Horoman peridotite (lherzolite), Hokkaido, Japan - olivine microfabric

The Horoman peridotite complex is located in south-central Hokkaido, about 180 kilometers southeast of Sapporo, near the small coastal town of Samani (See post of April 29, 2011: "The Town that Loves Peridotites"). The layered, gently warped peridotite body crops out over an area of approximately 8 km by 10 km, with a thickness of about 3.7 km (Sawaguchi, 2004). It is the largest ultramafic body in Japan and, because it is only weakly serpentinized, it is one of the largest relatively fresh peridotite bodies exposed on land anywhere in the world.

The Horoman complex is situated at the southern extremity of the 140-km-long Hidaka metamorphic belt, following the Hidaka mountain chain, which extends southward from the Ishikari mountain massif at the center of the island of Hokkaido. Although there are six other major ultramafic complexes lying along the Hidaka metamorphic belt, they are all much smaller than the Horoman complex. Since the 1970's, the geology of the Horoman complex has been studied intensively, and there is now a substantial body of literature on the subject.

The highest peaks within the Horoman exposure area are M. Pinneshiri, at 968 meters, and Mt. Apoi (Apoi-dake), at 811 meters. The topography is rugged, and from Mt. Apoi the land surface drops to sea level in less than 3 kilometers, an average slope of 15 degrees. However, as is evident in the photograph below, slopes are much steeper toward the top of the mountain ridge.

Looking from Mt. Apoi toward Mt. Pinneshiri, along the ridge-line through the Horoman peridotite complex.
Photo by geomantleh1           via www.lherzharz1.exblog.jp/

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Geology students from five Japanese universities at a dunite outcop along the Horoman River.
Photo by geomantleh1           via www.lherzharz1.exblog.jp/

The complex is in fault contact with a gabbro layer on the west, which separates it from unmetamorphosed sedimentary rocks. On the north, east, and south, the complex is in fault contact with schists, gneisses, and hornfels. The Horoman peridotite was initially formed in the upper mantle in the late Proterozoic, about 830 Ma. It was then exhumed to the plagioclase stability field, and later subducted to the garnet stability field in the upper mantle. A second generation of partial melting took place in the Cretaceous (~80 Ma), when a collision of two arc-trench systems created the Hidaka metamorphic belt. The body finally ascended upward again in the Miocene (~23 Ma), from the mantle to the crust, where it re-equilibrated in the spinel stability field.

At least three characteristics of the Horoman complex are somewhat unusual. First, there is  well-developed compositional layering throughout the body (Niida, 1974; Niida and Takazawa, 2007); Second, abundant spinel-plagioclase and spinel-pyroxene symplectite nodules occur in several of the peridotite layers (Morishita, 2000; Morishita and Arai, 2003); and third, there are three commingling types of olivine fabric in the peridotite (Niida, 1975). This post deals with the last-named characteristic.

The three types of olivine fabric identified by Niida (1975) are:
1. Primary olivine in large grains, often sheared or strained (image directly below).
2. Polygonal olivine grains, recrystallized (second image below).
3. Fine aggregates resulting from cataclasis and mylonitization (third image below).

Click on image to enlarge.          Photo © Daniel R. Snyder
Large primary olivine grain, recrystallized and showing deformation banding.
XPL digital mosaic. Imaged area approximately 4.8 mm by 5.5 mm.

Click on image to enlarge.          Photo © Daniel R. Snyder
"Polygonal" (subhedral) olivine inclusion in orthopyroxene. XPL.
Imaged area 1.3 mm by 1.7 mm.

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Click on image to enlarge.          Photo © Daniel R. Snyder
Fine olivine aggregate created by cataclasis and mylonization. XPL.
Imaged area 2.7 mm by 4 mm.

REFERENCES

Morishita, T., 2000, Three-dimensional Microstructure of Symplectite Minerals in the Horoman Peridotite: A preliminary Analysis; Jour. Geol Soc. Japan; 106:11, p. 800-811. (In English with Japanese abstract)

Morishita, T., and Arai, S., (2003), Evolution of spinel–pyroxene symplectite in spinel–lherzolites from the Horoman Complex, Japan; Contrib. Mineral. Petrol; 144, p. 509-22. (In English)

Niida, K., (1974), Structure of the Horoman Massif of the Hidaka Metamorphic Belt, Jour. Geol Soc. Japan; 80:1, p. 31-44. (In English)

Niida,  K., (1975), Textures and Olivine Fabrics of the Horoman Ultramafic Rocks, Japan; Jour. Japan. Assoc. Min. Petr. Econ, Geol.; 70, p. 265-285. (In English with Japanese abstract)

Niida, K.,  and Takazawa, E. (2007), Origin of Layering observed in the Horoman Peridotite Complex, Japan, Jour. Geol Soc. Japan; 113:Supplement, p. 167-184. (In Japanese except for some of the figure labels)

Takahashi, N., (1991), Origin of three peridotite suites from the Horoman peridotite complex, Hokkaido, Japan; Melting, melt segregation, and solidification processes in the upper mantle; Jour. Min.Petr. Econ. Geol.,  86: p. 199-215. (In English)

Sawaguchi, T., (2004), Deformation history and exhumation process of the Horoman Peridotite Complex, Hokkaido, Japan. Tectonophysics, 379, p. 109-126. (In English)