Geochemical and Sr–Nd–Pb isotopic characteristics of Paleocene plagioleucitites from the Eastern Pontides (NE Turkey)

Abstract

Abstract In the southern zone of the Eastern Pontides, an isolated small (~ 1.5 km 2 ) outcrop of plagioleucitites occurs in the Asutka thrust sheet. Field relations constrain the timing of volcanism between Maastrichtian and late Paleocene. The plagioleucitites consist of clinopyroxene, analcime (former leucite), Ti-magnetite, plagioclase, sanidine, apatite and accessory biotite. The rocks are represented by three geochemical and petrographic varieties: Types I and II are mostly blocks within epiclastic debris and type III are lava flows. All the rocks are considerably altered, whereby leucite is almost totally analcimized, apart from leucite inclusions within clinopyroxene that are remote from cracks. Furthermore, calcite and chlorite/smectite have been formed. Chemical effects of alteration are primarily reflected in an unsystematic variation of whole-rock K 2 O/Na 2 O ratios (0.12 to 1.71) and of the abundances of large-ion lithophile elements, such as Cs and K, and perhaps also Rb. Both the high modal abundance of (former) leucite and the composition of clinopyroxene indicate an ultrapotassic nature of the primary rocks. In primitive mantle-normalized element concentration diagrams, all samples are characterized by negative anomalies of Nb–Ta, Zr–Hf and Ti and a positive anomaly in Pb, testifying the orogenic nature of the volcanics. The rocks cover restricted ranges in all initial isotope ratios with 87 Sr/ 86 Sr(i) (60 Ma) ranging from 0.70537 to 0.70568, 143 Nd/ 144 Nd(i) from 0.512529 to 0.512585, 206 Pb/ 204 Pb(i) from 18.65 to 18.83, 207 Pb/ 204 Pb(i) from 15.65 to 15.66 and 208 Pb/ 204 Pb from 38.64 to 38.88. There is no obvious relationship between the degree of analcimization and the isotopic composition. In Pb–Pb variation diagrams, the rocks plot above the Northern Hemisphere Reference Line (NHRL). Chondrite-normalized rare earth element (REE) patterns show no significant Eu anomaly, but a strong enrichment of the LREE over the HREE with (La/Yb) cn = 11.6–14.2, whereby normalized concentrations of Er to Lu are nearly similar. The Everek Hanlari plagioleucitites represent the youngest products of Cretaceous to Paleocene arc magmatism of the Eastern Pontides, thus documenting the last stages of the Neotethyan subduction.