Redefinition of The Precambrian Tusas Mountain and Tres Piedras Granites, North-Central New Mexico

Abstract

Proterozoic granitic plutons of strongly contrasting textural and structural style in the Tusas Range previously have been correlated under the name Tres Piedras Granite. However, the foliated granite, in the village of Tres Piedras and in Tusas Canyon were imprinted by the same period of regional metamorphism (ca. 1,700 Ma) that de­ formed the wall rocks, whereas the granite porphyry stock at Tusas Mountain appears to be post-tectonic. Although the two kinds of granite are generally similar in terms of most major oxides, the granite at Tusas Mountain is slightly more calcic and is locally enriched in fluorine and a suite of trace elements indicative of late vapor-phase activity. Rb-Sr whole-rock analyses and U-Pb dating of zircon from the Tusas Mountain stock indicate an age between 1,500 and 1,430 Ma; thus the stock was emplaced during the second of the major Proterozoic intrusive episodes recognized in the southern Rocky Mountains. It may, therefore, be the only stock-sized pluton exposed west of the Rio Grande that was contemporaneous with a period of widespread pegmatite formation and metasomatic (sericitic) alteration of the rocks of the area. On the basis of this new work, we propose that the Tusas Mountain stock be referred to henceforth as the Tusas Mountain Granite, a new name, and that the term Tres Piedras Granite be applied only to the foliated granite of the 1,700 Ma intrusive episode.