HORNBLENDITES WITHIN OPHIOLITES OF CRETE, GREECE: EVIDENCE FOR AMPHIBOLE-RICH CUMULATES DERIVED FROM AN IRON-RICH THOLEIITIC MELT

Abstract

Massive hornblendites within Jurassic ophiolites near the Kerames village (Central Crete, Greece) represent cumulates derived from an iron-rich tholeiitic magma intruding lherzolitic peridotites. After intrusion, the rocks were affected by crystal-plastic deformation, which broadly overprinted the magmatic textures within a high-temperature shear zone. The dominant rock types are strongly foliated pargasite schists rich in accessory minerals, like ilmenite, titanite, apatite, occasionally bearing relics of primary plagioclase and clinopyroxene. Most of the pargasites are characterised by a marked zoning which indicates reequilibration at lower temperature. The rocks are characterised by a well defined correlation of FeOtot/(FeOtot+MgO) with TiO2, P2O5, Zr, Y, and light rare earth elements (positive) and with Cr and Ni (negative) indicating a typical magmatic differentiation trend. Major and trace element geochemistry reveal a single-stage magmatic evolution to extreme iron-rich compositions consistent with a model of fractional crystallization with amphibole as the dominant phase. Some highly differentiated rocks show chondrite-normalised REE patterns characterised by a strong enrichment of LREE, which is attributed to an accumulation of monazite and/or allanite. From experimental data we conclude that the Cretan hornblendites were generated under strongly reducing conditions from an iron-rich melt with a water content >4 wt% at temperatures around 1000°C. The inferred high water content is in accordance with an origin of the melt in a back-arc basin. In comparison with well-described examples of oxide gabbro occurrences from other ophiolites and from the modern oceanic crust, the evolution of a continuous sequence of ferrogabbroic composition with a thickness of some decameters is an outstanding feature of the investigated rocks from Crete. It seems probable that the hornblendite sequence represents a frozen cumulate volume derived from a heavy late-stage melt, that was not able to erupt. No chemical gradient (e.g., in the MgO/FeO ratio) with height was observed in the Cretan intrusive complex. Therefore, we assume that the intrusion is not the result of an in-situ solidification of one magma body, but the product of multiple intrusions of small magma batches with different grades of differentiation mixed prior to complete solidification by deformation and compaction processes.