Marcia Muñoz

The Andean margin of Central Chile (∼32°40′–34°30′S) records abundant Cenozoic arc magmatic activity with variable compositional characteristics. This is examined through the analysis of an extensive database of new and published whole‐rock geochemistry and Sr‐Nd isotopic compositions, in addition to zircon Hf and O isotopic compositions. The whole‐rock data record an increasing assimilation of continental crust material that developed through the compressional regime set in the margin in the Early Miocene that led to the construction of the modern Andes. The same is recorded by the Hf and O isotopic compositions, but these also allow identification of components involved in the magma genesis: (a) the sub arc mantle derived primary magmas, (b) a late Paleozoic—Early Triassic crystalline component, as that currently outcropping immediately to the east of the study area, (c) a deep enriched component, which likely corresponds to the Cuyania terrane, a Grenville age basement recognized further east in this Andean region, and (d) a deep juvenile low‐δ¹⁸O component, interpreted here as basement with oceanic crust affinities. This latter component reveals a possible composite nature for the Chilenia terrane, the postulated Grenville age basement in the region. The Cenozoic Andean arc magmas correspond to the differentiated products of an extensive intra‐crustal re‐working, taking place along tens of kilometers below their emplacement level, and whose main imprint is on the magmatic isotopic composition. Our results document the role of the continental crust, particularly regarding its evolving architecture and constitution, over the composition of arc magmas.

In the original article, the words “mantle source” appear in two places by mistake: Page 1108 (right column, end of first paragraph) and Page 1110 (left column, end of second paragraph). These two occurrences should read as “magmatic source.” In the “Conclusions” section, number (4), the words “Hf isotopic composition is inferred to fingerprint the source and O the isotope composition of the melting process” should read as “Hf isotopic composition is inferred to fingerprint the source, while O the melting process.” These details have been corrected only in this corrigendum to preserve the published version of record.

The scarce Eocene geological record in the Andean region of Central Chile ( c. 33.6° S) is represented by the Estratos del Cordón de los Ratones unit. This is a c. 450 m thick, mainly volcanic, continental series with a bimodal character that hosts numerous subvolcanic intrusions. The igneous products show compositional features indicating a co-genetic character for the whole suite, relatively juvenile mantle signatures and arc-like affinities. This, along with age determinations of c. 42 Ma, indicate that the unit corresponds to the middle Eocene arc in the region. The geochemical data obtained complement the evolving arc magmatic record throughout Andean evolution. This allows us to track: (1) the earliest Late Cretaceous orogenic event; (2) neutral tectonic conditions from the latest Cretaceous to mid-Eocene times; (3) the Oligocene–early Miocene extensional episode; and (4) the onset of the prevailing compression. The record also indicates that the extensional event started no earlier than the mid-Eocene, with the deposition of the Estratos del Cordón de los Ratones, and no later than the earliest Oligocene. In addition, orogenic processes developed from early Miocene times are highlighted relative to those developed during the early Late Cretaceous, at least in the arc region. Supplementary material: Analytical methods (Item 1), the full dataset of U–Pb age determinations by SHRIMP (Item 2) and LA-ICP-MS (Item 3), and additional geochronological diagrams (Item 4) are available at https://doi.org/10.6084/m9.figshare.c.4710305

Intrusive rocks related to porphyry copper mineralization are part of the wide diversity of subduction-related, mantle-derived, igneous rocks generated in convergent margin settings. What differentiates them from barren igneous rocks results ultimately from the multi-component and multi-stage processes that condition magma composition in these settings. Unfortunately, the petrogenetic history is largely obscured by the pervasive alteration that affects rocks in these deposits. We address this issue through the study of zircon grains from El Teniente, one of the largest known porphyry Cu–Mo deposits in the world. El Teniente belongs to the Miocene–Pliocene Cu–Mo belt of the Central Chilean Andes, which formed in a short timespan during the Cenozoic constructive period of the orogen. Previously U–Pb dated zircon grains were selected for re-examination of their morphological characteristics and in situ analysis of chemical (rare earth element, Hf, Y and Ti contents) and isotopic (Hf, O) composition. They are from six intermediate to felsic syn- to late-mineralization, intrusive units covering a timespan of ∼1·6 Myr. The El Teniente zircons have compositional and morphological characteristics indicating crystallization from a series of cogenetic melts. However, a minor hydrothermal imprint is documented in the presence of crystals with mottled surfaces that correspond to thin high U–Th overgrowth rims (low-luminescent features in cathodoluminescence images). In terms of any other chemical and isotopic characteristic, these are indistinguishable from the main mineral populations. Zircons define morphological and chemical trends reflecting an evolution towards more differentiated magma compositions, lower crystallization temperatures and increased cooling rates with decreasing age of intrusion. Hf and O isotopic compositions are remarkably uniform at grain, sample and deposit scale. This, together with the general absence of older inherited zircon components, the lack of correlations between isotopic signature and whole-rock composition and high initial ε_Hf values (total average 7·4 ± 1·2; 2σ), rules out involvement of any significant crustal contamination in the genesis of the El Teniente magmas. The Hf isotopic composition indicates a relatively juvenile source, but with some crustal residence time. The δ¹⁸O_Zrc weighted mean of 4·76 ± 0·12‰ (2σ; 61 analyses) is at the lower limit of the normal mantle zircon range of 5·3 ± 0·6‰ (2σ), and might reflect crystallization from low-¹⁸O magmas. Hf isotopic compositions have a restricted range in initial ε_Hf values between +6 and +10, identical to preceding Cenozoic barren magmatic activity in Central Chile. These igneous rocks are the product of nearly 25 Myr of subduction-related magmatic activity, developed under contrasting tectonic regimes and margin configurations. This suggests a primary control of the isotopic signature by a stable long-lived MASH-type (melting, assimilation, storage and homogenization) reservoir in the deep lithosphere. In the context of the Cenozoic evolution of Central Chile we argue that dehydration melting in the enriched MASH reservoir occurred as a consequence of increasing crustal thickness, and was prompted by a high-temperature thermal regime resulting from long-lasting preceding magmatism. This process can also fractionate O to generate low-¹⁸O magmas. At the time of El Teniente formation, dehydration melting occurred coevally with arc migration, which probably influenced the fertility of the magmas by increasing the melt component derived from this process relative to the component derived from primary basalt differentiation. At a regional scale, such reactions are expected to occur as a consequence of progressive crustal thickening during the constructive period of the Andes, and can explain the simultaneous generation of porphyry deposits in the Miocene–Pliocene Cu–Mo belt of Central Chile.