PALEOMAGNETIC AND PETROLOGICAL INVESTIGATION OF LONG MOUNTAIN GRANITE, WICHITA MOUNTAINS, OKLAHOMA

The Long Mountain Granite, a member of the Cambrian Wichita Granite Group, is exposed in the western Wichita Mountains, Oklahoma. The rock is red at the surface which grades into a dark gray to green core that has been exposed by quarrying operations. The contrasting exposures provide an opportunity to investigate the petrological and magnetic properties of the rock and how they have changed with alteration. The alkali feldspar granite is highly granophyric and fluorite-bearing with hedenbergite as the dominant mafic phase. The red and green granite have similar geochemical signatures. In the red granite, hematite occurs as fracture fill, grain boundary coatings and as slivers inserted along cleavage and exsolution planes in alkali feldspars. The Fe in the hematite appears to be sourced from the oxidation of magnetite and ilmenite and the breakdown of mafic minerals. Anisotropy of magnetic susceptibility analysis shows that the green granite contains what appears to be a primary magnetic fabric that is consistent with the sill-like emplacement of Wichita-group granites. Paleomagnetic analysis of the green granite yields a characteristic remanent magnetization (ChRM) with easterly declinations and steep down inclinations. This is interpreted as a primary Cambrian thermal remanent magnetization residing in magnetite, and the pole (8.8°S, 134.7°E) is consistent with several other Cambrian paleomagnetic poles of similar age. In contrast, the red granite has approximately two orders of magnitude lower magnetic susceptibility and natural remanent magnetization intensity. The degree of magnetic anisotropy is reduced relative to green granite and the fabrics are incompatible. The ChRM has southeasterly declinations and shallow inclinations with a late Paleozoic paleopole (44.9°N, 124.9°E) and is interpreted as a chemical remanent magnetization (CRM) residing in hematite. The CRM was caused by fluid interactions, either by high-temperature fluids mobilized by uplift or more likely by low-temperature weathering fluids while exposed near the surface in the late Paleozoic.