Collaborative Research: Identifying the Limitations and Expanding the Utility of Detrital Mineral U-Pb Geochronology to Tectonic Studies

利用碎屑矿物(特别是锆石)的放射性年龄进行的起源研究在过去十年中得到了极大的扩展，这主要是由于微观地质计时方法的进步。正是同样的性质，使锆石成为如此强大的地质年代学(极高的闭合温度和非常低的溶解度)，使其在碰撞造山作用中几乎是惰性的，除了上角闪岩到麻粒岩相和深熔作用。因此，在一些地体中可以经历碰撞、地壳增厚、加热和加载以及折返的整个周期，而使用传统的碎屑锆石分析方法在碎屑记录中不会有这种周期的记录。这项研究项目正在测试确定碎屑独居石的铀-钍-铅年龄的实用性，与只分析碎屑锆石核相比，它有可能提供更丰富的构造变质事件历史。此外，与通过分析较大颗粒的中心部分获得的造山带的年代学观点相比，极小的碎屑状锆石晶体和碎屑状锆石上的薄变质边缘提供了造山带本质上不同的年代学观点的假设正在得到验证。来自阿巴拉契亚山脉南部的碎屑沉积岩和冲积层正在被取样，以寻找它们的重矿物套装。之所以选择阿巴拉契亚造山带，是因为这条带经历了全方位的构造过程，而不是主要由诸如弧岩浆作用等单一过程构成。此外，为了充分检验这些假说，有必要从岩浆、变质和变形历史的详细资料已确定的地区收集碎屑数据。这将允许建立最可靠的方案，以最大限度地减少年龄偏差的影响，这些影响既包括碎屑矿物固有的影响，也包括在谷物选择过程中潜在的实验室诱导的偏差。只有从研究充分的多变形造山带获得碎屑年龄信息的结果，它们才能最好地应用于其他更不了解造山带的构造研究。这项研究的结果将对解释世界碎屑矿物年龄记录具有重要意义。这里开发的最成功地解决了目前在常规碎屑锆石数据库解释中存在的问题的程序和工具，将可供在任何造山带工作的任何地质时代的其他研究人员使用，以便最好地将碎屑矿物测年作为来源指示，作为板块重建的辅助工具，并更准确地阐明主要的地体构造热事件。

Provenance studies utilizing radiometric ages of detrital minerals (particularly zircon) have expanded greatly over the past decade, driven largely by advances in microgeochronometric methods. The very same properties that make zircon such a powerful geochronometer (extremely high closure temperature and very low solubility) render it virtually inert during collisional orogenesis, except at upper amphibolite to granulite facies and anatexis. Thus, an entire cycle of collision, crustal thickening, heating and loading, and exhumation could pass in some terranes and there would be no record of that cycle in the detrital record using conventional approaches for detrital zircon analysis. This research project is testing the utility of determining the Uranium-Thorium-Lead ages of detrital monazite, which holds the potential for providing a far richer history of tectonometamorphic events compared to analyzing only detrital zircon cores. Additionally, the hypothesis that very small detrital zircon crystals, and thin metamorphic rims on detrital zircon, provide a substantially different chronological view of an orogen compared to that obtained by analyzing the central portions of larger grains is being tested. Clastic sedimentary rocks and alluvium from the southern Appalachians are being sampled for their heavy mineral suites. The Appalachian orogen is chosen because this belt has experienced the full spectrum of tectonic processes, rather than being constructed largely by a single process such as arc magmatism. In addition, to fully test these hypotheses it is necessary to collect the detrital data from a region where the details of the magmatic, metamorphic, and deformational history are well established. This will allow establishment of the most robust protocols to minimize the effects of age bias, both those naturally inherent to detrital minerals, and potential laboratory induced bias during grain selection. Only once the results of detrital age information are available from a well-studied, polydeformed orogen can they be best applied to tectonic studies of other more poorly understood orogenic belts. The results of this study will have major implications for the interpretation of the worldwide detrital mineral age record. The procedures and tools developed here that are most successful for resolving the problems that currently exist with the interpretation of the conventional detrital zircon database will then be available for application by other researchers working in any orogenic belt, for any division of geologic time, to best use detrital mineral dating as a provenance indicator, as an aid in plate reconstructions, and for much more accurate elucidation of major terrane tectonothermal events.