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

在过去十年中，利用碎屑矿物（特别是锆石）的辐射年龄进行的物源研究已大大扩展，这主要是由于微地球测时方法的进步所推动的。使锆石成为如此强大的地质年代计的特性（极高的闭合温度和极低的溶解度）使其在碰撞造山过程中几乎呈惰性，除了上部角闪岩到麻粒岩相和深熔岩之外。因此，碰撞、地壳增厚、加热和加载以及折返的整个循环可能在某些地体中经过，并且使用传统的碎屑锆石分析方法在碎屑记录中不会有该循环的记录。该研究项目正在测试确定碎屑独居石铀-钍-铅年龄的实用性，与仅分析碎屑锆石岩心相比，它有可能提供更丰富的构造变质事件历史。此外，正在测试以下假设：与通过分析较大颗粒的中心部分获得的结果相比，非常小的碎屑锆石晶体和碎屑锆石上的薄变质边缘提供了截然不同的造山带年代学视图。正在对来自阿巴拉契亚山脉南部的碎屑沉积岩和冲积层进行取样，以获取其重矿物组。选择阿巴拉契亚造山带是因为该造山带经历了全方位的构造过程，而不是主要由弧岩浆作用等单一过程构成。此外，为了充分检验这些假设，有必要从岩浆、变质和变形历史细节已明确的地区收集碎屑数据。这将允许建立最稳健的协议，以最大限度地减少年龄偏差的影响，包括碎屑矿物固有的年龄偏差，以及谷物选择过程中潜在的实验室引起的偏差。只有从经过充分研究的多变形造山带获得碎屑年龄信息的结果后，才能最好地将其应用于其他知之甚少的造山带的构造研究。这项研究的结果将对全球碎屑矿物年龄记录的解释产生重大影响。这里开发的程序和工具最成功地解决了目前传统碎屑锆石数据库解释中存在的问题，然后可供在任何造山带、任何地质时期工作的其他研究人员应用，以最好地利用碎屑矿物测年作为来源指标，作为板块重建的帮助，并更准确地阐明主要地体构造热事件。

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.