Chemical features of orthopyroxene in peraluminous igneous rocks

Chemical compositions of orthopyroxene from peraluminous igneous rocks (PAI orthopyroxene) are compared with those from metalumimous igneous rocks (MAI orthopyroxene). Most of the PAI and MAI orthopyroxene coexists with garnet or Ca-rich clinopyroxene, respectively. PAI orthopyroxene samples are richer in Al₂O₃ and poorer in CaO than MAI orthopyroxene samples, and these two types of orthopyroxene are clearly separated in an Al-Ca plot. PAI orthopyroxene has Al > 2Ca and MAI orthopyroxene Al < 2Ca; that is, PAI orthopyroxene is corundum normative and MAI orthopyroxene is diopside normative. These compositional differences are understood by the position of orthopyroxene in the ACF diagram, where orthopyroxene is the only ferromagnesian mineral straddling the boundary between the corundumand diopside-normative fields. Compositional differences between PAI and MAI orthopyroxene samples reflect the magmas from which they crystallized.