Abstract

VALENCIA, Victor A. et al. U-Pb geochronology of granitoids in the north-western boundary of the Xolapa Terrane. Rev. mex. cienc. geol [online]. 2009, vol.26, n.1, pp.189-200. ISSN 2007-2902.

The Sierra Madre del Sur, a Mesozoic-Cenozoic magmatic arc in southern Mexico, was studied using U-Pb zircon geochronology. Undeformed to slightly deformed plutons from two transects were sampled at the limit between the Guerrero and Xolapa terranes, in order to constrain the magmatic history, nature of the basement and terrane boundaries. Four samples from the Zihuatanejo, Guerrero, transect within the Guerrero terrane, yielded crystallization ages of 41.8 ± 1.4, 43.4 ± 1.6, 40.8 ± 1.4 and, 41.8 ± 4.6 Ma. No inherited zircons were detected in these plutons indicating that pre-existing zircons from continental basement or sediments are not a significant component in these rocks. Five samples from the Atoyac, Guerrero transect within the Xolapa terrane, yielded crystallization ages of 53.5 ± 1.9, 52.7 ± 1.9, 57.3 ± 2.2, 54.4 ± 1.7, and 57.0 ± 2.1 Ma, analogous to the ages reported for the Acapulco intrusive. One sample of this transect yielded an age of 40.2 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 and, 143-153 Ma, indicate that the magmatism in the Xolapa terrane was active since the Jurassic, and that multiple episodes of magmatism occurred during the Cretaceous. Inherited zircons also indicate that processes of assimilation and recycling of previous intrusive bodies have played an important role in the evolution of the Xolapa Complex. Older Paleozoic (~320 Ma; ~360 Ma) and Grenvillian (~960-1085 Ma) inherited zircons ages suggest an affinity of the Xolapa Complex with the Acatlán and Oaxaca Complexes, even though the metasedimentary basement of the Xolapa complex (of unknown age) may be the source of these Paleozoic and Grenvillian zircons. The presence of inherited zircons in the Atoyac transect suggests that the limit between the Xolapa and Guerrero terranes is located between these two transects.

Keywords : U-Pb; zircon; arc magmatism; Xolapa; Mexico.

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