Lade Inhalt...

Precambrian Tectonics. Tectonics throughout the Hadean and Archean Eon

Seminararbeit 2016 49 Seiten

Geowissenschaften / Geographie - Allgemeines, Grundlagen

Leseprobe

TABLE OF CONTENTS

ABSTRACT

INTRODUCTION
FIRST CRATONS
PRECAMBRIAN HEAT FLOW

HADEAN EON 4,5 - 3,9 GA
EARTH’S OLDEST ROCKS STUDIES
TTGs Studies
Hadean detrital Zircon 9
HADEAN TECTONIC MODELS
TTGs
Zircon:

ARCHEAN EON 3,9 - 2,5 GA
GENERAL CHARACTERISTICS OF CRATONIC MANTLE LITHOSPHERE
GENERAL GEOLOGY OF ARCHEAN CRATONS
Greenstones
High-grade gneiss terrains
FORMATION OF THE ARCHEAN MANTLE LITHOSPHERE
FORMATION OF THE ARCHEAN CRUST
Dome and Keel Tectonics

ARCHEAN - PROTEROZOIC BOUNDARY

PROTEROZOIC EON 2,5 - 0,6 GA
GENERAL GEOLOGY OF PROTEROZOIC CRATONS
PROTEROZOIC PLATE TECTONICS

SUPERCONTINENTAL CYCLE

LATE ARCHEAN - EARLY PROTEROZOIC SUPERCONTINENTS
LATE PROTEROZOIC SUPERCONTINENTS

SUMMARY

BIBLIOGRAPHY

REFERENCES CITED

ABSTRACT

This assignment exhibits the period since Earth has formed until boundary conditions and tectonics started to stabilize. This period is called Precambrian and consists the 7/8 of Earth’s history. Precambrian lasts 3,9 Ga and is separated into three Eons, Hadean, Archean and Proterozoic with respect to time occurrence.

Earth loses heat through time due to gradually decrease in decay of radiogenic isotopes which are dispersed into the core, mantle and continental crust. Therefore, Earth composition and tectonics distinctly change. Tectonics generally change from vertical patterns to horizontal and the continental crust forms until the end of the Precambrian to the 82% of the overall crust, covering Earth now. Moreover, different movements of Supercontinents which fragmented, dispersed and collided where studied through geological, paleomagnetic and paleoclimatic studies.

INTRODUCTION

First Cratons

On the basis of isotopic data study, Condie (2005b) concluded that 39% of the continental crust formed during the Archean Eon, 31% during the Early Proterozoic and 12% during the Middle-Late Proterozoic. This means that 82% of the overall Earth’s continental crust was formed during Precambrian Period. This is prospective, considering that this period lasts 3,9 billion years, about 7/8 of total Earth’s age (Fig. 1). The formation of the first cratonic nucleii marks the transition from the Early Earth’s, Magma Ocean, geology when conditions were much hotter and energetic to a state where crustal preservation until present became possible. Main characteristic is a high velocity mantle root, stiff, chemically buoyant and highly resistant which extends to depths at least 200km (King, 2005). The chemical consistence of this mantle root contributed to the long-term survival of the Archean continental lithosphere (Carlson et. Al., 2005).

Fig. 1 Summarizing

illustration not visible in this excerpt

The age of the oldest rocks found was originally assumed to have been 4 billion years old: the Acasta gneisses of the Slave Craton in Northwestern Canada (Bowring & Williams, 1999). However, >4,4 detrital Zircon minerals found in the Yilgarn Craton of Western Australia (Wilde et. Al., 2001) and the Greenstone of Nuvvuagittuq Fold Belt which is located in the Minto Block, part of the NE Superior Province, Canada (David et al, 2009 GSA Bulletin) suggest that some continental crust may have formed as early as 4,4-4,5 Ga but still this interpretation is controversial and not yet determined. At the Archean - Proterozoic Boundary there is a gradual infer in tectonic patron observed which is caused by secular Earth cooling due to reduction of radiogenic isotopes. Central debating issues of Precambrian are how tectonics were before this period and if they were likely to the present, uniformitarian models and non-uniformitarian. Towards this determination considering the nature of Precambrian tectonic processes, three approaches have been adopted (Kröner, 1981; Cawood et al., 2006). First, a strictly uniformitarian model which supports that the same exact mechanisms of plate tectonics applied during Phanerozoic Period were also applying during Precambrian Period. This model best fits to interpret Proterozoic Orogenic Belts. Moreover it has been partly applied to Archean Cratons and also to some Hadean belts based on latest studies (T. Rushmer et. Al., 2013; 2014). Second, a moderate uniformitarian model was postulated, in which plate tectonic processes in the Precambrian were somewhat different from present justified by the physical conditions affecting the crust and mantle changes throughout geologic time. This model has been used in studies of both Archean and Early Proterozoic geology. Third model supports no similar tectonics to modern. This latter, non-uniformitarian approach most often is applied to the Hadean and Early - Middle Archean.

Precambrian Heat Flow

Heat Flow is the triggering factor for all major changes over Earth. The majority of terrestrial heat production comes from the decay of radioactive isotopes dispersed throughout the core, mantle, and continental crust which are not renewable. Consistent destruction and reduction of radiogenic isotopes over Earth lead to lower energetic states implying heat flow in the past must have been considerably greater. For an Earth model with a K/U ratio derived from measurements of crustal rocks, the heat flow in the crust at 4,0 Ga would have been three times greater than at the present day and at 2,5 Ga about two times the present value (Mareschal & Jaupart, 2006).

illustration not visible in this excerpt

Fig. 2 Variation of surface heat flow with time. Solid line, based on a chondritic model; dashed line, based on a K/U ratio derived from crustal rocks (redrawn from McKenzie & Weiss, 1975, with permission from Blackwell Publishing). Kearey et. Al..2008. Global Tectonics. Ch. 11, p.348

If the heat loss mostly occurred by the relatively inefficient mechanism of conduction then the lithosphere would have been warmer in present time. However, if the main mechanism of heat loss was convection beneath oceanic lithosphere, which is very effective at dissipating heat, then the continental lithosphere need not have been much hotter during Precambrian (Lenardic, 1998). Clarifying these aspects of the Archean thermal regime is essential in order to reconstruct tectonic processes in the ancient Earth and to assess whether they were different than they are today. Geologic evidence from many of the cratons, including an abundance of high temperature/low pressure metamorphic mineral assemblages and the intrusion of large volumes of granitoids, suggest relatively high (500-700 or 800°C) temperatures in the crust during Archean times, roughly similar to those which occur presently in regions of elevated geotherms. By contrast, geophysical surveys and isotopic studies of mantle nodules suggest that the cratonic mantle is strong and cool and that the geotherm has been relatively low since the Archean (Pearson et al., 2002; Carlson et. al., 2005). But it is determined to have been much thicker than current lithosphere through heat determining studies. According to Sleep (2003) model, a chemically buoyant layer of lithosphere forms a highly resistant lid above the convecting mantle, allowing it to maintain nearly constant thickness over time. These considerations illustrate how the formation and long-term survival of the cool mantle roots beneath the cratons has helped geoscientists constrain the mechanisms of heat transfer during Precambrian times.

Hargraves (1986) concluded that heat loss through the oceanic lithosphere is proportional to the cube root of the total length of the mid-ocean ridge. Assuming a nonexpanding Earth, the increased rate of plate production implies a similar increase in plate subduction rate. These computations suggest that some form of plate tectonics was taking place during the Precambrian at a much greater rate than today, main Archean characteristic. The fast rates suggest an image of the solid surface of the early Earth where the lithosphere was broken up into many small plates that contrasts with the relatively few large plates that exist presently.

Hadean Eon 4,5 - 3,9 Ga

Beginning with the oldest Earth’s history, Hadean Eon is the period Earth suffered consistent meteorite bombardments. Heat flow was a lot greater than modern times and was assumed to have been covered by magma oceans. However, there are no sufficient evidence to determine actual current conditions. Based on latest studies (T. Rushmer et. Al., 2013; 2014) there are serious implications of some kind of limited horizontal tectonics. Moreover other scientists don’t even consider the existence of Hadean Eon as a separate period different from Archean period and place Archean lower boundary to the age of 4,5 Ga. Interpretations for the Hadean Eon are keenly controversial.

Earth’s oldest rocks studies

TTGs Studies

The Earth’s oldest continental crust is composed primarily of granitic rocks known as tonalite-trondhjemite-granodiorite (TTG) (Jahn et al., 1981; Rudnick, 1995; Condie, 2005). These granites are classically associated with belts of metamorphosed volcanic rocks and sediments known as greenstones. Although it is generally believed that TTG magmas were produced by partial melting of a basaltic crust that predated them (Drummond and Defant, 1990; Martin, 1994; Rapp et al., 2003; Condie, 2005), the tectonic environment in which this occurred is not known. Furthermore, whether there is a direct genetic relationship between TTGs and their immediate greenstone host rocks is also problematic (Martin, Precambrian Tectonics 1987, 1994). These uncertainties have broader implications for studies of early Earth processes, including the question of when plate tectonics first became active.

T. Rushmer et. Al. (2013) have approached these issues by conducting partial melting experiments on two greenstones from one of the oldest known greenstone belts, the Nuvvuagittuq complex of northern Quebec (O’Neil et al., 2008, 2011). The Nuvvuagittuq greenstones include two stratigraphically and compositionally distinct groups (see O’Neil et al., 2011): these are (1) an incompatible element- depleted tholeiitic group, and (2) an overlying calc-alkaline group that is moderately enriched in incompatibles. Their objective was to test for a direct genetic link between the second overlying calc-alkaline group of Nuvvuagittuq greenstones and a closely associated but younger tonalite of the TTG series. TTGs are experimentally defined to have generated at 3,8 Ga by melting of 3,8 - 4,3 Ga mafic, arc-like, material as shown by expanded 142 Nd isotope data set and Sm/Nd systematics (O’Neil et al., 2008). At pressures of 0.5 to 2.0 GPa orthopyroxene ± olivine are the only near-liquidus phases, consistent with results for Tertiary boninites (Fig. 3). Under subliquidus conditions, at moderate to high degrees of partial-melting (34-47 %), melt coexists with clinopyroxene ± orthopyroxene ± plagioclase ± garnet. The melts are compositionally similar to TTGs, including the 3.66 Ga tonalite that encloses the Nuvvuagittuq greenstone belt (T. Rushmer et. Al., 2013).

illustration not visible in this excerpt

Fig. 3 Incompatible element concentrations in Nuvvuagittuq greenstones PC-227 and PC-162, and average Cenozoic boninite (using data from Cameron et al., 1983). Data are normalized to primitive mantle (Sun and McDonough, 1989) concentrations. T. Rushmer et. al., 2013. Hadean greenstones from the Nuvvuagittuq fold belt and the origin of the Earth’s early continental crust Thus in contrast to many previous proposals (e.g., Drummond and Defant, 1990; Foley et al., 2002), the distinctive compositional characteristics of TTGs (including low Nb, Ta, and Ti and high Rb, K, and Ba) may be the result of chemical fractionation that preceded TTG magma formation (T. Rushmer et. Al., 2013). This possibility is consistent with a number of other observations. One is that not all basalts can produce TTG-like melts. In fact, of the many experiments conducted on basaltic starting materials, very few have produced melts that genuinely resemble TTGs (T. Rushmer et. Al, 2013). It has also been shown that the concentrations of most incompatible elements in Archean TTGs are too high for them to have been produced by the partial melting of the K-poor tholeiites that predominate in most greenstone terrains, but instead require crustal sources that were already enriched in these components (Smithies et al., 2009). The fact of enriched Hadean crust predating the earliest known continental crust has been independently verified by studies of Pb, Nd, and Hf isotopes in Paleoarchean rocks and zircons (Andreasen and Sharma, 2009; Blichert-Toft and Albarede, 2008; Tessalina et al., 2010). Taken together, these observations demonstrate that many of the compositional characteristics of the early continental crust must have been inherited from previous cycles of mantle depletion and subsequent reenrichment (T. Rushmer et. Al., 2013).

However, there is sufficient evidence to suggest that, as in the modern Earth, the crust of the Hadean Earth was organized into a number of different tectonic environments and that one of these gave rise to the first continental crust. This is consistent with some form of horizontal tectonics and difficult to reconcile with models based purely on plume-driven tectonics (T. Rushmer et. Al., 2013). An alternative explanation based on isotope and geochronologic data is that the Nuvvuagittuq lavas were generated ca. 3.8 Ga, and that the variations in142 Nd isotopic compositions were inherited from primordial differentiation of the Earth’s mantle (Roth et al., 2013; Guitreau et al., 2013).

Hadean detrital Zircon

Numerous studies have utilized isotopic systems in both whole rocks and detrital minerals to investigate the geodynamics of the early Earth. For example, several groups have used the compositions of Hadean detrital zircons from the Jack Hills in Australia to argue that the crust of the early Earth was not so different from today (Mojzsis et al., 2001; Wilde et al., 2001; Harrison, 2009), whereas others have suggested that the early crust was more basaltic (e.g., Kemp et al., 2010). Some data sets have been further used to infer that subduction did not commence until 3 Ga (e.g., Dhuime et al., 2012) or even much later (Stern, 2005). Unfortunately, old rocks are rare and detrital minerals lack important geological context. T. Rushmer et. Al. (2015?) suggest that this geochemical stratigraphy might provide a more robust test of ancient tectonic settings than individual chemical or isotopic signatures in rocks or detrital minerals.

On a global basis, there are some long noted and intriguing similarities between (non-komatiitic) Archean mafic rocks and those found at present- day subduction zones. For fluid-immobile elements, these include elevated concentrations of Th and Zr relative to Nb, Ti, and Yb (Fig. 4), effectively the negative Nb and Ti anomalies that are characteristic of arc lavas and continental crust in general. There are also differences between the geochemical patterns of modern and Archean mafic rocks. For example, whereas modern mafic rocks show a clearly bimodal distribution between arc and non-arc settings, the Archean mafic rocks spread across these two fields (Fig. 4) (Moyen, 2013). Thus, geochemists have been appropriately prudent in assigning

illustration not visible in this excerpt

Fig. 4 Trace element discrimination diagrams showing comparison of individual geochemical signatures to tectonic settings for Archean rocks (Pearce, 2008).

modern arc basalts (red cloud) and mid-oceanic ridge basalts (MORB, blue cloud), non-komatiitic Archean basalts from GEOROC (green diamonds; Geochemistry of Rocks of the Oceans and Continents; http://georoc.mpchmainz.gwdg.de/georoc/), and mafic rocks from proto Izu-Bonin-Mariana arc (black circles) and Nuvvuagittuq (Quebec, Canada) supracrustal belt (white squares). Samples with SiO2 > 55 wt% are shown in gray. A: Th/Yb versus Nb/Yb diagram from Pearce (2008). B: Ti (ppm) versus Zr diagram from Pearce and Cann (1973). Field A is island arc tholeiite (IAT); field B is MORB; field C is calc-alkaline basalt (CAB); f eld D is ambiguous(intersection of previous three fields). T. Rushmer et. Al., 2015?. Heading down early on? Start of subduction on Earth.

The initiation of modern-day subduction has long been thought to be marked by eruption of distinctive lavas, called boninites, which are characterized by depletion in relatively compatible elements such as Ti. However, recent submersible exploration in the Izu-Bonin-Mariana forearc (Reagan et al., 2010; Ishizuka et al., 2011) found that the lowermost rocks are actually similar to mid- oceanic ridge basalts (MORBs). These are overlain successively by boninites associated with hydrothermal Fe-oxide and sulfide deposits (Ishizuka et al., 2008) and then by rocks of arc tholeiitic and calc-alkaline affinity that are typical of modern subduction systems (Fig. 5). The key outcome is that subduction initiation can now be identified by a geochemical stratigraphy (Stern et al., 2012), rather than the occurrence of one single lava type (e.g., boninites) or a selected trace element signature (such as negative Nb anomalies).

illustration not visible in this excerpt

Fig. 5 Despite possible 4,4 Ga difference in age, stratigraphic succession and trace element composition of old Earth rocks (O’Neil et al., 2011, 2012) are strikingly similar to those formed during inception of Izu-Bonin-Mariana (IBM) arc today (Reagan et al., 2010). Geochemical stratigraphy consists of four successive units that reflect transition from initial fracturing of an oceanic plate through commencement of fluid release, melting, and finally mature subduction. Trace element patterns are individual analyses: Nuvvuagittuq (Quebec, Canada) samples are PC132 (high Ti), PC432 (low Ti), and PC149 (low Ti enriched) from O’Neil et al. (2011); IBM samples are 974-R9 g (fore-arc basalt), 974-R4 (boninite), and GU9 (calc-alkaline) from Reagan et al. (2010) and Woodhead (1989). BIF—banded iron formation; LILE—large ion lithophile elements; HFSE—high field strength elements; REE—rare earth elements. Rushmer et. Al., 2015?. Heading down early on? Start of subduction on Earth.

The high-Ti tholeiites have flat Rare Earth Element (REE) and High Field Strength Element (HFSE) patterns that typify basalts generated from a relatively undepleted mantle. In contrast, the overlying low-Ti mafic rocks have high MgO and SiO2, are highly depleted in REEs and HFSEs, and have concave-upward REE patterns that are typical of boninites generated from previously depleted mantle. In both cases, they could represent hydrothermal activity associated with early extension, and perhaps the first introduction of subducted fluids (those associated with generation of the low-Ti lavas) (T. Rushmer et. Al., 2015?).

Some differences, such as the greater abundance of mafic sills and ultramafic rocks in the Nuvvuagittuq stratigraphy (Fig. 5), could indicate that there were local differences in magma generation during rifting and subduction.

Hadean tectonic models

There have been several geotectonical models associated either with the TTGs formation or the detrital zircon isotope according to above mentioned studies.

TTGs

Vertical Tectonic approach:

Mantle melting of mafic material derived from the lower mantle as shown by recent synthesis of TTG host rock and zircon Hf isotopes, Then evolving at shallower levels, or by direct partial melting, as suggested by experimental melting studies.

Process remains similar through time.

illustration not visible in this excerpt

Fig. 6 TTGs formation derived from mantle plume source. Vertical tectonic Model. (Guitreau et. Al., 2012) Rushmer et. Al., 2014. Onset of subduction and the genesis of TTGs (tonalities-trondhjemites- granodiorites) in the in the earliest Earth. (Powerpoint presentation)

Horizontal Tectonic approach:

The initiation of Hadean Eon subduction procedures was introduced by several scientists (Albarde, 1998, Boher et al., 1992; Condie 2005; Kerrich & Polat, 2006; Bedard, 2006; Polat, 2012; Rushmer et. Al., 2013).

illustration not visible in this excerpt

Fig. 7 TTGs formation during oceanic lithospheric subduction. Horizontal tectonic Model. Rushmer et. Al., 2014. Onset of subduction and the genesis of TTGs (tonalities-trondhjemites-granodiorites) in the in the earliest Earth. (Powerpoint presentation)

Main point of this approach is the role of oceanic plateaus and plumes and plate interaction (subduction zone geodynamics as today) during Early Earth geology. The subducted plate is dehydrating and partially melting (Fig. 7). The maficultramafic composition melts ascent until they reach the upper mantle- lower crust boundary where the magma is unterplating. After partial melting of this material TTGs are formed and intrude the crustal formations.

Zircon:

This model focuses on Zircon detrital isotope initiation and is mainly based on isotopic data determination from Jack Hills zircon study by Kemp et al., (2010). Earth composition is conducted by nascent hydrosphere, steam atmosphere and magma oceans (Fig. 8). Upper magma layer is altered and hydrated KREEPy rind with δ18 O 5-7%o underlined by thin KREEPy basaltic crust (dry δ18 O 5,3%o). KREEP is material having very high concentrations of incompatible elements; its name is the acronym for the incompatibles K, Rare-Earth-Elements (REE) and P. Hadean depleted mantle or magma-ocean cumulate begins ascending into upper KREEPy layers and erupts above them. The Zircon of Jack Hills is connected with the solidification and crystallization of this magma ocean (KREEPy layer) and a prolonged period of partial melting.

illustration not visible in this excerpt

Fig. 8 Hadean Zircon initiation: From Jack Hills zircon study (Kemp et al., 2010), solidification and crystallization of a magma ocean (KREEPy layer) and a prolonged period of partial melting.Rushmer et. Al., 2014. Onset of subduction and the genesis of TTGs (tonalities-trondhjemites-granodiorites) in the in the earliest Earth. (Powerpoint presentation)

Concluding for Hadean Eon:

Definite Hadean (primoridal) magma differentiation is observed. Non horizontal tectonics until Archean (Kerrich & Polat, 2006; Shirey et al.2008; Polat, 2012) is supported. Maybe some type of plates and some very limited horizontal tectonic movements’ implications (Turner et. Al., 2015?) (Fig. 9).

illustration not visible in this excerpt

Fig. 9 Hadean Earth’s image reconstruction. Tracy Rushmer. 2014. Onset of subduction and the genesis of TTGs (tonalities-trondhjemites-granodiorites) in the in the earliest Earth. (Powerpoint presentation)

[...]

Details

Seiten
49
Jahr
2016
ISBN (eBook)
9783668665439
ISBN (Buch)
9783668665446
Dateigröße
2.8 MB
Sprache
Englisch
Katalognummer
v417008
Institution / Hochschule
Ruhr-Universität Bochum – Institute of MIneralogy
Note
95%
Schlagworte
Plate Tectonics Structural geology Precambrian tectonics Archean Eon Hadean Eon Magmatismus Earth Science Earth evolution Proterozoik Eon

Autor

Teilen

Zurück

Titel: Precambrian Tectonics. Tectonics throughout the Hadean and Archean Eon