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K‐Ar Ages of Granitic Magmatism and Related Pegmatite Formation at the Umanotani‐Shiroyama Mine, Shimane Prefecture, SW Japan and Their Bearings on Cooling History
Author(s) -
Kihara Shouji,
Hoshino Kenichi,
Watanabe Makoto,
Nishido Hirotsugu,
Ishihara Shunso
Publication year - 2005
Publication title -
resource geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.597
H-Index - 43
eISSN - 1751-3928
pISSN - 1344-1698
DOI - 10.1111/j.1751-3928.2005.tb00234.x
Subject(s) - pegmatite , muscovite , geology , geochemistry , biotite , feldspar , quartz , microcline , zircon , tourmaline , ilmenite , magmatism , mineralogy , tectonics , seismology , paleontology
. The Umanotani‐Shiroyama pegmatite deposits, the largest producer of K‐feldspar and quartz in Japan, are of typical granitic pegmatite. Ilmenite‐series biotite granite and granite porphyry, hosting the ore deposits, and biotites separated from these rocks yielded K‐Ar ages ranging from 89.0 to 81.4 Ma and 95.2 to 93.7 Ma, respectively. Muscovite and K‐feldspar separated from the ore zone yielded K‐Ar ages with the range of 96.2 to 93.1 Ma and 87.3 to 80.7 Ma, respectively. Muscovites from quartz‐muscovite veins in the ore zone and in the granite porphyry yielded K‐Ar ages of 90.4 and 76.3 Ma, respectively. K‐feldspar is much younger in age than coexisting muscovite. It is noted that the K‐Ar ages of biotite separates and the whole‐rock ages are identical to those of muscovite and K‐feldspar in the ore zone, respectively. These time relations, as well as field occurrence, indicate that the formation of the pegmatite deposits at the Umanotani‐Shiroyama mine is closely related in space and time to a series of granitic magmatism of ilmenite‐series nature. Using closure temperatures of the K‐Ar system for biotite and K‐feldspar (microcline), cooling rate of the pegmatite deposits is estimated to be about 82d̀C/m.y. at the beginning, but slowed down to about 15d̀C/m.y. in the later period.