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Rubidium-Strontium Model of Formation of the Continental Crust and the Granite at the Island Arc

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概要 The following conclusions have been drawn through the study of a Rb-Sr whole rock age of granites from Southwest Japan and through the comparison, in terms of Rb and Sr contents, of different kinds of... unit of the earth's crust, i.e., the oceanic floor basalts (abbreviated as OFB), phanerozoic granites and recent volcanic rocks from island arcs and their corresponding orogenic belts at the continental margin. (1) The whole rock age and the initial Sr(87/86) ratio of granites from North Kyushu are 166±40 m.y. and 0.70414±0.00011 for the Itoshima granodiorite, 159±13 m.y. and 0.70584±0.00010 for the Haki granodiorite, and 134±26 m.y. and 0.70456±0.00010 for the Sawara granite respectively. The best estimate of the whole rock age of the Yatsushiro granite from Central Kyushu is 409±37 m.y. with an initial Sr(87/86) ratio of 0.7032±0.0006. A period of successive granitic intrusive activities in North Kyushu is about 32 m.y., as far as these whole rock age data are concerned. The difference between the Rb-Sr whole rock age and the mean K-Ar age of granites is nearly constant, unless they underwent later thermal effect. It is about 47 m.y. (2) The following relation between the whole rock age t (m.y.) and the initial strontum isotope ratio Sr(87/86)。 is found among the Yatsushiro, the Funatsu, the Haki, the Amami-oshima, the Minami-osumi granitic rocks. That is Sr(87/86)。=-1.15×10^<-5>t+0.70780. A Rb/Sr ratio calculated from the rate of growth of the Sr(87/86) ratio is about 0.278, which is very close to the recent estimates of the abundance ratio of both elements in the continental crust. (3) These granitic rocks, and the Sawara and the Itoshima granitic rocks are concluded to have been derived respectively from 400 and 230 m.y. old andesitic primary crusts with the Rb/Sr ratio of about 0.27. The initial Sr(87/86)ratio of the primary crust is estimated at 0.7033, which is larger than the mean of OFB by 0.0006. (4)The major chemical composition of a material which has contaminated volcanic rocks does not vary among young orogenic belts such as island arcs and mobile belts at the continental margin, and is andesitic or somewhat more acid with the Rb/Sr ratio close to the estimates of the abundance ratio of both elements in the continental crust. (5)The andesites of low Sr(87/86) ratios have possibly been derived from OFB under high pressure conditions. These andesites, however, are low in Rb content and therefore have low Rb/Sr ratios which are about one fifteenth of the mean Rb/Sr ratio of the continental crust. Accordingly, it is impossible to attribute the growth of the continental crust to the accretion of these andesites. (6)It is found that the melting proportion can attain an extent of about 80 percent of OFB. And the existence of the secondary mantle current driven by the gravitational rise of a large amount of derivatives from OFB is suggested in the upper mantle beneath the island arc. This hypothetical secondary mantle current provides a favourable condition for the formation of the continental crust. (7)Two possible alternative magmatic processes are proposed for the formation of the continental crust at the island arc. At first in both magmatic processes, OFB sink along the seismic zone into the mantle, being transformed into eclogite with the increase of depth, partially melt at depth of about 100-150 kms and then give rise to an andesitic magma. In one of the magmatic processes, this andesitic material rises in essentially solid state or in nearly solid state with an incipient melted portion through the mantle, reaches the bottom of the continental crust, and then remelts partially and mixes with a material produced by the previous analogous magmatic process. A residuum is transported by the secondary mantle current mostly toward the inner and partly toward the outer side of the island arc. The melt fraction will meet and then be mixed with the derivative material successively ascending from the seismic zone. This magmatic process is analogous to the zone refining processing of the semi-conductive material. With the proceeding of this kind of zone melting, water, primarily contained in OFB, is enriched in the liquid zone. This enrichment of water may promote the operation of this kind of the magmatic differentiation. After the operation of the zone melting for a definite period, the chemistry of the liquid zone converges into a definite composition which is equivalent to the estimated overall composition of the continental crust. In the other alternative magmatic process, a derivative material from OFB is remelted incipiently near the base of the continental crust. And the melt fraction is squeezed out from the material and then sticked at the base of the continental crust. The primary crust so formed has a specific Rb/Sr ratio irrespective of its position in the island arc. Rb and Sr contents in granites can be sufficiently explained in terms of the partial melting of the primary crust.続きを見る
目次 Ⅰ Introduction Ⅱ Determinations of a Whole Rock Age and an Initial Sr(87/86) Ratio of Granites Ⅲ The Evolution of the Initial Sr(87/86) Ratios of Granites Ⅳ Petrogenetic Implications of Rubidium and Strontium Contents in Volcanic Rocks Ⅴ Principal Equations in Rubidium and Strontium Concentrations Ⅵ The Secondary Mantle Current beneath the Island Arc Ⅶ Possible Schemes of Magmatic Process for the Formation of the Continental Crust Ⅷ The Formation of a Granite from the Primary Crust Ⅸ Concluding Remarks Acknowledgements References Cited

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登録日 2015.11.11
更新日 2021.02.18

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