Geochemistry and depositional environment of ferromanganoan metasediments on the Island of Kythnos, Cyclades, Greece
Chrysanthaki, A.I.; Baltatzis, Emm.
published: Jan 16, 2003
Samples of Fe-Mn metasediments (hematite quartzites, mica-bearing quartzites and garnet-bearing amphibole schists) from the lower sub-unit of Kythnos island, part of Attic-Cycladic complex of Alpine age, have been collected and analysed for 45 elements, including rare earth elements.The hematite quartzites and mica-bearing hematite quartzites occur as intercalations in the upper part of a metavolcanic sequence or in the lower part of an overlying metasedimentary sequence of the northern part of Kythnos island. They show trace elements concentration and rare earth element (REE) patterns similar to those of hydrothermal deposits. The garnet-bearing amphibole schists occur in the sedimentary sequence of the southeastern part of the island. They have lower Fe/Mn ratios and higher trace element and REE contents in comparison with the hematite-quartzites and mica-bearing hematite quartzites. According to these geochemical characteristics, the protolith of Fe-Mn metasediments may have been formed under oceanic hydrothermal activity. In this process, circulating heated seawater reacted with basic volcanic rocks to form metal bearing hydrothermal solutions enriched in Fe, Mn and trace elements. These hydrothermal solutions were discharged on the ocean floor, through shear and/or fracture zones in an environment of decreasing temperature. The decreasing negative Eu anomalies from the lower (hematite quartzites) to the upper part (garnet-bearing amphibole schists) of the sequence possibly indicate a change in the condition (T, fO2) of the deposition as well as an increase of the detrital-hydrogenous components in relation to the hydrothermal fluids.