The Sandia granite--Single or multiple plutons?
— Douglas G. Brookins and Arun Majumdar

The petrology and field relations of the Sandia granite have been studied by a number of workers (Lodewick, 1960; Green and Callender, 1973; Kelley and Northrop, 1975; Enz and others, 1979; Berkley and Callender, 1979; Condie and Budding, 1979). The Sandia granite is a gray to pink, medium- to coarse-grained, porphyritic rock. It consists mainly of distinctive microcline, quartz, oligoclase, and biotite. Ac-cessory minerals are sphene, magnetite, apatite, hornblende, muscovite, tourmaline, and pyrite. The average modal analysis for the granite is 35 percent quartz, 15 percent microcline, 35 percent plagioclase, 10 percent biotite, and 5 percent microperthite (Kelley and Northrop, 1975). The overall composition of the granite is from quartz monzonite to granodiorite, although there is pronounced local variation.

The northern contact of the Sandia granite with adjacent metamorphic rocks of the Juan Tabo series is rather sharp, and a distinct metamorphic aureole occurs near the granite boundary (Green and Callender, 1973). The southern contact of the granite with the Cibola gneiss is in part gradational, with microcline megacrysts of the pluton decreasing in size over a few meters from granite into the gneiss (Condie and Budding, 1979; Connolly, this guidebook). Lodewick (1960) relates this change to peripheral alkali metasomatism. Elsewhere, such as in Tijeras Canyon, there are unmistakable intrusive contacts of the granite into the Cibola gneiss (Taggart and Brookins, 1975; Connolly, 1982 and this guidebook). Further, Lodewick (1960) has reported sillimanite in the contact zone of the Cibola gneiss which may have formed due to contact metamorphism during intrusion by the Sandia granite. Berkley and Callender (1979) cite differences in the nature of the contacts (sharp versus gradational) of the Sandia granite with intruded rocks as evidence for granite emplacement in several discrete pulses, and they note that the granite has been emplaced as dikes and sills where the country rocks show dilation effects.

The Sandia granite is a product of magmatic crystallization as doc-umented by the studies of Enz and others (1979) who show that the quartz-microcline-oligoclase-biotite granites plot near the hypothetical minimum on the normative quartz-albite-orthoclase ternary. They fur-ther propose that the main body of granite crystallized in a relatively water-undersaturated condition, in accordance with the experimental evidence of Maaloe and Wyllie (1975).

Orbicular granites are known from several sites within the Sandia granite (see Affholter and Lambert, this guidebook). Thompson and Giles (1974) have suggested that the La Luz trail site orbicular granite is a product of metasomatism, but Enz and others (1979) provide evi-dence for an igneous origin. In the Juan Tabo picnic area, another occurrence of orbicular granite is composed essentially of alternating rings of quartzo-feldspathic material; whereas, the La Luz site occur-rence is characterized by alternating rings of mafic minerals with quartzo-feldspathic minerals. Affholter (1979) and Affholter and Lambert (this guidebook) have reported on quartzo-feldspathic orbicular granite from the southern part of the Sandia Mountains which is very much like the Juan Tabo occurrence.

The radiometric age of the Sandia granite is 1.44 ± 0.04 b.y. (billions of years) and is based on samples from widespread locations throughout the Sandia granite exposures. There appears to have been a mild (?) thermal event which affected the Sandia granite at about 1.35 b.y. to 1.375 b.y. (Brookins and others, 1975; Brookins and Majumdar, 1982). This event was sufficient to cause 'Ar (* = radiogenic) loss from bio-tites and some "87Sr loss from biotites as well. Muscovites from the metamorphic rocks and from pegmatites in the Rincon area yield K-Ar dates of about 1.375 b.y., slightly older than the reset biotites of 1.35 b.y. Sphene also yields a fission track date of 1.4 b.y. (in Naeser, 1971) which may be due to the same thermal event. Brookins (this guidebook) discusses this problem in more detail.