Boninitic and tholeiitic basaltic lavas and dikes from dispersed Jurassic East Othris ophiolitic units, Greece: petrogenesis and geodynamic implications

Pillow lavas, massive lava flows, and sub-volcanic dikes of tholeiitic basaltic composition are found to be members of the Vrinena, Aerino, Eretria, and Velestino dispersed Middle–Upper Jurassic ophiolitic units in East Othris. The Vrinena and Eretria ophiolitic units appear to have been emplaced onto the Pelagonian continental margin during the Upper Jurassic–Lower Cretaceous, whereas the Aerino and Velestino units seem to have been finally emplaced during post-Palaeocene times. Geochemically these are divided into two groups: Group I includes subduction-related boninites and low-Ti basalts from the Vrinena and Aerino units, and Group II high-Ti basalts show spreading-type characteristics occurring in the Eretria and Velestino units. Primary magma of the Group I volcanics appears to have been formed after high partial melting degrees (~18%) of a highly depleted harzburgitic mantle source, under relatively high temperatures (mantle potential temperature ~1372°C). Petrogenetic modelling also suggests that the primary magma of the Group II volcanics were formed after lower partial melting degrees (~7%) of a moderately depleted mantle source. The petrological and geochemical data from the East Othris dispersed and diversely emplaced ophiolitic units provide evidence of a common intra-oceanic supra-subduction zone (SSZ) origin within the Pindos oceanic strand of the Western Tethys. Specifically, Group I lavas and dikes from Vrinena seem to represent the extrusive part of an almost complete fore- to island-arc ophiolitic sequence. Dikes of Aerino most likely correspond to fore-arc magmatic material that intruded within exhumed serpentinized ultramafic rocks through a subduction channel that developed close to the slab and towards the fore-arc and the accretionary prism. The Group II volcanics either corresponded to a fore-arc magmatic expression, which extruded earlier than Group I volcanics and prior to the establishment of a mature subduction zone, or represent back-arc to island-arc magmatism that was contemporaneous to the fore-arc magmatic activity during rollback subduction.

在东奥塞里斯（East Othris）地区，具拉斑玄武质成分（tholeiitic basaltic composition）的枕状熔岩（Pillow lavas）、块状熔岩流及次火山岩脉（sub-volcanic dikes），均属于Vrinena、Aerino、Eretria及Velestino四个分散分布的中侏罗世-晚侏罗世蛇绿岩单元（ophiolitic unit）的组成部分。Vrinena与Eretria蛇绿岩单元似乎在晚侏罗世-早白垩世期间侵位于佩拉戈尼亚（Pelagonian）大陆边缘之上，而Aerino与Velestino单元则最终在古新世之后的时期完成侵位。
从地球化学角度可将其划分为两组：第一组包含来自Vrinena与Aerino单元的、与俯冲作用相关的玻安岩（boninite）及低钛玄武岩；第二组为高钛玄武岩，在Eretria与Velestino单元中产出并具有扩张型地球化学特征。第一组火山岩的原生岩浆，似乎形成于高度亏损的方辉橄榄岩型地幔源区（harzburgitic mantle source）经历高程度部分熔融（约18%）、且温度相对较高的条件下（地幔潜在温度约1372℃）。岩石成因模拟（petrogenetic modelling）结果同样表明，第二组火山岩的原生岩浆形成于中等亏损的地幔源区经历低程度部分熔融（约7%）的过程中。
来自东奥塞里斯地区分散分布、侵位时代各异的蛇绿岩单元的岩石学（petrological）与地球化学（geochemical）数据，证明其均形成于西特提斯洋宾多斯洋盆（Pindos oceanic strand）内部的统一洋内俯冲带之上环境（supra-subduction zone, SSZ）。具体而言，Vrinena单元的第一组熔岩与岩脉，近乎代表了一套完整的前弧-岛弧型蛇绿岩序列的喷出岩部分。Aerino单元的岩脉，极有可能对应于前弧岩浆物质：这些物质通过紧邻俯冲板片（slab）、朝向前弧与增生楔（accretionary prism）发育的俯冲通道（subduction channel），侵入至已剥露的蛇纹石化超镁铁质岩石（serpentinized ultramafic rocks）之中。第二组火山岩要么代表了早于第一组火山岩、且在成熟俯冲带形成之前喷发的前弧岩浆作用产物，要么代表了在俯冲回撤（rollback subduction）过程中与前弧岩浆活动同期发育的弧后-岛弧岩浆作用（back-arc to island-arc magmatism）。