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

Structural and metamorphic relationships in the Windmill Islands, Wilk es Land, east Antarctica show evidence for repeated tectonothermal act ivity. An early metamorphic peak (M(1)), characterized by sillimanite- biotite +/- cordierite assemblages in pelitic rocks was coeval with an d outlasted by D-1 deformation which formed a pervasive horizontal fab ric. 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 condi tions and is characterized by extensive in situ partial melting of qua rtzo-feldspathic units, a high-grade metamorphic overprint of M(1) pea k assemblages, and granite and charnockite emplacement. The M(2) overp rint increases in intensity from north to south and, in pelitic rocks, is characterized by the prograde breakdown of M(1) sillimanite-biotit e to M(2) garnet-cordierite, resulting in M(2) peak assemblages consis ting 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. bou dinaging of M(1) leucosomes) during the late stages of D-1 suggest tha t 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 str uctural and metamorphic events during the two cycles indicates they ma y have formed in two fundamentally different geodynamic environments. For example, the relative timing of M(1) and D-1 is typical of low-pre ssure high-temperature terrains, whereas the relative timing of D-2 an d M(2) is more typical of Barrovian-type terrains. Tectonic models con sistent with the grade and timing relationships of the two events are discussed.