始生代の泥質岩にみられる化学組成の多様性 : 南部アフリカ,カープバールクラトンおよびリンポポ帯の例

Major and minor elements chemistry of Archean pelitic rocks was analyzed by X-ray fluorescence spectrometer. Samples were collected from three different terranes in southern Africa; Barberton and Sutherland greenstone belts in the Kaapvaal Craton, granulite terrane of the Southern Marginal Zone of the Limpopo Belt, and supracrustal sequences of the Central Zone of the Limpopo Belt. Although bulk chemical composition varies from sample to sample, pelitic rocks from the Kaapvaal Craton exhibit similar composition to those from the Southern Marginal Zone. In contrast, those from the Central Zone have wide compositional range probably because they were supplied from many different sources.
An enrichment in Ni and Cr in pelitic rocks from the Kaapvaal Craton and Southern Marginal Zone was well observed. The Ni and Cr contents increase from 30 to 260 ppm and 10 to 580 ppm (Central Zone) to 280 to 550 ppm and 710 to 1410 ppm (Southern Marginal Zone and Kaapvaal Craton), respectively. The enrichment in Ni and Cr in particular pelitic rocks can be inferred from the presence of high Ni- and Cr-bearing rocks (komatiite?) on the earth surface during sedimentation.