New Mexico Geological Society Annual Spring Meeting — Abstracts

Porphyroblast-matrix textural relationships are often ambiguous and, used in isolation, can lead to conflicting interpretations of the geologic history of an area because deformation and metamorphism are generally heterogeneous in both space and time. This is true in Proterozoic rocks in southwestern North America where the debate about a tectonic versus anorogenic Middle Proterozoic geologic history continues. Some geologists describe the Middle Proterozoic as a time of tectonic quiescence whereas others suggest that the Proterozoic crust was reactivated by possibly several tectonic episodes.

Studies of the emplacement of the 1.43 Ga Priest quartz monzonite in central New Mexico help elucidate timing of tectonic events. This pluton is interpreted: 1) to have post-dated most of the regional (D₂) shortening deformation and some amphibolite-facies metamorphism; 2) to have been synchronous with continued or renewed NW-SE shortening and development of a 1-2 km wide metamorphic aureole; and 3) to have been overprinted by low-temperature deformation after cooling. Apparently conflicting porphyroblast-matrix assemblages and disequilibrium mineral assemblages can be reconciled with this polyphase history.

Evidence that a major component of the NW-SE shortening pre-dated pluton emplacement is: 1) the pluton cross-cuts the regional, subvertical S₂ fabric and isoclinal F₂ folds; 2) the pluton is generally unfoliated compared to the intensely deformed country rocks; and 3) most contactmetamorphic minerals (75%) epitaxially overgrow a differentiated S₂ cleavage. Pre-pluton metamorphism may be represented by kyanite, rare staurolites that are included in contactmetamorphic garnets, and cloudy, anhedral garnets that are locally overgrown by contactmetamorphic garnets.

Tectonism also accompanied plutonism. Poles to tabular and locally folded dikes define a great circle (northeast strike, subvertical dip), the pole to which is interpreted to record subhorizontal, NW-SE shortening. Some (25%) of the contact-metamorphic garnets and staurolites, especially in the area NW of the pluton, have straight inclusion trails that are subperpendicular to the subvertical "S₂" matrix fabric, suggesting early-syn-"S₂" growth. Although these might represent earlier metamorphic minerals, we prefer an interpretation that F₂ folds were tightened and "S₂" reactivated and intensified on the NW, high strain, side of the pluton during emplacement.

Post-emplacement deformation is recorded by discrete mylonite zones in the granite. Temperatures of less than 350°C are suggested by core and mantle structures, undulose extinction, and deformation bands in quartz, unrecovered kinked muscovite, and cataclastic deformation in feldspar grains. This low temperature deformation may be associated with retrogression of contact minerals and, possibly, with movement on the Monte Largo Canyon shear zone (4 km to the north). ⁴⁰Ar/³⁹Ar muscovite ages of about 1350 Ma suggest cooling through 350°C at this time and may give a maximum age for this low temperature deformation. Complex polyphase deformation and metamorphism around the 1.43 Ga Priest pluton can therefore be separated into pre-, syn-, and post-emplacement generations, but it remains unclear whether these represent discrete events or parts of a continuum.