The Kamuikotan zone in Hokkaido, Japan: tectonic mixing of high-pressure and low-pressure metamorphic rocks |
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| Authors: | HIDEO ISHIZUKA MASAYUKI IMAIZUMI NOBUO GOUCHI SHOHEI BANNO |
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| Institution: | Department of Geology, Kochi University, Kochi, 780 Japan; Ministry of Agriculture and Forestry, Tokyo, 101 Japan; Shiretoko Museum, Shari, Hokkaido, 099-41 Japan; Department of Geology and Mineralogy, Kyoto University, Kyoto, 606 Japan |
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| Abstract: | Abstract. In the Kamuikotan zone, central Hokkaido, Japan, two distinct types of metamorphic rocks are tectonically mixed up, along with a great quantity of ultramafic rocks; one type consists of high-pressure metamorphic rocks, and the other of low-pressure ones. The high-pressure metamorphic rocks are divided into two categories. (1) Prograde greenschist to glaucophaneschist facies rocks derived from mudstone, sandstone, limestone, a variety of basic rocks such as pillow and massive lavas, hyaloclastite and tuff, and radiolarian (Valanginian to Hauterivian) chert, among which the basic rocks and the chert, and occasionally the sandstone, occur as incoherent blocks (or inclusions) enveloped by mudstone. (2) Retrograde amphibolites with minor metachert and glaucophane-calcite rock, which are tectonic (or exotic) blocks enclosed within prograde mudstone or serpentinite, or separated from these prograde rocks by faults. The K-Ar ages of the prograde metamorphic rocks (72, 107 and 116 Ma on phengitic muscovites) are younger than those of the retrograde rocks (109, 132, 135 and 145 Ma on muscovites, and 120 Ma on hornblende). The low-pressure metamorphic rocks consist of the mafic members of an ophiolite sequence with a capping of radiolarian (Tithonian) chert with the metamorphic grade ranging from the zeolite facies, through the greenschist (partly, actinolite-calcic plagioclase) facies to the amphibolite (partly, hornblende-granulite) facies. The low-pressure metamorphism has a number of similarities with that described for'ocean-floor'metamorphism. The tectonic evolution of such a mixed-up zone is discussed in relation to Mesozoic plate motion. |
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| Keywords: | Key-words: high-pressure metamorphism Kamuikotan zone low-pressure metamorphism ophiolite tectonic block |
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