Precambrian rocks of a portion of the Pedernal Highlands, Torrance County, New Mexico
Dale G. Armstrong and R. J. Holcombe
The Pedernal Highlands are located in Torrance County, New Mexico, approximately 60 km east-southeast of Albuquerque (fig. 1). They represent a remnant of the ancestral "Rocky Mountains" and are seen today as gently rolling hills of eroded Precambrian rocks. The Pedernal Highlands form a narrow north-trending plateau about 70 km long and a few kilometers wide. The most prominent topographic feature is Pedernal Mountain with an elevation of 2,307 m, about 245 m higher than the surrounding plains.
The Precambrian core of the Pedernal Highlands is composed of metavolcanic, metasedimentary, and meta-intrusive rocks. In this study they are divided into four broad lithologic units which, from north to south, are: northern quartzite terrain, T-phase lithology, M-2 volcanics, and Crab Hill volcanics (fig. 1).
Metamorphic grade ranges from lower to middle greenschist facies. The predominant foliations are axial plane cleavages and transposition foliations which generally strike east and dip steeply to the south. Polyphase deformation has produced extremely complex outcrop pat-terns and rendered precise stratigraphic correlations nearly impossible. Post-metamorphic faulting is sparse except along the extreme western edge of the highlands. An Rb/Sr date of 1493±30 30 m.y. was obtained on a sample of M-2 metavolcanic rhyodacite (dated by R. L. Armstrong, University of British Columbia), and a metagranite in the southern part of the study area was dated at 1416± 100 m.y. It is possible that both dates have been partially reset by metamorphic recrystallization. Narrow dikes of post-metamorphic syenite are 469 7 m.y. old (Loring and Armstrong, 1980).
The Precambrian rocks of the Pedernal Highlands have been studied previously by Fallis (1958), Woodward and Fitzsimmons (1967), Gon-zales (1968), and Gonzales and Woodward (1972). These authors divided the Precambrian into granites, schists, cataclastic rocks, heterogeneous units, and quartzite.
- Armstrong, Dale G.; Holcombe, R. J., 1982, Precambrian rocks of a portion of the Pedernal Highlands, Torrance County, New Mexico, in: Albuquerque Country II, Grambling, J. A.; Wells, S. G., New Mexico Geological Society, Guidebook, 33rd Field Conference, pp. 203-210.