OKAYAMA UNIVERSITY Earth Science Report, Vol. 1, No. 1, 1-8, (1994) Petrography ofprimary peridotites from the Ohsa-yama area, Okayama Prefecture Toshio NOZAKA and Tsugio SHIBATA Department ofEarth Sciences, Faculty ofScience Okayama University Ultramafic rocks exposed around Mt. Ohsa (= Ohsa-yama), Okayama Prefecture, designated as "Ohsa­ yama ultramafic body" all together, are one ofthe Alpine-type peridotites in the Sangun metamorphic belt. They are intensely serpentinized and locally suffered contact metamorphism by younger granitic intrusions. In a por­ tion ofthe Ohsa-yama body where ithas notbeen affected by the contact metamorphism, the constituent minerals, texture and structure ofprimary ultramafic rocks have been locally preserved. Petrographic studies reveal that the primary ultramafic rocks ofthe Ohsa-yama body consist dominantly ofdunite and harzburgite possessing no obvious layering, and their constituent minerals are similar in composition to those of the Tari-Misaka and Ashidachi ultramafic bodies. These features indicate that unlike the Ochiai-Hokubo body, the Ohsa-yama ultra­ mafic body belongs to the "massive group" ofthe Arai's (1980) classification. Keywords: petrography, dunite, harzburgite, massive ultramafic body I. Introduction structure, and consist ofdunite, harzburgite and A number ofultramafic bodies occur in the chromitite. He contends that the former repre­ so-called "Sangun metamorphic belt" of high­ sents cumulates derived from crystallization of pressure type in the inner zone ofsouthwestern basaltic magma, the latter refractory residues Japan. Most of them are intensely serpenti­ left after partial fusion ofprimordial peridotites, nized; in addition, they are in many cases ther­ and both have initially constituted a part of an mally metamorphosed by younger granitic rocks ophiolite suite. (Research Group of Peridotite Intrusion, 1967). Compared to other regions in the Sangun Due to intense serpentinization and contact belt, ultramafic rocks are exposed most abun­ metamorphism, the primary petrologic charac­ dantly in the region from northwestern ters ofthese ultramafic bodies were generally ob­ Okayama Prefecture to northeastern Hiroshima scured, but original structure, texture and min­ Prefecture (Fig. 1). Among the relatively large erals have been preserved in some portions ofin­ ultramafic bodies in this region, the Tari-Misaka dividual bodies, hence making it possible to ob­ and Ashidachi bodies belong to the Arai's mas­ tain their primary characters (e.g., Igi and Abe, sive group, and the Ochiai-Hokubo body (also 1969; Arai, 1980). called "Onoro-yama body") belongs to the layered According to Arai (1980), the ultramafic group (Arai, 1980; Hamada, 1982); however, the bodies in the Sangun-Yamaguchi Zone can be nature of ultramafic rocks exposed around Mt. classified into two groups on the basis oforiginal Ohsa (= Ohsa-yama*), Okayama Prefecture (des­ structure and rock type; i.e., layered group and ignated hereafter as "Ohsa-yama ultramafic massive group. The layered bodies consist ofsev­ body") has not been clearly known. Petrography eral types of rocks such as lherzolite, dunite, ofsome peridotites in the Ohsa-yama bodyis pre­ wehrlite, chromitite, websterite and clinopyroxe­ sented here for the purpose ofclarification ofits nite. The massive bodies rarely show layered primary petrologic nature. * also spelled "Oosa-yama" in other papers. 2 Toshio NOZAKA and Tsugio SHIBATA ~\ Okayarru/ '------!----,.--,Pref < ( \ ... OH lOkm 133°30'E Fig. 1 Distribution of ultramafic rocks in the area ofnorthwestern Okayama and northeastern Hiroshima Prefectures (compiled after Hiroshima Prefecture, 1964; Igi and Sakamoto, 1977; and Mitsuno and Sugita,1980). Abbreviations for ultramafic bodies: TM, Tari-Msiaka body; AS, Ashidach body, OH, Ochiai-Hokubo body; OS, Ohsa-yama body. 11. Geological Setting is ca. 3 km wide from north to south and 5 km The Sangun metamorphic belt is charac­ long from east to west. The exposed body has an terized by glaucophanitic metamorphism (e.g., unusual, ax-like shape in plane (Fig. 2). Rock Hashimoto, 1968). Recent chronological studies outcrops are found sporadically throughout the reveal that the Sangun metamorphic rocks have body. The ultramafic rocks are intensely serpen­ a significantly wide range of radiometric ages tinized, except for those occurring at the south­ (Nishimura and Shibata, 1987). On the basis of western slope of Mt. Ohsa where the serpenti­ distribution of crystalline schists that show ap­ nites have reverted to compact peridotitic rocks proximately identical ages, Nishimura (1990) due to contact metamorphism by a younger gra­ subdivided the glaucophanitic terrane into three nitic intrusion. Many gabbroic and leucocratic regions; i.e., Sangun-Renge Belt (around 300 blocks with a few meters in diameter occur in the Ma), Suo Terrane (around 220 Ma) and Chizu Ohsa-yama body. These blocks consist of fine­ Terrane (around 180 Ma). Ophiolitic rocks such grained gabbro, jadeitite, albitite and rodingite, as serpentinite and metagabbro occurring in the and have been found only within the ultramafic glaucophanitic terrane are also different in age body. Therefore, they are thought to have been from one region to another, though they are included into or formed within the ultramafic older than the surrounding crystalline schists in body before its emplacement into the present po­ each region. For example, the Ohsa-yama ultra­ sition in the surrounding schists. mafic body is included in the 180 Ma Chizu Ter­ The Ohsa-yama ultramaficbody is in fault rane, and a block of basic schist in this body has contact with crystalline schists on the north and a K-Ar age ofca. 250 Ma (Watanabe et al., 1987). east ofthe body. The crystalline schists are com­ Also, the Tari-Misaka, Ashidachi and Ochiai­ posed dominantly ofpelitic and basic rocks with Hokubo ultramafic bodies belong to the Chizu small amounts of intercalated thin beds of Terrane, so that we infer thatthere exists a close psammitic and siliceous rocks. Their schistosity petrogenetic link among these ultramafic bodies. planes trend N-S or NE-SW and dip 30 to 60 0 NW. On a macroscopic scale, however, these Ill. General Geology ofthe Ohsa-yama Area schists form a large fold with its axial plane The Ohsa-yama ultramafic bodyis exposed trending E-Wand extending from the in an area around the summitofMt. Ohsa, which Katsuyama through Ohsa-yama to Hokubo areas Petrography ofprimary peridotites from the Ohsa-yama area 3 133"31'30''E r::::::l l:.:..:..:..:.: Alluvium t::+;+)+++++ Granite ~ + ~ Rhyolite H' ++++ ++++ r::::::l ++++ +++++ ~ Conglomerate +++++ ++t++ ++++++ § Pelitic schist +++++++++++ .. +.+++++++++ '-+++++++t++++ ///// ++++++ .. + .. ++ • ///// ........++...... Basic schist -.••+.++++++.+ .. ............ ++.1' •• " ++••+ ........................................+ ++ +++ ..... ++++++ .. ++ ++.+ .. Ultramafic rock ...................+ ++ ++ .. ........ + ............. t'" ++ .. ++ + ... .......................... + . ...... ++ ........... ++ .. ++ ++ ...................... +++ ++ .. ................. ++++ ++ .. ................... ++ .. + ++ ... ............ ++++ +++ ... + ... .. +++ ......... ++ .... ++++ ++ .+t•••••••• ++-+-.+-.+- ++ ++ ++ ++ ....................... +- .......... ++ ++ .. ++ .. ++...... .. +++ ++ ++ ++ ++ +++ ++ ++.+ ++ .. ++ .......+ ++ ++ + ++ ++++ .. 1 km. 35°4'N ++++":: ..••. ....+.++++::::::::::::::.. ++++.+++++++++++.::::t:::: Fig. 2 Geological sketch map ofthe Ohsa-yama (Mt. Ohsa) area. (Mitsuno and Sugita, 1980). The Ohsa-yama ul­ sion (Nozaka, 1987). The minerals formed bythe tramafic body is situated at the crest ofthis fold. contact metamorphism include orthopyroxene, The unusual shape ofthe Ohsa-yama body may olivine, talc and tremolite, and the recrystallized result from bending associated with the large­ Ohsa-yama rocks show distinct, granoblastic or scale folding. poikiloblastic textures. The ultramafi.c rocks oc­ Metamorphic minerals composing the curring in the eastern portion ofthe Ohsa-yama crystalline schists adjacent to the Ohsa-yama body are not affected by the contact metamor­ body include muscovite, chlorite and epidote in phism. Although these rocks have suffered in­ pelitic schists, and chlorite, epidote, actinolite, tense serpentinization, they still locally preserve albite, stilpnomelane, muscovite and locally the original textures and contain olivine, glaucophane in basic schists. A lawsonite-bear­ orthopyroxene, clinopyroxene and brown spinel ing basic schist has also been reported from the as relict minerals. Any layered structures on Ohsa-yama area (Hashimoto and Igi, 1970). Oc­ both mesoscopic and microscopic scales are not currence of these metamorphic minerals sug­ recognized in the Ohsa-yama body. gests high PIT metamorphism in this area. Olivine, the most abundant relict phase, is The Ohsa-yama body and the surrounding equidimensional in shape and a few millimeters schists are intruded by a biotite granite and in diameter. Pyroxenes have commonly exsolu­ hornblende-biotite granite, and overlain by tion lamellae; i.e., clinopyroxene lamellae in or­ unmetamorphosed conglomerates and rhyolites. thopyroxene, and vice versa. Brown spinel has These younger igneous and sedimentary rocks an equidimensional shape in a serpentine ma­ are Cretaceous in age (Mitsuno and Sugita, trix, and a vermicular shape within orin contact 1980). with clinopyroxene grains. The relict minerals are never indirect con­ IV. Petrography tact with each other due to replacement