Rapid Archean Diapirism - the Missing Link in Continent Formation?
TTG as Archean syn-orogenic granitoids
The Archean TTG (tonalite-trondhjemite-granodiorite) suite is the most common type of plutonic rock in the Archean cratonic rock record. The two end-member models for TTG formation are melting of the basaltic slab in a"hot subduction"; and intra-plate melting of basaltic rocks at the base of thick crust (oceanic plateau?). As the pressure requirements for some types of TTG magmas are substantial (> 15 kbar), this second model may require delaminated fragments of a mafic thick crust sinking into the mantle. If the thickening is created by convergence, this again comes close to a subduction process in character. Additionally, in the modern Earth, granites mostly form in collision zones, principally by biotite incongruent melting (either metasediments, or older continental crust); thus, the calc-alkaline series potassic granites are essentially a consequence of the presence of biotite in the deep metamorphic crust. In contrast, in the young Earth, were continental crust was less abundant and large sediment accumulations nearly absent, similar settings would have involved mafic rocks dominated by amphibole. Thus, even if Archean melting occurred in settings as diverse as those that typify anatexis related to modern convergent margins, most of the melts would nevertheless have been formed by incongruent melting of amphibole yielding sodic granitoids, of apparently uniform composition but with subtle differences. This complexity has been somewhat overlooked due to the focus on TTG magmas as a (potential) marker for subduction zones. However, when examined in detail a geochemical case can be made for TTG production within different geodynamic settings. This is perhaps best illustrated by the TTG rocks associated with the Barberton Granite-Greenstone Terrain (BGGT) of South Africa at ca. 3.2 Ga. A well- documented metamorphic and structural history allows for the identification of allochtonous terranes, docked along a crustal scale suture (the Inyoka-Inyoni fault system); paired metamorphic belts with contrasting geothermal gradients, separated by major structural boundaries; and, the existence of low geothermal gradients in the "lower plate"; all of which are synchronous with an important deformation phase, and the emplacement of voluminous pre- to late-tectonic TTG plutons. Within this setting TTG magmas vary considerably with regard to chemical parameters indicative of depth of source; the pre-tectonic plutons have elevated Sr, Al, Na and low Y and Yb and reflect deep melting (> 20 kbar), whereas the late-tectonic examples have geochemical characteristics (lower Sr, Al, Na and higher Y and Yb) consistent with shallower melting along hotter geotherms. Relatively low pressure Archean TTGs have been interpreted as being formed by melting of the base of the crust. However, in the light of the metamorphic and structural constrains existing for the evolution of the BGGT, the relatively elevated geotherms existing for the late tectonic TTGs are more likely to reflect orogenic to post-orogenic (post-collision collapse?) situations; they contrast with the lower geotherms that existed during the formation of the pre-tectonic TTGs, that are evocative of subduction zones. Collectively, the different TTG plutons record an orogenic cycle from subduction to collision; in the Archean as now, the main site of granitoid formation were orogenic domains, but the lack of voluminous continental crust and sediments resulted in sodic, not potassic, granitoids being formed.
Calc-Alkaline Interruptions of a Tholeiitic Sequence in the Chibougamau Area, Abitibi Belt: Stratigraphic and Geochemical Evidence Against an Arc Origin
Many think Archaean greenstones are tectonic collages of arcs and oceanic crust. Others propose in-situ growth and maturation of oceanic plateaux. In Chibougamau (NE Abitibi), the Obatogamau (tholeiitic) and Waconichi (calc-alkaline: ca 2727Ma) Fms constitute a 1st volcanic cycle; and the Bruneau (mostly tholeiitic) and Blondeau (mostly calc-alkaline) Fms a 2nd. The Lac Dore Complex (LDC, 2727Ma, mafic cumulates) intrudes beneath the Waconichi. The Bruneau-Blondeau contact is intruded by tholeiitic and calc-alkaline layered sills (2717Ma). Tholeiitic diabasic sills are common below this contact and may be feeders to basaltic lavas. The LDC is split by a tonalitic pluton (2714Ma) in an anticline core, and tonalitic cobbles occur in unconformably overlying strata. Obatogamau tholeiites have low, near-constant MgO (ca 5%), suggesting a quasi-steady-state plumbing system, with mantle inputs balanced by fractional crystallization. Higher Th in Lower Obatogamau lavas suggest crustal contamination, but Nd-isotopic data preclude involvement of old crust. Upwardly decreasing Zr/Y, TiO2 and P2O5 in Upper Obatogamau lavas suggest progressive source depletion. The Waconichi Fm. records arrival of felsic magmas. Near the LDC, these are Type III high-T soda- rhyolites (enriched, flat REE), that can be generated by low-P melting of local tholeiitic basalts. Type I-II intermediate to felsic tuffs (high L/HREE) dominate the distal (to the LDC) Waconichi; are underlain by a thin set of intermediate lavas (hybrids?); underlain in turn by a chert-exhalite that rests on Obatogamau basalts. The high L/HREE of Type I-II tuffs require garnetiferous residues and tholeiitic basalts are plausible sources. The absence of basalt in Upper Waconichi tuffs suggests an efficient crustal filter was established, preventing their eruption. Model melts from LDC cumulates resemble type I-II magmas. Since the LDC and Waconichi are coeval, the LDC may be the exhumed magma chamber in which Type I-II melts accumulated. Lowermost Bruneau basalts are MgO-rich (<12 wt%), and give way up-section to evolved basalts, but with Zr/Y ratios that define continuations of the Obatogamau trend. This suggests that the parental melt did not change significantly during the Waconichi event and that there is no major unconformity. We interpret the Waconichi as the result of the establishment of a major crustal magma chamber (LDC) that prevented eruption of basalt. The high-MgO pulse of the lower Bruneau Fm. would represent a sudden breakdown of this crustal filter, allowing unfractionated mantle melt to pass unhindered through the crust. The disappearance of high-MgO lavas and increase in TiO2 and P2O5 of the upper Bruneau Fm basalts suggest that a crustal plumbing system was re- established and that unfractionated melts were no longer able to erupt. Volcanics in the upper Bruneau and Blondeau Fms also contain evolved melts and interbedded tholeiitic and calc-alkaline volcanics. This suggests that; a) the mantle flux into the crustal plumbing system could not buffer melt compositions; b) mantle-derived melts became increasingly fractionated with time; c) that crustal inputs became more significant, although Nd isotopic data imply a near-juvenile source. Comparison with other tholeiitic/calc- alkaline packages in the Abitibi such as the Blake River Group, suggest that calc-alkaline magmatism does not represent episodically developed arc systems, but is the product of localized crustal reworking.
Evidence for Recycling of Eoarchean (3700 Ma) Continental Crust into Mesoarchean (3075 Ma) Mantle, Ivisaartoq Greenstone Belt, SW Greenland
The 3075 Ma Ivisaartoq greenstone belt, West Greenland, is composed mainly of pillow basalts, serpentinites, picrites, gabbros, and minor diorites, representing Mesoarchean supra-subduction zone oceanic crust. Many pillow basalts contain numerous millimetre- to centimetre-long polycrystalline felsic ellipsoidal inclusions, called 'ocelli', set in a dark green mafic matrix. Mineralogically ocelli are composed mainly of plagioclase, amphibole, epidote, and quartz, whereas the matrix consists of amphibole, plagioclase, and epidote. Contacts between ocelli and matrix are sharp to gradational. The ocelli-hosting pillow basalts contain rare small magmatic Eoarchean (ca. 3700 Ma) zircons. Given that there is no field evidence indicating that the Mesoarchean Ivisaartoq greenstone belt was deposited on Eoarchean continental crust, the zircon ages are interpreted as magmatic ages for recycled Eoarchean (ca. 3700 Ma) continental crust. In the least metasomatized pillow basalts and gabbros there is a strong negative correlation between the initial epsilon- Nd (+0.76 to +3.10) and depleted mantle model ages (3100-3800 Ma). Crustal contamination and metasomatic alteration can be ruled out as the cause of this co-variation. The negative correlation between the initial epsilon-Nd and depleted mantle model ages is interpreted to have resulted from variable mixing between the ca.3075 Ma depleted mantle and the ca. 3700 Ma recycled continental crust. We suggest that Eoarchean continental crust was recycled into Mesoarchean mantle either by delamination of the lower continental crust during continental rifting or by sediment subduction-erosion. Such recycling processes might have been responsible for the destruction of the early continental crust and for the generation of chemical and isotopic heterogeneities in the mantle.
Contrasting Tectonic Histories of Australia's Pilbara, Yilgarn and Gawler Cratons: Key Pieces of the Neoarchean to Early Paleoproterozoic Tectonic Puzzle
The Neoarchean record comprises 35 cratons. Most of these display Proterozoic rifted margins suggesting that they were fragments of supercratons or a supercontinent. Although it is possible to group cratons with similar tectonic histories, fundamental differences between the tectonic histories of some of the better known cratons suggest they evolved separately. Consequently a more complete history of Neoarchean tectonics may be recorded by the contemporary, but contrasting tectonic regimes preserved by individual cratons or groups of cratons. Australia's Pilbara, Yilgarn and Gawler cratons have contrasting tectonic histories that record different tectonic environments and stages of a full Neoarchean global tectonic cycle. The Pilbara Craton acted as stable continental lithosphere by 2.8 Ga and may have been part of the continent Vaalbara. The Kaapvaal Craton has a similar 2.8 to 2.6 Ga history and together these cratons provide evidence for Neoarchean continental rifting and breakup enhanced by mantle plume magmatism. The 2.59 to 2.40 Ga tectonic histories of these cratons reflect the conversion from passive margins of an internal ocean, with deposition of banded iron formations during a period of mantle plume activity, to foreland basin sedimentation culminating in collision with other cratons or terranes and continental stability. Granitoid-greenstone terranes world-wide record one of the most intense periods of generation and stabilisation of new continental crust preserved in the geological record, between 2.8 and 2.6 Ga. The eastern Yilgarn Craton, and many other Neorchean terranes show histories of magmatic arc and mantle plume magmatism and associated sedimentation culminating in orogeny, granitoid emplacement and terrane accretion, that parallel those of marginal basins to the Pacific during the Mesozoic breakup of Pangea and Cretaceous mantle plume activity. The formation of Kenorland by ~2.6 Ga is evidence that cratons started to aggregate at that time. The Gawler Craton, provides evidence for a second cycle of convergent margin tectonics and collision between 2.6 and ~2.40 Ga. The Gawler Craton contains 2.56 to 2.5 Ga ultramafic to felsic volcanic rocks (including ~2.51 Ga komatiites), metasedimentary rocks, and granitoids with compositions that are typical of Archean granitoid-greenstone terranes interpreted to have formed at convergent continental margins. These were deformed, intruded by granitoids and metamorphosed to high- grade (up to granulite facies) during the 2.48 to 2.42 Ga Sleafordian orogeny. Terranes in east Antarctica, India and China have similar histories culminating with orogeny and high-grade metamorphism after 2.5 Ga corresponding to the aggregation of Indian cratons within a larger continent. The tectonic histories of the Pilbara, Yilgarn and Gawler cratons represent the evolution of different tectonic environments and stages of a full 2.8 to 2.4 Ga global tectonic cycle involving the breakup of a pre-existing continent accompanied by growth and aggregation of cratons to form new continents and followed by continued convergence and collision of cratons and continents culminating in the possible formation of the Earth's first supercontinent by 2.4 Ga. The Earth's first widespread glaciation and oxidation of the atmosphere accompanied supercontinent formation.
Paleoproterozoic (~ 1.96 Ga) noritic magmatism in the northern margin of the North China craton: evidence of plume-continental interaction
The Paleoproterozoic (1.96 Ga) noritic magmatism in the northern margin of the North China craton presents as tens of dykes and plutons in the khondalite series (deposit at about 2.0 Ga), and hundreds of entrained dykes and bodies in the granitoid batholiths (solidified at 1.93-1.92 Ga). The dykes and plutons have a scale of up to 20 kilometers; whereas the entrained dykes and bodies vary from several kilometers to meters or even smaller. They have experienced regional high-grade metamorphism with a clockwise P-T path with the khondalite series (locally under ultrahigh-temperature conditions), and have strongly deformed both at about 1.93-1.92 Ga. The noritic intrusives belong to a tholeiitic series and can be delineated into two groups: a high- Mg group (6.20-22.85 wt% MgO and Mg numbers of 51-82) and a relatively low-Mg group (2.20-5.71 wt% MgO and Mg numbers of 35-57). The high-Mg group shows negative anomaly in Eu (Eu/Eu* = 0.53-0.72) and slight light REE enrichment (La/YbN = 0.56-1.53), and slight negative anomalies in HFSE compared with the neighboring elements in the spidergram. The Ndt (t = 1.96 Ga) values vary from -1.73 to 2.61. The low-Mg group shows varied Eu-anomalies (Eu/Eu* = 0.48-1.05) and are obviously enriched in light REEs (La/YbN = 1.51-11.98), and are negative anomalies in some HFSEs (e.g. Th, Nb) but slight negative or no anomalies in some others (e.g. Zr, Ti), compared with the neighboring elements in the spidergram. The åNdt (t = 1958 Ma) values are varied (-4.63--0.32). The granitoid batholith composes of calc-alkaline intermediate to acidic rocks, and it is characterized by S-type granites. It has low MgO contents (0.98-3.20 wt%) and Mg numbers (37-54), negative to positive Eu anomalies (Eu/Eu* = 0.14-1.23), enriched light REE, and negative anomalies of some HFSEs (e.g. Nb, Ti) but no or slight positive anomalies in others (e.g. Zr). The Ndt (t = 1.96 Ga) values vary from -5.39 to -1.15. The noritic intrusives have experienced a two-stage assimilation and fractional crystallization: assimilation and fractional crystallization of olivine and hypersthene at the first stage (the high-Mg group), and continuously assimilation and fractionation of olivine, clinopyroxene and plagioclase at the second stage (the low-Mg group). The calc-alkaline granitoid batholith is traditionally thought to be derivation of the metasediments. It has distinctly contaminated by the noritic magma. A magma mingling and partly mixing model can explain the occurrence and chemistry variations of the noritic intrusives and the granitoid batholith. The noritic magma has a mantle origination with possibly extremely high potential temperatures (> 1600°C) in the source region, which could be resulted from a 1.96-1.93 Ga Paleo-plume. And this plume could be also responsible for the extensive crustal anatexis, which have produced numerous crustal carbonatite dykes and extensive granitoid magmatism, and the regional (ultra) high-temperature metamorphism. This noritic magmatism event could have quickly followed a regional tectonic-thermal event (e.g. continental collision) during 2.0-1.95 Ga, and then have been followed by continuous uplifting during 1.92- 1.86 Ga, in the northern margin of the North China craton.
The Paleoproterozoic (1995-1950 Ma) Pre-Orogenic Supracrustal Sequences in the West Troms Gneiss Region, Arctic Norway
The West Troms gneiss region is exposed along the west coast of Troms in Arctic Norway. The region is characterized by Late Archean gneisses with varied protoliths, Late Archean greenstone belts, variably deformed and metamorphosed, high-strain supracrustal belts mainly with Paleoproterozoic deposition ages, and Svecofennian bimodal intrusions with ages around 1800 Ma. It is believed that some of the high-strain belts constitute terrane-boundaries separating Late Archean continental terranes. They were to be assembled and/or deformed during the global 1900-1750 orogenic cycle that led to the formation of Nuna, the first(?) supercontinent (Hoffman, 1997). In the Baltic Shield, these events are represented by the Svecofennian and Lapland-Kola orogenies. New geochronology from two such high-strain belts shows that both were formed prior to the Svecofennian/Lapland-Kola orogenies, in a period characterized by crustal extension and basin formation. The Mjelde belt, with its mafic magmatism, ultramafic rocks, and chemical and siliciclastic deposits has some typical characteristics of an oceanic terrane. The U-Pb-age of a gabbro in this assemblage is 1992 +/- 2 Ma. As such, it represents a rare correlative to mafic magmatism of similar age found in the Pechenga structure in Russia (Daly et al., 2006 and references therein), the Cape Smith ophiolite complex (1998 +/- 2 Ma) in the Superior Province of Canada (Parrish, 1989) and the 1992 +3/-2 Ma Scourie (Strathan) mafic dykes that represents the initial pre-orogenic rifting in the Lewisian Complex (Heaman and Tarney, 1989). The Torsnes belt, on the other hand, represents a continental extensional(?) basin influenced by mafic magmatism and with a maximum age of deposition defined by the youngest zircon at 1970 +/- 14 Ma, it correlates broadly with similar sequences in the Baltic Shield. However, the lack in Torsnes of the otherwise widespread 1950-1890 Ma zircon population indicates that the supracrustal series here may have been deposited 30-50 Myr earlier than elsewhere within the Baltic Shield. In that regard, The Loch Mare Group (Whitehouse et al., 1997) in the Lewisian complex is a better correlative. The new data from the West Troms Gneiss Region invite correlations with Paleoproterozoic orogenies elsewhere in the North Atlantic region, and also add detail to the puzzle of configuration of late Archean terranes in the region.
Paleomagnetism of Paleoproterozoic Cross-Cutting Dike Swarms from Dharwar Craton, India
We report new paleomagnetic and geochronological data from our ongoing study of mafic Proterozoic dikes intruding the Dharwar craton in Karnataka and Andhra Pradesh states in South India. Based on the observed field relationships, analyses of satellite images and airborne maps, we have identified at least three different cross-cutting Precambrian dike swarms (trending E-W, NE-SW and NW-SE). The observed cross-cutting relationships suggest that the NE-SW trending dikes were emplaced first, followed by the E-W dikes, and finally by the NW-SE dikes. Of these, the E-W trending quartz dolerites have been reliably dated at ∼2.37 Ga by the U-Pb method (eg., Halls et al., 2007) and the NW-SE trending dolerites have U-Pb ages at ∼2.18. Ga (French et al., 2004). Additional precise age determinations for the swarms may provide an independent test to the implied dike emplacement sequence. Paleomagnetic directions were measured by detailed alternating field (AF) and thermal demagnetization experiments. Most samples revealed two or three natural remanent magnetization components. For majority of the dikes, the characteristic remanent magnetization (ChRM) was isolated within a narrow temperature range above 550oC, or in the high-coercivity part of AF demagnetization spectra. The mean ChRM directions were found to be different for each of the studied swarms. This observation, coupled with baked contact test results for each swarm allows us to refine the apparent polar wander path for Dharwar craton during the Proterozoic. We will discuss implications of our results for studying the Proterozoic tectonic history of the Dharwar craton as well as for understanding the morphology and secular variation of the geomagnetic field in the Proterozoic. We will also report preliminary paleointensity determinations from the dike swarms. Different degree of feldspar clouding is manifested in mafic dikes of Dharwar craton (Halls et al., 2007). The clouding zonations, combined with paleomagnetic data, may provide important insight for deciphering the regional tectonic record of the Dharwar craton. Based on our petrological observations, we will present a novel model to explain the cloudiness in feldspars.