Timing of Paleozoic Metamorphism in the Jiaobei Terrane, East China: Evidence from Apatite U-Pb Age and Trace Element Composition

Abstract

The northward subduction of the Paleo-Tethys oceanic crust in the Paleozoic to Mesozoic is critical for the tectonic evolution of the Qinling-Tongbai-Hong’an-Dabie-Sulu-Imjingang-Gyeonggi orogenic belt. However, the Paleozoic geological fingerprint of the Paleo-Tethys oceanic crust subduction in the Dabie-Sulu orogenic belt remains obscure. In the present study, apatite from the Muniushan monzogranite in the Houkuang area was analyzed to constrain the age of metamorphism in the Jiaobei Terrane and is regarded as the response to the Paleo-Tethys oceanic crust subduction in the Early Permian. Muniushan apatite with obvious negative Eu anomaly is enriched in LREE and depleted in HREE. The chondrite-normalized REE patterns of apatite correspond with I-type granitoids and mafic igneous rocks, implying a magmatic origin. Igneous apatite grains have reset compositional zonation in the cathodoluminescence image. Apatites plotted on a support vector machine apatite classification biplot and Eu/Y-Ce discrimination diagram shows a tendency from the region of “mafic igneous rocks and I-type granitoids” to “low and middle metamorphic”. This evidence consistently suggests that the Muniushan apatite suffered metamorphism at a later stage. The twenty-six apatite grains from the Muniushan monzogranite yield a metamorphic age of 297 ± 8 Ma (by LA-ICP-MS U-Pb), which is different from the Muniushan zircon SHRIMP U-Pb results of 2110 ± 4 Ma, indicating the metamorphism occurred in the Early Permian and reset the U-Pb system of apatite. The Early Permian metamorphism that occurred in the Jiaobei Terrane is synchronous to the subduction of the Paleo-Tethys oceanic crust and is the response to the Paleo-Tethys oceanic crust subduction.