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PALEOMAGNETIC RESULTS FROM TWO HYDROTHERMALLY ALTERED UNITS

IN THE SUPERIOR PROVINCE

by

Vincenzo Francesco Costanzo-Alvarez

A Thesis submitted in conformity with the requirements for the Degree of Doctor of Philosophy in the

University of Toronto

© Copyright by Vinceuzo F. Costanzo-Alvarez 1990

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ABSTRACT

Two problems of Precambrian palcomagnctism, namely the scarcity of well-dated palcopolcs for this

geological era, and the aggravating effects of retrograde hydrothermal alteration on the rocks from which

the scanty palcomagnetic record available derives, arc evaluated by the study of two specific test-cases

in the northcentral and northwestern Superior Province. The fonnations studied arc: the unaltered

granitic batholiths and hydrothermally altered supracrustal units of the Archean Red Lake Grcenstone

Belt (northwestern Ontario), a major gold-producing region: and the Kcwccnawan-agc carbonatites and

Archean anorthositcs of the Kapuskasing Structural Zone (KSZ, northcentral Ontario), an early

Proterozoic belt that has been hypothesized as a cross-section of the continental crust.

For the Red Lake greenstone belt, natural rcmancnt magnetization (NRM) component RLR (D= 191°,

I=-76°, ~s=13°, k=41°, N=3 intrusions (11 sites)), a primary thennal remanence (TRM) carried by fine­

grained magnetite and deuterie hematite and dating from =2700 Ma (40Ar/39 Ar cooling age), has been

isolated in three granitic batholiths: Little Vermillion (VB), Hamell Lake(HL) and Killala Baird (KB).

The primary nature and thcm1al origin of this component is mainly indicated by the pristine appearance

of opaque minerals in thin section, its high cocrcivities (>30 mT), high unblocking temperatures

(>500°C) and strength of magnetic intensity (as high as 4500 mA/m). Component RLR thus represents

one of the oldest 40 Ar/39 Ar dated paleomagnetic components for Laurentia. In addition, and also in the

Red Lake grcenstone belt, NRM component RLG (0=180°, I=-58°, ~s=3°, k=902, N=4 intrusions (9

sites)), a thennochemical remanent magnetization (TCRM) carried by secondary hematite, has been

isolated. This component is the characteristic remanence of the Dome stock (DS), the Howey dioritc

(HD) and the Dickenson Mine's (DM) felsic and mafic dikes which have been hydrothermally altered.

An age of =2580 Ma is derived for this component by purely paleomagnetic means implying a

temperature of acquisition of =260°C. Its thermochemical origin was deduced mainly from the high

degree of alteration of opaque minerals and silicates observed in thin sections and the magnetic

properties of the RLG carriers, such as relatively high coercivi1ics (>40 mT) and unblocking

temperatures (>500°C) even though they have weak magnetization intensity values (as low as 3 mA/m).

A third rcmancncc RLN (D=352°, 1=79°, u"95=4°, k=l65, N=5 intrusions (34 sites)), is common in all

the intrusions sampled. It could, in some cases, be the normal-polarity counterpart or Rl.R, hut iis

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iii

gcnc1ally low cocrcivitics and unblocking temperatures favour a recent viscous origin in most cases.

The palcopolcs for components RLR and RLG form an age sequence on the late Archean-early

Proterozoic apparent polar wander path for Laurentia which is opposite to that previously accepted, but

which agrees with recent paleomagnetic results from other workers and with new U/Pb ages for the

Matachcwan/Hcarst dikes palcopolcs (=2450 Ma). The redefined APWP for the latest Archean and

earliest Proterozoic also suggests a rate of paleolatitudinal change for Laurcntia of "'2.5 cm/yr which

disagrees with previous higher estimates using the old definition of the path, but does agree with the rate

of paleolatitude change calculated for other cratons for the same interval of time and with modem plate

velocities.

For the southern lobe of KSZ (Chapleau Block), a steep up direction of remanence (component B)

had previously been isolated in the Archean Shawmere anorthosi~e by Costanzo-Alvarez & Dunlop

( 1988) and Symons ct al. ( 1988), and interpreted eitJ1er as a TCRM genetically linked with the intrusion

of the nearby = 1100 Ma old Nemegosenda carbonatite or as a TRM acquired during uplift and cooling

of the Chaplcau block at = 19 50 Ma. As pan of a test of the first hypothesis, the same direction of

remancnce (Bl) was isolated in both normal and reversed polarities (BIN: D=308°, !=52°, °'9s=3°,

k=800, N=4 intrusions (18 sites) and BIR: D=109°, l=-49°, °'9s=11°, k=40, N=4 intrusions (22 sites)),

as the oldest rcmanencc of a series of Keweenawan-age alkaline complexes that intrude the Chapleau

block, namely the Shenango River, Nemegosenda Lake and Lackner Lake alkaline complexes and the

Borden township magnetic anomaly.

Along with BIN and BIR, component B2N (D=68°, I=-4°, °'9s=22°, k=IO, N=5 intrusions (11 sites))

and B2R (D=280°, l=-2°, °'9s=21°, k=9, N=5 intrusions (13 sites)) have been isolated in both the

Shawmere anonhosite and the alkaline complexes. These are probably later magnetic acquisitions,

recording intermiuent alkaline intrusive activity. The corresponding paleopoles for components Bl and

132 trace out a portion of the Logan Loop from ca. 1100 Ma to 1000 Ma.

A gr.mulometric test, consisting of comparing relative lengths of TRM (or ARM) and magnetic

susceptibility anisotropy ellipsoids (Stephenson ct al, 1986), has been extensively used and developed

in order to assess the nature of the B TCRMs and the extent of hydrothermal alteration in the Shawmcrc

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iv

anorthosite. Granulometric, rock magnetic and petrographic analyses of the Shawmcre anorthosite

suggest that these TCRMs originated by growth and crystallization of secondary magnetite as a

consequence of extensive hydrothem1al alteration. TI1is alteration is hypothesized to have taken place

along a carbonatite trend parallel to the Ivanhoe Lake Cataclastic zone (ILCZ) connecting the alkaline

complexes in the Chapleau block. These analyses also allow a preliminary mapping or the extent of

hydrothermal alteration in the Shawmcre anorthositc away from the carbonatitc trend. In fact, for sites

close to foci of hydrothennal alteration such as the carbonatite trend itself and the ILCZ, different

generations of magnetites with different grain sizes can be detected coexisting with their primary

counterparts. Far away from these foci of hydrothennal activity, unaltered anorthositc samples have

preserved their primary magnetic mineralogies almost untouched, as reflected by their narrow ranges of

magnetite grain sizes around the PSD-SD threshold size. The modified Stephenson test pmves to be

especially suitable for the study of natural samples where traditional granulomctric techniques generally

fail to distinguish different fractions of magnetites with different grain sizes.

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ACKNOWLEDGMEJ.\'TS

Many people assisted me, in one way or another, for the completion of this thesis, and to all of

them I am grateful.

Foremost I want to thank my supervisor Professor David J. Dunlop. During these last five years

he provided me with continuous and insightful guidance, feed back, encouragement during stormy

times when I could not even sec the light at the end of the tunnel, generous financial support from

research grants and most important of all, the treasure of his lriendship.

Long enlightening discussions with Drs. M.P. Bates (University of Toronto), C.J. Hale (University

of Toronto), H.C. Halls (University of Toronto), H. Hyodo (Hiruzen Research Institute, Okayama

University, Japan), P.W. Layer (University of Alaska), L.J. Pesonen (Geological Survey of Finland),

H. Ueno (Kagoshima University, Japan), S. Xu (University of Toronto) and D. York (University of

Toronto) helped me to organize my ideas and to shape out tons of experimental data into a coherent

story.

Lauri Pesonen spent a complete week of almost 24 hours of work daily (and nightly) doing the

thermal runs of the Nemegosenda and Shenango carbonatites, he also kindly measured 21 of these

samples using his facilities in the Paleomagnetic Laboratories of the Geological Survey of Finland

in Espoo.

Martin Bates and Chris Hale were my "petrographic mentors" sharing with me their knowledge

in the subject and helping with the set!.!p, microphotography and analyses of opaques in thin sections

using transmitted (M.B.) and reflected light (C.H.) microscopy.

Sebastian Pfleiderer patiently spent almost a week fixing a plotting program for me. He also

:11lowed me to use his susceptibility bridge and software and shared with me his knowledge and

experience in the AMS subject.

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