U-Pb SHRIMP igneous zircon ages from metamorphic rocks between the Precordillera terrane and Gondwana margin, Sierras de La Huerta to Pie de Palo, northwest Argentina.

MCCLELLAND, W.C., ELLIS, J.R., ROESKE, S.M., VUJOVICH, G.I., NAIPAUER, M.

Montevideo

Conferencia; Gondwana 12 "Geological and Biological Heritage of Gondwana"; 2005

Academia Nacional de Ciencias

The Precordillera terrane of northwestern Argentina represents an allochthonous fragment accreted to the Paleozoic Gondwana margin. The origin of the terrane and timing of its accretion have long been debated. New U–Pb SHRIMP zircon results from granitoids and LA-ICPMS detrital zircon data from metasedimentary rocks in the suture zone suggest that rocks separating the Precordillera terrane from the Famatina arc margin represent, or were derived from, basement of uncertain affinity. Low-grade carbonate platform rocks of the Precordillera terrane are bound to the east by metamorphic rocks in the Sierra de Pie de Palo that are generally thought to represent Grenville-age Precordillera basement. However, this detrital zircon study combined with previously published work finds that the Grenville age is not unique to Laurentia and cannot be used to determine the boundaries of Paleozoic blocks in this part of South America. Detrital grains from four quartzite samples from Sierra de Pie de Palo dominantly range in age from 1000 to 1400 Ma with a prominent peak within this range at 1070 Ma, minor peaks at 1850 and 2710 Ma, and numerous single grains between 2415 and 3350 Ma. The detrital zircon signature from quartzite at Nikizanga has peaks at 530, 640, and 1025 Ma and single grains as old as 3400 Ma. Farther east, metamorphic rocks in Loma de las Chacras experienced complex metamorphism during which peak high-P conditions preceded peak high-T conditions. Metamorphism followed granite emplacement at 472 ± 3 Ma and the timing of peak-T conditions is estimated from leucosomes within metasedimentary rocks and amphibolite that yield U–Pb zircon ages of 461 ± 1.7 Ma and 456 ± 2, respectively. Three quartzite samples from Las Chacras have a 1000–1400 Ma range in detrital zircon ages with minor peaks at 2680 and 2900. Additional metagreywacke samples are dominated by 600–650 Ma and 515–540 Ma detrital zircons with minor peaks at 1180, 2030, and 2080 Ma. Metamorphic rocks of Las Chacras are structurally juxtaposed to the east with an apparently low-grade metaplutonic complex, informally referred to as the Resina complex, that yields U–Pb zircon ages of 839 ± 10, 1062 ±20, 1084 ± 4, 1091 ± 15, and 1099 ± 22 Ma. Oscillatory-zoned igneous zircon from all five dated samples lack evidence of rims recording Paleozoic metamorphic overgrowth. Metasedimentary rocks exposed at Pan de Azucar contain a dominant age cluster at 1105 Ma that is chronologically compatible with the Proterozoic igneous complex and may represent a supracrustal sequence deposited on it. Detrital zircon ages from Sierra de Pie de Palo and Las Chacras define a non-unique spectrum from Grenville-age and older sources that may be assigned either a Laurentian or Gondwanan affinity. The younger 550 and 650 Ma age clusters are consistent with peri-Gondwana and Gondwana signatures. The ca. 840 igneous age in the Resina Complex is not common in either the Precordillera terrane or the Gondwana margin exposed in the Sierras Pampeanas of northwest Argentina. These ages are however observed elsewhere along the Gondwana margin or in terranes thought be associated with it. Similarity in plutonic, leucosome, and metamorphic rim ages in Sierra de la Huerta, Loma de las Chacras, and Sierra de Pie de Palo suggests that accretion of the Precordillera was ongoing at 460 Ma and deformation within the suture zone continued through to 400 Ma. The fault-bound Resina complex may represent Pampean basement upon which the Famatina arc was built, Precordillera basement that differs in metamorphic history from Pie de Palo, or a distinct crustal fragment of possible Gondwana of peri-Gondwana affinity trapped within the suture zone between the Precordillera and Famatina arc. On the basis of the unique ca. 840 Ma age, the latter is currently favoured.