A rare earth element (REE) study was made by isotope-dilution mass spectrometry of plagioclase separates from a variety of cumulates stratigraphically spanning the Banded series of the Stillwater Complex, Montana. Evaluation of parent liquid REE patterns, calculated on the basis of published plagioclase-liquid partition coefficients, shows that the range of REE ratios is too large to be attributable to fractionation of a single magma type. At least two different parental melts were present throughout the Banded series. This finding supports hypotheses of previous workers that the Stillwater Complex formed from two different parent magma types, designated the anorthositic- or A-type liquid and the ultramafic- or U-type liquid. On the basis of our data, one melt has a REE pattern with a distinctive shallow slope and is represented by samples from the thick, massive Anorthosite zones I and II (AN I and AN II) of the Middle Banded series. Although samples from AN I and AN II are separated by as much as 1400 m stratigraphically, they have remarkably similar calculated parent liquid characteristics, with (Ce/Sm)n = 1.7-1.9, (Nd/Sm)(n) = 1.3-1.4 and (Ce/Yb)(n) = 2.9-4.6 (where n denotes chondrite-normalized). These calculated liquids are probably close to representing A-type magma. In addition, plagioclase-bronzite cumulates from Norite zones I and II (N I and N II), although thought to be U-type cumulates, contain plagioclase that has A-type REE characteristics, implying that A-type magmas were injected into the magma chamber during formation of those zones. In contrast, calculated parent liquids of cumulus augite-bearing rocks have REE patterns that display distinctly steeper slopes than the A-type REE pattern. The extreme is the calculated parent liquid of a plagioclase-bronzite-augite cumulate with (Ce/Sm)(n) = 2.9, (Nd/Sm)(n) = 1.7, and (Ce/Yb)(n) = 10.1. Analysis of published REE and Nd isotopic data for Stillwater cumulates reveals similarities between AN I, AN II, and other thin plagioclase cumulate layers in the Lower and Upper Banded series, which supports the notion that they were all derived from similar (A-type) parent melts. In contrast, plagioclase separates from cumulus augite-bearing rocks display light REE and Nd isotopic characteristics that are similar to U-type cumulates from the Ultramafic series as described by previous studies. Thus far, the only cumulates from the Banded series that display U-type REE and Nd isotopic characteristics are those that contain cumulus augite. Therefore, cumulus augite appears to be an important indicator of magmatic parentage. The REE and Nd isotopic ratios show erratic variation with stratigraphic position, indicating that the magmas from which the Banded series crystallized were injected at various levels into the magma chamber. Different cumulate types crystallized from discrete liquids, as indicated by the correlation between REE signature and cumulate type. Samples from Olivine-bearing zones III and IV (stratigraphically between AN I and AN II) display a range in REE ratios; e.g., (Ce/Sm)(n) = 1.8-2.8 and (Ce/Yb)(n) = 3.9-6.1, results that rule out the crystallization of the Middle Banded series from a single magma type. Furthermore, the possibility that AN I and AN II are directly related to the underlying Ultramafic series, either as flotation cumulates or as crystallization products of expelled liquids, is not substantiated by the REE data because the calculated parent magma of AN I and AN II was different from that of the Ultramafic series as defined by previous studies. The REE data of this study further constrain interpretations of published Pb isotopic data (Wooden et al., 1991)and indicate that the magmas from which the Stillwater Complex formed were derived from two sources that had only small differences in Pb isotopic composition. The REE and isotopic data, as well as crystallization sequences of the two main parental magmas, indicate that the magmas were probably derived from two closely related upper-mantle sources, one harzburgitic and the other lherzolitic in composition, resulting in the U-type and A-type magmas, from which orthopyroxene crystallized before and after clinopyroxene, respectively. Both sources had been enriched in large-ion lithophile elements, probably owing to mantle metasomatism.
