STRUCTURAL PETROLOGY OF PLANES OF LIQUID INCLUSIONS by ORVILLE

Planes of liquid inclusions are common in the quartz of igneous and metamorphic rocks. The planes have a high degree of preferred orientation, and it is believed that they represent fractures in the quartz grains that have been filled with intergranular liquid which subsequently changed to individual inclusions by differential solution and deposition of silica. The planes of inclusions commonly extend through several quartz grains of different orientation with no deflection at the grain boundaries, indicating that there is little or no crystallographic control of their orientation. The planes of inclusions have been found to have a remarkably unifom orientation over an area of 15 by 20 miles. They strike northwest and dip steeply to the northeast through the area, whereasthe primary structures, contacts and foliations strike north-south and dip westward, indicating that the planes of liquid inclusions have been superimposed on the primary structure. Detailed work on drag-folds indicates that the planes of inclusions may develop in the shear-directions of the deforming stress-pattern and that one set tends to predominate. The poles to the planes tend to lie in a partial girdle about the fold-axis. Some evidence was found indicating that the planes may develop in the tensional direction. Quartz, then, may fail by shear or tensional rupture and in certain oases by deformation which produces lamellae that may be a type of translation-gliding. Failure by rupture along crystallographic directions has been described by one investigator, and has been produced experimentally in the laboratory. These different phenomena are believed to represent failure under varying environmental conditions and the optimum development of any one type of failure depends on such factors as confining pressure, temperature, composition, and the presence (and character) of solutions. If several sets of planes of liquid inclusions develop at different times, the relative ages of the planes can be discerned from the character of the planes. Planes of inclusions that are very "young" appear as extremely thin sheets of liquid, whereas "old" planes are made up of inclusions that are nearly equant, with some of the inclusions having crystal planes as boundaries (negative crystals).