GEOCHEMISTRY AND MINERALOGY OF MAFIC, INTERMEDIATE, AND FELSIC ROCKS OF THE NORTH FLORIDA VOLCANIC SERIES

New geochemical analyses, quantitative mineral modes, and optical and electron petrography allow a more detailed characterization of the North Florida volcanic series (NFVS) which underlies early to mid-Paleozoic Suwannee basin strata. The NFVS contains a full suite of calc-alkaline extrusive rocks which have been variably altered at diagenetic to lower greenschist facies conditions.

Basaltic rocks from four wells across ~200 km are unlikely to be directly related but do share many textural, geochemical, and mineralogical characteristics. They are dominantly Pl- and Cpx-phyric, are LREE enriched (La_N/Yb_N = 2.7-4.9), excluding MORB affinity, and have Eu/Eu* values (0.92-1.07) suggesting little to no plagioclase fractionation. All are altered to 50-70% Ab-Qz-Chl-(Ilt)-(Pg)-(Dol)-(Cal). Samples from two wells also contain Act-Prh-Ep as well as Mag-Ttn symplectites.

Analyses of welded andesitic tuff across ~140 m in one well reveal relatively uniform compositions, including SiO₂ = 57.8-60.7% and TiO₂ = 0.70-0.86%, and suggest the recovered material belongs to a single large eruption. High La_N/Yb_N (6.0-6.2) are compatible with an arc setting and low Eu/Eu* (0.81-0.89) demonstrate Pl fractionation. Alteration to ~50% Chl-Ilt-Ep-(Cal) is pervasive. Actinolite is common in the deepest interval, suggesting a contact metamorphic gradient across the penetrated depth.

Felsic rocks are aphyric to weakly porphyritic dacites and rhyolites with subhedral Qz, Kfs, and Pl phenocrysts in fine-grained quartzofeldspathic matrices, altered 42-56% to Ilt-Kln-Hem. In one sample, feldspar has been completely replaced. REE concentrations of felsic rocks overlap with the more mafic lithologies, with more significant plagioclase fractionation shown by Eu/Eu* = 0.71-0.92.

Although petrogenetic relationships among these lithologies are unlikely, available evidence allows that they are common to a single collisional arc. The pervasive, low grade alteration is consistent with weathering and shallow burial. Metamorphic fabrics are not observed, and the occurrence of greenschist facies index minerals does not follow any clear map pattern such as proximity to known granitoids. Therefore, the localized, higher temperature alteration may be due to hydrothermal circulation related to NFVS activity.