THE STILLWATER COMPLEX, MONTANA: THE ROOT OF A FLOOD BASALT PROVINCE?

We have estimated the amount of magma that passed through the Stillwater Complex to form its Ultramafic Series. The estimate ranges from 70,000 to 210,000 km3. By comparison, the estimated eruptive volume of the Colombia River Flood basalt province is 170,000 km3. The estimate for the Stillwater volume is a minimum because the Ultramafic Series is about 20 percent of the thickness of the Complex where it is most complete and the chamber was larger then the exposed Complex now.

About 20 cyclic units compose much of the Ultramafic Series, and most workers interpret their presence as an indication that the magma chamber filled and vented many times. In addition, the Complex is almost 6 km thick at its thickest point and was probably at least 8 km originally. The maximum metamorphic pressure at the base is 3 kb (9 km). Thus, the top of the complex was very close to or even at the surface, and venting almost certainly took place to the surface. Estimating the volume of magma involved several factors: (1) the area of the Complex, (2) the thickness of the Ultramafic Series; and, (3) the amount of fractionation that took place to form the cyclic units. The area of the Complex as determined by gravity and magnetics is 2500 to 4400 km2. From measured sections, the average thickness of the Ultramafic Series is 1400 m. From detailed study of the very small amount of change in Mg/(Mg+Fe) in the cumulate olivine and orthopyroxene (Mg#=.86-.84), we conclude that no more than 3-5 percent fractionation took place during the deposition of the Ultramafic Series. Thus, 95 to 97 percent of the liquid was probably vented because the cumulates above in the Banded Series display a different crystallization order than in the Ultramafic Series. Given the dimensions and an estimate of the fraction of liquid vented, we arrive at the volume extruded. Recall this estimate is for only about 20 percent of the Complex, we do not yet have a method to estimate the volume for the rest of the Stillwater.

We find no trace of the basalt that almost certainly vented to the surface 2.7 billion years ago. In addition, there are no known flood basalt provinces that are boninitic as the parent composition of the Stillwater Complex was.