THE CHENGWATANA VOLCANICS, WISCONSIN AND MINNESOTA - PETROGENESIS OF THE SOUTHERNMOST VOLCANIC-ROCKS EXPOSED IN THE MIDCONTINENT RIFT

The southernmost exposed rocks of the North American Midcontinent rift system (1100 Ma) consist of 3000 m of mafic volcanic flows and minor interflow sediment exposed along the St. Croix River in Minnesota and Wisconsin. The flows are mostly high-Fe tholeiitic basalt with plagioc lase phenocrysts and ophitic to subophitic clinopyroxene. Abundant sec ondary chlorite, epidote, and actinolite indicate the group was metamo rphosed to greenschist facies (similar to 350 degrees C). Low sodium ( M4 site) and tetrahedral aluminum (Al-IV) contents of actinolite indic ate low-pressure metamorphism (0.25 GPa) and imply a geothermal gradie nt of 45-50 degrees C/km. Low magnesium (Mg# = 0.37-0.58) and Ni conte nts (36-185 ppm) indicate the basalts have undergone significant fract ionation and are not primary mantle melts. Incompatible element abunda nces are inversely correlated with Mg#, and most samples plot within e ither high or low trace element groups (e.g., Ti, P, Zr). The basalts are enriched in the light rare earth elements and Th, and are variably depleted in Ta and Nb relative to La and Th. Initial Nd-143/Nd-144 co mpositions of the group range from 0.51099 to 0.51122 (initial epsilon (Nd) = -4.5 to +0.1). Most flows have isotopic compositions within a r elatively limited range (initial epsilon(Nd) = -2.5 to -1.6), but exhi bit variable trace element abundances. Flows with the highest and lowe st initial Nd-143/Nd-144 ratios have isotopic compositions that are in versely correlated with trace element abundances and ratios (e.g., La/ Yb, Th/La, Th/Ta). The combined geochemical data suggest that the Chen gwatana basalts originated from plume-derived melts and underwent vari able fractional crystallization and crustal contamination. These melts may have interacted with lithospheric mantle enriched during Penokean subduction.