The Ediacaran margin of the proterozoic MARA block and the sedimentary basins of the Clymene Ocean: Insights from U-Pb zircon geochronology in the Puna-Sierras Pampeanas transition, NW Argentina

Abstract

The basement of the sierras of Fiambalá and Altohuasi, in the Puna-Sierras Pampeanas transition (NW Argentina), consists mainly of early Cambrian metamorphic rocks (Fiambalá Metasedimentary Complex; new name) and granitoids (Loconte Intrusive Complex; new name). The metasedimentary complex consists of: 1) the Tatón Unit (gneisses, metatexites, diatexites and mylonites); and 2) the Laguna Helada Unit (low- to medium-grade fine-grained metaconglomerates with subordinated metapsammites, slates, phyllites, schists, mylonites and marbles). U-Pb zircon geochronology performed in the two units yields similar results suggesting that they were sourced from the same basements and probably belong to a continuous stratigraphic pile prior to metamorphism and ductile shearing. These units show a main group of ages between ca. 0.95 and 1.10 Ga, and a secondary group at ca. 1.15–1.25 Ga, with a minor peak at ca. 1.33 Ga, and few Paleoproterozoic-Neoarchean ages. This, along with zircon metamorphic overgrowths of 526 ± 4 Ma found in the Tatón Unit, and regional correlations, indicate that sedimentation took place in Ediacaran to early Cambrian times, in a mixed carbonate-siliciclastic platform bordering the extinct Clymene Ocean. Comparisons (lithology, Sr-isotopes and detrital zircon data) with similar successions recognized elsewhere in the Sierras Pampeanas, southern Puna, Cordillera Frontal, and Cordillera Oriental suggest that the sediments came from the same source areas and that they were part of a common sedimentary environment, i.e., the Difunta Correa Basin. The main source areas were the Grenville and the Granite-Rhyolite provinces of Laurentia and the MARA block. This basin – now dismembered by Paleozoic deformation – is one of the many basins formed along the margins of the Clymene Ocean lying in NW Argentina, southern Brazil, Paraguay, and Río de la Plata craton. The closure of this ocean, during the early Cambrian Pampean orogeny resulted in the low- to high-grade metamorphism of the Fiambalá Metasedimentary Complex.