Sandia Granite and adjoining metamorphic rocks at southern Rincon Ridge

This year we are helping lead the 2026 New Mexico Geological Society Fall Field Conference. The first stop of this conference will be at the south end of Rincon Ridge. One of the neat things near Stop 1 is the contact between the Sandia Granite and its country rock, the older Juan Tabo metamorphic rocks that correlates to the 1.68–1.60 Ga Manzano Group (white line in Photo 1). The protolith of the Manzano Group, inferred to be mudstones and sandstones, were metamorphosed into schists and quartzites during the 1.65–1.60 Ga Mazatzal orogeny. During the subsequent 1.47–1.35 Ga Picuris orogeny (Hillenbrand et al., 2025 [https://doi.org/10.1111/ter.12748]), emplacement of the 1.45 Ga Sandia Granite metamorphosed some of these rocks, especially those near the contact, into gneiss and migmatite. Kirby et al. (1995; https://doi.org/10.1029/94TC02699) presents more details about the granite and related contact aureole. Descriptions and interpretations of these rocks and younger geologic features in the area can be found in the upcoming Conference guidebook (available for download from the New Mexico Geological Society webpage after mid-October). Another reference for the local geology is Dirk Van Hart’s excellent book: New Mexico’s Magnificent Sandia Mountain: The Complete Geologic Story.

For an enjoyable hike, you can acquaint yourself better with the Precambrian geology near Stop 1 by walking the granite-metamorphic rock contact north of Stop 1 to the cone-shaped peak depicted on Photo 1 (called Rincon high point). On this annotated photograph, Site A is the location of photographs 2 through 4. Site B (Photo 1) is where photographs 5 through 8 were taken. The location of the contact is shown on Geologic Map 78, authored by Sean Connell and published by the New Mexico Bureau of Geology (https://geoinfo.nmt.edu/publications/maps/geologic/gm/78/)

In Juan Tabo Canyon, you can constrain the contact based on float and outcrop. The contact can be mapped more accurately and directly observed near Forest Road 333, where its geometric irregularity at detailed scales (over ~100 m) suggests an intrusive (rather than fault) contact. Photo 2 shows the contact in a Forest Road 333 roadcut, and Photos 3 and 4 depict close-ups of the rocks on either side of the contact. The granite is slightly weathered (Photo 3) due to being near the topographic surface before excavation of Road 333 exposed it. Try to get your eye keyed into the minerals: the granite consists of orthoclase megacrysts in a finer grained matrix of sodic plagioclase (white), quartz (clear/grey), and biotite (the black grains) as the hydrous phase. The schist shown in Photo 4 is typical of the schist found throughout the Rincon Ridge area, where the minerals are generally about 1 mm long and include quartz, muscovite, and biotite (+/- amphibolite). Andalusite and sillimanite are also found in the schists and can be mapped as mineral isograds that record increasing metamorphic temperatures near the pluton (as shown in the Kirby et al., 1995, paper).

At Site B an illustrative boulder of schist sits near the bottom of Juan Tabo Canyon (Photo 5). Here, one can see two foliation surfaces: an earlier one (S1) defined by millimeter-scale mafic-rich bands alternating with quartz-rich bands, probably mimicking original bedding. Cross-cutting S1 at consistent angles (25–40 degrees) is a later foliation surface (S2), defined by bands of recrystallized mafic minerals. According to observations and measurements at Stop 1 of the NMGS Fall Field Conference, the S2 foliation occurred during emplacement of the Sandia pluton (and during the Picuris orogeny) but mostly before emplacement of NW-striking 1.43 Ga pegmatite/granite dikes. An example of a pegmatite crosscutting schist is shown in Photo 6.

At Site B are also exposures of injection migmatite (Photo 7), where granitic melts can be seen to crosscut the metamorphic rocks, sometimes following older foliation, and sometimes crosscutting it. This outcrop (not totally in-place) is quite close (~few hundred meters) west of the pluton contact.

Photos 8 and 9 show two types of inclusions found in the Sandia Granite. The blackish rock in Photo 8 is probably a mafic enclave, formed when mafic magmas are injected into and mingle with the granitic magma; they crystallize relatively rapidly because of their higher crystallization temperature. The enclave then floats in the felsic magma as a semi-solid mass until the surrounding granite crystallizes. You can see some hybridization between the two magmas at the contact. The white films on the enclave right of the finger is modern calcium carbonate precipitation.

Photo 9 shows a boulder-size, schistose xenolith of the Juan Tabo metamorphic complex. A xenolith is a chunk of the older host rock that gets entrained into the intruding magma. Note the light-colored bands in the xenolith, which are granitic injections some of which (upper 2 arrows) seem to come from the granite. The schistosity in the xenolith predates the granite yet the melts opportunistically followed zones of weakness while the xenolith was floating in the magma.

These outcrop features in the contact zone of the Sandia granite may get you thinking about lots of interesting aspects of the Precambrian basement rocks of New Mexico: such as the tectonic settings of the Picuris and Mazatzal orogenies, middle crustal processes at temperatures of 550-650 C and depths of ~ 10 km, how granite melts form and interact with their country rock, and many more still-active research questions. With a little practice, you can make many primary observations such as these while hiking in this beautiful part of the Sandia Mountains.

Next week’s postcard explores the geomorphology along this contact in Juan Tabo Canyon.