SHRIMP zircon U–Pb geochronology, elemental, and Nd isotopic geochemistry of the Neoproterozoic mafic dykes in the Yanbian area, SW China

Abstract

Sensitive High-Resolution Ion Microprobe (SHRIMP) U–Pb zircon ages, geochemical and Nd isotopic data are reported for two groups of Neoproterozoic mafic dykes in the Yanbian area of Sichuan Province, SW China. The Group 1 and Group 2 dykes were dated at 792 ± 13 Ma and 761 ± 14 Ma, respectively. While Group 1 and Group 2 rocks are commonly high in Fe 2O 3 and TiO 2 contents with similar MgO contents and Mg numbers, they exhibit distinct trace element features. Group 1 rocks with high ɛ Nd( T) values (+5.8 to +7.2) and total rare earth element (REE) contents (109–171 ppm), are characterized by LREE-enriched and “humped” trace-element patterns with slightly negative Nb–Ta anomaly, similar to typical alkaline basalts associated with continental rifts. Group 2 rocks have relatively low total REE contents (50.4–103 ppm) with relatively flat REE patterns and can be further divided into two sub-types. Group 2a rocks show MORB-like trace element patterns depleted in highly incompatible elements such as Th, Nb, Ta, La and Ce, whereas the Group 2b rocks exhibit flatter REE patterns than those of the Group 2a. The ɛ Nd( T) values of the Group 2a and Group 2b rocks are +5.4 to +7.2 and +4.5 to +5.3, respectively. The Group 1 and Group 2 dykes display different magma crystallization trends. Group 1 rocks were crystallized from a high-Ti alkaline basaltic magma that was generated by low degrees of partial melting of OIB-like mantle source in the garnet-bearing field. In contrast, the parental magma of Group 2 rocks is similar to those parental to low-Ti tholeiitic basaltic melt generated by relatively higher degrees of partial melting of the asthenospheric mantle source in the spinel-stable field. Group 1 and Group 2a rocks show the intraplate basalts affinity. It is therefore suggested that they originated from an intra-continental rift regime, which was likely related to a mantle plume underneath the supercontinent Rodinia.