Petrogenesis of late cretaceous arc volcanism in the Eastern Pontides, NE Turkey: insight from zircon U–Pb geochronology, whole-rock geochemistry and Hf-Sr-Nd isotopes

The Eastern Pontide Orogenic Belt (EPOB), a well-preserved palaeo-magmatic arc, exists south of the Black Sea, from Bulgaria to Georgia. The region, which forms an essential part of the Alpine-Himalayan belt, contains volcanic rocks formed in the Late Cretaceous. This paper aims to investigate the petrogenesis of the Late Cretaceous volcanic rocks using both new and previously existing geochemical and geochronological data. Based on the geochemical features, the studied volcanics are divided into Group I (dacite/rhyolite) and Group II (basalt/basaltic andesite). According to LA-ICP-MS zircon U–Pb data, the crystallization age of Group I is 83.43 ± 0.87 Ma. The εNd_(t) values of Group I are −1.4 and + 4.5, and the εNd_(t) value of Group II is +3.0. The Lu–Hf isotope data from zircon shows that Group I have positive εHf_(t) values (+14.20 - +12.15), and their crustal model age is 240–339 Ma. The εHf_(t) values indicate that the Group I rocks are derived from the melting of a depleted mantle source. Whole-rock geochemistry data show that Group II samples were formed by partial melting of metasomatised spinel-bearing lherzolites at shallow depths. It is thought that the Late Cretaceous volcanic rocks are the products of arc magmatism associated with the northward subduction of the Neotethys oceanic lithosphere under the Eurasian plate, and the EPOB was an active continental margin arc during the Late Cretaceous.

东蓬蒂德造山带（Eastern Pontide Orogenic Belt, EPOB）是一处保存完好的古岩浆弧，分布于黑海以南，范围从保加利亚延伸至格鲁吉亚。该区域作为阿尔卑斯-喜马拉雅造山带的重要组成部分，产出晚白垩世形成的火山岩。本文旨在借助新获取与已发表的地球化学及地质年代学数据，探究这套晚白垩世火山岩的岩石成因。基于地球化学特征，本次研究的火山岩可划分为I组（英安岩/流纹岩）与II组（玄武岩/玄武安山岩）。依据LA-ICP-MS（激光剥蚀电感耦合等离子体质谱法）锆石U-Pb定年数据，I组火山岩的结晶年龄为83.43±0.87 Ma。I组的εNd(t)值分别为-1.4与+4.5，II组的εNd(t)值为+3.0。锆石Lu-Hf同位素数据显示，I组火山岩具有正的εHf(t)值（+14.20 ~ +12.15），其地壳模式年龄为240~339 Ma。εHf(t)值表明，I组岩石起源于亏损地幔源区的熔融作用。全岩地球化学数据显示，II组样品形成于浅部环境下，经交代作用改造的含尖晶石二辉橄榄岩的部分熔融。研究认为，这套晚白垩世火山岩是新特提斯洋岩石圈在欧亚板块之下北向俯冲所对应的弧岩浆作用产物，且晚白垩世时期东蓬蒂德造山带为活动大陆边缘弧。