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���B�ɎY����N�V���ȍ~�̉ΎR��ނ̒n�����w�I�����̎���ϑJ��p���āCBack-arc basin�̊g��Ƃ���ɔ����}���g���̑Η��p�^�[���𐄒肵���D�ΎR��ނ̍̎�n��́C���݂̉ΎR�t�����g�ɑ΂����s�����50km�͈̔͂ɂ���ԁD�̎悵���ΎR��ޖ�100�‚Ɋւ��āC�������ώ@�C�听�����f�C���ʐ������f�C��y�ތ��f����ѓ��ʑ̔�̑�����s�����D

���B�̉ΎR�����́C6-7Ma�Ɏn�܂����t�B���s���C�v���[�g�̒��݂��݂�2Ma�Ɏn�܂���Okinawa Trough�̊g��ɖ��ڂɊ֘A���Ă���D�ΎR�����́C���o�N��Ɗ��ɂ��3�‚�phase�ɕ�������F�@1st phase (2.7-4.3Ma)�C�A2nd phase (1.2-2.3Ma)�C�B3rd phase (0.007-0.74Ma)�D1st phase�́CHigh Alumina Basalt (HAB)�n��̋P�Έ��R��ނ���̂Ƃ��Ă���D1st phase�̉ΎR��ނɂ́CAdakite���i����Sr/Y��FSr/Y�䁁37.55-61.77�j�o������DAdakite�̕��z�́C�ł��w�ʈ�Ɍ�����D2nd phase�����3rd phase�́C������ނƃf�C�T�C�g����̂Ƃ���Bimodal�ΎR�����ł���D3rd phase�̉ΎR��ނ́C2nd phase�̉ΎR��ނ��O�ʈ�ɕ��z����D2nd phase�����3rd phase�̌�����ނ́C��ɃJ�������ΒP�΋P�΂���Ȃ邪�C���̑��ɃJ�������Ό����₨��і���������������F�߂���D�����̌�����ނ́C�n�����w�I��������Low Alkali Tholeiite (LTH)�n��CHAB�n�񂨂��Alkali Basalt (AK)�n��ɑ�����D���ʌ��f�̓������CAK�n��̌������Oceanic Island Basalt (OIB)�ɁCHAB�n��̌������Island Arc Basalt (IAB)�ɂ��ꂼ�ꕪ�ނ����DLTH�n��̌�����ނ́COIB��IAB�̒��ԓI�ȓ��������D2nd phase�̌�����ނ́CNb/Y�䂪0.17-0.21�ł���CLa/Yb�䂪3.2-8.3�ł������D3rd phase�̌�����ނ́CNb/Y�䂪0.16-1.56�ł���CLa/Yb�䂪0.17-0.24�ł������D������ނ�Nb/Y��̕ω��́C�����⎿�}�O�}�̋N������������ƂƂ���HFSE�Ɍ͊����Ă������Ƃ������CLa/Yb��̕ω��́C�����⎿�}�O�}�̕����n�Z���x������ƂƂ��ɑ������Ă������Ƃ����ꂼ�ꎦ���Ă���Ƃ݂Ȃ���D

���B�ɕ��z����ΎR��ނ̒n�����w�I�����Ƃ��̎���ϑJ���C�}���g���̑Η��p�^�[���𐄒肵���DAdakite�̑��݂́C1st phase�ɂ����āC���ݍ��ރX���u�̗n�Z�𐶂��鍂���‹����w�ʈ�̃}���g���[���ɑ��݂������Ƃ𕨌��D2nd phase�ȍ~�̌�����ނ�1st phase�̉ΎR��ނɔ�בO�ʈ�ɕ��z���Ă��邱�Ƃ���C�ΎR��ނ͎���ƂƂ��ɕ��z���O�ʂֈړ������Ƃ݂Ȃ���D������ނ̓��ʑ̔��systematics���C���ׂĂ̌�����ނ��C����N�������ɗR�������Ƃ݂Ȃ��C���ʌ��f�g���ɂ݂�������̑����́Cmelt��extraction�ɂ��}���g���̌͊��ߒ��Ə㏸�ɔ����n�Z�x�̑����Ƃ��Đ��������D�‚܂�}���g���̑Η��p�^�[���́C�w�ʑ�����O�ʑ��ɗ��ꍞ��Counter Flow�ɂȂ�D���B�ɓW�J�����N�V���ȍ~�̉ΎR�����́C��������̍����}���g�������̈ڗ��ɂ���Ďx�z����Ă����Ɛ��������D

 

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Petrological and geochemical study of Iwato pyroclastic deposits

 

Abstract

In order to understand genesis of large-scale felsic magmatism, new analyses of 105 samples of essential fragments in pyroclastic flow deposits from the Aira caldera have been measured. These essential fragments are collected from twelve geological units (Fukuyama air-fall pumice deposit, Iwato pyroclastic flow deposit, Otsuka air-fall pumice deposit, Fukaminato air-fall pumice deposit, Arasaki pyroclastic flow deposit, Kenashino air fall pumice deposit, Osumi air-fall pumice deposit, Tarumizu pyroclastic flow deposit, Tsumaya pyroclastic flow deposit, Ito pyroclastic flow deposit, Moeshima pyroclastic flow deposit, Takano base surge deposit) eastern part of Kokubu and Tarumizu city. Each essential fragment consists mainly of plagioclase, orthopyroxene and clinopyroxene as a phenocryst. Hydrous mineral is only observed in pumice from Fukuyama air-fall pumice deposit.

The partial melting of sedimentary rocks has been conceivable for a formation mechanism of felsic magma. The major elements of the essential fragments show negative trends in compatible elements (e.g. TiO2, Fe2O3, MgO) vs. SiO2 diagram, and show positive trends in alkali elements (e.g. Na2O, K2O) vs. SiO2 diagram. All samples are plotted in I-type area defined by ACF diagram. On the basis of this diagram, source rocks for Aira caldera magma may be igneous rocks. Therefore, not sedimentary rocks but igneous rocks are most plausible source rocks for the magma.

Relation between K and Rb shows positive trend that has K/Rb=224.2 values. On the other hand, there is no correlation between Zr and Nb. This result is inconsistent with the correlation between LIL elements. Scoria of Iwato pyroclastic flow deposit and pumice of Moeshima pyroclastic flow deposit have higher Zr and Nb concentration in comparison with other eruptions. In addition, both eruptions have also higher Ga, REE, and Zn in concentration. This character can not explain even either mixing process or fractionation process.

The high concentration of HFSE, Ga, Zn and REE in igneous rock suite, however, have been admitted to A-type magma. The geochemical characters are also observed in the scoria of Iwato pyroclastic flow deposit, and pumice of Moeshima pyroclastic flow in this region. Namely, It is possible that scoria of Iwato pyroclastic flow deposit, pumice of Moeshima pyroclastic flow deposit and A-type magma are formed the same condition. Therefore, it is conceivable that the early stage and the latest stage of Aira caldera formation were dry and high temperature in the source region. Such high temperature and dry condition can explain inconsistency of the HFSE character of the magma. I propose that geochemical character of the Aira caldera magma may not inherit to their source character but may be depended on the physical conditions of the melting region.

 

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Petrological and geochemical study of Cretaceous granites in the northern part of Kumamoto, central Kyushu, Japan.

 

abstract

Petrological characteristics, whole rock chemistry and mineral chemistry are examined for Cretaceous granitic rocks, located in the northern part of Kumamoto, central Kyushu Japan to understand the petrogenesis of granitic rocks.

Tamana granodiorite indicates typical I-type granite and are estimated to be produced on condition of 0.25 Gpa and 780ºC. Kikuchi granite and Tsutsugatake granite indicate changing from I-type to S-type characteristics in composition grade into systematically. Geochemical observations implies that granitic rocks in this study are formed by in situ fractional crystallization of a single magma. The center of granitic magma chamber is thought to exist the northward of study area on the basis of their occurrence and changes in chemical compositions. The results also contain available information that suggest relations to granitic rocks located in the northern Kyushu, and petrogenesis of granitic rocks.

Both petrological and geochemical constraints apply to the forming processes of granitic rocks.

 

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K poor volcanic rocks from Yahazu volcano , western Kyushu , Japan: on landed back-arc basin volcanism

Abstract

 Yahazudake volcano (Kumamoto Pref.) must be formed by igneous activity which is related to the spreading of Okinawa Trough (East Sea).

 The volcanic rocks which belongs to island-arc system are characterized as follow ;(1) abundance of incompatible elements is tend to decrease from volcanic front side to back-arc side, (2) eruption volume exponensially increase from volcanic front side to back arc side. However , Pliocene-Pleistocene volcanic rocks in southwest Kyushu doesn�ft have these characteristics, notwithstanding, they are in the back-arc side sea. In the concrete, the volcanic rocks in Yahazudake which lies in back-arc side area, have significantly lower K2O contents than volcanic front sided area�fs volcanic rocks (cf. Ontake volcano, Kirisima volcanoes, and so on). Genesis of the volcanic rocks like this, must be different from that of island-arc volcanic rocks, and it requires another tectonic setting model Then, we can find that there are some relations between Yahazudake and Okinawa trough, which is like its location and generated age. So, in this report, I compare the chemical compositions of volcanic rocks from Yahazudake that from Okinawa trough. As a result of the comparison, I could make out they have the so similar source compositions; major and minor (include isotope) elements compositions. This suggests that there is petrogenetic casual nexus between them.

 

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Shigeru�@YAMAMOTO

Several types of xenolith have been found in the Quaternary andesites from Kinpo Volcano,Kyushu.The xenoliths have been classified mainly by the following 5types;(�@)Mafic Inclusion,(�A)Hb Gabbro,(�B)Px-Hb Gabbro,(�C)Basic Metamorphic Rocks,(�D)Pelitic Gneiss.The origin of them were seeked by the observation of the naked eye and the microscope,and the chemical analysis for the major elements and the rear earth elements.As a result,it was suggested that all types of xenolith except for Mafic Inclusion,which was cognate xenolith,were accidental xenolith.In addition,it was suggested by the bulk rock chemical analysis that the origin of basic rock xenolith,which consist of Hb Gabbro,Px-HbGabbro and Basic Metamorphic Rock,was MORB.It is suggested that all of the accedental xenoliths,which consist of the basic rock xenoliths and pelitic Gneiss,are derived from oceanic crust.