THE PETROGENETIC RELATIONSHIP BETWEEN THE TRES PIEDRAS AND TUSAS MOUNTAIN GRANITE ORTHOGNEISS, TUSAS MOUNTAINS, NEW MEXICO

Controversy over the nature and timing of assembly, and the reactivation of Proterozoic rocks exposed in the north-central New Mexico arises from a lack of clear diagnostic crosscutting relationships that have not been overprinted upon which to build tectonic histories. Rocks in this region comprise supracrustal metasedimentary and metavolcanic rocks deposited on intermediate to mafic composition island arc rocks at ca. 1750-1650 Ma. Regional metamorphic assemblages and structural fabrics developed during and after tectonic burial to mid-crustal levels. Pervasive, regional tectonothermal overprinting partially obscured earlier tectonics events and is constrained to 1450-1380 Ma from Ar/Ar Hornblende and Mica cooling ages.

Crosscutting relationships between supracrustal host rocks and two orthogneiss bodies in the northern Tusas Mountains, the Tres Piedras (TP) and Tusas Mountain (TM) metagranites, preserve information on the early tectonic history of the region. Previous studies in the region suggested separate tectonic histories for the TP and TM. Based largely on highly discordant zircon ages, ca. 1500 Ma Rb/Sr ages and mineral texture arguments, Wobus and Hedge (1982) assigned the age of the TM to be ca. 1490 Ma. However, new concordant U-PB zircon ages for the TM of 1693 ± 3 Ma suggest an older age for this body, in agreement with the timing of TP emplacement which consistently yields 1700-1650 Ma U/Pb zircon ages and ~1500 Ma Rb/Sr ages. We present new bulk composition and texture analysis to suggest that the TP and TM may share a petrogenetic relationship, are not tectonically or chronologically distinguishable, despite internal variations in major element composition. Preliminary REE Y and Sr data from both plutons are compositionally similar, and rapakivi and anti-rapakivi textures suggest that these variations are the result of magma mixing within both plutons. Spatially extensive textural analysis of the TM pluton has shown that the core of the pluton exposure is dominated by coarse-grained texture similar to the TP.

To clarify future discussions on the Proterozoic tectonics of New Mexico, we propose that the TM be considered a ca. 1690 Ma body, and that the name Tusas Mountain granite be removed, thus referring to the Tres Piedras granite at Tusas Mountain as it was defined before 1982.