CONGRATULATIONS TO GRASSHOPPERS&ANT FOR FTF!
Requirements
To earn credit for “finding” this Earth Cache (EC), hike/navigate to Ground Zero by parking at the recommended parking spot along FR333 and hiking the Granite Trail and a portion of the Juan Tabo Canyon Trail. See Figure 1 for an All-trails version of this route. The Granite and Juan Tabo Trails are the only “official” trails in the Rincon Hills area recognized by the National Forest Service, so the expectation is that you follow these trails for at least either the “out” or “back” leg of your trip. Note that there are several trails heading north for a portion of the distance, but the Granite Trail is the one that follows a nice scenic ridge line, and not one of the trails that follow drainages on either side.
If you follow the recommended hiking route, you will be traveling through “Mazatza-landia” for the entire trip. The initial descent into Juan Tabo Canyon along the Granite Trail going north from Scenic Point 2 has pretty spectacular views of Juan Tabo Canyon, Rincon Ridge and “Mazatza-landia”. Note that there is a steep stretch of trail along the final descent north of Scenic Point 3 into the Canyon, so the trail difficulty has been raised a bit for that reason.
Answer a few simple questions at a couple of Scenic view waypoints along the way, and then answer questions posed at Ground Zero, based on reading the Description and making your own Observations. One picture needs to be taken. Submit your answers and picture to the CO via the geocaching.com website.
Be careful in summer heat, as there is little shade along this route, although some can be found in Juan Tabo Canyon. The hike is a little over 1 mile one-way (taking the Granite Trail). It’s probably best to take your hike on a sunny day.
Background
Around Albuquerque, the Sandia Mountains and Sandia Granite get all the “glory”, but what about the Rincon and the metamorphic rocks there? Here “Rincon” refers to Rincon Ridge, which includes prominent Juan Tabo Peak, and the Rincon “Hills” situated between the southern part of the Ridge and FR333. The Sandia Granite crystalized into solid rock during the MESOproterozoic (~1.45 billions of years ago (Ga)); but the Rincon metamorphic rocks, which are much older, were formed during the PALEOproterozoic (~1.65 Ga). The granite and the metamorphics are very different types of rocks; however, their origins are underpinned by the same central phenomenon of Plate Tectonics.
The metamorphic rocks in the Rincon are, at the risk of oversimplification, characterized by fabrics with locally pervasive near-alignment of sparkly grains – as in schists – and by wavy and convoluted banding of light and dark minerals – as in gneisses – and by variations in between. Metamorphic rocks are formed when pre-existing rocks of any type are subjected to elevated heat and pressure without melting. These forces cause new minerals to form at the expense of some of the pre-existing ones, and new fabrics to be imparted into the rocks. Sophisticated geological analyses of the metamorphic rocks tell a story of a “terrane” [sic], or province, of which the Rincon is a local surface exposure, that was subjected to widespread regional metamorphism prior to emplacement of the Sandia Granite. But what caused all this metamorphism in the first place?
The fundamental answer to that question is that successive island arc and micro-continents were at one time colliding in a northwestward direction (using today’s geographic orientation) with a pre-existing continental landmass called Laurentia. The metamorphic rocks in the Rincon, as well as other metamorphic rocks in the Albuquerque region, are surface-exposed vestiges of a continent-wide metamorphic province called the “Mazatzal” – named after the southern Arizona Mazatzal (“Land of the Deer”) Mountains, which are the “type” locale for rocks originating during this period of tectonism.
Figure 2 from Blakey and Ranney (2018) shows a map of the North American craton by about 1 Ga. This is essentially a map of Laurentia at its greatest extent. The land area by ~1 Ga included the original Laurentia from ~2 Ga (shades of brown and tan), plus a series of new provinces that were accreted in succession from the southeast onto the continent later (shades of green). The Mazatzal is one of the younger “green” provinces. Note where New Mexico sits, and how far the Mazatzal province extends (all the way to the Canadian Atlantic seaboard!).
Figure 3 also from Blakey and Ranney (2018) shows a more artistic rendering of the paleogeography during the Paleoproterozoic ~1.68 Ga Mazatzal orogeny. The Canadian province and U.S. state borders are difficult to make out, so I highlighted approximately the New Mexico border (in yellow). This reference book is full of these kinds of maps from the Proterozoic through to the Present. They are very helpful for allowing the mind to better imagine how land surfaces and ocean extents may have appeared in the past, in this case the very distant past. Note the “Mazatzal Mountains” stretching through what is present-day New Mexico, and the “Mazatzal Arcs – Microcontinents” offshore to the southeast.
Figure 4 from Baer (2004) is a generalized geologic map that zooms into the Sandias (including the Rincon), the Manzanitas and Manzanos, and even the Los Pinos mountains to the south. The colors represent different zones and/or rock assemblages. Notice that the Sandia Pluton is labeled by the authors, and is the large red area at the northern end of the colored area. Can you find the Rincon? It is the blue color at the very northwestern end of the colored area. To make sure you can find where you are, I have labled (in red) the 4 main mountain ranges, and also added labels pointing to the Sandia Granite and the Rincon. This map shows (in the various colors) only those rocks that were formed during the Proterozoic. You may wonder at first why there are so few of these ancient rocks, considering the Mazatzal province existed in a wide swath across New Mexico, as was shown in prior figures. But that was a long time ago, and a lot has happened geologically speaking since to either hide/bury or erode away those rocks.
During the Mazatzal Orogeny of plate collisions, crustal squeezing and thickening caused large mountains to form and intermediate-depth rocks to become even more deeply buried and metamorphosed from heat and pressure. Figure 5 from Karlstrom (2016) shows a couple crustal-scale schematic cross sections that further help us to visualize what academics are interpreting was happening. The upper part of the figure (“A”) shows a proposed configuration during the Mazatzal orogen time frame, ~1.65 Ga. A key aspect to notice is that there are pieces of slab descending to the north below continental crust. Some slabs are shown as dipping along a low-angle, others more steeply: there are reasons why variations in slab dip have been conjectured. During this time period crustal slabs, such as island arcs, from the south (right) were moving northward (left) causing all sorts of “mayhem” along the collision zone, including subduction of oceanic igneous crust and trench sediments, intrusion of large magma bodies, and annexation of continental-type crust. The lower part of Figure 5 shows a later time period, ~1.4 Ga, and shows, among other things, the addition of more plutons of molten rock being injected into the shallow crustal levels: this would be the time frame of the formation of the Sandia Pluton and granite. I have labeled (in red) “Sandia Granite Pluton” along with an arrow that points to one of the plutons that schematically would represent the Sandia. Decades of field work, laboratory analysis, interpretation and re-interpretation by academics have contributed to the evolution in understanding of the various provinces being pieced together by plate tectonics.
The story of the Rincon metamorphics would not be complete without mentioning that when the Sandia Granite intruded the pre-existing metamorphics, a phenomenon called “contact metamorphism” took place. First, the pre-existing metamorphic rocks were metamorphosed over a huge region by “regional” metamorphism, as explained previously. Next, due to “contact” metamorphism, changes in mineralogy occured more locally due to close proximity to the hot magma that would form the Sandia Granite, and this effect has indeed been documented in the Rincon.
Since the Proterozoic, there have been subsequent periods of tectonic-related plate collision and plate rifting events that have affected the evolving North American continental craton. All this subsequent history is beyond the scope of the EC, of course. What is important to note, is that in general, erosion of mountains on an epic scale will help to eventually cause deep crustal roots along convergent plate boundaries to “rebound” with the removal of all the overlying weight. Hence, rocks that were previously deeply buried and metamorphosed will become exposed near or at the surface. Some of these rocks will have not been “chewed up” or altered further by subsequent tectonic events, and some of these rocks themselves will have not been eroded away. The result is that we can see and walk on some of the ancient rocks today, such as in the Rincon. So, the next time you hike the Rincon Hills, pause and think upon the grandiose plate tectonic happenings going on that created these rocks over a billion years ago!
The Rocks - Granite
it’s probably a good idea to have the Sandia Granite in mind when looking at the Rincon metamorphics to better appreciate how they differ. This EC presumes to some extent that many geocachers interested in this EC may be at least partly familiar with the ubiquitous Sandia Granite that many trails in the Sandias cross. Occasional visitors to the area may at least in part be familiar with granite rocks in general. Regardless, some pictures are provided in Figure 6. The perimeter of the recommended parking lot is also lined with large Sandia Granite boulders. Keep the appearance of the Sandia Granite in mind as you encounter the metamorphics along your hike.
The Geologic map by Connell (2006) describes the Sandia Granite as “Mesoproterozoic – Pink megacrystic (large crystal) biotite monzogranite and granodiorite; intrudes older granite and schist…”. The terms “monzogranite” and “granodiorite” mean that the amount of quartz as a percent of the total mineral makeup of the Sandia Granite, especially with regard to certain feldspars, is not as much as in a “true” granite. However, the term “granite” has been used colloquially for a long time when referring to the Sandia Granite.
The Rocks – Rincon Metamorphics
The Geologic map by Connell (2006) describes the metamorphic rocks of the Rincon as “Schist, undivided – Pelitic schist and phyllite; locally quartz bearing; commonly crenulated; zones of phyllite-to andalusite-bearing schist, to biotite and sillimanite schist characterize metamorphic aureole with Sandia granite…”. One thing to note is that this description appears to be rather more complicated than that for the Sandia Granite. Before looking at pictures, some descriptions and definitions follow here (note the underlined phrases are particularly relevant for this EC, and may help you think about what you are seeing in the field an how to answer some of the questions).
A pelite is a metamorphosed fine-grained sedimentary rock. “Crenulation” is a tight zig-zag-type pattern, or foliation, often found in schists and phyllites.
Phyllites are also fine-grained metamorphosed sedimentary rocks, similar to slates, but they tend to break apart into slabs (as do schists often) rather than into thinner sheets like slate does (slate being a higher-grade metamorphic rock).
Schists are metamorphic rocks that have a flaky and foliated wrinkled or wavy texture caused by a pervasive rough parallel alignment of flaky or platy minerals, such as micas. They can break apart into slab-like pieces, but not into thinner sheets.
Berkley and Callender (1979) further describe the Rincon metamorphic rocks as follows: “Pelitic phyllite, schist and gneiss” among other details. Also, “These units generally have a northeasterly strike (trend) and an easterly dip (tilt). Phyllite and gneiss are more abundant in the north, whereas schist and schistose gneiss predominate to the south”.
Something to note in this description is that these rocks also include gneisses, and even “schistose gneisses”, and that schist and schistose gneiss predominate to the south, which is where this EC is located. Generally, the metamorphics of the Rincon have been interpreted as having originally been mostly sedimentary rocks that may have been up to 2,000 meters thick.
Gneisses are metamorphic rocks that show distinct bands of minerals (often of alternating light and dark colors), and although they can have well-developed foliation (alignment of minerals), they don’t have a tendency to split along rough planes like schists do.
So what are Schistose Gneisses? Sometimes there is a “continuum” in mineral assemblages, textures, banding and colors grading between schists and gneisses. It’s not always clear to the non-academic when a rock should be called a gneiss or a schist. This appears to be true in the Rincon, at least to me. There are certainly rocks that can be reasonably classified as gneisses, and others that can be reasonably classified as schists, but others simply appear to warrant being classified as schistose gneisses. Since I found this classification in the literature applied to rocks in the Rincon, I’m happy to use “schistose gneiss” as a “catch-all” when in doubt, and you can too(!).
Many different assemblages of metamorphic minerals (some of which are also igneous minerals) are found in the Rincon metamorphics. These include chlorite, muscovite, biotite, andalusite, sillimanite, microcline, diopside, and others. The presence of certain combinations of these minerals are indicative of the combination of heat and pressure to which the rocks were subjected. Publications show NNE-trending parallel zones of these different assemblages; the NNE orientation is significant, as it has to do with the general WNW direction of plate collision during the Mazatzal orogeny. But diving further into this level of detail is beyond the scope of this EC.
The Rocks – Felsic Dikes
Various pegmatite, aplite and quartz veins, dikes, sills and lenses intrude the Rincon metamorphics. These are all “sialic”, generally meaning rich in quartz and light-colored feldspars. Pegmatites are coarse-grained, while aplites are uniformly fine-to-medium grained and appear “sugary” in texture. These sialic intrusions date from around or a bit after the intrusion of the Sandia Granite, and are generally similar in composition, but tend to lack abundant dark minerals that can be observed in the Sandia Granite in places. See “Yikes! White Dikes!” (GC9PY4C) for more details.
Figures 7, 8 & 9 show some pictures of gneiss, schist and schistose gneiss taken mostly in the Rincon, but some were taken in the Manzanos too for comparison. For purposes of this EC, let’s not worry about trying to distinguishing between phyllites and schists, and let’s not worry about other types of metamorphic rocks that are abundant in “Mazatza-landia”, such as amphibolites and quartzites, which tend to be scarce in the Rincon – these rocks can perhaps be the subject of other EC’s, some day. So, these pictures are not “textbook” examples, because they were taken by me in the field here. Sometimes lichens (various shades of green, black and even orange), weathering effects, and iron oxidation staining (orange-to-reddish), calcite staining (white-ish) and desert varnish (black manganese oxide) hide the true beauty of fresh rock faces. Some pictures were also taken at spots not on the Granite and Juan Tabo trails, so you may or may not see the same sorts of examples on your hike to the EC.
Further directions and questions
As you hike north along the Granite Trail, take mental notes of the types of rock you encounter along the way. Answer some questions at two of the Scenic waypoints, and then answer the bulk of questions at Ground Zero. GZ is located at what I think of as the physiographic north entrance to Juan Tabo Canyon itself. At GZ be sure to hike south along the trail for a way. You will have already hiked along this stretch if you came north on the Granite Trail.
Take a picture at Ground Zero
At GZ take a picture looking somewhat west, to show the prominent cliff/ledge of rocks at this location, including some sort of unique object (or even a fellow hiker) as scale, to demonstrate you made it deep into “Mazatza-landia”.
Optional Question for Scenic waypoint #1:
Q: Scattered around the ground are a bunch of rocks that contrast with most of the other rocks at this local rise along the trail: what color are they? What prominent land mass do you see on the horizon to the northwest beyond the Hills?
Optional Question for Scenic waypoint #2:
Q: What man-made feature is present at this location? There are some rocks you can see on the ground, without even leaving the trail, that contrast with most of the other rocks found along the trail: what color are they, and why do you think they are here?
Questions for Ground Zero:
Q: Do you see granite here at GZ, or have you seen granite along your journey? What is the gravel in the streambed and trail composed of? Why?
How would you describe the rocks that form the cliffs, ledges and outcrops here? To help form your description, just try to answer the following questions:
Q: What color are the rocks?
Q: What is the grain size of the rocks, fine-grained, medium-grained, large-grained?
Q: Do you see banding, and if so is it on a fine-scale or large-scale?
Q: Do you see layering of beds? If so, in what direction do the beds dip downwards, and at what angle (approximately) from horizontal?
Q: Do you see sparkly minerals shining in the sunlight?
Q: Do you see convoluted or wavy patterns, and if so is it on a fine-scale or large-scale?
Q: From GZ, hike about 45 paces (or about 110 feet) south and look west over to the bottom of the cliff/ledges there, and take notice of a rock-fall area. How would you describe the shapes of these pieces of fallen rock, for example slabs or thinner sheets, or maybe rounded boulders?
Q: So, if you were to classify the majority of rock at GZ and this stretch of upper Juan Tabo Canyon, how would you?
Q: Think back to your hike along the Granite Trail. Is that a good name for it?
Q: From GZ, can you see the massive Sandia Granite Pluton?
Q: Ancient metamorphic rocks also exist at the bottom of the Grand Canyon. Are these older or younger than the Rincon metamorphics? All you need to answer this question is Figure 2 and its description.
Permission
Permission for placement of this Earth Cache was obtained from Cibola National Forest, Sandia Ranger District.
References
Baer, S. H., 2004, Geologic and Tectonic Evolution of the Manzano Peak Quadrangle, central New Mexico, University of New Mexico, Earth and Planetary Sciences, Electronic Theses and Dissertations.
Berkely, J. and J. Callender, 1979, Precambrian Metamorphism in the Placitas-Juan Tabo Area, Northwestern Sandia Mountains, New Mexico, New Mexico Geol. Soc. Guidebook, 181-188.
Blakely, R. C. and W. D. Ranney, 2017, Ancient Landscapes of Western North America, A Geologic History with Paleogeographic Maps, Springer.
Bonewitz, R. L., 2012, Smithsonian Nature Guide to Rocks and Minerals, Dorling Kindersley.
Brandes, N., 2021, New Mexico Rocks!, Mountain Press Publishing Company.
Connell, S. D., 2008, Geologic map of the Albuquerque – Rio Rancho metropolitan area and vicinity, Bernalillo and Sandoval counties, New Mexico, New Mexico Bureau of Geology and Mineral Resources.
Daniel, C., et. al., 1995, The reconstruction of a middle Proterozoic orogenic belt in north-central New Mexico, New Mexico Geological Society.
Grambling, T. A., M. Holland, K. E. Karlstrom, G. E. Gehrels and M. Pecha, 2015, Revised Location for the Yavapai-Mazatzal Crustal Province Boundary in New Mexico: HF Isotopic Data form Proterozoic Rocks of the Nacimento Mountains, New Mexico Geological Society Guidebook, 66th Field Conference, Geolog of the Meadowlands, Las Vegas Region, p. 175-184.
Holland, M., K. E. Karlstrom, T. A. Grambling and M. T. Heizler, 2016, Geochronology of Proterozoic rocks of the Sandia-Manzano-Los Pinos Uplift, implications for the timing of crustal assembly of the southwestern United States, New Mexico Geological Society Guidebook, p. 161-168.
Julyan, R. and M. Stuever, eds., 2005, Field Guide to the Sandia Mountains, University of New Mexico Press, Albuquerque.
Karlstrom, K. E., M. L. Williams, M. T. Heizler, M. E. Holland, R. A. Grambling and J. M. Amato, 2016, U-Pb Monazite and 40AR/39AR Data Supporting Polyphase Tectonism in the Manzano Mountains: A Record of Both the Mazatzal (1.66-1.60 GA) and Picuris (1.45 GA) Orogenies, New Mexico Geological Society Guidebook, 67th Field Conference, Geology of the Belen Area, p. 177-184.
Kirby, E., K. Karlstrom and C. Andronicos, 1995, Tectonic setting of the Sandia pluton: an orogenic 1.4 Ga granite in New Mexico, Tectonics, 14, 1, 185-201.
Mazatzal Orogeny, Wikipedia, 2023.
Robb, L., 2021, Introduction to Ore-Forming Processes, 2nd Ed., Wiley Blackwell.