The effects of weathering on whole-rock Rb-Sr ages of granitic rocks

Abstract

Four weathered samples of the Cape Ann Granite from Massachusetts average ten percent younger than the age from the whole-rock isochron. Fourteen fresh samples from the Petersburg Granite, Virginia, do not define an isochron, but the average age of seven weathered samples is 8 to 15 percent younger than the average of the fresh samples. For Petersburg Granite, the minimum age and maximum (Sr-87/Sr-86) ~o~ ratio are 460 m.y. and 0.7245 respectively. Assuming a (Sr-87/Sr-86) ~o~ ratio of 0.705, the maximum age is 690 m.y. The Petersburg Granite is late Precambrian to early Paleozoic in age.