Petrology and tectonic implications of the transition from the staurolite-kyanite zone to the Wayah granulite-facies metamorphic core, southwest North Carolina Blue Ridge : including quantitative analysis of mineral homogeneity

Abstract

The Noland Creek-Wayah metamorphic transition, from the staurolite-kyanite zone to the Wayah granulite-facies metamorphic core, occupies the NW flank and core of high-grade metamorphism in the southwestern North Carolina Blue Ridge. Evidence indicates this complete transition was produced during a single, prograde, Paleozoic metamorphic event, and represents a continuous progression into the high-grade culmination of Paleozoic metamorphism in the southern Appalachian Blue Ridge. Mineralogical geo-thermometry and barometry yield conditions of 542-564 °C, 5.2-7.6 kb in the staurolite-kyanite zone, 561-627 °C, 4.9-6.6 kb in the kyanite-almandine zone, and 561-639 °C, 5.2-7.5 kb at the kyanite-aillimanite isograd., Results indicate the following range of conditions at the second-sillimanite isograd: T = 668-672 °C, P = 6.4-7.0 kb, and X(H2O) = 0.57-0.64. Calculated peak T and P reach 844 °C, 9.8 kb in the hornblende granulite facies core. Comparison of calculated static geothermal gradients and the best-fit peak P-T array ("peak thermal gradient") precludes an increased heat flux from any unrecognized magmatic source. Textural relationships and estimates of peak T and P remain compatible with a clockwise P-T-time path. The Wayah granulite-facies core provides an example of Phanerozoic granulites which formed without unusual tectonic or magmatic events. Similar granulite facies cores should occupy areas of pre-metamorphic-peak crustal thickening, and may be restricted to relatively inboard positions. Thin-section-scale mineral heterogeneity is quantified by the Homogeneity Index (HI). Crystal-chemical substitutions which control micro-variation are evaluated using correlation coefficients for covariations of individual cations and partial sums. Results suggest microscale tschermakitic substitutions in two of six biotites. Control of Ti exchange apparently resulted from a Ti-vacancy ([]) substitution of the form...