what happened 1.5 billion years ago? - mosaic
TRANSCRIPT
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
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