GEOCHEMISTRY OF LATE PROTEROZOIC BASALTIC ROCKS FROM SOUTHEASTERN NEW-BRUNSWICK, CANADA

Late Proterozoic volcanic rocks of the Coldbrook Group (∼630-600 Ma) from the Avalon Zone of the Appalachians in southeastern New Brunswick (Canada) are predominantly bimodal and occur in three belts/fault blocks (Eastern, Central and Western). The basaltic rocks were emplaced in a convergent plate margin setting and range from incompatible element-depleted island arc tholeiites in the Eastern belt to calc-alkalic types of the Western belt. The distinct compositional zonation of the basaltic rocks resembles the across-arc variation observed in recent volcanic arc systems. The trends include an increase of the LILE and HFSE abundances and the Zr/SiO2, Th/SiO2, La/Yb, Th/Nb, Th/Hf and Zr/Y ratios from the Eastern belt in the southeast through the Central belt to the Western belt in the northwest. The zonation of the Coldbrook Group is attributed to a northwest-dipping subduction zone during Late Proterozoic time. It is suggested that some of the Late Proterozoic basins in the Avalon Zone of the Northern Appalachians reached a stage of oceanization. © 1990.