Origin of Taylor creek rhyolite magma, black Rangef New Mexico, based on Nd-Sr isotope studies

Taylor Creek high-silica tin-bearing rhyolites are found in the northern Black Range of the Mogollon-Datil volcanic field in southwestern New Mexico, occurring near the stratigraphic top of a thick mid-Tertiary volcanic section. Initial tNd values for the high-silica rhyolite lavas range from -5.0 to -6.2, which are similar to those of the Garcia Camp tuff, a pyroclastic phase of the Taylor Creek Rhyolite. The older Kneeling Nun tuff, which crops out in the same area, also has a similar tNd( value, which indicates that the high-silica rhyolites and spatially associated silicic tuffs were derived from an isotopically similar source. Comparison with data from lower crustal xenoliths and data bearing on the isotopic compositions of the lower crust suggest that the melts were derived from 80 to 50 percent lower crustal sources. The Poverty Creek basaltic andesite and Bearwallow Mountain Formation andesite, stratigraphically below and above Taylor Creek Rhyolite, respectively, have more positive ε_Nd1 values of -4.7 and -2.3, respectively, indicating a greater mantle component. Initial ⁸⁷Sr/⁸⁶Sr ratios vary from 0.7046 to 0.7131 for the Taylor Creek Rhyolite. There is a broad positive correlation between initial ⁸⁷Sr/⁸⁶Sr and Sr content and a negative correlation with Rb, Ta, and Th content. These variations may be explained by late-stage upper crustal assimilation of radiogenic and relatively Sr-rich wall and roof rocks. Whole-rock Sr contents of the least radiogenic rocks as low as 3 ppm indicate that little assimilation would be required to affect the original Sr isotopic signature of the Taylor Creek Rhyolite magma. The Nd isotopes, however, were not measurably affected by the upper crustal processes.