REASSESSING THE SOURCE OF THE COLORADO MINERAL BELT USING THE WINDY GAP CONGLOMERATE

The Colorado Mineral Belt (COMB) is a northeast trending zone of Late Cretaceous to Early Tertiary (~75-50Ma) magmatism that accompanied the development of basement cored uplifts of the Laramide orogeny in Colorado. While the origin of COMB magmatism remains enigmatic despite decades of active study, many past studies of COMB generally relate magmatism to lower crustal melting (Simmons and Hedge, 1978, Stein and Crock, 1990). However, these studies are based largely on intermediate composition intrusive rocks that may have undergone significant crustal interaction, rather than on more mafic volcanic material, due in part to the lack of preserved volcanic material of this age available for geochemical study. The Late Cretaceous Windy Gap Member of the Middle Park Formation west of the Front Range provides an opportunity to further investigate the origins of Laramide magmatism. The Windy Gap Member is a volcaniclastic conglomerate located near Granby, Colorado, and is unique in that it is composed largely of Laramide COMB volcanic clasts. The Windy Gap Member also contains interbedded andesitic lava flows, among the few Laramide volcanic flows preserved in Colorado. Clast compositions include basaltic rocks (47 wt.% SiO2) (Larson and Drexler, 1988) with initial εNd (T) values ranging from -3.4 to -6.6 and uniform ⁸⁷Sr/⁸⁶Sr ratios (~0.705). These clasts are significant in that they reveal that melting in ultramafic, upper mantle likely occurred beneath the COMB and that the resulting magmas had relatively low εNd (T) values. The latter overlap those of intermediate intrusive rocks throughout COMB, yet the values do not extend through lower εNd (T) (-12) and higher initial ⁸⁷Sr/⁸⁶Sr ratios (0.70962) of mafic lower crustal xenoliths found in the State Line Kimberlite District (Farmer et al., 2005). We suggest, then, that COMB ultimately represents the product of a mantle melting event, and therefore any model for generating COMB magmatism must ultimately incorporate dynamics of the upper mantle.