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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}
}
  • Swarms of both reversed-polarity (R, older) and normal-polarity (N, younger) basaltic dikes help to define the evolution of the Minnesota portion of the Midcontinent Rift of North America. Each swarm, representing fissure-feeders for a package of overlying plateau lavas now eroded away, shows the direction of least principal stress at that time and place in the complex evolution of this abortive but nearly-successful rift. Paleomagnetic pole determinations for Carlton county (CC) and Grand Portage (GP) R dikes are coincident along the Logan Loop of the Proterozoic track, showing essential contemporaneity, though their trends are different (N.30/sup 0/E vs. N. 70-90/supmore » 0/E.) and they are 250 km apart. These poles match that of the R lavas of the North Shore Volcanic Group (NSVG) and imply a younger age than the R Logan sills and dikes. The geochemistry of the dikes (71 analyses) also correlates well with NSVG flows, ranging from olivine tholeiites to transitional basalts and basaltic andesites and is similar to tholeiites of Columbia River, Parana, and Tertiary No. Atlantic provinces. Though each swarm shows a range of compositions, some are dominantly more evolved, whereas others contain more primitive dikes with higher Al, Mg/Fe, Cr and Ni and lower Fe, Ti, P and LIL's. An early major episode of rifting during the R polarity interval was followed by at least one major N episode in Minnesota before the final one along the present Lake Superior syncline axis. Rifting directions and mantle sources were different for each episode as shown by cross-trending dike sets, indicating complex rift development.« less
  • Rift-related lavas of the North Shore Volcanic Group (NSVG) are intruded by plutonic rocks of the Duluth Complex along the unconformity between the NSVG and the underlying Proterozoic metasedimentary rocks (Animikie Group) and Archean volcano-sedimentary and plutonic rocks. Heat associated with the emplacement of the mafic intrusions generated fluid flow in the overlying plateau lavas. {delta}{sup 18}O values for whole rocks from the NSVG and hypabyssal sills range from 5.5 to 17.7{per_thousand} and 5.3 to 11.5{per_thousand}, respectively, and most values are higher than those considered normal for basaltic rocks (5.4 to 6.0{per_thousand}). In general, there is a positive correlation betweenmore » whole rock {delta}{sup 18}O and water content, which suggests that elevated {delta}{sup 18}O values are related primarily to secondary mineral growth and isotopic exchange during hydrothermal alteration and metamorphism. {delta}{sup 18}O{sub H{sub 2}O} values computed from amygdule-filling minerals such as smectite, chlorite, and epidote found in low- to high-temperature metamorphic zones range from {approximately}{minus}1 to 6{per_thousand} with an average value of {approximately}3{per_thousand}. Smectite in the lower-grade zones gives computed {delta}D{sub H{sub 2}O} values between {minus}26 and {minus}83{per_thousand}, whereas epidote in the higher-grade zones gives {delta}D{sub H{sub 2}O} values of {minus}15 to 6{per_thousand}. Fluid isotopic compositions computed from epidote and smectite values are suggestive of the involvement of at least two fluids during the early stages of amygdule filling. Fluid {delta}D and {delta}{sup 18}O values determined from epidote at the higher metamorphic grades indicate that seawater dominated the deeper portions of the system where greenschist facies assemblages and elevated {delta}{sup 18}O values were produced in flow interiors, as well as margins. Smectite isotopic compositions suggest that meteoric water was predominant in the shallower portions of the system. The increase in {delta}{sup 18}O values of massive flow interiors with depth is interpreted as a result of rock interaction with a fluid of constant oxygen isotopic composition with increasing temperature. The stable isotopic data are supportive of previous suggestions that seawater was involved in the hydrothermal system associated with the Midcontinent Rift. Although the origin of the seawater remains problematic, it appears that marine incursions may have occurred during the late stages of Portage Lake volcanism, and periodically thereafter.« less
  • Rb-Sr and O isotopic data for mid-Tertiary alkalic rocks from the Trans-Pecos magmatic province of west Texas demonstrate that hydrothermal alteration and fluid/rock (cation exchange) interactions have affected the isotope geochemistry of these rocks. Strontium and O isotopic data for late-stage minerals in an alkali basalt (hawaiite) still record two episodes of fluid/rock interactions. These data suggest that later meteoric fluids introduced Sr with a Cretaceous marine {sup 87}Sr/{sup 86}Sr ratio into minerals with significant cation exchange capacity. Dilute HCl leaching experiments demonstrate the removal of this labile or exchangeable Sr from the alkali basalt. Rb-Sr isotopic data for themore » leached alkali basalt and handpicked calcite record a crystallization age of 42 Ma, consistent with K-Ar data for an unaltered alkali basalt (hawaiite) dike from the same area (42.6 {plus minus} 1.3 Ma). Leaching experiments on one phonolite suggest the Sr isotopic variability in unleached phonolite and nepheline trachyte samples may be attributed to Sr in secondary calcite and zeolites, which have an upper Cretaceous marine {sup 87}Sr/{sup 86}Sr ratio. Rb-Sr isotopic data for leached phonolite and sanidine separate yield an age of 36.5 {plus minus} 0.8 Ma, within analytical uncertainty of a K-Ar biotite age (36.0 {plus minus} 1.1 Ma) of another phonolite. These leaching experiments demonstrate that the Rb-Sr isotopic systematics of hydrothermally-altered continental alkalic rocks may be significantly improved, providing more reliable geochronologic and isotopic tracer information necessary in constructing precise models of mantle sources.« less
  • The Proterozoic Midcontinent Rift System of North America is remarkably similar to Phanerozoic rifted continental margins and flood basalt provinces. Like the younger analogues, the volcanism within this older rift can be explained by decompression melting and rapid extrusion of igneous material during lithospheric extension above a broad, asthenospheric, thermal anomaly which the authors call the Keweenaw hot spot. Great Lakes International Multidisciplinary Program on Crustal evolution seismic reflection profiles constrain end-member models of melt thickness and stretching factors, which yield an inferred mantle potential temperature of 1,500-1,570C during rifting. Combined gravity modeling and subsidence calculations are consistent with stretchingmore » factors that reached 3 or 4 before rifting ceased, and much of the lower crust beneath the rift consists of relatively high density intruded or underplated synrift igneous material. The isotopic signature of Keweenawan volcanic rocks, presented in a companion paper by Nicholson and Shirey (this issue), is consistent with the model of passive rifting above an asthenospheric mantle plume.« less