GEOCHEMISTRY OF MIGMATITES FOR NE GEORGIA - THE ATHENS GNEISS (AG)

We report whole-rock major and trace element chemical data for some upper amphibolite grade migmatites from the Southern Appalachian Inner Piedmont core in the vicinity of Athens, Georgia. These migmatites are composed of bio + plg + kfs + qtz + ms ± grt ± sill and occur as both metatexite migmatites and diatexite migmatites, often with a gradation from one type to the other within an outcrop.

Peak P-T conditions of 790±50°C and 8.3±0.5 kbar, estimated on the basis of mineral equilibria (Mirante and Patiño-Douce, 2000) are sufficient to have allowed the melting of the inferred protolith, an amphibolite-grade biotite gneiss, and subsequent segregation of the melt into melanosomes and leucosomes.

Metatexite leucosomes are enriched in SiO2 and depleted in FeO, MgO, Rb, Ba, and LREE relative to the inferred protolith. Metatexite melanosome is enriched in FeO, MgO, Ni, Co, Cr, Nb, Ta, and HREE and depleted in SiO2, CaO, and Na2O relative to the inferred protolith. This is consistent with our supposition that the leucosomes represent a granitic melt that was extracted and quickly separated during high-grade metamorphism leaving behind a more mafic melanosome restite.

Diatexite migmatites are only slightly enriched in FeO and HREE and depleted in Na2O and K2O. This suggests that diatexite migmatites are melt-depleted rocks, with the alkalis being removed during partial melting and FeO and the HREE being concentrated in the residual or peritectic phases.

Major element data from the leucosomes when compared with published experimental data similar rock types both with and without water (Patiño-Douce and Beard, 1995; Patiño-Douce, 1996), shows that the leucosomes have similar alkali contents as melting experiments with 1-4 wt% added H2O. This suggests that water activities during melting were high; although H2O may not have occurred as a free phase. This agrees with estimates of water activities, aH20=0.6-0.9, that were calculated using an independent equilibrium reaction. These findings demonstrate the importance of the presence of H2O during melting in the Inner Piedmont core.