The Coastal Complex (CC) and Bay of Islands Complex (BOIC) represent two supra-subduction zone (SSZ) forearc ophiolitic assemblages of different ages within the Taconic Humber Arm Allochthon of the Western Newfoundland Appalachians. To further constrain age relationships between the CC and BOIC and earliest initiation of Iapetus subduction, we acquired LA-ICP-MS U-Pb zircon ages for eleven plagiogranites sampled along strike of the CC belt. These ages combined with three prior legacy ages establish an age range of 514.3–502.7 Ma for the six CC plagiogranite plutons. The oldest precise ca. 514.3 Ma CC pluton age represents the oldest Taconic arc-related plutonic activity yet documented in Western Newfoundland and provides a minimum age for the intruded CC ophiolite basement and initiation of westerly vergent subduction within the peri-Laurentian Iapetus Ocean. This confirms the Early Cambrian CC mafic–ultramafic basement is significantly older than the juxtaposed SSZ BOIC, which formed by later rifting of the CC forearc along younger forearc spreading center in the Late Cambrian by ca. 488.3 Ma. After subduction initiation a prolonged early period CC forearc silicic magmatism lasted at least ∼ 11.6 Ma until ca. 502.7 Ma, a range consistent with age spans of magmatism in modern juvenile forearcs. We also reinvestigated the Central Newfoundland Twillingate Granite batholith thought to be one of the earliest major felsic batholiths formed within the adjacent juvenile arc massif of the Notre Dame Subzone, part of the same peri-Laurentian arc. We determined a precise 3-sample composite age of 504.3 ± 1.8 Ma that signals the formation of earliest Notre Dame arc edifice developed by Middle Cambrian. It overlaps the end-stage CC forearc plagiogranite magmatism and places a minimum mid-Cambrian age constraint on intruded ophiolitic arc tholeiitic and boninitic pillow lavas of the Sleepy Cove and correlated Lushs Bight volcanics, suggesting basement continuity with the CC.

海岸杂岩体 (CC) 和岛屿湾杂岩体 (BOIC) 代表了西纽芬兰阿巴拉契亚山脉 Taconic Humber Arm Allochthon 内不同年龄的两个超俯冲带 (SSZ) 弧前蛇绿岩组合。为了进一步限制 CC 和 BOIC 之间的年龄关系以及最早开始的 Iapetus 俯冲，我们获得了沿 CC 带走向采样的 11 个斜花岗岩的 LA-ICP-MS U-Pb 锆石年龄。这些年龄与三个先前的遗留年龄相结合，确定了六个 CC 斜花岗岩岩体的年龄范围为 514.3-502.7 Ma。最古老的精确约。514.3 Ma CC 深成岩年龄代表了纽芬兰西部迄今为止记录的最古老的与 Taconic 弧相关的深成活动，并为侵入的 CC 蛇绿岩基底和在peri -Laurentian Iapetus 海洋。这证实了早寒武世CC镁铁质-超镁铁质基底明显比并列的SSZ BOIC古老，后者是由CC前弧沿晚寒武世较年轻的前弧扩展中心后期裂谷形成的。488.3 毫安。在俯冲开始后，一个延长的早期 CC 前弧硅质岩浆作用持续了至少 ∼ 11.6 Ma 直到 ca。502.7 Ma，这个范围与现代少年前弧岩浆作用的年龄跨度一致。我们还重新调查了纽芬兰中部斜纹花岗岩岩基，该岩基被认为是在圣母子带相邻幼年弧地块内形成的最早的主要长英质岩基之一，该地块属于同一外围- 劳伦斯弧。我们确定了 504.3 ± 1.8 Ma 的精确 3 样本复合年龄，这标志着中寒武世开发的最早的圣母院弧形大厦的形成。它与终期 CC 前弧斜花岗岩岩浆作用重叠，并对 Sleepy Cove 和相关的 Lushs Bight 火山岩侵入的蛇绿质弧拉斑斑岩和博尼质枕状熔岩施加了最小的中寒武世年龄限制，表明地下室与 CC 的连续性。