Melt Diversity and Magmatic Evolution in the Dali Picrites, Emeishan Large Igneous Province

Picrites are potentially near-primary melts that offer rare insights into the earliest stages of magmatic evolution. However, without detailed investigation of magmatic processes, robust records of early melt evolution and mantle melting behavior cannot be acquired. Here petrological and geochemical interrogations of the Dali picrites were conducted to investigate the magmatic processes. Compositional variation observed in the Dali picrites is dominated by variable accumulation of a wehrlitic assemblage, transported by a low MgO carrier melt. Groundmass clinopyroxenes were produced by direct crystallization of the carrier melt. In contrast, compositions of olivine-hosted melt inclusions and clinopyroxene macrocrysts indicate that olivine and clinopyroxene macrocrysts crystallized from diverse melt batches that are genetically related to the carrier melt, which formed as a consequence of magma mixing. The compositional range of melts in equilibrium with clinopyroxene macrocrysts with Mg#<88 (>88) is similar to (notably smaller than) the range of melt inclusions hosted in olivine macrocrysts with equivalent forsterite values. This observation can be explained by coupling between major and trace elements during partial melting in the mantle. Overall, a process involving periodic magma replenishment, tapping, and fractional crystallization in deep magma chamber(s) can explain the compositional variations recorded in the Dali macrocryst assemblage. Close genetic relationships between macrocrysts and carrier melts appear to be common to many primitive mafic rocks, which may reflect common deep magma chamber processes whereby diverse mantle-derived melts are injected and evolved, and the primitive macrocrysts thus formed are subsequently transported by mixed liquids, producing macrocryst-bearing primitive mafic rocks.