The discovery of orbicular acid hypabyssal rocks in Wulagaisumu area of Inner Mongolia and their petrographic characteristics

The orbicular acid hypabyssal rocks outcropped in Ulgai area of Da Hinggan Mountains were discovered for the first time. They are composed of host rock (micro fine-grained graphic granite), orbicular rocks and matrix among orbicular rocks (alkaline rhyolite porphyry and micro fine-grained granite). Based on the differences between orbicular rocks'concentric shells and nuclear bulges, the authors divided the orbicular rocks into three types, i.e., spherules with non-shell layer, orbicular rocks with single shell layer, and orbicular rocks with multiple shell layers. Spherules with non-shell layer and non nuclear, whose diameters are between 0.5mm and 2mm, are distributed in alkaline rhyolite porphyry and micro fine-grained granite and are composed of radial felsic fiber, iron fiber and needle columnar alkali amphibole in alternate distribution. Orbicular rocks with single shell layer, whose diameters are between 0.5cm and 2cm, are distributed in alkaline rhyolite porphyry. With thin single shell layer presented in the form of long and needle columnar alkali amphibole around orbicular rocks'tangential direction, the orbicular rocks, composed of multiple spherules, exhibit radial and sector structure. However, orbicular rocks with multiple shell layers, whose diameters are between 2cm and 8cm, are distributed in alkaline rhyolite porphyry and can be divided into inner, middle and outer layer. The inner layer, composed of felsic and dark mineral cryptocrystalline aggregates and amphibole, has obvious layering structure. The middle layer is composed of a large quantity of stomatal amygdales in imbricate and raindrop forms, with felsic cryptocrystalline aggregates between them. The outer layer is composed of felsic cryptocrystalline aggregates in the cloud-like, cotton wool and radial forms. According to these petrographic characteristics, the authors preliminarily hold that the orbicular acid hypabyssal rocks were formed by magmatic crystallization in an extensional environment, the gas-rich hydrothermal fluid activity was strong at the late stage, and the minerals in orbicular rocks might have crystallized from the inner part to the outer part.