GEOCHEMISTRY OF MIGMATITES AND DIKES FROM THE CENTRAL BLUE RIDGE PROVINCE: CONSTRAINTS ON PARTIAL MELTING IN THE CRUST

The Winding Stair Gap (WSG), Savannah Church and Sylva outcrops in the central Blue Ridge expose pelitic (Grt – Bt – Sill), semipelitic (Grt – Hb – Bt - Plg), and quartzofeldspathic (Qtz – Plg – Bt) schists, gneisses and granofelses formed under upper amphibolite to granulite facies conditions. These rocks show evidence of partial melting which gives rise to two types of migmatites: stromatic metatexites with alternating layers of concordant melanosomes and leucosomes, and diatexites with chaotic, somewhat discordant leucosomal entities with residual melanocratic minerals. All rocks are crosscut by leucocratic dikes of varying sizes throughout these three outcrops. Chemical ages on monazites from a pelitic melanosome from WSG suggest that peak metamorphism and partial melting occurred 438 ± 5 Ma, and was followed by a weaker thermal overprint at 360 ± 5 Ma¹. ID-TIMS and SHRIMP ages on leucosomal zircon from a diatexite yield an age of 460 ± 12 Ma².

Bulk rock chemical analysis shows that the leucosomes, whether stromatic metatexites or dikes, classify mainly as peraluminous diorites to monzonites, and have similar trace and RE element compositions. Almost all stromatic metatexites and leucocratic dikes are more enriched in LREE and show a positive Eu anomaly compared to corresponding melanosomes. Isocon diagrams for related leucosome/melanosome pairs show that Na, Ca, Sr ± Si are always preferentially incorporated in the melt, whereas Ti, Mg, Fe, Mn, and Y are retained by the pelitic melanosome. Leucosomes with semipelitic/ Hb – bearing melanosomes concentrate Na, Ba, Sr, and Si relative to the melanosome. Zr saturation temperatures for stromatic leucosomes and dikes are similar within error (790 ± 50°C), and slightly lower than peak temperatures (850 ± 30°C at 9 ± 1 kbar) calculated for WSG^1,2. These results suggest that the stromatic metatexites and dikes developed by peritectic melting at similar conditions. Whereas stromatic leucosomes formed by partial melting of pelitic and Hb-bearing semipelitic rocks, the dikes most likely represent melt extracted from pelitic rocks from the deeper parts of the section. On the other hand, the quartzofeldspathic gneisses do not appear to have produced significant melt.

¹ El-Shazly et al. (2011). JMG 29, 753 – 780.

² Moecher et al. (2004). J. Geol., 112, 289 – 304.