A Triassic large igneous province in the Pontides, northern Turkey: geochemical data for its tectonic setting

A strongly deformed and metamorphosed Triassic oceanic seamount(s) and plateau succession extends as an east–west belt for 1100 km along the Pontides of northern Turkey. This succession, known widely as the Nilüfer unit, consists mainly of metabasic lava and tuff–marble–phyllite association including tectonic slices of ultramafic rock and gabbro. According to the conodont findings the unit formed during the Early to Mid-Triassic, and the isotopic age data indicate that it underwent high-pressure greenschist facies metamorphism during the latest Triassic period. The metavolcanic rocks form over 80% of the sequence. The Nilüfer unit covers an area of 120,000 km², with the volume of mafic lava estimated as 2×10⁵ km³. Such a huge volcanic pile has erupted rapidly in a relatively short period during the Early to Mid-Triassic (approx. 10 Ma). Hypotheses for the origin of the Nilüfer unit include a ‘seamount’, ‘intra-arc and/or fore-arc basin’, ‘oceanic plateau’, and ‘Early Triassic rift’. The geochemistry of metabasites and that of relict magmatic clinopyroxenes indicate that there are two main mafic rock groups in the Nilüfer unit displaying tholeiitic and alkaline affinities. No metabasite and clinopyroxene sample display typical orogenic basalt affinity or a subduction signature. Geochemical data obtained in this study are consistent with the derivation of the metabasites from the topmost extrusive layers of an oceanic plateau (LIP) together with the volcanic rocks of seamount(s).