Geophysical properties of continental crust depend on the nature of crustal evolution. This is well illustrated by examination of two crustal cross-sections (1), the combined Ivrea-Verbano zone (IVZ) and Strona-Ceneri zone (SCZ) of northern Italy and the Pikwitonei granulite belt (PGB) and Cross Lake subprovince (CLS) of Manitoba. These two cross-sections are of particular interest because the IVZ and SCZ developed during Phanerozoic time whereas the PGB-CLS is an example of Archean crustal evolution. Consequently, each cross-section is geologically distinctive and, thus, exhibits very different geophysical properties such as density, seismic velocity, heat production, and magnetism. Results of geological investigations of each area are given

Fountain, D. M.

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47
GEOPHYSICAL CONSEQUENCES OF PHANEROZOIC AND ARCHEAN CRUSTAL EVOLU-
TION: EVIDENCE FROM CRUSTAL CROSS-SECTIONS; David M. Fountain, Dept. of
Geology and Geophysics, Program for Crustal Studies, Univ. of Wyoming,
Laramie, WY 82071
Geophysical properties of continental crust depend on the nature of
crustal evolution. This is well illustrated by examination of two crustal
cross-sections (1), the combined Ivrea-Verbano zone (IVZ) and Strona-Ceneri
zone (SCZ) of northern Italy and the Pikwitonei granulite belt (PGB) and
Cross Lake subprovince (CLS) of Manitoba. These two cross-sections are
of particular interest because the IVZ and SCZ developed during Phanerozoic
time whereas the PGB-CLS is an example of Archean crustal evolution.
Consequently, each cross-section is geologically distinctive and, thus,
exhibits very different geophysical properties such as density, seismic
velocity, heat production, and magnetism.
Perhaps the best known cross-section of the crust is the IVZ-SCZ of
northern Italy (Figure 1). Deeper crustal levels are represented by the
granulite and upper amphibolite facies ultramafic, mafic and pelitic rocks
of the IVZ that experienced peak metamorphic conditions of 9-11 kb and
700°-820°C during Caledonian time (2,3). Isotopic data indicate that
these high-grade rocks resided at lower to middle crustal levels until
Jurassic time. Amphibolite and greenschist facies pelitic schists and
gneisses, intruded by post-metamorphic granitic plutons, dominate higher
crustal levels exposed in the SCZ. Peak metamorphism in the SCZ was also
Caledonian (2). The IVZ and SCZ are separated by a vertical mylonite zone,
the Pogallo line, which Hodges and Fountain (4) interpret as a Jurassic
low-angle normal fault rotated into its present position during Alpine
deformation. The Pogallo line is but one manifestation of the rift event
that created the Tethys Ocean.
Ivrco Zont
pelitic rocks
mode rocks
rocks
Strona - Ctntn Zone
toeisses and schists
Permian wlcanics and
Mesozotc sediments
Insubnc Lin*
o mcroprote samples ®
Figure 1. Sketch map of IVZ and
SCZ from Hunziker and
Zingg (2).
The early history of the IVZ and SCZ is somewhat enigmatic but was
dominated by deposition of a thick pelitic package of sediments between 480
and 700 Ma (2). Deep burial and amphibolite facies metamorphism preceded
https://ntrs.nasa.gov/search.jsp?R=19840012887 2020-03-20T23:50:39+00:00Z
GEOPHYSICS AND CRUSTAL EVOLUTION
48 Fountain, D.M.
intrusion of mafic and ultramafic magmas and peak metamorphisin during Cale-
donian time (2). Schmid (5) speculated that granulite facies pelitic
gneisses were de-granitized during this event. Post-metamorphic intrusion
of granites suggests Andean margin conditions prevailed later in the Paleo-
zoic. Rifting, normal faulting and formation of a Tethyan trailing margin
modified this Paleozoic crustal block during early Mesozoic time.
Geophysical properties of the IVZ and SCZ are a consequence of its
Phanerozoic evolution. Seismic velocities (6) for mafic and ultramafic
rocks are expectedly high (7.2-8.4 km/sec). Pelitic granulites also have
high velocities (7.0-7.6 km/sec) because of high garnet and sillimanite
content. Deep crustal levels are characterized by high P-wave velocities.
In contrast, schists, gneisses and granites of the SCZ have P-wave velocities
around 6.4-6.5 km/sec, causing a marked seismic distinction between the
lower crust and higher levels. Investigations by Wasilewski and Fountain
(7) indicate that magnetite is the magnetic phase in the granulites and
that susceptibilities are high in mafic granulites and very low in higher
level rocks of the SCZ. The deep crust represented by the IVZ, therefore,
is also magnetically distinct and could cause long-wavelength magnetic
anomalies if it resided in areas of normal geothermal gradients. Published
heat production data (8) indicate that heat production is low in mafic
granulites (0.8 HGU), moderately high in upper amphibolite pelitic gneisses
and SCZ lithologies (5-6 HGU) and high for post-metamorphic granites (7 HGU).
No data is available for pelitic granulites. These data suggest that there
may be a significant stepwise decrease in heat production at deep crustal
levels although analysis of high-grade pelitic rocks is necessary to confirm
this.
Deep levels of Archean crust are exposed in the Pikwitonei granulite
belt of Manitoba (Figure 2). About 80% of the PGB gneisses are silicic
granulites (enderbites and charnockites) that surround much less abundant,
discontinuous layers of granulite facies paragneisses, iron formation,
and mafic and ultramafic rocks (9) that are similar to nearby lower grade
greenstone belt lithologies. The CLS, representing shallower crustal levels,
lies to the southeast of the PGB and consists of several greenstone belts
surrounded by granites and granitic gneisses. Metamorphic grade in these
belts tends to decrease to the southeast.
Figure 2. Sketch map of geolo-
gical domains of
northwestern Super-
ior Province from
Weber and Scoates
(9).
SUPERIOR PROVMCE
. BOlMURY OF GCOLOGCAL OGHAM
fiWLTED. APPROMMATe
GEOPHYSICS CRUSTAL EVOLUTION
49Fountain, D.M.
Geologic investigations in the PGB and CLS (9, 10) suggest that there
is no fundamental difference between the two terrains other than variation
of raetamorphic grade and differences in volumetric abundances of certain
supracrustal lithologies. Mapping by Hubregtse (10) in the Cross Lake
area demonstrated that low-grade greenstone belts can be traced across
the orthopyroxene isograd into the PGB, suggesting that the PGB and CLS
share a common history. Isotopic data (11) suggest an approximate 2.8
Ga age of metamorphism for PGB enderbites. Ages of metaplutonic rocks
of CLS are between 2.7-3.0 Ga (11).
The bimodal compositional nature of rocks of the PGB and CLS produce
a distinctive geophysical character. Enderbites and granitic rocks surround-
ing supracrustal belts have similar mineralogy, low densities (mean density
= 2.68 g/cm ) and thus, low seismic velocities (estimated at approximately
6.5 km/sec). This is in contrast to high densities (2.8-3.1 g/cm ) and
seismic velocities (estimated at approximately 6.8-7.2 km/sec) for the
supracrustal belts. Overall, this crustal section is dominated by a low
P-wave velocity matrix that surrounds discontinuous, high velocity layers,
thus there is little seismic distinction between the upper, middle and
lower crust. Work in progress (Shive and Williams) indicates that magnetite
is also the magnetic phase in PGB enderbites but susceptibilities are much
less than IVZ granulites, suggesting that lower crustal magnetic patterns
will differ substantially for the two crustal cross-sections. Work on
heat production is also in progress. Preliminary data indicate no signifi-
cant difference in K^O content between PGB enderbites and CLS granitic
rocks suggesitng there may be no major change in heat production across
the amphibolite-granulite facies boundary.
The IVZ and SCZ represent Phanerozoic crust in which mafic and ultra-
mafic magmas were added to deeper portions of a modestly metamorphosed
pelitic package of sediments. Physical properties of mafic, ultramafic
and de-granitized pelitic rocks are distinctively different than upper
level granites and pelitic schists and gneisses resulting in a geophysically
zoned crust. This zonation is enhanced by the Pogallo line, a Jurassic
low-angle normal fault. In contrast, the granite-greenstone belt mode
of evolution of the PGB and CLS created a crustal cross-section with little
geophysical distinction between various crustal levels although significant
lithologic heterogeneity exists. In the future, geophysical techniques
may have the resolution necessary to detect differences such as these thus
permitting routine analysis of crustal evolution through use of geophysical
surveys.
REFERENCES
(1) Fountain D.M. and Salisbury M.H. (1981) Exposed cross-section through
the continental crust: Implications for crustal structure,
petrology, and evolution. Earth Planet. Sci. Lett., p. 263-277.
(2) Hunziker J.C. and Zingg A. (1980) Lower Paleozoic amphibolite to
granulite facies metamorphism in the Ivrea Zone (Southern Alps,
Northern Italy). Schweiz. mineral, petrogr. Mitt., p. 181-213.
(3) Schmid R. and Wood B.J. (1976) Phase relationships in granulitic meta-
pelites from the Ivrea-Verbano zone (Northern Italy). Contr.
Mineral. Petrol., p. 255-279.
(4) Hodges K.V. and Fountain D.M. (manuscript) The Pogallo Line, South
Alps, Northern Italy: An intermediate crustal level, low-angle
normal fault?, submitted to Geology.
(5) Schmid R. (1978/1979) Are the metapelites of the Ivrea-Verbano zone
restites?. Mem. Sci. geol., p. 67-69.
GEOPHYSICS CRUSTAL EVOLUTION
Fountain, D.M.
(6) Fountain D.M. (1976) The Ivrea-Verbano and Strona-Ceneri Zones,
Northern Italy: A cross-section of the continental crust—New
evidence from seismic velocities of rock samples. Tectonophysics,
p. 145-165.
(7) Wasilewski P. and Fountain D.M. (1982) The Ivrea Zone as a model for
the distribution of magnetization in the continental crust.
Geophys. Res. Lett., p. 333-336.
(8) Hohndorf A. (1975) Radioaktive Warmproduktion in Gesteinen der Ivrea-
Zone. Schweiz. mineral, petrogr. Mitt., p. 89-102.
(9) Weber W. and Scoates R.F.J. (1978) Archean and Proterozoic metamorphism
in the northwestern Superior Province and along the Churchill-
Superior boundary, Manitoba». Geol. Surv. Canada Paper 78-10,
p. 5-16.
(10) Hubregtse J.J.M.W. (1980) The Archean Pikwitonei granulite domain and
its position at the margin of the northwestern Superior Province
(Central Manitoba). Manitoba Mineral Resources Division, Geologi-
cal Paper GP80-3. 16 p.
(11) Weber W. and Loveridge W.D. (1981) Geological relationships and Rb-Sr
age studies of the Cauchon Lake Tonalite and Pikwitonei Enderbite
in the northwestern Superior Province, Central Manitoba.
Geol. Surv. Canada Paper 81-1C. p. 115-121.


Geophysical consequences of phanerozoic and Archean crustal evolution: Evidence from crustal cross-sections

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