Petrogenesis of Dark Enclaves and Magmatic Processes in the Early Paleozoic Fushui Mafic Complex from the Qinling Orogenic Belt, Central China

Bokang Zhu ^1,2* Xiaoying Liao ^1,2* Yongsheng Gai ^1,2 Liang Liu ^1,2 Ge Wang ^1,2 Sang Wan Pak ^1,2 Wenqiang Yang ^1,2

• ¹ Northwest University, Xi'an, China
• ² State Key Laboratory of Continental Dynamics, Northwest University, Xi'an, China

The enclaves in igneous rocks provide valuable insights into the petrogenetic processes, and the origin and evolution of magmas. The Fushui Complex in the Qinling Orogenic Belt contains many dark enclaves, including hornblendite and dark gabbro. In this study, we investigates these enclaves and the host rocks, aiming to explore their petrogenesis and relationship. On this basis, we further restored the magmatic evolution of the complex. Zircon U-Pb dating show the host rocks are crystallized at 484-492 Ma, and the hornblendite enclaves share relatively consistent crystallized ages around 500 Ma. However, some of the zircons in the hornblendite enclaves record a younger crystallized age of 475 Ma, suggesting a continuous magmatic process. The host rocks and dark gabbro enclaves exhibit arc-like trace-element signatures. In contrast, the hornblendite enclaves exhibit more variable, likely reflecting different degrees of magma mixing. Some hornblendite enclaves share similar geochemical characteristics with the host rocks, while most are enriched in Th and U, depleted in HFSE and Sr, and show slightly enriched in LREE or flat REE distribution patterns. Compared to the host rocks, the hornblendites exhibit more depleted Sr-Nd isotope compositions. Trace element modeling indicates that the Fushui Complex was originated from a metasomatized mantle, influenced by subducted oceanic and continental crust. The hornblendite enclaves, characterized by orthocumulate texture, the earlier crystallization age, are interpreted as the early cumulates formed in a deep magma chamber, and the maximum crystallization temperature and pressure were 871 °C and 13.7 kbar. The dark gabbro enclaves are characterized by fine-grained textures, field occurrence indicative of late-stage crystallization, and geochemical similarities with the host rock. These features suggest that they are the product of the rapid crystallization of the host magma at the edge of the magma chamber. Their crystallization temperature and pressure are 852 °C and 11.9 kbar. Integrating geochronological, geochemical, mineralogical data with previous studies, we identify three magmatic intrusion events and a greenschist-to amphibolite-facies metamorphic overprint event within the Fushui Complex. This study represents the first detailed investigation of dark enclaves in the Fushui Complex and provide new insight into their petrogenesis and the magmatic evolution.