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

汤普森9628520一个多世纪以来，地质学家们一直聚集在马萨诸塞州昆西的Squaw Head Seacliff，就斯昆特“蒂利特”可能的冰川起源进行争论。尽管最近的沉积学观点最终倾向于冰川岩沉积，但通过测定这块岩石的年代，仍有许多东西需要了解。例如，SQuantum年龄数据将为波士顿周围著名的约625-590 Ma的阿瓦隆岩浆弧杂岩之后沉积-火山事件的时间提供第一个放射性证据。SQuantum“Tillite”将在麻省理工学院的放射性同位素实验室使用高精度的锆石U-Pb年代学方法进行测年。将对火山岩碎屑以及来自Squantum二辉橄榄岩的碎屑锆石进行分析，以确定其最大沉积年龄。在更南边的Houghs Neck，昆西的火山流与砾岩SQuantum层序的互层也将被确定日期。这一结果将更直接地限制沉积年龄，从而限制该地区的冰川年龄。人们将通过碎屑状的锆石研究来寻求更广泛的Santum来源的观点。对于一位从事了二十年本科生教学工作的首席研究员来说，这个项目为他提供了在麻省理工学院放射性同位素实验室学习U-Pb年代学方法的机会，这是他职业生涯中的一次重大进步。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，肯定会出现更广泛和更具启发性的研究问题。这样的研究结果肯定会回到原点，让一位智力充沛的教师回到卫尔斯理的课堂上。

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.