Hf isotopes in detrital zircons from accretionary complex rocks of Alexander Island, Antarctic Peninsula
Abstract
The LeMay Group of Alexander Island, Antarctic Peninsula (AP), has been interpreted as an accretionary complex associated with the AP magmatic arc. More recent models suggest that these rocks form part of an exotic terrane, which may have been deposited and accreted elsewhere on the Gondwana margin, and moved into its present position as a result of Mesozoic strike-slip movements. To test these models, we have combined SHRIMP dating with laser-ablation Hf-isotope analysis of LeMay Group detrital zircons, in order to characterise the geochemical evolution of possible source regions. Detrital zircons from the older, Triassic, part of the LeMay Group are dominated by Permian and Lower Palaeozoic grains, with a sparse Proterozoic component; their age distribution is remarkably similar to that of Torlesse rocks of New Zealand. Zircons in the younger, Cretaceous, part of the LeMay Group are dominated by Middle Jurassic grains, with a few Early Cretaceous and Palaeozoic grains, and a significant Proterozoic and Archaean component that appears be derived by recycling of Palaeozoic sedimentary rocks from within Gondwana. Zircons in AP granites show consistent variations in ɛHf with age, which parallel well-established Nd-isotope evolution curves for the peninsula. Detrital zircons in Triassic LeMay Group sediments plot on identical trends to the granites through the Palaeozoic; Cambrian grains have ɛHf ≈ 0, rising to +7 in Carboniferous times, and falling to c. 0 in Permian times. Detrital zircons from the Cretaceous, outboard part of the LeMay Group show decreasing ɛHf through the Mesozoic, in marked contrast to the well-defined trend of increasing ɛHf shown by AP granites at that time. We conclude that the Triassic LeMay Group formed as an accretionary complex associated with the AP magmatic arc. However, the Cretaceous LeMay Group sediments were not derived from the AP; we suggest that they were deposited elsewhere, and transported into their present position as a result of Mesozoic strike-slip movements.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....9257M