POLYMETAMORPHIC EVOLUTION OF THE PAN-AFRICAN BASEMENT AND PALAEOZOIC–EARLY TERTIARY COVER SERIES OF THE MENDERES MASSIF

Abstract

The Menderes Massif exposing in the Western Anatolia substantially presents a complex tectonostratigraphy as a result of Late Alpine compressional tectonism. The lithostratigraphical succession of this crystalline complex can be divided into two units: 1- The Pan-African basement (core series) and 2-Palaeozoic - Early Tertiary metasedimentary rocks (cover series). The Pan-African basement of the Menderes Massif is made up of a Late Neoproterozoic metaclastic sequence consisting of paragneisses and conformably overlying micaschists. This high-grade metaclastic sequence is extensively migmatized and intruded by the syn- to post-Pan-African gabbros and granitoids. The primary contact relationship between the core and cover series is a regional unconformity in character. The Palaeozoic (?Upper Devonian - Permian) cover units, which are cut by the intrusion of Triassic leucocratic metagranites are consisted of phyllites, quartzites and marbles. The Mesozoic cover units are characterized by Triassic to Upper Cretaceous platform-type thick marbles at lower levels of the sequence. Upper Campanian - Upper Maastrichtian pelagic carbonates and the overlying Middle Paleocene - Eocene flysch-type blocky unit constitute the uppermost units of cover series. Relic mineral assemblages observed in the PanAfrican basement reveal a complex polyphase metamorphic evolution of this basement under granulite, eclogite and amphibolite-facies conditions. The high temperature metamorphism developing under granulite facies is characterized by the presence of hypersthene type pyroxene. Pelitic granulites, orthopyroxene gneisses, orthopyroxene paragneisses and metagabbroic / metanoritic rocks form typical granulite-facies relics observed in the massif. Geothermobarometric estimations characterize an average temperature of 730 °C and pressure of 6 kbar for the granulite-facies metamorphism. By means of SHRIMP II method, clustering ages of 583±5.7 Ma were dated from the outer parts zircons in pelitic granulites which have no zoning but have overgrown under granulite facies. High grade metamorphism relics in the Pan-African basement are characterized by eclogite and eclogitic metagabbros. Fully recrystallized, fine grained massif eclogites, with non bearing relic texture belonging to protolith are composed of 'omphacite (jd40-52) + garnet + clinozoisite + amphibole + quartz + rutile'. However; relic texture and minerals are extensively observed in metagabbros derived from eclogitic gabbros. The pressuretemperature (P-T) conditions of the Pan-African high-pressure metamorphism were estimated as 644°C with a minimum pressure of about 15 kbar, which corresponds to a burial depth of about 50 km. 206Pb/ 238U zircon ages obtained from eclogitic metagabbros by TIMS yield 529.9±22 Ma, reveal the high-pressure metamorphism as Pan-African in age. The Barrowian type medium pressure metamorphism reaching up migmatization stage in which anatectic granites developed caused extensive retrogradations. Geothermobarometric estimates from garnet amphibolites, retrograded from eclogites indicate that this metamorphism developed under P/T conditions of 7 kbar in pressure and 628°C in temperature. The crystallization ages of these anatectic granites range from 551 to 540 Ma. They were generated by migmatization of paragneisses and reveal that this medium-pressure event is related to the last stage of polyphase Pan-African metamorphism. All metamorphic ages obtained from the Pan-African basement are compatible with the latest stages of assemblage of Gondwana super continent. It is considered that protoliths of paragneisses and schists of the Pan-African basement were deposited on a passive continental margin of a basin occurring between East and West Gondwana during the Late Proterozoic time (Mozambique Ocean). The Pan-African basement of the Menderes Massif was deeply buried and metamorphosed under granulite, eclogite and amphibolite-facies conditions as a result of the closure of this basin and collision of East and West Gondwana during Late Neoproterozoic time. Both core and cover series of the Menderes Massif were affected by an Alpine aged old regional metamorphism. In the Palaeozoic sequence of cover series, this metamorphism is characterized by a Barrowian type medium-pressure metamorphism. This metamorphism developed under greenschist to lower amphibolite-facies (6 kbar in pressure / 430-550 °C in temperature) and described by the occurrences of garnet, staurolite and kyanite (disthene) in phyllites. MesozoicEarly Tertiary cover series at the southern part of Çine submassif contain data associated with Alpine aged HP/LT metamorphism. Carpholite-kyanite assemblage within Triassic quartz metaconglomerates shows a metamorphism under a pressure of 10-12 kbar and temperature of 440 °C corresponding to a minimum depth of 30 km. So far, there has not been detected any data for an Alpine HP / LT metamorphism, neither in the Pan-African basement nor in the Palaeozoic sequence of the Menderes Massif. Based on the fossil content obtained from youngest unit of the cover series and from the oldest non metamorphic sedimentary cover on the massif, the Alpine metamorphism can biostratigraphically be constrained into Eocene and Oligocene time interval. Few isotopic data (37±1 Ma, Late Eocene Rb/Sr biotite age; 36±2 Ma, Middle Eocene Ar/Ar muscovite age; 43-37 Ma, Eocene Ar/Ar muscovite age) related to Alpine metamorphism are compatible with the related time interval. The Alpine metamorphisms of tectonical zones belonging to Anatolides is substantially associated with the closure of the northern branch of Neothethys Ocean and with the collision in Paleogene. In such a tectonic model, the segment of the Anatolide-Tauride platform corresponding to the Menderes Massif was subjected to intense internal imbrication during the subduction process of the northern branch of Neotethys and the following period in which the continental collision occurred. The tecto