DI13A-2633
Evolution of the mantle source in an evolving arc–backarc system (Torres del Paine, Patagonia): Evidence from Hf isotopes in zircon

Monday, 14 December 2015
Poster Hall (Moscone South)
Tanya A Ewing1, Othmar Muntener1, Julien Leuthold1,2, Lukas P Baumgartner1, Benita Putlitz3, François-Xavier d'Abzac4 and Massimo Chiaradia5, (1)University of Lausanne, Lausanne, Switzerland, (2)ETH Zürich, Institute of Geochemistry and Petrology, Zürich, Switzerland, (3)University of Lausanne, Lausanne, Switzerland, (4)University of Geneva, Department of Earth Sciences, Geneva, Switzerland, (5)University of Geneva, Geneva, Switzerland
Abstract:
The Miocene Torres del Paine intrusive complex (TPIC) in Patagonia is a transitional alkaline backarc intrusion1 emplaced on short timescales of 162 ± 11 ka2. It is subdivided into two units with distinct ages of ~12.6 Ma and ~12.45 Ma1. Smaller intrusive bodies in the area record a change in chemistry from calc-alkaline at ~16 Ma, to transitional alkaline at ~12.5 Ma.

Zircons from ~16 Ma intrusives and the 12.6 Ma part of the TPIC have remarkably consistent, slightly enriched Hf isotope compositions with εHf(i) of –1 to +2. An abrupt shift towards more juvenile Hf isotope compositions is observed in the ~12.45 Ma part of the TPIC, with εHf(i) of +3 to +6. Bulk rock Nd and Sr isotopes for the TPIC show the same shift towards more juvenile compositions at this time1. The long-term consistency of εHf(i) from 16 to 12.6 Ma is surprising, given that in the same period the bulk rock chemistry changes from calc-alkaline to transitional alkaline. Conversely, the major shift in εHf(i) is not correlated with any change in bulk rock chemistry, which remains transitional alkaline from 12.6 to 12.45 Ma.

The decoupling of major element chemical evolution and Hf isotope signatures suggests that the subsequent rapid influx of juvenile material recorded by our Hf isotope data must have occurred by renewed mantle melting. Subduction of the Chile ridge at ~12.5 Ma in this area caused arc magmatism to move westwards and back-arc extension to initiate. We propose that the first TPIC magmas (12.6 Ma) came from a mantle wedge with a residual subduction signature. Subsequent melting of more juvenile mantle, less contaminated by a subduction component, generated the 12.45 Ma TPIC magmas. These results demonstrate that magmatic complexes such as the TPIC may tap distinct mantle sources even on very short timescales, fingerprinting arc–backarc transition processes.

1Leuthold et al., 2013, JPET, 54: 273–303

2Leuthold et al., 2012, EPSL, 325: 85–92

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