CHEMICAL REMANENT MAGNETIZATIONS ASSOCIATED WITH PALEO-WEATHERING SURFACES

Weathering fluids have long been identified as agents of remagnetization; authigenic hematite precipitation at the weathering surface can hold a chemical remanent magnetization (CRM) that dates the magnetic mineral formation. Widespread Permian-Triassic CRMs are found in basement rocks below weathering surfaces in Europe and North America. The identification and dating of these paleotopographic surfaces may have consequences in both paleogeographic reconstructions as well as implications for climatic conditions during the chemical weathering event. Red Cambrian granites from several localities in the Wichita Mountains (southern Oklahoma) hold a late Permian CRM (D = 150.1º, I = 2.1 º, k = 123.6; α₉₅ = 8.3 º) that resides in hematite. The red granites are found as a weathering profile up to 200m thick. At Long Mountain, quarrying operations reveal that at depth the granites are green in color and hold a magnetization with easterly declinations and steep down inclinations that resides in magnetite and is interpreted to be Cambrian in age. The red granite contains abundant secondary hematite which occurs in fractures, and along cleavage and exsolution planes in alkali feldspars. The Fe in the hematite appears to be sourced from the oxidation alteration of magnetite and ilmenite and the breakdown of mafic minerals. The Permian CRM in the red granite is interpreted to be related to low-temperature weathering fluids that affected the granite exposed near the surface in the Permian. Preliminary data from reddened Dalradian schist below an unconformity overlain by Permian-Triassic sandstones on the Kintyre peninsula in Scotland contain a late Permian-early Triassic CRM (D = 184.6º, I = -33.3º) that resides in hematite. The red sandstone has a similar CRM. The red schist contains dolomite intergrown with authigenic hematite. Gray schist contains magnetite but does not contain a stable magnetization. The Permo-Triassic CRM is seen world-wide and defines the paleotopographic surface during this long-lived chemical weathering event. This Permo-Triassic enhanced weathering surface may be unique in geologic time. Other unconformity surfaces in Scotland are also being investigated as part of this study to determine if similar weathering conditions may be recorded during other geologic time intervals.