Reset Rb/Sr whole-rock systems and chemical control

Abstract

THE susceptibility of fine-grained, volcanic rocks to open-system behaviour during regional metamorphic activity has long been suspected in Rb/Sr geochronology^1–5. The evidence that we present here shows that not only can open-system behaviour occur, but that it is partly a function of bulk chemical composition. We have studied the Burlington Peninsula area of northern Newfoundland (Fig. 1) which is underlain by the Cape St John Group, a volcanic pile of sub-aerial, calc-alkalic volcanic and sedimentary material, about 3,500 m thick. Coarse calcareous units and minor conglomeratic beds form its basal part; overlying units consist of basic volcanic flows and sills, commonly vesicular, associated with andesitic and silicic pyroclastic rocks and flows. The Cape St John Group is similar to calc-alkalic rocks associated with modern island arcs, although there is a predominance of high-silica rocks in the former; a marked compositional gap between 62 and 67% SiO₂ for the Cape St John succession corresponds to a lack of high-silica andesite and low-silica dacite and illustrates its bimodal nature. Sub- to low-greenschist facies metamorphism has affected the entire volcanic pile, and the metamorphic grade gradually increases from the southern part of the peninsula to the north, where it reaches amphibolite facies. An east-west trending syncline forms the major structure in the area and at least four phases of folding have been documented₆. The Cape St John Group rests unconformably on the Snooks Arm Group (Arenig)^7,8 and should, therefore, be younger than about 500 Myr.