High-precision CA-TIMS 206Pb/238U zircon dates clarify the age and tectonic significance of the Cambridge Formation which is poorly exposed in the Boston Basin, eastern Massachusetts, but transected by ∼50 km of tunnels beneath the mainland and Boston Harbor. The youngest detrital zircon in a sample from the northern Braintree Weymouth Tunnel establishes a maximum depositional age of 584.09 ± 1.98 Ma, consistent with sources in sills of that age in underlying Roxbury Conglomerate. A 551.22 ± 0.20 Ma ash bed from the Mystic Quarry in Somerville, Massachusetts lies near the top of an approximately 5350 m thick, dominantly argillaceous section measured in subsurface cross sections. These were constructed from attitudes reported in pre-1960 tunnels and from mapping logs obtained from tunnels completed decades later during the federally ordered clean-up of Boston Harbor. A 488.58 ± 0.16 Ma aplite sill intruding argillite ∼800 m above the ash bed sets the minimum depositional age on the north side of the Basin. A tighter constraint comes from trilobite-bearing strata of the lower Cambrian Weymouth Formation located south of Boston that overlies the Cambridge Formation without obvious break in the Braintree Weymouth Tunnel. If Cambridge deposition was continuous after 584 Ma, the depositional interval would exceed 40 million years. An estimated ∼20 Ma depositional hiatus seems more likely because the base of the Cambridge Formation appears to define a regional unconformity above which argillite rests variously on magmatic arc-related units of both the 595 to 584 Ma Roxbury Conglomerate and the 597 to 593 Ma Lynn-Mattapan Volcanic Complex. Cambridge deposition set in once arc activity in more northerly “West” Avalonian terranes extending through Atlantic Canada to the Avalon Peninsula, Newfoundland had given way to wrench faulting and bimodal magmatism. This regime is manifested structurally in Boston-area tunnels by later-reactivated normal faults in which hanging wall blocks of Cambridge argillite were originally downthrown relative to older footwall units. Pyroclastic volcanic textures and thin basaltic flows with soft sediment contacts are present in argillite of the City Tunnel Extension, and whole rock major element and REE compositions reveal mixed terrigenous and volcanic components deposited under marine conditions throughout the Basin. Proposed sources for the latter are voluminous eruptions recorded in the 560 to 550 Ma Coldbrook Group in New Brunswick's Caledonia terrane.

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新英格兰东南部阿瓦隆阶弧到地台的转变：美国马萨诸塞州波士顿盆地埃迪卡拉纪“泥质岩”的 U-Pb 年代学和地层学

高精度 CA-TIMS 206Pb/238U 锆石日期阐明了剑桥地层的年龄和构造意义，该地层在马萨诸塞州东部波士顿盆地出露不良，但被大陆和波士顿港下方约 50 公里的隧道横断。来自布伦特里韦茅斯隧道北部样品中最年轻的碎屑锆石确定最大沉积年龄为 584.09 ± 1.98 Ma，与下伏 Roxbury 砾岩中该年龄的岩床来源一致。来自马萨诸塞州萨默维尔神秘采石场的 551.22 ± 0.20 Ma 灰层位于约 5350 m 厚、主要为泥质部分的顶部附近，在地下横截面中测量。这些是根据 1960 年之前隧道中报告的态度以及从几十年后联邦命令清理波士顿港期间完成的隧道中获得的测绘日志构建的。一个 488.58 ± 0.16 Ma 的 aplite 基岩侵入灰层上方约 800 m 的泥质岩，设定了盆地北侧的最小沉积年龄。一个更严格的限制来自位于波士顿南部的下寒武纪韦茅斯组的含三叶虫的地层，它覆盖在剑桥组上，布伦特里韦茅斯隧道没有明显的中断。如果剑桥沉积在 584 Ma 之后是连续的，那么沉积间隔将超过 4000 万年。大约 20 Ma 的沉积中断似乎更有可能，因为剑桥组的底部似乎定义了一个区域不整合面，在该不整合面之上，泥质岩以不同的方式位于 595 至 584 Ma Roxbury 砾岩和 597 至 593 Ma Lynn 的岩浆弧相关单元上-马塔潘火山群。剑桥沉积在更偏北的“西”阿瓦隆地体中形成弧形活动，通过大西洋加拿大延伸到阿瓦隆半岛，纽芬兰已经让位于扳手断层和双峰岩浆作用。这种情况在波士顿地区隧道的结构上表现为后来重新激活的正断层，在这些断层中，剑桥泥质岩的悬壁块最初相对于较旧的下盘单元被倒塌。城市隧道延长线的泥质岩中存在火山碎屑火山结构和薄玄武岩流与软沉积物接触，全岩主要元素和稀土元素组成显示整个盆地在海洋条件下沉积的混合陆源和火山成分。后者的建议来源是记录在新不伦瑞克省喀里多尼亚地体的 560 至 550 Ma Coldbrook Group 中的大量喷发。