Rapid Rejuvenation of the Source of a Backarc Sheeted Magmatic Complex (Torres del Paine, Patagonia): Evidence From Hf isotopes in Zircon

Abstract:

The Miocene Torres del Paine intrusive complex (TPIC) in Patagonia is a spectacularly exposed example of a bimodal shallow crustal laccolith, made up of a sill complex and a subvertical feeder system. The TPIC was emplaced in a back-arc setting, but slightly older arc-related intrusive units in this area testify to an earlier shift from an arc to a backarc setting. The entire ~88 km³ main complex was emplaced over short time scales of 162 ± 11 ka between ~12.4 and 12.6 Ma, with mafic units from the feeder zone found to be older than mafic units from the sill complex^1,2. We aim to assess whether successive pulses of mafic magmatism can tap different geochemical reservoirs in sheeted magmatic complexes emplaced on such short timescales.

Hf isotope compositions of individual zircons from mafic units from both the feeder zone and the sill complex were determined by solution MC-ICPMS. Zircons from all units have Hf isotope compositions that indicate a slightly enriched mantle source. Zircons from the mafic sill complex units have higher (more juvenile) initial ε_Hf than zircons from feeder zone mafic units. The shift towards more depleted Hf isotope compositions in the sill complex units, which are younger, demonstrates the rapid input of new juvenile material into the source region between ~12.6 Ma and ~12.5 Ma. A similar shift is also seen in bulk rock Nd and Sr isotope data for related samples³.

The Hf isotope data demonstrate that significant variability in source geochemistry is possible for sheeted magmatic complexes built up on very short timescales. Analysis of zircons from a range of dikes and intrusive bodies external to the main Torres del Paine complex, with ages that span ~12–29 Ma, will provide a more complete picture in time and space of the geochemical evolution of this magmatic system as it switches between an arc and backarc setting.

¹Leuthold et al., 2012, EPSL, 325: 85–92

²Michel et al., 2008, Geology, 36: 459–462

³Leuthold et al., 2013, JPET, 54: 273–303