Age and implication of Late Ediacaran dykes in the İstanbul-Zonguldak Tectonic Unit (NW Turkey): Implications for the rifting of the Rheic Ocean

The İstanbul-Zonguldak Tectonic Unit is a part of Avalonia-Carolina and represents Far East Avalonia. It includes Ordovician to Carboniferous deposits which unconformably overlie Late Neoproterozoic metamorphic rocks. Timing of its detachment from West Gondwana-land, thus rifting of the Rheic Ocean, is poorly known. Here, I report on dacite, diabase and andesite dykes of Late Ediacaran igneous crystallization ages from the basement rocks and rift-deposits in the İstanbul-Zonguldak Tectonic Unit (NW Turkey). They are folded about a consistent axis and have porphyritic to intersertal textures. U-Pb dating on igneous zircons from folded dykes yielded Late Ediacaran ages of ca. 556.2 ± 2.2 Ma (2σ) and 552.1 ± 2.8 to 549.2 ± 2.3 Ma (2σ). Late Ediacaran dykes show calc-alkaline and alkaline affinities, and contain with-in plate components. Rift-related Late Ediacaran magmatism shows that the Yellice arc changed from an arc to a back-arc basin and the rift-deposits formed during the late Ediacaran due to the arc rifting. In conjunction with the data from literature, I suggest that the rift-related magmatism is related to the rifting event during the late Ediacaran, leading to the detachment of this continental block from the West Gondwana-land, thus to the opening of the Rheic Ocean, and Early Cambrian deformation event correspond to the adding of the Kraishte terrane with Far East Avalonia. The docking of the Kraishte terrane led to the deformation of Late Ediacaran dykes. Overall, these data indicate that the depositional time of sedimentary rocks in the continental fragment should have started during the late Ediacaran instead of Ordovician.

伊斯坦布尔-宗古尔达克构造单元（İstanbul-Zonguldak Tectonic Unit）是阿瓦隆尼亚-卡罗莱纳（Avalonia-Carolina）的组成部分，代表东阿瓦隆尼亚（Far East Avalonia）。该构造单元包含奥陶纪（Ordovician）至石炭纪（Carboniferous）沉积地层，其不整合覆于新元古代晚期变质岩之上。目前对于该构造单元从西冈瓦纳大陆分离的时间（亦即瑞亚克洋（Rheic Ocean）的张裂时间）尚不清楚。
本文报道了产自土耳其西北部（NW Turkey）伊斯坦布尔-宗古尔达克构造单元基底岩石及裂谷沉积中的晚埃迪卡拉纪火成结晶年龄的英安岩（dacite）、辉绿岩（diabase）和安山岩（andesite）岩脉。这些岩脉沿统一轴向褶皱，发育斑状至间粒结构（intersertal texture）。对褶皱岩脉中的火成锆石（zircon）开展U-Pb定年（U-Pb dating），获得了晚埃迪卡拉纪年龄：约556.2±2.2 Ma（2σ）以及552.1±2.8 Ma至549.2±2.3 Ma（2σ）。晚埃迪卡拉纪岩脉兼具钙碱性（calc-alkaline）与碱性（alkaline）亲和性，且含有板内（within-plate）组分。
与裂谷相关的晚埃迪卡拉纪岩浆作用表明，耶利采弧（Yellice arc）已从弧环境转变为弧后盆地（back-arc basin），且裂谷沉积形成于晚埃迪卡拉纪，源于弧张裂作用。结合已有文献数据，本文认为该裂谷相关岩浆作用与晚埃迪卡拉纪的张裂事件存在关联，该事件导致该大陆块从西冈瓦纳大陆分离，进而开启了瑞亚克洋；而早寒武世变形事件则对应克拉伊什泰地体（Kraishte terrane）与东阿瓦隆尼亚的拼合。克拉伊什泰地体的拼合引发了晚埃迪卡拉纪岩脉的变形。
总体而言，本次研究数据表明，该大陆碎块中沉积岩的沉积时间应始于晚埃迪卡拉纪，而非此前认为的奥陶纪。