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Title: Geochemistry of hypabyssal rocks of the Midcontinent Rift system in Minnesota, and implications for a Keweenawan magmatic ``family tree``

Abstract

The hypabyssal rocks associated with the Keweenawan (1.1 Ga) Midcontinent Rift along the Minnesota shore of Lake Superior are a distinct suite within the rock associations of this region. These rocks are found predominantly as ophitic diabase dikes and sills of various sizes, ranging from a few meters to several hundred meters across. Chilled margins were sampled and analyzed by neutron activation analysis and microprobe fused-bead techniques for bulk chemistry. Mineral compositions were obtained by electron microprobe. Variations in composition were found that are consistent with fractionation. Major-element modeling of fractionation indicates that the majority of the hypabyssal rocks formed at moderate pressures ({approximately}6 kbar), although a number show evidence of fractionation at near-surface levels, and some deeper ({approximately}10 kbar). Resorption features seen in plagioclase phenocrysts are evidence for magmatic evolution at varying levels in the crust. It is possible to relate the varied hypabyssal rocks to a single primary parent through polybaric fractionation. This parent is a high-Al primitive olivine tholeiite--a magma composition common among the volcanic rocks associated with the Midcontinent Rift. Trace-element modeling with this same parent composition yields results consistent with the formation of some hypabyssal rocks as products of a periodically tapped and replenished, constantlymore » fractionating magma chamber, which can decouple the behavior of major and trace elements.« less

Authors:
 [1]
  1. Oak Ridge National Lab., TN (United States)
Publication Date:
Sponsoring Org.:
USDOE
OSTI Identifier:
316236
Resource Type:
Journal Article
Resource Relation:
Journal Name: International Geology Review; Journal Volume: 40; Journal Issue: 11; Other Information: PBD: Nov 1998
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; MINNESOTA; RIFT ZONES; MINERALOGY; ORIGIN; MAGMATISM; GEOLOGIC HISTORY

Citation Formats

Jerde, E.A. Geochemistry of hypabyssal rocks of the Midcontinent Rift system in Minnesota, and implications for a Keweenawan magmatic ``family tree``. United States: N. p., 1998. Web. doi:10.1080/00206819809465248.
Jerde, E.A. Geochemistry of hypabyssal rocks of the Midcontinent Rift system in Minnesota, and implications for a Keweenawan magmatic ``family tree``. United States. doi:10.1080/00206819809465248.
Jerde, E.A. Sun . "Geochemistry of hypabyssal rocks of the Midcontinent Rift system in Minnesota, and implications for a Keweenawan magmatic ``family tree``". United States. doi:10.1080/00206819809465248.
@article{osti_316236,
title = {Geochemistry of hypabyssal rocks of the Midcontinent Rift system in Minnesota, and implications for a Keweenawan magmatic ``family tree``},
author = {Jerde, E.A.},
abstractNote = {The hypabyssal rocks associated with the Keweenawan (1.1 Ga) Midcontinent Rift along the Minnesota shore of Lake Superior are a distinct suite within the rock associations of this region. These rocks are found predominantly as ophitic diabase dikes and sills of various sizes, ranging from a few meters to several hundred meters across. Chilled margins were sampled and analyzed by neutron activation analysis and microprobe fused-bead techniques for bulk chemistry. Mineral compositions were obtained by electron microprobe. Variations in composition were found that are consistent with fractionation. Major-element modeling of fractionation indicates that the majority of the hypabyssal rocks formed at moderate pressures ({approximately}6 kbar), although a number show evidence of fractionation at near-surface levels, and some deeper ({approximately}10 kbar). Resorption features seen in plagioclase phenocrysts are evidence for magmatic evolution at varying levels in the crust. It is possible to relate the varied hypabyssal rocks to a single primary parent through polybaric fractionation. This parent is a high-Al primitive olivine tholeiite--a magma composition common among the volcanic rocks associated with the Midcontinent Rift. Trace-element modeling with this same parent composition yields results consistent with the formation of some hypabyssal rocks as products of a periodically tapped and replenished, constantly fractionating magma chamber, which can decouple the behavior of major and trace elements.},
doi = {10.1080/00206819809465248},
journal = {International Geology Review},
number = 11,
volume = 40,
place = {United States},
year = {Sun Nov 01 00:00:00 EST 1998},
month = {Sun Nov 01 00:00:00 EST 1998}
}