Download What happened 1.5 billion years ago?

What happened
1.5 billion years ago?
8
MOSAIC Mar/Apr 1975
In 1770, the Moravian missionary
Wolfe collected a beautifully iridescent mineral near Nain, Labrador,
and took it back to Europe, where it
attracted the lively attention of mineralogists. The mineral was soon found
to be a type of feldspar, and it was
named labradorite in honor of its type
locality. Neither Brother Wolfe nor the
early mineralogists could have known
that the origin of labradorite and its
widespread host rocks in Labrador would
remain, two centuries later, one of geology's major unsolved mysteries.
The mystery revolves principally
around three puzzles:
• The rocks, known as anorthosites,
occur in a few areas as gigantic
bodies (many kilometers across)
called batholiths. Although they
crystallized from huge volumes of
molten magma deep within the
Earth's crust, they have no known
volcanic (lava) equivalents on the
Earth's surface. Other igneous rocks
do have surface equivalents. If a
magma can produce anorthosite
within the Earth, what stops it from
spewing anorthosite out at the surface?
• The anorthosite batholiths are always accompanied by somewhat
younger granitic rocks.
• Anorthosite batholiths occur in just
a few geographic locations, and all
were apparently formed between
1.1 and 1.5 billion years ago. This
limited occurrence in both place and
time suggests a unique—and considering their size, profound—event
in the evolution of the Earth's continental crust.
In addition to the anorthosite batholiths, smaller anorthosite bodies were
formed about 2.7 billion years ago or
more. Their origin is clear—they crystallized from essentially basaltic magmas
—and they differ from batholiths both in
their structure and chemical composition.
Made for the fjords. The 51-foot Pitsiulak,
built specifically as a base for geological
research along the superb rock exposures
of Labrador's coast, rests at her home base
of Wyatt Harbour.
These bodies appear to be quite similar
to the anorthosites about four billion
years old that dominate the lunar highlands.
Anorthosite batholiths occur in belts
in the Northern and Southern Hemispheres, and there is always the possibility that additional batholiths will be
discovered when better techniques are
developed for probing deeper into the
Earth or when more surface information
permits constructing a model or theory
to predict the existence of anorthosite
batholiths at those depths. The belts are
more pronounced when plotted on the
now well-known maps that reconstruct
the continents before they began to drift
apart 180 to 200 million years ago. The
Northern Hemisphere belt, as plotted by
Norman Herz of the University of Georgia, extends across northern Europe, the
Outer Hebrides, Greenland, Canada, and
the United States. The belt is difficult to
trace once it leaves eastern Canada, but
it appears to split into two branches, one
going southwest as far as Virginia, the
other west to Minnesota. Another belt
may swing up through Nebraska, Wyoming, Montana, and Idaho, with a couple of seemingly isolated batholiths in
California, Mexico, and Colombia. There
may be another belt in eastern Siberia.
A predrift reconstruction of the Southern Hemisphere shows a belt across
Brazil, Angola, Tanzania, Madagascar,
Antarctica, India, and possibly Australia.
Arguments over the origin of anorthosite batholiths have raged for 50 years,
and no wonder, for an adequate picture
of the Earth's history can't be drawn
without an explanation of such abundant rocks. Almost every famous petrologist (specialist in the origin of rocks)
has speculated on their origin—but few
have studied the rocks extensively in the
field. The classical studies were done on
the relatively small and accessible batholiths in the Adirondack Mountains of
New York by Arthur F. Buddington, now
retired from Princeton University.
The problem in studying anorthosites
in the Adirondacks is that they were
subjected to the intense heat and pressure of metamorphism about one billion
MOSAIC Mar/Apr 1975
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Anorthosite belts. The groupings of known
anorthosite bodies on the Earth are even
more pronounced on maps that show the
locations of continents before they began
to drift apart some 200 million years ago.
(After Herz)
years ago. According to Stearns A.
Morse of the University of Massachusetts, "studying such metamorphosed
rocks is a bit like trying to deduce the
properties of milk from cottage cheese.
Fresh milk is better for the study of
milk, and fresh rocks are better for
geology."
Morse thinks the "fresh milk" is to be
found in Labrador, the only part of the
North American .anorthosite belt not
metamorphosed. Until recently, Labrador's cool, damp climate and inaccessibility have discouraged geologists. The
notable exception was E. P. Wheeler II,
of Cornell University, who explored and
mapped there for 48 years until his death
in the fall of 1974. Working winter and
summer, living off the land, exploring
far reaches in sledge trips, and spending
countless hours in the laboratory, he
produced the most detailed maps of any
anorthosite body in the world.
But since 1971, a research project
headed by Morse and supported by NSF
has fielded 15 to 20 geologists (from six
or eight universities) each summer to
Nain, site of a 10,000-square-kilometer
anorthosite batholith. Additional studies
of two smaller batholiths—at Harp Lake
and Lake Michikamau—are being done
by R. F. Emslie of the Geological Survey
of Canada. The work at Nain draws
heavily on Wheeler's work, and drew as
well on Wheeler himself, who took part
in the project during the first three
summers.
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MOSAIC Mar/Apr 1975
Describing a geologic feature of the
physical scale of the Nain batholith
poses problems not only of logistics but
also of human comprehension—especially when the description must cover
internal details of the anorthosite rocks
themselves as well as the regional setting
in which the rocks were emplaced. But
Nain is an excellent study site. The
superb rock exposures along the fjords
and island shores give a good picture of
the geographic variations in anorthosite
structure and composition. And deep
valleys eroded over millenia provide a
vital third dimension.
Because of these shoreline exposures,
the project is conducted from a 51-foot
boat, the Pitsiulak, built specifically for
this research. In the often bad weather
of Labrador, where even float-equipped
aircraft can be grounded for weeks at a
time, the boat provides dependable field
transportation and permits maximum
time for fieldwork during the short
season.
Using shore facilities and the Pitsiulak
for laboratory facilities and logistic support, Morse and his associates are studying the formation of the anorthosites a
billion or so years ago. They're also
studying very ancient (at least 3.5 billion
years old) rocks comparable to the
Earth's oldest (3.8 billion years old)
rocks just across the Davis Strait in
Greenland. (Rocks of about the same
age have more recently been discovered
in Minnesota.) The similarity between
the rocks of Labrador and Greenland
suggest that the two sides of the Strait
have the same geologic history—not unexpected since Greenland broke loose
from the North American continent 70
or 80 million years ago. Thus, Labrador
offers the setting for the study of more
than three-quarters of the Earth's 4.5billion-year history.
Crystallizing magmas
One of the oldest and most attractive
explanations for the anorthosite batholiths is that they represent an accumulation of plagioclase feldspar crystals that
formed from a common type of magma.
Being of the same density as magma, the
feldspar crystals might float or be carried
upward by convection. The crystals of
the denser iron-magnesium minerals in
the magma—pyroxene and olivine, for
example—would sink to the bottom of
the magma and be concentrated there.
The attractiveness of this theory has always been dimmed by the failure of
geologists to identify those denser minerals in suitable amounts. In the case
of metamorphosed batholiths there is
some evidence that anorthosite bodies
have been detached from their roots during structural deformation. But if that's
so, then an unmetamorphosed batholith
such as Nain should show evidence of
denser roots. The Canadian Earth Physics Branch, assisted by members of the
Nain Project, has started long-term grav-
ity studies that will help characterize the
lower reaches of the Nain anorthosite.
Meanwhile, the Nain Project has conducted field and mineralogical studies
that strongly support the existence somewhere of a denser counterpart. Field
workers found large crystals of pyroxene
locally in anorthosite, and their composition indicates that they grew along
with the feldspars, rather than after
them. That would indicate presence of
the pyroxene constituents in the parent
magma. Bolstering this evidence is the
discovery by Morse and his coworkers
of angular pockets of fine-grained
pyroxene-feldspar rock among large crystals of accumulated feldspar. The shape
and grain size of these angular pockets
suggest to Morse that they represent
parental liquid trapped between feldspar
crystals. If this is true, the parent magma
was norite, a reasonably common variety
of basalt magma.
These angular patches, which show
the former presence of magmatic liquids,
are rarely seen where metamorphism
has caused recrystallization and destroyed original textures. The abundant
evidence of liquid in the Nain batholith
has closed off a whole category of blind
alleys of conjecture that endeavored to
make anorthosites by metamorphic transformation of more common rocks.
Another debate has long raged over
whether water is essential in molten
magma for the formation of the coarse
feldspar crystals found in anorthosites.
It is not, according to evidence found by
Hope Davies, a graduate student at the
University of Massachusetts and one of
several women who have participated in
the Nain Project. Analyzing rocks from
the Kiglapait intrusion, a formation adjacent to the Nain Massif and similar
to it in structure, age, and other petrographic characteristics, she determined
that the apparent upper limit of water
in the Kiglapait magma was a scant three
parts per million—instead of the 20,000
parts per million that characterize "wet"
magmas. The large crystals in the Kiglapait intrusion puzzle lunar geologists,
however, since the anorthosite crystals
found in the dry lunar environment are
small.
Establishing a benchmark
The Kiglapait intrusion, although not
a part of the main anorthosite body itself, is an important part of the Nain
Project. The Kiglapait formation belongs to a class of igneous bodies known
as layered intrusions, which clearly display their history of crystallization in a
sequence of crystal layers, presumably
deposited with the aid of convection currents. Morse discovered the intrusion in
the summer of 1957 while he was a graduate student at McGill University working as a petrologist for British Newfoundland Exploration, Ltd.
The Kiglapait intrusion serves as the
control on chemistry of the Nain anorthosite. It represents a basaltic magma
emplaced in the Earth's crust and fractionally crystallized in place—that is, as
it slowly cooled, successions of different
mineral species crystallized—for a million years. Therefore, it should have
produced all the mineral compositions
possible from a basaltic magma. From
the data Morse has gathered on Kiglapait, he can specify the relative concentrations of some 15 elements in rocks
and their evolving parent magmas as a
function of time and falling crystallization temperature over the complete range
of crystallization history.
Rock history. These rocks on Uighordlekh
Island (a part of the Nain batholith)
illustrate the crystallization of anorthosife
from a basaltic magma. The lighter rocks
are pure anorthosite, rich in feldspar, and
they crystallized first. As the magma
cooled, it became richer in pyroxene (dark
in the photo). This noritic magma (simiiar
in composition to basaltic magma) invaded
and broke up the older anorthositic rock.
MOSAIC Mar/Apr 1975
11
Perfect exposure. Millions of years ago
this vertical dike of dark basaltic material
cut through crystal rocks near Sagiek Bay,
For scaie, note the two men in the canoe
in the foreground.
Since the Nain Anorthosite Project
began, field teams and staff members
working from the Pitsiulak have discovered at least a dozen layered intrusions.
"The Nain area," Morse says, "is turning out to be a Veritable garden of layered intrusions. Only in southwest
Greenland is there a comparable swarm
of known intrusions." He believes the
systematic study of these intrusions in
the Nain area promises to help clarify
not only the igneous history of the anorthosite massif but also to help establish further principles of how valuable
chemical elements are concentrated in
crystallization processes.
The existence of the many layered intrusions indicates that the Nain anorthosite complex is the result of pulse after
pulse of magma being forced into different sites in the area. Most of the intrusions are undeformed, indicating long
periods (thousands to millions of years)
of quiescent conditions while they crystallized.
The magmas were apparently emplaced at depths of ten to 17 kilometers,
according to studies made by J. A. Speer
and J. H. Berg (graduate students at Virginia Polytechnic Institute and the University of Massachusetts, respectively)
on the metamorphism of country rocks
adjacent to the Kiglapait intrusion and
elsewhere. That conclusion is bolstered
in independent findings by Douglas
Smith of the University of Texas that
pressures thought to correspond to those
depths are necessary to stabilize the ironrich pyroxenes of certain igneous rocks
closely associated with anorthosite. Since
the Earth's crust averages 35 kilometers
in thickness, reaching 50 or more kilometers in places, anorthosites were probably emplaced less than halfway down
in the crust, disproving the long-held
assumption that great crustal depths
were essential to anorthosite genesis.
These many intrusions are also proving to represent parent magmas with a
Stringy layers. Black fjyraxene, which
crystallized from magma, is seen in an
outcrop of the Nain anorthosite. These
layers were probably once nearly
horizontal, but were tilted by Safer Earth
crustal movements. A geologist's pick is
shown for scale.
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MOSAIC Mar/Apr 1975
Labrador Life Styles
a Boat in Labrador
Sometimes gray sea, gray sky, gray rain,
And when it's rough you're ill;
Or .else the sea and the sky are blue;
And the sun shines with a will.
•—Elise E. Morse, age 12
Learning to live comfortably is essential for the work of the Nain
Project to be effective. And working
effectively is essential, since the field
season is limited by the breaking up
of ice in June or even July and the
onset of gales and squalls in September, During the season, daytime temperatures in the 50's and 60's are
common—and the days are 20 hours
long. Rainy and foggy days with
temperatures in the 40's are also
common. So far, the Main Project has
been fortunate—1973, for example,
was the best summer in 50 years. The
ice broke up in early June, and the
weather was so consistently good that
field parties 'Were forced to use fair
days for office work, a luxury rarely
afforded, in Labrador. The weather in
1974 was also good, but the icepack
was so big and persistent that it delayed getting the field parties settled
and plagued the Pitsiulak through
July and even into August.
The Main Project relies heavily on
the Pitsiulak, the Eskimo name for
the black guillemot, a charming arctic
bird with the habit of emerging explosively from, seemingly bare rocky
outcrops along the shore to confound
the visitor with his aerobatics and his
ability to disappear just as quickly—•
"like the flash of insight that cheers
the geologist at one outcrop," offers
Morse, "only to vanish in confusion
at the next."
The Pitsiulak is a. modified design
of a Newfoundland fishing boat, with
8.7-knot cruising speed, 1,000-mile
Chef's special. Members of the Nain
project prepare fresh rock cod for chowder.
Local transportation. Some of the team
row through chunks of pack ice to set up a
camp ashore.
range, ice sheathing, and standard
navigational equipment. Her laboratory facilities short circuit the usual
six-month delay between field observations arid preliminary analytical results—an especially important consideration in anorthosite studies, for
some of the most interesting information comes from, mineral compositions
not apparent in the field. The vessel
also serves as a mobile base camp.
The Pitsiulak can sleep ten, feed
eight at a sitting, and accommodate
all the project's staff members at conferences held occasionally during the
season. Her crew is small—master,
pilot-engineer, cook, and geologist.
Morse, who learned to navigate the
coast of Labrador during his summers
as a college student, serves as master.
His wife served as cook on at least
part of the first four summers. The
Pitsiulak's 16-cubic-foot refrigeratorfreezer helps provide variety for the
crew's diet, but freeze-dried meats
and vegetables are the mainstays for
the field parties. To break the monotony, the Pitsiulak's crew, which at
times includes the Morse's three
school-age daughters, tries to maintain, a supply of fresh fish. The favorite is arctic char—"a magnificent delicacy, somewhat like salmon," Morse
says. No produce is grown in Labrador, although some is brought by ship
to Nain, an Eskimo village of 300.
Wild berries and mushrooms are
sometimes available.
Most of the half dozen or so oneand two-person field parties sent out
each summer by the Nain Project
work from camps at or near shoreline, accessible from a shallow-draft
vessel such as the Pitsiulak. Periodically, she resupplies the camps or
moves them to another location,
usually with the assistance of canoes.
Gales can be a problem—two tents
were blown down one summer—but
aside from a couple of minor injuries,
the field parties and the Pitsiulak's
crew as well have enjoyed good health.
At the end of the summer the
Pitsiulak is hauled out on a Canadian
government slip at Nain. Via bush
aircraft, members of the Nain Project
fly to Goose Bay, Labrador, then, continue on commercial airlines to Montreal and back to their universities.
Along with their usual academic duties, they face the tasks of analyzing
the thousands of rock samples sent
from Nain and of planning next
year's efforts to learn more about
anorthosite genesis.
MOSAIC Mar/Apr 1975
1
range of compositions. Moreover, an
area in a single intrusion can have compositions differing widely from the mean
for the intrusion. Thus, a diverse group
of magmas and processes played a part
in anorthosite formation, rather than a
single magma undergoing a unique process of differentiation. This in turn suggests that a favorable set of conditions
existed for the formation of anorthosites
from a range of magma types, and that
factors such as depth, cooling rate, and
oxidation state were more important than
the exact magma composition.
Morse leans to the idea that the magmas were basaltic. Many angular patches
that are apparently derived from trapped
parental liquid are basaltic in composition. In addition, Berg found what Morse
considers powerful proof that at least
one intrusion originated from basaltic
magma. At the "chill margin," where
hot magma contacts colder country rock
and crystallizes quickly, the rock composition is assumed to be representative
of the magma as a whole. In his studies
of one of the layered intrusions in the
Nain batholith, Berg found the composition at the chill margin to be that of a
basaltic magma.
The granitic connection
The Nain Project is also beginning to
supply hard information on another of
the major questions raised by anorthosite batholiths—their universal association with younger granitic rocks. Two
theories have been put forth: that the
two were derived from the same magma;
and that anorthosite was derived from
one magma and the granitic rocks from
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MOSAIC Mar/Apr 1975
Channel scour, As magma rapidly flowed
here in the Hettasch intrusion, it deposited
crystals of olivine and plagiociase. Such
ripply features clearly show both the
former presence of basaltic magma and the
presence of strong currents that may have
helped concentrate plagiociase elsewhere
to form anorthosite.
a second that followed in the conduit
set up by the first.
Morse thinks both mechanisms may
have been at work, with the second more
important. The other two major contributors to the Nain Project—Wheeler
and Dirk de Waard of Syracuse University—have advanced the first theory. In
mapping a layered body on Barth Island
(a few kilometers from the Village of
Nain), de Waard found evidence that
small amounts of granitic rock were produced by fractional crystallization of a
single magma. Morse feels, however,
that so much granitic rock is associated
with the anorthosite batholiths that the
parent magma of the granites could not
have been basaltic. Instead, it would
have to have been more granitic. J. M.
Barton of the University of Massachusetts is conducting radioisotope agedating and geochemical studies in hopes
of helping resolve the one-parent vs.
two-parent problem, but Morse sometimes wonders if the entire granitic question may turn out to be a red herring and
have nothing important to say about the
fundamental question of how and why
the anorthosite batholiths were formed.
That how-and-why question is probably the key to understanding the limited
distribution of anorthosite batholiths in
time and space. The fact that high pressures and temperatures were necessary
for their formation suggests to Morse
the possibility of some unusual tectonic
or thermal event. Others have suggested
a cataclysmic event such as a meteorite
impact, or the birth of the Earth-Moon
system. At the other end of the spectrum, it's also speculated that anorthosite formation and emplacement might
have been a normal event for an early
time in Earth history when a higher geothermal gradient existed than at present.
"What all our speculation amounts
to," Morse says, "is mere arm waving.
We're no smarter if we don't have more
facts on which to base those speculations. That's what the Nain Project is
all about."
Some of the facts develop from analyses performed aboard the Pitsiulak. The
vessel's laboratory is equipped with a
rock crusher and other equipment
needed to prepare samples for microscopic examination and identification. In
a typical season, several tons of samples
are collected in the Nain area, but only
a small portion of them can be analyzed
on the Pitsiulak. One or two tons can
be stored in the Pitsiulak's stern, which
helps her steering, but the rest are periodically packed into five-gallon steel
drums and sent by sea freight to the universities participating in the Nain Project.
Back in Amherst Morse found that
with the optical methods he had been
using he could analyze only about 25
samples a day. To speed up the analyses, he has been renting an automated
electron probe that quickly analyzes the
four minerals he's primarily interested
in. By early 1975 he hopes to have his
own unit in operation, which should be
able to process as many as 150 samples
per day. Morse points out that it would
take 30 years of full-time work to do
ten analyses for every square kilometer
of the Nain anorthosite's 10,000-squarekilometer area. "We haven't any such
goal, of course, but the size of the problem and the diversity of the rocks clearly
demand that we be able to acquire a lot
of data at low cost. As we do, we will
undoubtedly trim much mystery from
the problem and move past the more
pernicious blind alleys. But inasmuch as
batholith-type anorthosite is bound up
in some special and nonrepeating way
with the evolution of the continental
crust, there will be impressive challenges
to geological thought for a long time
after today's foremost questions are
answered." •