Objective The Jiadan guyot, an important part of the Mid−Pacific Mountains, hosts basalts containing well−preserved clinopyroxene phenocrysts that offer critical constraints on the magmatic evolution of oceanic island basalts in this region.

Methods Based on detailed petrographic observations and in situ mineral chemical analyses, we classify the clinopyroxene phenocrysts into four distinct populations according to their textural features and zoning patterns.

Results Type Ⅰ phenocrysts exhibit normal zoning, with rounded, irregularly resorbed high−Mg^# cores surrounded by clear, low−Mg^# mantles and rims. Types Ⅱ and Ⅲ also show normal zoning, characterized by relatively high−Mg^# cores and thin, lower−Mg^# rims, whereas Type Ⅳ crystals are euhedral and unzoned. Crystallization conditions estimated from high−Mg^# cores of Types Ⅰ and Ⅲ yield the highest temperatures and pressures (~1170 ℃ and ~3.50 kbar), whereas lower values (~1070 ℃ and ~0.72 kbar) are recorded in the mantles of Type I, the cores of Types Ⅱ–Ⅲ with lower Mg^#, and the Type Ⅳ phenocrysts, with further declines toward the rims. Compositionally, Mg^# and Cr decrease from core to rim, whereas Al₂O₃, TiO₂, and ΣREE contents increase. Notably, all textural domains (core, mantle, and rim) across the different types share similar chondrite−normalized REE patterns.

Conclusions These systematic variations indicate that the Jiadan basalts are differentiation products derived from a single mantle−derived parental magma, with fractional crystallization proceeding under progressively decreasing temperature and pressure during magma ascent.