The provenance and chemical variation of sandstones associated with the Mid-continent Rift System, U.S.A.

Sandstones occurring along the N portion of this Precambrian rift system have been analysed petrographically and geochemically. After the initial intrusion of abundant basalts along the rift, dominantly volcaniclastic sandstones of the Oronto group were deposited; these sandstones are covered by quartzose and subarkosic sandstones of the Bayfield group. Thus the sandstones of the Oronto group were derived from earlier basalts and those of the Bayfield group were derived from Precambrian granitic gneisses on the rift flanks. The chemical variation of the sandstones closely reflects the changing detrital modes with time. The average size of the negative Eu anomaly in the Oronto sandstones is significantly less (Eu/Eu (super *) 0.79 + or - 0.13) than that of the Bayfield group (0.57 + or - 0.09), also suggesting a more basic source for the former. From these models it is suggested that the volcaniclastic sandstones of the lower part of the Oronto grop were derived from approximately 80-90% basalt and 10-20% granitic rocks, whereas the rest of the Oronto group and the overlying Bayfield group may have formed by mixing approximately 30-60% basalt and 40-70% granitic rocks.