Geochemistry of eocene calc-alkaline volcanic rocks from the Kastamonu area, Northern Turkey
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Radiogenic nuclide
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We determined K-Ar ages for four andesites of the Hisatsu volcanic rocks from Minamata and for rhyolite of the Gesujima welded tuff from Amakusa Shimoshima, southwest Kyushu Japan. The andesites include three Yahazudake volcanic rocks and one Ontake volcanic rocks. The obtained K-Ar ages for the Yahazudake type andesite range from 1.98 to 2.08 Ma. The ages of Ontake type andesite and Gesujima welded tuff are 2.15 Ma and 2.89 Ma respectively. From our new K-Ar age data combined with previously reported geological, petrological and chronological data of the Hisatsu volcanic rocks, we discussed the space and time distribution of these volcanic rocks. Andesites from Hisatsu volcanic rocks are divided into two categories, “Ontake type andesite (K2O>1.7 wt. %)” and “Yahazudake type andesite (K2O<1.2 wt. %)”, based on potassium content in andesite at given SiO2 range (57-63 wt.%). In terms of this criterion, most of the Hisatsu volcanic rocks and andesites from Kirishima volcano are defined as the “Ontake type andesite”. Consequently, the Ontake type andesite distributes whole area of the Hisatsu volcanic province extending from Nagashima area to Kirishima volcano. Andesites of this type, as a whole, have a positive correlation between latest eruption age of each volcano and the distance from the present volcanic front. This spatial arrangement of the Ontake type andesite implies that the width of volcanic zone has narrowed from Nagashima to Kirishima volcano during last three million years. Although the distribution area of Yahazudake type andesites overlaps the Ontake type andesites, the volcanism of the Yahazudake type andesites is quite limited in age ranging from 1.6 to 2.0 Ma. The production of each magma types may belong to different tectonic events.
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Miocene volcanic rocks were discovered in the Geiyo Islands, western Setouchi region of southwest Japan. These occur as dikes or volcanic necks with marginal pyroclastic rocks. The volcanic rocks are composed of high-alkali tholeiitic olivine basalt, olivine andesite and bronzite andesite, in which the olivine andesite from Oge-shima chemically corresponds to high magnesian andesite. Whole rock K-Ar age determinations were carried out on olivine basalts, olivine andesites and bronzite andesites. An olivine andesite from Osakishimo-shima and two bronzite andesites from Okamura-shima yielded late Miocene ages of 8.2±0.7 Ma, 7.8±0.4 Ma and 8.0±0.4 Ma, respectively. It follows that subalkaline volcanism distinctively occurred at late Miocene age in the western Setouchi region. Accordingly, we give a name of "Geiyo volcanic rocks" to these volcanic rocks, which include late Miocene high-alkali tholeiitic olivine basalt reported from the adjacent area. On the other hand, olivine basalts from Okubi-shima and Ko-oge-shima give middle Miocene ages of 12.8±0.6 Ma and 15.4±0.8 Ma, respectively. In addition, an olivine andesite from Oge-shima has been dated as 17.4±0.9 Ma, indicating early Miocene age. On the basis of the radiometric ages, petrography and whole rock chemistry, these volcanic rocks are considered to be a member of the Setouchi volcanic rocks.
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We determined K-Ar ages for four andesites of the Hisatsu volcanic rocks from Minamata and for rhyolite of the Gesujima welded tuff from Amakusa Shimoshima, southwest Kyushu Japan. The andesites include three Yahazudake volcanic rocks and one Ontake volcanic rocks. The obtained K-Ar ages for the Yahazudake type andesite range from 1.98 to 2.08 Ma. The ages of Ontake type andesite and Gesujima welded tuff are 2.15 Ma and 2.89 Ma respectively. From our new K-Ar age data combined with previously reported geological, petrological and chronological data of the Hisatsu volcanic rocks, we discussed the space and time distribution of these volcanic rocks. Andesites from Hisatsu volcanic rocks are divided into two categories, “Ontake type andesite (K2O>1.7 wt. %)” and “Yahazudake type andesite (K2O<1.2 wt. %)”, based on potassium content in andesite at given SiO2 range (57-63 wt.%). In terms of this criterion, most of the Hisatsu volcanic rocks and andesites from Kirishima volcano are defined as the “Ontake type andesite”. Consequently, the Ontake type andesite distributes whole area of the Hisatsu volcanic province extending from Nagashima area to Kirishima volcano. Andesites of this type, as a whole, have a positive correlation between latest eruption age of each volcano and the distance from the present volcanic front. This spatial arrangement of the Ontake type andesite implies that the width of volcanic zone has narrowed from Nagashima to Kirishima volcano during last three million years. Although the distribution area of Yahazudake type andesites overlaps the Ontake type andesites, the volcanism of the Yahazudake type andesites is quite limited in age ranging from 1.6 to 2.0 Ma. The production of each magma types may belong to different tectonic events.
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Abstract Andesite volcanoes of the North Island of New Zealand are divided on the basis of their potash contents into two groups; a low potash group which includes the Northland, Coromandel, and Taupo Volcanic Zone andesites, and a high potash group which contains the volcanoes of western Taranaki. There are no significant differences among the chemistries of the low potash andesites from the different regions, leading to the proposition that they have a similar origin. It is suggested that an andesite "source zone" developed in the upper mantle beneath the old arcuate structure during Miocene time and has propagated eastwards across the North Island. The high potash andesites appear to be associated with events beneath the old arcuate structure rather than with the active margin of the east coast of the North Island. The chemistries of the New Zealand andesites are compared with analyses from other active regions and the invariance of the soda content is contrasted with the large potash variation.
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K-Ar dating and petrologic description were carried out on the Inase valcanic rocks from eastern part of the Kitakami district of Iwate Prefecture, Northeast Japan. We obtained K-Ar whole rock ages of 15.1±0.4 Ma and 15.5±0.4 Ma for two andesites. The results of the present and previous K-Ar age detamination indicate that the volcanic activities in the Inase district took place in the range of 15.5-13.2 Ma. The chemical compositions of representative 32 andesitic samples were determined by XRF technique. The upper Inase volcanics composed of tholeiitic andesites with SiO2 of 53.61-60.61% have chemical characters similar broadly to those of the Quaternary andesites from the NE Japan arc, whereas the lower Inase ones composed also of tholeiitic andesites with SiO2 of 53.86-55.28% are characterized by high contents of HFS elements such as P, Ti, Nb, Y and Zr, suggesting that the latter are different in chemical compositions from the typical island arc andesites. We estimated the trace element composition of the primary magmas of the lower and upper Inase volcanics using the method of Sakuyama and Nesbitt (1986) and Yoshida and Aoki (1988). The process identification diagram for the primary magma compositions of the Inase volcanics and the Quaternary volcanics in the NE Japan arc and the surrounding areas suggests that the upper Inase primary magma may be derived from the magma source chemically similar to the present sub-continental mantle beneath the NE Japan arc, while the lower Inase primary magma may be derived from the magma source which is more enriched in HFS element contents than the sub-continental mantle.
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Abstract The volcanic rocks of the Taupo Zone are mostly acid tuffs and flows, with less abundant andesites, minor dacites, and rare basalts. A preliminary examination has been made of distribution of Cr, V, Sr, Rb, Cs, Li, Ba, and Zr in selected rocks from the Taupo Zone. The data support the hypothesis that the Taupo Zone volcanics belong to a continuous chemical sequence, non-porphyritic aluminous basalt-andesite-dacite-calc-alkaline rhyolite, with accumulative olivine and pyroxene andesites. Low total alkalis, low Zr, high K/Rb ratios and high Ba contents distinguish the acid rocks of the Taupo Zone from those in other New Zealand Tertiary provinces. Low concentrations of K, Rb, Cs, and Zr, high K/Rb ratios and low Rb/Sr ratios, compared with values for the average continental crust and average granite, suggest a subsialic source for the Taupo volcanics. Sr87/Sr86 ratios of 0.7047 for basalt and 0.7066 for rhyolite, together with the overall chemistry, indicate that the acid rocks are not the fusion products of ancient sial or reworked material derived from this and suggest a common source for both the acid and basic rocks in the Taupo Zone.
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