GARNET'S AFFINITY FOR OVERSTEPPING ITS BOUNDARIES: AN EXAMINATION OF GARNET NUCLEATION CONDITIONS AT OVERSTEPPED CONDITIONS WITHIN A TYPICAL GARNET ZONE IN CENTRAL VERMONT

Evaluation of the reaction affinity required for garnet nucleation at overstepped conditions was performed on a series of metapelitic rocks from the Connecticut Valley Synclinorium (CVS), central Vermont. The pressure and temperature conditions at which garnet nucleated were constrained by the intersection of quartz inclusion Raman barometry (QuiG) isochores and garnet core Fe/(Fe+Mg) isopleths. The inferred ∆T from nucleation to peak T was determined by comparing core and rim garnet Fe/(Fe+Mg) isopleths along an isobaric path. Textural analysis of the present matrix and inclusion suite indicated garnet nucleated from a matrix assemblage of Qtz, Chl, Bt, Ms, Pl, Ilm, ± Ep. Mineral assemblage diagrams (MADs) were constructed in the MnNCKFMASH system to model how much the P-T conditions of garnet nucleation overstepped the equilibrium isograd, and to quantify the reaction affinity required to nucleate garnet at these overstepped conditions.

Preliminary QuiG pressure results and MAD diagram analysis indicate that garnet did not nucleate at or near equilibrium isograd conditions. Garnet porphyroblasts in samples OW-5C and OW-4 collected from both sides of the Richardson Memorial Contact at Bethel VT, nucleated at conditions up to 10 kbars and 530 °C on the west side and 10.5 kbars and 540 °C on the east side, respectively. The OW-5C rim Fe/Fe+Mg isopleth indicates the garnet reached 560 °C, while the OW-4 Fe/Fe+Mg isopleth records a possible peak T of 575 °C. Further east, sample TM-916C from the Townshend Brownington Syncline nucleated near 11 kbars and 550 °C followed by heating to 590 °C. Sample TM-675 from the South Royalton quadrangle yields nucleation conditions of ~10.5 kbar and 550 °C, followed by heating to 600 °C. At these nucleation conditions, reaction affinities of 2.1, 2.0 1.5, and 2.7 kJ/mol O, respectively, were required for garnet nucleation. Overall, garnet nucleated within a pressure range of 9-11 kbars, and temperature range of 530 to 560 °C. This P-T range required at least several kbars of pressure overstepping, tens of degrees of temperature overstepping in some samples, and 1.50 to 2.66 kJ/mol O of affinity for garnet nucleation. Garnet nucleation conditions indicate these rocks reached greater depths than previously assumed, requiring reevaluation of the tectonic history and paragenesis of the CVS.