CAA: U-Pb Geochronology of Squantum 'Tillite' and Related Rocks Around Boston, MA: Implications for Terminal Proterozoic Glaciation and Supercontinent Breakup

一个多世纪以来，地质学家们一直聚集在马萨诸塞州昆西市的Squaw Head海崖，争论量子“Tillite”可能的冰川起源。虽然最近的沉积学观点最终倾向于冰川期海洋沉积，但通过测定这块岩石的年代，还有很多东西需要了解。例如，量子年龄数据将为沉积-火山事件的时间提供第一个放射性证据，以确定波士顿周围著名的约625-590 Ma阿瓦洛尼亚岩浆弧复合体的年代。量子“Tillite”将在麻省理工学院的放射性成因同位素实验室使用高精度U-Pb锆石年代学方法进行年代测定。火山碎屑和碎屑锆石将被分析，以确定其最大沉积年龄。火山流与昆西更南部的houghsneck的砾岩量子层序相互作用，也将确定年代。这一结果将更直接地约束沉积时代，从而约束该地区的冰川时代。将通过碎屑锆石研究寻求更广泛的量子来源观点。对于一个已经花了20年时间教授本科生的首席研究员来说，这个项目提供的在麻省理工学院放射性成因同位素实验室学习U-Pb地质年代学方法的机会代表了一个重大的职业发展。一旦高精度的年龄测定能够与现有的野外、岩石学和地球化学数据相结合，肯定会出现更广泛、更有刺激性的研究问题。这样的研究结果肯定会使一个智力充沛的教师回到韦尔斯利的课堂上来。

Thompson 9628520 For more than a century, geologists have congregated at the Squaw Head seacliff in Quincy, Massachusetts to argue over the possible glacial origin of the Squantum "Tillite". Although recent sedimentological opinion has finally titled toward glaciomarine deposition, much remains to be learned by dating this rock. Squantum age data, for example, will provide the first radiometric evidence for the timing of sedimentary-volcanic events post-dating the well known ca. 625-590 Ma Avalonian magmatic arc complex around Boston. The Squantum "Tillite" will be dated in the Radiogenic Isotope Laboratory at MIT using methods of high precision U-Pb zircon geochronology. Volcanic rock clasts as well as detrital zircons from Squantum diamictite will be analyzed to fix its maximum depositional age. Volcanic flows interbedded with a conglomeratic Squantum sequence farther south in Houghs Neck, Quincy will also be dated. This result will constrain the age of deposition, and thus the age glaciation in this area, more directly. A broader view of Squantum provenance will be sought via detrital zircon studies. For a Principal Investigator who has spent twenty years teaching undergraduates, the opportunity presented by this project to learn the methods of U-Pb geochronology in MIT's Radiogenic Isotope Laboratory represents a major career advancement. Much broader and more stimulating research questions will certainly arise once high precision age determinations can be integrated with existing field, petrographic and geochemical data. The results of such research will surely come full circle by returning an intellectually energized instructor to Wellesley classrooms.