‰ªŽR‘åŠw—�Šw•”’n‹…‰ÈŠw‹³Žº Acta Medica Okayama 1340-7414 16 1 2009 Sr–Nd isotopic compositions of Paleoproterozoic metavolcanic rocks from the southern Ashanti volcanic belt, Ghana 9 28 EN Samuel Dampare Tsugio Shibata Daniel Asiedu Osamu Okano Johnson Manu Patrick Sakyi 10.18926/ESR/19849 Neodymium (Nd) and strontium (Sr) isotopic data are presented for Paleoproterozoic metavolcanic rocks in the southern part of the Ashanti volcanic belt of Ghana. The metavolcanic rocks are predominantly basalts/basaltic andesites and andesites with minor dacites. Two types of basalts/basaltic andesites (B/A), Type I and Type II, have been identified. The Type I B/A are stratigraphically overlain by the Type II B/A, followed by the andesites and the dacites. The analyzed volcanic rocks commonly have low initial (87)Sr/(86)Sr ratios consistent with previous studies on Paleoproterozoic rocks from the West African craton. The LREE-depleted, tholeiitic Type I B/A exhibit back-arc basin geochemical signatures and show high positive epsilon Nd (i.e., ƒÃ(Nd) (2.1 Ga) = +3.89 to +7.21), which suggest a long term depleted source and also indicate that they were produced in an entirely oceanic environment devoid of influence of continental crust. The isotope signatures are thus consistent with the previously published trace element data of the Type I basalts/basaltic andesites in suggesting that their parent magma was generated from a depleted mantle. The Type I B/A have Nd model ages (T(DM2)) of 1.83–2.09 Ga similar to their formation ages, suggesting that they were juvenile at their time of formation. The andesites and the Type II B/A andesites show LREE-enriched patterns and exhibit characteristics of subduction zone-related magmas, and show initial ƒÃ(Nd) (2.1Ga) values of –1.15 to + 1.35 and Nd model ages (T(DM2)) of 2.32–2.58 Ga. The LREE-enriched dacitic porphyry also exhibits characteristics of subduction zone-related magmas, and have initial ƒÃ(Nd) (2.1Ga) value of –2.24 and Nd model ages (T(DM2)) of 2.64 Ga. The Nd isotopic data confirms the juvenile character of the Birimian crust, but also suggests some contributions of a pre-Birimian crustal material (or Archean?) in the genesis of some of the metavolcanic rocks. Our isotopic result is consistent with the island arc complex model which views Paleoproteozoic terranes of West Africa in the context of subduction–accretion processes. No potential conflict of interest relevant to this article was reported.

‰ªŽR‘åŠw—�Šw•”’n‹…‰ÈŠw‹³Žº Acta Medica Okayama 1340-7414 16 1 2009 Provenance of Early Cretaceous Hayama Formation, Okayama Prefecture, Inner Zone of Southwest Japan: constraints from modal mineralogy and mineral chemistry of derived detrital grains 29 42 EN Daniel K. Asiedu Shigeyuki Suzuki Tsugio Shibata 10.18926/ESR/19850 Petrographic and phase chemistry studies of detrital grains were carried out on sandstones from the Lower Cretaceous Hayama Formation, Inner Zone of Southwest Japan, to determine their provenance and the tectonic setting during the early Cretaceous. The results of the modal mineralogy suggest that the Hayama Formation has magmatic arc provenance and that deposition of the sediments took place in the back-arc areas with detritus mostly derived from the magmatic arc and rifted continental margins. The chemical compositions of chromian spinel, chlorite and sphene indicate that significant proportions of the detrital grains were derived from mafic and/or ultramafic sources. The source areas are the mafic and ophiolitic rocks in the Sangun-Renge and Akiyoshi terranes and the felsic volcanic rocks probably from either the Akiyoshi terrane or a source not presently exposed in southwest Japan. However, minor amounts of the detritus were derived from the basement rocks; i.e., carbonates and siliciclastic rocks of the Akiyoshi terrane and the metamorphosed mafic rocks of the Chizu terrane. No potential conflict of interest relevant to this article was reported.

‰ªŽR‘åŠw—�Šw•”’n‹…‰ÈŠw‹³Žº Acta Medica Okayama 1340-7414 12 1 2005 Major-element geochemistry of Proterozoic Prince's Town granitoid from the southern Ashanti volcanic belt, Ghana 15 30 EN Tsugio Shibata 10.18926/ESR/13853 The Pateoproterozoic metavolcanic rocks of the southern Ashanti greenstone belt of Ghana are intruded by three major suites of granitoids, locally called Prince's Town, Dixcove and Ketan plutons. The Prince's Town pluton is the largest intrusive body in the Axim area, and tends to separate the Axim volcanic branch from the Cape Three Points branch. The Pluton consists of granitic to dioritic rocks, which are generally massive but occasionally display alignment of ferromagnesian minerals. The rocks contain mainly plagioclase, K-feldspar, quartz, amphibole, biotite and opaques. The feldspars are mostly sericitized and saussuritized, and alteration of amphibole and biotite to epidote and chlorite is common. Accessory minerals include apatite, sphene and zircon. The geochemical data indicate that the rocks are tonalitic to granodioritic in composoition, metaluminous (ASI<1) and have I-type characteristics. The granitoids have the SiO2 content of 63-70% ; total iron, as Fe2 O3 of 3.10-5.80% ; (Na2O+K2O) content of 5.01-6.96% and Na2O/K2O ratios from 1.34 to 2.70 ; and are characterized by Mg# ranging from 53 to 48. The Fe*(=FeOtot/FeOtot+MgO) and modified alkali-lime index (MALI) of the rocks indicate that the Prince's Town pluton is dominantly magnesian and calcic in nature. Higher values in molar CaO/(MgO+FeOtot) coupled with low molar AI2O3/(MgO+FeOtot) may suggest their derivation from partial melting of metabasaltic to metatonalitic sourcc, with a possible contribution from metagreywacke, but preclude any contribution from metapelitic sources. The Birimaian metavolcanic rocks are the likely source material candidate for the rocks. CIPW norm calculations yielded a crystallization temperature of�`650-685�Ž and a pressure of 4-7kb for the rocks, suggesting a lower crustal souree. The Prince's Town plutonic rocks also show characteristice of plutons emplaced in a volcanic are tectonic setting environment. This observation is largely consistent with previous studies conducted on granitoids from other parts of the southern Ashanti greenstone belt c and the belt-type granitoids of Ghana as a whole. No potential conflict of interest relevant to this article was reported.

‰ªŽR‘åŠw—�Šw•”’n‹…‰ÈŠw‹³Žº Acta Medica Okayama 1340-7414 12 1 2005 Heavy mineral analysis of alluvial aedinents from the Akwatia area of the Birim diamonndierous field, Ghana 7 14 EN Tsugio Shibata 10.18926/ESR/13851 Heavy mineral analysis was carried out on Pliocene to Recent alluvial sediments from the Birim diamondiferous field od Ghana. The main objective of the study was to examine the mineral composition of heavy fractions in order to identify : (1) the heavy mineral assenblage that occur in the sediments, (2) particular diamond indicator minerals associated with the diamonds, and (3) the provenance of the alluvial sediments. The heavy minerals are essentially composed of staurolite, ilmenite and magnetite in varying proportions, with trace amounts of leucoxene, rutile, garnet and zircon. The heavy mineral assenblage and chemical composition of ilmenite and gamet suggest their derivation from phyllites and schists which reflect directly the composition of the basement rocks developed in the study area. The absence of diamond indicatior minerals such as apatite, pyrope garmet, chromian spinel, and picroilmenite in the heavy fraction is unlikely to be due to their destruction during intense weathering and/or diagenesis but rarher their non-occurrence in the area. No potential conflict of interest relevant to this article was reported.

‰ªŽR‘åŠw–„‘ •¶‰»�à’²�¸Œ¤‹†ƒZƒ“ƒ^�[ Acta Medica Okayama 16 2003 ’Ó‡‰ª‘åˆâ�Õ 11�@�\‘æ10�E12ŽŸ’²�¸�\�@�i•ÛŒ’ŠÇ—�ƒZƒ“ƒ^�[�V‰c�@•�‘®�}�‘ŠÙ�V‰c�j EN No potential conflict of interest relevant to this article was reported.

‰ªŽR‘åŠw—�Šw•”’n‹…‰ÈŠw‹³Žº Acta Medica Okayama 1340-7414 3 1 1996 Provenance of Lower Cretaceous sediments in the Nariwa and Hokubo areas, Okayama Prefecture, deduced from detrital modes and geochemistry of sandstones 1 16 EN Shigeyuki Suzuki Tsugio Shibata Petrographic and geochemical study of sandstones from the Kenseki Formation have shown that the sandstones are compositionally immature. The immaturity is reflected geochemically by their low SiO2 contents (52-66 wt%) and petrographically by low modal percents of quartz and K-feldspar, and high modal percents of plagioclase and volcanic lithic fragmants. The Kensaki sandstones are, however, poor in Na2O (up to 2.1 wt%). Both petrography and geochemistry suggest a heterogeneous source lithologies of acidic and basic volcanics, sedimentary, and ultramafic rocks. Petrographic evidence is supplied by quartz and plagioclase of volcanic origin, acidic volcanic fragments, basic volcanic fragments, volcanic glass, serpentinite fragments and detrital spinel grains. Geochemical evidence is provided by high FeO* (total iron as FeO), MgO, TiO2, CaO and K2O contents. Petrographic and geochemical study of the Kenseki sandstones indicate calc-alkaline oceanic island are provenance. The sediments were locally derived, with Alpine-type ultramafic rocks exposed in the north and volcanic materials of the Akiyoshi Belt supplying the bulk of the detritus. No potential conflict of interest relevant to this article was reported.

‰ªŽR‘åŠw—�Šw•”’n‹…‰ÈŠw‹³Žº Acta Medica Okayama 1340-7414 2 1 1995 Mineral paragenesis in thermally metamorphosed serpentinites, Ohsa-yama, Okayama Prefecture 1 12 EN Toshio Nozaka Tsugio Shibata The Ohsa-yama ultramafic body, which consists of several types of serpentinized peridotites, underwent contact metamorphism caused by a Cretaceous granite intrusion ; this metamorphism resulted in the formation of contact aureole, 1.5-2.0 km wide, around the granite intrusion, and produced progressive mineral changes in metaserpentinites toward the contact between the Ohsa-yama body and the granite intrusion. On the basis of analysis of mineral paragenetic relations, the Ohsa-yama ultramafic body can be divided into three zones with progressive changes in mineral assemblages as follows : Zone ‡T : serpentine �} chlorite �} brucite Zone ‡U : olivine + talc �} tremolite �} chlorite Zone ‡V : olivine + orthopyroxene �} tremolite �} spinel Zone ‡T corresponds to the parts unaffected by the thermal event, and Zones ‡U and ‡V correspond to the thermally metamorphosed parts of the Ohsa-yama body. The results obtained in this study are generally consistent with those of the previous studies on metamorphic peridotites from the Sangun and Muzuru zones. No potential conflict of interest relevant to this article was reported.

‰ªŽR‘åŠw—�Šw•”’n‹…‰ÈŠw‹³Žº Acta Medica Okayama 1340-7414 1 1 1994 Petrography of primary peridotites from the Ohsa-yama area, Okayama Prefecture 1 8 EN Toshio Nozaka Tsugio Shibata Ultramafic rocks exposed around Mt. Ohsa(= Ohsa-yama), Okayama prefecture, designated as "Ohsa-yama ultramafic body" all together, are one of the 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 of the ohsa-yama body where it has been affected by the contact metamorphism, the constituent minerals, texture and structure of primary ultramafic rocks have been locally preserved. Petrographic studies revead that the primary ultramafic rocks of the ohsa-yama body consist domimantly of dunite 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" of the Arai's (1980) classification. No potential conflict of interest relevant to this article was reported.

‰ªŽR‘åŠw‰·�òŒ¤‹†�Š Acta Medica Okayama 0369-7142 54 1984 ‰ªŽRŒ§‹v•ÄŒS’؈ä’nˆæ‚ÌŽO•”•Ï�¬Šâ—Þ‚ÌŽc‘¶‰Î�¬�z•¨ 43 53 EN Qing-Yi Li Tsugio Shibata Chiharu Mitsuno The Sangun metamorphic rocks of the Maniwa Group are well developed in the Tsuboi district, Okayama Prefecture. These metamorphic rocks consist mainly of psammitic and pelitic sedimentary rocks and lavas and pyroclastics of basic to acidic compositions that recrystallized during the Sangun low-grade regional metamorphism. The metamorphic rocks in this district underwent to a considerable extent a mineralogical and textural transformation, but still preserve in parts their original, mineralogical and textural features. Unaltered, relict clinopyroxenes and hornblendes are commonly observed in the metamorphic rocks derived from basic lavas and intermediate pyroclastics. Microprobe analyses of these relict minerals suggest that their host rocks were probably derivatives of magma(s) with a non-alkaline affinity. No potential conflict of interest relevant to this article was reported.