Geochemistry and alteration patterns in the early Mesozoic Cordilleran arc and arc-related rocks: evidence for sources of detritus in continental successions

The Upper Triassic Chinle Formation on the Colorado Plateau and El Antimonio Group in northwest Sonora, Mexico, contain abundant coarse volcanic detritus. Detailed geochemistry of 15 clasts from coarse-grained units in the middle Chinle Formation and eight clasts of the El Antimonio Group provide information about potential source rocks in the Triassic volcanic arc along the Cordilleran margin. These data are complemented by new analyses of 27 granite and 10 volcanic rock samples from the eastern Sierra Nevada. Clasts from the Chinle Formation range in age from 218 - 235 Ma. Samples are dacite to rhyolite and are strongly altered with respect to Na, K, Rb, and Sr. In some cases clasts are highly silicified; SiO2 values are as high as 79%. Na2O values range from 0.71 - 9.62%, K2O ranges from 0.6 - 6.44, Rb from 0 - 220 ppm, and Sr from 59 - 283 ppm. Sr initial ratios vary from 0.708123 - 0.709412. El Antimonio clasts (280 Ma on one clast) are dacite and are much less altered than Chinle clasts (Na2O = 4.25% - 6.94%; K2O = 2.21% - 3.19%; Rb = 61 ppm - 95 ppm; Sr = 416 ppm - 713 ppm), although initial Sr ratios are equally high (0.707103 - 0.709783). Coeval granites from the central Sierra Nevada plot in standard fields on total alkali-silica and variation diagrams; Sr initial ratios are 0.70236 - 0.70628. Triassic rhyolitic ignimbrite from the Sierra Nevada is generally similar to the granitic rocks, with Na2O depletion in some rocks (as low as 0.06%). ɛNd values in the Sierran igneous rocks vary from +0.29 (2 samples) to -4.58. The Chinle and El Antimonio clasts have more negative values, from -3.21 to -8.35. These data, when taken together with previously published results from Triassic plutons in the Mojave desert, suggest broad relations between clasts and remnants of the arc. ɛNd values from Chinle and El Antimonio clasts suggest the possibility of sources in the San Gabriel or Granite Mtn suite, where ɛNd values are -2 to -8 and in the San Bernardino suites, which has values of -7 to -10. The suggested correlations for Chinle clasts, however, are not supported by Th/U values in zircons from these samples, which suggest an unexposed source to the east, in westernmost Arizona. The high initial Sr values in Chinle and El Antimonio clasts suggest the interaction of clasts with either seawater or highly alkaline waters in an arid environment. The upper Chinle Formation reflects arid conditions, and the Sri values correspond to a Cretaceous age, including when marine conditions existed on the Colorado Plateau.