T169 Proterozoic Crustal Evolution of Southern Laurentia: Tectonic Settings, Petrology, Geochemistry, and Geochronology I

Tuesday, 2 November 2010
8:00 AM - 12:00 PM
Room 302

INVERTED METAMORPHISM AND DECOMPRESSION IN THE SOUTHERN TUSAS MOUNTAINS, NEW MEXICO

Paper number: T169-7
Program: Technical Sessions
Time: 9:40 AM – 9:55 AM

Authors

Abstract

Two outstanding problems for understanding the Proterozoic assembly of the southwest are (1) how regional temperatures of 500-600oC were achieved at relatively shallow depths (10-12 km, 3.5-4.0 kbar), and (2) whether this metamorphism occurred at 1.6 Ga or 1.4 Ga. In our study of the metamorphism and deformation history of the Cerro Colorado area, southern Tusas Mountains, New Mexico, we use mineral reaction textures coupled with equilibrium phase diagrams to determine peak metamorphic conditions. We determine that the area underwent decompression from about 6 to 4 kbar at about 600oC, during ductile thrusting. Inverted metamorphism involving sillimanite-garnet schists (625oC) structurally above staurolite-garnet schists (550oC) suggests thrusting. Evidence for decompression is recorded by the reactions muscovite + garnet = staurolite + biotite, which is negative in P-T space, muscovite+ garnet = sillimanite + biotite, which is nearly horizontal in P-T space, and kyanite partially to totally psuedomorphed by sillimanite.

The primary reaction texture indicating decompression near peak metamorphic temperature is the replacement of garnet by clots of sillimanite, surrounded by halos of biotite. The sillimanite clots, most without relic garnet in the cores and with highly variable aspect ratios, are aligned. They define a lineation that formed during D2, along with S2, the dominant foliation. The sillimanite lineation and the S2 foliation are folded by a map scale F3 antiform, with sillimanite continuing to grow duringD3. Kyanite was deformed and was partially to totally replaced by sillimanite during D2. Early garnet growth continued through D2. A second generation of garnet grew during or after D3. Both generations of garnet show remarkably minor chemical zoning considering the relatively low temperatures (550-625oC) of peak metamorphism. This implies the rocks were held near peak metamorphic temperatures for long enough for any growth zoning to have been relaxed by diffusion.

We suggest that the sillimanite schists were juxtaposed above the staurolite schists during regional top to north thrusting. The rocks remained at peak temperatures for long enough that the garnets were homogenized.