Scattered exposures of Precambrian quartzite, gneiss, and a siliceous volcanic rock that has been metasomatized into quartz-feldspar-muscovite schist are found in La Madera quadrangle in the Tusas Mountains of north-central New Mexico. These rocks have been multiply deformed and invaded by swarms of pegmatite bodies. Complexly zoned pegmatites probably originated during a period of folding; later pegmatites are undeformed, sharply transecting dikes. Rb-Sr isotopic data from whole rocks, metamorphic muscovite, and pegmatite “book” muscovite plot on isochrons that indicate an age of 1,425 m.y. Thus igneous intrusion and final stages of metamorphism appear to have ceased within a time interval too short to be resolved by the Rb-Sr method. Initial Sr87/Sr86 ratios from schist at two localities (0.720 and 0.716) are exceptionally high, suggesting that a lengthy crustal history took place earlier than the regional metamorphism. Results of a qualitative calculation indicate a primary age of the rocks of about 1,600 m.y. or older. The ages noted here are similar to those of Precambrian rocks over a wide area of Colorado and New Mexico.

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