Decoupling of metamorphic zircon U-Pb ages and P-T paths in the Dunhuang metamorphic complex, northwestern China

• Published in: Precambrian research 2022-09, Vol.379, p.106783, Article 106783
• Main Authors: Si, Yang, Ge, Rongfeng, Zhou, Teng, Wang, Yue
• Format: Article
• Language: English
• Subjects: Dunhuang Block; Hot orogen; Metamorphic P-T path; Phase equilibrium modeling; Sm-Nd isochron
• ISSN: 0301-9268; 1872-7433
• DOI: 10.1016/j.precamres.2022.106783

•Metamorphic zircon ages and REE data indicate 1.9–1.8 Ga metamorphism.•Garnet, titanite and apatite ages indicate thermal overprinting during 345–236 Ma.•Decoupling of metamorphic zircon U-Pb ages and P-T paths due to thermal overprinting.•The Dunhuang area was like a long-lived hot continental arc during the Paleozoic. U-Pb dating of metamorphic zircon is commonly used to determine the timing of high-grade metamorphism, but linking zircon U-Pb ages to metamorphic P-T paths can be challenging, especially in metamorphic terranes where multiple metamorphic events result in various degrees of re-equilibration and complex metamorphic zircon growth/recrystallization. Here we carried out a detailed P-T path study, as well as zircon/titanite/apatite U-Pb dating and mineral Sm-Nd isochron dating, for garnet amphibolites from the Dunhuang Block, northwestern China, where late Paleoproterozoic metamorphism was extensively overprinted by a Paleozoic tectonothermal event and the P-T conditions of the earlier event remain elusive. Petrographic observation, phase equilibria modeling and geothermobarometry show that the garnet amphibolites share a clockwise P-T path, from 675 – 689 °C/10.0 – 11.6 kbar to 780 – 807 °C/9.9 – 11.8 kbar for the prograde to peak metamorphism, followed by significant cooling and decompression to 612 – 689 °C/4.1 – 6.0 kbar, indicating an orogenic setting. U-Pb dating of metamorphic zircons yields two groups of upper intercept ages of ca. 1.82 – 1.85 and 1.95 Ga. The ca. 1.82 – 1.85 Ga metamorphic zircons have flat HREE patterns and weak Eu* anomalies, suggesting zircon growth when abundant garnet but little plagioclase was present, consistent with high-pressure metamorphism. In contrast, the ca. 1.95 Ga zircons have steep HREE and negative Eu anomalies, suggesting formation at relatively low-pressures. These zircon results provide evidence for 1.8 – 1.9 Ga metamorphism, possibly due to variable crustal thickening during a late Paleoproterozoic orogenic event. However, garnet porphyroblasts and the matrix minerals yield a Sm-Nd isochron age of 333.5 ± 4.0 Ma, whereas metamorphic titanites record a U-Pb age of 345.4 ± 2.7 Ma. Considering the high closure temperatures of the garnet Sm-Nd (650 – 720℃) and titanite U-Pb (710 – 780℃) systems, our data suggest that the late Paleoproterozoic metamorphic mineral assemblages were likely completely re-equilibrated by a Paleozoic tectonothermal event and that the P-T path documented above is a result of