Paper No. 2
Presentation Time: 8:00 AM-12:00 PM
ALTERED AMPHIBOLITE HYPOTHESIS FOR THE ORIGIN OF TODD-TYPE CHLORITE BODIES IN THE ASHE FORMATION, NW NORTH CAROLINA: EVIDENCE FROM AMPHIBOLE MINERALOGY AND PARAGENETIC SEQUENCE
Scotford and Williams (1983) divided chlorite bodies found within the Late Proterozoic-Early Paleozoic Ashe Formation in NW North Carolina into Edmonds- and Todd-type based on modal mineralogy and bulk geochemistry that are interpreted to reflect the degree of metasomatic alteration of an ultramafic protolith. Todd-type chlorites have a sheet-like geometry, m- to several 10's of m-thick and 10's of m to several km long and are parallel to the regional Ashe foliation. Where contact relationships can be determined, Todd-type chlorite gneisses are in sharp contact with coarse-grained amphibolites and Todd-type chlorite schists are in sharp contact with fine-grained amphibolites and locally contain included decimeter-size, angular blocks of fine-grained amphibolite. Sampled Todd-type chlorite gneisses and their adjacent coarse-grained amphibolites have very similar amphibole mineralogies and paragenetic sequences. Both have relict magnesiohornblende grains, rich in Fe/Ti-oxide inclusions, as the initial amphibole phase which is overgrown and largely replaced by Mg-rich actinolite (0.879+0.027 Mg/Mg+Fe). In some amphibolites the actinolite is overgrown by an inclusion-free magnesiohornblende. Cummingtonite is the final amphibole phase in both of these lithologies; it replaces the actinolite and chlorite grains. Sampled Todd-type chlorite schists and their adjacent fine-grained amphibolites also have similar amphibole mineralogies and paragenetic sequences. Both consist of a variety of hornblendes (magnesiohornblende, tschermakite, and pargasite) that are overgrown or replaced with cummingtonite. Relict olivine was not found in either the chlorite gneisses or schists. These similarities in amphibole mineralogies and paragenetic sequences argue that the adjacent coarse- and fine-grained amphibolites are most likely the protoliths for the Todd-type chlorite bodies, rather than ultramafics which have undergone metasomatic alteration.