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Geological Society, London, Special Publications; 2005; v. 246; p. 381-416;
DOI: 10.1144/GSL.SP.2005.246.01.16
© 2005 Geological Society of London

Topics and Methodologies

Provenance studies of very low- to low-grade metasedimentary rocks of the Puncoviscana complex, northwest Argentina

Udo Zimmermann

Department of Geology, University of Johannesburg, PO Box 524, Auckland Park 2006, Johannesburg, South Africa uz{at}rau.na.ac.za

A provenance study of Neoproterozoic to Lower Cambrian rocks for the entire Puncoviscana Basin was conducted, using 119 samples from 15 different outcrops. Petrographic data (Qt60–80, F15–35, L5–20, P/F 0.2–0.4, Lv/L = 0) show a composition comparable to foreland-basin successions. Lithoclasts are of metamorphic and metasedimentary origin. Volcanic debris is detected only in the form of sanidine, and volcanic lithoclasts were probably decomposed to form pseudo-matrix. Framework clasts are sub-angular to sub-rounded, and the rocks are poorly sorted. Major element geochemistry shows a moderate to high Chemical Index of Alteration (56–77) and failed to provide coherent provenance and rock classification. Trace element geochemistry suggests a rhyodacitic composition overall. Rare earth element patterns are comparable to those of model upper continental crust (UCC), as are concentrations of Nb, Ta, Ti, Th-Sc and Eu/Eu* (0.45–0.87; 95% between 0.4 and 0.7); reworking signatures are not detected. The uniform mineralogical and geochemical composition reflects supra-crustal source(s) for the entire basin, including significant metamorphic rock debris. The Puncoviscana complex is interpreted as a peripheral Pampean foreland basin, fed mainly from an eastern fold-thrust belt, but includes relicts of pre- and syn-collisional magmatic activity as well. A source area of UCC composition to the west is represented by the Arequipa block.





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