Abstract:

It is very important for understanding the igneous processes to discover that several crystal populations can be found in an igneous sample. The discovery makes genetic mineralogy again key to revealing the history of a magmatic system, however, this importance is not reflected in the literature. A typical example is the debate on the origin of cumulates in so-called mafic layered intrusions that wether the cumulated minerals are antecrysts or liquidus of the parent magma. In this contribution, we attempt to clarify the origin of cumulate formation in mafic layered intrusions in the Pan-Xi region, Sichuan Province, and to emphasize the importance of genetic mineralogy. Microscopic observations suggest that clinopyroxene cumulate is rich in Fe-Ti oxide exsolution lamellaes (lamellae-bearing pyroxene), indicating the magmatic condition of crystallizing lamellae-bearing pyroxene is obviously different from crystallizing clinopyroxene (lamellae-free pyroxene), which is co-crystallized with plagioclase. The olivine grains included in lamellae-free pyroxene and plagioclase are neaer-rounded, which suggests thermodynamic disequilibrium between olivine and melt. The Fe-Mg partitioning relation between olivine and melt reveals that liquidus olivine Fo value inferred from the parental magma is much lower than the upper limit of the composition range (Fo₆₁-Fo₈₁) of olivine in cumulate. In other words, the observed olivine grains, partly at least, are not of the liquidus phase of the host intrusion. Correlations between Mg^#[Mg/(Mg+Fe)] and minor elements (especially Ni) also suggest multiple olivine crystal populations formed under different thermodynamic conditions. Crystal settling analysis indicates that crystals precipitating from the host magma may not experience rapid gradational settling and cumulate. All the evidences show that the cumulate-forming mineral crystals are mainly arrived from different chambers at depths of the magmatic system, and they are of antecryst and transferred to the terminal magma chamber. Consequently, the cumulated minerals have larger initial grain radius, which is helpful for their rapid settling to the bottom of the intrusion at an earlier stage to form cumulate.

Key words: cumulate, olivine, antecryst, genetic mineralogy, Pan-Xi region