Ediacaran-Early Paleozoic sedimentation of Southwest Gondwana: evidence from detrital zircons in the Eastern Sierras Pampeanas

CARLOS WASHINGTON RAPELA; SEBASTIÁN OSVALDO VERDECCHIA; ROBERT JOHN PANKHURST; EDGARDO BALDO; CESAR CASQUET; GALINDO CARMEN; JUAN ANDRES DAHLQUIST; MARK FANNING

San Pablo

Simposio; IX South American Symposium on Isotope Geology; 2014

Universidad Federal de San Pablo

Basement exposures in the Eastern Sierras Pampeanas (ESP) at 26º-33º S present a unique opportunity to compare sediment sources along a significant stretch of the Pampean belt. This has been accomplished on the basis of U-Pb SHRIMP detrital zircon ages for a large number of samples over a large region, including comparisons with NW Argentina and Patagonia.. High-grade metasedimentary rocks of the ESP and low-grade rocks in NW Argentina can be subdivided into those deposited before, or simultaneously with, the Early Cambrian Pampean orogeny and those deposited after the orogeny (Late Early Cambrian to Early Ordovician). Preliminary conclusions are as follow: (a) The patterns of ESP metasedimentary rocks affected by the Pampean orogeny are similar to those of the tract of the Puncoviscana Formation that was involved in the Pampean orogeny. Similarly, the patterns of the ESP metasedimentary rock deposited after the Pampean deformation are comparable in part to those of the Late Cambrian Mesón Group. This strongly suggests that the ESP is composed of metasedimentary sequences equivalent to those located at higher structural levels in NW Argentina. (b) Detrital zircon patterns of sediments deposited after Pampean deformation show in general the following characteristics: (i) the abundance of the Mesoproterozoic group (970-1080 Ma) strongly decreases to less than 15%; (ii) the largest peak is Early Cambrian ~ 525 Ma, which together with a 600-640 Ma Neoproterozoic component dominates the pattern; (iii) there is a 2170-2190 Ma minor peak in the age range of the Río de la Plata craton. These characteristics indicate that both the Pampean belt and the Río de la Plata craton were already exposed in Late Cambrian times. The post-Pampean pattern is indistinguishable from that of the metasedimentary rocks of the North Patagonian Massif, suggesting provenance from similar sources.