THE GEOLOGY AND GEOCHRONOLOGY OF THE PAN-AFRICAN AND TRIASSIC METAGRANITOIDS IN THE MENDERES MASSIF, WESTERN ANATOLIA, TURKEY

The Menderes Massif is a metamorphic complex cropping out on a large region in the Alpine orogenic belt in Western Anatolia. The massif mainly, is made up of a Precambrian basement and the overlying Palaeozoic-Early Tertiary cover series. The basement comprises Late Proterozoic metaclastics composed of paragneiss and high grade micaschists, syn to post-tectonical Pan-African orthogneisses that have intruded into them, and metagabbros which have partly turned into eclogitic form. Cover series unconformably overlying the basement are divided into two units, in Palaeozoic and Mesozoic-Early Tertiary ages. The basement and cover series were influenced by an effective Alpine contractional deformation and a regional metamorphism in Tertiary time. Geological and geochronological data in the Menderes Massif, indicate the presence of three acidic magmatic activities of i) Pan-African, ii) Triassic and iii) Miocene in ages. The acidic magmatics forming the protolithes of Pan-African orthogneisses can be divided into three main types according to their textural and mineralogical compositions as; 1) Biotite orthogneiss, 2) Tourmaline leucocratic orhogneiss and 3) Amphibole orthogneiss. The relations of primary granites of these orthogneisses present a clear intrusive contact relationship among them and with the metaclastics of Late Proterozoic age. The contact relationships show that these orthogneiss types can be ordered as biotite orthogneisses, tourmaline leucocratic orthogneisses and amphibole orthogneisses ranging from old to young, with respect to relative aging. Although there has been some problems originating from the definition of samples dated in previous studies, it is noted that radiometric data mainly show a consistency with this relative relationship (biotite orthogneiss: 550-570 Ma; tourmaline leucocratic orthogneiss: 541-547 Ma and amphibole orthogneiss: 531 Ma). These radiometric data indicate that different orthogneiss types in the Massif are differentiated products of the same Late Proterozoic - Early Cambrian acidic magmatic activity. When the paleogeographical position of the Massif in Early Cambrian and the close temporal relation of the metamorphic stage of cover series and the acidic magmatism are assessed with the geochemical character of orthogneisses, this widespread magmatic activity with an average age of about 550 Ma which is related to the Pan-African orogenesis can be attributed to proccesses of closure of the Mozambique Ocean, collision of East and West Gondwanaland, crustal thickening and partial melting of the lower crust. Triassic leucocratic orthogneisses constitutes the second effective acidic magmatic activity in the Menderes Massif with dimensions of 6-7 km. These plutonic leucocratic orthogneisses in 6-7 km dimensions are exposed in Ödemiþ-Kiraz and Demirci-Gördes submassifs. They show well-preserved intrusive contact relationships with Late Proterozoic metaclastics of the Pan-African basement and with metasediments of Late Palaeozoic cover series. Geochemical data show that protolithes of leucocratic orthogneisses are in calc alkaline and S type in character. Based on single zircon Pb/Pb evaporation method, these were radiometrically dated as ranging from 227 to 246 Ma. These ages are interpreted as the age of emplacement of protolithes of orthogneisses in Middle Triassic. The existence of Early-Middle Triassic magmatic activity is widely known not only in the Massif but also at the tectonical zones of the Anatolides, at the Karaburun peninsula, in Cyclades and at the inner Hellenides. When the regional character of Triassic magmatic activities is considered, it is suggested that there is a close genetical relationship between these leucocratic orthogneisses and Triassic magmatics. It is also considered that it can be attributed to the opening of Neothethys Ocean.