The origin and evolution of the Menderes Massif, W-Turkey: A rubidium/strontium and oxygen isotope study

Abstract

The Menderes Massif, located in the Alpine orogen of Western Turkey, has three litho-structural units, a basal »gneiss core«, a »schist mantle« covering the gneiss core, and an overlying »marble mantle«. This study reports isotopic data from samples of metasediments and migmatites from the southern (Type A) and of tonalitic to granitic metaigneous rocks from the northern Menderes Massif (Type B). Type A rocks define a Rb/Sr whole rock isochron age of 502±10 Ma and an initial87Sr/86Sr ratio of 0.71174±73. This is interpreted as the age of migmatization. Type B rocks give a Rb/Sr whole rock isochron age of 471±9 Ma with an initial87Sr/86Sr ratio of 0.70684±25, which we interpret as the intrusion age. K-White micas from rock type A give an average Rb/Sr mineral age of 56±1 Ma, which can be interpreted as the formation age of the Alpine mineral paragenesis. K-white micas from rock type B give Rb/Sr mixed ages of 120–230 Ma between Alpine and Pan-African orogenies. Biotites from both rock types yield an average Rb/Sr age of 37±1 Ma, which is interpreted as an Alpine cooling age. The oxygen isotope ratios of type A rocks range from 10.4 to 14.6‰, and of type B from 8.9 to 11.8‰. The oxygen and strontium isotopic compositions together clearly distinguish rocks of types A and B. Compositions of type A rocks suggest a crustal origin: type B rocks have compositions transitional between those of crustal and mantle-derived materials.