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 εHf(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.
Highlights
The isotopic compositions (87 Sr/86 Sr ≥ 0.7050; εNd ≤ −2; εHf(t) ≤ +2) of some recently eruptedAndean volcanic rocks indicate that they have incorporated continental crust [1,2,3,4,5]
The εHf(t) for all twelve samples ranges from an average of +9.1 for the four samples of ≥8.9 Ma Teniente Mafic Complex basalts and gabbros down to +1.6 for the 2.3 Ma olivine-bearing basaltic andesite lava flow (Table 4), which is the last manifestation of magmatic activity in the region of El Teniente prior to the eastward migration of the volcanic arc
We suggest instead that the near constancy of the Hf isotopic composition of the 7.1 to 4.8 Ma felsic plutons in the El Teniente deposit is a result of mixing, storage and homogenization of mantle-derived magmas in a large vertically zoned upper crustal magma chamber (Figure 10) which developed as a result of compressive deformation after 7 Ma (Figure 8)
Summary
Andean volcanic rocks indicate that they have incorporated continental crust [1,2,3,4,5]. Two different processes for the incorporation of crust into Andean magmas have been proposed (Figure 1). 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]. PEELED, involves a mantle source region contaminated by crustal components subducted below the mantle. Minerals 2019, 9, 550 x FOR PEER REVIEW involves a mantle source region contaminated by crustal components subducted below the mantle wedge released into into the the wedge wedge by by either either melting melting or or dehydration dehydration and and volatile volatile transport transport [3,4].
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