The age and significance of the Puncoviscana Formation with respect to Neoproteozoic to Cambrian tectonic evolution of the Proto-Andean margin of Gondwana.

MONICA ESCAYOLA; CEES VAN STAAL

Orange, Sidney, Australia

Conferencia; Mcquarie Arc Conference, Orange, Sidney Australia; 2009

Geological Survey of Australia

We present a new tectonic model for the Pampean-Tilcarian accretion of the Arequipa-Antofalla-Western Pampia (AA-WP) 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 and conventional single grain TIMS of  Puncoviscana Formation, which are based on  recently discovered felsic tuffs and mafic volcaniclastic rocks (~ 537±1 Ma) in the unit’s  type locality suggests that the Puncoviscana Formation 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 ~650-570 Ma island arc (Eastern Pampia arc), built upon the rifted, leading edge of the AA-WP. The c. 537±5 Ma felsic tuffs are interpreted to represent the products of 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 SHRIMP and conventional single grain zircon age of the post-collision Canani tonalite (524±1 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 529-520 Ma  zircon ages for post-collision peraluminous granites and tonalites  in the Eastern Pampean Ranges to the south indicates that the synorogenic Puncoviscana basin formed between 540 and 524Ma, progressively cannibalizing its orogenic hinterland over time. In addition, the Tilcarian and Pampean orogenies represent the same event. We suggest that AA-WP 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 AA-WP, 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 AA-WP, but returned later to Laurentia during Iapetus’ closure.