7. AN ELECTRON MICROPROBE STUDY OF THE AMAZON FAN¹

Futoshi Nanayama²

ABSTRACT

Detrital grains were examined in 38 sediment samples from Holes 931B, 936A, and 944A of Ocean Drilling Program Leg 155 on the Amazon Fan, adjacent to the South American continental margin. All samples are Quaternary in age (<360 ka). Optically, the fine sand and silt fraction consists of quartz (70% to 80%), feldspar (8% to 15%) and mica (~1%). The light heavy minerals form 5% to 7% of the sediment and consist of zircon, tourmaline, hornblende, clinopyroxene, orthopyroxene, staurolite, kyanite, sillimanite, chloritoid, and garnet; opaques form 2% to 5%. The quartz grains are both subangular and wellrounded with iron-rich surface coatings. Thus, the sands of the Amazon Fan are quartz arenite to subarkose. The modal and chemical compositions of the Amazon Fan sands show no vertical and temporal variations throughout the sections and stratigraphic horizons.

In order to understand the provenance of the sediments, compositions of constituent mineral grains were analyzed using the electron microprobe. From this study, five mineral assemblages have been recognized that are characteristic of medium-pressure type metamorphic rocks, high-pressure type metamorphic rocks, granitic rocks, and recycled and arc-volcanic sources. Possible sources for the assemblages are Precambrian rocks of the Guiana and Brazilian Shields, the foreland region of the Andes, arc-volcanic rocks of the Andean Cordillera, and Paleozoic to Tertiary sediments of the Amazon Basin.

¹Flood, R.D., Piper, D.J.W., Klaus, A., and Peterson, L.C. (Eds.), 1997. Proc. ODP, Sci. Results, 155: College Station, TX (Ocean Drilling Program).
²Fuel Geology Section, Fuel Resources Department, Geological Survey of Japan, Tsukuba 305, Japan. nanayama@gsj.go.jp