Precambrian time

Several rock types yield information on the range of environments that may have existed during Precambrian time. Evolution of the atmosphere is recorded by banded-iron formations (BIFs), paleosols (buried soil horizons), and red beds, whereas tillites (sedimentary rocks formed by the lithification of glacial till) provide clues to the climatic patterns that occurred during Precambrian glaciations.


Paleogeography

One of the most important factors controlling the nature of sediments deposited today is continental drift. This follows from the fact that the continents are distributed at different latitudes, and latitudinal position affects the temperature of oceanic waters along continental margins (the combined area of the continental shelf and continental slope); in short, sedimentary deposition is climatically sensitive. At present, most carbonates and oxidized red soils are being deposited within 30 degrees of the Equator, phosphorites within 45 degrees, and evaporites within 50 degrees. Most fossil carbonates, evaporites, phosphorites, and red beds of Phanerozoic age dating back to the Cambrian have a similar bimodal distribution with respect to their paleoequators. If the uniformitarian principle that the present is the key to the past is valid (meaning the same geologic processes occurring today occurred in the past), then sediments laid down during the Precambrian would have likewise been controlled by the movement and geographic position of the continents. Thus, it can be inferred that the extensive evaporites dating to 3.5 billion years ago from the Pilbara region of Western Australia could not have been formed within or near the poles. It can also be inferred that stromatolite-bearing dolomites of Riphean rock, a sedimentary sequence spanning the period from 1.65 billion to 800 million years ago, were deposited in warm, tropical waters. Riphean rock is primarily located in the East European craton, which extends from Denmark to the Ural Mountains, and in the Siberian craton in Russia.

Today, phosphate sediments are deposited primarily along the western side of continents. This is the result of high biological productivity in nearby surface waters due to the upwelling of nutrient-rich currents that are moving toward the Equator. The major phosphorite deposits of the Aravalli mountain belt of Rajasthan in northwestern India, which date from the Proterozoic Eon, are associated with stromatolite-rich dolomites. They were most likely deposited on the western side of a continental landmass that resided in the tropics.


Paleoclimate

EVOLUTION OF THE ATMOSPHERE AND OCEAN

During the long course of Precambrian time, the climatic conditions of the Earth changed considerably. Evidence of this can be seen in the sedimentary record, which documents appreciable changes in the composition of the atmosphere and oceans over time.


Oxygenation of the atmosphere

Earth almost certainly possessed a reducing atmosphere before 2.5 billion years ago. The Sun’s radiation produced organic compounds from reducing gases—methane (CH4) and ammonia (NH3). The minerals uraninite (UO2) and pyrite (FeS2) are easily destroyed in an oxidizing atmosphere; confirmation of a reducing atmosphere is provided by unoxidized grains of these minerals in 3.0-billion-year-old sediments. However, the presence of many types of filamentous microfossils dated to 3.45 billion years ago in the cherts of the Pilbara region suggests that photosynthesis had begun to release oxygen into the atmosphere by that time. The presence of fossil molecules in the cell walls of 2.5-billion year-old blue-green algae (cyanobacteria) establishes the existence of rare oxygen-producing organisms by that period.

Oceans of the Archean Eon (4.0 to 2.5 billion years ago) contained much volcanic-derived ferrous iron (Fe2+), which was deposited as hematite (Fe2O3) in BIFs. The oxygen that combined the ferrous iron was provided as a waste product of cyanobacterial metabolism. A major burst in the deposition of BIFs from 3.1 billion to 2.5 billion years ago—peaking about 2.7 billion years ago—cleared the oceans of ferrous iron. This enabled the atmospheric oxygen level to increaseappreciably. By the time of the widespread appearance of eukaryotes at 1.8 billion years ago, oxygen concentration had risen to 10 percent of present atmospheric level (PAL). These relatively high concentrations were sufficient for oxidative weathering to take place, as evidenced by hematite-rich fossil soils (paleosols) and red beds (sandstones with hematite-coated quartz grains). A second major peak, which raised atmospheric oxygen levels to 50 percent PAL, was reached by 600 million years ago. It was denoted by the first appearance of animal life (metazoans) requiring sufficient oxygen for the production of collagen and the subsequent formation of skeletons. Furthermore, in the stratosphere during the Precambrian, free oxygen began to form a layer of ozone (O3), which currently acts as a protective shield against the Sun’s ultraviolet rays.


Development of the ocean

The origin of Earth’s oceans occurred earlier than that of the oldest sedimentary rocks. The 3.85-billion-year-old sediments at Isua in western Greenland contain BIFs that were deposited in water. These sediments, which include abraded detrital zircon grains that indicate water transport, are interbedded with basaltic lavas with pillow structures that form when lavas are extruded under water. The stability of liquid water (that is, its continuous presence on Earth) implies that surface seawater temperatures were similar to those of the present.
Differences in the chemical composition of Archean and Proterozoic sedimentary rocks point to two different mechanisms for controlling seawater composition between the two Precambrian eons. During the Archean, seawater composition was primarily influenced by the pumping of water through basaltic oceanic crust, such as occurs today at oceanic spreading centres. In contrast, during the Proterozoic, the controlling factor was river discharge off stable continental margins, which first developed after 2.5 billion years ago. The present-day oceans maintain their salinity levels by a balance between salts delivered by freshwater runoff from the continents and the deposition of minerals from seawater.


CLIMATIC CONDITIONS

A major factor controlling the climate during the Precambrian was the tectonic arrangement of continents. At times of supercontinent formation (at 2.5 billion, 2.1 to 1.8 billion, and 1.0 billion to 900 million years ago), the total number of volcanoes was limited; there were few island arcs (long, curved island chains associated with intense volcanic and seismic activity), and the overall length of oceanic spreading ridges was relatively short. This relative shortage of volcanoes resulted in low emissions of the greenhouse gas carbon dioxide (CO2). This contributed to low surface temperatures and extensive glaciations. In contrast, at times of continental breakup, which led to maximum rates of seafloor spreading and subduction (at 2.3 to 1.8 billion, 1.7 to 1.2 billion, and 800 to 500 million years ago), there were high emissions of CO2 from numerous volcanoes in oceanic ridges and island arcs. The atmospheric greenhouse effect was enhanced, warming Earth’s surface, and glaciation was absent. These latter conditions also applied to the Archean Eon prior to the formation of continents.


Temperature and rainfall

The discovery of 3.85-billion-year-old marine sediments and pillow lavas in Greenland indicates the existence of liquid water and implies a surface temperature above 0 °C (32 °F) during the early part of Precambrian time. The presence of 3.5-billion-year-old stromatolites in Australia suggests a surface temperature of about 7 °C (45 °F). Extreme greenhouse conditions in the Archean caused by elevated atmospheric levels of carbon dioxide from intense volcanism (effusion of lava from submarine fissures) kept surface temperatures high enough for the evolution of life. They counteracted the reduced solar luminosity (rate of total energy output from the Sun), which ranged from 70 to 80 percent of the present value. Without these extreme greenhouse conditions, liquid water would not have occurred on the Earth’s surface.

In contrast, direct evidence of rainfall in the geological record is very difficult to find. Some limited evidence has been provided by well-preserved rain pits in 1.8-billion-year-old rocks in southwestern Greenland.


Worldwide glaciations

The presence of tillites (glacial sediments) indicates that extensive glaciations occurred several times during the Precambrian. Glacial deposits are not necessarily limited to high latitudes. In general, they are complementary to the carbonates, evaporites, and red beds that are climatically sensitive and restricted to low latitudes.

The oldest known glaciation took place 2.9 billion years ago in South Africa during the Late Archean; the evidence is provided by glacial deposits in sediments of the Pongola Rift in southern Africa. The most extensive early Precambrian Huronian glaciation occurred 2.3 billion years ago during the early Proterozoic. It can be recognized from the rocks and structures that the glaciers and ice sheets left behind in parts of Western Australia, Finland, southern Africa, and North America. The most extensive occurrences are found in North America in a belt nearly 3,000 km (1,800 miles) long extending from Chibougamau in Quebec through Ontario to Michigan and southwestward to the Medicine Bow Mountains of Wyoming. This probably represents the area of the original ice sheet. Most details are known from the Gowganda Formation in Ontario, which contains glacial deposits that are up to 3,000 metres (9,850 feet) thick and that occupy an area of about 20,000 square km (7,700 square miles); the entire glacial event may have covered an area of more than 2.5 million square km. Paleomagnetic studies indicate that the Gowganda glaciation occurred near the paleoequator. Similar, roughly contemporaneous glacial deposits can be found in other parts of the world, suggesting that there was at least one extensive glaciation during the early Proterozoic.

The largest glaciation in the history of the Earth occurred during the late Proterozoic in the period between 1 billion and 600 million years ago. It left its mark almost everywhere. One of the best-described occurrences is in the Flinders Range of South Australia, where there is a sequence 4 km (2.5 miles) thick of tillites and varved sediments occupying an area of 400 by 500 km (250 by 300 miles). Detailed stratigraphy and isotopic dating show that three worldwide glaciations took place: the Sturtian glaciation (750 to 700 million years ago), the Varanger-Marinoan ice ages (625 to 580 million years ago), and the Sinian glaciation (600 to 550 million years ago).
What is the explanation for all these occurrences of glacial deposits? Some paleomagnetic studies have shown that the tillites in Scotland, Norway, Greenland, central Africa, North America, and South Australia were deposited in low or near-equatorial paleolatitudes. Such conclusions are, however, controversial, because it has also been suggested that the positions of the northern and southern magnetic poles may have migrated across the globe, leaving a record of glaciations in both high and low latitudes. There is the possibility that floating ice sheets could have traveled to low latitudes, depositing glacial sediments and dropstones below them. Whatever the answer, the existence of such vast quantities of tillites and of such extensive glaciations is intriguing. It has been suggested that they record the existence of a frozen “snowball” Earth.


Precambrian life

Precambrian rocks were originally defined to predate the Cambrian Period and therefore all life, although the term Proterozoic was later coined from the Greek for “early life.” It is now known that Precambrian rocks contain evidence of the very beginnings of life on Earth (and thus the record of its evolution for more than 3.5 billion years), the explosion of life-forms without skeletons before the Cambrian, and even the development of sexual reproduction.

The earliest signs of life on Earth are in western Greenland where apatite (calcium phosphate) grains within a 3.85-billion-year-old meta-sedimentary rock have carbon isotope ratios that indicate an organic origin. The presence of organic hydrocarbon droplets in kerogenous sediments has been found in the 3.46-billion-year-old Warrawoona Group in the Pilbara craton of Western Australia. These are small amounts of Archean oil.

The first fossil evidence of terrestrial life is found in the early Archean sedimentary rocks of the greenstone-granite belts (metamorphosed oceanic crust and island arc complexes) of the Barberton craton in South Africa and in the Warrawoona Group, which are both about 3.5 billion years old. There are two types of these early, simple, biological structures: microfossils and stromatolites (sheetlike mats precipitated by communities of microorganisms).


Microfossils and stromatolites

The microfossils occur in cherts and shales and are of two varieties. One type consists of spherical carbonaceous aggregates, or spheroids, which may measure as much as 20 mm (0.8 inch) in diameter. These resemble algae and cysts of flagellates and are widely regarded as biogenic (produced by living organisms). The other variety of microfossils is made up of carbonaceous filamentous threads, which are curving, hollow tubes up to 150 micrometres (0.006 inch) long. Most likely, these tubes are the fossil remains of filamentous organisms. Hundreds of them have been found in some rock layers. The 2.8-billion-year-old gold reefs (conglomerate beds with rich gold deposits) of the Witwatersrand Basin in South Africa contain carbonaceous columnar microfossils up to 7 mm (slightly less than 0.3 inch) long that resemble modern algae, fungi, and lichens. They probably extracted gold from their environment in much the same way that modern fungi and lichens do.

Stromatolites are stratiform, domal, or columnar structures made from sheetlike mats precipitated by communities of microorganisms, particularly filamentous blue-green algae. The early Archean examples form domes as tall as about 10 cm (4 inches). Stromatolites occur in many of the world’s greenstone-granite belts. In the 2.7-billion-year-old Steep Rock Lake belt in Ontario, Can., they reach 3 metres (9 feet) in height and diameter.

Stromatolites continued to form all the way through the geologic record and today grow in warm intertidal waters, as exemplified by those of Shark Bay in Western Australia. They provide indisputable evidence that life had begun on Earth using algal photosynthesis in complex, integrated biological communities by 3.5 billion years ago.

These Archean organisms were prokaryotes that were incapable of cell division. They were relatively resistant to ultraviolet radiation and thus were able to survive during Earth’s early history when the atmosphere lacked an ozone layer. The prokaryotes were predominant until about 1.7 billion to 1.9 billion years ago, when they were overtaken by the eukaryotes (organisms possessing nucleated cells). The latter made use of oxygen in metabolism and for growth and thus developed profusely in the increasingly oxygen-rich atmosphere of the early Proterozoic. The eukaryotes were capable of cell division, which allowed DNA (deoxyribonucleic acid), the genetic coding material, to be passed on to succeeding generations.

By early Proterozoic time both microfossils and stromatolites had proliferated. The best-known occurrence of microorganisms is in the 2-billion-year-old, stromatolite-bearing Gunflint iron formation in the Huronian Basin of southern Ontario. These microbial fossils include some 30 different types with spheroidal, filamentous, and sporelike forms up to about 20 micrometres (0.0008 inch) across. Sixteen species in 14 genera have been classified so far. Microfossils of this kind are abundant, contain beautifully preserved organic matter, and are extremely similar to such present-day microorganisms as blue-green algae and microbacteria. There are comparable microfossils from the early Proterozoic in Minnesota and Michigan in the United States, the Belcher Islands in Hudson Bay in Canada, southern Greenland, Western Australia, and northern China. These microbiota lived at the time of the transition in the chemical composition of the atmosphere when oxygen began accumulating for the first time.

During the late Proterozoic, stromatolites reached their peak of development, became distributed worldwide, and diversified into complex, branching forms. From about 700 million years ago, however, they began to decline significantly in number. Possibly the newly arrived metazoans (multicelled organisms whose cells are differentiated into tissues and organs) ate the stromatolitic algae, and their profuse growth destroyed the habitats of the latter.

There is the intriguing question as to when sexual division arose in life-forms. In the late 1960s, American paleobiologist J. William Schopf pointed out that the abundant microflora of the 900-million-year-old Bitter Springs Formation of central Australia includes some eukaryotic algae that have cells in various stages of division arranged into tetrahedral sporelike forms. These resemble the tetrad of spore cells of living plants known to develop by sexual division. In effect, by the end of the Precambrian the conditions were set for the explosion of life at the start of the Phanerozoic Eon.


Ediacaran fossils

Metazoans developed rapidly from the beginning of the Cambrian, when organisms acquired the ability to produce the protein collagen and, thus, skeletons and shells. However, more-primitive metazoans without skeletons—the Ediacara fauna—appeared earlier (more than 600 million years ago), after the end of the Varanger-Marinoan ice age at 580 million years ago and before the onset of the Cambrian Period at 542 million years ago. They are found as impressions of soft-bodied, multicellular animals in the rocks and have the form of tiny blobs, circular discs, or plantlike fronds ranging from less than 1 cm (less than 0.4 inch) to more than 1 metre (about 3 feet) long. The type locality is the Ediacara Hills in South Australia, where over 1,500 well-preserved specimens have been collected, resulting in the naming of more than 60 species and 30 genera. They occur in a quartzite that is stratigraphically situated some 500 metres (1,600 feet) below the base of the Cambrian System. These organisms resemble modern jellyfish, worms, sponges, seaweed, sea anemones, and sea pens. Comparable impressions in the youngest Precambrian sediments have been found in over 30 localities from every continent except Antarctica. Ediacaran fossils have been discovered in areas such as Charnwood in central England, Ukraine, Iran, the Ural Mountains and the White Sea coast in Russia, Namibia, Newfoundland, the Mackenzie and Wernecke mountains in northwestern Canada, the Yangtze valley in China, and North Carolina in the southeastern United States. Ediacaran fossils have been deposited in environments ranging from tidal marine habitats to the deep seafloor. The Ediacaran organisms were probably the ancestors of shelled organisms that mark the beginning of the Phanerozoic.


Precambrian geology

Major subdivisions of the Precambrian System

By international agreement, Precambrian time is divided into the Archean Eon (occurring between roughly 4.0 billion years ago and 2.5 billion years ago) and Proterozoic Eon (occurring between 2.5 billion and 542 million years ago). After the Precambrian, geologic time intervals are commonly subdivided on the basis of the fossil record. The paucity of Precambrian fossils, however, precludes the creation of small-scale subdivisions (epochs and ages) in this time period. Instead, relative chronologies of events have been produced for different regions based on such field relationships as unconformities (interruption in the accumulation of sedimentary rock due to erosion or nondeposition) and crosscutting dikes (intrusions of igneous rock that burrow through cracks in the original structures of surrounding rock). These field relationships, combined with the isotopic age determinations of specific rocks, allow for some correlation between neighbouring regions. The International Commission on Stratigraphy (ISC) and International Union of Geological Sciences (IUGS) divide the Archean Eon into the Eoarchean (approximately 4.0 billion to 3.6 billion years ago), Paleoarchean (3.6 billion to 3.2 billion years ago), Mesoarchean (3.2 billion to 2.8 billion years ago), and Neoarchean (2.8 billion to 2.5 billion years ago) eras. Likewise, they divide the Proterozoic Eon into the Paleoproterozoic (2.5 billion to 1.6 billion years ago), Mesoproterozoic (1.6 billion to 1 billion years ago), and Neoproterozoic (1 billion to 542 million years ago) eras. These definitions are based on isotopic age determinations.


Oldest minerals and rocks

The oldest minerals on Earth, detrital zircons from western Australia, crystallized about 4.4 billion years ago. They occur within sedimentary sandstones and conglomerates dated to about 3.3 billion years ago, but the environment in which they were formed is totally unknown. The rocks from which they came may have been destroyed by some kind of tectonic process or by a meteorite impact that spared individual zircon crystals. On the other hand, rocks containing these minerals may still exist on Earth’s surface but simply have not been found. Perhaps their very absence is indicative of something important about early terrestrial processes. Comparisons with the Moon indicate that the Earth must have been subjected to an enormous number of meteorite impacts about 4 billion years ago, but there is no geologic evidence of such events.

The oldest known rocks on Earth are the faux amphibolite volcanic deposits of the Nuvvuagittuq greenstone belt in Quebec, Canada; they are estimated to be 4.28 billion years old. The age of these rocks was estimated using a radiometric dating technique that measures the ratio of the rare-earth elements neodymium and samarium present in a sample.

The Acasta gneisses, found near Canada’s Great Slave Lake, are also among the world’s oldest rocks. Their age has been established radiometrically at 4.0 to 3.9 billion years. The Acasta gneisses are granitic and contain a single relict zircon crystal, which has been dated to 4.2 billion years ago and formed from granitic magma. They are thought to have evolved from older basaltic material in the crust that was melted and remelted by tectonic processes.


Significant geologic events

DISTINCTIVE FEATURES

The Archean and Proterozoic eons within Precambrian time are very different and must be considered separately. The Archean-Proterozoic boundary constitutes a major turning point in Earth history. Before that time the crust of the Earth was in the process of growing, and so there were no large, stable continents. Afterward, when such continents had emerged, orogenic belts were able to form on the margins of and between continental blocks.
There are two types of Archean orogenic belts. The first occurs in upper crustal greenstone-granite belts rich in volcanic rocks that are probably primitive types of oceanic crust and island arcs (long, curved island chains associated with intense volcanic and seismic activity) that formed during the early rapid stage of crustal growth. The second occurs in granulite-gneiss belts that were recrystallized in the Archean mid-lower crust under metamorphic conditions associated with high-temperature granulite and amphibolite facies. Thus, granulites, which typically contain the high-temperature mineral hypersthene (a type of pyroxene), are a characteristic feature of many Precambrian orogenic belts that have been deeply eroded. In Phanerozoic orogenic belts, granulites are rare.

There are several other rock types that developed primarily during the Precambrian but rarely later. This restriction is a result of the unique conditions that prevailed during Precambrian time. For example, banded-iron formations are ferruginous sediments that were deposited on the margins of early, iron-rich oceans. Anorthosite, which consists largely of plagioclase, forms large bodies in several Proterozoic belts. Komatiite, a magnesium-rich, high-temperature volcanic rock derived from very hot mantle (part of the Earth between the crust and the core), was extruded in abundance during the early Precambrian when the heat flow of the Earth was higher than it is today. Blueschist, which contains the blue mineral glaucophane, forms in subduction zones under high pressures and low temperatures, and its rare occurrence in Precambrian rocks may indicate that temperatures in early subduction zones were too high for its formation.

The bulk of many of the world’s valuable mineral deposits (for example, those of gold, nickel, chromite, copper, and iron) also formed during the Precambrian. These concentrations are a reflection of distinctive Precambrian sedimentary and magmatic rocks and their environments of formation.


ARCHEAN CRUSTAL GROWTH

During the first third of geologic history (that is, until about 2.5 billion years ago), the Earth developed in a broadly similar manner. Greenstone-granite belts (metamorphosed oceanic crust and island arc complexes) formed in the upper Archean crust, and granulite-gneiss belts formed in the mid-lower crust. This was a time when the overall rate of heat production by the breakdown of radioactive isotopes was several times greater than it is today. This condition was manifested by very rapid tectonic processes, probably by some sort of primitive plate tectonics (more-modern plate-tectonic processes could not occur until the crust became cooler and more rigid). Most of the heat that escapes from Earth’s interior today does so at oceanic ridges. This manner of heat loss probably occurred during the Archean in much larger amounts. The oceanic ridges of the Archean were more abundant, longer, and opened faster than those in the modern oceans, and oceanic plateaus derived from hot mantle plumes (slowly rising currents of highly viscous mantle material) were more common. Although the amount of newly generated crust was probably enormous, a large part of this material was inevitably destroyed by equally rapid plate subduction processes. The main results of this early growth that still remain today are the many island arcs and oceanic plateaus in greenstone-granite belts and the voluminous Andean-type tonalites (a granitic-type rock rich in plagioclase feldspar) that were deformed to orthogneiss (gneiss derived from igneous rocks) in granulite-gneiss belts. Although most of the Archean oceanic crust was subducted, a few ophiolitic-type complexes have been preserved in greenstone-granite belts.

The late Archean (Neoarchean Era) was an important interval of time because it marks the beginning of the major changeover from Archean to Proterozoic types of crustal growth. The formation of the first major rifts characterized the significant events of this time. The first major rift valley known in the world, the Pongola Rift, emerged along the border of present-day Swaziland and South Africa; the intrusion of the first major basic dikes (such as the Great Dyke, which transects the entire Zimbabwe craton) and the first large stratiform layered igneous complexes (such as the Stillwater in Montana) formed; and the formation of the first large sedimentary basins (for example, the Witwatersrand in South Africa) also occurred. All of these structures indicate that the continental crust had reached a mature stage with considerable stability and rigidity for the first time during the late Archean. The Neoarchean represents the culmination that followed the rapid tectonic processes of the early Archean (Eoarchean and Paleoarchean) and middle Archean (Mesoarchean) eras. Because crustal growth took place at different times throughout the world, similar structures can be found in the early Proterozoic (Paleoproterozoic) Era.


PROTEROZOIC PLATE MOVEMENTS

During the early Proterozoic, large amounts of quartzite, carbonate, and shale were deposited on the shelves and margins of many continental blocks. This would be consistent with the breakup of a supercontinent into several smaller continents with long continental margins (combined areas of continental shelf and continental slope). Examples of shelf sequences of this kind are found along the margins of orogenic (mountain) belts, such as the Wopmay, bordering Canada’s Slave province, and also the Labrador Trough, bordering the Superior province.
The existence of stable continental blocks by the early Proterozoic allowed orogenic belts to develop at their margins by some form of collision tectonics. This was the first time that long, linear orogenic belts could form by “modern” tectonic processes that involved seafloor spreading, ophiolite obduction, subduction, and landmass collisions. Subduction lead to the creation of island arcs and Andean-type (formed by subduction at the continental margin) granitic batholiths. In addition, the collision of arcs and continents could now give rise to both sutures with ophiolites and to Himalayan-type (formed by continent-to-continent collision) thrust belts with abundant crustal-melt granites. These were key events in the evolution of the continents, and such processes have continued throughout Earth history.

During the late Proterozoic (Neoproterozoic Era), some orogenic belts, like the Pan-African belts of Saudi Arabia and East Africa, continued to develop. The intense crustal growth and the many orogenic belts that formed throughout the Proterozoic began to create large continental blocks, which amalgamated to produce a new supercontinent by the end of the Precambrian. Therefore, in the late Proterozoic many sedimentary basins were infilled with conglomerates and sandstones due to the deposition of material eroded from higher elevations. For example, the Riphean sequence in Russia and also the Sinian sequence in China were able to form on extensive cratons of continental crust.


Occurrence and distribution of Precambrian rocks

Precambrian rocks, as a whole, occur in a wide variety of shapes and sizes. There are extensive Archean regions, up to a few thousands of kilometres across, that may contain either greenstone-granite belts or granulite-gneiss belts or both. These regions are variously designated in different parts of the world as cratons, shields, provinces, or blocks. Some examples include: the North Atlantic craton that incorporates northwestern Scotland, central Greenland, and Labrador; the Kaapvaal and Zimbabwean cratons in southern Africa; the Dharwar craton in India; the Aldan and Anabar shields in Siberia in Russia; the Baltic Shield that includes much of Sweden, Finland, and the Kola Peninsula of far northern Russia; the Superior and Slave provinces in Canada; and the Yilgarn and Pilbara blocks in Western Australia. Linear belts, up to several thousand kilometres long, that are frequently though not exclusively of Proterozoic age include the Limpopo, Mozambique, and Damaran belts in Africa, the Labrador Trough in Canada, and the Eastern Ghats belt in India. Several small relict areas, spanning a few hundred kilometres across, exist within or against Phanerozoic orogenic belts and include the Lofoten islands of Norway, the Lewisian Complex in northwestern Scotland, and the Adirondack Mountains in the northeastern United States. Nevertheless, some extensive areas of Precambrian rocks, such as under the European and Russian platforms and under the central United States, remain overlain by a blanket of Phanerozoic sediments.


Archean rock types

Archean rocks occur in greenstone-granite belts that represent the upper crust, in granulite-gneiss belts that formed in the mid-lower crust, and in sedimentary basins, basic dikes, and layered complexes that were either deposited on or intruded into the first two types of belts.


GREENSTONE-GRANITE BELTS

These belts occur on most continents. The largest extend several hundred kilometres in length and measure several hundred metres in width. Today many greenstone-granite belts are regarded as tectonic “slices” of oceanic and island arc crust that have been thrust together to form tectonic collages similar to those in belts found in the present-day Pacific Ocean.

The greenstone sequence in many belts is divisible into a lower volcanic group and an upper sedimentary group. The volcanics are made up of lavas that are ultramafic (silica content less than 45 percent) and basaltic (silica content of 45 to 52 percent). The uppermost sediments are typically terrigenous (land-derived) shales, sandstones, quartzites, wackes, and conglomerates. All the greenstone sequences have undergone recrystallization during the metamorphism of greenschist facies at relatively low temperatures and pressures. In fact, the presence of the three green metamorphic minerals chlorite, hornblende, and epidote has given rise to the term greenstone for the recrystallized basaltic volcanics. Granitic rocks and gneisses occur within, adjacent to, and between many greenstone sequences.


Economic significance of Archean greenstone-granite deposits

Abundant mineralization has occurred in greenstone-granite belts. These belts constitute one of the world’s principal depositories of gold, silver, chromium, nickel, copper, and zinc. In the past they were termed gold belts because of the gold rushes of the 19th century that took place in areas such as Kalgoorlie in the Yilgarn belt of Western Australia, the Barberton belt of South Africa, and Val d’Or in the Abitibi belt of southern Canada. The mineral deposits occur in all the major rock groups: chromite, nickel, asbestos, magnesite, and talc in ultramafic lavas; gold, silver, copper, and zinc in basaltic to rhyolitic volcanics; iron ore, manganese, and barite in sediments; and lithium, tantalum, beryllium, tin, molybdenum, and bismuth in granites and associated pegmatites. Important occurrences are chromite at Selukwe in Zimbabwe, nickel at Kambalda in southwestern Australia, tantalum in Manitoba in Canada, and copper-zinc at Timmins and Noranda in the Canadian Abitibi belt.


Greenstone-granite rock types

The volcanics that comprise the lower portion of a greenstone sequence are made up of lavas noted for magnesian komatiites (ultramafic extrusive igneous rocks) that probably formed in the oceanic crust that are overlain by basalts, andesites, and rhyolites whose chemical composition is much like that of modern island arcs. Especially important is the presence in the Isua, Barberton, and Yellowknife belts of sheeted basic dike complexes cutting across gabbros and overlain by pillow-bearing basalts (basalts extruded underwater that form characteristic pillow-shaped hummocks). Volcanic sequences are capped by oceanic cherts and terrigenous sedimentary groups. The overall stratigraphy suggests an evolution from extensive submarine eruptions of komatiite and basalt (ocean floor) to more-localized stratovolcanoes (volcanoes constructed from alternating layers of ash and lava), which become increasingly emergent with intervening and overlying clastic sediments (clay-, silt-, and sand-sized sediments) that were deposited in trenches at the mouths of subduction zones. There are, however, regional differences in the volcanic and sedimentary makeup of some belts. The older belts in southern Africa and Australia have more komatiites, basalts, shallow-water banded-iron formations, cherts, and evaporites and fewer terrigenous (land-derived) sediments. On the other hand, the younger belts in North America have a higher proportion of andesites, rhyolites, and terrigenous and turbidite debris (sediments delivered to the deep ocean by density currents) but fewer shallow-water sediments. These differences reflect a change from the older oceanic-type volcanism (effusion of lava from submarine fissures) to the younger, more arc-type phenomena such as explosive eruption of pyroclastic materials (incandescent material ejected during violent eruptions) and lava flows from steep volcanic cones. Additional changes include an increase in the amount of trench (subduction zone) turbidites and graywackes and an increase in the availability of continental crust as a source for terrigenous debris.

Ultramafic rocks (rocks with a very low silica content—less than 45 percent) are commonly altered to talc schists and tremolite-actinolite schists. There are some indications that several phases of metamorphism exist—namely, seafloor metamorphism associated with the action of hydrothermal brines that could occur at oceanic ridges, syntectonic metamorphism related to thrust-nappe tectonics, and local thermal contact metamorphism caused by intrusive granitic plutons pushing into cooler surrounding rock.

Granitic rocks and gneisses are associated with many greenstone sequences. Some paragneisses (gneisses metamorphosed from sedimentary rocks), as in the Quetico belt in Canada, are derived from wackes. They were probably deposited in an ocean trench or accretionary prism (a mass of accumulating sediments on the inner trench wall in a subduction zone) at the mouth of a subduction zone between the island arcs of the adjacent greenstone sequences. Many early granitic plutons were deformed and converted into orthogneiss (gneisses metamorphosed from igneous rocks). Late plutons commonly intruded the greenstones that were downfolded in synclines (an upward concave fold of rock) between them, or they intruded along the borders of the belts, deflecting them into irregular shapes.


Structure and formation of greenstone-granite belts

The structure of many belts is complex. Their stratigraphic successions are upside-down and deformed by thrusts and major horizontal folds (nappes). They have been subsequently refolded by upright anticlines (convex folds of rock) and synclines. The result of this thrusting is the repetition of the same stratigraphic successions on top of one another, creating a massive deposit of material up to 10 to 20 km (6 to 12 miles) thick. Also, there may be thrusts along the base of the belts, as in the case of Barberton, showing that they have been transported from elsewhere. In other instances, the thrusts may occur along the borders of the belts, indicating that they have been forced against and over adjacent gneissic belts. The conclusion from structural studies is that many belts have undergone intense subhorizontal deformation during thrust transport.

Clearly, there are different types of greenstone-granite belts. To understand their origin and mode of evolution, it is necessary to correlate them with comparable modern analogues. Some, like the Barberton and Yellowknife belts, consist of oceanic-type crust and have sheeted dike swarms that occur in many ophiolites of Mesozoic-Cenozoic origin, such as in the Troodos Mountains in Cyprus. They are the hallmark of a modern oceanic crust that formed at an oceanic ridge. Also, like modern ophiolites, a few seem to have been covered by thrusting onto continental crust. Many belts, such as the Isua belt of Greenland and those in the Superior province of Canada, are very similar to modern island arcs. Geochemical data are revealing that some lavas were derived from depths of 1,000 to 2,700 km (620 to 1,680 miles) in the Earth’s mantle and not from shallower subduction zones, which are commonly 600 km (about 373 miles) deep. These rocks are comparable to oceanic plateaus in modern oceanic crust that were formed from plumes of hot magma from the very deep mantle. The Wawa belt, for example, has been shown to consist of an immature island arc built on oceanic plateau crust and overlain by a more mature arc. The Abitibi belt began as oceanic crust with island arcs and oceanic plateaus. Between the Wawa and Wabigoon island arcs lies the Quetico belt, consisting of metamorphosed turbidites and slices of volcanics that probably developed in a regularly overlapping accretionary prism in an arc-trench system, as seen today in the Japanese arcs. The Pilbara belts are similar to modern active continental margins, and they have been interthrust with older continental orthogneisses to form very thick crustal piles intruded by diapiric crustal-melt granites. This scenario is quite comparable to that of a Himalayan type of orogenic belt formed by collisional tectonics. In conclusion, most greenstone-granite belts are today regarded by geologists as different parts of interthrust oceanic crust–accretionary prism structures within island arcs of oceanic plateau systems that collided with continental gneissic blocks.


Age and occurrence of greenstone-granite belts

Greenstone-granite belts developed at many different times throughout the long Archean Eon. The Isua greenstone belt in West Greenland is about 3.85 billion years old. In the Zimbabwean craton, they formed over three successive periods: the Selukwe belt about 3.8 to 3.75 billion years ago, the Belingwean belts about 2.9 billion years ago, and the Bulawayan-Shamvaian belts about 2.7 to 2.6 billion years ago. The Barberton belt in the Kaapvaal craton and the Warrawoona belt in the Pilbara block are 3.5 billion years old. Globally, the most important period of formation was from 2.7 to 2.6 billion years ago, especially in the Slave and Superior provinces of North America, the Yilgarn block in Australia, and the Dharwar craton in India. Some of the better-documented belts seem to have formed within about 50 million years. It is important to note that while the Bulawayan-Shamvaian belts were forming in the Zimbabwean craton, flat-lying sediments and volcanics were laid down in the Pongola Rift and the Witwatersrand Basin not far to the north.

Greenstone-granite belts range from aggregates of several belts (as in the southern Superior province of Canada) to irregular, even triangular-shaped belts (as in the Barberton in South Africa) to synclinal basins (as in the Indian Dharwar craton). The irregular and synclinal shapes are commonly caused by the diapiric intrusion of younger granites.

Important occurrences are the Barberton belt in South Africa; the Sebakwian, Belingwean, and Bulawayan-Shamvaian belts of Zimbabwe; the Yellowknife belts in the Slave province of Canada; the Abitibi, Wawa, Wabigoon, and Quetico belts of the Superior province of Canada; the Dharwar belts in India; and the Warrawoona and Yilgarn belts in Australia.


GRANULITE-GNEISS BELTS

The granulites, gneisses, and associated rocks in these belts were metamorphosed to a high grade in deep levels of the Archean crust; metamorphism occurred at a temperature of 750 to 980 °C (1,380 to 1,800 °F) and at a depth of about 15 to 30 km (9 to 19 miles). These belts, therefore, represent sections of the continents that have been highly uplifted, with the result that the upper crust made up of volcanics, sediments, and granites has been eroded. Accordingly, the granulite-gneiss belts are very different from the greenstone-granite belts. Granulite-gneiss belts may be regarded as variably preserved sections of continental cratons.


Economic significance of Archean granulite-gneiss deposits

The mid-lower crust is relatively barren of ore deposits as compared to the upper crust with its sizable concentrations of greenstones and granites, and therefore little mineralization is found in the granulite-gneiss belts. The few exceptions include a nickel–copper sulfide deposit at Selebi-Pikwe in the Limpopo belt in Botswana that is economic to mine, and banded-iron formations in gneisses in the eastern Hubei and Liaoning provinces of northwestern China that form the foundation of a major steel industry. There are subeconomic quantities of chromitite in the anorthosites of western Greenland, southern India, and the Limpopo belt; iron from a banded-iron formation at Isua in western Greenland; and tungsten in amphibolites of western Greenland.


Granulite-gneiss rock types

Orthogneisses of deformed and recrystallized tonalite (a granitic-type rock rich in plagioclase feldspar) and granite constitute the most common rock type. The geochemical signature of these rocks closely resembles that of modern equivalents that occur in granitic batholiths in the Andes. Where such rocks have been metamorphosed under conditions associated with amphibolite facies, they contain hornblende, biotite, or a combination of the two. However, where they have been subjected to conditions of higher temperature associated with the granulite facies, the rocks contain pyroxene and hypersthene and so can be called granulites.

The granulites and gneisses enclose a wide variety of other minor rock types in layers and lenses. These types include schists and paragneisses that were originally deposited on the Earth’s surface as shales and which now contain high-temperature metamorphic minerals such as biotite, garnet, cordierite, staurolite, sillimanite, or kyanite. There also are quartzites, which were once sandstones or cherts; marbles (either limestones or dolomites); and banded-iron formations. Commonly intercalated with these metasediments are amphibolites, which locally contain relict pillow structures, demonstrating that they are derived from basaltic lavas extruded underwater. These amphibolites have a trace element chemistry quite similar to that of modern seafloor basalts. The amphibolites are often accompanied by chromite-layered anorthosite, gabbro, and ultramafic rocks such as peridotite and dunite. All these rocks occur in layered igneous complexes, which in their well-preserved state may be up to 2 km (1.2 miles) thick and 100 km (60 miles) long. Such complexes occur at Fiskenaesset in western Greenland, in the Limpopo belt of southern Africa, and in southern India. These complexes may have formed at an oceanic ridge in a magma chamber that also fed the basaltic lavas, or they may be parts of oceanic plateaus. In many cases, the complexes, basaltic amphibolites, and sediments were extensively intruded by the tonalites and granites that were later deformed and recrystallized. The result of this is that all of these rocks may now occur as metre-sized lenses in the orthogneisses and granulites.


Structure and occurrence of granulite-gneiss belts

The structure of the granulite-gneiss belts is extremely complex because the constituent rocks have been highly deformed several times. In all likelihood the basalts and layered complexes from the oceanic crust were interthrust with shallow-water limestones, sandstones, and shales; with tonalites and granites from Andean-type batholiths; and with older basement rocks from a continental margin. All these rocks, which are now mutually conformable (parallel to one another with uninterrupted deposition), were folded in horizontal nappes and then refolded. The picture that emerges is one of a very mobile Earth, where newly formed rocks were routinely compressed and thrust against other rocks.

Granulite-gneiss belts occur in a variety of environments. These may be extensive regions, such as the North Atlantic craton, which measures 1,000 by 2,000 km (about 620 by 1,240 miles) across and, before the opening of the Atlantic Ocean, was contiguous with the Scourian Complex of northwestern Scotland, the central part of Greenland, and the coast of Labrador; the Aldan and Ukrainian shields of continental Europe; the North China craton; large parts of the Superior province of Canada; the Yilgarn block in Australia; and the Limpopo belt in southern Africa. They may be confined to small areas such as the Ancient Gneiss Complex of Swaziland, the Minnesota River valley and the Beartooth Mountains of the United States, the Peninsular gneisses and Sargur supracrustals of southern India, the English River gneisses of Ontario in Canada that form a narrow strip between greenstone-granite belts, the Sand River gneisses that occupy a small area between greenstone-granite belts in Zimbabwe, and the Napier Complex in Enderby Land in Antarctica. Granulite-gneiss belts are commonly surrounded by younger, mostly Proterozoic belts that contain remobilized relicts of the Archean rocks, and the granulites and gneisses must underlie many Archean greenstone-granite belts and blankets of Phanerozoic sediment.


Age and correlation of granulite-gneiss belts

Isotopic age determinations from the granulite-gneiss belts record an evolution from about 4.0 to 2.5 billion years ago—more than a third of geologic time. Most important are the few but well-constrained age determinations of detrital zircons at Mount Narryer and Jack Hills in Western Australia that are more than 4 billion years old. Several regions have a history that began in the period dating from 3.9 to 3.6 billion years ago—western Greenland, Labrador, the Limpopo belt, Enderby Land, the North China craton, and the Aldan Shield. Most regions of the world experienced a major tectonic event that may have involved intrusion, metamorphism, and deformation during the period between 3.1 and 2.8 billion years ago; some of these regions, like the Scourian in northwestern Scotland, show no evidence of any older crustal growth. The best-documented region is in western Greenland, which has a long and complicated history from 3.85 to 2.5 billion years ago.

It is impossible to correlate the rocks in different granulite-gneiss belts. One granitic gneiss is essentially the same as another but may be of vastly different age. There is a marked similarity in the anorthosites in various belts throughout the world, and their similar relationship with the gneisses suggests that the belts have undergone comparable stages of evolution, although each has its own distinctive features. Little correlation can be made with rocks of Mesozoic-Cenozoic age because few modern orogenic belts have been eroded sufficiently to expose their mid-lower crust. The lack of modern analogues for comparison makes it particularly difficult to interpret the mode of origin and evolution of the Archean granulite-gneiss belts.


SEDIMENTARY BASINS, BASIC DIKES, AND LAYERED COMPLEXES

During middle and late Archean time (3 to 2.5 billion years ago), relatively stable, post-orogenic conditions developed locally in the upper crust, especially in southern Africa, where the development of greenstone-granite and granulite-gneiss belts was completed much earlier than in other parts of the world. The final chapters of Archean crustal evolution can be followed by considering specific key sedimentary basins, basic (basaltic) dikes, and layered complexes.

Along the border of Swaziland and South Africa is the Pongola Rift, which is the oldest such continental trough in the world; it is 2.95 billion years old, having formed only 50 million years after the thrusting of adjacent greenstone-granite belts. If there were earlier rifts, they have not survived, or, more likely, this was the first time in Earth history that the upper crust was sufficiently stable and rigid for a rift to form. It is 30 km (19 miles) wide, 130 km (81 miles) long, and within it is a sequence of lavas and sediments that is 11 km (7 miles) thick. It seems most likely that the rift developed as the result of the collapse of an overthickened crust following the long period of Archean crustal growth and thrusting in the Kaapvaal craton.

The 200-by-350-km (124-by-217-mile) Witwatersrand Basin contains an 11-km- (7-mile-) thick sequence of lavas and sediments that are 3 billion years old. The basin is famous for its very large deposits of gold and uranium that occur as detrital minerals in conglomerates. These minerals were derived by erosion of the surrounding greenstone-granite belts and transported by rivers into the shoreline of the basin. In all probability, the gold originally came from the komatiitic and basaltic lavas in the early Archean oceanic crust.

The Great Dyke, thought to be about 2.5 billion years old, transects the entire Zimbabwe craton. It is 480 km (about 300 miles) long, 8 km (5 miles) wide, and made up of layered ultrabasic rocks—gabbros and norites. The ultrabasic rocks have several layers of chromite and an extensive platinum-bearing layer that form economic deposits. The Great Dyke represents a rift that has been filled in with magma that was probably derived from a deep mantle plume.

The Stillwater Complex is a famous, 2.7-billion-year-old, layered ultrabasic-basic intrusion in the Beartooth Mountains of Montana in the United States. It is 48 km (30 miles) long and has a stratigraphic thickness of 6 km (3.7 miles). It was intruded as a subhorizontal body of magma that underwent crystal settling to form the layered structure. It is notable for a 3-metre- (9-foot-) thick layer enriched in platinum minerals that forms a major economic deposit.

The basins, dikes, and complexes described above cannot be mutually correlated. They most resemble equivalent structures that formed at the end of plate-tectonic cycles in the Phanerozoic. They represent the culmination of Archean crustal growth.


Proterozoic rock types

What happened geologically at the time of the Archean-Proterozoic boundary 2.5 billion years ago is uncertain. It seems to have been a period of little tectonic activity, and so it is possible that the earlier intensive Archean crustal growth had caused the amalgamation of continental fragments into a supercontinent, perhaps similar to Pangea of Permian-Triassic times. The fragmentation of this supercontinent and the formation of new oceans gave rise to many continental margins upon which a variety of distinctive sediments were deposited. Much evidence suggests that in the period from 2.5 billion to 570 million years ago Proterozoic oceans were formed and destroyed by plate-tectonic processes and that most Proterozoic orogenic belts arose by collisional tectonics. Sedimentary, igneous, and metamorphic rocks that formed during this period are widespread throughout the world. There are many swarms of basic dikes, important sedimentary rifts, basins, and layered igneous complexes, as well as many orogenic belts. The rocks commonly occur in orogenic belts that wrap around the borders of Archean cratons. The characteristic types of Proterozoic rocks are considered below, as are classic examples of their occurrence in orogenic belts. The following types of rocks were formed during the early, middle, and late Proterozoic, indicating that similar conditions and environments existed throughout this long period of time.


BASIC DIKES

The continents were sufficiently stable and rigid during the Proterozoic Eon for an extremely large number of basic dikes to be intruded into parallel, extensional fractures in major swarms. Individual dikes measure up to several hundred metres in width and length, and there may be hundreds or even thousands of dikes in a swarm, some having transcontinental dimensions. For example, the 1.2-billion-year-old Mackenzie swarm is more than 500 km (311 miles) wide and 3,000 km (1,864 miles) long and extends in a northwesterly direction across the whole of Canada from the Arctic to the Great Lakes. The 1.95-billion-year-old Kangamiut swarm in western Greenland is only about 250 km (155 miles) long but is one of the world’s densest continental dike swarms. Many of the major dike swarms were intruded on the continental margins of Proterozoic oceans in a manner similar to the dikes that border the present-day Atlantic Ocean and were similarly the result of the rise of mantle plumes into the crust.


LAYERED IGNEOUS INTRUSIONS

There are several very important layered, mafic to ultramafic intrusions of Proterozoic age that were formed by the accumulation of crystals in large magma chambers. The well-known ones are several tens or even hundreds of kilometres across, have a dikelike or sheetlike (stratiform) shape, and contain major economic mineral deposits. The largest and most famous is the Bushveld Complex in South Africa, which is 9 km (5.6 miles) thick and covers an area of 66,000 square km (about 25,500 square miles). It was intruded nearly 2.1 billion years ago and is the largest repository of magmatic ore deposits in the world. The Bushveld Complex consists of stratiform layers of dunite, norite (a type of gabbro rich in orthopyroxene), anorthosite, and ferrodiorite (an iron-rich intrusive igneous rock that is basic to intermediate in composition) and contains deposits of chromite, iron, titanium, vanadium, nickel, and—most important of all—platinum. The Sudbury Complex in southern Canada, which is about 1.9 billion years old, is a basin-shaped body that extends up to 60 km (37 miles) across. It consists mostly of layered norite and has deposits of copper, nickel, cobalt, gold, and platinum. It is noted for its high-pressure structures and other manifestations of shock metamorphism, which suggest that the intrusion was produced by an enormous meteorite impact.


SHELF-TYPE SEDIMENTS

Quartzites, dolomites, shales, and banded-iron formations make up sequences that reach up to 10 km (6.2 miles) in thickness and that amount to more than 60 percent of Proterozoic sediments. Minor sediments include sandstones, conglomerates, red beds, evaporites, and cherts. The quartzites typically have cross-bedding and ripple marks, which are indicative of tidal action, and the dolomites often contain stromatolites similar to those that grow today in intertidal waters. Also present in the dolomites are phosphorites that are similar to those deposited on shallow continental margins against areas of oceanic upwelling during the Phanerozoic. Several early-middle Proterozoic examples of such dolomites have been found in Finland and northern Australia, as well as in the Marquette Range of Michigan in the United States, in the Aravalli Range of Rajasthan in northwestern India, and at Hamersley and Broken Hill in Australia. Other constituents of these dolomites include evaporites that contain casts and relicts of halite, gypsum, and anhydrite. Examples occur at Mount Isa in Australia (1.6 billion years old) and in the Belcher Group in Canada (1.8 billion years old). These evaporites were deposited by brines in very shallow pools such as those encountered today in the Persian Gulf.


OPHIOLITES

Phanerozoic ophiolites are considered to be fragments of ocean floor that have been trapped between island arcs and continental plates that collided or that have been thrust onto the shelf sediments of continental margins. They consist of a downward sequence of oceanic sediments such as cherts, pillow-bearing basalts, sheeted basic dikes, gabbros, and certain ultramafic rocks (such as serpentinized harzburgite, which is primarily made of olivine and orthopyroxene; and lherzolite, which is mainly composed of olivine, clinopyroxene, and orthopyroxene). Comparable ophiolites occur in several Proterozoic orogenic belts and provide strong evidence of the existence of oceanic plates similar to those of today. The oldest is an ophiolite in the Cape Smith belt on the south side of Hudson Bay in Canada whose age has been firmly established at 1.999 billion years. There is a 1.96-billion-year-old ophiolite in the Svecofennian belt of southern Finland, but most Proterozoic ophiolites are 1 billion to 570 million years old and occur in the Pan-African belts of Saudi Arabia, Egypt, Yemen, and The Sudan, where they occur in sutures between a variety of island arcs.


GREENSTONES AND GRANITES

Greenstone-granite belts such as those of the Archean continued to form in the Proterozoic, albeit in greatly reduced amounts. They are characterized by abundant volcanic rocks that include pillowed subaqueous basalt flows and subaerial and subaqueous volcaniclastic rocks. Magnesian komatiites are for the most part absent, however. Intrusive plutons are typically made of granodiorite. Examples occur at Flin Flon in central Canada, in the Birrimian Group in West Africa, and in the Pan-African belts of the Arabian-Nubian Shield. Generally, such rocks resemble those in modern island arcs and back-arc basins, and the presence of remnants of oceanic plateau is suspected.


GRANULITES AND GNEISSES

These highly deformed and metamorphosed rocks are similar to those of the Archean Eon and occur in many Proterozoic orogenic belts such as the Grenville in Canada, the Pan-African Mozambique belt in eastern Africa and Madagascar, the Musgrave and Arunta ranges in Australia, and in Lapland in the northern Baltic Shield. They were brought up from the mid-lower crust on major thrusts as a result of continental collisions.


OROGENIC BELTS

One of the world’s classic Proterozoic orogenic belts is the Wopmay Orogen, which is situated in the Arctic in the northwestern part of the Canadian Shield. This beautifully exposed belt formed within a relatively short time (between 1.97 and 1.84 billion years ago) and provides convincing evidence of tectonic activity of a modern form in the early Proterozoic. On the eastern continental margin here are red beds (sandstones) that pass oceanward and westward into stromatolite-rich dolomites deposited on the continental shelf to a thickness of 4 km (2.5 miles); these dolomites pass into submarine turbidite fans that were deposited on the continental rise. An island arc and a continental margin are located to the west. The history of the Wopmay Orogen can be best interpreted in terms of subduction of oceanic crust and collision tectonics.

The Svecofennian Orogen of the Baltic Shield extends in a southeasterly direction from northern Sweden through southern Finland to the adjoining part of western Russia. It formed in the period from 1.9 to 1.7 billion years ago. A major lineament across southern Finland consists of the suture zone on which occur ophiolite complexes representing the remains of oceanic crust. At Outokumpu there is copper mineralization in these oceanic crust rocks similar to that in the Cretaceous ophiolite at Troodos in Cyprus. On the northern side of the suture is a shelf-type sequence of sediments; on the southern side is a volcanic-plutonic arc. To the south of this arc lies a broad zone with thrusted gneisses intruded by tin-bearing crustal-melt granites, called rapakivi granites after their coarse, zoned feldspar megacrysts (that is, crystals that are significantly larger than the surrounding fine-grained matrix). The rocks in this zone probably formed as a result of mantle plume activity.

The Grenville Orogen is a deeply eroded and highly uplifted orogenic belt that extends from Labrador in northeastern Canada to the Adirondack Mountains and southwestward under the coastal plain of the eastern United States. It developed from about 1.5 to 1 billion years ago. Apart from an island arc situated today in Ontario, most of the Grenville Orogen consists of highly metamorphosed and deformed gneisses and granulites that have been brought to the present surface on major thrusts from the mid-lower crust. A result of the terminal continental collision that occurred at about 1.1 billion years ago was the formation of the Midcontinent (or Keweenawan) rift system that extends southward for more than 2,000 km (about 1,240 miles) from Lake Superior.
A type of crustal growth—one very different from that described above—took place in what are now Saudi Arabia, Egypt, Yemen, and The Sudan in the period from 1.1 billion to 500 million years ago. This entire shield, called the Arabian-Nubian Shield, is dominated by volcanic lavas, tuffs (consolidated rocks consisting of pyroclastic fragments and ash), and granitic plutons that formed in a variety of island arcs separated by several sutures along which many ophiolite complexes occur. Some of the ophiolites contain a complete stratigraphy that is widely accepted as a section through the oceanic upper mantle and crust. The final collision of the arcs was associated with widespread thrusting and followed by the intrusion of granitic plutons containing tungsten, tin, uranium, and niobium ore deposits. The island arcs grew from the subduction of oceanic crust in a manner quite comparable to that taking place today throughout Indonesia.

The Mozambique belt is one of the many Pan-African orogenic belts that formed in the period between 1 billion and 500 million years ago. It extends along the eastern border of Africa from Ethiopia to Kenya and Tanzania. It consists largely of highly metamorphosed, mid-crustal gneisses deformed by eastward-dipping thrusts very similar to the thrusts on the southern side of the Himalayas (formed as a result of the collision of India with Tibet during the early Cenozoic Era). To the east on the island of Madagascar, mid-crustal gneisses of similar age were brought to the surface by major late extensional collapse of the orogenic belt.

During the middle and late Proterozoic, thick sequences of sediment were deposited in many basins throughout Asia. The Riphean sequence spans the period from 1.6 billion to 800 million years ago and occurs primarily in Russia. The Sinian sequence in China extends from 800 to 570 million years ago, toward the end of the Precambrian time. The sediments are terrigenous debris characterized by conglomerates, sandstone, siltstone, and shale, some of which are oxidized red beds, along with stromatolite-rich dolomite. Total thicknesses reach over 10 km (6.2 miles). The terrigenous sediments were derived from the erosion of Proterozoic orogenic belts.


GLACIAL SEDIMENTS

Evidence of the oldest known glaciation, which occurred 2.9 billion years ago, is preserved in the Pongola Rift in South Africa, though most Precambrian glaciations occurred during the Proterozoic. Evidence that ancient deposits are of glacial origin is obtained by comparing them with those left behind by the Quaternary ice sheets and with deposits associated with modern glaciers. The main sediments left behind by early Proterozoic glaciers are tillites containing rock fragments ranging in size from pebbles to boulders and distributed randomly in a fine-grained silty matrix. The surfaces of some pebbles have parallel scratches caused by having been rubbed against harder pebbles during ice transport. Locally, the basement rocks below the tillite also have been scratched, or striated, by the movement of the overlying boulder-strewn ice. Another type of glacial deposit is a varved (laminated) sediment composed of alternating millimetre-to-centimetre-thick layers of silt and clay, which closely resemble the layered varves that are laid down in modern glacial lakes at the front of retreating glaciers or ice sheets. Each of these layers defines an annual accumulation of sediment. Varved sediments may contain dropstones, which are fragments of rock that have dropped from an overlying floating ice sheet and that have sunk into and depressed the layers beneath them. When all these features are found together, they provide good evidence of ancient glaciations.

The most extensive early Proterozoic Huronian glaciation occurred 2.3 billion years ago in what is now northern North America. Glacial deposits, similar in age to those of the Huronian, are located in the Transvaal and Cape regions of South Africa, where they reach only 30 metres (100 feet) in thickness but extend over an area of 20,000 square km (7,700 square miles). Such deposits are also encountered in the Hamersley Basin of Western Australia, in east-central Finland and the adjoining part of northwestern Russia, near Lake Baikal in Siberia, and in central India, suggesting the occurrence of a wide-spread glaciation.

Evidence for the largest glaciation in Earth’s history, known as the Snowball Earth event, dates from the late Proterozoic between 1 billion and 600 million years ago. The principal occurrences of these global glacial deposits are in Europe (Scotland, Ireland, Sweden, Norway, France, the Czech Republic, and Slovakia), the Western Cordillera (Yukon, Can., to California, U.S.) of western North America and the Appalachians of the United States, eastern Greenland, Brazil, much of Africa (Congo [Brazzaville], Angola, Namibia, Zambia, Congo [Kinshasa], and South Africa), and much of Russia, China, and Australia. In addition to the Flinders Range deposits described above (see Worldwide glaciations), other notable deposits include the Port Askaig tillite on the island of Islay off northwestern Scotland, which is only 750 metres (2,460 feet) thick but records 17 ice advances and retreats and 27 periglacial periods (which are indicated by infilled polygons that formed under ice-free permafrost conditions). There are two major tillites in central Africa and Namibia (910 to 870 and 720 to 700 million years old, respectively) and two other such consolidated tills in eastern Greenland.


Correlation of Precambrian strata

The fact that Phanerozoic sediments have been so successfully subdivided and correlated is attributable to the presence of abundant fossil remains of life-forms that evolved and underwent changes over time. Precambrian sediments lack such fossils, thus preventing any comparable correlations. There are, however, stromatolites in Precambrian sediments ranging in age from about 3.5 billion to 540 million years that reached their peak of development in the Proterozoic. Stromatolites underwent evolutionary changes sufficient for Russian biostratigraphers to use to subdivide the Riphean sequence into four main zones throughout widely separated areas of former Soviet territory. Similar stromatolite-based stratigraphic divisions have been recognized in the Norwegian islands of Spitsbergen, China, and Australia. This stromatolite biostratigraphy still has relatively limited application, however. As a consequence, it is the chronometric time scale that is used to subdivide Precambrian time and to correlate rocks from region to region and from continent to continent.

The rocks within Proterozoic orogenic belts are invariably too deformed to allow correlation of units between different belts. Nonetheless, the techniques of geochronology—in particular, zircon dating—have improved considerably in recent years, with the result that rocks of approximately similar age on different continents can be mutually compared and regarded as equivalent. The isotopic dating of Archean rocks, especially with the use of zircons, has enabled similarities and differences in age to be determined, thereby aiding correlation.
Establishing Precambrian boundaries

There is no record of tectonic activity of any sort at the time corresponding to the Archean-Proterozoic boundary—about 2.5 billion years ago. This probably means that a supercontinent had been created by the amalgamation of innumerable smaller continental blocks and island arcs. Accordingly, this was a period of tectonic stability that may have been comparable to the Permian-Triassic when the supercontinent of Pangea existed. The main geologic events would have been the intrusion of basic dikes and the formation of sedimentary basins such as the Huronian on the U.S.-Canadian border, into which large volumes of clastic sediment (that is, sediment of predominantly clay, silt, and sand sizes) were deposited. Such sediments would have been derived by erosion of high plateaus and mountains that are characteristic of a large continental mass.

Brian Frederick Windley