Forearc Basin Provenance Analysis by SHRIMP Detrital Zircon Geochronology

摘要砂岩中颗粒的组成可以诊断这些颗粒的原始来源，就像火山弧侵蚀的火山颗粒一样。传统上，这种分析是用标准的光学显微镜进行的，但所有火山弧的沙子成分普遍相似，往往导致具体来源的模糊。确定单个颗粒年龄的能力将提供更大的解释杠杆，而这种能力存在于敏感高分辨率离子微探针（SHRIMP）中。将SHRIMP技术应用于地质特征良好的与弧有关的沉积盆地（即沉积物来源独立已知的盆地），可以作为一个标准，用来衡量对不太了解但类似的盆地的研究。加利福尼亚大峡谷盆地的中生代地层提供了这样一个机会，因为沉积物的散装颗粒和微量元素组成已经确定，并且邻近的Sierran和Klamath弧的沉积物来源通常是已知的。因此，对大峡谷锆石沙粒年龄的SHRIMP研究将形成古弧相关盆地沉积物来源关系模型的基础，并为火山弧的长期侵蚀速率和衍生沉积物的沉积提供见解。随后，将Great Valley模型与来自华盛顿和不列颠哥伦比亚省中生代Methow盆地的锆石颗粒SHRIMP分析结果进行比较。如果与大峡谷的研究结果一致，该方法的结果可能会证明分析方法的更广泛的实用性，并有助于解决该地区地质演化的一些具体问题。

Project Title: Forearc Basin Provenance Analysis by SHRIMP Detrital Zircon GeochronologyPI: Stephan GrahamABSTRACT The composition of grains in sandstone can be diagnostic of the original source of those grains, as in the case of volcanic grains eroded from volcanic arcs. Conventionally, such analyses are conducted with a standard optical microscope, but the broad compositional similarity of sand from all volcanic arcs often results in ambiguity as to specific sources. Greater interpretational leverage would be provided by an ability to determine the ages of individual grains, and that capability exists in the Sensitive High Resolution Ion Microprobe (SHRIMP). The application of SHRIMP technology to a geologically well-characterized arc-related sedimentary basin (i.e., one for which the sources of sediment are independently known) could serve as a standard against which studies of poorly known but analogous basins might be gauged. The Mesozoic strata of the Great Valley basin of California present such an opportunity, because the bulk-grain and trace element compositions of sediment are already determined, and the sources of sediment in the adjacent Sierran and Klamath arcs are generally known. Thus, SHRIMP study of the ages of Great Valley zircon sand grains will form the basis of a model of sediment source relations in an ancient arc-related basin, as well as provide insights into long-term rates of erosion of volcanic arcs and sedimentation of derivative sediments. The Great Valley model subsequently will be compared with results of SHRIMP analysis of zircon grains from the Mesozoic Methow basin of Washington and British Columbia. If consistent with the Great Valley study, the Methow results may demonstrate the broader utility of the analytical method, as well as contribute to solving some specific problems of the geologic evolution of that region.