U-PB ZIRCON GEOCHRONOLOGY AND SM-ND SYSTEMATICS OF PALEOPROTEROZOIC ROCKS, MT. OURAY, COLORADO

Bimodal metavolcanic rocks in Pass Creek Valley, Colorado, which include komatiites, komatiitic basalts, tholeiitic basalts, and felsic volcaniclastics, are petrographically and chemically similar to the 1728 Ma rocks near Salida and the 1730-1740 Ma Cochetopa Succession near Gunnison (c.f. Sauer, 1988; Bickford and Boardman, 1984). The U-Pb ages of 7 detrital zircons, derived from a felsic metavolcaniclastic unit, yield a regressed upper-intercept age of 1719 ± 21 Ma. Although both prismatic and equant grains were analyzed, their ages are similar. All but one analysis yielded uncharacteristically low 206Pb/204Pb ratios, which contributed to the relatively large uncertainties in calculated ages. Because the zircons were derived from a sedimentary volcaniclastic rock, the ages do not rule out the possibility that older or younger detrital zircons might be present, but the cluster of ages suggests a model in which all the zircons were derived from a nearby igneous source. Thus, the age probably constrains the age of the interlayered mafic meta-volcanic rocks. Sm-Nd data for two felsic rocks yield e Nd(t) values of 3.75 and 3.80 and T(DM) ages of 1765 and 1761. e Nd(t) values for the felsic rocks plot near the depleted mantle curve and overlap the lower range of values for the mafic suite (3.7-4.68), suggesting similar mantle separation ages for both. These isotopic characteristics, which are similar to recently reported data for Paleoproterozoic volcanic and plutonic rocks in Salida and Gunnison, suggest that the rocks are mostly of juvenile mantle derivation. The age of the Pass Creek rocks suggests that they are coeval with the Gunnison and Salida suites. The presence of komatiites and komatiitic basalts within all three areas further supports the hypothesis that the three areas are cogenetic.