2. Mineralogy – Petrology – Geochemistry - SWISS GEOSCIENCE ...

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Symposium 1: Structural Geology, Tectonics and Geodynamics
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A late Paleozoic back-arc basin on the western Gondwana margin: age
and geodynamic setting of Permo-Carboniferous sedimentary rocks of
South-East Peru.
Mariël Reitsma 1 , Richard Spikings 1 , Urs Schaltegger 1 & Alexey Ulianov 2
1 University of Geneva, Earth and Environmental Sciences, Rue des Maraîchers 13, Genève (martje.reitsma@unige.ch)
2 University of Lausanne, Department of Mineralogy and Geochemistry, Lausanne
After a long period of igneous quiescence during the Siluro-Devonian, magmatic arc activity recommenced along the western
Gondwana margin in the early Carboniferous (~345 Ma; Bahlburg et al., 2009). During the same period sediments of
the Ambo Group accumulated in the dominantly continental basins of Peru. At many locations plant remains are preserved
indicating a late Viséan – earliest Serpukhovian age and a warm-temperate, humid climate (Azcuy and di Pasquo,
2005). In the Pennsylvanian a marine transgression covered a significant part of Peru and largely overflowed the limits of
the Mississippian Ambo basins. Transgression continued and reached its maximum extent in the lower Permian. The
platform carbonates deposited in this epeiric sea are referred to as the (Tarma)-Copacabana Group. The upper levels of the
Copacabana Group are assigned to the Artinskian (lower Permian) based on palynology and foraminifera (Doubinger and
Marocco, 1981).
In this study we present Laser Ablation ICP-MS U-Pb age determinations on detrital and volcanic zircons, and whole rock
geochemical compositions on lavas from 5 stratigraphic sections and several key samples from central and south-east Peru.
For the first time isotopic age constraints for the Ambo and Copacabana groups are available. Maximum detrital ages for
the Ambo sandstones fit well with ages assigned to the plant fossils in the literature. Subsequent flooding of the Ambo
basins at the Mississippian-Pennsylvanian boundary coincides with global sea level rise (Haq and Schutter, 2008). However,
our study shows that the basin did not remain exclusively marine till the Artinskian but rather experienced important sea
level fluctuations tentatively coupled to Permo-Carboniferous glaciation cycles. Most significantly we discovered that the
Copacabana Group ends with a continental interval of red bed and volcanic deposits that was previously attributed to the
Triassic Mitu Group. Emersion of the Copacabana basin continued until sediments were exposed above base level creating
the erosional hiatus with the overlying Mitu Group, this disruption coincides with the eustatic low across the Permo-
Triassic boundary (Haq and Schutter, 2008).
U-Pb ages for the Ambo and Copacabana groups overlap with those for the peraluminous granitoids of the Cusco-
Vilcabamba area obtained in this study. The Permo-Carboniferous plutons of north and central Peru have been interpreted
as a continental arc based solely on their whole rock geochemistry (Nb, Ta, Pb anomalies; Miskovic et al., 2009). However,
we argue that the same geochemical signature can be explained by recycling of continental crust in the melt. We therefore
reinterpret the late Paleozoic granitoids and sediments from the Eastern Cordillera as remnants of a back-arc system.
The research area is too far inboard from the present day trench (~400 km) to be simply related to a subduction zone,
neither is there evidence for Phanerozoic terrane accretation to the Peruvian margin. It seems more likely that the coexisting
Permo-Carboniferous continental arc was removed at the time of extensive subduction erosion associated with the
break-up of Pangea.
REFERENCES
Azcuy, C.L., Di Pasquo, M. & Valdivia Ampuero, H. 2002: Late Carboniferous miospores from the Tarma Formation, Pongo
de Mainique, Peru: review of palaeobotany and palynology, 118, 1-28.
Bahlburg, H., Vervoort, J.D., Du Frane, S.A., Bock, B., Augustsson, C. & Reimann, C.R. 2009: Timing of crust formation and
recycling in accretionary orogens: Insights learned from the western margin of South America: Earth-Science
Reviews, 97, 215-241.
Doubinger, J. & Marocco, R. 1981: Contenu Palynologique du Groupe Copacabana (Permien Inférieur et Moyen) sur la
bordure Sud de la Cordillère de Vilcabamba, région de Cuzco (Pérou): International Journal of Earth Sciences, 70,
1086-1099.
Haq, B.U. & Schutter, S.R. 2008: A chronology of Paleozoic sea-level changes: Science, 322, 64-68.
Miskovic, A., Spikings, R., Chew, D.M., Kosler, J., Ulianov, A. & Schaltegger, U. 2009: Tectonomagmatic evolution of
Western Amazonia: Geochemical characterization and zircon U-Pb geochronologic constraints from the Peruvian
Eastern Cordilleran granitoids: Geological Society of America Bulletin, 121, 1298-1324.
Swiss Geoscience Meeting 2011
Platform Geosciences, Swiss Academy of Science, SCNAT