Radiometric age and Sr isotope characteristics of volcanic rocks from the Japan Sea floor.

Abstract

Based on the radiometric age (mostly K-Ar and 40Ar 39Ar ages) and isotopic ratio data for volcanic rocks recovered from the Japan Sea floor, the formation age and the characteristics of magma sources for these volcanic rocks are discussed.Most volcanic rocks from the Yamato and Japan Basins, where oceanic crusts are presumed to exist, show radiometric ages of less than 20Ma. Igneous rocks dredged from such areas as continental remains are conjectured show radiometric ages of more than 100 Ma. Volcanic activity in the Yamato Bank area along the borderland of the Yamato Basin seems to have ceased after 20Ma, which might have related to the formation of the Yamato Basin. Combining such data with the detailed information on the obtained ages, the formation of the Yamato Basin has been inferred to have started prior to 17Ma and not later than around 25Ma. The Japan Basin probably formed almost at the same period or slightly earlier.87Sr/86Sr ratios for volcanic rocks are systematically different between the Yamato Basin and the Yamato Bank together with continental shelf areas: the former indicate values of 0.7035-0.7040, whereas the latter of higher than 0.7045. This might reflect the difference in the degree of incorporation of continental crustal materials.

Nd and Sr isotopes and K-Ar ages of the Ulreungdo alkali volcanic rocks in the East Sea, South Korea.

“…This maybe support above conclusion. On the basis of Sr iso tope composition, Kaneoka (1990) already pro posed that continental crustal contamination would be more responsible for 87Sr/86Sr values (0.7036-0.7039) of volcanic rocks than subconti nental lithosphere mixing in the backarc basin. The 87Sr/86Sr values in the Ulreungdo fall into the simi lar initial 87Sr/86Sr ratios range from 0.7042 to 0.7067 of Cretaceous granites in the continental Kyongsang basin apart from about 130 km from the Ulreungdo and their 6Nd values are between -6.6 and 0.4.…”

“…Based on the isotopic and geochemical char acteristics, several processes and sources of Ulreungdo alkali basaltic magma have proposed: MORB type mantle source (Nakamura et al, 1990), island arc type (Fujibayash et al, 1988) and crustal contamination (Kaneoka, 1990). Shimazu et al (1990b) ing of the Sea of Japan and East Sea.…”

“…Many magmatic processes have been invoked in order to explain the variable isotopic ratios and major and trace elements geochemistry. Inter preted results of previous investigations include: 1) the Ulreungdo alkali volcanics as a product of interaction between a MORB type mantle and a deep mantle plume (Nakamura et al, 1990), 2) partial melting of metasomatized old mantle lithosphere with subduction related fluids (Tatsumoto and Nakamura, 1991), 3) contamina tion by continental crustal materials for mantle derived basaltic magmas (Kaneoka, 1990) and (Nakamura et al ., 1990) and of the continental fragments, rifted continental crust and submarine volcanoes (Tamaki , 1988) in the East Sea and Sea of Japan are also shown.…”

“…No abso lute age data is available to establish the volcanostratigraphy. The Ulreungdo alkali vol canic rocks are characterized by enriched Sr iso topic compositions and extremely high 0208Pb/ 204Pb values (Fujibayashi et al ., 1988;Nakamura et al, 1990;Kaneoka, 1990;Tatsumoto and Nakamura, 1991). These isotopic features are cou pled with major and trace elements geochemical patterns, which compare with other volcanics emplaced in the backarc basin tectonic settings.…”

“…Most alkali volcanic activities in the backarc basin in terms of the petrogenesis, chemical and isotopical characteristics and tectonic framework are closely related to the opening of the Sea of Japan and East Sea Wasserburg, 1981, 1986;Fujibayashi et al, 1988;Nohda et al, 1988;Tamaki, 1988;Morris and Kagami, 1989;Nakamura et al, 1989;Tatsumi et al, 1989;Kaneoka, 1990;Kaneoka et al, 1990;Nakamura et al, 1990;Shimazu et al, 1990b;Otofuji et al, 1991;Tatsumoto and Nakamura, 1991;Cousens and Allan, 1992;Shimazu et al, 1993;Jolivet et al, 1991Jolivet et al, , 1994Uto et al, 1994;Kaneko et al, 1996;Morris et al, 1997). Although several tens of prehistorical submarine volcanoes found in the backarc basin of the Japanese island, surface vol canoes on the sea surface cropped out extremely limited numbers such as the Ulreungdo, Dogdo and Oki islands.…”