U/PB CHARACTERIZATION OF SIERRA DE MAZ: IMPLICATIONS FOR THE TIMING OF TERRANE ACCRETION AND TRANSLATION ON THE WESTERN MARGIN OF GONDWANA

The Paleozoic evolution of the western Gondwana margin involved accretion and translation of the Cuyania terrane and MARA block between 515-400 Ma. The Valle Fertil fault zone (VFFZ) of central Argentina marks the boundary between accreted terranes and the Pampean margin. The boundary projects northward to the Sierra de Maz region where relations between the Famatina arc on the Pampean margin, allochthonous material, and structures that accommodated accretion and translation are poorly known. Sierra de Maz is transected by the Maz shear zone, which separates a western clastic assemblage with a simple metamorphic history from an eastern assemblage of orthogneiss and paragneiss of complex deformational and metamorphic history.

U/Pb analysis of zircon from granulite facies tonalite-diorite orthogneiss defines ages between 1221 ± 10 and 1276 ± 13 Ma, with xenocrystic cores derived from a ca 1865 Ma protolith. Leucosomes cross-cutting orthogneiss units yield ages ca 1200 Ma. The units were intruded by a K-feldspar granite at 1090 ± 11 Ma and by 827 ± 32 and 845 ± 9 Ma pegmatites. Deformed granitic and pegmatitic bodies parallel to the foliation of the Proterozoic basement complex have emplacement ages between 422 ± 9 and 431 ± 3 Ma. Metamorphic rims on both detrital and igneous zircon from all units range from 399 ± 9 to 425 ± 7 Ma, consistent with leucosome ages. In-situ U/Pb analyses of titanite from samples within the Maz shear zone and orthogneiss units define ages of of 433 ± 9 and 410 ± 16 Ma for grains with pre- and syn-deformational relationships with the foliation. East of the main sierra in granulite facies units of poorly constrained origin, a leucosome and an undeformed granite yielded ages of 455 ± 3 and 418 ± 4 Ma. The U/Pb data suggests that the Proterozoic basement and Maz shear zone experienced widespread metamorphism and deformation from ca 430 to 420 Ma. Metamorphic assemblages and kinematic indicators (see abstracts Tholt et al., 2017 and Houlihan et al., 2017) supported by Ti-in-Zircon thermometry suggest this took place at amphibolite facies conditions in a sinistral transpressional regime. The magmatic and metamorphic history is similar to that observed west of the VFFZ to the south, suggesting that Sierra de Maz records a complex Silurian history of convergence and translation of the MARA block along the Pampean margin.