Revisiting accretionary history and magma sources in the Southern Andes: time variation of “typical Andean granites”

RAPELA, C.W.; PANKHURST, R.J; DAHLQUIST, J.A.; BALDO, E.G; CASQUET, C.; GALINDO, C.

Niza, Francia

Simposio; 7th International Symposium on Andean Geodynamics; 2008

ORSTOM

The isotopic and chemical characteristics of the granites emplaced in the different pre-Andean episodes described above are compared with the typical Andean I-type granites emplaced near the continental margin after the break-up of Gondwana. The latter are mostly Cretaceous in age at 30º-34ºS, but a complete record from 185 Ma to Tertiary is exposed in the Patagonian batholith and subcordilleran belts (Rapela et al., 2005, Hervé et al., 2007). The Pampean and Famatinian rocks have chemical and isotopic characteristics that contrast with the younger Andean bodies. The older pre-Andean rocks show a wide silica range and, although metaluminous Itype varieties from gabbro to granodiorites are abundant, cordierite-bearing S-type granites are also conspicuous. S-type granites are rare in the Carboniferous and Andean granites, indicating that melting of sedimentary material was not common in these episodes. The _Ndt values decreases with time, suggesting derivation from progressively more primitive and depleted sources. Only the younger Gondwanan and the majority of the Andean granites plot in the “mantle array” of the (87Sr/86Sr)0 – _Ndt isotopic diagram, in contrast to the Palaeozoic granites, most of which lie outside the mantle field, with _Ndt < -2 . This is a remarkably consistent feature of the Pampean and Famatinian events, as they include abundant amphibole-bearing and noritic gabbros with less than 50% SiO2 that share the same crustal signature as the intermediate rocks. As there is no evidence for massive in situ contamination during emplacement in the upper crust, this signature must reflect the composition of the middle or lower crust (Pankhurst et al. 1998). Depleted mantle model ages (TDM) for most of the Cambrian and Ordovician rocks, both I- and S-types, are in the interval 1400–1700 Ma indicating involvement of Palaeo- to Mesoproterozoic sources. Altogether the chemical and isotopic evidence suggests that the Pampean and Famatinian episodes did not involve significant recycling of young underplated material. Rather, it indicates melting of an old crustal section, including the underlying subcontinental mantle, to produce the basic rocks with enriched isotopic signatures. Although isotopically less evolved than the Cambrian–Ordovician granites, the Carboniferous coastal batholiths of Chile also plot off the “mantle array”, but with younger (mostly Neoproterozoic) model ages. Recycling of the immature 1000–1200 Ma juvenile Grenvillian lithosphere in which they are emplaced seems to fit the source isotopic constraints. Only the Andean and younger Gondwanan granites show depleted signatures (Parada et al. 1999): this is not only a characteristic of central Chile but also in the Patagonian Andes (Pankhurst et al. 1999, Rapela et al. 2005, Hervé et al., 2007). Remarkably, the Andean granites also show a change in isotopic composition with time. For example the Sm–Nd relationships of granitoids from the Patagonian batholith at 44º–46ºS indicate source compositions that change from slightly LIL-enriched for the Jurassic and Early Cretaceous rocks, to significantly depleted for the Late Cretaceous to Early Miocene plutons (_Ndt values between +4 and +6), the latter in turn very similar to those of the Tertiary to Recent mafic strato-volcanoes of the Southern Volcanic Zones (Pankhurst et al. 1999). This cannot be explained by upper or lower crustal contamination and it has been suggested that melting occurs in progressively more LIL-depleted mantle sources underlying the Patagonian batholith (Rapela et al.,2005). The obvious conclusion is that under the label of “typical I-type Andean granite” there is a wide range of isotopic compositions that shows a general variation towards more depleted mantle sources with time. Isotopic equivalents of modern Andean granites are uncommon or absent in the extensive Early Paleozoic metaluminous suites.