Neodymium, strontium, and lead isotopes in the Maloin Ranch Pluton, Wyoming: Implications for the origin of evolved rocks at anorthosite margins

Abstract

Neodymium, strontium, and lead isotopic data are used in this study to investigate the origin of chemically evolved rocks in the Maloin Ranch Pluton, a composite body that borders and intrudes the Laramie Anorthosite. The range of rock types present is similar to that found at the margins of Proterozoic anorthosites throughout the world. In the Maloin Ranch Pluton, these include ferrodiorite at the base of the intrusion, overlain progressively by fine-grained monzonite, monzosyenite, and porphyritic granite. Biotite gabbro and fine-grained granitic dikes are present locally at various levels of this sequence. The origin of the evolved rocks and their possible relation to associated anorthositic bodies has been much debated. In the Maloin Ranch Pluton, each rock type has distinct isotopic characteristics which, together with trace-element data previously reported, suggest different source characteristics for each member. Strontium and neodymium isotopic data for Maloin Ranch ferrodiorite and Laramie anorthositic rocks show considerable overlap, consistent with a comagmatic relation. Biotite gabbro is chemically and isotopically the most primitive rock type in the Maloin Ranch Pluton. The data suggest that biotite gabbro has a mantle source, but has undergone extensive fractionation in the crust. Our results suggest that the remainder (and bulk) of the intrusion formed by partial melting of the lower crust due to the emplacement of the Laramie Anorthosite. Maloin Ranch fine-grained monzonite is chemically more evolved than the underlying ferrodiorite (i.e., lower Mg #, higher silica and alkalis), yet has more primitive isotopic characteristics (lower I Sr ; higher epsilon Nd). Formation of fine-grained monzonite by in situ fractionation of ferrodiorite is precluded by both the isotopic data and by mass balance. The isotopic data do not rule out formation of fine-grained monzonite by fractionation or contamination of ferrodiorite at depth, but this model requires that the ferrodiorite is isotopically heterogeneous and that very large volumes of ferrodiorite were present. Alternatively, trace-element and isotopic characteristics of the fine-grained monzonite are explained by partial melting of mantle-derived lower crust, added to the margin of the Archean Wyoming craton at about 1.8 Ga. Neodymium, strontium, and lead isotope data for Maloin Ranch monzosyenite and porphyritic granite also suggest a lower crustal source. These data indicate a > 1.8 Ga source component for the Maloin Ranch monzosyenites, but do not require input of Archean crust from the Wyoming province to the north.