K-Ar Ages of Volcanic Rocks in the Samburu Hills Area, Northern Kenya

44 volcanic rocks collected systematically from the Samburu Hills area, west of Baragoi, Northern Kenya were dated with the K-Ar age determination method to examine the chronology of fossil hominoids discovered in the Japan-Kenya Expedition, 1982. The K-Ar data suggest that Samburu hominoids at the site SH-22 are from 7.1±0.5 to 10.7±0.6 Ma in age and the Kenyapithecus at the Site BG-X from 12.6±0.6 to 14.9±0.6 Ma in age

Petrography, K-Ar age, and chemistry of Yoshino-dai lavas in the Aira caldera

The Aira caldera was formed about 22, 000 years ago in association with large-scale pyroclastic eruptions of more than 400km^3 felsic ejecta. Petrography was described and major/trace element compositions and K-Ar ages were determined for Aira precaldera lavas in the Yoshino-dai area, in order to examine mechanisms by which a series of the Aira precaldera and climactic magmas were produced. Precaldera lavas in the Yoshino-dai area mainly consist of augite-olivine basalt and two-pyroxene andesite, as previous workers indicated. K-Ar ages of precaldera basalts and andesites are >0.5 Ma and, in contrast, K-Ar age of precaldera rhyolite is <0.5 Ma and eruptions of precaldera dacites and minor pyroclastic flows may have occurred after ～ 0.5 Ma. The precaldera and climactic ejecta show a single chemical trend with the cusp at SiO^2～65 wt. % for some elements. It follows that a series of Aira precaldera magmas was not produced by a single process such as fractional crystallization from basaltic magmas or two-end-member mixing between basaltic magmas and rhyolitic magmas. The generation of a series of Aira precaldera magmas requires at least two processes; production of basaltic to andesitic magmas before ～ 0.5 Ma and of dacitic to rhyolitic magmas after ～ 0.5 Ma. The possibility that the state of the magmatic system beneath the Aira caldera may have changed at ～ 0.5 Ma remains

K-Ar geochronology of a middle Miocene submarine volcano-plutonic complex in southwest Japan

A volcano-plutonic complex in the Susa area, southwest Japan, consists of the Yamashima andesites, the Koyama gabbros and syn-plutonic porphyrite dykes derived from a common basaltic andesite magma. The complex is closely associated with middle Miocene turbidite deposits. The Yamashima andesites are composed mainly of basaltic andesite feeder dykes, massive submarine lavas with hyaloclastites, and their reworked deposits. The lavas and deposits immediately overlie turbidite deposits, indicating submarine volcanic activity. The Koyama gabbros formed hornfels by contact metamorphism of the surrounding turbidites and andesites. Highly purified clinopyroxene and plagioclase mineral separates from the Yamashima andesites were dated by a K-Ar method using an ultra-low blank K analysis procedure. Ages obtained from duplicate analyses are 16.5±1.5, 15.2±1.4, 15.8±1.7, and 16.5±2.0 Ma for clinopyroxene, and 14.2±0.8, 15.2±0.9, and 15.6±0.9 Ma for plagioclase. The clinopyroxene and plagioclase data define a mineral isochron age of 14.7±0.9 (1[sigma]) Ma with an initial 40Ar/36Ar ratio of 297.3±2.4 (1[sigma]), suggesting that clinopyroxene has no excess argon and can be reliably dated by K-Ar. Most of the groundmass ages are considerably younger (12.1-14.6) than the isochron age, perhaps due to argon loss during alteration. The gabbros give ages of 14.2±0.3 and 14.1±0.3 Ma for biotite, and 13.7±0.3 and 13.7±0.7 Ma for green hornblende. The porphyrite dyke yields an age of 12.5±0.3 Ma for the groundmass, and the pelitic hornfels gives a biotite age of 14.8±0.3 Ma. Our new K-Ar ages, together with previous studies, show that a series of geological events took place in the Susa area between 16 and 13 Ma. Conglomerates and sandstones were deposited in the beginning of marine transgression. Subsequent abrupt deepening led to deposition of a thick black shale unit, turbidite deposits and large-scale submarine channel-fill deposits. Coeval igneous activity formed the volcano-plutonic complex. The magmato-tectonic event was synchronous with the opening of the Japan Sea and the associated clockwise rotation of the southwest Japan arc sliver, recording a unique tectonic setting