ABSTRACT

Poly-metamorphism has been observed in many Precambrian metamorphic terranes. The metamorphic overprinting in the poly-metamorphic rocks makes it difficult to determine the timing and P − T path of each metamorphic event. In this study, in situ U–Pb dating of zircon, xenotime, and monazite, with simultaneously determination of trace element compositions, is utilized to provide new constraints on the timing of discrete metamorphic events involving reactivation of the Jiao-Liao-Ji Belt, the eastern edge of the North China Craton. Detrital zircons from metasandstones in the Gaixian Formation record a mixed provenance that includes late Neoarchean (ca. 2532 Ma) and middle Paleoproterozoic (ca. 2181 Ma). Xenotimes from the Gaixian pelitic schists yield an upper intercept age of 1906 ± 25 Ma, and this age is interpreted as the timing of greenschist-facies regional metamorphism. However, monazites from the same pelitic schists yield a distinctly younger age of 242 Ma, with late Paleoproterozoic inherited ages of 1894 and 1809 Ma, which are broadly consistent with the xenotime ages. In addition, magmatic zircons from a granitic mylonite vein yield a similar age of 245 Ma, making it coeval with the monazite. The interpretation of these ages is also reinforced by the mineral assemblage of inclusions in the dated accessory mineral grains. These new geochronological data, combined with published data from the Jiao-Liao-Ji Belt and the related regions, suggest that the Gaixian Formation in the Yanghe area was likely some time deposited at 2181–1900 Ma, and experienced both late Paleoproterozoic and Triassic low-grade metamorphic events related to two distinct orogenic phases in the eastern North China Craton.

摘要

在许多前寒武纪变质地层中已经观察到多变质作用。多变质岩中的变质叠印使得难以确定时机和P-T每个变质事件的路径。在这项研究中，利用锆石，xenotime和独居石的原位U–Pb测年，同时测定痕量元素的组成，对涉及胶辽—冀基带活化的离散变质事件的时间提供了新的限制，华北克拉通的东部边缘。盖县组元砂岩中的碎屑锆石记录了混合的物源，其中包括新新世晚期（约2532 Ma）和中古元古代（约2181 Ma）。盖县片岩片岩的Xenotimes的上交年龄为1906±25 Ma，该年龄被解释为绿片岩相区域变质的时间。但是，来自相同的古生代片岩的独居石的年龄显然要小得多，为242 Ma，晚古元古代的遗传年龄为1894和1809 Ma，这与xenotime时代大致相符。此外，花岗岩花岗石脉中的岩浆锆石产生的年龄约为245 Ma，使其与独居石处于同一时代。对这些年龄的解释也因过时的副矿物颗粒中夹杂物的矿物组合而得到加强。这些新的年代学数据，再加上胶辽－冀基带及相关地区的已发布数据，表明洋河地区的盖县组可能在2181〜1900 Ma沉积了一段时间，并经历了古元古代晚期和三叠纪低潮。华北克拉通东部两个明显的造山相有关的高等级变质事件。对这些年龄的解释也因过时的副矿物颗粒中夹杂物的矿物组合而得到加强。这些新的年代学数据，再加上胶辽－冀基带及相关地区的已发布数据，表明洋河地区的盖县组可能在2181〜1900 Ma沉积了一段时间，并经历了古元古代晚期和三叠纪低潮。华北克拉通东部两个明显的造山相有关的高等级变质事件。对这些年龄的解释也因过时的副矿物颗粒中夹杂物的矿物组合而得到加强。这些新的年代学数据，再加上胶辽－冀基带及相关地区的已发布数据，表明洋河地区的盖县组可能在2181〜1900 Ma沉积了一段时间，并经历了古元古代晚期和三叠纪低潮。华北克拉通东部两个明显的造山相有关的高等级变质事件。