Geochemical fingerprints of Late Triassic calc-alkaline lamprophyres from the Eastern Pontides, NE Turkey: A key to understanding lamprophyre formation in a subduction-related environment

Abstract

The Eastern Pontides in NE Turkey is one of the major orogenic belts in Anatolia. In this paper, we report our new 40Ar/39Ar dating, mineral chemistry, major and trace elements and Sr–Nd–Pb isotopic analyses of the lamprophyre intrusions in this region. The lamprophyres are widely scattered and intrude Late Carboniferous granitoid rocks. The lamprophyres exhibit fine-grained textures and are mineralogically uniform. Hornblende 40Ar/39Ar dating yielded a plateau age of 216.01±10.64Ma. Based on their geochemistry, mineral compositions and paragenesis, the lamprophyres are classified as calc-alkaline lamprophyres in general and spessartites in particular, which are rich in large ion lithophile elements (e.g., Rb, Ba, K) but depleted in Nb and Ti. Our samples exhibit moderate fractionation in LREE patterns approximately 100 times that of chondrite but HREE abundances less than 10 times that of chondrite. These calc-alkaline lamprophyres display a range of ISr (216Ma) values from 0.70619 to 0.71291 and εNd (216Ma) values from −1.4 to 4.1, with TDM=1.11 to 2.20Ga. Their Pb isotopic ratios indicate an enriched mantle source. The enrichment process is related to metasomatism of a subcontinental lithospheric mantle source, which is caused by a large quantity of H2O-rich fluids, rather than sediments released from oceanic crust at depth during the closure of the Paleotethys Ocean in Triassic times. All of the geochemical data and the trace element modeling suggest that the primary magma of the calc-alkaline to high-K calc-alkaline spessartites was generated at depth by a low degree of partial melting (~1–10%) of a previously enriched lithospheric mantle wedge consisting of phlogopite-bearing spinel peridotite. The ascendance of a hot asthenosphere triggered by extensional events caused partial melting of mantle material. The rising melts were accompanied by fractional crystallization and crustal contamination en route to the surface. All of the geochemical features combined with regional data suggest that the Eastern Pontides calc-alkaline lamprophyres originated in an extensional environment along an active continental margin throughout the Late Triassic. Such an extensional event, causing upwelling of hot asthenosphere, led to the opening of the northern branch of the Neotethys as a back-arc basin farther south of the Eastern Pontides.