RB SR CHRONOLOGY AND CRYSTAL-CHEMISTRY OF CELADONITES FROM THE PARANACONTINENTAL THOLEIITES, BRAZIL/

Celadonites from the Parana flood basalts have been dated by internal Rb/Sr isochrons, s using the ammonium acetate leaching technique previ ously used by Hart and Staudigel (1986) for celadonites from DSDP hole s. The residues after leaching have very high Rb-87/Sr-86 ratios (18.7 -1900), and very radiogenic Sr-87/Sr-86 (0.73-2.96). Electron micropro be analyses show that potassium, and thus also presumably rubidium, be came enriched in these minerals during the transformation of a protoce ladonite smectite phase into celadonite. Our leaching experiment sugge sts that radiogenic Sr-87 and rubidium are very resistant to chemical fractionation during leaching and thus may have remained in closed-sys tem for Rb/Sr chronology. Ages varying from 107 +/- 1 to 81 +/- 1 Ma h ave been obtained at two different sites, one on high-Ti basalts in th e northern Parana basin and the other on low-Ti basalts in the souther n basin. These ages are significantly younger than the eruption of the flood basalts (127-137 Ma), and in this respect are in good agreement with results obtained on celadonites from oceanic ridges. Initial Sr- 87/Sr-86 ratios are distinctly higher in the celadonites than in the h ost basalts, probably due to contamination by radiogenic strontium tra nsported by the hydrothermal fluids. Our results suggest that mineralo gy and isotopic composition are different in celadonites from the nort hern and southern Parana basins, although this needs to be confirmed o n a larger dataset. Copyright (C) 1997 Elsevier Science Ltd.