A REVISED VIEW OF PROTEROZOIC REGIONAL METAMORPHISM IN NORTHERN NEW MEXICO AS CONSTRAINED BY THE PETROLOGIC AND GEOCHRONOLOGIC INTERROGATION OF GARNET GROWTH

In this investigation, a combined geochronologic and petrologic study of deformation and metamorphism was undertaken in the Tusas and Picuris mountains of New Mexico in order to clarify the early Proterozoic history of North America. The timing of garnet growth in the Vadito and Hondo groups was constrained using Lu/Hf geochronology (see Aronoff et al., this meeting), identifying the onset of regional amphibolite facies metamorphism. Petrologic observations were used to determine both the timing of garnet growth relative to deformation, and the peak conditions experienced during metamorphism.

Results of this study show that regional amphibolite facies metamorphism occurred in these mountain ranges at ca. 1.4 Ga. Garnet growth at this time was intratectonic, occurring between development of D2 and D3 deformation events, based on the relationship between the matrix foliation and the foliations preserved in garnets during growth. Typical peak assemblages in both mountain ranges include garnet, staurolite, biotite, muscovite, and quartz ± tourmaline. Garnet and biotite grew through the breakdown of chlorite in the presence of muscovite. Staurolite grew through the breakdown of garnet in the presence of chlorite. These reactions are consistent with peak temperatures above 550ºC. In one sample, breakdown of garnet and muscovite to form sillimanite and biotite provides evidence for decompression. In total, these observations indicate a clockwise pressure-temperature path reaching amphibolite facies metamorphism in the Tusas and Picuris Mountains. These estimates of peak conditions will be verified using isochemical phase diagrams based on the whole rock chemistry for each sample to further constrain conditions during metamorphism.

The combined petrologic and geochronologic observations presented here and by Aronoff et al. (this meeting) portray a single amphibolite facies regional metamorphic event at ca. 1.4 Ga. There is no evidence preserved in these samples for earlier high grade regional metamorphism. However, the rocks were deformed prior to garnet growth. Thus, this study suggests that northern New Mexico experienced a single episode of deep burial at ca. 1.4 Ga and not a polyphase metamorphic history with long-lived mid-crustal residence prior to 1.4 Ga.