -
Ichikazu Hayase, Susumu Nohda1969 Volume 3 Issue 1 Pages 45-52
Published: 1969
Released on J-STAGE: April 08, 2008
JOURNAL FREE ACCESSThe Kurosegawa structural belt, linearly aligned along the outer zone of S. W. Japan from Kyushu and Shikoku to the Kinki district of Honshu Island contains the oldest rock of Japan both from the paleontological (Silurian fossiliferous limestone) and the geochronological (Rb-Sr age of biotite from the Mitaki granite) points of view. The Rb-Sr age of the biotite is 428±4 m.y. and that of potassium feldspar in the Kurosegawa belt is 288 ± 19 m.y. The latter age seems to coincide with the age of upheaval of the belt. As is revealed by microscopic examination, potassium feldspar is characterized by alteration and replacement at low temperature.View full abstractDownload PDF (3286K) -
Mitsunobu Tatsumoto, Roy J. Knight1969 Volume 3 Issue 1 Pages 53-86
Published: 1969
Released on J-STAGE: April 08, 2008
JOURNAL FREE ACCESSThe isotopic composition of lead and concentrations of lead, uranium, and thorium were determined in tholeiitic and high-alumina basalts, and their calc-alkali rock series, from central Japan. The isotopic composition of lead of high alumina basalts is similar to that of tholeiites from adjacent areas, whereas their silicic differentiates (calc-alkali rock series) are rich in 207Pb and 208Pb. This is interpreted as a result of crustal contamination. The isotopic composition of lead in the primary basalts gradually decreases in radiogenic character from the Pacific Ocean side to the Japan Sea side, whereas the observed 238U/204Pb and 232Th/238U ratios in the basalts increase in the same direction. This inverse correlation can be interpreted as resulting from differentiation of the upper mantle about 3.6b.y. ago, with tholeiite (Pacific side) generated from a shallower zone than the alkali basalt (Japan Sea side). The magma generation is associated with a process which extracts Pb preferentially to U and Th at shallower depth and U and Th preferentially to Pb at deeper depth in the past (multi differentiation for the source region) or at the magma generation stage. An alternative interpretation of this inverse correlation is that the ocean floor is being thrust under the Japanese Island arcs and the isotopic difference is produced by the degree of mixing of mantle material under the arcs with thrust material.View full abstractDownload PDF (17360K)
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