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

Southern Andes: accretionary history of the basement blocks The composition and distribution of Andean magmas are strongly influenced by the age and extent of the different basement blocks beneath the modern Andes. In  particular, radical  revision of southwestern  Gonwana assembly models depicted in recent studies has to be taken into account  when  considering the variation with time of the early pre-Andean and Andean subduction-related granitic magmas (Fig. 1):  (i) Palaeomagnetic studies indicate that one of the most important assembly episodes occurred during the Pampean-Araguaia collisional orogeny (540-520 Ma),  between an Amazonia craton group and the West Africa, Congo-São Francisco, Paraná and Río de la Plata cratons (Trindade et al., 2006 and references therein); (ii) The Amazonia craton group included the Arequipa-Antofalla  and Western Sierras Pampeanas basement blocks (Rapela et al., 2007), for which a common metamorphic and magmatic history has been established (Casquet et al., 2006; 2008); (iii) Further evidence shows that the large Neoproterozoic turbiditic sequence of the Eastern Sierras Pampeanas (Pampean belt),  now bounded to the east by the Palaeoproterozoic Rio de la Plata craton, is a transcurrent terrane resulting from right-lateral movements along the SW Gondwana margin (Rapela et al., 2007). This dextral displacement was  associated with the oblique collision of the Western Sierras Pampeanas during the Pampean–Araguaia  orogeny, following closure of the intervening Clymene ocean (Fig. 2); (iv) South   Figure 1. Group of cratons and minor blocks amalgamated in the Pampean-Araguaia orogeny (540-515 Ma) (modified from Trindade et al., 2006 and  Rapela et al., 2007). TB = Transbrasiliano shear zone.     Figure 2. Precambrian and Early Palaeozoic terrains in the Southern Andes, disclosed by back-thrusting above Miocene “flat-slab” subduction (28º - 33ºS).   of 36ºS,  the last recognised  accretion to the already assembled Gondwana took place during mid-Carboniferous time, when Early Carboniferous I-type granites representing a subduction-related magmatic arc was followed by a  collision between continental areas typified by the Deseado and North Patagonian massifs (Pankhurst et al., 2006).   Subduction associated with pre-Andean episodes was related to Wilson cycles of ocean opening and closing,  where the final event is either a continent-continent collision or large-scale  back-arc closure. At 30º-34ºS,  three main episodes of pre-Andean  plate convergence are well established: (1) Pampean: 540-528 Ma subduction, followed by oblique continent-continent collision at 528-515 Ma. The supercontinent grew westwards by lateral accretion of the Western Sierras Pampeanas Grenvillian block (Rapela et al., 2007), including the Precordillera (Fig.2). (2) Famatinian: 484-463 Ma convergent episode associated with the opening and closing of a large back-arc basin  in Early to Mid Ordovician times (Pankhurst et al., 2000; Dahlquist et al., 2008) (Fig.2). (3) Gondwanan: 320-190 Ma. After the intrusion of Devonian and  Early Carboniferous (c. 380 and 340 Ma) intra-plate A-type granites in the Sierras Pampeanas, a new subduction regime started along the palaeo-Pacific margin in Late Carboniferous times (c. 320 Ma), which included younger pulses (Parada et al., 1999).  At 33ºS the Late Palaeozoic batholiths occur both in the coast range of Chile and in the Frontal Cordillera, suggesting that no major continental accretion took place after the collision of the Western Sierras Pampeanas and associated Grenvillian blocks. Isotopic and chemical variations of the subduction-related granites 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 I-type 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 eNdt 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 – eNdt isotopic diagram,  in contrast to the Palaeozoic granites, most of which lie outside the mantle field, with eNdt < -2 (Fig. 3).  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   Figure 3. Variation of  eNdt versus initial 87Sr/86Sr for the granitic rocks emplaced during the main convergence episodes in the Andean sector at 28º- 33ºS (a) and  Patagonia (b). Data sources are Pankhurst et al., 1999; 2000, Parada et al., 1999, Rapela et al., 2005, Hervé et al., 2007 and references therein.     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 (eNdt 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) (Fig. 3). 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.