The White Tank Mountains: Anatomy of a Metamorphic Core Complex

My last post set the stage for this report, but this time I did a lot of walking to get the facts. A short drive took me to White Tank Mountain Regional Park, about 30 miles west of Phoenix. I studied geology at Arizona State University in Tempe…it must have been 40 years ago, and Metamorphic Core Complexes (MCC) were a big thing then. Let’s start with a map (actually several maps that focus ever closer on the field area).

Figure 1. Overview of the area. The White Tank MCC is located west of Phoenix (lower left inset). The mountain range is dissected in a SW-NE lineation and is steeper on the east side than the west. The lower-right inset shows the two locations where I examined the rocks. Site 1 is more of an overview, and Site 2 is where I looked closely at the mineralogical and textural variations within an intrusive rock body of Cretaceous age.

Now for a view from the ground.

Figure 2. View looking north from Site 2. The mountains in the distance are the Central Highlands, which I discussed in my last post. Many of the same Precambrian rocks are present in the White Tank Mountains, but with a different story to tell.

Site 1.

The White Tank MCC is located within the Basin and Range Province. In the middle of a low-lying flat desert, MCCs appeared within the last 60 my, in close proximity to a region defined by faulting and the uplift of Precambrian rocks on a huge scale. Time to look at some rocks.

Figure 3. A relatively small outcrop at Site 1 (see Fig. 1 for location). The entire road cut was about 200 yards in length.

Figure 3 reveals a complex pattern of deformation and magmatism. The most striking feature of this outcrop is the brilliant white veins that cut across the dark rocks, and folded in a crazy pattern on the right side. What is going on here?

The background is that the dark rocks are Precambrian metamorphic rocks and granites. As I discussed before, these rocks were deformed several times during the billion years spanning the resetting of their radiometric clocks and the tectonics associated with the uplift of the Colorado Plateau. The lighter-colored rocks are of Cretaceous age, injected as veins into preexisting weak fracture zones.

Figure 5. This photo shows the sharp contact between the white intrusion and the dark-colored Precambrian rocks, creating a weak zone that led to the erosional feature seen in the photo.
Figure 6. The dark rock is Precambrian gneiss that has been reheated and deformed ductile. Note the sharp contact with the intrusive granite. The older rock did not melt during intrusion.
Figure 7. Hand sample of the Cretaceous granite, showing alignment of platy minerals. This is a good example of syntectonic intrusion.The magma was intruded while the deeply buried rocks were being deformed.
Figure 8. I’m not certain how to interpret this sample, which is less than a foot in diameter. The best I can do (without getting carried away) is to suggest that none of these rocks were molten except for the white one on the left (the intrusive granite), which may have cooled sufficiently to behave the same as the Precambrian granites it was invading. It really is a remarkable juxtaposition of rock types and mineralogies. Note the block of intrusive granite embedded in a dark matrix of (supposedly) Precambrian gneiss.
Figure 9. This photo says it all. Note the arched intrusive rock in the lower center of the photo. This is an anticline, which results from compressional stress. It has been rotated because there is no source of compressional stress from that angle (about 20 degrees from vertical). Just above it, a similar vein of white rock has been kinked into a “V” pointing to the right. At the top of the photo, there appears to be an “X” which is a common jointing pattern. The only way to reconcile these disparate textures in such a small area (the image is about 4 feet across) identifying the folded veins as part of the Precambrian folding of metamorphic and igneous rocks. The unfolded veins are associated with intrusion of the White Tank granite in the late Cretaceous to Tertiary periods.

SIte 2.

Figure 10. Location map for textures seen along Mesquite Trail. See text for explanation.

Figure 10 reveals some of the field textures seen along the trail, starting at the uphill end, where medium and small blocks litter the landscape as the granite weathers in place (Fig. 11). Panel A (Fig. 10) shows a sharp contact between the main granite and a whiter material with microcrystalline structure similar to what we saw at Site 1 (Fig. 3). As the magma was intruded, the melt was fractionating into a component with a lower melting temperature and so it filled fractures, which indicates there was tectonic movement at that time. Panel B is evidence of syntectonic intrusion because it shows a lineation that was present in many of the rocks. Panel C is a close up (5x magnification) showing larger crystals in a finer matrix. All of it is feldspar (dominated by Na also known as albite). Panel D is a good exposure of the relationship between the finer grained material that forms veins in the main rock. Panel E shows a salt-and-pepper texture that dominates the rocks along the trail.

Figure 11. Hill side covered with large blocks

A close up of a fresh surface near Sample D (see Fig. 10 for location) shows simple mineralogy of the main rock body (Fig. 12). I would estimate 70-80% albite, >15% quartz, and minor biotite (a platy mineral, a variety of mica).

Figure 12. Close up (5x magnification) of granite, showing the dominance of feldspar.

The mineralogical composition can be used to classify this granite as tonalite. Tonalite is a granite that contains no potassium feldspar (no pink color), very little quartz, and mostly feldspar containing sodium and calcium. These rocks originate deep in the earth where ocean crust (basalt) melts and rises, losing much of the iron and magnesium it originally contained. The tectonic setting is an island arc, like Japan.

Figure 13. Close up (5x magnification) showing weathering of the granite in place. The larger crystals of feldspar and the quartz are more resistant, producing a gravel product with lots of fine grained material (mud).


Just as in the Central Highlands, sediments were deposited here in ocean basins as long ago as 2.8 by and subsequently buried and deformed, changing through heat and pressure into gniess, and injected with veins of quartz and feldspar during metamorphism. Several orogenic events followed, deforming the assemblage further. It was exhumed slowly over the ensuing billion years, and eventually injected with a tonalite granite created deep beneath an island arc. This was the time when the White Tank granite was emplaced. This magma was intruded when the region was undergoing extensional stress (pulling apart) as part of the adjustment to complex tectonic process associated with uplift of the Colorado Plateau.

Rather than breaking into irregular blocks as in the Central Highlands, the White Tank mountains (and other MCCs in the western Cordillera), were formed by the uplift of Precambrian basement along deep sub-horizontal surfaces called detachment faults. In other words, the crust stretched in the Basin and Range rather than breaking into fragments.

I haven’t attempted to describe the complex tectonics of central Arizona, a task that is well beyond my experience. This is a topic that is hotly debated in the geological community to this day. This has only been a brief effort to relate the rocks I saw with my own eyes to what is known about the history of the earth.

Always listen to the rocks…

Trackbacks / Pingbacks

  1. A Typical Basin and Range | Timothy R. Keen - January 22, 2022

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: