The Xishan fayalite-bearing subvolcanic rocks originated from the mixing of two batches of crustal magmas.

Two populations of zircon U-Pb ages and initial Hf isotope compositions distinguish these two magma pulses.

Non-equilibrium partitioning of oxygen isotopes had happened among zircon, fayalite, and ferrosilite.

All the fayalite-bearing felsic rocks worldwide formed under high temperature, low-moderate water, and low oxygen fugacity.

Aforementioned restricted physical conditions can explain the limited volumes of such rocks in nature.