THE RECORD OF EAST GONDWANA MARGIN CYCLES IN WEST ANTARCTICA: EVIDENCE FROM MARIE BYRD LAND

The Tasman Orogen of East Gondwana is a product of cycles of convergence, HTLP metamorphism, magmatism and rapid extensional collapse throughout Paleozoic time. Most-studied in Australia, the alternation of extension and contraction stages caused deposition and transformation of voluminous turbidites (e.g. Lachlan Group) into continental crust, aiding the growth and stabilization of the East Gondwana margin. New U-Pb SHRIMP zircon data from Antarctica reveal the eastward continuation of the orogen, where along-strike variations in the character of the lithosphere and in accretionary margin processes can be assessed.

New U-Pb SHRIMP zircon geochronology data from turbidite-migmatite-granite rocks of Marie Byrd Land, West Antarctica, provide a temporal connection to the central Lachlan belt of Australia and Western Province of New Zealand. Primary igneous zircons from granodiorite and monzogranite yield SHRIMP U-Pb ages of 369 ± 2.5 Ma (n=10) and 366±4 Ma (n=12), respectively. The calcalkaline rocks developed migmatic foliation and were cut by discordant leucogranites that occupy structural sites. Four leucogranites yield a narrow range of ages from 365 to 359 Ma. A K-feldspar porphyritic granite--determined on the basis of isotopes and geochemistry to be a product of melting of Paleozoic turbidite sediments-- yields a U Pb age of 353±3 Ma (n=8) for igneous zircons. This fairly short span of 15 to 20 million years of magmatism and crustal melting overlapped in time with New Zealand Karamea suite emplacement from 371 to 358 Ma. So we may explore the derivation of the plutonic phases of Marie Byrd Land from crustal or mantle sources, acquisition of SHRIMP δ18O and LA-MC-ICP-MS εHf data upon previously dated zircons is underway.

There is a diminished record of Tasman Orogen accretionary cycles in Australia after Late Paleozoic time, but geological records from Antarctica and New Zealand make it clear that the cyclic processes that brought about growth of the East Gondwana margin stepped outboard and continued into Mesozoic time. Core complexes, gneiss domes, and granites of 118 to 96 Ma define a high temperature plutonic-metamorphic belt that borders the culminating extensional province that is still preserved along the margin: the Cretaceous West Antarctic rift system and Tasman Sea.