Formation of the Fine Scale Rhythmic Layering of the Stillwater Complex, Montana

The Stillwater Complex (SC) is a Precambrian layered mafic intrusion which outcrops along the northern edge of the Beartooth Mountains in southwestern Montana. The SC is broken down into 5 zones, the Basal Series, the Ultramafic Series, the Lower Banded Series, the Middle Banded Series, and the Upper Banded Series. The focus in this study is on the fine scale rhythmic layering found in the Lower Banded Series of the SC. These fine scale rhythmic bands repeat a doublet pattern of alternating, millimeter- to centimeter-scale plagioclase and pyroxene bands. The pyroxene bands are usually 1 to 2 cm thick and separated by 2 to 4 cm thick plagioclase bands. The purpose of this study is to determine the mechanisms (i.e., physical or chemical) which formed the fine scale rhythmic bands of the SC. Utilizing a combination of full thin section quantitative element maps, EPMA major element and LA-ICP-MS trace element analyses of plagioclase and pyroxene, and various geothermometers and geospeedometers, we have determined the crystallization history of our sample. The calculated crystallization temperature for coexistent plagioclase and pyroxene is 1220±13°C based on rare earth element (REE) partitioning. The calculated temperatures based on Fe-Mg partitioning between coexisting pyroxene are ~835°C. Based on this difference in equilibration temperature, differences in REE and Fe-Mg diffusion rates, and geospeedometry models of Sun and Lissenberg (2018), the high temperature cooling rate is in the range of 0.0011 - 0.0102 °C/year. The rapid cooling rate indicates crystallization of plagioclase and pyroxene over a short time interval near the solidus temperature. Textural relationships and crystallization temperatures are inconsistent with compositional layering during crystallization and instead support sub-solidus equilibration and formation of the fine rhythmic layering in the Lower Banded Series.