Journal of Mineralogical and Petrological Sciences
Online ISSN : 1349-3825
Print ISSN : 1345-6296
ISSN-L : 1345-6296
最新号
選択された号の論文の6件中1~6を表示しています
Editorial
ORIGINAL ARTICLE
  • Shengnan SUN, Zuozhen HAN, Zhigang SONG
    2022 年 117 巻 1 号 論文ID: 211224
    発行日: 2022年
    公開日: 2022/06/04
    ジャーナル フリー
    電子付録

    The Permo–Triassic was a crucial interval for the Changchun–Yanji suture zone in NE China since it witnessed the late–stage tectonic evolution of the Paleo–Asian Ocean (PAO). Thus, provenance analysis of Triassic sedimentary rocks in this region can shed light on the eventual closure process of the PAO. In this study, we carried out petrological, geochemical, and detrital zircon U–Pb geochronological research on two (meta)–sedimentary rocks from the Yitong–Gongzhuling area located in the westernmost section of the Changchun–Yanji suture zone. LA–ICP–MS U–Pb ages of detrital zircons from the two samples indicate a middle Triassic maximum depositional age (~ 240 Ma). The two samples present a most significant Permo–Triassic (299–240 Ma) detrital zircon population that corresponds to the multi–phase subduction and collision–related magmatism along the Changchun–Yanji suture zone. In addition, a late Silurian–early Devonian (429–390 Ma) detrital zircon population is also present and presumably fed from a pre–existing accreted arc terrane along the northern margin of the North China Craton (NCC). In contrast to some previously reported Permo–Triassic sedimentary rocks in adjacent areas, our new results show an absence of ~ 2.5 and ~ 1.8 Ga ages characteristic of NCC magmatism or metamorphism. The results imply that the Yitong–Gongzhuling area probably has a higher relief than the adjacent area during the middle Triassic, preventing the deposition of materials from the NCC to the south. This is further evidenced by the proximal orogenic source, short–distance transportation, and rapid accumulation for the studied samples as indicated by the subhedral to anhedral and poor roundness morphology of detrital zircons. The inferred regional uplift was hypothesized to be caused by the continued transpression during the PAO final closure based on the widely distribute early–middle Triassic syn–collisional granitic rocks along the Changchun–Yanji suture zone.

LETTER
  • Yusaku TANAKA, Keisuke ESHIMA, Masaaki OWADA
    2022 年 117 巻 1 号 論文ID: 210831
    発行日: 2022年
    公開日: 2022/06/04
    ジャーナル フリー

    The metamorphic complex from the Mt. Tenzan area in northern Kyushu consists mainly of mafic rocks with small amounts of siliceous, calc–silicate, and ultramafic rocks. These lithofacies can be recognized as an ophiolitic complex. Metamorphosed mafic rocks are divided into two types, amphibolites I and II, which are probably derived from supracrustal and intrusive rocks, respectively. The geochemical data of both amphibolites plotted within the field between mid–ocean ridge and island arc basalts; such geochemical features resemble those of back–arc basin basalts. As the metamorphic complex was intruded by Cretaceous granitoids, protoliths of the complex could have been formed prior to the Cretaceous. The protolith lithofacies assemblage and geochemical constraints of the Tenzan metamorphic complex indicate the correlation with the Yakuno ophiolite rather than the Oeyama ophiolite.

LETTER
  • Shota H. OKUMURA, Satoshi OKUMURA, Akira MIYAKE
    2022 年 117 巻 1 号 論文ID: 211219
    発行日: 2022年
    公開日: 2022/06/04
    ジャーナル フリー

    Groundmass pyroxene crystals in pumice from the 1914 eruption of Sakurajima in Japan show varied combinations of crystallographic faces (i.e., ‘tracht’). To investigate whether the groundmass pyroxene tracht depends on magma decompression conditions, we performed isothermal single–step decompression experiments on hydrous Sakurajima dacite magma. The magma was held under water–saturated conditions at 920 °C, 120 MPa, and oxygen fugacity conditions no more oxidizing than one log unit above Ni–NiO equilibrium for 24 h. Then, a control experiment was immediately quenched, whereas others were decompressed to final pressures of 20, 10, or 5 MPa and held for 3 h before quenching. Groundmass pyroxenes in the control experiment and that quenched at 20 MPa showed octagonal shapes, whereas those decompressed to lower pressures characteristically had hexagonal shapes. Some pyroxenes in the 20 MPa experiment were hexagonal near plagioclase crystals because plagioclase crystallization locally increased the supersaturation of pyroxene in the melt. We conclude that the tracht of groundmass pyroxenes changes from octagonal to hexagonal as the degree of effective undercooling increases and thus reflects the decompression history of a magma during its ascent in a volcanic conduit.

LETTER
LETTER
  • Sota NIKI, Kenta YOSHIDA, Hikaru SAWADA, Ryosuke OYANAGI, Takafumi HIR ...
    2022 年 117 巻 1 号 論文ID: 210814
    発行日: 2022年
    公開日: 2022/05/18
    ジャーナル フリー
    電子付録

    Here we first report the in situ U–Pb dating of metamorphic grossular garnet (Grs) with distinction between internal zonation textures. The studied Grs occurs in an eclogite–facies marble collected from the eastern Iratsu body of the Sanbagawa metamorphic terrane, Japan. The Grs has a patchy texture, predominantly with pure Grs cores and andradite (Adr)–rich rims formed during eclogite–facies and exhumation stages, respectively. The U–Pb ages for the Grs core and Adr–rich rim were 97 ± 10 and 106 ± 16 Ma (95% confidence level), respectively. Despite the compositional zoning formed under different PT conditions, the U–Pb ages of the core and rim were in similar values within analytical uncertainties. This decoupling of chemical zonation and U–Pb ages implies that the U–Pb chronological signatures of rims were inherited from cores owing to the redistribution of radiogenic Pb in cores during the rim formation through fluid–mediated dissolution and reprecipitation. The Grs U–Pb age (97 ± 10 Ma) thus directly corresponds to previously reported PT conditions of the core formation during the eclogite–facies metamorphism. This advantage of Grt petrochronology as the combination of radiometric ages obtained by in situ analysis and PT conditions deduced from paragenesis can contribute to reconstruct reliable metamorphic histories.

feedback
Top