|Budget Amount *help
¥3,300,000 (Direct Cost : ¥3,300,000)
Fiscal Year 1997 : ¥1,500,000 (Direct Cost : ¥1,500,000)
Fiscal Year 1996 : ¥1,800,000 (Direct Cost : ¥1,800,000)
Formation and evolution of the continental crust were isotopically and geochemically studied. Some of the resuIts are summarized as follows.
1.Neodymium and cerium isotopic ratios, rare earth element (REE) abundances and major element compositions were determined for Precambrian orthogneiss and granite clasts in the Jurassic Kamiaso conglomerate from central Japan. Sm-Nd data on these rocks give an age of (2.07(]SY.+-。[)0.06) Ga (2sigma) with epsilon<@D2Nd@>D2 (2.07Ga)=-1.9(]SY.+-。[)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 age of the original granitic rocks. The initial value of epsilon<@D2Nd@>D2 (2.07Ga)=-1.9 shows slightly enriched or chondritic feature for source of origibal granitic rocks at 2.07 Ga.
2.Isotopic data of Sm-Nd, La-Ce and Rb-Sr systems, REE abundances and major element compositions are reported for
the Archean Marble Bar Chert from the Towers Formation of the Salgash Subgroup, Warrawoona Group, Pilbara Block, Western Australia. The Marble Bar Chert records two Sm-Nd ages ; 3.2(]SY.+-。[)0.3 Ga for red (or black) and white banded cherts and 2.5(]SY.+-。[)0.2 Gafor yellowish gray cherts. The older age of 3.2(]SY.+-。[)0.3 Ga can be regarded as the formation time of the banded cherts. On the other hand, the younger age of 2.5(]SY.+-。[)0.2 Ga for the yellowish gray cherts is thought to reflect the time of later thermal events and the cherts do not retain their original chemical features. The 3.2 Ga Sm-Nd age for the Marble Bar banded cherts is the first Archean record obtained for the Archean cherts ; their initial epsilon<@D2Nd@>D2 value of +1.0(]SY.+-。[)3.0 holds the original nature at their formation without severe geochemical disturbance.
3.REE abundances, Sr isotopic data and major element compositions are reported for the Permian and Triassic cherts from Kuma and Saiki areas in the Southern Chichibu (Sambosan) terrane, the Outer Zone of Southwest Japan. Our geochemical and isotopic data of the Sambosan cherts suggest a change from ocean-basin floor deposition for the Permian to hemi-pelagic for the Triassic cherts. Less