MESOPROTEROZOIC METAMORPHISM AND AR-40 AR-39 THERMAL HISTORY OF THE 1.4 GA PRIEST PLUTON, MANZANO MOUNTAINS, NEW-MEXICO/

Paleoproterozoic supracrustal rocks in the aureole of the 1.43 +/- 0.0 1 Ca Priest pluton provide evidence for a polyphase Proterozoic metamo rphic history in central New Mexico. This history involved pre-, syn-, and post-emplacement metamorphic mineral growth. The 1-2 km wide Prie st pluton contact aureole is characterized by two isograds (first appe arance of sillimanite and staurolite, respectively). Carnet-biotite te mperatures combined with assemblage data (sillimanite + K-feldspar) in dicate an increase from 525 degrees to similar to 650 degrees C toward the pluton at pressures of similar to 4 kb, sug gesting emplacement d epths of similar to 12 km. An Ar-40/Ar-39 age of 1.44 +/- 0.005 Ca for hornblende in the aureole suggests rapid cooling through similar to 5 00 degrees C following pluton emplacement. The well-defined thermal au reole can be used to help constrain both pre- and post-emplacement met amorphisms. Pre-pluton metamorphism is recorded by amphibolite-facies inclusions inside contact metamorphic porphyroblasts. Included mineral s are interpreted to be art of a regional metamorphic assemblage becau se similar minerals are found outside the aureole. P-T estimates outsi de the aureole suggest pre-pluton regional conditions near 525 degrees C and 4 kb. Post-emplacement greenschist-facies retrograde metamor ph ism was superimposed on the northern and southern parts of the contact aureole and is characterized by the replacement of contact-metamorphi c porphyroblasts with muscovite and/or chlorite. Retrogression was mos t likely due to continued cooling following pluton emplacement. Ar-40/ Ar-39 muscovite plateau ages from the aureole range from 1.39 to 1.36 Ga. These data bracket the retrograde metamorphism between 1.43 and 1. 39-1.36 Ga, which suggest that ambient temperatures following pluton e mplacement were about 350 degrees C for a prolonged (30 m.y.) period, indicating slow cooling following 1.4 Ga plutonism.