THE CHEMISTRY AND TECTONIC SETTING OF ORDOVICIAN VOLCANIC-ROCKS IN NORTHERN MAINE AND THEIR RELATIONSHIP TO CONTEMPORARY VOLCANIC-ROCKS IN NORTHERN NEW-BRUNSWICK

A geochemical study of Ordovician volcanic rocks in northern Maine sho ws that the mafic rocks can be divided geographically into three group s dominated by tholeiite. Two of these groups (Stacyville-Lobster Moun tain group and Munsungun-Winterville group) are chemically similar but geographically separated and were erupted in a within-plate setting. A third, dissimilar volcanic group (Pinkham Road group), intervening b etween the other two, is characterized by rifted island-arc magmatism. Wherever exposed, boundaries between these groups are tectonic and tr end northeast-southwest. The within-plate basalt groups are thought to be related to each other and to correlate with coeval basalts in the Tetagouche Group of northern New Brunswick, where they are associated with similar faunas. By contrast the rifted island-arc basalts of the Pinkham Road group closely resemble some basalt suites from the Fourni er Group in New Brunswick. Unlike sequences in New Brunswick, alkalic basalts are relatively scarce in Maine, and no equivalent of MORB-like Fournier basalts has been seen. Our correlations suggest that the rel ationships of Ordovician basalt formations in northern Maine need revi sion and question tectonic interpretations based on paleomagnetism.