Petrology and geochemistry of anorthosites in the Stillwater Complex, Montana, and implications for the petrogenesis of the banded series.

Abstract

The petrogenesis of the Stillwater Complex (Montana) and its anorthosites (AN I and AN II) is controversial. The present study uses petrographic and trace element geochemical data to constrain the origin of AN I and AN II and the Banded series. Multiphase (clinopyroxene + ilmenite + apatite) magmatic inclusions occur within cumulus plagioclase grains from AN I and AN II and other rocks in the Middle Banded series. The mineralogy, high MnO content (up to 7.5 wt%) of the ilmenite, and the presence of baddeleyite (ZrO$\sb2$) are interpreted to indicate formation of the inclusions from droplets of an immiscible Fe-Mg-Ti-Mn-REE-P-Zr-enriched depolymerized silicate liquid that exsolved from a highly polymerized aluminosilicate melt from which AN I and AN II crystallized. A lack of correlation between P and REE contents of plagioclase indicates that the amount of apatite is too low to affect the plagioclase REE signature. Rare earth element (REE), Sr isotopic, and new interpretations of published REE, Nd, and Pb isotopic data indicate similarities between plagioclase separates from similar lithology. The pC's (A-type cumulates), including AN I and AN II, formed from melts that had lower Ce/Sm$\sb{\rm n}$ and Nd/Sm$\sb{\rm n}$ and higher $\epsilon$Nd than cumulus augite-bearing rocks, particularly pbaC's (U-type cumulates). Previous models for the formation of AN I and AN II as flotation cumulates from Ultramafic series magma, or as sills, are ruled out. A stratified liquid within the magma chamber is proposed. Different lithologies crystallized from discrete layers which differed in major element, REE and Nd isotopic composition. The magmas were derived from two LILE-enriched sources which differed subtly in Pb isotopic composition. One was harzburgitic and the other lherzolitic, resulting in U-type and A-type magmas, from which orthopyroxene crystallized before and after clinopyroxene, respectively. Trace element contents, including Sr, Ba, Ti, and Mg/(Mg + Fe) do not allow distinction between A- and U-type plagioclase.