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Structure and geochemistry of the Nemo area Black Hills, South Dakota, United States of America
Abstract
The Black Hills are located near the trace of the Proterozoic plate boundary (North America Central Plain Conductivity Anomaly) and may be the only exposure along the Trans-Hudson Orogenic belt (1850 Ma). Thus, the Black Hills should contain important aspects for tectonic interpretation of Early Proterozoic plate movement. With this objective, this investigation gathered structural information on the strain history. The Nemo Iron Formation is believed to have been deposited before the Early Proterozoic Boxelder Creek Quartzite. Using stratigraphic evidence alone, it is difficult to separate the Nemo Iron Formation from the younger Benchmark Iron Formation of the Nemo area. Therefore, the present study used geochemical and structural analysis to supplement Redden's purely stratigraphic study. The geochemical results show that the Benchmark Iron Formation and the Nemo Iron Formation have: (1) different REE distribution characteristics, (2) different europium and cerium anomalies, which indicate a different chemical precipitation environment. REE patterns in the ferruginous schist intercalated with the Nemo Iron Formation are typically Archean in character. In contrast, sedimentary rocks deposited after the Boxelder Creek Quartzite show post-Archean REE characteristics. TEM analysis revealed that basal $\langle a\rangle$ slip systems were activated in the quartz of the banded iron formations of the Nemo area during a later folding event (F$\sb2$ or F$\sb3$ period). These slip systems are favored by lower temperature conditions. The dominant grain boundary type of the hematite-rich layers is a low angle grain boundary indicating that the degree of dynamic recovery was extensive. There are two peaks in the misorientation-angle distribution of grain boundaries within the hematite layer. One peak indicates low-angle (1 $\sim$ 15$\sp\circ)$ grain boundaries and the other indicates high-angle (${\approx}55\sp\circ)$ grain boundaries. This bimodal distribution is attributed to subgrain rotation during recrystallization. Quartz lattice preferred orientations suggest that dominant symmetrical fabrics were caused by east-west directed pure shear during the F$\sb2$ event. The antisymmetrical fabrics are interpreted as resulting from strain localization during the ductile F$\sb2$ deformation.
Subject Area
Geology|Geochemistry|Geophysics
Recommended Citation
Lee, Kyung Ho, "Structure and geochemistry of the Nemo area Black Hills, South Dakota, United States of America" (1996). ETD collection for University of Nebraska-Lincoln. AAI9839145.
https://digitalcommons.unl.edu/dissertations/AAI9839145