GSA Connects 2021 in Portland, Oregon

Paper No. 188-6
Presentation Time: 2:30 PM-6:30 PM


MENNENGA, Madeline, Geology Department, Grand Valley State University, 1 Campus Drive, Allendale, MI 49401, PETERSON, Ginny, Geology Department, Grand Valley State University, Geology Department, Grand Valley State University, Allendale, MI 49401 and RAHL, Jeffrey, Department of Geology, Washington and Lee University, 204 W Washington St, Lexington, VA 24450-2116

Interpretation of data from Electron Backscatter Diffraction (EBSD), thin section observations, and paleopiezometry has led to a better understanding of the deformation and emplacement history of ultramafic rocks. The relatively pristine olivine-rich ultramafic bodies preserved in the western Blue Ridge of North Carolina lend themselves to these textural studies. Our previous work on one of the largest, the Buck Creek (BC) ultramafic body indicates formation as cumulate oceanic crust and emplacement into the lower Laurentian continental crust, with consistent (9 samples) B-type Olivine fabrics ((010)[001] slip system), low water content, and recrystallized grain sizes from thin sections (97-150 μm) that imply stresses ~32-46 MPa.

This study explores olivine analyses from dunite deposits NE of Buck Creek (Deposit #9 – D9, Mincey Mine - MM, Moore’s Knob - MK, Webster-Addie - WA, Dark Ridge - DR), with a revised approach to determine recrystallized grain size for paleopiezometry using EBSD data. We adapted published MATLAB/MTEX code (for quartz) that uses grain orientation spread to separate recrystallized and relict grains and calculate statistical grain-size data for olivine. Median recrystallized grain size was used in 3 different paleopiezometers. Recrystallized grain size/stress estimates for BC samples using thin section and EBSD methods are similar; the EBSD method is more quantitative and includes significantly more data points. Median grain sizes vary from 75-150 μm, corresponding to stresses of 32-56 MPa. Crystallographic Preferred Orientation (CPO) plots and Inverse Pole figures from EBSD data indicate C- or E-type fabrics - [001](100) or [100](001) slip for D9, WA, and DR deposits and A-type fabrics – [100](010) slip for MM and MK deposits.

Recrystallized grain size/stress estimates for BC samples overlap with those determine for the 5 deposits in this study; however, the slip systems active during deformation are distinctly different for BC (B-type) vs. the other deposits (C/E- or A-type), suggesting different tectonic settings for deformation of the different deposits.