STRATIGRAPHY OF THE PAN – AFRICAN BASEMENT OF THE MENDERES MASSIF AND THE RELATIONSHIP WITH LATE NEOPROTEROZOIC/CAMBRIAN EVOLUTION OF THE GONDWANA

Abstract

The Menderes Massif, exposed in the Western Anatolia, is tectonically overlain to the north by the Afyon Zone and to the south by the Lycian Nappes. In the northwest, two high-pressure units, the Cycladic Complex and overlying the Lycian Nappes, as well as the nappes of Ýzmir - Ankara Zone tectonically overlay the Menderes Massif. The metamorphic rock succession of the Massif can be divided into two main units: 1-PanAfrican basement (core series) and 2-Palaeozoic - Early Tertiary metasedimentary rocks (cover series). The PanAfrican basement shows a stratigraphy consisting of a partly migmatized metaclastic sequence and polymetamorphic basic and acidic igneous rocks that intruded into these metaclastics. This metaclastic sequence, which is composed of paragneiss and conformably overlying schist units reaches a minimum thickness of eight kilometers and forms the oldest rocks of the Pan-African basement of the Menderes Massif. Field studies and geochronological data suggest that the protoliths of the paragneisses are predominantly clastic sediments of litharenitic composition. Frequently, the paragneisses alternate and interfinger in all directions with micaschists and biotite-albite schists, originating from mudstone and subarkosic sandstone, respectively. The paragneisses are conformably overlain by a schist unit. The originally gradual sedimentary contact is represented by a paragneiss and schist intercalation. The schists are dominated by micaschists with biotite-albite schist layers, derived from mudstone and subarkose, respectively. In the Menderes Massif, the paragneisses are generally extensively migmatized. Widespread anatectic granites occur as irregular-shaped bodies with migmatized margins. The detrital zircons of the paragneisses yielded ages scattered between 610 - 2558 Ma. Detrital zircons of the schist unit of the Pan-African basement were dated at 592 - 3239 Ma. The intrusion ages of the orthogneiss showing clear intrusive contact relationship with metaclastic sequence range from 570 to 520 Ma. The granulite facies metamorphism of the paragneiss unit was dated at 583±5.7 Ma. These geochronological evidence and the contact relationships clearly reveal that the deposition age of the protoliths of the metaclastic sequence of the Pan-African basement can be constrained to Late Neoproterozoic between ca 590 - 580 Ma. The metaclastic sequence is intruded by large granitoid bodies and gabbroic stocks. The intrusion ages of the granitic precursors now represented by orthogneisses with changed mineralogical compositions and primary textures range from 520 to 570 Ma with a major event at about 550 Ma. These granites, which can be divided into three main groups (biotite orthogneiss, amphibole orthogneiss and tourmaline leucocratic orthogneiss) are the products of a poly-phase Pan-African acidic magmatic activity that intruded the metaclastic sequence. They are syn- to post-metamorphic intrusions with respect to the Pan-African orogeny. The basic meta-igneous rocks occurring in the Pan-African basement have gabbroic to noritic composition. They display massive cores and foliated margins that consist mainly of garnet amphibolites with relics of eclogites revealing the polymetamorphic history of the Pan-African basement. Field evidence and radiometric age data indicate that the primary contact relationship between the Pan-African basement and the Palaeozoic cover series in the Menderes Massif was an unconformity (Supra-Pan-African unconformity) and the cover series were sourced from the Pan-African basement. The protoliths of the paragneiss and schist of the Pan-African basement were deposited on the passive continental margin of a basin occurring between East and West Gondwana during the Late Neoproterozoic time (Mozambique ocean). All the magmatic and metamorphic ages obtained from Pan-African basement coincide with the closure of this ocean and the final assemblage of the Gondwana supercontinent during Late NeoproterozoicCambrian time.