Probing the composition of unexposed basement, South Portuguese Zone, southern Iberia: Implications for the connections between the Appalachian and Variscan orogens

摘要

Geochemistry and Sm-Nd and U-Pb (magmatic zircon) isotope data from a postcollisional batholith that crosscuts the allochthonous South Portuguese Zone (SPZ) of southern Iberia suggest that the basement is compositionally more juvenile than the exposed upper crust. The SPZ is an allochthonous terrane of the late Paleozoic Variscan orogen. The oldest exposed units in the SPZ are Late Devonian continental clastics, and as a result, the origins of the SPZ are unknown. Multifaceted inherited zircon cores from a granitoid batholith (Sierra Norte Batholith, SNB) reveal Neoproterozoic (ca. 561- 647 Ma) and Mesoproterozoic ages (ca. 1075 - ca. 1116 Ma). Granitoid samples are characterized by εNd values ranging from +1.4 to -9.6 and model ages ca. 0.76-1.8 Ga. Conversely, the exposed Late Devonian clastics of the SPZ are characterized by more negative εNd values (-7.5 to -10.4). Taken together, U-Pb and Sm-Nd data indicate the lower crust that melted to yield the SNB was (i) Neoproterozoic (ca. 560-650 Ma) to Mesoproterozoic (ca. 1.0-1.2 Ga) in age, (ii) was not compositionally similar to the overlying Devono-Carboniferous continental detritus but was instead more juvenile, with model ages between ca. 0.9-1.2 Ga. This unusual relationship is similar to the relationship between the relatively juvenile basement and ancient upper crust documented in the exposed portion of the Meguma terrane in the northern Appalachians, which paleogeographic reconstructions show was immediately outboard of southern Iberia in the Late Devonian.