ABSTRACT

Neodymium and cerium isotopic ratios, rare earth clement abundances and major element compositions were determined for Precambrian orthogneiss and granite clasts in the Jurassic Kamiaso conglomerate from central Japan. Three types of REE patterns are observed for the Kamiaso samples: (1) reverse S-shaped patterns with high enrichment of LREE, (2) patterns consisting of upward convex La-Nd span and reverse S-shaped Nd-Lu span, and (3) patterns consisting of two or three rectilinear lines with an inflection at Ho (and Nd). Sm-Nd data on these rocks give an age of 2.07 ± 0.06 Ga (2σ) with ε_Nd (2.07 Ga) = –1.9 ± 0.4. The Sm-Nd age is in accordance with the Rb-Sr whole-rock age of 2.05 Ga for quartzo-feldsparthic gneiss clasts by Shibata and Adachi (1974), confirming that the age of ca. 2.05 Ga indicates the time of formation of the original granitic rocks. The initial value of ε_Nd (2.07 Ga) = –1.9 shows slightly enriched or chondritic feature for the source of original granitic rocks at 2.07 Ga. This study shows that Precambrian episodes and protolithic features can be understood from Precambrian orthogneiss and granite clasts in Mesozoic conglomerates, since the clasts have not undergone post-depositional thermo-tectonic influences.