EDIACARAN·EARLY PALEOZOIC SEDIMENTATION OF SOUTHWEST GONDWANA: EVIDENCE FROM DETRITAL ZIRCONS IN THE EASTERN SIERRAS PAMPEANAS

RAPELA, C.W.; VERDECCHIA, S.; PANKHURST, R.J.; BALDO, E.; CASQUET , C.; GALINDO, C.; DAHLQUIST, J.A.; FANNING, C.M.

San Pablo

Simposio; 9th South American Symposium on Isotope Geology; 2014

Universidad de San Pablo USP

Basement exposures in the Eastern Sierras Pampeanas (ESP) at 26°-33° 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 follows: (a) The patterns of ESP metasedimentary rocks affected by the Pampean orogeny are similar tothose of thetract ofthe Puncoviscana Formation that was involvedinthe Pampean orogeny. Similarly,the patterns ofthe ESP metasedimentary rock deposited after the Pampean deformation are comparable in part to those ofthe Late Cambrian Meson Group. This strongly suggests that the ESP iscomposed of metasedimentary sequences equivalent to those located at higher structural levels in NWArgentina. (b) Detrital zircon patterns ofsediments 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 EarlyCambrian- 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 ofthe Rio de la Plata craton. These characteristics indicatethat both the Pampean belt and the Rio de la Plata craton were already exposed in Late Cambrian times. The postPampean pattern is indistinguishablefrom that ofthe metasedimentary rocks ofthe North Patagonian Massif, suggesting provenance from similar sources.