The discovery of orbicular granodiorite and its petrographic characteristics in Huangling, Hubei

The orbicular granodiorite outcropped between Neoproterozoic phyric granodiorite and quartz diorite in an extremely small place of Huangling area, the Yangtze continental nucleus, was discovered for the first time. This kind of orbicular granodiorite is composed of host rock, orbs and matrix among orbs. Though the orbs are round, oval or irregular in shape, they are all composed of nuclear bulges and concentric shells. Based on the discrepancies of their nuclear bulges and spherical shells, the authors divide the orbs into three types, i.e., orbs with multiple shell layers and concentric structure, orbs with single shell layer structure, and orbs with no obvious structure of shell layers. Composed of dark layers with dark minerals like amphibole and biotite, the single shell layer exhibits radial structure. However, the shell of multiple shell layers is composed of rhythmites comprising layers of dark minerals like amphibole and biotite as well as light-color layers comprising dominantly plagilclase with no or very few dark minerals, exhibiting comb or radial structure. The composition of nuclear bulge might have come from comagma or heterogeneous xenolith, amphibolite and enclaves rich in mica. In the orbs with single shell layer, the acidity increases constantly from nuclear bulge through shell to matrix, and the inner part grows in radial patterns, indicating the growth of magmatic crystallization from the inner part to the outer part. As for the rhythmites in the orbs with multiple shell layers, the authors preliminarily hold that they might have resulted from the crystallization of magma rich in hydrothermal solution in a turbulent environment, the minerals in the shell might have crystallized from the inner part to the outer part, and the crystallization of nuclear bulges lasted relatively longer time.