In the latest Quest tagged: , , , , ,

In the latest Quest

Posted by in Earth science, Environment, Featured, Life, Science in Society

The Barberton Granites


What was happening in the very earliest part of the Earth’s history? How important was plate convergence and subduction in the formation of the Earth’s crust? Geologists from Stellenbosch University have made some important discoveries.

Plate tectonics is recognised as the central geological process of the modern Earth. However, if, and exactly how, plate tectonics happened during the oldest period of Earth’s history for which a solid rock record remains – the Archaean era (4.0-2.5 billion years ago) – is under dispute.

This map shows the position of global convergent plate margins (55 000 km long) as jagged lines

Subduction

The process of subduction is important in Earth’s history because where plates move apart, new oceanic crust is created and where plates converge, continental crust can be created and recycled, while oceanic crust is consumed. Along these convergent plate margins oceanic lithosphere (the crust plus the top brittle portion of the upper mantle) is subducted to become part of the mantle again. Buoyant island arcs that are part of the subducted plate become accreted to buoyant continental crust and ultimately, as the ocean basin closes, continental crustal blocks collide to build mountain belts.

Accretion is a physical process by which solid rock material is added to a tectonic plate or a landmass.  Accretion is a physical process by which solid rock material is added to a tectonic plate or a landmass.  The down-going slab – called the subducting plate – is overidden by the leading edge of the other plate. The slab sinks at an angle of 25-45º to the surface of the Earth. Once the slab reaches a depth of between 80-120 km, the basalt of the oceanic slab is converted to a very dense metamorphic rock, eclogite. As a result, the density of the oceanic lithosphere increases and it is carried into the mantle by virtue of its higher density (slab pull) and by downwelling convective currents within the mantle, as well as a push effect resulting from the fact than the mid-ocean ridges, where oceanic crust is created by plate divergence, stand higher than the surrounding oceanic crust (ridge push). Subduction zones are the regions where the Earth’s lithosphere, oceanic crust, sedimentary layers and some trapped water are recycled into the deep mantle. A geothermal gradient is the rate of increasing temperature in relation to the depth in the Earth’s interior.

These diagrams show how subduction creates island arcs and ocean trenches at continental boundaries.

These diagrams show how subduction creates island arcs and ocean trenches at continental boundaries.

 

Subduction in geological time

Without subduction, plate tectonics could not exist. But what was happening at the very earliest period of the Earth’s history? This period of Earth’s history is puzzling because evidence exists in the rock record which appears to demand a plate tectonic explanation, yet there is a complete lack of some types of the evidence that is typical of modern subduction zones. The South African geological record is rich in exceptionally well-preserved Archaean rocks and these help shed light on this problem. In the Barberton greenstone belt there is evidence that blocks of volcanic and sedimentary rock strata of different ages have been brought together by the lateral motion of plates. In essence, this evidence consists of upper crustal rocks, formed in different places and times, which were brought together along a prominent fault approximately 3.2 billion years ago. Importantly though, two key types of subduction zone evidence from the modern Earth may not exist in the Archaean rock record. The first such evidence is the presence of blueschists, which are metamorphic rocks formed under the very high pressures and low temperatures that exist only in subduction zones. Blueschists are unknown from the Archaean.

Eclogite sample with garnet (red) and omphacite (greyish-green) as prominent crystals in the rock. The sky-blue crystals are kyanite. Some white quartz is seen too – it was probably once coesite. A few gold-white phengite mica minerals can be seen at the top. Coin of 1 euro (23 mm) for scale. Image: Wikimedia commons

However, eclogites, which form under pressuretemperature conditions defining similar geothermal gradients to those along which blueschists form, certainly formed during the Archaean. We know this because eclogites which formed more than 3.0 billion years ago are unearthed by kimberlite magmas, which erupt through the lithospheric keels that underlie the old continents. The second piece of evidence relates to opheolites, which are sections of the Earth’s oceanic crust and underlying upper mantle that have been lifted up and exposed above sea level as a result of thrusting of the opheolite onto the continental crust during island arc accretion or during collisions between continents. Some rock successions from Barberton and elsewhere have been proposed as representing opheolites, but these are different in rock structure and chemistry to typical opheolites and their origin remains controversial. As a result, the existence of subduction during the Archaean era is questioned. Kimberlites are a type of volcanic rock, best known for containing diamonds and other gem stones.

The Barberton terrain

The Barberton terrain is part of an area called the Kaapvaal craton.

The present-day Kaapvaal craton. Image: Wikimedia commons

The Kaapvaal craton, as well as the Pilbara craton in Western Australia, parts of Canada and west Greenland, represent the few remaining areas of 3.6-3.2 billion-year-old crust on Earth. In the past few years South African geologists, Jean-François Moyen, Gary Stevens and Alex Kisters, have found rocks that record pressures of 1.2-1.5 GPa at temperatures of 600-650 ºC among amphibolites found in the crust of the mid-Archaean Barberton granitoidgreenstone terrain. These high-pressure amphibolites contain garnet and epidote in addition to the hornblende that normally characterises amphibolites. Amphibolite is a metamorphic rock that is made up mainly of hornblende amphibole. They are typical of a particular set of pressure and temperature conditions – called the amphibolite facies. The conditions under which these rocks formed suggest geothermal gradients of 12-15 ºC, which are similar to those found in recent subduction zones. In particular, they are similar to the conditions that exist in subduction zones where young oceanic crust is subducted. In the Barberton greenstone belt, the formation of these highpressure amphibolites coincided with the accretion of blocks with different age and stratigraphy (terranes) in the overlying Barberton greenstone belt. Terrane in geology is short for tectonostratigraphic terrane, which is a fragment of crustal material, formed on, or broken off from, one tectonic plate and accreted or ‘sutured’ to crust lying on another plate. This crustal fragment retains its own distinctive geological history, which is different from that of the surrounding areas. It is these high-pressure and lowtemperature conditions that provide the metamorphic evidence for a cold and strong lithosphere, as well as
subduction-driven tectonic processes during the evolution of the early Earth. The lack of blueschists in the Archean rock record reflects the fact that subducted oceanic crust was hotter, as a consequence of a generally higher geothermal gradient, and possibly also because it was young. Within the Barberton greenstone belt we have crustal rocks that contain evidence that appears to demand that a lateral tectonic process operated on Earth earlier than 3 230 million years. This evidence suggests that both the continental crust and the eclogitic domains in lithospheric keels beneath the continents were produced as a result of subduction zones, which created both the magmas that built the bouyant continental crust and the metamorphic conditions under which the eclogites were stabilised. The resultant old continental crust, underlain by thick lithosphere, has proved to be incredibly long lived, surviving 3.0 billion years of Earth’s turbulent tectonic processes.
Gary Stevens is the SARChI professor in Experimental Petrology, Alex Kisters is Associate Professor of Structural Geology (Stellenbosch University) and J-F Moyen is Associate Professor of Petrology at St Éttiene Université in France. At the time of conducting this research all were based at the Centre for Crustal Petrology at Stellenbosch University.