PHYSICAL, MINERALOGICAL, AND CHEMICAL TRENDS DURING RETROGRADE METAMORPHISM OF METAULTRAMAFIC ROCKS, BLUE RIDGE BELT, NORTH CAROLINA, USA

Blue Ridge metaultramafic (MU) rocks — associated with metamafites, pelitic schists, and quartzofeldspathic gneisses and semischists — underwent fluid-facilitated, metasomatic, retrograde metamorphism during Taconic, Acadian, and Alleghenian orogenies in the southern Appalachian region. A number of physical, mineralogical, and chemical changes in the MU rock bodies, rocks, and minerals characterized the metamorphism as the rocks changed from eclogite/granulite? facies conditions, and responded to upper and lower amphibolite facies and greenschist facies conditions. Hydration increased over time resulting in changes from olivine-rich rocks to Mg-phyllosilicate schists and serpentinites. Average rock body aspect ratios changed from ~ 2 to 6 to 17 and maximum aspect ratios changed from 5 to 15 to 35. Calculated and estimated P,T conditions changed from 15kb/750°C to 7-10kb/700°C to < 4kb/< 400°C. Maximum whole rock wt.% Al2O3 rose from 2 to 8, while maximum CaO rose from < 1 to 10. In spinels, TiO2 wt % [~0.05 to 10+], total iron, and Cr number rose; as wt. % Cr [~ 60 to 0], wt. % Al [~ 10 to <1.0], and Mg number fell. The latter data highlight the mobility of Al, Ca, and Ti during metamorphism of these rocks and signal caution in the use of chemical-petrotectonic setting diagrams for determining sites of origin of ultramafic protoliths. The oldest assemblages characterized by olivine, pyroxenes, and Al-richer, Cr spinel cores may represent metamorphic rocks rather than igneous protoliths.