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＜紀要論文＞
長崎県西彼杵半島面高に分布する枕状溶岩の組織と化学組成
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| 概要 | In the northern part of the Nishisonogi Peninsula, basaltic rocks of late Cenozoic are\ idely distributed, overlying the Mesozoic metamorphic rocks and Tertiary sedimentary rocks. A bed of pillow-lava... of 2 to 3 m thick, which was reported from the Omodaka area for the first time by TAJIMA (1987), develops just below a massive lava-flow, covering the late Cenozoic sedimentary rocks. The pillow lava is composed of pillow lobes with radial section of 0.5 to 1.5 m across and hyaloclastites of sand to ash size. The pillow lobes are piled up successively and tangled intricately. One lobe often diverges into multiple lobes. The surface structures such as corrugation, ropy wrinkles, tensional cracks and contraction cracks, develop well on the glassy surface of lobes. Some pillow lobes thrust out from the base of the massive lava-flow which was chilled and jointed at the bottom. A pillow lobe contains microphenocrysts of plagioclase and orthopyroxene embedded in groundmass which is composed of plagioclase, clinopyroxene (pigeonite to augite), spherulite and glass. The crystallinity of groundmass changes the texture systematically from the margin of a pillow lobe to the core; from glass only in the outermost skin through glass+ plagioclase + spherulite to hyalo-ophitic/intergranular in the interior. In the interior part 2-2.5 cm from the surface, clinopyroxene forms fan-shaped spherulite and groundmass plagioclase is tabular and rarely skeletal in form. There, the size of spherulite increases inward. The core of a pillow lobe is holocrystalline and the constituent minerals show polyhedral form. These textural changes reflect a monotonous decrease in cooling rate from the outermost skin to the core due to quenching in water. The pillow lava studied here is a basaltic andesite with the composition of low-alkali tholeiite. It is most impoverished in K_2O/SiO_2 among basaltic rocks of late Cenozoic in northwest Kyushu (Matsuura basalts). The temperature for the final-stage of plagioclase microphenocryst crystallization was estimated as high as 1200℃ by the method of KUDO and WEILL (1970). These observed features suggest the following mechanism for pillow lava formation that a lava-flow of low viscosity entered successively into a shallow pond, resulting in the formation of pillow lava in which pillow lobes grew within free spaces in water and piled up one afrer another, and then, the successive lava-flows covered a bed of the pillow lava which had filled up the pond. The high temperature of above 1200℃ caused the basaltic andesite fluid enough to form pillow lobes.続きを見る |
| 目次 | I. まえがき II. 地質概説 III. 枕状溶岩の産状と内部組織 IV. 枕状溶岩とその構成鉱物の化学組成 V. 考察 |
詳細
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| 登録日 | 2021.10.20 |
| 更新日 | 2022.10.25 |