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.

东奥蒂斯山（East Othris）范围内分散产出的中侏罗世-晚侏罗世蛇绿岩单元（ophiolitic units）包含四个成员：Vrinena、Aerino、Eretria与Velestino，其岩石类型均为拉斑玄武质成分的枕状熔岩、块状熔岩流及次火山岩脉。Vrinena与Eretria蛇绿岩单元似乎在晚侏罗世-早白垩世期间侵位至佩拉贡尼亚大陆边缘（Pelagonian continental margin）之上，而Aerino与Velestino单元则最终在古新世之后完成侵位。地球化学上，这些岩石可划分为两个群组：第一群组包含产自Vrinena与Aerino单元的、与俯冲作用相关的玻安岩（boninites）及低钛玄武岩；第二群组的高钛玄武岩则表现出扩张型地球化学特征，产出在Eretria与Velestino单元中。第一群组火山岩的原生岩浆，形成于相对高温（地幔位温约1372℃）环境下，源自高度亏损的方辉橄榄岩型地幔源区经约18%的高程度部分熔融作用。岩石成因模拟结果同样表明，第二群组火山岩的原生岩浆，形成于中等亏损地幔源区经约7%的低程度部分熔融作用之后。东奥蒂斯山分散分布、侵位时代各异的蛇绿岩单元所获得的岩石学与地球化学数据，证明其均起源于西特提斯洋（Western Tethys）品都斯洋域（Pindos oceanic strand）内统一的洋内俯冲带之上（supra-subduction zone, SSZ）环境。具体而言，产自Vrinena的第一群组熔岩与岩脉，几乎代表了一套完整的前弧至岛弧型蛇绿岩序列的喷出岩部分。Aerino单元的岩脉，极有可能对应着通过近板片俯冲通道侵入至剥露的蛇纹石化超镁铁岩中的前弧岩浆物质，该俯冲通道朝向弧前与增生楔（accretionary prism）发育。第二群组火山岩，要么代表早于第一群组火山岩、且在成熟俯冲带形成之前喷出的前弧岩浆作用产物，要么对应着与俯冲回撤（rollback subduction）期间弧前岩浆活动同期的弧后至岛弧岩浆作用。