STRUCTURAL AND METAMORPHIC GEOLOGY OF THE WINDMILL-ISLANDS, EAST ANTARCTICA - FIELD EVIDENCE FOR REPEATED TECTONOTHERMAL ACTIVITY

Structural and metamorphic relationships in the Windmill Islands, Wilkes Land, east Antarctica show evidence for repeated tectonothermal activity. An early metamorphic peak (M(1)), characterized by sillimanite-biotite +/- cordierite assemblages in pelitic rocks was coeval with and outlasted by D-1 deformation which formed a pervasive horizontal fabric. A major shortening phase (D-2) refolded the M(1)/D-1 fabric. The syn- and post-D-2 thermal peak (M(2)) occurred at largely static conditions and is characterized by extensive in situ partial melting of quartzo-feldspathic units, a high-grade metamorphic overprint of M(1) peak assemblages, and granite and charnockite emplacement. The M(2) overprint increases in intensity from north to south and, in pelitic rocks, is characterized by the prograde breakdown of M(1) sillimanite-biotite to M(2) garnet-cordierite, resulting in M(2) peak assemblages consisting of garnet-cordierite-biotite, garnet-cordierite-orthopyroxene and cordierite-orthopyroxene. Pinnitization of early M(1) cordierite and competency relationships between M(1) leuco- and melanosomes (e.g. boudinaging of M(1) leucosomes) during the late stages of D-1 suggest that a cooling phase separates M(1)/D-1 and D-2/M(2) into two independent tectonothermal events. The contrast in the relative timing of the structural and metamorphic events during the two cycles indicates they may have formed in two fundamentally different geodynamic environments. For example, the relative timing of M(1) and D-1 is typical of low-pressure high-temperature terrains, whereas the relative timing of D-2 and M(2) is more typical of Barrovian-type terrains. Tectonic models consistent with the grade and timing relationships of the two events are discussed.