Differentiation of a Gabbro Sill in the Oregon Coast Range by Differentiation of a Gabbro Sill in the Oregon Coast Range by Crystallization-Zone Settling
By NORMAN S. MacLEOD
GEOLOGICAL SURVEY P R O F E S S I O N A L P A P E R 1165
UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1981 UNITED STATES DEPARTMENT OF THE INTERIOR JAMES G. WATT, Secretary
GEOLOGICAL SURVEY Doyle G. Frederick, Acting Director
Library of Congress catalog-card No. 81-6000^9
For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS
Page Page Abstract ...... 1 Order of crystallization ...... 8 Introduction ...... 1 Petrochemistry ...... 8 Geologic setting ...... 1 Composition of the chilled border ...... 8 General features ...... 2 Composition of rocks from the interior of the sill ...... 9 Petrography ...... 3 Compositional variations with height ...... 9 Chilled basalt ...... 3 Bulk composition of the sill ...... 10 Gabbro ...... 3 Residual liquid compositions ...... 12 Ferrogranophyre ...... 3 Chemical variation ...... 14 Pegmatite ...... :...... 6 Process of differentiation ...... 15 Granophyre ...... 6 Summary ...... 21 Mineralogical data ...... 6 References cited ...... 22 Textural relations ...... 7
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