Paleozoic crustal and Triassic rift evolution of the proto-Andes from detrital heavy minerals
Authors: Bahlburg Heinrich , Panca Fernando 
Published: February 2023

&lt;p&gt;The central proto-Andean margin of South America evolved as an external accretionary orogen since the Late Neoproterozoic. During the Paleozoic and Triassic, the evolution of the margin and its orogenic basins was linked to alternating states of extension and shortening. The most prominent arc system is the Late Cambrian-Ordovician Famatinian arc which extended along the entire margin. The Silurian and 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 was most likely connected to a lull phase of arc activity.&lt;/p&gt;
&lt;p&gt;From the Late Carboniferous to the Triassic, the Gondwanide Orogeny presents a highly variable record along the central proto-Andean margin. An arc system with episodic activity was present in northern Chile and absent from Bolivia. In Peru the setting has been considered enigmatic with interpretations varying between backarc basin and continental rift.&lt;/p&gt;
&lt;p&gt;We demonstrate through a study of the late Permian and Triassic Mitu Group which is exposed for 1500 km in NW-SE orientation along the length of Peru that the Mitu basin formed as a backarc basin. This interpretation is compelled by the existence of eastward dipping subduction zones, and respective movements of the oceanic plate system, south and north of the Mitu basin, in Chile and the northern Andes, respectively.&lt;/p&gt;
&lt;p&gt;The Mitu Group is characterized by highly variable alluvial and aeolian facies with angular unconformities at base and top of the group. It appears to be arranged in several subbasins. Mafic alkaline and calc-alkaline lavas, predominantly of within-plate OIB affinity, and felsic ignimbrites are intercalated episodically. U-Pb zircon ages of the latter allow for a chronostratigraphic subdivision of the subbasin fills.&lt;/p&gt;
&lt;p&gt;Detrital zircon age distributions of Paleozoic central Andean strata show a typical South American, i.e. 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.&lt;/p&gt;
&lt;p&gt;This conforms to the widely held view that accretionary orogens are the sites of the production of juvenile continental crust. The proto-Andean 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.&lt;/p&gt;