Characterizing the relationship of part of the Inner Piedmont and Pine Mountain window, Georgia, from detailed geologic mapping, geochemistry, geochronology, and structural analysis at the southwestern end of the Cat Square terrane

Geologic investigations at the southwestern end of the Cat Square terrane (CSt) in central Georgia provided new insight into the complex tectonic history of the Pine Mountain window (PMw) and neighboring Inner Piedmont terranes. Detailed 1:24,000-scale geologic mapping of an approximately 180 km2 area near Barnesville, Georgia has provided the foundation for further geochemical, geochronologic, and structural analyses refining models for tectonic development and emplacement of the CSt and PMw.

Whole-rock geochemical analyses of several CSt amphibolites yielded data suggesting a continental back-arc setting for genesis of the Cat Square basin (CSb). Major element data confirm derivation from a basaltic protolith and support the hypothesis that CSt amphibolites represent vestiges of ocean crust once comprising the CSb floor. Mid-ocean ridge basalt normalized multi-element diagrams and tectonic discriminant diagrams indicate that, while the CSb likely developed as a back-arc basin, basalt generated was not homogenous and involved mixing of at least two distinct magma components.

Zircon geochronologic analyses of select lithologies provided new data regarding the crystallization and deformation history of basement units of the PMw, plus a maximum depositional age of the CSb. SHRIMP-RG ages of zircons from basement felsic gneiss, the Woodland Gneiss, and basement tonalite yielded approximate ages of 1158, 1040, and 1011 Ma, respectively. Analysis of three detrital samples of CSt biotite gneiss via LA-ICPMS yielded a 602 ± 8 Ma age for the youngest zircon and peak ages of 1356, 1232, 1171, 1054, 1030, 954, and 858 Ma for the suite. The older of the two ages provides a robust upper limit on the depositional age of the CSb, but does not refute the younger interpreted age of the basin proposed by others.

Structural analysis of the study area supports the hypothesis that the PMw is structurally complex and is framed by at least three faults of different ages. These interpretations have advanced our understanding of the tectonic history of the PMw from once inferring a simple erosional hole in a thrust sheet to now suggesting the Pine Mountain terrane was a microcontinent rifted from the Laurentian margin during the breakup of Rodinia.