Chromite-bearing spessartites from Kasuga-mura,Japan, and their bearing on possible mantle origin andesite |
| |
Authors: | Kazuhiro Suzuki Keiichi Shiraki |
| |
Institution: | (1) Department of Astronomy and Earth Sciences, Tokyo Gakugei University, Koganei-shi, 184 Tokyo, Japan;(2) Department of Earth Sciences, Faculty of Science, Nagoya University, Chikusa-ku, 464 Nagoya, Japan;(3) Present address: Department of Earth Sciences, Faculty of Science, Nagoya University, Chikusa-ku, 464 Nagoya, Japan |
| |
Abstract: | An unusual association of chromite and hornblende was found in the spessartites of andesite composition, occurring as a dike
swarm associated with a Cretaceous granite batholith. The spessartites are largely porphyritic with phenocrysts of either
hornblende or augite. One dike, comprising a finegrained spessartite, exhibits distinct chilled selvages of aphanitic facies.
The chromites in the fine-grained and augite-spessartites are significantly higher in Cr/ (Cr+Al) than those occurring rarely
as inclusions in the phenocrystic hornblendes in the hornblende spessartite, although both are similar in Mg/ (Mg+Fe), Fe2O3, and TiO2. The phenocrystic hornblendes are titaniferous pargasite with high Mg/ (Mg+Fe), and differ in their higher octahedral Al
from the groundmass hornblendes including those in the fine-grained spessartite. The crystallization sequence in the phenocrystic
hornblende-bearing spessartites is Al-rich chromite, phenocrystic hornblende, and plagioclase without pyroxene, suggesting
a high water content in the magma and the start of the crystallization at relatively high pressures. The finegrained spessartite
from which the porphyritic spessartites have been derived by fractionation of dominant mafic minerals, has the high Mg-value
and Cr content equivalent to those in primitive, undifferentiated basalts, although still andesitic in SiO2 content. Chemically similar magnesian andesites, although uncommon, found in some orogenic calc-alkalic suites may represent
a magma composition in equilibrium with mantle peridotite under the condition of high water pressures. |
| |
Keywords: | |
本文献已被 SpringerLink 等数据库收录! |
|