A PALEOMAGNETIC AND PETROGRAPHIC ANALYSIS OF UNCONFORMITY SURFACES IN NEVADA AND MISSOURI: POSSIBLE IMPLICATIONS FOR PALEOWEATHERING PROCESSES

Paleomagnetic and petrographic analyses were performed on two rock suites to determine the extent of subaerial exposure of Precambrian crystalline basement rocks prior to deposition of sedimentary cover. At Frenchman Mountain, Nevada, a ChRM held primarily in magnetite in Vishnu Group schists and granites exhibits northeasterly declinations and moderately steep up inclinations and yields a paleopole at 16.3°S, 129.9°W (d_p= 7.3, d_m= 10.2), near previously reported Mesoproterozoic pole positions. This magnetization is interpreted as a TRM and was likely acquired during cooling after peak metamorphism. Petrographic analysis of Vishnu specimens confirms the presence of Ti-magnetite as an accessory mineral and the magnetic carrier. Magnetizations in the overlying Tapeats Sandstone, although stable, contain scattered directions and are interpreted as the product of time-progressive acquisition of magnetizations during growth of authigenic hematite. Local climatic conditions or the presence of unknown eroded sedimentary units may have prevented the basement from acquiring or preserving a magnetization during subaerial exposure in the Neoproterozoic.

In southeastern Missouri, paleomagnetic directions in the Grassy Mountain Ignimbrite (GMI) and Butler Hill Granite (BHG) are westerly and moderately steep down, corresponding to a paleopole at 3.4°S, 145.1°W (d_p= 1.8, d_m= 2.8), agreeing with a previously published primary Mesoproterozoic (1.476 ± 16 Ma) magnetization. This is interpreted as a primary magnetization held in magnetite acquired during emplacement and eruption. Petrographic and rock magnetic analyses confirm magnetite as the magnetic carrier. The overlying Lamotte Sandstone contains a stable magnetization with southeasterly declination and moderate down inclination, corresponding to a Silurian or Mississippian paleopole at 14.7°N, 126°W (d_p= 8.7, d_m= 13.6). This is interpreted as a CRM held in magnetite, acquired during the late Devonian to early Mississippian movement of Fe-rich ore-forming hydrothermal fluids. The silica-rich, low-permeability GMI likely served as a protective seal for the basement and prevented penetration of supergene or ore-forming fluids into the upper basement rocks, preserving the primary magnetization held in the GMI and the BHG.