TEXTURAL CONSTRAINS ON THE P-T CONDITIONS OF GARNET NUCLEATION AND APPEARANCE OF RUTILE IN CONNECTICUT VALLEY SYNCLINORIUM, VT METAPELITES, AND COOLING RATES FROM PEAK T

Metapelite samples from the St-Ky zone of the Connecticut Valley Synclinorium (CVS), central VT, contain garnet porphyroblasts that grew during the transition from ilmenite to rutile. Garnet cores in several samples are inclusion free or contain sparse quartz, ilmenite, zircon, and epidote inclusions followed by inclusion rich outer mantles that contain plentiful quartz inclusions and rutile. Zr-in-rutile (ZiR) thermometry and quartz in garnet (QuiG) Raman barometry provided robust constraints on the P-T of both garnet growth and rutile nucleation. These conditions are inferred to be very near the nucleation conditions of garnet. The three samples examined for this study nucleated garnet within a P-T range of 8.3-9.5 kbars and 560-575 °C, and required 300-600 J/mol O of affinity to nucleate. The affinities presented in this study are consistent with the range of affinity reported for metapelitic samples in contact aureoles. These results are inconsistent with the notion that garnet must nucleate near chemical equilibrium.

Two samples (TM-921 and TM-678) have distinct upticks in spessartine (sps) on their rims, which were characterized by diffusion modeling to extract cooling rates from peak temperature. TM-678 displays retrograde replacement of the original garnet by a combination of muscovite, biotite, plagioclase, and low grossular garnet. Using a peak T of 565 °C the sps enrichment profile is well fit by a segmented cooling T-t path defined by initial slow cooling from peak and rim resorption followed by rapid cooling from 550 °C to 500 °C to preserve the sps uptick. The total cooling time from 565 to 500 °C occurred over ~3.3 Ma over a cooling rate of ~19 °C/Ma. Sample TM-921 also has a distinct outer uptick in spessartine over a 200 um broad sps enriched zone. The sharp enrichment occurred as a result of rim resorption during retrogression within the outer 50 um of the garnet rim. This preserved profile is well fit by a similar cooling trend as TM-678. Using the ZiR temperature of 575 °C as the peak temperature, the back diffusion profile formed over a cooling time of 3.1 Ma with a cooling rate of 24 °C/Ma.