Volcanic rocks from Northern Mariana Islands

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  • 北マリアナ諸島の火山岩類
  • キタ マリアナ ショトウ ノ カザンガンルイ

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Abstract

Eighteen volcanic rock samples, collected from Northern Mariana Islands by the present authors, were identified as follows: hypersthene augite andesite from Moug Island, olivine-bearing augite andesite, olivine-bearing augite basalt from Agrigan Island, olivine-bearing augite basalt, olivine basalt from Pagan Island, clivine-bearing orthopyroxene augite andesite from Sarigan Island, and olivinebearing bronzite augite andesite, orthopyroxene augite andesite and olivine-bearing orthopyroxene augite andesite from Anatahan Island, respectively. As the result of chemical analysis, it was clarified that these rocks contain 49.07% to 63.63% of SiO2, 16.14% to 20.85% of Al2O3, and 2.22% to 6.74% of K2O + Na2O, and all of them were verified to have some normative quartz and hypersthene. These rocks can be divided into two types, namely, those of high alkali tholeiite series and low alkali tholeiite one. On the AMF diagram (Fig. 3), these volcanic rocks are plotted nearly in the same zone of differentiation of those from Pagan Island which shows a typical tholeiitic trend. This fact suggests that all the rocks had the same crystallization differentiation trend. Comparing the chemical composition of the volcanic rocks from Northern Mariana Islands with those from Izu Islands by means of the Oxide-Solidification Index diagram, the former rocks are characterized by low contents of SiO2, FeO+Fe2O3, FeO and TiO2, and high contents of Al2O3, CaO, K2O and Na2O, and nearly the same contents of MgO, MnO and P2O5. And then, on the SiO2-Al2O3-K2O+Na2O diagram (Fig. 8), the rocks from Northern Mariana Islands are plotted to the K2O+Na2O side comparing with those from Izu Islands. This suggests that the volcanic rocks from Northern Mariana Islands and those from Izu Islands were of the different types of crystallization differentiation. The so-called OMM rock type (AoKi, 1969, 1970 a, 1970 b) therefore, might be divided into two different types. It is not clear whether such different types of rocks have been derived from only one kind of magma or two. In any case, the difference of geological structures between the region of Northern Mariana Islands and that of Izu Islands, which is presumed from the distribution of the earthquake hypocenters (KATSUMATA et al., 1968), might have some effect on the derivation of such difference of rock types.

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