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Volume 77, Issue 12
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A petrological study of the Funagata pigeonite basaltsAkihiko Fujinawa1982 Volume 77 Issue 12 Pages 419-437
Published: December 05, 1982
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSIn order to understand the generation and differentiation processes of the island arc tholeiite magma, it is essential to reveal the crystallization sequence of the minerals in the shallow magma chamber.
For that purpose, a microprobe study has been undertaken on three Funagata pigeonite basalts which are the most primitive basalts among the island arc tholeiitic series in the Nasu zone, NE Japan. Firstly, the crystallization sequence recognized in each sample has been clarified based on chemical compositions and textural relations. Secondly, information on the near equilibrium crystallization in the shallow magma chamber has been extracted and connected.
The crystallization sequence obtained through the study is as follows.
Stage A: Plagioclase (An90+), olivine (Fo78), bronzite (Wo4.5 En76Fs19.5) and pigeonite (Wo9.5En71Fs19.5) coexist with liquid. At this stage, olivine has already been in reaction relation with liquid to form bronzite and/or pigeonite. Bronzite is also in reaction relation with liquid to form pigeonite.
Stage B: Mafic minerals gradually enrich in Fe2+. When the Mg/(Mg+Fe) ratio of the pigeonite reaches 0.23, augite begins to crystallize. Penecontemporaneously with the start of the augite crystallization, olivine (Fo75) is fractionated from the liquid. An mole % of the plagioclase is nearly constant.
Stage C: The three pyroxenes further enrich in Fe2+. Three pyroxene coexistence continues until the Mg/(Mg+Fe) of the pigeonite reaches 0.26. Chemical composition of the plagioclase is nearly constant (An90).
Experimental data indicate that the crystallization sequence obtained here must reflect the notable change of the pyroxene phase relation in the early stage differentiation of the Funagata tholeiitic basalt magma. This type of change in pyroxene phase relation is interpreted as being due to the successive decrease of the liquidus and solidus temperatures of pyroxenes, accompanied with the proceeding of the differentiation.View full abstractDownload PDF (1281K) -
HISAO TANAKA, SATOSHI KANISAWA, HITOSHI ONUKI1982 Volume 77 Issue 12 Pages 438-454
Published: December 05, 1982
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSThe Nishidohira cortlandtitic mass of probable Late Mesozoic age in the southern Abukuma Mountains has been characterized on the basis of field evidence, microscopic observations and chemical analyses of rocks and minerals. Though the mass is small, it is composed of various rock types.
Cortlandtite, quartz hornblende gabbro, monzonite and syenite are products of clinopy-roxene dominated fractionation of a picrite-basaltic magma. Whereas amphibole-dominated fractionation played an important role in the formation of hornblende gabbro, quartz diorite and granite from a fairly MgO-poor basaltic magma. The two rock groups are probably not comagmatic, but might have originated from a common parental material enriched in K and Rb and depleted in Ni. The two effective fractionations had occurred before the intrusion to the present position.
The cortlandtite has crystallized under conditions of 7-9 kb>Ptotal>5±2 kb, PH2O≥2-3 kb, and had equilibrated at temperatures from 1000 to less than 750°C. Prevailing high PH2O and slow cooling of the mass brought about considerably low equilibration temperatures. When the almost congealed cortlandtite had intruded at about 5±2 kb, the latest liquid of the clinopyroxene-dominated fractionation was segregated to form the syenite and monzonite.View full abstractDownload PDF (1219K) -
HARUO OHASHI, MASAMI SEKITA1982 Volume 77 Issue 12 Pages 455-459
Published: December 05, 1982
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESS
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