The Hf and O isotope record of long-lasting accretionary orogens: The example of the Proterozoic and Paleozoic-Triassic central South America

Bahlburg, H; Kemp, AIS; Fanning, CM; Martin, L

Abstract

The southwestern margin of Amazonia hosted accretionary orogens for most of the past 2 Ga. A succession of accretionary mountain belts evolved from 2 Ga to 1 Ga, here informally grouped in the Terra Amazonica Orogen. It occupies large tracts of the Amazonian Shield, is poorly exposed and often inaccessible. Its evolution ended when Amazonia collided with Laurentia at 1 Ga in Rodinia. After the dispersal of Rodinia, an accretionary margin was re-established at 0.65 Ga, starting the proto-Andean accretionary Terra Australis Orogen. Large volumes of polycyclic detritus derived ultimately from the Terra Amazonica Orogen was trapped in the sedimentary basins of the Terra Australis Orogen. Using this detritus we assess the crustal evolution of both orogens with new zircon U-Pb age (n = 12752), Hf isotope (n=5502) and O isotope (igneous n = 855, detrital n = 957) databases for zircons including our own new analyses and literature data. For the Terra Amazonica Orogen three subcycles of zircon eHf(t) values define a cumulative trend from strongly unradiogenic to radiogenic values. In accretionary orogens, such trends indicate the progressive removal of lower crust and lithospheric mantle of the upper plate during subduction and their replacement by new radiogenic crust. Early Neoproterozoic rifting of Rodinia registered a drop in mean eHf(t) values by eleven units from +4 to -7. This is superseded by a two-step increase to values between 0 and +5 after the inception of the active margin of the Terra Australis Orogen, registering one large cycle with two subcycles from negative to positive values typical of accretionary orogens. Zircon d18O data show a flat mean trend at slightly elevated values of 6.3 ‰ over the first 800 Myr of the Terra Amazonica Orogen, increasing to ca. 7.3‰ towards collision with Laurentia. Encompassing the Terra Amazonica Orogen and Neoproterozoic rifting, mean d18O from detrital zircons is consistently lower than the igneous mean, indicating a wider distribution of mantle-like rocks than previously considered. d18O of the Terra Australis Orogen trends from supracrustal 8‰ to more mantle-like values around 6.5‰, paralleling the accretionary orogen trend from unradiogenic to radiogenic eHf(t) values. Global zircon d18O data suggest a gradual increase of d18O after 2.5 Ga connected to progressive hydration and intra-crustal reworking of the continental crust. Our data register the sudden appearance of elevated d18O values up to 10‰ at 2.5 Ga. Amazonia has been an independently drifting entity with an active margin at least from the late Archean to the collision with Laurentia. d18O values up to 10‰ at 2.5 Ga may have appeared because the upper crust of Amazonia had already experienced alteration and crustal recycling at an accretionary margin since long before 2.5 Ga. The Amazonian and proto-Andean detrital zircon age record is statistically similar to the global record. We argue that large scale continent-wide sedimentary reworking of detritus results in efficient mixing of the detrital zircons from accretionary and collisional orogens. We conclude that the global detrital zircon age record represents the averaged record of the successive supercontinent cycles.