『Abstract
Geochemical mapping of northern Honshu in the Northeast Japan
Arc was carried out using stream sediments at a sampling density
of one sample per 130 km2. More than 50 elements were
determined in 395 stream sediment samples (＜0.18 mm fraction).
In geochemical maps, areas with especially low concentrations
of large ion lithophile elements (LILE), Be and Li widely overlap
with the distribution of Quaternary volcanic rocks along the volcanic
front. The areas rich in mafic elements are associated with mafic
rocks in many cases. On a regional scale, Ni, Cr and Cu contents
are higher in the eastern Paleozoic-Mesozoic basement zone, Pb
and Tl tend to be higher on the western zones, and Zn and Cd are
high in the western back-arc zone. The areas especially rich in
Cu, Zn, Cd, Pb, Bi and Tl coincide with the distribution of large
mineral deposits. High concentrations related to Kuroko, hydrothermal-vein,
and skarn-type deposits are recognized. High values of As and
Sb are related to active geothermal areas near volcanoes and ore
deposits. Chemical variations of K, Ce, Th and Sn in the stream
sediments are concordant with chemical variations in major rocks.
The median and mean concentrations for the stream sediments in
northern Honshu, showing arc signatures, are lower in Rb, Cs,
Th, Li, L, Be, Ta, LREE, Ni, Hg and Sn, and higher in Sc, Ca and
Cd relative to the whole area of Japan, largely because of the
contribution of Cenozoic island-arc volcanic rocks that are generally
poor in incompatible elements. The averaged chemical compositions
of the stream sediments for the geologic zones show systematic
variations of many elements. The contrasting variations of LREE
and Th contents, which are lower in the zones of Cenozoic rocks
relative to the zones of pre-Neogene basements, reflect the regional
variations in the main rocks, and also reflect the change of geological
settings of the studied area from the continental margin to an
island arc during the Cenozoic.』
1. Introduction
2. Geological setting
3. Sampling and analyses
4. Map presentation
5. Results
5.1. Chemical compositions of the stream sediments from northern
Honshu
5.2. Geochemical maps
5.2.1. Lithophile elements abundant in felsic rocks
5.2.2. Lithophile and siderophile elements abundant in mafic
rocks
5.2.3. Chalcophile elements
5.3. Chemical variations according to the geologic zones
6. Discussion
6.1. Comparison with the chemical compositions of major rocks
6.2. Regional geochemical characteristics and controlling factors
6.3. Comparison with crustal abundances
6.4. Implication of regional variations of LREE and Th
7. Conclusions
Acknowledgments
References