The timing and kinematics of terrane accretion and translation on the Paleozoic margin of Gondwana remains contentious. Multiple models exist for the accretion of the MARA and Precordillera terranes, but deviate in timing and kinematics of accretion, the nature of involved terranes, Laurentian versus Gondwanan origin, and the extent of translation along the margin. The Valle Fertil Fault Zone (VFFZ) marks the suture zone between the Pampean margin and the MARA terrane, which was accreted and translated via thrusting or transpression between 515-400 Ma. The VFFZ projects northward to the Sierra de Maz region, which is transected by ductile shear zones that lie along strike with the boundary and separate assemblages with varied magmatic and metamorphic history. Sierra de Maz is underlain by Mesoproterozoic-Neoproterozoic igneous and metamorphic units assigned to the MARA terrane, known as the Maz and El Taco Complexes, which are juxtaposed with El Zaino Complex, a clastic assemblage that has a simple metamorphic history. Ramaditas lies to the east of the main sierra, and is comprised of granulite facies units with poorly constrained origin.
Characterization of Sierra de Maz and Ramaditas involved U/Pb analysis of 17 igneous zircon and 2 in-situ titanite samples by SHRIMP-RG methods, 6 igneous zircon and 8 detrital zircon samples by LA-ICP-MS methods, and trace element analyses of selected samples. Microstructural analyses and Ti-in-Zr geothermometry were used to analyze the timing of metamorphic conditions and kinematics. This collaborative study was performed in conjunction with petrologic analysis, geochronologic analyses of Lu/Hf garnet and Ar/Ar suites, and detailed structural analyses executed by two M.S. students from Western Washington University and University of California-Davis.
The U/Pb data suggests that the Proterozoic basement and Maz Shear Zone experienced widespread metamorphism and deformation from ca. 430 to 420 Ma in the Maz and El Zaino Complexes, and from ca. 430-460 Ma in the Taco Complex and Ramaditas. Metamorphic assemblages and kinematic indicators suggest this took place at granulite to greenschist facies conditions in a sinistral transpressional regime, with localized dextral slip. 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 late Ordovician to early Devonian history of convergence and translation of the MARA terrane along the Pampean margin.