U-PB SINGLE CRYSTAL SYSTEMATICS: CONSTRAINTS FOR THE MAGMATISM AND NORTHWEST BOUNDARY OF THE XOLAPA COMPLEX, MEXICO

Sierra Madre del Sur, a Mesozoic-Cenozoic magmatic arc in southern Mexico was studied for U-Pb geochronology. Plutonic and metaplutonic units from two transects were sampled at the northwest limit of the Xolapa Complex in order to constraint magmatic history, nature of the basement and terrane boundaries. Four samples from the northern transect near Zihuatanejo, Guerrero, yield crystallization ages of 41.8 ± 1.4, 43.4 ± 1.6, 40.5 ± 1.4, 43.0 ± 2 Ma with no inherited zircons, suggesting that pre-existing continental basement or sediments are not a significant component in these rocks. Five of six samples from the second transect near Atoyac, Guerrero, yield crystallization ages of 53.0 ± 1.9, 52.7 ± 1.9, 56.7 ± 2.4, 54.4 ± 1.7, 58.1 ± 2.1 Ma, analogous to the ages reported for the Acapulco Intrusive. The remaining sample yielded an age of 40 Ma with an inherited component of 58–64 Ma, similar to the ages determined for the first five samples. Several clusters of Mesozoic inherited zircons with ages of 72-74 Ma, 83-87 Ma, 90-92 Ma, 105-111 Ma, 143-153 Ma, indicate that the magmatism was active since the late Jurassic, and that multiple episodes of magmatism have occurred during the late Cretaceous. These clusters of inherited zircons also indicate that processes of assimilation and recycling have played an important role in the evolution of the Xolapa Complex. Older Paleozoic (~320Ma; ~360Ma) and Grenvillian (~960-1085 Ma) inherited zircons ages suggest an affinity of the Xolapa Complex with the Acatlan Complex and/or Oaxacan complex. The presence of inherited zircons in the southern transect suggests that the limit between the Xolapa and Guerrero terrane is located between these two transects as previously proposed by Campa and Coney (1983).

Campa, M., and Coney, P. J., 1983, Tectono-stratigraphic terranes and mineral resource distribution in Mexico.: Canadian Journal of Earth Sciences, v. 20, p. 1040-1051.