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Volume 82, Issue 5
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M.A. PARADA, I. ARACENA, HISAO TANAKA1987 Volume 82 Issue 5 Pages 177-188
Published: May 05, 1987
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSThe Tertiary Chuquicamata Plutonic Complex, in which the Chuquicamata porphyry copper deposit occurs, is described from petrographical and mineralogical viewpoints. The complex is composed of the Fortuna Monzonite, Chuqui Granite Porphyry and Este Monzogranite. The Chuqui Granite Porphyry, the host rock of the mineralization, is divided into the porphyry I, II and III. Potassic, propylitic and phyllic alteration of varying degrees prevails throughout the complex.
Amphibole from the Fortuna Monzonite, the most basic rock type, has Mg/(Mg+Fe*) ratios higher than 0.74 and high Si atoms. Biotites contained in all the rock types exhibit Mg/(Mg+Fe*) ratios higher than 0.58. The high Mg/(Mg+Fe*) ratios of the mafic silicates, high Fe2O3/FeO whole rock ratios, the presence of magnetite and high magnetic susceptibility indicate that the Chuquicamata rocks were formed at high oxygen fugacities between the Qz-Mt-Fa and Mt-Hm oxygen buffer reactions. Two-feldspar geothermometer of Stormer (1975) gave fairly low equilibration temperatures from 550° to 350°C. The lowest temperatures were obtained for the twofeldspar pairs from the porphyry I, which is consistent with the lowest TiO2 contents of their biotites.
The early plagioclase crystallization in the Fortuna Monzonite, and the quartz crystallization before biotite in the Chuqui Granite Porphyry imply that the Chuquicamata magmas were not saturated with H2O throughout their entire congelation. Because only a little amount of magmatic fluid is available, the presence of meteoric fluids is speculated for the cause of the hydrothermal alteration and subsolidus equilibration.View full abstractDownload PDF (773K) -
HIROMITSU TANIGUCHI, TSUTOMU MURASE1987 Volume 82 Issue 5 Pages 189-202
Published: May 05, 1987
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSIn the present study, the effects of free volume on the viscous behavior of magmatic melts are discussed on the basis of free-volume theory. The most important conclusion is that the change of viscosity with temperature, pressure, and composition is related approximately to the change of free volume in the melts.
It has been found that an increase of temperature and mafic components induces an increase of free volume in magmatic melts, and it is well known that they also tend to cause a decrease of viscosity. The relation between free volume and viscosity is interpreted as a reflection of the greater ease of movement of constituent units in melts that have more interstitial space.
Pressure effects on viscosity are also interpreted as fundamentally based on the change of free volume. Melts having an abundance of free volume at 1 atm are depolymerized melts and show an increase of viscosity with pressure owing to a decrease of free volume. On the other hand, in polymerized melts having a small free volume at low pressures, the controlling process of viscous flow is not the change of free volume but making or breaking of bonds between constituent units of the melt. Sharma et al. (1979) suggested on the basis of Raman spectroscopy that the decrease of viscosity of polymerized melts at high pressures was caused by deformation of constituent unit accompanying the weakening of Si-O-Si bond. Our explanation of the behavior of polymerized melts is in harmony with their interpretation.View full abstractDownload PDF (991K) -
Kazuya Kubo1987 Volume 82 Issue 5 Pages 203-206
Published: May 05, 1987
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSTuff breccia was found in Yamagata-mura, Kunohe-gun, Iwate-Prefecture. This tuff breccia contains many granitic and andesitic rocks with a small numbr of chert fragments. The matrix part of the tuff breccia is characterized by the presence of coarse, euhedral or corroded hornblende crystals.
K-Ar age of the hornblende is dated as 16.1±2.0 Ma, and therefore, the tuff breccia is considered to be formed at the Middle Miocene in age. At the northern Kitakami Mountains, the area of Neogene magmatism should have been more extended 10 km to the east than that was considered before.View full abstractDownload PDF (321K)
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