Diagenesis and very low-grade metamorphism of the Antalya unit: mineralogical evidence of Triassic rifting, Alanya-Gazipaşa, central Taurus belt, Turkey

Abstract

The Paleozoic-Lower Mesozoic formations of the Antalya Unit in southern Turkey comprise diagenetic sediments, shale, mudstone, shale, siltstone, sandstone and scarce spiculite, together with very low-grade metamorphic lithologies, slate, metamudstone, metasiltstone, metasandstone and metalimestone. These diagenetic and metamorphic lithologies are dominant in the northern (Demirtaş-Alanya) and southern (Gazipaşa) parts of the unit, respectively. The main minerals of the meta-sedimentary rocks are phyllosilicates (2M 1+1M d+1M illite, Ia chlorite, C–V, C–S, I–S), quartz, plagioclase, calcite and dolomite. The Cambrian–Cretaceous units of the Antalya Unit differ from the Devonian–Cretaceous northern Bolkardağı and Aladağ units with respect to age, lithology and phyllosilicate assemblage, but show a considerable similarity to the metamorphic Permian–Triassic Mahmutlar Nappe of the Alanya Unit in their phyllosilicate paragenesis. Although the Antalya and Precambrian–Tertiary Geyikdağı units seem to have similar lithologies and phyllosilicate associations, there are some distinct mineralogical differences, such as phyllosilicate polytypes and degree of diagenetic/metamorphic alteration. Ophiolitic fragments, olistostromal lithologies, phyllosilicate parageneses, illite polytype variations, anchi-epimetamorphic illite crystallinity, and relatively low b values suggest that the Paleozoic-Lower Mesozoic parts of the Antalya Unit were metamorphosed in anticlockwise P–T–t paths in a rifting-associated extensional basin. The grade of diagenetic/metamorphic alteration increases from north to south in the Antalya Unit, related to the higher heat flow due to the southward deepening of the basin. Vertical diagenetic/metamorphic inversion between the Lower Paleozoic and Triassic units is related to higher the heat flux, which affected the Triassic sediments, rather than to thrust tectonics.