Structural and metamorphic geology of the Windmill Islands, east Antarctica: Field evidence for repeated tectonothermal activity

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Abstract
Structural and metamorphic relationships in the Windmill Islands, Wilkes Land, east Antarctica show evidence for repeated tectonothermal activity. An early metamorphic peak (M1), characterized by sillimanite‐biotite ± cordierite assemblages in pelitic rocks was coeval with and outlasted by D1 deformation which formed a pervasive horizontal fabric. A major shortening phase (D2) refolded the M1/D1 fabric. The syn‐ and post‐D2 thermal peak (M2) occurred at largely static conditions and is characterized by extensive in situ partial melting of quartzo‐feldspathic units, a high‐grade metamorphic overprint of M1 peak assemblages, and granite and charnockite emplacement. The M2 overprint increases in intensity from north to south and, in pelitic rocks, is characterized by the prograde breakdown of M1 sillimanite‐biotite to M2 garnet‐cordierite, resulting in M2 peak assemblages consisting of garnet‐cordierite‐biotite, garnet‐cordierite‐orthopyroxene and cordierite‐orthopyroxene. Pinnitization of early M1 cordierite and competency relationships between M1 leuco‐ and melanosomes (e.g. boudinaging of M1 leucosomes) during the late stages of D1 suggest that a cooling phase separates M1/D1 and D2/M2 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 M1 and D1 is typical of low‐pressure high‐temperature terrains, whereas the relative timing of D2 and M2 is more typical of Barrovian‐type terrains. Tectonic models consistent with the grade and timing relationships of the two events are discussed.