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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

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