WHERE ARE THE CPX-BEARING ROCKS IN BLUE RIDGE METAULTRAMAFIC ROCK BODIES?: INSIGHTS FROM THE WEBSTER-ADDIE COMPLEX, NC

The Webster-Addie ultramafic complex consists primarily of metadunite, but, unique to metaultramafic bodies of the Blue Ridge belt, it also contains a significant amount of clinopyroxene-rich rocks (chiefly olivine metawebsterite and olivine metaclinopyroxenite). The latter are interpreted to represent cpx-rich ultramafic cumulates derived from the lower part of the plutonic complex that overlies the basal metamorphic dunite in a typical ophiolite sequence.

Metamorphism of the cpx-rich rocks has involved the following: (1) exsolution in the pyroxenes (both cpx and opx); (2) partial replacement of cpx by calcic amphibole (edenite/tremolite); and (3) growth of tremolite mantles around edenite cores. Two-pyroxene geothermometry combined with the absence of anthophyllite in associated metadunites indicates peak metamorphic conditions were 795-835˚C at ~11 kb, consistent with the upper amphibolite facies of the enclosing schists.

Whereas other Blue Ridge metaultramafic bodies are virtually devoid of cpx, calcic amphibole crystals similar to those found replacing pyroxene at Webster-Addie are a minor but widespread constituent. We suggest that such calcic amphiboles may reflect the former presence of clinopyroxene. The significant Ca-content of various metaultramafic chlorite-talc-amphibole schists can similarly be explained if the protoliths were cpx-bearing ultramafic rocks and, during metamorphism and accompanying hydration, the cpx recrystallized to calcic amphibole.