Geochemistry and Rb-Sr dating of the Muruvik rhyolite tuff, Trondheimsfjord, Central Norway.

Abstract

A geochemical study of a low greenschist facies, felsic lapill-ash tuff fromnear the stratigraphic top of the Ordovician Lower Hovin Group at Muruvik,Trondheimsfjord, has shown that the pyroclastite is clearly calc-alkaline andfalls in the field of high-K rhyolites in classification schemes for volcanicrocks. The chemical composition of the Muruvik tuff compares well with that offelsic lavas and pyroclastic rocks from the cordilleras of western North andSouth America. The data lend further support to an earlier paleotectonic modelinvolving generation of these mature-arc, Late Ordovician volcanites along themargin of a continent or microcontinent during the later stages of infillingof a back-arc marginal basin. An attempt at Rb-Sr dating of the rhyolite tuffproduced a 9-point errorchron of 410+-27Ma. As the tuff appears to have adepositional age of Late Caradoc (c.445-440Ma), based on faunas in an adjacentpelite along strike from Muruvik, then the Rb-Sr age is interpreted to datethe Caledonian (Scandian) greenschist facies metamorphism of this rock.