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

Abstract

Rapid changes in geochemical and isotopic signatures of arc-related magmatic products can be used to trace magmatic processes in subduction zones across many scales, from the regional response of magmatism to large-scale geodynamic changes in the subduction system, to the emplacement of single intrusions. In this contribution, we use the south Patagonian subduction system as a natural laboratory to investigate magmatic processes in continental arcs. We use diverse intrusions and dikes from the retro-arc region at 49−51°S to investigate these processes both at the subduction zone scale, and within the exceptionally well-exposed Torres del Paine sheeted intrusion. We present Hf isotope data for zircon from 30 to 12 Ma magmatic units that were emplaced ∼50 km inboard of the main subduction-related batholith. These samples record an ∼18 m.y. period during which the region experienced profound geodynamic changes, resulting in transient migration of arc magmatism into the retro-arc, which then vanished to be replaced by more alkaline retro-arc magmatism. Integrating published whole rock geochemistry, we show that the Hf isotope signatures of these magmatic units directly record their mantle sources, with negligible assimilation of continental crust into magmas during transport through and storage in the crust. This allows us to trace the appearance and disappearance of the subduction component in the retro-arc mantle. Our data show that migration of calc-alkaline magmatism into the retro-arc produced magmas with a more enriched Hf isotope composition that was remarkably consistent over ∼200 km and >4 m.y. (eHf(i) of −1 to +2.5). These signatures record addition of subducted continental crust to the mantle wedge during a period of subduction erosion that was associated with arc migration, and show that only a few m.y. of fluxing by a subduction component is sufficient to leave a distinct Hf isotope imprint on a mantle wedge previously unmodified by subduction. The Torres del Paine laccolith was built up by discrete pulses of magmatism in <200 k.y. Our data show isotopic differences between magmatic batches, and an abrupt shift to more juvenile Hf isotope compositions during the buildup of the youngest part of this magmatic complex, recording the rapid input of new mantle-derived melts during its formation. This rapid rejuvenation occurred within 20 ± 10 k.y., demonstrating that different batches of magmatism within a single intrusive complex can tap geochemically distinct mantle reservoirs on very short timescales of <200 k.y.