Thursday, July 11, 2013

Horoman peridotite (lherzolite) - full thin section

 For a general description of the Horoman peridotite body, see the post of July 2, 2013: "Horoman Peridotite ... - olivine microfabfric".

This image shows a full thin section of Horoman peridotite. The 29-mm long dimension is actually shorter than a typical thin section, which is usually between 38 mm and 42 mm. The coarse texture of the peridotite is evident in this image. For example, the large olivine grain just left of center is about 10 mm in length. Most of the olivine grains are strained, as indicated by their banded coloration (deformation banding). The pyroxene grains also show deformation banding, but much less obviously than the olivine.

FULL THIN SECTION
Click on image to enlarge.          Photo © Daniel R. Snyder
Horoman peridotite (lherzolite), Hokkaido, Japan. Full thin section, XPL macrophotograph. Brightly-colored grains are olivine; gray grains are pyroxenes. Smaller, rounded brown grains are symplectite nodules. Imaged area approximately 21 mm by 29 mm.



REFERENCES

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., (1984), Petrology of the Horoman Ultramafic Rocks in the Hidaka Metamorphic Belt, Hokkaido, Japan, Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy, 21(2):197-250. (In English)

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)

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

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) 


Wednesday, July 10, 2013

Horoman peridotite - compositional layering

For a general description of the Horoman peridotite body, see the post of July 2, 2013: "Horoman Peridotite ... - olivine microfabfric".

As shown on the map by Niida (1974) the rocks of the Horoman peridotite body are mainly plagioclase lherzolite, lherzolite, and dunite, as well as small amounts of gabbro and pyroxenite. Interlayered plagioclase lherzolite and dunite account for almost all of the bedrock surface in the northern three-quarters of the body, from south of Mt. Apoi to north of Mt. Pinneshiri, a distance of about seven kilometers. Thus, these two rock types dominate the lithology of the body. The remaining southern one-quarter of the surface is mainly interlayered lherzolite and dunite. You can download Prof. Niida's 1984 paper, including the dramatic geological map, from: http://eprints.lib.hokudai.ac.jp/dspace/handle/2115/36729  This will take you to the abstract of Dr. Niida's paper in HUSCAP, Hokkaido University Scholarly and Academic Papers. To download the entire PDF (11.4 mb), click  the "View/Open" box. The map is on the fourth page (page 200).



Compositional layering in the upper zone of the Horoman peridotite complex.
This is the best picture of layering in the Horoman complex that I have seen. If
you have a better one, send it to me and I'll post it. If you happen to be the man
in the photograph, please let me know so I can identify you.
Photo: geomantleh1 via lherzharz1.exblog.jp


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University students on a field trip in the Horoman peridotite complex. Layering is clearly
visible in the fallen rock at right. Photo: geomantleh1 via lherzharz1.exblog.jp



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REFERENCES

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., (1984), Petrology of the Horoman Ultramafic Rocks in the Hidaka Metamorphic Belt, Hokkaido, Japan, Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy, 21(2):197-250. (In English)

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)





Tuesday, July 9, 2013

Horoman peridotite (lherzolite), Hokkaido, Japan - symplectite pseudomorphs after garnet.

For a general description of the Horoman peridotite body, see the post of July 2, 2013: "Horoman Peridotite ... - olivine microfabfric".

Symplectite nodules (intergrowths of fine-grained minerals - in this example, spinel and clinopyroxene) are abundant in some layers of the Horoman peridotite complex. Because of their mineral assemblages and bulk chemical composition, these nodules are thought to be pseudomorphs after pyrope garnet.

Click on image to enlarge.          Photo © Daniel R. Snyder
Three large symplectite nodules in the Horoman peridotite body.  Brightly-colored
grains are olivine; gray grains are pyroxenes. XPL. Imaged area 2.7 mm by 4 mm.



Below: Higher-magnification image (10x objective) of a small symplectite nodule.

Click on image to enlarge.          Photo © Daniel R. Snyder
XPL. Imaged area 0.5 mm by 0.8 mm.


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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)

Odashima, N., Morishita, T., Ozawa, k., Nagahara, H., Tsuchiyama, A., and Nagashima, R., (2008), Formation and deformation mechanisms of pyroxene-spinel in an ascending mantle, the Horoman peridotite complex, Japan: an EBSD (electron backscatter diffraction) study; Jour. Mineral. Petrol. Sci., 103, p. 1-15. (In English)

Tuesday, July 2, 2013

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)