Paper No. 7
Presentation Time: 8:00 AM-6:00 PM
EFFECTS OF DISSOLUTION-PRECIPITATION CREEP on THE CRYSTALLOGRAPHIC PREFERRED ORIENTATION OF QUARTZ WITHIN THE PURGATORY CONGLOMERATE, RI
MCPHERREN, Eric D., Department of Geology and Geophysics, Boston College, 140 Commonwealth Avenue, Chestnut Hill, MA 02467, KUIPER, Yvette D., Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, CO 80401 and BEANE, Rachel J., Earth and Oceanographic Science, Bowdoin College, Brunswick, ME 04011, eric.mcpherren.1@bc.edu
Crystallographic Preferred Orientations (CPO) are common in deformed rocks and usually result from crystal plastic deformation by dislocation creep. Whether deformation mechanisms that occur at lower differential stress and lower temperature than dislocation creep, such as Dissolution-Precipitation Creep (DPC), are able to produce a CPO is less certain. DPC has caused substantial removal and reprecipitation of quartz within the Purgatory Conglomerate of Rhode Island. This conglomerate, within the southeastern part of the Pennsylvanian Narragansett basin, experienced folding during the Alleghanian orogeny. DPC dissolved quartz along cobble surfaces perpendicular to the shortening direction and precipitated quartz as overgrowths at the ends of the cobbles (strain shadows) parallel to the maximum extension direction. Strain within the southeastern portion of the Narragansett basin increases from west to east and is associated with a metamorphic gradient from very low grade greenschist facies in the west to the lower biotite zone in the east. We conclude that the degree of cobble rotation and their orientations with respect to the instantaneous shortening axes of the region played an integral role in how DPC changed the shape of the cobbles.
The Purgatory Conglomerate offers a unique opportunity to study the effects of dissolution and precipitation as a result of DPC, because quartz within the cobbles experienced dissolution only, while precipitation occurred in the strain shadows. Electron Backscatter Diffraction (EBSD) analysis tested whether quartz dissolution within the cobbles and/or quartz precipitation within the strain shadows resulted in CPO development. Quartz grain c-axis orientations of various domains within the cobbles and within the strain shadows are random, irrespective of the degree of strain, cobble alignment and metamorphic grade. The existence of discrete mica selvages along the cobble margins, suggest that quartz dissolution only occurred along the cobble surface and did not affect the grains, or result in a CPO, in the cobble’s interior. Quartz precipitation within the strain shadows did not result in a CPO, probably because the strain shadows are truly localized regions of low strain and differential stress, allowing quartz grain growth in random orientations.
