The role of magma mixing in generating the Eocene potassic quartz syenite and mafic microgranular enclaves in the Ailao Shan–Red River shear zone, SW China: Insights from in situ Sr isotopes of plagioclase

Abstract

Understanding the origins of potassic rocks and mafic microgranular enclaves (MMEs) can provide important information for revealing their petrogenesis and regional tectonic evolution. We studied the Shilicun quartz syenite and MME and their plagioclases to elucidate their origins and investigate crust–mantle interactions in the southern Ailao Shan–Red River (ASRR) shear zone. Zircon U–Pb ages indicate the coeval formation of the quartz syenite and MME during the Eocene (approximately 35 Ma). The MMEs exhibit spheroidal shapes and contain acicular apatites, indicating that they represent globules of the coeval mafic melt injected into and partly assimilated by the felsic magmas. They are characterized by high contents of MgO (2.9–5.1 wt%), Ni (90–172 ppm), and Cr (105–180 ppm), as well as high Ba–Sr contents. The MME samples yield ε_Hf(t) values of –2.2 to + 0.5 and ε_Nd(t) values of –5.7 to –6.4, with high (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.70734–0.70758. These features suggest that they were derived from metasomatized enriched lithospheric mantle. The quartz syenites are characterized by high SiO₂ (68.5–69.0 wt%) and Al₂O₃ (15.6–16.2 wt%) contents, Sr/Y ratios (40.6–73.3), and low MgO, Cr, and Ni contents, as well as low Mg# values. They have a low (⁸⁷Sr/⁸⁶Sr)_i ratio of 0.70716 and an ε_Nd(t) value of –4.5. Type I plagioclase from the quartz syenite exhibits reverse zoning and shows sieve textures and resorption surfaces in the core, indicative of rapid cooling and disequilibrium mixing between mafic and felsic magmas. It has significantly different (⁸⁷Sr/⁸⁶Sr)_i ratios (0.70666–0.70726) among the core (0.70666–0.70674), mantle (0.70692–0.70726), and rim (0.70714–0.70715), with the An content increasing from the core (An_10–13) to the mantle (An_14–15) and then decreasing to the rim (An_9–10). Type II plagioclase exhibits a normal zoning texture, with the An content decreasing from the core (An_13–15) to the rim (An_12–13). It has a small variation in (⁸⁷Sr/⁸⁶Sr)_i ratios from 0.70708 in the core to 0.70722 at the rim. Type III plagioclase from the MME is normally zoned and has high (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.70719–0.70742, with the An content decreasing from the core (An₁₇) to the rim (An₈). The large variations in the Sr isotopes of different types of plagioclases record magma mixing processes. Therefore, we suggest that the quartz syenites were formed by partial melting of thickened lower crust, with the involvement of mantle-derived melts that are characterized by high (⁸⁷Sr/⁸⁶Sr)_i ratios. We propose that partial removal of the lower continental lithospheric mantle induced asthenosphere upwelling, which triggered partial melting of the residual metasomatized lithospheric mantle. These mantle-derived magmas provide heat and material for the partial melting of thickened lower crust and ultimately generate diverse potassic rocks along the ASRR shear zone.