New Mexico Geological Society Annual Spring Meeting — Abstracts

Granulite facies metamorphism affected at least 3,000 km³ of the northern Taos and western Cimarron Mountains. Previous work has failed to recognize this extreme metamorphic grade because pervasive retrogression largely obliterated minerals definitive of the granulite facies.

The granulite facies is best defined by the first appearance of pyroxene in rocks of mafic bulk composition. Pyroxene has been identified in three specimens of mafic gneiss collected near the village of Costilla. Two contain the peak metamorphic assemblage clinopyroxene-hornblende-plagioclase; the third has orthopyroxene-clinopyroxene-plagioclase. These are the first reported occurrences of Proterozoic
granulites in New Mexico.

Other rock types document that metamorphism occurred in the granulite facies over most of the northern Taos and western Cimarron Mountains. Aluminous gneisses typically contain sillimanite coexisting with K feldspar, and migmatites occur in the latter area. Both features require temperatures exceeding 650°C. Pelitic gneisses commonly contain garnet-plagioclase-biotite-sillimanite-quartz, a mineralogy amenable to quantitative thermobarometry. Microprobe data indicate peak metamorphic conditions of 700-800°C, 9-11 kb at Costilla; 650-750°C (P unknown) in the northern Latir Peaks Wilderness; and 660°C, 6 kb northwest of Red River. Each P-T determination is compatible with the granulite facies.

The three samples of mafic granulite from Costilla are weakly deformed and show minor signs of retrogression: partial pseudomorphs of blue-green hornblende after pyroxene, and cross-cutting veins of retrograde epidote. The granulites occur as texturally massive lenses and
pods within larger bodies of intensely foliated and lineated mafic gneiss. The tectonized gneiss consists largely of blue-green hornblende,
plagioclase and chlorite, minerals characteristic of the lower amphibolite facies. Field relations therefore indicate that the lower grade minerals were derived from mafic granulites by retrograde metamorphism and ductile shearing. This interpretation is supported by petrography: brown cores found in some blue-green hornblendea appear to be relicts of the Ti-rich amphibole stable in the lower granulite facies. Retrogression affecting mafic rocks more intensely than other lithologies explains the abundance of felsic granulites, the scarcity of mafic granulites, and suggests that more mafic granulites might be found in structural environments that experienced low strain.

Retrograde formation of amphibolite from granulite requires the addition of water, apparently along ductile shear zones focussed on the
edges of the thin mafic granulites. The shear zones formed as horizontal surfaces with top-to-the-south movement. The largest ot these, exposed in scattered outcrops across most of the northern Taos and western Cimarron Mountains, truncates the top of the zone of granulite facies rocks. Peak metamorphism above this shear zone reeched 500°C, 4 kb. The similarity of these peak conditions to lower-plate retrograde conditions, and the Byntectonic textures shown by peak metamorphic minerals in the upper plate, indicate that the structurally higher rocks were metamophosed as the shear zone developed. The Proterozoic basement of New Mexico may contain other unrecognized, subhorizontal tectonic boundaries.