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Oxygen isotope and some other geochemical evidence for the origin of two contrasting types of granitic rocks of Japan

Hiroji Honma
Hiroji Honma
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January 01, 1983

There are two different groups of Late Cretaceous to Early Tertiary granitic rocks in Japan: (1) intrusions into non-metamorphic terranes, with σ18O of quartz ranging from 8.5 to 10.0 o/oo, and (2) granitic rocks of low P/T type metamorphic belts, with σ18O of quartz ranging from 11.5 to 14.5 o/oo.

Granitic rocks of the Chugoku non-metamorphic belt are only slightly (0.8 to 1.0 o/oo) enriched in l8O compared with associated basic plutonic rocks having low 18O/16O ratios (whole-rock σ18O: 6.2-1.5 o/oo). The paucity of 18O enrichment is thought to be associated with magma crystallization and selective settling of certain minerals. Contribution of crustal rocks to the chemistry of granites is detected only locally and on a small scale.

In contrast, granite magmas in and around the Ryoke metamorphic belt are enriched in 18O, whereas the surrounding metamorphic and migmatitic rocks are considerably depleted in 18O compared with their original state. Other geochemical evidence, such as the NH4 content and major element chemistry of biotites, also indicates a significant contribution by sedimentary rocks to the chemistry of granitic rocks of the Ryoke belt.

Characteristic geological and geochemical features of granitic rocks of the Ryoke belt may be related to the special geological situation of this belt. For example, the tectonic movement in the Ryoke belt may have enhanced the mobility of fluid and, thus, increased the interaction between granite magma and country rocks.

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GSA Memoirs

Circum-Pacific Plutonic Terranes

J. A. Roddick
J. A. Roddick
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Geological Society of America
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Publication date:
January 01, 1983




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