ARAGONITIC DASYCLADALEAN ALGAE FROM THE UPPER PERMIAN OF SICHUAN, CHINA

Exceptionally well-preserved specimens of Mizzia yabei (Karpinsky) wer e discovered in bituminous packstone intercalations of the early Upper Permian Maokou Formation of Anxian County (Sichuan, China). Staining techniques and x-ray diffraction reveal that the organic encrustation around the central stem and the lateral branches consists of aragonite . Average aragonite contents range between 27% and 40%. The bulk of th e remaining 60-73% calcite represents cement. The total Sr-content is 3,400 ppm, indicating 8,000-10,000 ppm for the aragonite fraction. The organically precipitated aragonite was deposited as closely packed fi bres, 10-20 mu m long and 0.5-1 mu m wide. The fibres are arranged in clinogonal (water-jet) fascicles, which diverge towards the central (s par-filled) stem. This microstructure shows that the mineralization of the mucilage started at the periphery of the extracellular encrustati on and progressed toward its interior. Diagenetic alterations of the o riginal aragonite include various stages of breakdown of the fibres in to 0.5-1 mu m large granules (micritization), minor syntaxial growth o f the fibres and local transformation of acicular aragonite into neomo rphic calcite. The dasycladaleans from the Permian of Sichuan appear t o be by far the oldest representatives of this order with preserved ar agonitic mineralogy and microstructure. Comparably well-preserved exam ples are only known from Eocene and younger deposits.