Central America Crustal Evolution
U-Pb geochronology and Nd-Sr isotopic analyses as documented for the first time an extensive Precambrian crustal history in the Chortis Block. Consideration of age patterns observed in Guatemala and in southern Mexico suggest genetic relationships
Nelson B.K., Herrmann U.R., Gordon M.B. and Ratschbacher L. (1997)
Sm-Nd and U-Pb evidence for Proterozoic crust in the Chortis Block, Central America: Comparison with the crustal history of southern Mexico: Terra Nova, v. 9, Abstract Supplement No 1, p. 496.
The Chortis block constitutes most of Central America and comprises the western part of the Caribbean plate. It is separated from North America by the Motagua-Polochic (MPFZ) and Swan Fault Zones. It probably extends south into northern Costa Rica under the cover of Cenozoic volcanics. U/Pb and Sm/Nd data from southern Mexico documents a widespread Grenville basement. Although Chortis basement age is poorly known, some tectonic scenarios relate it to southern Mexico. Identifying a pre-Paleozoic Chortis crustal history is critical for understanding the development of Central American crust.
Basement exposed south of the MPFZ in Guatemala and northern Honduras comprises high-grade sequences of pelitic metasedimentary units (gneisses, migmatites), quartzofeldspathic gneisses, and younger phyllite and quartzite sequences. Previously reported early Paleozoic ages (305 Ma) from deformed metaigneous rock cross-cutting the metamorphic complex provided a minimum age. Undeformed granitic and tonalitic plutons yield K-Ar and Rb-Sr dates of 35-100 Ma.
Our U/Pb zircon dates and Sm-Nd data establish that the Chortis crust in Guatemala and Honduras is mainly of Grenville age or has an inherited Grenville component. Samples from migmatites, gneisses and metaigneous rocks have Grenville crystallization ages or inherited zircons, later overgrown during Paleozoic, Mesozoic and Cenozoic metamorphism and migmatization. Upper intercept dates are 950-1250 Ma. Whether inheritance or crystallization ages, some of these samples have concordant Nd model ages at 0.9 to 1.1 Ga. Pre-Grenville crust may also be present as an inherited component, indicated by Nd model ages of 1.4-1.6 Ga (eNd (1.1 Ga) = 0.4 - 1.3). In addition to the Grenville ages, crystallization ages group around 400 Ma, 230 Ma and between 70 and 40 Ma.
These data are comparable to southern Mexican coastal complexes. The Oaxaca, Acatlan and Xolapa complexes have Proterozoic basement or inherited Proterozoic crust (0.9 to 1.1 Ga Nd model ages and upper U-Pb concordia intercepts). Both the Oaxaca and Acatlan complexes record Paleozoic metamorphism and migmatization (300 to 400 Ma) and, in the coastal Xolapa complex, granites related to magmatic arc activity yield Mesozoic-Cenozoic dates.