Neoproteozoic to Cambrian tectonic evolution of the Proto-Andean margin of Gondwana: implications for the opening of Iapetus.

VAN STAAL C. R.; ESCAYOLA M. P.

Toronto

Congreso; AGU Meetings Toronto; 2009

AGU - GAC - MAC

We present a new tectonic model for the Pampean accretion of the Arequipa-Antofalla-Pampia (AAP) ribbon continent to the Proto-Andean margin of Gondwana represented by the Amazonia and Rio de La Plata cratons, based on our studies of the Puncoviscana Formation and adjacent units in northern and central Argentina.  A compilation of existing detrital zircon ages of the Puncoviscana Formation and correlative units along strike in the Pampean orogenic belt to the south combined with our new U-Pb SHRIMP zircon ages of Puncoviscana Formation felsic tuffs and mafic volcaniclastic rocks (~ 535 Ma) suggests this unit mainly represents an Early Cambrian arc-trench gap to foreland basin succession formed during east-directed closure of a late Neoproterozoic oceanic back-arc basin. The back-arc basin, which probably remained relatively narrow, initially had opened behind an east-facing ~ 650-580 Ma island arc (eastern Pampia arc), built upon the rifted, leading edge of the AAP. The c. 531+-3.5 Ma felsic tuffs are interpreted to represent the products a new, short-lived Early Cambrian magmatic arc built upon the composite Proto-Andean margin, following Late Neoproterozoic, soft-accretion of the eastern Pampia arc and a subduction polarity reversal. Puncoviscana Formation conglomerates and mafic volcanics previously interpreted as early rift-related deposits are better interpreted as late-orogenic basin fills and/or were deposited after basin closure. Our new U-Pb zircon age of the post-collision Canani tonalite (~517 Ma), which intruded into Tilcarian deformed  Puncoviscana Formation rocks in the north westernmost part of Argentina in the Puncoviscana type locality, combined with the existing 529-517 Ma  zircon ages for post-collision peraluminous granites and tonalites  in the Sierra de Cordoba in central Argentina indicates that the synorogenic Puncoviscana basin formed between 540 and 517 Ma, progressively cannibalizing its orogenic hinterland over time. In addition, the Tilcarian and Pampean orogenies represent the same event. We suggest that AAP rifted-off from Laurentia between 700 and 650 Ma, shortly after Amazonia’s departure during Rodinia’s break-up. We emphasize that it is the departure of AAP, not Amazonia that opened Iapetus in the Late Neoproterozoic. We also suggest that the Ganderia terrane in the northern Appalachians, originally formed an extension of the AAP, but returned later to Laurentia during Iapetus’ closure.