USE OF ZIRCON GEOCHEMISTRY TO TIE VOLCANIC DETRITUS TO SOURCE PLUTONIC ROCKS: AN EXAMPLE FROM PERMIAN NORTHWESTERN SONORA, MEXICO

Whole rock geochemistry together with analysis of zircons from the Los Tanques pluton and two clasts from the Permo-Triassic El Antimonio Formation in northwest Sonora suggest that arc volcanism was co-eval with and likely related to crystalization of plutons. In addition to previously reported SHRIMP-RG dates on one clast and on the nearby Los Tanques pluton (280 ± 3 Ma and ~ 270 Ma, respectively, Riggs et al., 2009, GSA Ann Mtg v. 41 p. 540), a second clast has been dated by LA-ICP-MS at 273 ± 6 Ma.

Major element geochemistry and rare earth element concentrations and ratios from zircons suggest that the clasts and the pluton, separated by ~120 km, could be related. Plutonic samples are more silicic than clasts (73% vs. ~64%); variation diagrams show an expected decrease in TiO₂ and Na₂O, and increase in K₂O with increasing SiO₂, Zircon chemistry, however, yields a more detailed picture. Both clasts and pluton have low Th/U (0.01 – 0.5, with most zircons yielding ratios <0.25). These ratios, when plotted against concentrations of Hf (11000 – 15000 ppm) and to Yb/Gd (20 – 60), show very strong overlap.

The relation between the western Mexico Permian arc and plutonic rocks as old as 260 Ma in southwestern California is less certain. Ages obtained thus far do not overlap. Si/Al are similar between California and Sonoran rocks, but when compared to Fe+Mg+Mn+Ti group distinctively. Th/U ratios in zircons are higher in the California portion of the Permo-Triassic arc, ranging from 0.2 – 2 (overall average ~0.8). Yb/Gd and Hf concentrations are lower (Hf as low as ~7000 ppm; Yb/Gd = 5 – 38). Differences in zircon chemistry between the California and Sonoran parts of the arc are most easily attributed to chemical characteristics of intruded Proterozoic crust.