EMPLACEMENT OF THE DEVONIAN RABUN GRANODIORITE, NORTHEAST GEORGIA AND NORTH CAROLINA BLUE RIDGE: DETAILED GEOLOGIC MAPPING, AND STRUCTURAL, AND PETROLOGIC ANALYSIS

Numerous hypotheses exist regarding the origin, ascent, and emplacement of granitic magmas. The two end-member ascent mechanisms for transport of granitic magma through the crust are diapiric rise and dike-transport. Other mechanisms include stoping, transport, and emplacement along faults or other pre-existing fractures, tectonic squeezing, and ballooning (related to emplacement). Magma ascent mechanisms are clearly controlled to some extent by tectonic setting (e.g., tensional vs. contractional settings). Large plutons emplaced at transcurrent faults or during crustal thinning may be explained by siphoning of magma along fractures that tap deep-seated sources. Recent modeling and field studies (e.g., Great Tonalite sill, SE Alaska and British Columbia) have shown plutons emplaced in contractional settings during orogenesis often take advantage of crustal-scale faults as conduits from their source to final emplacement site. Plutons emplaced in compressional regimes not associated with crustal-scale faults have remained more difficult to explain. The Rabun Granodiorite, a ~374 Ma, linear, two-phase (megacrystic and fine-grained), polydeformed, catazonal pluton emplaced syntectonically during the Neoacadian orogeny, shows no evidence of emplacement along a preexisting weakness zone. The Rabun Granodiorite is also not a pluton tilted on its side, like the Bergell pluton in Switzerland. Detailed geologic mapping and structural analysis in NE Georgia and SW North Carolina reveals a dike-transport and possible ballooning mechanism for ascent and emplacement of the Rabun at mid-to lower crustal level. Neither inward dipping foliation nor vertical stretching lineations are present in the pluton, nor are strong shear zones or rim synforms related to a passing diapir present in the country rocks near the contact. Evidence for dike ascent is observed near contacts where pervasive screens of wall rock and thin layers of strongly deformed granite are in contact with and truncated by layers of less deformed granite. Primary magmatic foliation is preserved in the interior of the pluton, while a strong, solid-state plastic flow deformation is present near the contacts. Contact relationships observed around the Rabun pluton provide further support for a multi-pulse, dike ascent, ballooning emplacement mechanism for mid- to lower crustal plutons.