GEOCHEMICAL VARIATION OF GRAND MESA VOLCANIC FIELD, WESTERN COLORADO

This study represents a major-element geochemical study of the Grand Mesa volcanic field (GMVF; area ~155 km²) from 52 locations, plus a drill core. Analyses were done by ICP-MS (N = 81) and energy-dispersive X-ray fluorescence in vacuum (N = 350). Twenty-five ICP-MS samples were used to calibrate the XRF spectra and convert them to semi-quantitative oxide values. The results show that the GMVF can be subdivided into three parts. Area 1 (largest) consists of the Flowing Park and Palisade lobes and their confluence. Flow-sequence thickness ranges from 56 m (6 flows) to 187 m (26 flows) and decreases from east to west. Individual flows are laterally extensive, have well-defined vertical and lateral boundaries, and are associated with thin (<1 m) sedimentary interbeds. Argon-argon dates (17 flows) range from 9.45 to 10.99 Ma (mean = 10.17 Ma). A TSA plot (SiO₂ v Na₂O+K₂O in wt%) shows that Area 1 samples (N = 281) are mainly basaltic andesite (BA, 55.2%) and basalt (B, 29.5%), followed by potassic trachy-basalt (PTB, 8.5%) and shoshonite (SH, 6.8%). Area 2 is a probable vent complex and centers around Crag Crest (CC). The flow sequence is about 100 m thick and consists of a chaotic mix of cooling units with agglutinated pyroclastics. A 10-m-wide, north-trending dike is exposed at the base of CC, but does not completely penetrate the flow sequence. Age dates (2 dikes, 1 flow) range from 10.49 to 10.82 (mean = 10.61). Samples from Area 2 (N = 105) are dominated by SH (81.9%), followed by PTB (5.7%), latite (5.7%), BA (4.8%) and B (1.9%). Area 3 consists of eastern flow outliers such as Leon Peak and Mt. Darline. Total flow thickness ranges from 15 to 140 m and decreases from west to east. The west-trending Lombard dike occurs near Mt. Darline, but is geochemically different from the adjacent flows. Age dates (6 flows, 1 dike) for Area 3 range from 10.10 to 10.41 Ma (mean = 10.26 Ma). Samples (N = 45) are mainly BA (44.4%), followed by SH (24.4%), PTB (20.1%) and B (11.1%). Results of this study confirm that the eruptive history of GMVF is more complicated than previously thought. The geochemical characteristics of the vent complex (Area 2) are considerably different from the flows in Areas 1 and 3. This suggests that other vent areas remain to be discovered or that magmatic differentiation occurred during eruption history.