Geochronology and geochemistry characteristics of Neoproterozoic gabbros in Western Yangtze Block and its implication for an injection of mafic magma into the active continental margin

Preserved numerous intermediate to acid igneous rocks with minor basic to ultrabasic rocks, the Western Yangtze Block (WYB) is the window to see the evolutional process of South China Craton during Precambrian era. Here, we report geochronology and geochemistry data from gabbros in the south of Dechang County in SW Sichuan Province. Field investigation and zircon U–Pb age from a gabbro sample suggest that these mafic rocks were emplaced during the early Neoproterozoic era (831.8 ± 2.6 Ma). These gabbros show low contents of SiO2, K2O, and Na2O. They are enriched in Th, and LREEs but depleted in Nb, Ta, Zr, and Ti, showing right-inclined primitive mantle-normalized Rare Earth Elements (REE) patterns with negative to positive weak Eu anomalies. Trace-elemental geochemical features, multiple linear regression, and ratio-ratio plots point to the idea that the primary magma of these gabbros originated from the partial melting of a mantle metasomatized by subduction fluid and contaminated by subducted sediments. The geotectonic discrimination diagrams suggest that these Neoproterozoic gabbros formed at an island arc above a subduction zone in the active continental margin. In addition, we found that the arc-related ~ 830 Ma gabbros in WYB could be divided into two groups. The tholeiite Guandaoshan gabbros showing a flat REE pattern is chemically discerned from the calc-alkaline Dechang and Shimian gabbros having right-inclined REE patterns. We, therefore, thought that a slab window beneath WYB caused Guandaoshan gabbros before it migrated northward to form the Mianning and Shimian A-type granites later.