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)

Second alteration cycle - serpentine altering to anthophyllite (Presque Isle peridotite)

Click on image to enlarge.          Photo © Daniel R. Snyder

The grain shown in this image, originally olivine, was altered to serpentine, most of which has been altered in turn to anthophyllite. That is, I thought it was anthophyllite, but then I read Michael Lewan's (1972)* M. S. thesis, in which he reported NO anthophyllite in the three samples of Presque Isle peridotite for which he carried out modal analysis. However, all three samples contained small amounts of chlorite (7.3, 7.1, and 5.4 percent volume. I've never seen chlorite that looks like this, but maybe... If you know, please send me a comment.

Tiny red or orange anthophyllite(?) crystals occupy most of the former olivine grain, except for the two remaining areas of serpentine, gray-green in the XPL image above (lower left and upper right). The image below, also XPL, shows most of the anthophyllite(?) crystals (those aligned from upper left to lower right, or vice versa) at extinction - as extinct as they get. The serpentine areas show more clearly in this view, while the anthophyllite(?) crystals that are "at extinction" have dark outlines of varying thickness.

Click on image to enlarge.          Photo © Daniel R. Snyder

In the PPL image below, the serpentine areas are relatively featureless, while elsewhere the elongated crystals of anthophyllite(?) show distinct outlines. Yellow-orange color in PPL image is due to iron-oxide staining from hematite. Black spots are magnetite. Presque Isle peridotite, Marquette County, northern Michigan. Imaged area 0.5 mm x 0.8 mm.

Click on image to enlarge.          Photo © Daniel R. Snyder

*Lewan, Michael D., 1972, Metasomatism and Weathering of the Presque Isle Serpentinized Peridotite, Marquette, Michigan, unpublished M.S. thesis, Michigan Technological University.

Presque Isle peridotite - full thin section

Click on image to enlarge.          Photo © Daniel R. Snyder

This image shows the texture of the peridotite, in which olivine and pyroxene have been almost entirely pseudomorphed by serpentine, much of which has been replaced in turn by amphiboles. Light-colored areas are carbonate veins. Marquette County, northern Michigan. XPL Macrophotograph. Imaged area 25 mm x 41 mm.

Plagioclase in Yellow Dog peridotite (plagioclase-bearing lherzolite)

Click on image to enlarge.          Photo © Daniel R. Snyder

 Orthopyroxene at upper left (gray), enclosing plagioclase laths; clinopyroxene (magenta) at lower right; olivine at lower left, upper right, and top center. Yellow Dog Plains, Marquette County, northern Michigan. XPL. Imaged area 2.7 mm x 4 mm (2X objective).

This is an unusually rich concentration of plagioclase (center) for the Yellow Dog peridotite. Most of the plagioclase in my samples is scattered as single laths, perhaps two or three small grains in a typical frame this size.  Older geology texts rule out ANY plagioclase if a rock is to be called "peridotite", but the IUGS classification allows up to 10 percent, in which case they are called "plagioclase-bearing ____". However, the consultant's report states that the plagioclase can average 25 to 30 percent "over significant intervals". In order to accommodate this, the consultant uses a maximum of 30% plagioclase for peridotite. The report defines rocks with between 10% and 30% plagioclase as "feldspathic peridotite".  (This report is on the Web. You can get it in PDF format by Googling "Eagle Deposit Geology".) Under the IUGS classification, these rocks would either be olivine gabbro, olivine norite, or olivine gabbronorite.

Deformation bands in olivine

Click on image to enlarge.          Photo © Daniel R. Snyder

In this image of dunite, several large olivine grains (center, top left, top right, bottom right) show deformation banding. Webster-Addie ultramafic body, Blue Ridge Mountains, Jackson County, Western north Carolina. XPL. Imaged area 2.7 mm x 4 mm.

Twin Sisters dunite - full thin section

Click on image to enlarge.          Photo ©  Daniel R. Snyder

Thin section of Twin Sisters dunite showing strained olivine and pyroxene grains with deformation banding. Cascade Mountains, Whatcom County, northwestern Washington. Macrophotograph, XPL. Imaged area 21 mm x 40 mm. Link to Dan McShane's Twin Sisters page.

Pyroxene in dunite

Click on the image to enlarge.           Photo: Dan Snyder

A large orthopyroxene grain in dunite. According to the IUGS classification, a dunite is a rock made up of ferromagnesian minerals, mainly olivine and pyroxene, of which more than 90% is olivine (discounting accessory minerals). In practice, serpentine inferred to be derived from olivine is counted as olivine. Although a pyroxene grain dominates this photograph, there are only a few such grains in the sample.  Webster-Addie ultramafic body, Jackson County, North Carolina. Reflected-light photomicrograph of polished block. Ordinary light. Imaged area 3.3 mm x 4 mm.

Yellow Dog peridotite (plagioclase-bearing lherzolite)

Click on the image to enlarge.           Photo © Daniel R. Snyder

The Yellow Dog peridotite, thought to be approximately 1.1 Ga in age, is composed  of partly serpentinized peridotite, containing 40 to 50 percent olivine (1/2 to 2/3 serpentinitzed) and 30 percent pyroxenes consisting of subequal amounts of orthopyroxene and clinopyroxene. (Klasner, et al., 1979)*. In addition, it contains 5 to10 percent plagioclase, which would classify it as a "plagioclase-bearing lherzolite" according to the IUGS criteria.

The peridotite body was first described by William Morris in his 1977 M.S. thesis**. He named it the "Yellow Dog Plains peridotite". The surface outcrop is roughly oval-shaped in horizontal cross-section. Doug Hull's comment below is valid.  Appendix C (Geology) to the mining development permit request is now on the Web. You can get it in PDF format by Googling "Eagle Deposit Geology". This report includes cross-sections, at irregular horizontal intervals and at 50-meter vertical intervals, of what is referred to as the "Eagle Deposit". These diagrams justifiy calling it a dike.The report states that the "intrusion at the surface"  extends 480 meters in length and is 100 meters wide at its thickest point. The peridotite intrudes metasediments that are at least 1.9 Ga in age.  

The image above shows two heavily fractured olivine macrocrysts, which were most likely carried upward by magma originating in the upper mantle. A serpentine meshwork fills most of the fractures, and stringers of magnetite occupy the larger fractures in the crystal on the right. Yellow Dog Plains, Marquette County, northern Michigan. XPL. Imaged area 2.7 mm x 4 mm.

Below is an optical scan of a fractured surface of a hand sample of Yellow Dog peridotite. 2400 dpi optical scan, Imaged area 17 mm x 17 mm. 

Click on the image to enlarge.          Photo © Daniel R. Snyder

*John S. Klasner, David. W. Snider, W. F. Cannon, and John F. Slack (1979), The Yellow Dog Peridotite and a Possible Buried Igneous Complex of Lower Keweenawan Age in the Northern Peninsula of Michigan, Geological Survey Division, Michigan Department of Natural Resources, 38 p.

**Morris, William J., (1977) Geochemistry and Origin of the Yellow Dog Plains Peridotite, Marquette County, Northern Michigan, unpublished master's thesis, Michigan State University, 82p.