花崗岩の成因とSr・Pb同位体

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Isotopic studes of Sr and Pb on granitic rocks are reviewed and their contributions to the problems of the origin of granitic rocks are evaluated. Many works accumulated for the latest decade on various acidic rocks show that through the geologic ages since crustal formation the greater part of acidic rocks had similar Sr^<87>/Sr^<86> ratios at the time of their formation. It has also been clarified that the initial ratios were slightly higher than the Sr evolution curve of source material of basaltic rocks probably in the upper mantle. Various explanations have been offered to interprete the small difference between the initial ratios of granitic rocks and estimated Sr isotopic composition of mantle material. Thus some have considered that the magmas originated in the source region similar to that of basaltic magmas were contaminated with Sr of crustal materials, the others having held the view that acidic magmas were originated in the source underneath the continent where Rb/Sr ratio is supposed to be higher than that underneath the oceanic region. From the Rb-Sr isotopic studies on the late Mesozoic acidic igneous rocks in Nohi district of central Japan, it has been observed that the whole rock isochron ages of some rhyolites and associated granites are significantly older (in the order of 10^7〜10^8 years) than their mineral ages. It is emphasized, in this connection, that in the age determination by Rb-Sr method the ages obtained from single mineral or mineral isochron should be distinguished from that gained from whole rock isochron, as the former ages indicate the time of crystallization of rocks, the latter showing the time of differentiation of magmas from certain source material. In order to explain the time discrepancy, one may consider that the acidic magmas originated in great depth needed long time in the cource of upheaval towards the crust or they spent long time in some porsions of the upper mantle. In the opinion of the authors, similar relation seems to be recognized in the late Mesozoic acidic rocks from Roskruge Mts., Arizona reported by Bikerman (1967), and in some other instances. It is noticeable that the above time discrepancy may explain the difference between the "initial" Sr^<87>/Sr^<86> ratios in granitic rocks and the Sr evolution curve of mantle material. That is, the time interval between the generation and solidification of magma may lower the initial ratios at the time of magma generation according to the Rb/Sr ratio in each rock. The above hypothesis may be examined with further works especially on the granitic rocks which are relatively young in age and have suffered no secondary changes.
日本地質学会の論文
1969-03-25

花崗岩の成因とSr・Pb同位体

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