Ricardo H. Omarini

Geodynamical evolution of Central Andes at 24°S as inferred by magma composition along the Calama-Olacapato-El Toro transversal volcanic belt

Abstract Miocene to Recent volcanism on the Puna plateau (Central Andes) developed in three geological settings: (a) volcanic arc in the Western Cordillera (Miocene–Recent); (b) trans-arc along the main NW–SE transverse fault systems (Miocene); and (c) back-arc, mainly monogenic volcanic centres (Pliocene–Quaternary). We have studied the evolution of the arc–trans-arc volcanism along one of the most extensive transverse structures of Central Andes, the Calama–Olacapato–El Toro, at 24°S. Compositional variations from arc to trans-arc volcanism provide insights into petrogenesis and magma source regions. Puntas Negras and Rincon volcanic centres are arc-type and have typical calc-alkaline geochemical and Sr–Nd–Pb isotopic characteristics. East of the arc, lavas of the Tul-Tul, Del Medio and Pocitos complexes (TUMEPO) are heavy rare earth element-depleted and could be derived from 20–30% of partial melting of a lower crustal garnet-bearing metabasite. These liquids could be variably mixed with arc magmas at the base of the crust (MASH). This suggests important contributions from lower crustal sources to TUMEPO centres. Products at the Quevar and Aguas Calientes volcanic complexes to the east of TUMEPO show a prominent upper crustal signature (high 86Sr/87Sr, low 143Nd/144Nd) and could represent mixtures of 20–30% TUMEPO-type liquids with up to 70–80% of upper crustal melts. We propose a geodynamic model to explain geochemical variations for the arc–trans-arc transverse volcanism from the Upper Miocene to Recent. In our model, arc volcanism is linked to dehydration of the subducting Nazca plate, which produces typical calc-alkaline compositions. During the Upper Miocene (10–5 Ma), lithospheric evolution in the Puna plateau was dominated by thickening of ductile lower crust and thinning of the lithosphere. Lower crustal melting was promoted by concomitant asthenospheric upwelling and water release from the amphibolite–eclogite transformation, yielding TUMEPO magmas with lower crustal signatures. At the same time, the eastern sector of the Puna plateau experienced westward underthrusting of the Brazilian shield and upper crustal brittle deformation. Partial melts from underthrust upper crustal wedges mixed with lower crustal magmas to produce lavas at Aguas Calientes and Quevar. During Pliocene to Quaternary, delamination of the lithosphere and lowermost crust promoted a widespread monogenetic ocean island basalt-type and shoshonitic volcanism.

The geochemical variations of the upper cenozoic volcanism along the Calama–Olacapato–El Toro transversal fault system in central Andes (~24°S): petrogenetic and geodynamic implications

In this paper, we present new geochemical and Sr–Nd isotopic data for several Upper Miocene volcanic centres aligned along one of the most extensive transcurrent lineament in the Central Andes, the Calama–Olacapato–El Toro (COT). The transversal volcanic belt along COT is constituted by large composite volcanoes and a caldera structure; they are, from NW to SE, Puntas Negras, Rincon, Tul Tul, Del Medio and Pocitos (TUMEPO), Quevar Aguas Calientes and Tastil. In order to compare chemical data from the different centres along the COT transect, differentiation effects were minimised by using data extrapolated at 60% SiO2 with least-square regression method. In the western sector of the COT, the volcanic products of Puntas Negras and Rincon show relatively high K2O and 87Sr/86Sr and low Rb/Cs, Ta/Th, La/Yb, 143Nd/144Nd. To the east, the TUMEPO products have high Sr and 143Nd/144Nd, La/Yb and Ba/Rb and low Y, 87Sr/86Sr. In the easternmost COT sector, Quevar, Aguas Calientes and Tastil volcanic complexes exhibit low La/Yb, high87Sr/Sr86 and low 143Nd/144Nd. On the basis of these data, we propose a petrogenetic and geodynamical model for Central Andes at 24°S. In correspondence of Miocene–Quaternary volcanic arc (Puntas Negras and Rincon), the magmas inherited a calcalkaline signature partly modified by upper crustal and/or sediment assimilation. In the central eastern sector, melting, assimilation, storage and homogenisation (MASH) processes occurred at the base of a thickened crust. In this COT sector, TUMEPO products show an evident lower crust signature and could be considered representative for MASH derived magmas. In the easternmost sector, Quevar, Aguas Calientes and Tastil products could represent magmas generated by partial melting of underthrusted Brasilian shield and mixed with magmas derived by MASH processes.

Miocene magmatism and tectonics of the easternmost sector of the Calama–Olacapato–El Toro fault system in Central Andes at ~24°S: Insights into the evolution of the Eastern Cordillera

The Miocene Las Burras–Almagro–El Toro magmatic complex lies ~300 km to the east of the Central Andes volcanic arc, in the easternmost sector of the transverse Calama–Olacapato–El Toro fault zone. The magmatic rocks of the Las Burras–Almagro–El Toro complex comprise a monzogabbro to monzogranite laccolith like intrusion and basaltic andesite to dacite volcanic rocks that include seven lithostratigraphic members. New Rb-Sr dates indicate that the intrusive rocks are ca. 14 Ma, and K-Ar dates suggest emplacement ages of ca. 12.8–6.4 Ma for the volcanic rocks. The emplacement of the intrusion was controlled by N-S–striking strike-slip faults in a context of oblique convergence; the volcanism, which occurred along WNW-ESE– and N-S–striking extensional faults, relates to the Calama–Olacapato–El Toro fault zone. Two magmatic phases were recognized. Intrusive and volcanic rocks of the older magmatic phase (ca. 14–13 Ma) are characterized by Ba/Nb (7–14), La/Ta (11–18), and isotopic ratios ( 87 Sr/ 86 Sr: 0.704339–0.705281, 143 Nd/ 144 Nd: 0.512713–0.512598), which are intraplate characteristics. The source of the older magmas was isotopically depleted lithospheric mantle rich in K, Rb, and Th. Energy constrained–assimilation and fractional crystallization (EC-AFC) modeling indicates that fractional crystallization and crustal assimilation moderately modified magma composition during its residence in the crust. The products of the younger magmatic phase (ca. 11–6 Ma) have higher Ba/Nb (24–42) and La/Ta (24–30) and 87 Sr/ 86 Sr (0.706738–0.708729) and lower 143 Nd/ 144 Nd (0.512433–0.512360). The results of EC-AFC modeling exclude a significant role for the upper crust in the generation of the most primitive magmas of this phase. Their compositions can be explained by (1) contamination of the primary magmas having originated in a depleted mantle with a mafic crust, or (2) the contribution of isotopically enriched mantle zones. Shallow differentiation and moderate contamination by continental crust can explain the composition of the intermediate and evolved products of the younger phase. The variation of the magma source characteristics at 11 Ma is discussed in the frame of the complex geo-dynamical setting in this region.

Pliocene intraplate-type volcanism in the Andean foreland at 26°10′S, 64°40′W (NW Argentina): Implications for magmatic and structural evolution of the Central Andes

The Antilla magmatic complex (26°10′S, 64°40′W, NW Argentina) attests to magma eruption at ca. 4.7 Ma in the Central Andes backarc region, 300 km E of the active arc. The Antilla lavas have an alkaline, predominantly mafic composition and record the most primitive isotopic ratios ( 87 Sr/ 86 Sr = 0.704360 and 143 Nd/ 144 Nd = 0.512764) of Central Andes Neogene backarc volcanism between 24°S and 27°S. They show trace-element patterns recalling backarc Pliocene-Quaternary intraplate mafic rocks, but they show lower silica and higher alkali contents, and are interpreted to derive from the depleted subcontinental mantle. A revision of the structural and volcanological characteristics of the Central Andes between 24°S and 27°S shows that this region was, during the Miocene-Pliocene, the site of lithospheric processes that account for partial melting in the mantle wedge, in the subcontinental mantle, and in the continental crust. The existing geophysical and petrological data agree with a model in which magma production was related to a process of lithospheric delamination. The Antilla rocks are the easternmost volcanic products with intraplate characteristics, located beside a large zone of partial melting of the continental crust, at the intersection of the NW-trending Archibarca and NE-trending Tucuman transversal lineaments. Their age of 4.7 Ma corresponds to the acme of mafic monogenetic and silicic ignimbrite volcanism in the backarc at the same latitude. This provides new constraints for the spatial and temporal reconstruction of deformation events in the crust and of the lithospheric delamination process and its bearing on magmatic activity.