Geoscience reports of Shimane University

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Geoscience reports of Shimane University 18
1999-12-25 発行

琵琶湖北東部,後期白亜紀貝月山花崗岩体

Late Cretaceous Kaizukiyama granite,northeast of LakeBiwa,Southwest Japan
Sugii, Kanji
Sawada, Yoshihiro
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Description
The Kaizukiyama granite is distributed northeast of Lake Biwa and was intruded into the Jurassic accretionary complex of the Mino-Tamba Belt.The grantic mass crops out over an area of approximately 120k㎡,(14 km N-S,11.5 km E-W).lt consists mainly of equigranular coarse-grained hornblende bearing biotite granite to granodiorite,associated with porphyritic hornblende bearing biotite granite-granodiorite,medium-grained muscovite granite,and granite porphyry.
Equigranular hornblende bearing biotite granite-granodiorite of the Kaizukiyama granite consists mainly Plagioclase,K-feldspar and quartz,with subordinate biotite and hornblende,and accessory opaque minerals,apatite,zircon,monazite,allanite and tourmaline.Porphyritic hornblende bearing biotite granite-granodiotite have the same mineralogy,but contain K-feldspar megacrysts up to 7 cm in length.Medium-grained muscovite granite occurs in the central part of the Kaizukiyama granite as schlieren less than a few tens of meters in width,and consists mainly of plagioclase,K-feldspar,quartz and muscovite,with accessory biotite,garnet,opaque minerals,apatite,zircon and monazite.
Color index(modal composition of mafic minerals)and modal plagioclase / K-feldspar ratio of slab samples and magnetic susceptibility tend to decrease from the margin to core,and with increasing of altitude(from 250 m to 1200 m).
SiO_2 contents of hornblende bearing biotite granite−granodiorite and muscovitegranite from the Kaizukiyama granite are 66.52-72.23 wt% and 75.25-75.87 wt%,respectively.Muscovite granite plots with in the S-type granite field on the ACF diagram of White and Chappell(1977).
NCID
AA11455176