Hornblende and Plagioclase Micro-Texture and Compositions: Evidence for Magma Mixing in High-Mg Adakitic Pluton, North China Craton

In this study, we performed microtextural, major/minor element, and Sr-isotope analyses on hornblende and plagioclase (as phenocrysts, groundmass, or inclusions) from the Early Cretaceous Jiagou pluton (eastern North China Craton), to elucidate the magma source, possible magma mixing process, and the transition from low-Mg to high-Mg adakitic magmas. Petrographic study and electron microprobe (EMP) analyses reveal well-defined compositional zoning in hornblende and plagioclase phenocrysts. Outward from the core (first zone), the second and third zones show pronounced oscillatory zoning and significant variations in Mg# and An%, while the fourth zone is relatively homogeneous. A corroded albitic plagioclase core with sieve texture is enclosed in the first zone and locally intergrows with worm-like quartz streaks and fine hornblende inclusions, featuring Mg# = 81 (core) and 62 (rim). The new plagioclase infill has An% = 14–41. The corroded plagioclase has an initial ⁸⁷Sr/⁸⁶Sr = 0.7074, while that of zoned phenocrystic plagioclase ranges from 0.7068 to 0.7079, suggesting EMI and EMII mantle input. Inclusion hornblende is low in Ti and Cr, while phenocrystic hornblende shows higher Cr in the first zone and lower Cr in the outer zones. The newly discovered mafic microgranular enclaves (MMEs) and regional geochemical data suggest three major magma mixing events. The felsic parental magma was likely originated from a mixed EMI–EMII mantle source before mixing with a mafic magma derived from the partial melting of, successively, a low-Cr and a high-Cr peridotite. Our findings support a petrogenetic model of lower crustal delamination and highlight the critical role of repeated mafic injections in generating high-Mg adakitic magmas.