8. Grano-diorite

Grano Sign

Grano-diorite with pegmatitic dikes from Garner, NC

Rock type: 
Igneous 

Description: 
A boulder that has two different rock types in it: grano-diorite and pegmatite. The grano-diorite is a coarse-grained rock with gray, pink, black, and white speckles and makes up the majority of the boulder. Cutting through the grano-diorite are thick lines of pegmatite, which have extremely coarse grains in white and pink.

Figure 1: The grano-diorite boulder at Northern Wake Campus, with two prominent pegmatite dikes visible on the front surface.

Figure 1: The grano-diorite boulder at Northern Wake Campus, with two prominent pegmatite dikes visible on the front surface.


Figure 2: Grano-diorite boulder at Southern Wake (Main) Campus.

Figure 2: Grano-diorite boulder at Southern Wake (Main) Campus.


Unique features: 
In the grano-diorite, the visible grains of quartz, feldspar, and other minerals, and in the pegmatite, the large feldspar grains.

Figure 3: The surface of the grano-diorite. Notice the visible crystals and their various colors.

Figure 3: The surface of the grano-diorite with a black ballpoint pen for scale.  Notice the visible crystals and their various colors.


Figure 4: A close up image of the grano-diorite showing the minerals.

Figure 4: A close up image of the grano-diorite showing the minerals. In this image, you can see the minerals feldspar, quartz, hornblende, and amphibole.


 

Figure 5: The surface of the pegmatite.

Figure 5: The surface of the pegmatite. Almost all of the minerals are light colored, so you have to look harder to distinguish minerals from each other. However, the minerals are very large.


Figure 6: An close-up of the pegmatite showing the large sizes of some of these minerals.

Figure 6: An close-up of the pegmatite showing the large sizes of some of these minerals. Black ballpoint pen cap for scale.


How did it form? 
Both grano-diorite and pegmatite are igneous rocks that form when molten magma cools slowly deep within the Earth’s crust. These rocks have slightly different chemical compositions, and one rock is cutting through the other, which suggests that there were several steps involved in forming this rock. 

How would a geologist figure out how it formed using rock characteristics?
Examining the grano-diorite, we notice that it has visible grains and the light colors of the minerals. The large, visible grains would suggest that the rock cooled slowly deep within the crust. Laboratory experiments have shown that the length of cooling of magma determines the size of the resulting mineral grains; the longer the cooling, the larger the minerals are able to grow. The light color of the minerals suggests that the source for the magma that cooled to form this rock was likely derived from melting of continental crust. 

Examining the pegmatite, we notice the extremely large minerals, most of which are over three centimeters long. Minerals this large are unusual, and can only form when there is a long amount of time to allow the minerals to grow and the presence of super-heated water to help organize the elements in the melt into minerals.  

The pegmatite is composed mostly of quartz and feldspar, which is a different chemical composition than the grano-diorite. This suggests that the pegmatite had a different magma source than the grano-diorite. In this case, it may have come from melting of pre-existing continental crust. 

Finally, geologists would notice that the pegmatite cuts through the grano-diorite, which indicates that the grano-diorite formed first, and was likely fully cooled into a rock before the pegmatite intruded, flowing along existing fractures and widening them to create dikes through the grano-diorite.

Figure 7 An image of the grano-diorite with the lighter-colored pegmatite cutting through it

Figure 7 An image of the grano-diorite with the lighter-colored pegmatite cutting through it. The fact that the pegmatite cuts through the grano-diorite suggests that the grano-diorite formed first, and the pegmatite formed later after that grano-diorite was already a rock. 


Other interesting information? 
This rock was quarried from the Garner quarry, which contains an incredible amount of this material. It is thought that the quarry pit has dug just into the top of what is called a “batholith”, a gigantic underground form of course-grained igneous rocks. This particular batholith likely formed when two continents collided, causing friction along their edges. This friction would have generated incredible amounts of heat, causing some of the rocks in the continental crust to melt, and then cool back down to form the grano-diorite. The pegmatite would have intruded later, due to some other plate tectonic event.