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

在过去的十年中，利用碎屑矿物（特别是锆石）的放射性年龄进行的物源研究得到了极大的发展，这主要是由于微地质年代学方法的进步。正是这些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）使其在碰撞造山作用期间几乎呈惰性，但在上部角闪岩到麻粒岩相和深熔作用中除外。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碎屑岩中通过，而使用常规的碎屑锆石分析方法在碎屑记录中将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的铀-钍-铅年龄的实用性，与仅分析碎屑锆石芯相比，它具有提供更丰富的构造变质事件历史的潜力。此外，假设非常小的碎屑锆石晶体，和薄变质边缘碎屑锆石，提供了一个造山带的年代学观点相比，通过分析较大颗粒的中心部分得到的，有很大的不同正在测试。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，以获得其重矿物套件。之所以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，有必要收集来自一个地区的碎屑数据，其中详细的岩浆，变质和变形的历史已经确立。这将允许建立最强大的协议，以尽量减少年龄偏差的影响，无论是那些自然固有的碎屑矿物，和潜在的实验室诱导的偏见在粮食选择。只有从一个经过充分研究的多变形造山带中获得碎屑年龄信息的结果，才能将其最好地应用于对其他认识较少的造山带的构造研究。这项研究的结果将有重大意义的解释全球碎屑矿物年龄记录。在这里开发的程序和工具，是最成功的解决问题，目前存在的传统碎屑锆石数据库的解释，然后将可用于其他研究人员的应用工作在任何造山带，为任何划分的地质时代，最好地使用碎屑矿物定年作为一个物源指标，作为一个援助，在板块重建，更准确地阐明了主要的构造事件。

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.