YOUNGER SEAFLOOR FROM AN OLDER PART OF AN ANCIENT OCEAN BASIN: THE CASE OF THE PCF AT THE GEORGIA SOUTHERN APPALACHIANS

The Pumkinvine Creek Formation (PCF) consists of interbedded amphibolites and metasedimentary rocks. The amphibolites are one among many mafic and ultramfic bodies that are present just east of an inferred Taconian Suture that is represented by the Hayesville Thrust Fault and its equivalents including the Allatoona Fault. The PCF amphibolites are interlayred with metafelsites that contain relict volcanic and/or pyroclastic textures. The chemistries of samples from these fine-grained protoliths would represent magmas from which they formed perhaps modified much later by metamorphism. The felsites have moderately high over all chondrite-normalized REE ratios and patterns characterized by moderate LREE enrichment and EU depletion. They are depleted in titanium, and have chemistries all together indicating a suprasubduction calcalkaline suite. Chondrite-normalized REE patterns of the amphibolites are flat similar to those of oceanic tholeiites and their overall values are quite low. Because the REE have low fluid /rock partition coefficients, it would take large volumes of water than present in regional metamorphic event to reduce the REE abundance in the rocks. Accordingly, the REE abundance is dependent on the magma source, and the low REE abundance of the PCF amphibolites likely indicates a primitive mantle source. Similar to the PCF amphibolites, moderate to high TiO2, and flat REE patterns and low rock/chondrite ratios in some of the amphibolites of Lake Chatuge and Chunky Gal Mountains of Northern Georgia and North Carolina were likely derived from primitive mantle sources. A primitive mantle source for the 460 million year old PCF likely indicates magma generation by rifting (back arc style) from the older part of an ocean similar in setting to the modern western Pacific Ocean, which is older than the eastern part.