Assessment of Paleozoic terrane accretion along the southern central Andes using detrital zircon geochronology

Tuesday, 15 December 2015
Poster Hall (Moscone South)
Ryan McKenzie1,2, Brian K Horton3, Facundo Fuentes4, Julie C Fosdick5, Tomas Capaldi2, Daniel F Stockli2 and Patricia M Alvarado6, (1)Yale University, Geology and Geophysics, New Haven, CT, United States, (2)University of Texas at Austin, Department of Geological Sciences, Austin, TX, United States, (3)University of Texas at Austin, Department of Geological Sciences and Institute for Geophysics, Austin, TX, United States, (4)YPF, Buenos Aires, Argentina, (5)Indiana University Bloomington, Bloomington, IN, United States, (6)CONICET and Universidad Nacional de San Juan, Departamento de GeofĂ­sica y AstronomĂ­a, FCEFN, San Juan, Argentina
Two distinct Paleozoic terranes known as Cuyania and Chilenia occupy the southern central Andes of Argentina and Chile. Because the proposed terrane boundaries coincide with major structural elements of the modern Andean system at 30-36°S, it is important to understand their origins and potential role in guiding later Andean deformation. The Cuyania terrane of western Argentina encompasses the Precordillera (PC) and a thick-skinned thrust block of the western Sierras Pampeanas, persisting southward to the San Rafael Basin (SRB). Although recently challenged, Cuyania has been long considered a piece of southern Laurentia that rifted away during the early Cambrian and collided with the Argentine margin during the Ordovician. Chilenia is situated west of Cuyania and includes the Frontal Cordillera (FC) and Andean magmatic arc. This less-studied terrane was potentially accreted during an enigmatic Devonian orogenic event.

We present new detrital zircon U-Pb age data from siliciclastic sedimentary rocks that span the entire Paleozoic to Triassic from the FC, PC, and SRB. Cambrian rocks of the PC exhibit similar zircon age distributions with prominent ~1.4 and subordinate ~1.1 Ga populations, which are distinct from other Paleozoic strata. Plutonic rocks with these ages are common in southern Laurentia, whereas ~1.4 Ga zircons are uncommon in South American age distributions. This supports a Laurentian origin for Cuyania in isolation from Argentina during the Cambrian. Upper Paleozoic strata from the PC, FC, and SRB all yield similar age data suggesting shared provenance across the proposed Cuyania-Chilenia suture. Age distributions also notably lack Devonian-age grains. The regional paucity of Devonian plutonic rocks and detrital zircon casts doubt on a possible arc system between these terranes at this time, a key requisite for the mid-Paleozoic transfer and accretion of Chilenia to the Argentine margin. Collectively, these data question the precise boundaries of the Chilenia terrane.