Injection gneiss is a type of gneissic rock formation that is characterized by the presence of numerous veins or injections of igneous material, such as granite or diorite, within a pre-existing gneissic rock matrix. It is a metamorphic rock, formed through the process of contact metamorphism, where the igneous melts, intrude into the surrounding gneissic rock, and cool and solidify, creating the distinct veins or injections.
The term "injection" in injection gneiss refers to the intrusion of the igneous melts into the gneissic host rock. These injections can vary in size from thin veins to more massive bodies, and their orientation within the gneiss can greatly influence the rock's overall appearance. Injection gneiss often exhibits a banded or foliated texture due to the parallel alignment of mineral crystals that developed during metamorphism, and these bands may be accentuated by the presence of the injection veins.
Injection gneiss commonly displays a granitic or granodioritic composition, reflecting the composition of the injected magma. The gneissic matrix surrounding the injections is typically composed of alternating layers of light-colored minerals, such as quartz and feldspar, and dark-colored minerals, such as biotite and hornblende.
This type of rock is commonly found in regions with a complex geological history, where multiple episodes of heat and pressure have occurred, leading to metamorphism. Injection gneiss is often valued for its aesthetic appeal, with the contrast between the injected veins and the gneissic host rock creating visually striking patterns and textures.
