Major-, minor-, and trace-element abundance data on 220, late Cenozoic, basaltic rocks in and around the Carbondale and Eagle collapse centers in west-central Colorado are combined with isotopic, age, and petrographic data to correlate lava flows and establish the timing and minimum areal extent of collapse events associated with removal of Pennsylvanian evaporite. On the basis of these data, 46 distinct compositional groups of volcanic rocks were identified. The rocks within each group, which are represented by at least two samples, have compositions and ages that are indistinguishable from each other and are either (1) undifferentiated samples from the same eruption and possibly from outcrops of the same flow, (2) differentiated from the same magma batch erupted at different times or (3) related to each other by very small differences in the degree of partial melting. The areal extent of chemically correlated and dated volcanic flows in the region was established and these results were used to recognize and understand many of the collapse events as described in companion papers in this volume.

Compositional data are also used to infer the petrogenetic processes that generated the parental magmas. Subtle but significant differences among rocks that are broadly similar in geochemical and isotopic composition and were erupted over a small time interval (<0.5 m.y.) suggests that the mantle source region of these magmas is quite heterogeneous. Over the past 11 m.y., the lavas became less mafic and more enriched in incompatible trace elements. This heterogeneity is attributed to variable contributions of subducted material in the lithosphere during the melting processes. To account for its isotopic features, the source material must be at least mid-Proterozoic in age. A melt contribution from underlying asthenospheric mantle can not be ruled out but none of the volcanic rocks have clear characteristics of oceanic-island basalts or mid-oceanic-ridge basalts. The trace-element compositions of rocks that were sampled from multiple stacked flows are indistinguishable from one another.This suggests that lower and/or upper crustal assimilation had a minor effect on their overall chemistry. However, Pb isotopic data suggest that crustal contamination of at least some samples did occur.