Research Collection Journal Article The zircon Hf isotope archive of rapidly changing mantle sources in the south Patagonian retro-arc Author(s): Ewing, Tanya A.; Müntener, Othmar; Leuthold, Juerg; Ramírez de Arellano, Cristóbal; Baumgartner, Lukas P.; Schaltegger, Urs Publication Date: 2018 Permanent Link: https://doi.org/10.3929/ethz-b-000305252 Originally published in: GSA Bulletin , http://doi.org/10.1130/b31983.1 Rights / License: In Copyright - Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library ewing-B31983.1 1st pages / 1 of 22 Rapidly changing mantle sources in the south Patagonian retro-arc The zircon Hf isotope archive of rapidly changing mantle sources in the south Patagonian retro-arc Tanya A. Ewing1,†, Othmar Müntener1, Julien Leuthold2, Cristóbal Ramírez de Arellano3, Lukas P. Baumgartner1, and Urs Schaltegger4 1Institute of Earth Sciences, University of Lausanne, 1015 Lausanne, Switzerland 2Institute of Geochemistry and Petrology, ETH Zürich, 8092 Zurich, Switzerland 3Faculty of Engineering, Universidad Andres Bello, Santiago, Chile 4Department of Earth Sciences, University of Geneva, 1205 Geneva, Switzerland ABSTRACT enriched Hf isotope composition that was et al., 2004; Leuthold et al., 2012; Michel et al., remarkably consistent over ~200 km and 2008; Schaltegger et al., 2009; Schmitt et al., Rapid changes in geochemical and isotopic >4 m.y. (εHf(i) of –1 to +2.5). These signatures 2011; Schoene et al., 2012). A question that signatures of arc-related magmatic products record addition of subducted continental has received less attention is whether different can be used to trace magmatic processes in crust to the mantle wedge during a period of batches of magma within a sheeted complex tap subduction zones across many scales, from subduction erosion that was associated with the same reservoir, or are fed by geochemically the regional response of magmatism to large- arc migration, and show that only a few m.y. different sources. Hafnium (Hf) isotopes are a scale geodynamic changes in the subduction of fluxing by a subduction component is suf- powerful tool to address this question, provid- system, to the emplacement of single intru- ficient to leave a distinct Hf isotope imprint ing a robust isotopic fingerprint of the magmatic sions. In this contribution, we use the south on a mantle wedge previously unmodified by source that traces the relative contributions from Patagonian subduction system as a natural subduction. The Torres del Paine laccolith continental crust and mantle reservoirs. The laboratory to investigate magmatic processes was built up by discrete pulses of magma- mineral zircon is a particularly attractive target in continental arcs. We use diverse intru- tism in <200 k.y. Our data show isotopic dif- for Hf isotope analysis, as it contains weight sions and dikes from the retro-arc region at ferences between magmatic batches, and an percent of HfO2 (Hoskin and Schaltegger, 2003) 49–51°S to investigate these processes both abrupt shift to more juvenile Hf isotope com- and is extremely robust both physically and at the subduction zone scale, and within the positions during the buildup of the youngest chemically, preserving its Hf isotope signature exceptionally well-exposed Torres del Paine part of this magmatic complex, recording under all known crustal conditions (Hoskin and sheeted intrusion. We present Hf isotope data the rapid input of new mantle-derived melts Black, 2000; Kinny et al., 1991). Coupled Hf for zircon from 30 to 12 Ma magmatic units during its formation. This rapid rejuvenation isotope and U–Pb age data can be used to trace that were emplaced ~50 km inboard of the occurred within 20 ± 10 k.y., demonstrating changes in magma source composition over main subduction-related batholith. These that different batches of magmatism within time, whether on the scale of a single pluton samples record an ~18 m.y. period dur- a single intrusive complex can tap geochemi- (e.g., Broderick et al., 2015), a batholith (e.g., ing which the region experienced profound cally distinct mantle reservoirs on very short Kemp et al., 2007), or the continental crust (e.g., geodynamic changes, resulting in transient timescales of <200 k.y. Dhuime et al., 2012). migration of arc magmatism into the retro- The Torres del Paine intrusive complex arc, which then vanished to be replaced by 1. INTRODUCTION (TPIC) is a shallow crustal sheeted intrusive more alkaline retro-arc magmatism. Inte- complex that was emplaced in Patagonia on grating published whole rock geochemistry, Sheeted intrusive complexes offer an oppor- rapid timescales of ca. 160 k.y. (Leuthold et al., we show that the Hf isotope signatures of tunity to investigate the architecture of a pluton, 2012; Michel et al., 2008) (Figs. 1 and 2). Field these magmatic units directly record their and constrain the timing, frequency, and mag- and geochronological evidence testify to its mantle sources, with negligible assimilation nitude of successive episodes of magmatism. emplacement in multiple discrete batches of of continental crust into magmas during The traditional view of plutons as large entities magma (Leuthold et al., 2013; Leuthold et al., transport through and storage in the crust. emplaced en masse then slowly cooled has been 2012; Michael, 1991; Michel et al., 2008). It This allows us to trace the appearance and challenged in favor of more complex, polyphase is spectacularly exposed in three dimensions disappearance of the subduction component models. Single-crystal U–Pb geochronology due to kilometer-scale vertical relief and ex- in the retro-arc mantle. Our data show that of zircon has demonstrated that different units tensive glacier-denuded outcrops. The TPIC migration of calc-alkaline magmatism into within a single intrusive complex can have re- was emplaced in the retro-arc region of a long- the retro-arc produced magmas with a more solvably different ages, complementing field lived continental subduction zone in southern evidence for the emplacement of such com- Patagonia (South America). In this contribu- †Present address: Institute of Geological Sciences, plexes in discrete magma batches (e.g., Barboni tion, we present zircon Hf isotope data for University of Bern, 3012 Bern, Switzerland; tanya et al., 2015; Broderick et al., 2015; Claiborne different units from the 12–13 Ma Torres del .ewing@ geo .unibe .ch et al., 2010; Coleman et al., 2004; Glazner Paine intrusive complex. We also present Hf GSA Bulletin; May/June 2018; v. 130; no. 5/6; p. 1–22; https://doi.org/10.1130/B31983.1; 11 figures; 2 tables; Data Repository item 2018281.; published online 22 November 2017 . Geological Society of America Bulletin, v. XXX, no. X/X 1 © 2018 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license (remove if not applicable). ewing-B31983.1 1st pages / 2 of 22 Ewing et al. 78°W 74°W 70°W 66°W Nazca plate 46°S Plio–Miocene retro-arc intrusions 46°S Chile Ridge Antarctic plate ~3 Ma P 48°S Taito FZ a 48°S Tres Montes FZ ~6 Ma t a N g CHPC Esmeralda FZ o ~12 Ma n Fig. 3 50°S i 50°S a Madre de Dios FZ n Fig. 2 TPIC b 13–14 Ma a 52°S t 52°S h o l South Patagonian batholith i t American Plateau basalts Desolacion FZ h plate Oligocene/early Miocene Late Miocene/Pliocene 15–14 Ma Pliocene/Recent Quaternary volcanoes Scotia plate Miocene & Pliocene intrusions Front of fold & thrust belt 02100 00 km Figure 1. Map of southern Patagonia, modified after Ramírez de Arellano et al. (2012), showing the Patagonian batholith, Plio–Miocene intrusions inboard of the batholith, and the Quaternary volcanoes that represent the modern active arc in this area. Plateau basalts after Kay et al. (2004); Eocene plateaus are not shown. The Torres del Paine intrusive complex (TPIC) and Chaltén plutonic complex (CHPC) are shown as triangles with a bold outline. Black rectangles indicate the approximate locations (not to scale) of detailed maps of the two complexes presented in Figures 2 and 3, respectively. The present day positions of the Chile spreading ridge (solid lines), as well as the major fracture zones (FZ) (dashed lines), are shown along with separated segments of the Chile ridge subducted at different times, are shown along with the time of subduction for each segment (after Cande and Leslie, 1986; Guivel et al., 2006; Kay et al., 2004). isotope data for a number of small intrusions on short timescales (<200 k.y.) during the and show that more crustal Hf isotope compo- and dikes that are found within ~15 km of the progressive emplacement of a single sheeted sitions record addition of a subducted conti- TPIC, with ages spanning 12–30 Ma; and for a intrusive complex within the retro-arc. We nental crustal component to the mantle wedge. 16–17 Ma intrusive complex (Chaltén plutonic demonstrate that the longer-term evolution of Our data reveal an abrupt shift to more juvenile complex) that crops out ~200 km to the north. the Hf isotope composition of magmatism on a Hf isotope compositions in the youngest part Our data resolve the evolving Hf isotope signa- regional scale is unaffected by crustal assimila- of the TPIC, recording input of more primi- ture of retro-arc magmatism over a ca. 18 m.y. tion, with remarkably consistent εHf(i) on large tive melts derived directly from the mantle. period of its history, during which the region temporal and spatial scales of ca. 4 m.y. and We thus demonstrate that different batches of witnessed profound geodynamic changes as- 200 km. Our data thus allow us to trace chang- magma within a single intrusive complex can sociated with changing convergence rates and ing crustal inputs into the retro-arc mantle of a tap distinct mantle geochemical reservoirs, subduction of an active spreading ridge; and single continental arc over a ca.