September 26, 2020
A fiery ball of molten lava – the beginning of our planet, four and a half billion years ago. There was no atmosphere, no oceans, and no rock, as we know it; the conditions were too hot for the molten material to form solids. But, slowly, things cooled, and five hundred million years later, areas of the earth’s crust had solidified. Amazingly, remnants of these first rocks have remained intact through the ages, and are now being studied by geologists looking for hints of that nascent world. The oldest rocks have been found in Australia, scattered crystals that date back 4.28 billion years. But located in the Canadian Arctic are rocks over 4 billion years old that form a solid mass, the heart of one of the first continents on Earth, perhaps the first1.
Just north of Yellowknife, in the Northwest Territories, Canadian geology sleuths Wouter Bleeker and Richard Stern have been examining the composition of these primeval rocks to piece together the first 1.5 billion years of the ancient continent’s history. The rocks stretch continuously for hundreds of kilometers – they are not scattered remnants, but a solid chunk of a continent (Fig. 1)!
The piece of ancient crust is made of gneiss, a metamorphic rock that forms under high temperatures and pressure. The top of this layer is uneven, an indication that it has been exposed to the elements for some time; it has been weathered.
The central and often dominant feature of most continents is their vast Precambrian Shield area; examples include the Canadian Shield, Brazilian Shield, African Shield, and Australian Shield. In these rocks, dating reveals ages of 1 billion to 4.28 billion years, and they have been little affected by tectonic events postdating the Cambrian. But these shield areas are themselves complex. They consist of vast areas of granitic or granodioritic gneisses. Inside them, between them, and overlapping onto them are belts of sedimentary rocks quite like those in modern sedimentary belts of the Pacific margin or the European Alps. These rocks are frequently metamorphosed in the green schist, amphibolite, and granulite facies. Low-temperature facies and, in particular, low-temperature–high-pressure facies are missing—or have not yet been found. From marginal areas of these stable shield areas, a complex array of processes has been documented covering the past few hundred million years. The Caledonian Orogeny (at the close of the Silurian Period) produced tectonic-metamorphic events along the east coast of North America, Greenland, the British Isles, Fennoscandia (the region made up of Scandinavia, Finland, and northwestern Russia), Central Asia, and Australia. The Hercynian, or Variscan, orogeny followed about 300 million years ago, affecting subparallel regions and the Urals and European Alps. In fact, the shield margins appear to have been subjected to a more or less constant battering by forces both destroying and rebuilding the margins of these proto-continents. As geologists study Precambrian areas in detail, the number of metamorphic and orogenic events recognized on a global scale increases.
Some of the layers on top of the gneiss are much younger–about 2.8 billion years old. This gap in time likely represents a period of uplift, when forces from the mantle below forced the continent upwards, exposing its surface to the elements. After millennia of erosion, the continent then sank below an ocean and sedimentary layers – sandstone, and other, iron-rich rocks – began to accumulate. The 2.8 billion year old layers are volcanic, indicating that the wafer of rock was ripped apart, and lava from below flowed over the crust’s surface.
Along the eastern flank of this archaic landmass, the layers of gneiss, sandstone, and volcanic material are all missing. This is a sure indication that the 4 billion year old continent was once bigger–that the Canadian chunk is only a portion of a larger continent, perhaps even a supercontinent. Other pieces of this continent may still exist! In places as far afield as Wyoming and Zimbabwe, rocks dating to the same age and with the same layering pattern have been found. The history of this archaic landmass is obviously a convoluted one, but it just might be the information gleaned from the relic located in Canada’s arctic that illuminates it.
Metamorphic reactions can be classified into two types that show different degrees of sensitivity to temperature and pressure changes: net-transfer reactions and exchange reactions. Net-transfer reactions involve the breakdown of preexisting mineral Phases and corresponding nucleation and growth of new phases. (Nucleation is the process in which a crystal begins to grow from one or more points, or nuclei.) They can be either solid-solid reactions (mineral A + mineral B = mineral C + mineral D) or de-volatilization reactions (hydrous mineral A = anhydrous mineral B + water), but in either case they require significant breaking of bonds and reorganization of material in the rock. They may depend most strongly on either temperature or pressure changes. In general, devolatilization reactions are temperature-sensitive, reflecting the large increase in entropy (disorder) that accompanies release of structurally bound hydroxyl groups (OH−) from minerals to produce molecular water. Net-transfer reactions that involve a significant change in density of the participating mineral phases are typically more sensitive to changes in pressure than in temperature. An example is the transformation of albite (NaAlSi3O8) to the sodic pyroxene jadeite (NaAlSi2O6) plus quartz (SiO2). Albite and quartz have similar densities, of about 2.6 grams per cubic cm (1.5 ounces per cubic inch), whereas jadeite has a density of 3.3 grams per cubic cm (1.9 ounces per cubic inch). The increased density reflects the closer packing of atoms in the jadeite structure. Not surprisingly, the denser phase jadeite is produced during subduction zone (high-pressure) metamorphism. Net-transfer reactions always involve a change in mineral assemblage, and textural evidence of the reaction often remains in the sample; isograd reactions are invariably net-transfer reactions.
In contrast to net-transfer reactions, exchange reactions involve redistribution of atoms between preexisting phases rather than nucleation and growth of new phases. The reactions result simply in compositional changes of minerals already present in the rock and do not modify the mineral assemblage. For example, the reaction involving almandine garnet (Fe3Al2Si3O12) and magnesium biotite (KMg3AlSi3O10(OH)2) that yields pyrope garnet (Mg3Al2Si3O12) and iron biotite (KFe3AlSi3O10(OH)2) results in redistribution of iron and magnesium between garnet and biotite but creates no new phases. This reaction is limited by the rates at which iron and magnesium can diffuse through the garnet and biotite structures. Because diffusion processes are strongly controlled by temperature but are nearly unaffected by pressure, exchange reactions are typically sensitive to changes only in metamorphic temperature. Exchange reactions leave no textural record in the sample and can be determined only by detailed microanalysis of the constituent mineral phases. The compositions of minerals as controlled by exchange reactions can provide a useful record of the temperature history of a metamorphic sample.
The types of reactions cited here are typical of all metamorphic changes. Gases are lost (hydrous minerals lose water, carbonates lose carbon dioxide), and mineral phases undergo polymorphic or other structural changes; low-volume, dense mineral species are formed by high pressures, and less dense phases are favored by high temperatures. Considering the immense chemical and mineralogical complexity of Earth’s Crust, it is clear that the number of possible reactions is vast. In any given complex column of crustal materials, some chemical reaction is likely for almost any incremental change in pressure and temperature. This is a fact of immense importance in unraveling the history and mechanics of Earth, for such changes constitute a vital record and are the primary reason for the study of metamorphic rocks.
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