The lower Miocene lavas (23--20 Ma) of the Flat Tops volcanic field, NW Colorado, are one of the earliest and most voluminous phases of magmatic activity associated with the development of the Rio Grande rift. Flow-by-flow collections of the lavas at Flander, Trappers Lake, provide evidence for complex open-system magma chamber processes. Geochemically, the lavas range from basalts to shoshonites and have concentrations of major and compatible trace elements that are buffered at higher levels than expected for simple fractional crystallization. Cyclic repetitions (at least five) in the geochemical stratigraphy of the Flander lavas suggest that they are the result of replenishment fractional crystallization (RFC). Variations in some incomptible trace element ratios, such as Ta/Yb and La/Yb, and radiogenic isotopes (87Sr/86Sr=0.70458--0.70607 143Nd/144Nd =0.51226--0.51241) suggest that the lavas have also been contaminated by approximately 10% of Proterozoic 87Sr-rich upper-crust. In addition, anomalously K-rich lava flows with low 87Sr/86Sr occur close to the base and at the top of the succession.

It is argued that these are the products of mixing between magmas within the Flat Tops reservoir system and an influx of strongly potassic melt from lithospheric mantle; without subsequent geochemical overprint by the effects of crustal contamination and RFC. All of the lavas have incompatible trace element ratios that mostly resemble those of calc-alkaline basalts; e.g., their chondrite-normalized patterns have troughs at Nb and Ta. These features could result from several alternative processes, such as melting within convecting mantle above a low-angle subducted slab; reaction between various asthenospheric melts and overlying lithospheric mantle; and fusion of metasomatized lithospheric mantle as a result of decompression during regional extension. ¿American Geophysical Union 1991