RARE-EARTH ELEMENT EVIDENCE FOR THE PETROGENESIS OF THE BANDED SERIESOF THE STILLWATER COMPLEX, MONTANA, AND ITS ANORTHOSITES

A rare earth element (REE) study was made by isotope-dilution mass spe ctrometry of plagioclase separates from a variety of cumulates stratig raphically spanning the Banded series of the Stillwater Complex, Monta na. 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 pre sent throughout the Banded series. This finding supports hypotheses of previous workers that the Stillwater Complex formed from two differen t parent magma types, designated the anorthositic- or A-type liquid an d 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 a nd 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 charac teristics, implying that A-type magmas were injected into the magma ch amber 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 cumulat e with (Ce/Sm)(n) = 2.9, (Nd/Sm)(n) = 1.7, and (Ce/Yb)(n) = 10.1. Anal ysis of published REE and Nd isotopic data for Stillwater cumulates re veals similarities between AN I, AN II, and other thin plagioclase cum ulate 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 d isplay light REE and Nd isotopic characteristics that are similar to U -type cumulates from the Ultramafic series as described by previous st udies. Thus far, the only cumulates from the Banded series that displa y U-type REE and Nd isotopic characteristics are those that contain cu mulus augite. Therefore, cumulus augite appears to be an important ind icator of magmatic parentage. The REE and Nd isotopic ratios show erra tic variation with stratigraphic position, indicating that the magmas from which the Banded series crystallized were injected at various lev els into the magma chamber. Different cumulate types crystallized from discrete liquids, as indicated by the correlation between REE signatu re and cumulate type. Samples from Olivine-bearing zones III and IV (s tratigraphically 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 ru le out the crystallization of the Middle Banded series from a single m agma type. Furthermore, the possibility that AN I and AN II are direct ly related to the underlying Ultramafic series, either as flotation cu mulates or as crystallization products of expelled liquids, is not sub stantiated 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 int erpretations of published Pb isotopic data (Wooden et al., 1991)and in dicate that the magmas from which the Stillwater Complex formed were d erived from two sources that had only small differences in Pb isotopic composition. The REE and isotopic data, as well as crystallization se quences of the two main parental magmas, indicate that the magmas were probably derived from two closely related upper-mantle sources, one h arzburgitic and the other lherzolitic in composition, resulting in the U-type and A-type magmas, from which orthopyroxene crystallized befor e and after clinopyroxene, respectively. Both sources had been enriche d in large-ion lithophile elements, probably owing to mantle metasomat ism.