Source and fractionation controls on subduction-related plutons and dike swarms in southern Patagonia (Torres del Paine area) and the low Nb/Ta of upper crustal igneous rocks

Abstract

The subduction system in southern Patagonia provides direct evidence for the variability of the position of an active continental arc with respect to the subducting plate through time, but the consequences on the arc magmatic record are less well studied. Here we present a geochemical and geochronological study on small plutons and dykes from the upper crust of the southern Patagonian Andes at ~ 51°S, which formed as a result of the subduction of the Nazca and Antarctic plates beneath the South American continent. In situ U–Pb geochronology on zircons and bulk rock geochemical data of plutonic and dyke rocks are used to constrain the magmatic evolution of the retro-arc over the last 30 Ma. We demonstrate that these combined U–Pb and geochemical data for magmatic rocks track the temporal and spatial migration of the active arc, and associated retro-arc magmatism. Our dataset indicates that the rear-arc area is characterized by small volumes of alkaline basaltic magmas at 29–30 Ma that are characterized by low La/Nb and Th/Nb ratios with negligible arc signatures. Subsequent progressive eastward migration of the active arc culminated with the emplacement of calc-alkaline plutons and dikes ~ 17–16 Ma with elevated La/Nb and Th/Nb ratios and typical subduction signatures constraining the easternmost position of the southern Patagonian batholith at that time. Geochemical data on the post-16 Ma igneous rocks including the Torres del Paine laccolith indicate an evolution to transitional K-rich calc-alkaline magmatism at 12.5 ± 0.2 Ma. We show that trace element ratios such as Nb/Ta and Dy/Yb systematically decrease with increasing SiO2, for both the 17–16 Ma calc-alkaline and the 12–13 Ma K-rich transitional magmatism. In contrast, Th/Nb and La/Nb monitor the changes in the source composition of these magmas. We suggest that the transition from the common calc-alkaline to K-rich transitional magmatism involves a change in the source component, while the trace element ratios, such as Nb/Ta and Dy/Yb, of derivative higher silica content liquids are controlled by similar fractionating mineral assemblages. Analysis of a global compilation of Nb/Ta ratios of arc magmatic rocks and simple geochemical models indicate that amphibole and variable amounts of biotite exert a major control on the low Dy/Yb and Nb/Ta of derivative granitic liquids. Lastly, we suggest that the low Nb/Ta ratio of silica-rich magmas is a natural consequence of biotite fractionation and that alternative models such as amphibolite melting in subduction zones and diffusive fractionation are not required to explain the Nb/Ta ratio of the upper continental crust.Electronic supplementary materialThe online version of this article (10.1007/s00410-018-1467-0) contains supplementary material, which is available to authorized users.

The zircon Hf isotope archive of rapidly changing mantle sources in the south Patagonian retro-arc

“…∼12 Ma dikes and monzonite from the area surrounding the TPIC Sample 11-TPM-94 is from a small exposure of a monzonitic intrusion in Valle Bader that bears some similarities to parts of the TPIC. It was dated at 12.4 ± 0.2 Ma (Müntener et al, 2018), within error of both the mafic sill complex and the feeder zone of the TPIC. Its whole rock chemistry is similar to that of the TPIC (Müntener et al, 2018).…”

“…It was dated at 12.4 ± 0.2 Ma (Müntener et al, 2018), within error of both the mafic sill complex and the feeder zone of the TPIC. Its whole rock chemistry is similar to that of the TPIC (Müntener et al, 2018). The monzonite is largely hidden under glacial moraine, obscuring any field relations with other units including the TPIC.…”

“…This sample gave a zircon U-Pb age of 12.8 ± 0.2 Ma (Müntener et al, 2018). A number of such bimodal dikes, which have microgranitic centres and basaltic andesite borders, have been documented (Müntener et al, 2018). At least ten such dikes have been observed crosscutting granites of the TPIC in various localities throughout the area, implying that at least some bimodal dikes postdate these granites, and providing indirect evidence that the analysed sample probably postdates Granite III.…”

“…11-TPM-32A is a sample from the felsic part of a bimodal dike cutting metasediments of the Cerro Toro formation on the western flank of Monte Almirante Nieto. This sample gave a zircon U-Pb age of 12.8 ± 0.2 Ma (Müntener et al, 2018). A number of such bimodal dikes, which have microgranitic centres and basaltic andesite borders, have been documented (Müntener et al, 2018).…”

“…2, main text). Both have transitional alkaline whole rock geochemistry similar to that of the TPIC (Müntener et al, 2018). 11-TPM-32A is a sample from the felsic part of a bimodal dike cutting metasediments of the Cerro Toro formation on the western flank of Monte Almirante Nieto.…”