Metamorphic evolution and reworking of the Sleaford Complex metapelites in the southern Eyre Peninsula, South Australia*

Abstract

The partial melting processes of Al‐rich and cordierite‐absent garnet–orthopyroxene‐bearing leucosome assemblages in the reworked Archaean Sleaford Complex in southern Eyre Peninsula, South Australia are examined. The Kimban Orogeny produced a pervasive development of garnet–K‐feldspar‐dominated leucosome assemblages. The textural relationships suggest that the peak assemblage garnet–orthopyroxene–K‐feldspar involved a melt phase and formed through the divariant partial melting reaction garnet + biotite = orthopyroxene + K‐feldspar + liquid in the KFMASH system, during a prograde KD 1 heating event. A cordierite‐bearing metapelite also preserves the peak KD 1 assemblage of garnet–cordierite–orthopyroxene–K‐feldspar, and the textural relationships suggest that peak assemblages formed through the crossing of the univariant partial melting reaction garnet + biotite = orthopyroxene + cordierite + K‐feldspar + liquid in the KFMASH system, during the KD 1 prograde heating stage. It is suggested that peak metamorphic P–T conditions reached 850–900°C and 0.9–1.0 GPa, and that the preservation of peak assemblages means that some melt was lost near or during the metamorphic peak. The pervasive interstitial K‐feldspar grains nucleated synchronously with decompression, and the second generation of small euhedral garnets are inferred to have formed through decompression‐generated partial melting during a KD 2 high‐strain shear event. The P–T conditions of 700–750°C and 0.5–0.6 GPa were attained during the KD 2 high‐grade shear event. The inferred clockwise P–T path from KD 1 to KD 2 is compatible with a collisional tectonic model during the Kimban Orogeny ( ca 1730–1700 Ma), whereas syn‐KD 2 decompression‐cooling was a result of rapid tectonic exhumation of the terrain.