The mafic Peine Group and related Triassic intrusions share most of the chemical characterics of suprasubduction zone magmas, such as pronounced negative Nb and Ta anomalies and enriched LREE (Figs. 3.35 and 3.39) and low ratios of Nb/U and Ce/Pb (Figs. 3.34 and 3.40a). These characteristics also persist in the continental crust (e.g., Arculus, 1981; Brenan et al., 1995), but to a lesser extent than in
primary arc magmatic rocks (e.g., Nb/Nb* = 0.54 [lower crust]; 0.37 [upper crust]; McLennan et al., 2006; compared to Nb/Nb* mode = 0.28, and 70% < 0.37 for magmatic arcs [n=5447; GEOROC online database).
Figure 3.39. Chemistry of the upper Peine Group and related intrusions shown as a spider diagrams normalised to primitive mantle (Sun and McDonough, 1989). A) Basaltic rocks of the Cascasca dykes and lower Yabricoya formation. B) Intermediate to felsic Yabricoya formation and lower Triassic porphyritic intrusive rocks. Reference fields are shown for the range of lower to middle Peine Group andesites (from Fig. 3.37) and Triassic basalts (from A). Data marked with ansterisk (*) are from Masterman (2003), from which the Collahuasi Porphyry is represented as the average of six analyses. The observed subduction-like character could record active subduction-related magmatism, recycling of fossilised metasomatised mantle belonging to an older subduction zone, or crustal melt extraction (under certain conditions). Permian sub-crustal lithosphere may have preserved domains of arc-related volatile- and LREE-enriched, rutile-bearing metasomatised mantle that had formed during early
Figure 3.40. Trace element ratios indicating a supra-subduction zone origin for the Yabricoya forma- tion and related rocks. A) Nb/U v. Ce/Pb, with reference fields as shown in Figure 3.37. B) V/Sc v. SiO2 wt%, with a reference field for global arc magmas, adapted from Aeolus-Lee et al. (2005).
A. B.
Paleozoic arc magmatism (Famatina Arc, section 2.2.1). The Choiyoi Arc at least locally overlaps the inferred distribution of these older arc rocks (e.g., Damm et al., 1990), and hence this scenario is broadly permitted by the relative positions of the Lower and Upper Paleozoic arcs. However, where melting ‘fossilised’ metasomatised mantle has been demonstrated, such as in eastern Anatolia, the distribution of
inherited subduction-like characteristics is highly variable in space (e.g., Keskin, 2003). There are not substantial differences between the mafic Peine Group rocks from the four locations in northern Chile for which data are available, so this scenario is unlikely, though it cannot be explicitly ruled out.
Reworking of the crust is an attractive explanation because of the peraluminous composition of the mafic Peine Group. Similar magma compositions could be uniquely be derived by partial melting of the mafic lower crust if the residual mineralogy contained rutile and/or garnet to cause the relative Nb-Ta depletion and REE fractionation. However, that too is unlikely since the western Andean margin crust did not achieve thicknesses sufficient to stabilise those minerals during the Paleozoic (Omarini et al., 1999; Xiong et al., 2005; Alvarado et al., 2007). Several other mineralogical criteria could be placed on a hypothetical gneissic or granulite residue to drive the other arc-like characteristics, but the coincidence of these
becomes increasingly improbable. Instead, the observed Ta-Nb-(U), Pb/Ce, and LREE anomalies are best explained if a component of the parental Peine Group magma formed in the metasomatised mantle wedge above an active subduction zone. In contrast to the mafic rocks, the felsic lower Peine Group are interpreted to
be largely the products of intracrustal melting, consistent with the previous
interpretations drawn for the Peine Group and/or north Chilean Permian plutons (Kay et al., 1989; Brown, 1991; Parada et al., 1991; Lucassen et al., 1999a; Depine et al., 2005; Munizaga et al., 2008). Partial melting of the crust may have been mediated by the ascent of the basaltic arc magma, and mixing between these and intracrustal felsic melts adequately accounts for the aluminosity of all lower Peine Group rocks, the unusual alkali Harker trends (Fig. 3.33a), and the Sr-Nd radiogenic isotope composition of the felsic Peine Group at Collahuasi (Fig. 3.36).
Breitkreuz and Zeil (1994) inferred that the wide distribution of contemporaneous sedimentary strata across northern Chile marked incipient arc-parallel rifting along strike of a magmatic rift basin in southern Peru (e.g., Kontak et al., 1985). Indeed some aspects of the Peine Group chemistry support this hypothesis. Half the lower and middle Peine group mafic rocks have V > 290 ppm (n= 6) and Ti/V in the range 17-42 (average 30), and several samples have Zr/Y ~4. These values lie at the edge of, or just outside of the ranges for typical subduction-related basalts and within the ranges of rift-related basalts (e.g., Pearce, 1976, 1982). Similar HFSE compositions are recorded among Cretaceous to Miocene basaltic rocks of the South Central Andes (Nyström et al., 2003; Hollings et al., 2005) and Quaternary andesites of the Taupo Volcanic Zone, New Zealand (Price et al., 2005). These are appropriate comparators to the Peine Group in terms of the thickness of the continental column through which magmas must have ascended to erupt at the surface. In both cases magmatism occurred above an active subduction zone where continental extension encroached into the magmatic arc front (Yañez et al., 2002; Nyström et al., 2003; Price et al., 2005). The consequent upwelling of athenosphere would have driven anomalous heat flow and provided an environment that favoured intracrustal melting during arc magma ascent and storage.
New geochemical analyses of the middle and lower Peine Group therefore support the idea that the Choiyoi Arc was built above an active subduction zone that
underwent contemporaneous extension; the ‘arc-graben’ of Breitkreuz and Zeil (1994). The lower and middle Peine Group mafic magmas are inferred to be two component hybrids of tholeiitic arc basaltic magma, and a crustal alkali rhyolite melt. The voluminous felsic lower Peine Group reflects the products of largely undiluted variations of this intracrustal melt. Following the San Rafael tectonic event, the relative significance of intracrustal melting decreased, as is evidenced by the more primitive and less peraluminous (partly metaluminous) Yabricoya formation and related intrusions. The primary arc basalt composition changed to high-K calc-alkaline at the same time, plausibly by way of mineralogical changes related to higher pressures (greater depths) of volatile expuslion from the subducting slab (Schmidt, 1996), or of melt generation in the mantle wedge (Wyllie and Sekine, 1982). The implied deeper subduction processes during the terminal stages of Peine Group magmatism may therefore record the magmatic response to the onset of slab rollback along the Choiyoi Arc. Mafic dykes also cut the lower Triassic Este Granodiorite at Chuquicamata (Tomlinson et al., 2001a), and therefore emplacement of relatively primitive magmas at this time appears to have been at a regional phenomenon. However, Choiyoi Arc rocks locally became more anatectic in character at approximately the same time (Kay et al., 1989; Brown, 1991; Depine et al., 2005). The arc may therefore have been substantially segmented, and rollback may have been diachronous as a result.