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Title: 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. In the Maloin Ranch Pluton, these include ferrodiorite at the base of the intrusion, overalain 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. The authors' 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. Trace-elementmore » and isotopic characteristics of the fine-grained monzonite are explained by partial melting of mantle-dervied 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.« less

Authors:
;  [1];  [2];  [3]
  1. (State Univ. of New York, Stony Brook (United States))
  2. (Univ. of Wyoming, Laramie (United States))
  3. (Univ. of Idaho, Moscow (United States))
Publication Date:
OSTI Identifier:
6967762
Resource Type:
Journal Article
Resource Relation:
Journal Name: Geochimica et Cosmochimica Acta; (United States); Journal Volume: 55:8
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; PLUTONIC ROCKS; PETROCHEMISTRY; PETROGENESIS; WYOMING; GEOLOGIC FORMATIONS; CHEMICAL COMPOSITION; GEOCHEMISTRY; ISOTOPE RATIO; LEAD ISOTOPES; NEODYMIUM ISOTOPES; STRONTIUM ISOTOPES; ALKALINE EARTH ISOTOPES; CHEMISTRY; DEVELOPED COUNTRIES; FEDERAL REGION VIII; IGNEOUS ROCKS; ISOTOPES; NORTH AMERICA; ROCKS; USA; 580000* - Geosciences

Citation Formats

Kolker, A., Hanson, G.N., Frost, C.D., and Geist, D.J. Neodymium, strontium, and lead isotopes in the Maloin Ranch Pluton, Wyoming: Implications for the origin of evolved rocks at anorthosite margins. United States: N. p., 1991. Web. doi:10.1016/0016-7037(91)90104-D.
Kolker, A., Hanson, G.N., Frost, C.D., & Geist, D.J. Neodymium, strontium, and lead isotopes in the Maloin Ranch Pluton, Wyoming: Implications for the origin of evolved rocks at anorthosite margins. United States. doi:10.1016/0016-7037(91)90104-D.
Kolker, A., Hanson, G.N., Frost, C.D., and Geist, D.J. Thu . "Neodymium, strontium, and lead isotopes in the Maloin Ranch Pluton, Wyoming: Implications for the origin of evolved rocks at anorthosite margins". United States. doi:10.1016/0016-7037(91)90104-D.
@article{osti_6967762,
title = {Neodymium, strontium, and lead isotopes in the Maloin Ranch Pluton, Wyoming: Implications for the origin of evolved rocks at anorthosite margins},
author = {Kolker, A. and Hanson, G.N. and Frost, C.D. and Geist, D.J.},
abstractNote = {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. In the Maloin Ranch Pluton, these include ferrodiorite at the base of the intrusion, overalain 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. The authors' 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. Trace-element and isotopic characteristics of the fine-grained monzonite are explained by partial melting of mantle-dervied 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.},
doi = {10.1016/0016-7037(91)90104-D},
journal = {Geochimica et Cosmochimica Acta; (United States)},
number = ,
volume = 55:8,
place = {United States},
year = {Thu Aug 01 00:00:00 EDT 1991},
month = {Thu Aug 01 00:00:00 EDT 1991}
}
  • The Maloin Ranch pluton is a layered composite intrusion at the southeast margin of the main body of the Laramie Anorthosite. The layering defines a body shaped like a half bowl, which contains ferrodiorite at its base, overlain progressively by fine-grained monzonite and porphyritic monzonite, monzosyenite, and porphyritic granite. The trace element data presented here, together with isotopic data for these rocks (Kolker, 1989) show that each of the members of this composite intrusion has a separate origin. Field, geochemical, and isotopic evidence are consistent with a comagmatic relation between ferrodiorite and anorthositic rocks, either by fractionation or immiscibility. Themore » fine-grained monzonite is not the result of fractionation of ferrodiorite, or mixing of ferrodiorite with a more evolved magma, such as monzosyenite or granite. At intermediate levels in the intrusion, porphyritic monzonite formed by mixing of fine-grained monzonite (or biotite gabbro) and monzosyenite magmas. Textures, REE patterns, and other trace-element data show that the monzosyenite is a feldspar cumulate, with a highly variable proportion of liquid. Trace-element and isotopic characteristics of the Maloin monzosyenite are consistent with derivation from a more evolved deep crustal source or more extensive fractionation, compared to the fine-grained monzonite. REE data suggest that some of the overlying porphyritic granite is a late-stage segregation related to the monzosyenite and not to the overlying Sherman Granite.« less
  • In order to provide a precise age for the Red Mountain pluton, the youngest of the syenitic bodies of the Laramie Anorthosite Complex, Wyoming, zircons were extracted and analyzed for U-Pb isotopes. Zircon grains were selected and individually analyzed in order to minimize potential complications of inherited components. No grains with inherited cores were identified, either by microscopic examination or from the isotopic data. On a concordia diagram, the U-Pb data form a slightly discordant array. A best-fit line intersects concordia at 1,439{sup +7}{sub {minus}6} Ma and 102 {plus minus} 64 Ma (95% confidence level). The upper intercept is interpretedmore » as the crystallization age of the Red Mountain pluton and provides a younger age limit for the rest of the Laramie Anorthosite Complex.« less
  • Anorthosites and gabbros from the Anorthosite Complex (LAC) have Nd and Sr isotopic ratios that are similar to values for undepleted mantle, but variation within the anorthosite-gabbro suite suggests that their isotopic ratios are affected by assimilated country rock. Some of the chemically evolved rocks have isotopic ratios indistinguishable from those of the anorthosites and gabbros, although in general the chemically evolved rocks contain a greater component of crustal material. Each of the chemically evolved satellite intrusions have distinct Sr and Nd isotopic ratios, indicating that each intrusion evolved independently. The isotopic data for the chemically evolved rocks are bestmore » explained by assimilation of various amounts of crustal material, followed in some instances by crystal fractionation and further assimilation at the level of emplacement. Isotopic ratios in two oxide-rich rocks are identical to those of the anorthosites, consistent with the hypothesis that they are late-stage fractionates of the anorthosite series.« less
  • Two major suits of mafic volcanics and intrusive rocks are juxtaposed across the Florence-Niagara Shear Zone within the central portion of the early Proterozoic Lake Superior Belt. To the north of this shear zone, the volcanics were erupted onto previously existing Archean crust during the rifting stages of a major Early Proterozoic rifting event circa 2.0 Ga ago. The volcanics and intrusives found just to the south of the shear zone however, are substantially different in character, and are part of a large magmatic terrane which resembles a modern volcanic arc. Neodymium isotopic systematics from tholeiites of the southern terranemore » yield an isochron age of 1.87 {plus minus} 0.05 Ga, and an {epsilon}{sub Nd} = +4.2. Such a large positive {epsilon}{sub Nd} is indicative of derivation from a mantle very depleted in light rare earth elements (LREE), and is similar to values predicted by some models for the source regions for mid-ocean ridge basalts (MORB) at 1.9 Ga. This characteristic, combined with the general geochemical characteristics and tectonic setting of these basalts, suggest that they may well have been erupted onto oceanic crust, and derived from an early Proterozoic depleted mantle type reservoir. The overall tectonic affinity of this southern terrane however, appears related to island arcs. In contrast, the Nd isotopic systematics of continental tholeiites from the northern terrane do not form an isochron, but rather indicate mixing with a continental crustal LREE enriched reservoir. Two component mixing models utilizing Nd isotopes, along with major and trace elements have been developed for the basalts of this northern terrane, and suggest that the source of the basaltic end-member of this mixing suite was the same as that which generated the LREE depleted basalts of the southern magmatic belt.« less