Bahlburg,Heinrich1, Panca,Fernando 1
1Institut für Geologie und Paläontologie, Westfälische Wilhelms-Universität, Münster, Germany
The central Andean margin of South America evolved as an external accretionary orogen since the Late Neoproterozoic. During the Paleozoic and Triassic evolution, the evolution of the margin and its orogenic basins was linked to alternating extensional and compressional states. The most prominent arc system is the Late Cambrian-Ordovician Famatinian arc which extended along the entire margin. The Devonian is here a time of tectonic, magmatic and metamorphic quiescence allowing for the steady accumulation of shallow marine facies on a stable platform west of the Ordovician orogen. This setting appears like a passive margin but in the geologic context has to be considered enigmatic.
Connected to the Gondwanide Orogeny, a Late Carboniferous to Triassic arc system with episodic activity was present in northern Chile and absent from Bolivia. In southern Peru it is preserved as Late Carboniferous to Early Permian backarc magmatic rocks.
In the late Permian and Triassic, extensional alluvial basins hosting within-plate magmatic rocks developed here. The latter evolution is represented by the Mitu Group. Its depocenters are considered rift and not backarc basins, mainly because a contemporaneous magmatic arc is nowhere exposed and unknown.
Accretionary orogens are considered to be the sites of the production of juvenile continental crust. The Famatinian and Late Carboniferous to Early Permian magmatic arcs, however, are characterized by a marked scarcity of mafic intrusive and extrusive rocks. The detrital heavy mineral spectra reflect this with Nb/Cr ratios in detrital rutiles, for example, indicating predominant input from felsic rocks. Detrital zircon U-Pb age distributions denote the arcs as prolific zircon sources. The epsHf(t) isotope values of the zircons derived from the arcs, and the Famatinian arc in particular, are strongly negative. In both cases a pronounced recycling of older continental crust is indicated. Detrital zircons of Devonian age are very scarce.
The late Permian and Triassic Mitu Group basin system extends for 1500 km in NW-SE orientation along the length of Peru. It is characterized by highly variable alluvial facies with angular unconformities at base and top. The group appears to be arranged in several subbasins. Mafic alkaline and calc-alkaline lavas, and felsic ignimbrites are intercalated episodically. U-Pb zircon ages of the latter allow for a stratigraphic subdivision of the subbasin fills.
Detrital zircon age distributions of the Paleozoic central Andes show a typical South American, Amazonian provenance, with ages ranging from the Archean to the maximum ages of deposition. In the Mitu Group, maximum depositional ages indicate a variable onset of deposition between 260 and 216 Ma, locally continuing into the Jurassic. The epsHf(t) isotope values of late Permian and Triassic zircons form vertical arrays between ca. -10 and +6 and indicate a significantly more pronounced juvenile component in Mitu rocks than in the older arc rocks.
Heinrich Bahlburg studies clastic sedimentary basins and the crustal evolution of accretionary orogens. He combines field work and facies analysis with the geochemical and geochronological analysis of heavy minerals. This interest took him to orogens in Alaska, Europe, and for over 30 years to the central Andes.