Triassic volcanism in the Afyon Zone (NW Türkiye): Evidence from U-Pb zircon ages, Sr-Nd isotopes, and geochemistry of an enriched mantle source

Abstract

In the northwestern part of the Afyon Zone, along the northern margin of the Anatolide-Tauride Block, which is generally regarded as a fragment of northern Gondwana during the Late Paleozoic, Triassic metavolcanic rocks are extensively exposed. These rocks comprise felsic and basic types distinguished by mineralogical and geochemical variations. Zircon U-Pb dating of the felsic metavolcanics yields an age of 233.4 +/- 2.1 Ma, indicating a Triassic magmatic episode. Metavolcanics have high potassium-shoshonitic and calc-alkaline characteristics. They are enriched in large-ion lithophile elements (LILE), and depleted in high-field-strength elements (HFSE). They also show enrichment in light rare earth elements (LREE) relative to heavy rare earth elements (HREE). The relative enrichment of low-field-strength elements (LFSE) over HFSE, along with average (La/Lu)N ratios of 3-25, reflects typical calc-alkaline volcanism. The geochemical trends suggest that assimilation and/or crustal contamination may have occurred during magma evolution. Furthermore, they are considered to be related to the island arc and resemble Tholeiitic-type mid-ocean ridge basalts. Isotope data supports an enriched mantle source. The initial Sr-8(7)/Sr-8(6) isotope ratios are 0.711429-0.716276 in metadacite and 0.709275-0.714190 in metabasic. The 143Nd/144Nd isotope ratios vary from 0.512349 to 0.512369 in metadacite, and from 0.512463 to 0.512509 in metabasic. The geochemical and isotopic data suggest that the parental magmas were derived from an enriched mantle source metasomatized by subduction-related components beneath the Afyon Zone. The northward subduction of the Paleozoic Tethyan oceanic lithosphere under the southern margin of Laurasia during the Triassic is interpreted to have triggered a regionally rift-related regime along the northern margin of Gondwana. The Triassic metavolcanic rocks are interpreted to represent the earliest magmatic products associated with the Paleotethys Ocean crust, formed along the northern margin of Gondwana.