Geochemical and geochronological study of the Danburg and Sandy Hill granitoids and associated mafic enclaves, northeast Georgia

by (Douglas Kent), 1957- Dvoracek

A comprehensive data set of whole rock and mineral chemistry has been collected from the Alleghanian Danburg pluton and associated igneous rocks. The pluton has been dated at 304 +/- 5 Ma, with an initial Sr ratio of 0.70476 +/- 0.00016. Amphiboleplagioclase thermobarometry indicates the early magma crystallized at pressures and temperatures similar to nearby Alleghanian granites. Mafic enclaves in the Danburg pluton are shown to result from quenching of an intermediate or gabbroic magma that likely had peralkaline affinities. These unusual compositions produced texturally rare enclaves with high HFSE contents and abundant titanite. The proximity of a syenitic stock adjacent to the Danburg pluton suggests that this magma may be the source of the mafic enclave magma. The Danburg pluton is an LIL-enriched body with trace element chemistry similar to subduction related magmas. However, overall geochemical, isotopic, and field relations favor a post-subduction environment for magma genesis. It is therefore proposed that the Danburg pluton formed after the cessation of subduction, possibly in response to crustal thickening or intrusion of more mafic magma into the crust following the collision of Laurentia and Gondwana. Geochemical signatures suggestive of subduction regimes are likely inherited from the source area. The Sandy Hill pluton is a leucocratic envelope on the south and southeastern margin of the Danburg body. Whole rock and mineral chemistry, coupled with isotopic data, indicate that the Sandy Hill pluton is an approximately contemporaneous intrusion but is not genetically related to magma that produced the Danburg pluton.