EXOTIC CRUSTAL FRAGMENTS ON THE PACIFIC MARGIN OF GONDWANA

IGCP Project 436 aims to compare evidence for the magmatic and tectonic history of Gondwana growth and break-up along its proto-Pacific margin (southern South America, West Antarctica and Australasia). Terrane accretion is well established for New Zealand (Permian–Cretaceous), but has only recently been proposed for the Antarctic Peninsula. We are using U-Pb microprobe (SHRIMP) age patterns of detrital/inherited zircons in metasediments and granites of the Antarctic Peninsula to assess the contribution of Gondwana sources to the different terranes. (1) The Eastern Domain is parautochthonous to Gondwana (?equivalent to the Western Province of New Zealand, the Ross Province of Marie Byrd Land, parts of Argentina Patagonia and the Eastern Metamorphic Series of southern Chile). Its zircons have complex age patterns dominated by peaks in the range 500–700 Ma, suggesting derivation from Pan-African orogenic belts within Gondwana. (2) The Central Domain is a suspect microcontinental arc terrane, like the Median Tectonic Zone of New Zealand, the Amundsen Province of Marie Byrd Land, and the Coastal Cordillera of northern Chile. Triassic-Early Jurassic S-types granites here dominantly contain inherited zircons of 470 and 530 Ma, with minor components between 260 and 560 Ma, representing an active marginal arc without the full range of Gondwana sources. (3) The Western Domain is characterised by turbiditic metasedimentary rocks, with strong similarities to the Eastern Province of New Zealand, the Western Metamorphic Series of south-central Chile, and the Chonos accretionary complex of northwestern Patagonia. A Western Domain sandstone has a zircon age distribution similar to that of the Central Domain, but with younger peaks, including a major one at 265 Ma. Predominantly Permian provenance is common in the accretionary complexes of Chile and New Zealand. The absence of complex Gondwana-derived zircons in the Central and Western domains could reflect variations in drainage patterns and source exposure, but is also consistent with separation between an allochthonous micro-continent and parautochthonous Gondwana.