This short article presents some diagrams showing the compositional variations of primary and metamorphic olivine, orthopyroxene and clinopyroxene in peridotites and serpentinites from thermally metamorphosed ultramafic complexes in SW Japan. In contrast to olivine, which shows a gradual change of chemical composition corresponding with metamorphic grade, orthopyroxene and clinopyroxene show clear differences in composition between primary and metamorphic phases. Compared with primary pyroxenes, even though their compositions could be variable depending on original rock composition, metamorphic orthopyroxene and
metamorphic clinopyroxene is clearly deficient in Cr(2)O(3) and CaO, and in Cr(2)O(3) and Al(2)O(3), respectively. These characteristics are useful for the discrimination between the pyroxenes of different origin

Nozaka, Toshio

English

Okayama University Digital Information Repository

西南日本の熱変成超苦鉄質岩体中のかんらん石と輝石の組成変化に関する覚書

Okayama University Scientific Achievement Repository

OKAYAMA University Earth Science Reports,  Vol. 17, No. 1, 1-5, (2010) A note on compositional variation of olivine and pyroxene in thermally metamorphosed ultramafic complexes from SW Japan  Toshio NOZAKA*  This short article presents some diagrams showing the compositional variations of primary and metamorphic olivine, orthopyroxene and clinopyroxene in peridotites and serpentinites from thermally metamorphosed ultramafic complexes in SW Japan. In contrast to olivine, which shows a gradual change of chemical composition corresponding with metamorphic grade, orthopyroxene and clinopyroxene show clear differences in composition between primary and metamorphic phases. Compared with primary pyroxenes, even though their compositions could be variable depending on original rock composition, metamorphic orthopyroxene and metamorphic clinopyroxene is clearly deficient in Cr2O3 and CaO, and in Cr2O3 and Al2O3, respectively. These characteristics are useful for the discrimination between the pyroxenes of different origin.  Keywords: chemical composition, olivine, orthopyroxene, clinopyroxene, metaperidotite   I. Introduction  Prograde and retrograde metamorphism of mantle peridotites is important for understanding several aspects of global mass balance, and geological and geophysical processes at variable tectonic settings. The most effective retrogression process of peridotites is serpentinization, which commonly forms serpentine, brucite, chlorite, secondary clinopyroxene, magnetite, and minor amounts of Fe- or Fe-Ni sulfides/metals. Relict grains of primary minerals originated in mantle peridotites, such as olivine, orthopyroxene, clinopyroxene and chromian spinel or chromite, locally occurs where serpentinization is incompletion. On the other hand, prograde metamorphism after serpentinization commonly takes place as well in most of peridotites exposed on land, and forms variable mineral association of olivine, orthopyroxene, Mg-Fe- amphibole, Ca-amphibole, talc and Al-spinel, depending on pressure-temperature conditions and whole-rock compositions. As a consequence, the simultaneous occurrence of primary and metamorphic olivine and pyroxene is commonly observed in ultramafic complexes that were metamorphosed after partial serpentinization. The distinction between these minerals of different origin is necessary for understanding the metamorphic processes and tectonic history of the lithospheric mantle. Many of ultramafic complexes exposed in SW Japan were originated from the upper mantle and were thermally metamorphosed after serpentinization. This short article presents some compositional diagrams showing characteristics of primary and metamorphic olivine, orthopyroxene and clinopyroxene in these ultramafic complexes for coming comparative studies.  II. Outline of regional geology and thermal metamorphism  Ultramafic complexes are sporadically exposed in so-called “Sangun high-P/T metamorphic belt” in SW Japan (Fig.1; Igi and Sakamoto, 1977; Hattori, 1978; Mitsuno and Uchida, 1980; Ishiwatari, 1989, 1990; Nishimura, 1998; Takeuchi, 2002; Teraoka et al., 1996). The ultramafic complexes consist mainly of serpentinized harzburgite and dunite that contain small amounts of relict primary minerals. Arai (1980) interpreted the peridotites as refractory residues left after partial melting of primitive peridotites and as dismembered ophiolites embedded in Paleozoic accretionary complexes. The ultramafic complexes and surrounding regional metamorphic or Paleozoic sedimentary rocks were thermally metamorphosed by Cretaceous intrusions of granitic, dioritic or, locally, more basic rocks. Each of the thermally metamorphosed ultramafic complexes in SW Japan can be divided into four or five metamorphic zones (Fig. 2; Arai, 1975; Matsumoto et al. 1995; Nozaka and Shibata, 1995; Nozaka, 2003). Zone I is almost unaffected by thermal metamorphism, whereas Zones II, III, IV and V are characterized by the first appearance of metamorphic olivine, talc, anthophyllite and orthopyroxene, respectively, indicating increasing temperatures of metamorphism from Zone II to V. In Zone V, cordierite, aluminous spinel and clinopyroxene rarely occurs in aluminous rocks (Arai, 1975; Nozaka and Shibata, 1995). The cordierite-bearing rocks look to be derived from chromitite (Arai, 1975). The existence of relict grains of primary olivine, orthopyroxene, clinopyroxene, and chromian spinel or chromite is obvious in less serpentinized peridotites in Zone I and more sparsely in Zone II. The primary silicate * Department of Earth Sciences, Faculty of Science, Okayama University, Okayama 700-8530, Japan 2 minerals are relatively coarse-grained and free from opaque inclusions. Primary orthopyroxene and clinopyroxene are prismatic in shape, and each of them has exsolution lamella of the other (e.g., Nozaka and Shibata, 1995). Metamorphic minerals produced by thermal metamorphism can be distinguished from primary minerals by means of spatial distribution and mode of occurrence (e.g., Nozaka and Shibata, 1995). Metamorphic olivine is commonly fine-grained and contains many inclusions of opaque minerals (mostly magnetite). Metamorphic orthopyroxene, which occurs exclusively in Zone V, is free from the exsolution lamella of clinopyroxene, contains abundant opaque inclusions, and frequently forms radial or fan-shaped aggregates. Clinopyroxene looks to be absent in thermally metamorphosed rocks with normal peridotitic composition. Instead, secondary (retrograde) clinopyroxene, which looks to coexist with serpentine, occurs as fine-grained acicular or rhomboid crystals, selvages replacing primary clinopyroxene, or aggregates forming veinlets set in a serpentine matrix in Zone I and low-grade parts of Zone II. The secondary clinopyroxene lacks exsolution lamella of orthopyroxene.  III. Compositional variations of olivine and pyroxenes     Chemical compositions of minerals were analyzed using a JEOL JXA-733 electron microprobe at Okayama University (for procedures, see Nozaka, 2005). Chemical compositions of primary minerals derived from residual mantle peridotites are almost homogeneous, whereas those of metamorphic minerals are significantly heterogeneous even within individual grains. Some chemical variables were selected for the illustration of the compositional variations of minerals (Figs. 3-5). Among the ultramafic complexes investigated, the Oeyama, Wakasa and Happo complexes (Fig. 1) show evidence for crystallization from differentiated basaltic melts and/or re-equilibration during high-temperature retrogression Figure 1. Distribution of ultramafic complexes and the Sangun high-P/T metamorphic belt in SW Japan (Igi and Sakamoto,1977; Hattori, 1978; Mitsuno and Uchida, 1980; Ishiwatari, 1989, 1990; Nishimura, 1998; Takeuchi, 2002; Teraoka et al.,1996). Abbreviations for ultramafic complexes: TM, Tari-Misaka; TS, Takase; OS, Ohsayama; WS, Wakasa; SK, Sekinomiya; OE,Oeyama; HP, Happo.  Figure 2. Distribution of metamorphic silicate minerals inperidotites/serpentinites from each zone of thermallymetamorphosed ultramafic complexes in SW Japan(compiled after Arai, 1975; Matsumoto et al. 1995; Nozakaand Shibata, 1995; Nozaka, 2003). Abbreviations forminerals: Ath, anthophyllite; Cam, calcic amphibole(tremolite up to lower-grade Zone V; hornblende inupper-grade Zone V); Cpx, clinopyroxene; Ol, olivine; Opx,orthopyroxene; Srp, serpentine; Tlc, talc. Primary mineralsare omitted from this figure. Toshio NOZAKA 3 (Nozaka, 2005; Nozaka and Ito, in press). Minerals from these complexes are omitted from Figs. 3-5, except for primary orthopyroxene from an Oeyama harzburgite that is similar to mantle peridotites in mineral assemblage, texture and mineral composition, and for secondary clinopyroxene coexisting clearly with serpentine in the Diopside Zone of the Happo complex. Olivine in Zones II-V shows variations in NiO, MnO and Fo contents (Fig. 3). Corresponding with increasing metamorphic grade, the compositions of metamorphic olivine become less variable and approach those of primary olivine, suggesting the progress of intracrystalline cation diffusion and homogenization of olivine (see also, Nozaka, in press). The simultaneous occurrence of primary and metamorphic olivine is evident in Zone II (Nozaka, 2003). The frequency of the occurrence of primary olivine in Zones I and II suggests that relict grains of primary olivine are not uncommon in higher grade zones as well. However, the existence of primary olivine in these zones has been obscured，probably because of the progress of cation diffusion between primary and metamorphic grains of olivine.  Orthopyroxene in Zone V shows significant variation in Al2O3 and Cr2O3 contents, and XMg [=Mg/(Mg+Fe)], compared with primary orthopyroxene (Fig. 4). However, in the Al2O3-XMg diagram, there is an overlapping range of composition between primary and metamorphic orthopyroxene, and it is impossible to distinguish the two groups of orthopyroxene by means of these variables. In contrast, the CaO-Cr2O3 diagram shows no overlap of the ranges of two types of orthopyroxene (Fig. 4). Although the electron-probe analyses of primary orthopyroxene may be affected by adjacent clinopyroxene lamella, analyses using a defocused, broad beam (30 μm in diameter) indicate that the involvement of lamellar clinopyroxene yields an enrichment of Ca without significant changes in other elements, and the composition of orthopyroxene itself can be known by careful examination of data. It is clear that deficiency in CaO is a prominent feature of metamorphic orthopyroxene, and the CaO-Cr2O3 diagram is useful for the discrimination of orthopyroxene (Fig. 4). The existence of relict primary orthopyroxene in contact aureoles is expected as the case of olivine. However, relict grains of primary orthopyroxene have never found in Zones III-V. The primary orthopyroxene was probably decomposed by the formation of talc in Zone III and anthophyllite (though sluggish; see Nozaka, in press) in Zone IV, and by the recrystallization of orthopyroxene in Zone V. Clinopyroxene formed during retrograde alteration or serpentinization occurs in a minor amount in low-grade, Zones I and II. Where original peridotites are relatively rich in Ca like the Happo complex, the occurrence of secondary clinopyroxene is frequent. The XMg of Compositional variation of olivine and pyroxene in thermally metamorphosed ultramafic complexesFigure 3. NiO and MnO vs. Fo contents of olivine in serpentinized mantle peridotites and thermally metamorphosedserpentinites/peridotites from SW Japan. Composition range of metamorphic olivine from each zone is drawn excluding a pointwith the highest NiO content, which was obtained in the proximity to an opaque inclusion, in order to show a general tendency ofthe shift of compositional range with increasing metamorphic grade.  4 clinopyroxene that coexists with serpentine is commonly Figure 4. Al2O3 vs. XMg [=Mg/(Mg+Fe)] and CaO vs. Cr2O3 contents of orthopyroxene in serpentinized mantle peridotites andthermally metamorphosed serpentinites/peridotites from SW Japan. Broad-beam (30 μm in diameter) probe analyses of primary orthopyroxene are shown in order to know the effect of thin exsolution lamella of clinopyroxene. Figure 5. Al2O3 vs. XMg and Al2O3 vs. Cr2O3 contents of clinopyroxene in serpentinized mantle peridotites from SW Japan.Metamorphic clinopyroxene with low XMg (< 0.9) forms veinlet aggregates in a serpentine matrix. The compositions of retrogradeclinopyroxene from the Happo ultramafic complex, in which primary clinopyroxene has not been found, are also shown forcomparison. Toshio NOZAKA5 clinopyroxene that coexists with serpentine is commonly high (> 0.95), but some grains of secondary clinopyroxene, particularly those of veinlet aggregates have XMg lower than primary clinopyroxene (Fig. 5). Although the compositions of primary clinopyroxene are variable, secondary clinopyroxene is distinguishable by means of the deficiencies in Al2O3 and Cr2O3 (Fig. 5). Because of the presence of the overlap of XMg, the more useful for the discrimination between primary and secondary (i.e., retrograde) clinopyroxene is the Al2O3-Cr2O3 diagram (Fig.5).   IV. Summary  The range of compositional variation of olivine in thermally metamorphosed serpentinites, as described in previous studies, gradually changes corresponding with metamorphic grade, and in high-grade zones, metamorphic olivine is indistinguishable in composition from primary olivine. In contrast, orthopyroxene and clinopyroxene show clear differences in composition between primary and metamorphic phases. Compared with primary pyroxenes, even though their compositions could be variable depending on original rock composition (i.e., degree of depletion), metamorphic orthopyroxene and metamorphic (retrograde) clinopyroxene is clearly deficient in Cr2O3 and CaO, and in Cr2O3 and Al2O3, respectively. These characteristics are useful for the discrimination between the pyroxenes of different origin.  Acknowledgments  A part of the data set used in this article came from unpublished theses of Okayama University. I am grateful to M. Saito, Y. Yamaguchi, K. Tanaka, M. Komoda, S. Nitta, Y. Ito, N. Fujiwara, S. Kondo and Y. Yoshii for assistance in field and laboratory works. This study was partly supported by funding from the Wesco Scientific Promotion Foundation.  References  Arai, S. (1975) Contact metamorphosed dunite-harzburgite complex in the Chugoku district, western Japan. Contributions to Mineralogy and Petrology, 52, 1-16. Arai, S. (1980) Dunite-harzburgite-chromitite complexes as refractory residue in the Sangun-Yamaguchi zone, western Japan. Journal of Petrology, 21, 141-165. Hattori, H. (1978) Quadrangle series, 1:50,000; Geology of the Kami-iwami district. Geological Survey of Japan, Tsukuba. Igi, S. and Sakamoto, T. (1977) Quadrangle series, 1:50,000; Geology of the Tari district. Geological Survey of Japan, Tsukuba. Ishiwatari, A. (1989) Ophiolites of Japan. Journal of Geography, 98, 290-303 (in Japanese with English abstract). Ishiwatari, A. (1990) Yakuno ophiolite and related rocks in Maizuru terrane. In: Pre-Cretaceous terranes of Japan (Ichikawa, K. et al. Eds.). Publication of IGCP #224, Osaka City University, Osaka, 109-120. Matsumoto, I., Arai, S., Muraoka, H., and Yamauchi, H. (1995) Petrological characteristics of the dunite-harzburgite- chromitite complexes of the Sangun zone, Southwest Japan. Journal of Mineralogy, Petrology and Economic Geology, 90, 13-26 (in Japanese with English abstract). Mitsuno, C. and Uchida, T. (1980) Subsurface geological map, 1:50,000; Katsuyama. Okayama Prefecture, Okayama. Nishimura, Y. (1998) Geotectonic subdivision and areal extent of the Sangun belt, Inner Zone of Southwest Japan. Journal of Metamorphic Geology, 16, 129-140. Nozaka, T. (2003) Compositional heterogeneity of olivine in thermally metamorphosed serpentinite from Southwest Japan. American Mineralogist, 88, 1377-1384. Nozaka, T. (2005) Metamorphic history of serpentinite mylonites from the Happo ultramafic complex, central Japan. Journal of Metamorphic Geology, 23, 711-723. Nozaka, T. (in press) Constraints on anthophyllite formation in thermally metamorphosed peridotites from southwestern Japan. Journal of Metamorphic Geology. Nozaka, T. and Ito, Y. (in press) Cleavable olivine in serpentinite mylonites from the Oeyama ophiolite. Journal of Mineralogical and Petrological Sciences. Nozaka, T. and Shibata, T. (1995) Mineral paragenesis in thermally metamorphosed serpentinites, Ohsa-yama, Okayama Prefecture. Okayama University Earth Science Reports, 2, 1-11. Takeuchi, M. (2002) The Paleozoic. In: Geology of the Shiroumadake district, quadrangle series, 1:50,000 (Nakano, S. et al. Eds.). pp. 105, Geological Survey of Japan, Tsukuba, 9-21 (in Japanese). Teraoka, Y., Matsuura, H., Makimoto, H., Yoshida, F., Kamitani, M., Hiroshima, T., Komazawa, M. and Shichi, R. (1996) Geological Map of Japan 1:200,000 Takahashi. Geological Survey of Japan. Tsukuba.  Compositional variation of olivine and pyroxene in thermally metamorphosed ultramafic complexes

Compositional heterogeneity of olivine in thermally metamorphosed serpentinite from Southwest Japan.

Contact metamorphosed dunite-harzburgite complex in the Chugoku district, western Japan. Contributions to Mineralogy and Petrology,

Dunite-harzburgite-chromitite complexes as refractory residue in the Sangun-Yamaguchi zone, western Japan.

Geological Map of Japan 1:200,000 Takahashi. Geological Survey of Japan. Tsukuba. Compositional variation of olivine and pyroxene in thermally metamorphosed ultramafic complexes

Geotectonic subdivision and areal extent of the Sangun belt, Inner Zone of Southwest Japan.

Metamorphic history of serpentinite mylonites from the Happo ultramafic complex, central Japan.

Mineral paragenesis in thermally metamorphosed serpentinites, Ohsa-yama, Okayama Prefecture.

Ophiolites of Japan.

Petrological characteristics of the dunite-harzburgitechromitite complexes of the Sangun zone, Southwest Japan.

Quadrangle series, 1:50,000;

Subsurface geological map, 1:50,000;

The Paleozoic. In: Geology of the Shiroumadake district, quadrangle series,

Yakuno ophiolite and related rocks in Maizuru terrane. In: Pre-Cretaceous terranes of Japan

西南日本の熱変成超苦鉄質岩体中のかんらん石と輝石の組成変化に関する覚書

Abstract

Similar works

Full text

Available Versions

Okayama University Digital Information Repository

Okayama University Scientific Achievement Repository