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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���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
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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.
���c���� �C�_
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.
�e�r����C�_
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.