COEXISTING DIOPSIDE/SPINEL AND TSCHERMAKITE/SPINEL SYMPLECTITES IN PLAGIOCLASE DOMAINS OF GRANULITE FACIES METATROCTOLITES FROM THE BUCK CREEK BODY, NORTH CAROLINA BLUE RIDGE

The Buck Creek body consists of metamorphosed dunites and troctolites, and is the ultramafic portion of the Chunky Gal Mountain mafic-ultramafic complex of the North Carolina Blue Ridge. Although amphibolite facies retrogression is common, many Buck Creek troctolites preserve textures and mineral assemblages characteristic of early granulite facies metamorphism that is well documented in the region. As a result of granulite facies metamorphism, olivine developed orthopyroxene rims, and calcic plagioclase was rimmed and partially to completely replaced by complex, fibrous symplectites. X-ray element maps and detailed microprobe line traverses show that diopside-spinel (Di/Spl) and tschermakitic amphibole-spinel (Tsch/Spl) symplectites occur as separate grains in the reacted plagioclase domains and must have grown simultaneously to replace plagioclase at high temperature. Di/Spl and Tsch/Spl grains occur in sharp contact with one another and there is no evidence of replacement of one by the other.

Although clinopyroxene-spinel symplectite commonly replaces plagioclase at granulite facies conditions, tschermakitic amphibole was apparently required to accommodate minor Na in the plagioclase (An~90) of Buck Creek troctolites that could not be taken up by the clinopyroxene at the moderate pressure of the granulite facies metamorphism. The Di/Spl symplectite in plagioclase of Buck Creek troctolites has a higher proportion of spinel than does the Tsch/Spl symplectite. The difference in proportion of spinel in the two different kinds of symplectite must result from the fact that more of the Al in plagioclase is taken up in the tschermakitic amphibole than in the diopside.

These observations show that plagioclase replacement in these troctolites was dependent on local mass balance of elements like Na and Al in addition to P/T stability of replacement minerals. Reaction within the plagioclase domains was nearly isochemical except for addition of Mg and Fe from external breakdown of Olivine to Orthopyroxene.