What happened 1.5 billion years ago?
In 1770, the Moravian missionary Wolfe collected a beautifully
irides cent mineral near Nain, Labrador,
and took it back to Europe, where it attracted the lively attention
of miner alogists. 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 geol
ogy's major unsolved mysteries.
The mystery revolves principal ly 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 sur
face?
• The anorthosite batholiths are al ways 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 con sidering their size, profound—event in the
evolution of the Earth's con tinental crust.
In addition to the anorthosite batho liths, smaller anorthosite
bodies were formed about 2.7 billion years ago or more. Their
origin is clear—they crys tallized 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 high lands.
Anorthosite batholiths occur in belts in the Northern and Southern
Hemi spheres, and there is always the possi bility 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 Geor gia, 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, Wy oming, Montana, and Idaho,
with a cou ple of seemingly isolated batholiths in California,
Mexico, and Colombia. There may be another belt in eastern Siberia.
A predrift reconstruction of the South ern Hemisphere shows a belt
across Brazil, Angola, Tanzania, Madagascar, Antarctica, India, and
possibly Australia.
Arguments over the origin of anortho site 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 abun dant
rocks. Almost every famous petrol- ogist (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 batho liths 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 pres sure of
metamorphism about one billion
MOSAIC Mar/Apr 1975 9
years ago. According to Stearns A. Morse of the University of
Massachu setts, "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, Labra dor's cool, damp climate and inaccessi
bility 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.
Describing a geologic feature of the physical scale of the Nain
batholith poses problems not only of logistics but also of human
comprehension—espe cially 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 sup port, Morse and his associates are study ing 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
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)
the rocks of Labrador and Greenland suggest that the two sides of
the Strait have the same geologic history—not un expected 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 4.5- billion-year
history.
Crystallizing magmas
One of the oldest and most attractive explanations for the
anorthosite batho liths is that they represent an accumula tion
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 al ways
been dimmed by the failure of geologists to identify those denser
min erals in suitable amounts. In the case of metamorphosed
batholiths there is some evidence that anorthosite bodies have been
detached from their roots dur ing structural deformation. But if
that's so, then an unmetamorphosed batholith such as Nain should
show evidence of denser roots. The Canadian Earth Phys ics Branch,
assisted by members of the Nain Project, has started long-term
grav-
10 MOSAIC Mar/Apr 1975
ity studies that will help characterize the lower reaches of the
Nain anorthosite.
Meanwhile, the Nain Project has con ducted field and mineralogical
studies that strongly support the existence some where of a denser
counterpart. Field workers found large crystals of pyroxene locally
in anorthosite, and their compo sition 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 crys tals 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 de stroyed 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 trans formation 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 ad
jacent to the Nain Massif and similar to it in structure, age, and
other petro- graphic 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 Kigla
pait 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 it self, is an important part of the Nain
Project. The Kiglapait formation be longs to a class of igneous
bodies known as layered intrusions, which clearly dis play their
history of crystallization in a sequence of crystal layers,
presumably deposited with the aid of convection cur rents. Morse
discovered the intrusion in the summer of 1957 while he was a grad
uate student at McGill University work ing as a petrologist for
British New foundland Exploration, Ltd.
The Kiglapait intrusion serves as the control on chemistry of the
Nain anor thosite. It represents a basaltic magma emplaced in the
Earth's crust and frac tionally crystallized in place—that is, as
it slowly cooled, successions of different mineral species
crystallized—for a mil lion years. Therefore, it should have
produced all the mineral compositions possible from a basaltic
magma. From the data Morse has gathered on Kigla pait, he can
specify the relative concen trations of some 15 elements in rocks
and their evolving parent magmas as a function of time and falling
crystalliza tion 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 discov ered at least a
dozen layered intrusions. "The Nain area," Morse says, "is turn
ing out to be a Veritable garden of lay ered 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 an orthosite massif but also to help estab lish
further principles of how valuable chemical elements are
concentrated in crystallization processes.
The existence of the many layered in trusions indicates that the
Nain anortho site complex is the result of pulse after pulse of
magma being forced into differ ent sites in the area. Most of the
intru sions are undeformed, indicating long periods (thousands to
millions of years) of quiescent conditions while they crys
tallized.
The magmas were apparently em- placed at depths of ten to 17
kilometers, according to studies made by J. A. Speer and J. H. Berg
(graduate students at Vir ginia Polytechnic Institute and the Uni
versity 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 iron-
rich pyroxenes of certain igneous rocks closely associated with
anorthosite. Since the Earth's crust averages 35 kilometers in
thickness, reaching 50 or more kilo meters in places, anorthosites
were prob ably 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 prov ing 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.
12 MOSAIC Mar/Apr 1975
Labrador Life Styles
Chef's special. Members of the Nain project prepare fresh rock cod
for chowder.
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 es sential 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 Septem ber,
During the season, daytime tem peratures 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 de layed 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 ex plosively 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
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
labora tory facilities short circuit the usual six-month delay
between field obser vations arid preliminary analytical re
sults—an especially important consid eration in anorthosite
studies, for some of the most interesting informa tion 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 con ferences 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 refrigerator- freezer helps provide
variety for the crew's diet, but freeze-dried meats and vegetables
are the mainstays for the field parties. To break the mo notony,
the Pitsiulak's crew, which at times includes the Morse's three
school-age daughters, tries to main tain, a supply of fresh fish.
The favor ite is arctic char—"a magnificent del icacy, somewhat
like salmon," Morse says. No produce is grown in Labra dor,
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 one- and two-person field parties sent
out each summer by the Nain Project work from camps at or near
shore line, accessible from a shallow-draft vessel such as the
Pitsiulak. Period ically, 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, con tinue on commercial
airlines to Mon treal and back to their universities. Along with
their usual academic du ties, 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 com positions 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 proc ess of differentiation. This in turn sug
gests 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 mag mas were basaltic. Many
angular patches that are apparently derived from trapped parental
liquid are basaltic in composi tion. 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 com position 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 composi tion 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 anortho site
batholiths—their universal associa tion 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
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 con tributors to the Nain
Project—Wheeler and Dirk de Waard of Syracuse Univer sity—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 pro duced 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 Massachu setts is conducting
radioisotope age- dating and geochemical studies in hopes of
helping resolve the one-parent vs. two-parent problem, but Morse
some times wonders if the entire granitic ques tion 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 prob ably the key to understanding
the limited distribution of anorthosite batholiths in time and
space. The fact that high pres sures 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 spec trum, it's also
speculated that anortho site formation and emplacement might have
been a normal event for an early time in Earth history when a
higher geo- thermal 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 specula tions. That's what the Nain Project is all
about."
Some of the facts develop from anal yses performed aboard the
Pitsiulak. The vessel's laboratory is equipped with a rock crusher
and other equipment needed to prepare samples for micro scopic
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 peri odically packed into five-gallon steel drums and
sent by sea freight to the uni versities 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 anal yses, 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-square- kilometer area. "We haven't any such goal, of
course, but the size of the prob lem 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." •
14 MOSAIC Mar/Apr 1975