minerals Article Implications of Hf Isotopes for the Evolution of the Mantle Source of Magmas Associated with the Giant El Teniente Cu-Mo Megabreccia Deposit, Central Chile Charles R. Stern 1,* , Kwan-Nang Pang 2, Hao-Yang Lee 2, M. Alexandra Skewes 1 and Alejandra Arévalo 3 1 Department of Geological Sciences, University of Colorado, Boulder, CO 80309-0399, USA; [email protected] 2 Institute of Earth Sciences, Academia Sinica, Taipei City 115, Taiwan; [email protected] (K.-N.P.); [email protected] (H.-Y.L.) 3 Escuela de Ingeniería, Universidad de O’Higgins, VI Región 2910000, Chile; [email protected] * Correspondence: [email protected]; Tel.: +1-303-492-7170 Received: 27 July 2019; Accepted: 10 September 2019; Published: 12 September 2019 Abstract: We have determined the Hf isotopic compositions of 12 samples associated with the giant El Teniente Cu-Mo megabreccia deposit, central Chile. The samples range in age from 8.9 to 2.3 Ma and ≥ provide information about the temporal evolution of their magmatic sources from the Late Miocene to Pliocene. Together with previously published data, the new analysis indicates a temporal decrease of 10 "Hf(t) units, from +11.6 down to +1.6, in the 12.7 m.y. from 15 to 2.3 Ma. These variations imply increasing incorporation of continental crust through time in the magmas that formed these rocks. The fact that the samples include mantle-derived olivine basalts and olivine lamprophyres suggests that these continental components were incorporated into their mantle source, and not by intra-crustal contamination (MASH). We attribute the increase, between the Middle Miocene and Pliocene, of crustal components in the subarc mantle source below El Teniente to be due to increased rates of subduction erosion and transport of crust into the mantle. The deposit formed above a large, long-lived, vertically zoned magma chamber that developed due to compressive deformation and persisted between ~7 to 4.6 Ma. Progressively more hydrous mantle-derived mafic magmas feed this chamber from below, providing heat, H2O, S and metals, but no unique “fertile” Cu-rich magma was involved in the formation of the deposit. As the volume of these mantle-derived magmas decreased from the Late Miocene into the Pliocene, the chamber crystallized and solidified, producing felsic plutons and large metal-rich magmatic-hydrothermal breccias that emplaced Cu and S into the older ( 8.9 Ma) mafic host rocks of this megabreccia deposit. ≥ Keywords: El Teniente Cu-Mo deposit; Andean magmatism; subduction erosion; mantle source region contamination; Hafnium isotopes 1. Introduction The isotopic compositions (87Sr/86Sr 0.7050; " 2; " +2) of some recently erupted ≥ Nd ≤ − Hf(t) ≤ Andean volcanic rocks indicate that they have incorporated continental crust [1–5]. Two different processes for the incorporation of crust into Andean magmas have been proposed (Figure1). One, MASH (Mixing, Assimilation, Storage and Homogenization), involves intra-crustal assimilation as mantle-derived magmas rise from their source through the crust to the surface [2]. Another, PEELED, involves a mantle source region contaminated by crustal components subducted below the mantle Minerals 2019, 9, 550; doi:10.3390/min9090550 www.mdpi.com/journal/minerals Minerals 2019, 9, 550x FOR PEER REVIEW 2 of 23 25 involves a mantle source region contaminated by crustal components subducted below the mantle wedge and then released into the wedge by either me meltinglting or or dehydration dehydration and and volatile volatile transport transport [3,4]. [3,4]. These two didifferentfferent models have very didifferentfferent implicationsimplications for geochemical cycling associated with subduction zones, and for the growth and evolution ofof bothboth thethe continentalcontinental crustcrust andand mantlemantle [[5].5]. Figure 1. 1. TwoTwo alternative alternative explanations, explanations, as originally as originally illustra illustratedted by Hickey-Vargas by Hickey-Vargas in 1991 in [1], 1991 for the [1], incorporationfor the incorporation of continental of continental crust crustinto intoAndean Andean magmas. magmas. MASHED MASHED on onthe the left left involves involves Mixing, Mixing, Assimilation, Storage and Homogenization in a MASH zone located at either the lower crust/mantle crust/mantle boundary or within the crust [[2].2]. PEELED, PEELED, on the the right, right, involves involves recy recyclingcling of subduction eroded crust into the mantle wedge sourcesource [[3–5].3–5]. J–K and P indicateindicate the position of the Jurassic-CretaceousJurassic-Cretaceous and Pliocene plutonic belts,belts, respectively,respectively, in in the the Andes Andes of of northern northern Chile. Chile. The The recently recently active active volcanic volcanic arc arc is ~250is ~250 km km east east of of the the Jurassic Jurassic arc arc as as a a result result in in part part of of subductionsubduction erosionerosion [[6].6]. (Reprinted from Nature, volume 350, Hickey-Vargas, R. Peeled or MASHed?, page 350, Copyright 1991, with permission from Springer Nature.). The amount of crustal components incorporated into recently erupted Andean volcanic rocks varies along strike from south-to-north,south-to-north, as do various geologic and tectonictectonic features such as crustal thickness and age, subduction angle, trench depth and sediment fill, fill, and the localized presence of buoyant features such as oceanic seamount chains and spreading ridges being subducted below the South American continental margin (Figure 22;;[ [4]).4]). For For instance, instance, in in the the Central Central Volcanic Volcanic Zone (CVZ) of northern Chile, the crust is much thicker ( 70 km) than below the central part of the Southern of northern Chile, the crust is much thicker (≥70 km) than below the central part of the Southern Volcanic Zone ( 35 km; CSVZ; Figure2) and the isotopic compositions of the recently erupted volcanic Volcanic Zone ≤(≤35 km; CSVZ; Figure 2) and the isotopic compositions of the recently erupted volcanicrocks suggests rocks moresuggests crustal more components crustal components have been incorporatedhave been incorporated in CVZ than CSVZin CVZ magmas than CSVZ [3–5]. Thismagmas could [3–5]. reflect This greater could amountsreflect greater of crustal amounts assimilation of crustal by assimilation mantle-derived by mantle-derived magmas as they magmas rose asthrough they rose the through thicker CVZthe thicker crust, CVZ but incrust, this but same in th region,is same the region, trench the is trench devoid is ofdevoid sediment of sediment due to duethe aridto the conditions arid conditions in northern in northern Chile, and Chile, compared and compared to the CSVZ, to the much CSVZ, more much significant more significant amounts 3 amountsof subduction of subduction erosion, erosion, at rates estimatedat rates estimated to be 50–70 to be km50–70/km km/m.y.3/km/m.y. since since ~150 ~150 Ma (Figure Ma (Figure2;[ 5 ]),2; [5]),have have truncated truncated the continentalthe continental margin margin [6] and [6] and caused caused the arcthe toarc migrate to migrate eastward eastward ~250 ~250 km km since since the Jurassicthe Jurassic (Figure (Figure1). The 1). subductionThe subduction of tectonically of tectonic erodedally eroded crust has crust been has invoked been invoked not only not to explainonly to explainthe crustal the components crustal components in CVZ magmasin CVZ magmas [3–5,7,8], [3–5,7,8], but also thebut upliftalso the and uplift crustal and thickening crustal thickening observed observedin this part in of this the part Andes of the [9]. Andes [9]. In the same way, atat thethe northernnorthern endend of thethe Andean SVZ (NSVZ; Figures 22 and and3 ),3), where where the the MASH model was first presented [2], the crust thickens to 60 km compared to 35 km below the MASH model was first presented [2], the crust thickens to ≥≥60 km compared to ≤≤35 km below the CSVZ, and the isotopic compositions of recently erupted volcanic rocks indicate that they contain greater amount of crustal componentscomponents [[3–5,10,11].3–5,10,11]. However, However, the subductionsubduction of the Juan FernFernándezández Ridge hashas causedcaused thethe subductionsubduction angle angle to to flatten, flatten, increasing increasing the the rate rate of subductionof subduction erosion erosion west west of the of 3 theNSVZ NSVZ to between to between 115and230 115 and230 km / kmkm/m.y.3/km/m.y. over theover last the 10 last m.y., 10 during m.y., whichduring time which the time active the volcanic active volcanicarc between arc 33–34between◦ S migrated33–34° S ~50migrated km to ~50 the eastkm to (Figure the east3). Stern(Figure [ 3 3).–5] Stern therefore [3–5] proposed therefore that proposed crustal thatcomponents crustal components were preferentially were preferentially incorporated incorporated in NSVZ compared in NSVZ to CSVZ compared magmas to CSVZ by increased magmas rates by increasedof subduction rates erosion of subduction and mantle erosion source regionand mantle contamination source region by subducted contamination components, by subducted and not by components,intra-crustal MASHand not processes. by intra-crustal MASH processes. Temporal variations in the isotopic compositions of Andean igneous rocks also are not unambiguously related to either changes in crustal thickness or rates of subduction erosion. Below the NSVZ, in the region where the El Teniente deposit formed in the Late Miocene (Figures 2 and 3), Minerals 2019, 9, x FOR PEER REVIEW 3 of 23 the crust has thickened and been uplifted since the Middle Miocene [12–20], and Sr, Nd and Pb isotopic data indicate increased proportions of continental crust in the igneous rocks emplaced through time (Figures 4 and 5; [10,21–24]). However, as described above, the southward migration of the locus of subduction of the Juan Fernández Ridge has also caused, during this same time period, Mineralsflattening2019 of, 9 ,the 550 angle of subduction, >50 km of eastward migration of the volcanic arc, and increased3 of 25 rates of subduction erosion (Figure 3; [3–5,10,25–27]).