Fujikawa is a 128 km-long river, which originates from the ridge between Nagano and Yamanashi Prefectures and flows across Yamanashi and Shizuoka Prefectures into Suruga Bay, covering 3,990 km²of valley. The river is called as Kamanashigawa in its upstream, then merges with Fuefukigawa and Shiokawa rivers in the southern Kofu basin, and changes its name to Fujikawa. It merges with another tributary Shibakawa-river, originated from Mt. Fuji, at 15 km upstream from its mouth. Unlike its width, the usual volume of the water is small, and wide river banks covered by boulders are spreading even close to its mouth.

Geology of the Fujikawa-river
The most part of the valley of the Fujikawa-river is covered by the Shimanto Supergroup, a Cretaceous to Tertiary accretionary belt. Acceretionary belt is characteristic to the subduction zones like Japanese Islands. Majority of the terrigenous sediments such as mud and sand subducts into the mantle from trench, however, a part of them is off-scraped and accreted at the edge of the hangingwall continental plate. This is accretionary belt. Not only the terrigenous sediments, but also the shallow portion of oceanic plate such as greenstone (altered basalt) and chert are also off-scraped and accreted. During the subduction and the subsequent uplift to the surface, accretionary belt rocks are compressed by stresses of the subductiong plate that causes intense deformation on them. These processes often destroy and reconstruct the original oceanic stratigraphy (greenstone, chert, hemipelagic sediments, trench tubidite; from bottom to top) to form an occurrence that blocks and thin slices of oceanic crust and chert scattering in the sandstone/mudstone matrix. This occurrence is called as mélange, which is characteristic to accretionary complex. For this reason, boulders of both oceanic (greenstone and chert), and terrigenous (sandstone and mudstone) origins are found on the banks of the Fujikawa-river.
On the other hand, some Neogene volcanic rocks of basalt to andesite compositions are distributed along Fuefukigawa-river, and lavas and volcaniclastics of Mt. Fuji origins are distributed along Shibakawa-river. And Tertiary granitoids (granit, granodiorite and diorite) are distributed surrounding Kofu Basin. These rocks give a wide variety to the boulders of the Fujikawa-river.

* See textbook for geologic maps (PDF)

Boulders and cobbles of the Fujikawa-river can be divided into three groups.

Group 1: Oceanic origin (deepsea ~ trench)
Oceanic basement basalt (now altered to greenstone) and overlying chert, and sedimentary rocks (conglomerate, sandstone and mudstone).

Group 2: Mt. Fuji volcanics
Basaltic ~ andesitic compositions. Vesicular appearance (gas void) is characteristic.

Group 3: Relatively young volcanic rocks erupted on-land and intrusives
Basalt (now altered to greenstone), andesite and granodiorite. Greenstones of this type can be distinguished from oceanic origin by containing white zeolite vains.

Greenstone
An ex-basalt, with hydrothermal alteration. Right one-third of the field of view is zeolite vein.
Upper) open polar, lower) crossed polars.

Diorite
Coarse-grained crystals of plagioclase and hornblende form an equigranular texture.
Upper) open polar, lower) crossed polars.

Quartz and feldspars occupy the most part. Quartz is derived from sandstones and granitoids, albite (Ca-free plagioclase) is from low-grade metamorphic rocks and other plagioclase are from young volcanics.

Heavy Mineral Modal Proportion

Clinopyroxene, amphibole and orthopyroxene are the dominant minerals associated with subordinate epidote, ilmenite, micas, chlorite, garnet and zircon. Clinopyroxene, amphibole and orthopyroxene are of tephra origin, and epidote, garnet and chlorite are of metamoephic origin, and micas and zircon are of sedimentary rock origin.

Orthopyroxene composition (Mg number) has two peaks at 72 and 68. The Mg-rich ones are derived from basalt, and the Mg-poor ones are from andesite both of Mt. Fuji origin. Not only volcaniclastics but also tephras are distributed along the Fujikawa-river. Most orthopyroxenes observed in the river sands are euhedral, which is considered to be of tephra origin. Only one grain of cummingtonite is observed, which is derived from granodiorite.

Because monazite is very rare in Fujikawa-river, we cannot discuss about monazite age. But about zircon, the majority is of very young ages (less than 25 Ma) , which is very contrasting to those of Abekawa-river. These young zircons are probably derived from granitoids distributed near the boundary to the Yamanashi Prefecture.