PETROLOGY, GEOCHEMISTRY, AND GEOCHRONOLOGY OF THE MIDDLE ORDOVICIAN DELHI SYENITE, NE GEORGIA

During a survey of Alleghanian plutons in NE Georgia, an undeformed intrusive body informally known as the Delhi syenite (intrudes the Charlotte Belt) was sampled for precise dating by SHRIMP-RG. This intrusion was previously mapped as either synchronous with the nearby, Alleghanian Danburg pluton (305.8+/-4.2Ma) OR as part of the Late Silurian-Early Devonian Concord plutonic suite of more than 20 gabbroic-syenitic plutons. We report a new SHRIMP-RG ²⁰⁷Pb-corrected ²⁰⁶Pb/²³⁸U zircon age for the Delhi syenite of 468.9±6.7Ma (no inheritance), which indicates instead a Taconic affiliation. Geochemically, the Delhi pluton is a ferroan, alkalic, metaluminous syenite with sodic amphibole and biotite. It is classified as a ferroan, or A-Type, granite characteristic of anorogenic settings. The Delhi syenite plots as shoshonitic (K₂O vs SiO₂) and as a WPG (Nb-Y). Few similarly aged plutons occur in the Charlotte Belt; the Barber (470±24Ma, Sm-Nd) and Southmont plutons (479Ma, U-Pb) in NC, and the Lowry’s North pluton in SC (486Ma, U-Pb). The plutons were postulated to be part of the central Virginia plutonic-volcanic belt which is interpreted to be associated with the late-Cambrian plutonism and volcanism during the development of a continental margin arc. However, the Charlotte belt is also suggested to not be spatially related to the Laurentian margin at this time (Sinha et al., 1989).

The exact timing of the Charlotte Belt’s accretion to Laurentia is not well established in the southern Appalachians. The prevailing models suggest the docking of Carolina during the Late Ordovician-Early Silurian (Pollock et al., 2012) contrasted by the model proposed by Sinha et al. (2012) that stresses accretion of the Charlotte Belt during the Acadian. In either model, the Delhi would have been emplaced prior to the collision of the Charlotte belt with Laurentia. Geochemically, the Delhi most resembles granites formed in an anorogenic setting, but alkalic rocks with shoshonitic affinity also occur as late differentiates in calc-alkaline suites; the paucity of similarly aged plutons in this belt precludes definitive classification of the tectonomagmatic setting of origin. More comprehensive trace element and isotopic analysis may provide additional constraints on the tectonic setting of emplacement for the Delhi syenite.