New Mexico Geological Society Annual Spring Meeting — Abstracts

Three different tectonometamorphic units are juxtaposed across ductile shear zones in the northern Taos Range. Published U-Pb zircon ages differ systematically across the shear zones, suggesting that these structural boundaries may represent major terrane boundaries. The first unit is dominated by high-grade gneiss and is exposed north of Questa. Quantitative thermobarometry on pelitic gneiss (Grt-Bt-Pl-Q-Sil-Kfs) yields pressures and temperatures of 9-11 kb and 700-800°C. Mafic gneisses are largely retrograded to hornblende and plagioclase, although pyroxene-bearing rocks are present locally. The second unit is a muscovitic quartzite and is exposed both north and south of Questa. Aluminous layers with Ky-An-Sil or St-Cld-Ky-An indicate P-T conditions near the aluminum silicate triple point (ca. 4 kb, 500°C). The third unit is exposed south of Questa and includes mafic and felsic metavolcanic rocks with rare pelitic and ultramafic schist. Mineral assemblages (Hb-Chl-Pl; Sil-St-Grt-Bt-Chl-Ms-Pl-Q) indicate P-T conditions in the lower amphibolite facies.

The contact bewteen the first and second units north of Questa is a subhorizontal, top-to-the-southeat mylonitic shear zone, interpreted to represent a ductile extensional fault (D₁). P-T paths from the underlying gneissic rocks show decompression. Contacts between the second and third units south of Questa define another subhorizontal ductile shear zone (D₁?; vergence not yet determined) across which metamorphic grade does not change. The second unit overlies the third unit along this shear zone. Both subhorizontal shear zones have been folded twice: first about easterly to northeasterly axial planes overturned to the south (D₂), and second about steep, northerly axial planes (D₃). The ductile shear zone separating the first and third units is quite different. Although this boundary is partly obscured by Tertiary volcanism near Questa, preliminary work has linked it with tectonized Precambrian rocks with lineations that plunge gently. Foliation is steep and trends WNW; asymmetric quartz ribbons indicate right-lateral motion. Movement on this WNW strike-slip shear zone is tentatively ascribed to D₄.