Journal of Mineralogical and Petrological Sciences
Online ISSN : 1349-3825
Print ISSN : 1345-6296
ISSN-L : 1345-6296
  • Keisuke ESHIMA
    論文ID: 200917
    発行日: 2021年
    [早期公開] 公開日: 2021/04/21
    ジャーナル フリー 早期公開

    The Shaku–dake body represents a two–pyroxene diorite body (TPD) located in the Northern Kyushu region in southwest Japan and is a member of the Cretaceous Northern Kyushu batholith. Layered structures and feeder dikes in the wall of the TPD body are recognized as magma plumbing systems that result from magma pulses. The magma pulses were likely provided from localized anomalous spots, which have been recognized as the horizontal variations of modal, major, and trace element compositions in the TPD body. Vertical variation in the TPD body indicates irregular changes, of which differentiated facies rich in modal quartz and K–feldspar appear at altitudes of 300–450 m; however, mafic facies rich in modal pyroxenes and MgO, Cr, and Ni contents are also present at the same altitude. Field observation, petrography, and geochemical features suggest that the TPD body could have been formed by multiple magma injections from several sites. On the other hand, the exsolution texture of clinopyroxene appears in the center of the TPD body, indicating slow cooling. Overall, the multiple magma injections prolonged the supra–solidus state. A sheet–on–sheet intrusion model was the most plausible mechanism for addition of andesitic magma to the magma reservoir. Multiple magma pulses may have been involved in the formation of the TPD magma chamber, where the wall of the TPD body cooled quickly and left a layered structure; additionally, the magma pulses may have caused slow cooling in the TPD body interior.

  • Kenta KAWAGUCHI, Pham MINH, Pham Trung HIEU, Truong Chi CUONG, Kaushik ...
    論文ID: 200916
    発行日: 2021年
    [早期公開] 公開日: 2021/03/30
    ジャーナル フリー 早期公開

    The Kontum Massif and the Truong Son Belt, central Vietnam are the magmatic–metamorphic massifs (belts) of the Indochina Block. These two massifs (belts) underwent two independent orogenic events during Ordovician–Silurian and Permian–Triassic ages. However, due to the strong overprint of these two orogenic events, the evidence of any preexisting (e.g., Precambrian) tectono–thermal events have become extremely poor. Hence, the Precambrian age components of the Indochina Block have not been fully revealed, and their implication is not well–understood. It is well known that such ‘lost memories’ of the older continental rocks or source crustal materials are sometimes preserved in the sedimentary basins. Keeping that goal in mind, we have investigated the LA–ICP–MS detrital zircon U–Pb dating for three sedimentary and metasedimentary rocks in the Kontum Massif and southern part of the Truong Son Belt, Indochina Block, central Vietnam to unravel the Precambrian episodes of the Indochina Block, if any. Two Triassic (meta)–sedimentary rocks reveal two significant age clusters of latest Carboniferous to Triassic (~ 300–230 Ma) and Early Paleozoic (~ 480–410 Ma), with the conspicuous lack of the 400–320 Ma zircon grains. Abundant Permian detrital zircon grains in Triassic metaquartzite from the Kontum Massif document that the subduction–related magmatism before the continental collision played an important role in the growth of the Kontum Massif. In contrast to these two (meta)–sedimentary rocks, the detrital zircon spectra of metaquartzite from the Kontum Massif show the most significant age peak of ~ 1780 Ma and abundant Archean zircon grains (16%). Furthermore, the oldest detrital zircon shows the Paleoarchean age (207Pb/206Pb spot age of 3314 ± 22 Ma). The metaquartzite was metamorphosed in the Early Paleozoic (~ 414 Ma). The youngest detrital zircon grain yields the 207Pb/206Pb spot age of 1359 ± 85 Ma. Hence, the approximate depositional timing of the protolith of metaquartzite was in Mesoproterozoic time. The newly obtained detrital zircon data indicate a significant contribution of the Paleoarchean to Paleoproterozoic crustal materials during the deposition of the quartzite in the Kontum Massif. The Kontum Massif had developed as a different unit in the Precambrian period from the Truong Son Belt as well as the southwestern Yangtze Block due to their different Precambrian provenance. This study reveals a new clue that the Kontum Massif was one of the Precambrian blocks in the Southeast Asian massifs.