ABSTRACT

The graywackes of the eastern Klamath subprovince are dominated by calc-alkaline volcani-clastic fragments and range in age from Ordovician through Jurassic. The Ordovician rocks are apparently more influenced by "cratonic" plutonic-metamorphic sources than younger graywackes of the subprovince suggesting a different environment of origin for the Ordovician graywackes. Although albitization occasionally masks original composition, major and trace element distribution of the least albitized graywackes and volcanic rocks is typical of island-arc calc-alkaline volcanic rocks.

Petrographic study of volcanic-rich graywacke indicates that there were four episodes of increased calc-alkaline volcanism in the subprovince. These occurred during the (1) late Silurian-early Devonian, (2) Carboniferous, (3) Permo-Triassic, and (4) Jurassic. There was less sedimentary material added to the subprovince with time and occasionally plutonic rock fragments were added to the formations. The episodes of increased volcanism may reflect an eastern Klamath island-arc. The arc seems to have been isolated from strong cratonic provenance after the Devonian.