Abstract

Whole-rock samples of the post-Middle Cambrian to Middle or Late Pennsylvanian Blue Hills igneous complex were analyzed for Rb, Sr, and Sr isotopic composition. An apparent Rb-Sr age of 365 ± 7 m.y. and an initial Sr87/Sr86 ratio of 0.703 ± 0.004 was obtained for the Quincy Granite. The Blue Hill Granite Porphyry has an apparent isochron age of 282 ± 8 m.y. and an initial Sr87/Sr86 ratio of 0.717 ± 0.002, although there is considerable scatter of data points. The Blue Hills aporhyolite does not yield an isochron, and the “ages” range from 235 to 420 m.y.

Published K-Ar hornblende ages and Pb207-Pb206 zircon ages for the Quincy Granite are 442 ± 22 m.y. and 450 ± 50 m.y., respectively. These ages plus the scatter of the Blue Hills whole-rock Rb-Sr data show that these rocks have not remained closed systems with respect to Rb and Sr since crystallization. We suggest that the low ages are due to loss of *Sr87 (radiogenic) and possibly some common Sr from the Blue Hills lithologic units.

First Page Preview

First page PDF preview
You do not currently have access to this article.