a b s t r a c tThis paper reports the results of clay mineral analysis (the amount of clay fraction, clay mineral assemblages, illite crystallinity) of samples collected from a drilled core (Rabibhawan (RB) core) located in the westcentral part of the Kathmandu Basin on the southern slope of the Central Himalaya. The amount of clay fraction in the core sediments between 12 m and 45 m depth (corresponding to ca. 17-76 ka), which belong to the Kalimati Formation, is variable and shows three clay-poor zones (19-31 ka, 44-51 ka, and 66-75 ka). The variations correspond with those of illite crystallinity index (Lanson index (LI) and modified Lanson index (MLI)) and kaolinite/illite ratio as well as the fossil pollen and diatom records reported by previous workers. These data reveal the following transformations occurring during the weathering process in this area: micas ðmainly muscoviteÞ→illiteð→illite−smectite mixed layer mineral ðR = 1ÞÞ→kaoliniteThe sedimentation rate (~50 cm/kyr) of clay-poor zones that correspond to dry climate intervals is only half that of clay-rich zones (~120 cm/kyr) that correspond to wet climate intervals, indicating weakened chemical weathering and erosion and low suspended discharge during dry climate intervals. The clay-poor zones commonly show unique laminite beds with very fine, authigenic calcite, which was probably precipitated under calm and high calcite concentration conditions caused by low precipitation and run-off. The variations between dry and wet conditions in this area as deduced from clay minerals appear to follow the Indian Summer Monsoon Index (ISMI) (30°N-30°S, 1 July) and northern hemisphere summer insolation (NHSI) signals (30°N) at 1 July, especially during the dry climate zones, whereas the wet maxima of the wet climate zones somewhat deviate from the strongest NHSI. On the other hand, the dry-wet records lead markedly the SPECMAP stack (by about 5000 years). These results suggest that the Indian summer monsoon precipitation was strongly controlled by the NHSI or summer insolation difference between the HimalayanTibetan Plateau and the subtropical Indian Ocean, showing a major fluctuation on the 23,000 years precessional cycle, and that it was not driven by changes in high-latitude ice volume, although the records of clay mineral indices during the wet intervals leave a question that other factors, in addition to insolation forcing, may play important roles in weathering, erosion, and sedimentation processes.

Mutsu Hiuchidake Volcano has an erosion caldera which shows a horseshoe like geomorphological feature toward east. Tertiary strata as a basement are distributed in the caldera. Tertialy strata and a part of pyroclastic deposits of the Mutsu Hiuchidake Volcano have altered strongly to moderately by hydrothermal activities related to the volcanism. This study is to clarify a progress history of the alteration by using a geological mapping, thermoluminescence (TL) dating, x ray diffraction analysis, an infrared reflection absorption analysis and a fluid inclusion study.Highly altered zone is recognized in the area of midstream to upstream along the Ohakagawa and the Koakagawa within erosion caldera. The argillic alteration zone surround a silicification zone in the highly altered area shows a circular distribution. The strongly altered areas are along NNW SSE to NNE SSW fractures. The alteration areas were divided into the smectite, kaolinite, alunite and pyrophyllite zones.The kaolinite and alunite zones give the TL ages of quartz 67 13 ka (KG 5), 88 18 ka (OG 4) and 91 23 ka (OG 1). The smectite zone within the argillic alteration zone of outside of collapse caldera, yield the ages 752 215 ka (SO 2) and 615 197 ka (KG 1). These TL ages suggest the hydrothermal activity end at 70 to 90 ka.The existence of pyrophyllite suggests that hydrothermal temperatures were 200 to 250 C in these area. This is supported from the homogenization temperatures of fluid inclusions in calcite, 242 C in average.

The estimation of crystallinity and relative amount of clay minerals in the Kathmandu Basin sediments were made by using decomposition procedure of X-ray diffraction (XRD) patterns. Based on this study an attempt has been made to reconstruct the palaeoclimatic variation recorded in the clay minerals. The variations of the two illite crystallinity indices, Lanson index (LI) and modified Lanson index (MLI) of the drill-core sediments between 5 m and 49 m depth are linked to those of the relative amount of smectite to illitie minerals or to chlorite. The relative amount of kaolinite to illitic minerals or to chlorite roughly showed mirror image of the relative amount of smectite to illitie minerals or chlorite, respectively. The variation of the hydrolysis condition inferred from these indices were congruous with the variation of δ18O from planktonic foraminifers of deep sea sediments in the Arabian Sea, confirming that the climatic variations during the last ~450 kyr in the Kathmandu Basin were closely related to global climate, without apparent local climatic changes. In addition, they depicted that it was dry during the cold climate and wet during the warm climate in the Kathmandu Basin.