Zircon saturation temperatures (T Zr) calculated from bulkrock compositions provide minimum estimates of temperature if the magma was undersaturated, but maxima if it was saturated. For plutons with abundant inherited zircon, T Zr provides a useful estimate of initial magma temperature at the source, an important parameter that is otherwise inaccessible. Among 54 investigated plutons, there is a clear distinction between T Zr for inheritancerich (mean 766 ؇C) and inheritance-poor (mean 837 ؇C) granitoids. The latter were probably undersaturated in zircon at the source, and hence the calculated T Zr is likely to be an underestimate of their initial temperature. These data suggest fundamentally different mechanisms of magma generation, transport, and emplacement. ''Hot'' felsic magmas with minimal inheritance probably require advective heat input into the crust, are crystal poor, and readily erupt, whereas ''cold,'' inheritance-rich magmas require fluid influx, are richer in crystals, and are unlikely to erupt.

Zircons in transport in the modern Amazon River range from coarse silt to medium sand. Older grains are smaller on average: Mesozoic and Cenozoic grains have average equivalent spherical diameter (ESD) 122 ± 42 µm (lower fi ne sand), whereas grains >2000 Ma have average ESD 67 ± 14 µm (upper coarse silt). As a full Wentworth size class separates the two values, zircons in these age populations are hydraulically distinct.Host sand size is correlated with average size of co-transported zircons, implying hydrodynamic fractionation. Zircon size is positively correlated with percent medium sand, and inversely correlated with percent very fi ne sand (p <0.0001 in both cases). In samples with >50% medium sand, average zircon size is 100 µm, compared with 80 µm in samples with >50% very fi ne sand. We infer from these data that zircon deposition is not size-blind, and that zircons track with hydraulically comparable sand grains. As different aged grains tend to have different characteristic sizes, this indicates the possibility of hydrodynamic fractionation of age populations.Five samples representing different hydrodynamic microenvironments of a single dune present signifi cantly different detrital zircon age spectra, apparently the result of hydraulic processes. Peak mismatch (age peaks failing to overlap at 2σ level) is the most common disparity; but age populations present in some samples are missing from other samples. The lack of correspondence among the samples appears to exceed that attributable to random sampling. We conclude that hydrodynamic fractionation of zircons and zircon-age populations does occur. Zircon size should therefore be taken into consideration in detrital zircon provenance analysis.

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We use new U-Pb detrital zircon (DZ) geochronology from the Pleistocene Amazon submarine fan (n = 1352 grains), integrated with onshore DZ age data, to propose a sedimentary model for sea level-modulated and hydroclimate-modulated sediment transfer in Earth's largest source-to-sink system. DZ ages from the modern Amazon River sediment display a progressive downstream dilution by older cratonic zircons, leading to the expectation of a submarine fan with high proportions of craton-derived sediment. Our new DZ age data from the submarine fan and mixture modeling suggest that higher proportions of sediment were supplied from the distant central Andes to the Amazon fan during the last two glacioeustatic lowstands, and thus the observed DZ age spectra of the modern lower Amazon River indicate a relative increase in craton-derived sediment during the Holocene. We interpret that during interglacials, when sea level was high and the submarine fan inactive, the lower Amazon River did not efficiently transfer sand-sized sediment to the margin and thus became enriched in craton-derived sediment. During sea-level lowstands, increased gradients and incision in the lower Amazon River due to base-level lowering resulted in enhanced connectivity and transfer of Andes-sourced zircons to the deep sea. These results are also consistent with interpreted patterns of Andean-Amazon hydroclimate anti-phasing (enhanced precipitation in the central Andes and increased aridity in the northern Amazon Basin) during the Last Glacial Maximum. Our results suggest that sand-sized sediment in the Amazon submarine fan records multi-millennial patterns of sea level and South American hydroclimate.

The Amazonian Craton is an old geological feature of Archaean/Proterozoic age that has determined the character of fl uvial systems in Amazonia throughout most of its past. This situation radically changed during the Cenozoic, when uplift of the Andes reshaped the relief and drainage patterns of northern South America. Here we review the sedimentary characteristics of Amazonian rivers and compare these with four fl uvial depositional settings from the Meso-Cenozoic sedimentary record. These sedimentary units are the Alter do Chão Formation (Brazil, Late Cretaceous-Paleogene), the Petaca Formation (Bolivia, Late Oligocene to Middle Miocene), the Mariñame and Apaporis Sand Units (Colombia, Miocene), and the Iquitos White Sand Unit (Peru, Late Miocene-Pliocene). This review illustrates that the river systems born on the craton share features such as sediment texture and composition, depositional environments and transport directions. Evidence for the diminished role of cratonic fl uvial systems and the onset of Neogene Andean uplift can be identifi ed in the sedimentary record by changes in sediment provenance and transport directions. Although the Andean uplift and related processes discontinued the major Amazonian-born fl uvial systems it also created new topographic features such as the Iquitos and Fitzcarrald Arches. These newly formed reliefs triggered a new generation of rivers, some of which are presently known as biodiversity hotspots.