CLINOPYROXENE COMPOSITIONS OF THE ISOTROPIC GABBROS FROM THE SARIKARAMAN OPHIOLITE: NEW EVIDENCE ON SUPRA-SUBDUCTION ZONE TYPE MAGMA GENESIS IN CENTRAL ANATOLIA

Abstract

The middle Turonian-early to Campanian Sarikaraman Ophiolite (SO) is one of the least dismembered ophiolitic units of the allochthonous Central Anatolian Ophiolites, and retains a recognizable pseudostratigraphy. The lowermost part of the SO is characterized by gabbros which are cut by random or en-echelon pegmatite veins. The clinopyroxenes within the isotropic gabbro sequence are small, colorless relicts with cloudy appearance. In most of the samples, they resulted completely replaced by successive generation of green to brown amphiboles. The EMP analyses of fresh clinopyroxenes from 5 selected samples display diopsitic to augitic compositions within the range of En45Fs5Wo49 to En50Fs9Wo41. They are Ca-rich (Wo41-49) and Na poor (Na2O<0.55) and characterized by high-Mg (Mg# = 81.4-91.19) and low-Ti (<0.68 wt %) contents. The composition of the SO clinopyroxene is characterized by low to very low values of Ti, and is thus similar to other well-known supra-subduction zone-type ophiolites, such as Troodos, Pindos, and Oman. When the concentration of clinopyroxene of the SO is compared to modern basalts formed in different settings (MORB, IAT and BON, Beccaluva et al., 1989), the SO samples match the island arc basalts, with some shift towards the boninite field. The most significant chemical characteristic of the clinopyroxene of the SO is the low-Ti content, which resulted 2-3 times lower of the values typically found in MORB basalts. The latter feature is also consistent with the generally low Ti content found in the basaltic lavas of the SO, and might indicate that the primary melt was probably formed by partial melting of a severely depleted upper mantle. In conclusion, the occurrence of a high-Mg and low-Ti magma constrains the formation of the Sarikaraman ophiolitic gabbro to a supra-subduction zone setting. The clinopyroxenes of the Sarikaraman Ophiolite show similar chemical compositions of other supra-subduction zone-type, Eastern Mediterranean ophiolites (e.g. Troodos, Varinous, Pindos and Oman ophiolites) that show island-arc affinity.