New Mexico Geological Society Annual Spring Meeting — Abstracts

The timing of plutonism, deformation, and metamorphism is not wen understood in many Proterozoic rocks in New Mexico. Early (ca. 1.65 Ga) Proterozoic events represent the formation ofcontinental lithosphere and were accompanied by synchronous plutonism, deformation, and prograde metamorphism. In the Middle (ca. 1 Ga) Proterozoic, voluminous granitic plutons were accompanied by synchronous deformation and regional metamorphism. The relationship and relative importance ofthese two events is a topic of debate, however.

The Manzanita pluton and surrounding country rocks are moderately to strongly deformed, with a dominant NE-SW, 45° to 60° south-dipping fabric. Shear sense indicators, including C/S fabrics, mantled porphyroclasts, and lattice-preferred orientation ofminerals show a top-to-the-north shear sense. Quartz and feldspar textures observed indicate deformation occurred from at least 500°C down through 250-300°C. Regional metamorphism in the area is greenschist grade. The aureole on the south side of the Manzanita pluton shows increased metamorphic grade as evidenced by garnet, andalusite with embayed kyanite, hornblende, and sillimanite. Some large andalusite minerals exhibit a bladed form similar to kyanite. P-T estimates for the aureole are approximately 600°C and less than 2.5 kbars based on assemblages with potassium feldspar and andalusite. We interpret the embayed kyanite and bladed habit of andalusite to mean that the rocks experienced a higher pressure history prior to pluton emplacement. Garnets from the aureole show inclusion patterns best explained by syntectonic growth. Our interpretation is that the rocks were at mid-crustal levels (10-15 km) similar to other early Proterozoic rocks in New Mexico, then were brought to shallow crustal levels (less than 6 km) during synchronous deformation and pluton emplacement.

Igneous zircons in the Manzanita pluton analyzed by U-Pb have yielded a preliminary age of 1636±34 Ma. Hornblendes from samples collected from the southern aureole analyzed by ⁴⁰Ar/³⁹Ar have yielded an age of 1432±8 Ma. Preliminary data, however, show that the hornblendes are only in the contact aureole, grading to actinolite farther from the pluton, although some actinolites have hornblende rims, probably from later regional heating. The hornblendes are thus genetically related to the pluton, and grew at approximately 1636 Ma. The 432±8 Ma ⁴⁰Ar/³⁹Ar apparent age of the hornblende is interpreted to reflect argon loss at this time related to a relatively high temperature
(~500°C) thermal event associated with regional 1.4 Ga activity.