REU: Geologic Evolution of the Robertson River Suite: Implications for the Timing, Petrogenesis, and Thermal History of the Late Proterozoic Rifting of Laurentia

罗伯逊河组（RRS，又名罗伯逊河组）包括一系列的铝质到过碱性花岗岩和花岗岩，暴露在弗吉尼亚州北部和中部的蓝岭背斜（BRA）的核心内。前人的大尺度勘测表明，RRS侵入Grenville时代（1.0-1.1 G.a）片麻岩，并被大量绿岩岩脉侵入，其中一些绿岩岩脉可能是BRA盖层Catoctin变质火山的补给源。因此，RRS的年龄仅受到野外证据的广泛限制，它比格伦维里亚变质作用更年轻，比至少大部分前寒武纪晚期的Catoctin组更古老。以前尝试使用辐射测量技术来确定更精确的套具就位时间，结果产生了相互矛盾的数据，跨度为730至570 ma。这些数据不一致的主要原因在于这些研究要么1)假设地层是均匀的；因此，通过从整个露头带收集的多个样本来准确地表征，或者2)承认套件的明显异质性，但试图将数据从一个位置推断到整个体。然而，根据该研究小组最近在该套北部完成的更详细的野外、岩石学和地球化学研究的新证据，这两种方法都不合适。这些数据表明，RRS包含一系列不同的岩体，这些岩体记录了一个明显复杂的岩石成因历史，其时间间隔可能相当长。在此基础上，本研究拟开展以下工作：1)完成该套南段1:24 000比尺的填图；2)利用岩石学和地球化学数据确定组成岩体的性质；3)获得与RRS侵位时间和持续时间相关的精确同位素数据。此外，本研究获得的石油化学和同位素数据将用于完善该组合独特的岩石成因演化模型和可能源区的同位素特征。综上所述，本研究结果将为该造山带侵位的合适成岩模型提供坚实的基础，为确定前陆裂谷的最大年龄提供重要的约束条件。3)建立数据库，评价该岩浆作用与Catoctin组内的大量长英质熔岩以及阿巴拉契亚中部其他地区出现的年龄和成分相似的长英质侵入岩的关系；4)确定岩浆源区的具体同位素特征；5)建立了这类组成独特的造山带成岩演化的地质合理模式。

The Robertson River Suite (RRS, a.k.a. Robertson River Formation) encompasses a series of metaluminous to peralkaline granitoids and felsites exposed within the core of the Blue Ridge anticlinorium (BRA) in northern and central Virginia. Broad- scale reconnaissance mapping by previous workers has established that the RRS is intrusive into Grenville age (1.0-1.1 G.a.) gneisses and is intruded by numerous greenstone dikes, some of which may have served as feeders to the Catoctin metavolcanics of the BRA cover sequence. The age of the RRS is thus only broadly constrained by field evidence as younger than Grenvillian metamorphism and older than at least most of the late Precambrian Catoctin Formation. Previous attempts using radiometric techniques to determine a more exact timing for emplacement of the suite have produced mutually contradictory data spanning the range 730 to 570 M.a. Much of the reason for these inconsistent data lies in the fact that these studies have either 1) assumed the formation to be homogeneous, and thus accurately characterized by multiple samples collected from throughout the outcrop belt, or 2) acknowledged the apparent heterogeneity of the suite and nevertheless attempted to extrapolate data from one location to the entire body. Neither approach is appropriate, however, in light of new evidence from more detailed field, petrologic, and geochemical studies recently completed in the northern portion of the suite by this research group. These data demonstrate that the RRS encompasses a series of distinct plutons that records an obviously complex petrogenetic history spanning a time interval which may have been considerable. This study proposes to build on this work by 1) completing 1:24,000-scale mapping of the southern portion of the suite, 2) using petrologic and geochemical data to define the nature of the constituent plutons, and 3) obtaining precise and accurate isotopic data bearing on the timing and duration of RRS emplacement. Furthermore, the petrochemical and isotopic data to be obtained in this study will be applied to refine models for the petrogenetic evolution of this compositionally distinctive suite and the isotopic characteristics of probable source regions. In summary, the results of this stud will 1) provide a firm foundation for appropriate petrogenetic models concerning emplacement of this anorogenic suite, 2) place important constraints on the maximum age of prelapetan rifting, 3) establish a database on which to evaluate the relationship of this plutonism to the voluminous felsic lavas included within the Catoctin Formation and felsic intrusives of similar (?) age and composition occurring elsewhere in the central Appalachians, 4) define specific isotopic characteristics of the source areas of magma generation, and 5) lead to the development of geologically reasonable models concerning the petrogenetic evolution of such compositionally unique anorogenic suites.