Detailed Geochemistry and K-Ar Geochronology of the Metamorphic Sole Rocks and Their Mafic Dykes from the Mersin Ophiolite, Southern Turkey

The metamorphic sole rocks at the base of mantle peridotites from the Mersin ophiolite consist of amphibolites and metasedimentary lithologies. Mineral parageneses in the metamorphic sole rocks exhibit amphibolite and greenschist facies assemblages. Geothermobarometric studies based on mineral assemblages and chemical compositions of minerals indicate that average metamorphic temperature during the metamorphism was 522 ± 15 °C and the pressure was less than 5 kb. Amphibolites from the metamorphic sole rocks exhibit geochemical characteristics of a supra-subduction zone (SSZ) type ophiolite, based on their major, trace and rare earth element (REE) compositions. The Th/Nb ratios of the amphibolites are higher than the average mid-ocean ridge basalt (MORB) and ocean island basalt (OIB) values. This may suggest that they were probably derived from an enriched mantle source modified by the addition of subduction component. Island arc tholeiite (IAT), OIB and MORB-like geochemistry of the amphibolites suggest that protoliths of these rocks were formed in a SSZ environment similar to the South Sandwich arc-basin system from South Atlantic ocean and the Mariana Trough from the Western Pacific. Isolated dolerite dykes intrude both the metamorphic sole rocks and the ophiolitic units at different structural levels. Dolerite dykes cutting the metamorphic sole rocks exhibit IAT-like geochemistry. They are enriched in large-ion-lithophile elements (LILE), depleted in high-field-strength elements (HFSE) and have relatively flat REE patterns, which also confirm their subduction-related origin. Double subduction is inferred here to explain the generation of the metamorphic sole rocks and dykes in the Neotethyan ocean, since the metamorphic sole rocks exhibit SSZ characteristics and were intruded by unmetamorphosed IAT-like dolerite dykes.