Early Precambrian Crust Research Articles

THE AGE OF ZIRCON FROM CRUSTAL GRANITOIDS OF THE VOVCHA AND HULIAIPOLE BLOCKS: THE HISTORY OF THE EARTH CRUST FORMATION OF THE AZOV DOMAIN OF THE UKRAINIAN SHIELD

The studied area comprises the junction zone of the Huliaipole granite-greenstone block with the Vovcha and Remivka granulitegneiss blocks. The Vovcha and Remivka blocks are composed of Archean quartz-feldspathic gneisses (>80%), also containing remnants of high-metamorphic rocks of the West-Azov series. The Huliaipole Block is composed of rocks of the West-Azov series and a Mesoarchaean granite-greenstone complex (3,2–3,0 Ga), which includes the Kosivtseve greenstone structure and TTG rocks of the Shevchenko complex. Granitoids are widespread in the area. Archean TTG rocks of the Shevchenko Complex were formed by partial melting of a mafic protolith. The later granitoids of the southern part of the Vovcha block and Dobropillya Complex are still poorly studied. According to petrological data, granitoids of the Dobropillya Complex were formed due to high degree (>50%) melting of the older crust, which consisted of granitoids and metamorphosed ultramafic and mafic rocks. In the Dobropillya granitoids, inherited zircon significantly prevails and, therefore, it is hard to determine the time of crystallization of these rocks. Seventynine LA-ICP-MS determinations of U-Pb isotope ages were performed on zircon from granite-porphyry of the Vovcha block. The age of this rock has been estimated by the modal 207Pb/206Pb age of the main zircon population as 2840±10 Ma. A small amount of older (up to 3624 Ma) grains has also been detected. Seventy-six LA-ICP-MS U-Pb age determinations were performed on zircon from granodiorite of the second intrusive phase of the Dobropillya massif. In this rock, four main zircon populations can be distinguished. The two oldest peaks (3905 and 3435 Ma) correspond to zircons inherited from an ancient protolith. The main zircon population has a weighted average 207Pb/206Pb age of 2847±20 Ma. Thus, the main zircon populations in the crustal granitoids of both the Vovcha and Huliaipole blocks have an age of 2.8–2.9 Ga. Granitoids of this age have not yet been found in this area and, therefore, there is no clear interpretation of their source. The Archean crust, from which the crustal granitoids of the Vovcha and Huliaipole blocks might have been melted, also included Eoarchean (3.9 and 3.8 Ga), Paleoarchean (3.6–3.2 Ga), and Mesoarchean (3.15–2.95 Ga) rocks. A small group of younger (2350–2135 Ma) zircons reflects the influence of Paleoproterozoic processes, either partial loss of radiogenic lead or the crystallization of a new zircon population. The obtained geochronological data indicates that the Early Precambrian crust of the west-Azov block was formed from 3.9 to 2.1 Ga.

Geochemistry and Isotope Composition of Paleoproterozoic Granites and Felsic Volcanics of the Elash Graben: Evidence of the Heterogeneity of the Early Precambrian Crust

Abstract —The paper presents results of a petrogeochemical and isotope–geochronological study of the granite–leucogranite association of the Pavlov massif and felsic volcanics from the Elash graben (Biryusa block, southwest of the Siberian craton). A characteristic feature of the granite–leucogranites is their spatial and temporal association with vein aplites and pegmatites of the East Sayan rare-metal province. The U–Pb age of zircon from granites of the Pavlov massif (1852 ± 5 Ma) is close to the age of the pegmatites of the Vishnyakovskoe rare-metal deposit (1838 ± 3 Ma). The predominant biotite porphyritic granites and leucogranites of the Pavlov massif show variable alkali ratios (K2O/Na2O = 1.1–2.3) and ferroan (Fe*) index and a peraluminous composition; they are comparable with S-granites. The studied rhyolites of the Tagul River (SiO2 = 71–76%) show a low ferroan index, a high K2O/Na2O ratio (1.6–4.0), low (La/Yb)n values (4.3–10.5), and a clear Eu minimum (Eu/Eu* = 0.3–0.5); they are similar to highly fractionated I-granites. All coeval late Paleoproterozoic (1.88–1.85 Ga) granites and felsic volcanics of the Elash graben have distinct differences in composition, especially in the ferroan index and HREE contents, owing to variations in the source composition and melting conditions during their formation at postcollisions extension. The wide range of the isotope parameters of granites and felsic volcanic rocks (εNd from +2.0 to –3.7) and zircons (εHf from +3.0 to +0.8, granites of the Toporok massif) indicates the heterogeneity of the crustal basement of the Elash graben, which formed both in the Archean and in the Paleoproterozoic.

Tectonic framework for metallogeny of precambrian structures on the example Kolyma and Omolon terranes (North-East of the Asia)

On example Omolon and Kolyma terranes (North-East of the Asia) showed a influence of the regional tectonic position and geodynamic evolution of Precambrian blocks on the structure of the ore deposits. The first of that is stable consolidated structure, and formation of basement was full completed to early of Neoproterozoic. The second example is composite terrane, similar to fold-trusted belt. Terranes with consolidated Early Precambrian crust was a stable structure for the all of geological history. Further ages of the compression and tension not to make any significance changes of that. Formed by this processes ore-controlled and ore-hosted structures correspond a deep and steep faults of the basement. By this pathways, subsequent tectonic and magmatic events permeated far into the terranes. That was favourable for forming large and hi-grade deposits but restrict the range of deposit types. Contrariwise, tectonic structure of the trust-faulted terranes was mobile for the whole timing scale. There Precambrian rocks compose of allokhtones, and trust-fault structure was multiple renewed. That was promote of forming wide range types of mineralization, but caused of dissipation of metals on numerous small-scale structures. Consequently, age and level of consolidation of the basement is a one of the most important factors of metallogeny of the Precambrian terranes. That defined the style of faulting structure (domination of steep or shallow faults), depth of the faults, morphology and geometry of the ore bodies.

The structure of the Western and Central parts of the Ukrainian schield. Controversial issues

Some controversial issues of composing the Scheme of the fault megablock tectonics of the western half of the Ukrainian shield in connection with the decision of the Interdepartmental Tectonic Committee of Ukraine on drawing up the Tectonic map of the Ukrainian shield at a scale of 1:500 000 have been considered. Geophysical data must be more fully taken into account and used in detail and objectively reflecting both the surface and the deep structure of the Earth’s crust of the shield while drawing up such a Scheme. The latest maps of anomalous magnetic and gravitational fields, a map of the topography of the Moho surface and other materials are given, the analysis of which gives grounds for a number of conclusions on specific structures of the western and central parts of the region. It has been established that the formation of a typical megablock structure of the Ukrainian shield began not earlier than in the Neoarchean. It is difficult unambiguously to establish and trace the investigated Archean fault zones in the part of the shield at the present time, and sometimes practically it is almost impossible, since the Early Proterozoic processes of regional granitization and dynamometamorphism radically changed the composition and structure of the earth’s crust. Geophysical data, supported by geological and tectonophysical observations, clearly record the Proterozoic fault zones, which began to form after 2.5 billion years ago. They cross and deform both the Archean and the Early Proterozoic granitoid complexes, accompanied by intensive diffusion, metasomatism and ore formation. It is proved that the Archean-Early Proterozoic structural-formation complexes (SFC), mapped within the shield, cannot and should not be artificially linked with the Proterozoic fault-megablock structure, which was formed at the end of the Early Proterozoic and superimposed on the SFC. It seems like the dubious and outdated thesis about the subhorizontal-floor structure of the Early Precambrian crust of the Ukrainian shield, according to which the structural floors of the crust are formed one above another by stratometamorphic complexes characterized by a directly proportional relationship of age and the degree of regional metamorphism of the rocks. This concept does not take into account the significant role of horizontal motions of lithospheric blocks, which cause the redistribution and localization of PT conditions in the earth’s crust.

Origin of the Early Sial Crust and U–Pb Isotope–Geochemical Heterogeneity of the Earth’s Mantle

It is shown that presence of the Early Precambrian sial crust in the Indo–Atlantic segment of the Earth and its absence in the Pacific has been caused by geochemical differences in the mantle underlying these segments. These differences were examined on the basis of Nd–Hf and U–Pb isotopes in modern basalts. The U–Pb isotope system is of particular interest, since uranium is a member of a group of heat-generating radioactive elements providing heat for plumes. It is shown that in the Indo–Atlantic segment, a distribution of areas of the modern HIMU type mantle is typical, while it is almost completely absent in the Pacific segment. In the Archean, in the upper HIMU type paleo-mantle areas, plume generation and formation of the primordial basic crust occurred; this was followed by its remelting resulting in the appearance of an early sial crust forming cratons of the Indo–Atlantic segment.

The growth, reworking and metamorphism of early Precambrian crust in the Jiaobei terrane, the North China Craton: Constraints from U–Th–Pb and Lu–Hf isotopic systematics, and REE concentrations of zircon from Archean granitoid gneisses

Archean granitoid (tonalite–trondhjemite–granodiorite, TTG) gneisses are widely distributed in the early Precambrian metamorphic basement of the Jiaobei terrane, the North China Craton (NCC), and record the formation and evolution of the continental crust in the early Precambrian. In this study, zircon cathodoluminescence (CL) images have assisted in obtaining U–Th–Pb and Lu–Hf isotopic systematics, and rare earth element (REE) concentrations from distinct zircon domains of complex zircons, and the results have been integrated in order to unravel the growth, reworking and metamorphism of the early Precambrian crust in the Jiaobei terrane. Zircons from seven representative Archean granitoid samples, collected from the Jiaobei terrane, record three groups of magmatic ages of ∼2.9, ∼2.7 and ∼2.5Ga, and two groups of metamorphic ages of ∼2.5 and ∼1.86Ga. The ∼2.5Ga metamorphic event is thought to be linked to underplating of large amounts of mantle-derived magma, and the ∼1.86Ga metamorphic event to arc (or continent)–continent collision. Magmatic zircons have positive ɛHf(t) values (+0.7 to +12.58, average +5.5), and two-stage Hf model ages cluster at ∼2.7–2.8 and ∼2.9–3.0Ga. This indicates that the Archean granitoid rocks were derived from partial melting of juvenile crust that was formed mainly during ∼2.7–2.8 and ∼2.9–3.0Ga which represent two periods of growth of juvenile crust. The most significant period of crustal growth is ∼2.7–2.8Ga, whereas ∼2.5Ga represents a period of reworking of the ∼2.7–2.8Ga juvenile crust in the Jiaobei terrane, as well as other areas in the NCC.

3D model of deep structure of the Early Precambrian crust in the East European Craton and paleogeodynamic implications

The integral 3D model of the deep structure of the Early Precambrian crust in the East European Craton is based on interpretation of the 1-EU, 4B, and TATSEIS seismic CDP profiles in Russia and the adjacent territory of Finland (FIRE project). The geological interpretation of seismic images of the crust is carried out in combination with consideration of geological and geophysical data on the structure of the Fennoscandian Shield and the basement of the East European platform. The model displays tectonically delaminated crust with a predominance of low-angle boundaries between the main tectonic units and the complex structure of the crust-mantle interface, allowing correlation of the deep structure of the Archean Kola, Karelian, and Kursk granite-greenstone terrane with the Volgo-Uralia granulite-gneiss terrane, as well as the Paleoproterozoic intracontinental collision orogens (the Lapland-Mid-Russia-South Baltia orogen and the East Voronezh and Ryazan-Saratov orogens) with the Svecofennian accretionary orogen. The lower crustal “layer” at the base of the Paleoproterozoic orogens and Archean cratons was formed in the Early Paleoproterozoic as a result of underplating and intraplating by mantle-plume mafic magmas and granulite-facies metamorphism. The increase in the thickness of this “layer” was related to hummocking of the lower crustal sheets along with reverse and thrust faulting in the upper crust. The middle crust was distinguished by lower rigidity and affected by ductile deformation. The crust of the Svecofennian Orogen is composed of tectonic sheets plunging to the northeast and consisting of island-arc, backarc, and other types of rocks. These sheets are traced in seismic sections to the crust-mantle interface.

The Belomorian eclogite province: Unique evidence of Meso-Neoarchaean subduction and collision

The Belomorian eclogite province (BEP) recently revealed in the eastern part of the Fennoscandian shield is a unique Archaean object. The age of the crust eclogites known in the world outside the BEP does not exceed two billion years [1], which corresponds to the middle of the Paleoproterozoic. Eclogites with an age not less than 2.72 Ga [2] were found for the first time within the Belomorian province. The studies of the BEP open principally new opportunities, first, for the reconstruction of the geodynamic processes in the his� tory of the Early Precambrian crust of the region, and second, for a more correct concept about the geody� namics of the Early Precambrian as a whole, because the lack of reliable findings of Archaean eclogites is one of the arguments against the reality of subduction and application of plate tectonics to the modeling of the Archaean geodynamics.

Formation stages of the Early Precambrian crust in the Sharyzhalgai basement uplift, southwestern Siberian craton: Synthesis of Sm-Nd and U-Pb data

Newly obtained U-Pb and Sm-Nd isotopic data on Early Precambrian metamorphic and granitoid complexes in the southwestern margin of the Siberian craton (Sharyzhalgai basement uplift) are synthe-sized in order to elucidate the crustal evolution starting at the Paleoarchean (∼3.6 Ga) to Late Paleoproterozoic (∼1.85 Ga), evaluate the lateral extent of the Paleo-Mesoarchean crust, and identify major stages in its growth and recycling. Two crustal growth stages were determined in the Onot and Bulun granite-greenstone terranes: at 3.6–3.3 and 2.8–2.9 Ga. The earliest recycling processes (at ∼3.4 and 3.2 Ga) involved partial melting, metamorphism, and migmatization and produced a stable continental crust. Crustal growth in the Mesoarchean (∼2.8–2.9 Ga) due to basaltoid magmatism was associated with the recycling of the Paleoarchean crust, which served as a source of felsic melts and of detrital material for terrigenous sediments. The Archean crust of the Irkut granulite-gneiss terrane was formed by two pulses of intermediate-felsic and basic volcanism at ∼3.6-3.4 and ∼2.7 Ga. In the terminal Archean (at ∼2.55 Ga), the preexisting crust was involved in metamorphic and magmatic processes. Traces of recycling of the Paleoproterozoic crust are identified in the isotopic parameters of the intermediate-felsic granulites. Two discrete stages in the influx of juvenile material are identified in the Paleoarchean: at ∼2.0 and 1.88–1.85 Ga, with the latter stage associated with the large-scale recycling of the Archean crust during the origin of granitoids.

Asteroid impact connections of crustal evolution*

Advances in isotopic age determinations increasingly point to the episodic nature of the evolution of lithosphere and crust, and in this paper well-documented field and geochemical evidence is to be reviewed, which indicates that at least some of these episodes were associated with large asteroid impacts. Observed overlaps between early Precambrian impacts [∼3.47, ∼2.63, ∼2.56, ∼2.48, ∼2.023 Ga (Vredefort) and 1.85 Ga (Sudbury)] and isotopic age frequency peaks, defined within age errors, while not constituting proof of cause–effect relationships, invite tests of field evidence relevant to this question. Evidence for major dynamic and thermal effects of large impact clusters on the early Precambrian crust is provided by ejecta fallout units associated with: (i) unconformities; (ii) tsunami boulder debris; (iii) compositional contrasts between supracrustal sequences that underlie and overlie the ejecta units; (iv) onset of episodes of iron-rich sedimentation; and (v) near-contemporaneous intrusion of granitoid magmas. An impact cluster at ∼3.26–3.24 Ga documented in the Barberton greenstone belt, South Africa, is associated with unconformities and granite activity correlated with unconformities and olistostromes in the Pilbara Craton, Western Australia. In both cratons a 300 Ma period of greenstone–granite evolution is terminated abruptly by unconformities overlain by impact ejecta, turbidite and banded iron-formation. The 3.26 and 3.24 Ga terminations involve major faulting, uplift, erosion, and the onset of high-energy sedimentation which includes detrital components from contemporaneous granites. The onset of ferruginous sedimentation, including banded iron-formation, in the wake of the ∼3.47, ∼3.26, ∼3.24, ∼2.63 and ∼2.56 Ga impacts suggests weathering and soluble transport of ferrous oxide under low-oxidation atmosphere and hydrosphere conditions, possibly reflecting extensive mafic volcanic activity triggered by the impacts. Extensive dyke formation during 2.48–2.42 Ga (Matachewan, Scourie, Karelian, Widgiemooltha, Bangalore, Antarctica dykes) may be related to deep crust/mantle fractures triggered by the ∼2.48 Dales Gorge mega-impact. Tentative observations are consistent with, but do not demonstrate, possible overlaps between Phanerozoic impacts and the onset of faulting and plate tectonics episodes.

Collisional sutures in the early precambrian crust as a factor responsible for localization of diamondiferous kimberlites in the northern East European Platform

Collisional sutures in the early precambrian crust as a factor responsible for localization of diamondiferous kimberlites in the northern East European Platform

Metaterrigenous rocks of the Irkut granulite-gneiss Block as indicators of the evolution of the Early Precambrian crust

Major, trace element, and Sm-Nd isotope data are presented for the garnet-biotite and cordierite-garnet-biotite gneisses from the Early Precambrian granulite complex of the Irkut Block (Sharyzhalgai Uplift, Siberian Craton). The garnet-biotite and cordierite-bearing gneisses of the Irkut Block were formed owing to the granulite metamorphism of metaterrigenous rocks. The chemical index of weathering and the content of clayey (pelitic) components in the normative mineral composition increase from the garnet-biotite gneisses to the cordierite-bearing gneisses, thus reflecting the maturation degree of initial sediments. Protoliths of the studied paragneisses correspond to a rock series ranging from the graywacke siltstones to clayey rocks. The trace and rare-earth element distribution indicates that the terrigenous material of the paragneisses was derived from felsic and mafic provenance. Increase in contents of Fe, Ti, Cr, Ni, and Sc and the Cr/Th ratio and decrease in the La/Sc ratio from the garnet-biotite to the cordierite-bearing gneisses reflect growth of the abundance of mafic rocks in the provenance. Potential sources of the detrital material were intermediate-felsic and mafic volcanic rocks (orthogneisses and basic crystalline schists) of the Irkut Block. The paragneisses show a distinct negative Eu anomaly (Eu/Eu* = 0.38–0.85), which suggests the input of crustal melting products, such as the potassium granites. A wide range of model Nd age (TNd(DM) = 2.4–3.1 Ga) of the paragneisses indicates the Archean to Early Paleoproterozoic age of their protoliths. The complex of isotopic, geochemical, and geochronological data, as well as the character of association of metaterrigenous rocks (mature pelites and carbonate rocks included), implies that sedimentation was separated in time from volcanism. The sedimentation was preceded by metamorphism, granite formation, and tectonic stabilization of the Irkut Block crust.

A three-dimensional model of the Early Precambrian crust under the southeastern Fennoscandian Shield: Karelia craton and Belomorian tectonic province

The Early Precambrian structures of the Archaean Karelia craton and Belomorian tectonic province have been investigated by reflection seismology. Integration of the existing geological database for these two entities and adjacent tectonic units with the seismic data for the crust and upper mantle along the 1-EU geotraverse and cross-traverse 4B totaling c.1300 km allows development of a three-dimensional model of the deep structure of the southeastern Fennoscandian Shield. Interpretation of the seismic data provides evidence that the Early Precambrian crust is characterized by inclined layering. The crustal architecture is a combination of thrust–nappe and thrust–underthrust structural assemblages consisting of Neoarchaean and Palaeoproterozoic rocks that were subsequently further deformed during uplift and emplacement of granite-gneiss domes. The new model substantially modifies previous conceptions of this part of the shield as being constituted of blocks with sub-vertical margins and individual internal layering. It is apparent that the surface traces of gently dipping boundaries are defined by the erosion level and consequently these boundaries are not those of particular “tectonic blocks” in the traditional sense.

Evolution of the Snow Lake portion of the Palaeoproterozoic Flin Flon and Kisseynew belts, Trans-Hudson Orogen, Manitoba, Canada

Understanding growth of early Precambrian crust requires knowledge of the evolution of granite-greenstone and metasedimentary belts, two of the fundamental building blocks of early Precambrian cratons. UPb geochronologic data from the Snow Lake area, which straddles the Flin Flon granite-greenstone and Kisseynew metasedimentary belts of the Trans-Hudson Orogen, documents a 60–70 Ma period of Palaeoproterozoic crust formation from inception of these belts in ocean basins through their eventual structural telescoping during ocean basin consumption and collision with bounding Archaean cratons. Earliest volcanic and plutonic rocks of the Flin Flon granite-greenstone terrane at Snow Lake formed ∼1.89 Ga, mainly in a juvenile oceanic arc tectonic setting. Sedimentary rocks of the Kisseynew belt, previously thought to be contemporaneous with Snow Lake area arc volcanism, are up to 30–50 Ma younger (∼1.86-1.84 Ga) and postdate deformation associated with ∼1.88-1.87 Ga arc-arc collisions and ∼1.87-1.84 Ga calc-alkaline plutonism that affected much of the western Flin Flon belt. Deformation at Snow Lake occurred during two widely spaced episodes, both of which effect the post 1.86-1.84 Ga turbidite sequence and, thus, postdate 1.88-1.87 Ga intraoceanic accretion in the western Flin Flon Belt. The older deformational episode involved isoclinal folding formed during ∼1.84 Ga convergence of the Kisseynew and Flin Flon belts and thrusting of Kisseynew sedimentary lithologies southwards onto the Flin Flon volcanic rocks. The younger deformational episode occurred 15–25 Ma later and involved further folding and southwest thrusting, accompanied by a ∼1.81 Ga high grade thermal metamorphic episode, centred in the Kisseynew Belt. Only minor increase of crustal material, in the form of late intrusions, were added following this episode.

Depth extent of cratons as inferred from tomographic studies

We address the question of the depth extent of mantle high-velocity zones under ancient cratons by using seismic velocity maps from recent mantle tomographic studies. We divided old continents into two age provinces: 800–1700 Ma (Middle Proterozoic) and older than 1700 Ma (Archean and Early Proterozoic). The areas included in these age provinces are cross-correlated with the tomography at different depths to quantify the global occurrence of high-velocity anomaly (HVA) extensions beneath cratons. Statistically significant HVAs underlie the oldest cratons to a depth of 250 km. There is no significant consistent correlation between tomography and younger cratons. The older cratons have correlated HVAs extending to depths varying from <100 to 450 km. There is no significant difference in the average velocity anomaly beneath the Early Proterozoic part and the Archean part of the Canadian Shield, whereas the Middle Proterozoic part does not contain any HVA. The mantle under Early Precambrian crust is distinctly different from mantle elsewhere. We propose two different hypotheses to explain these results: (1) cratons of Early Proterozoic age or older contain ancient and deep, physically attached roots, or (2) these cratons contain a small “permanent” physical root that induces cold downwellings beneath it. A root thickness of ∼200 km can be explained by an FeO-poor, olivine-rich mineralogy, whereas a requirement of high strength probably demands, in addition, a volatile-poor composition. Different root depths for cratons of similar age may be related to their location in the various supercontinents in which they were involved, and to their subsequent deformational history, or they may represent different snapshots in time of transient “roots,” such as thermal boundary layers.

Metamorphic evolution of the Early Precambrian crust of the northeastern Baltic Shield

(1990). Metamorphic evolution of the Early Precambrian crust of the northeastern Baltic Shield. Geologiska Foreningen i Stockholm Forhandlingar: Vol. 112, No. 3, pp. 295-296.

Vertical zonation and petrogenesis of the early Precambrian crust in Western Australia

Variations in metamorphic grade, structural style, isotopic ages and granite geochemistry observed within the Yilgarn craton, and between the Yilgarn and Pilbara cratons, Western Australia, are interpreted in terms of vertical zonation of the Archaean crust. We correlate the gneiss-granulite suite of the Wheat Belt (southwestern Yilgarn) with concealed coeval infracrustal roots of the low-grade granite—greenstone Kalgoorlie terrain (eastern Yilgarn). Differences between the Pilbara, Southern Cross and Laverton granite—greenstone blocks and the downfaulted linear greenstone belts of the Kalgoorlie block are interpreted in terms of deeper-level exposure in the first three blocks. Ultramafic—mafic volcanic sequences in the Yilgarn craton can be divided into at least two major groups — the lower greenstones, regarded as relicts of a once extensive simatic crust, and the significantly younger upper greenstones, believed to have formed within linear troughs following the intrusion of Na-rich granites. At least three major Archaean granite phases occur in Western Australia: (1) 3.1-2.9 b.y. old (recognized to date only in the western Yilgarn and in the Pilbara craton); (2) 2.8-2.7 b.y. old, and (3) 2.6 b.y. old (the two latter phases can only be separated from each other in the eastern Yilgarn, and phase (3) is also identified in the Pilbara). In the main, granites of phases (1) and (2) are Na-rich and those of phase (3) are K-rich. There is evidence for a secular increase in Rb levels and initial 87Sr/ 86Sr ratios. It is suggested that the K-rich granites grade down into Na-rich granites, and the former were generated by ensialic anatexis resulting in upward migration of K, Rb, U, and Th-enriched magmas. A review of data from several Archaean cratons in other continents suggests that evidence from these regions can be interpreted in terms of the general model of crustal evolution proposed for Western Australia. Implications of this model concerning petrogenesis of Archaean plutonic and volcanic suites, geothermal gradients and tectonic evolution of greenstone belts are discussed. Partial melting associated with mantle diapirism is thought to have given rise to the ultramafic—mafic volcanic cycles. Widespread subsidence and partial melting of this crust yielded Na-rich acid magmas. The development of the upper greenstones was confined to linear belts in a partly cratonized crustal environment. About 2.6 b.y. ago a rise in the geothermal gradient resulted in regional metamorphism and crusctal anatexis which gave rise to the K-rich granites.

Geochemistry and origin of the early Precambrian crust of northeastern Minnesota

Igneous rocks of the Early Precambrian greenstone-granite complex of northeastern Minnesota consist of four major groups: 1. (1) metavolcanic tholeiitic basalt and basaltic andesite; 2. (2) metavolcanic trondhjemite gneiss, intrusive tonalite, extrusive and hypabyssal dacite and rhyodacite; 3. (3) intrusive quartz monzonite and; 4. (4) post-kinematic intrusive syenodiorite and syenite. Major and rare earth elements, Rb, Sr and Ba were precisely determined for rocks of each group. Quantitative trace-element modeling is combined with isotopic and experimentalpetrologio evidence to delimit the origin of the individual rock types: 1. (1) basalt by shallow melting of mantle peridotite followed by fractional crystallization to produce basaltic andesite; 2. (2) trondhjemite, tonalite and dacite by partial melting of eclogite at mantle depths; 3. (3) quartz monzonite by partial melting of short-lived graywacke at crustal depths; and 4. (4) syenodiorite and syenite by partial melting of a mixed peridotite-eclogite source at mantle depths. Initial Sr 87 Sr 88 ratios of previously determined isochrons for all rock types are less than 0.701, and preclude magma origin by remelting of older granitic crust. The tectonothermal event 2700–2750 m.y. ago in northeastern Minnesota and adjacent Ontario represents addition of new erustal material from the mantle.

The deformation patterns of two Archaean greenstone belts in Rhodesia and Botswana

Abstract Variations in intensity of deformation have been examined in the Fort Victoria greenstone belt in southern Rhodesia and in the Tati belt in northeast Botswana, both of which are on the diffuse northern border of the Limpopo mobile belt. Much of the deformation is related to widespread Limpopo events, and not to diapiric granite intrusions previously considered of major importance in the development of the Early Precambrian crust. Variations in intensity of deformation are due to the form of the major structures and to localised major ductile shear zones which cut both granites and greenstone belts. The pattern of deformation within the belts depends partly on how the granites behaved. Whether the schist belts deformed homogeneously or were part rotated and cut by shear zones depended on whether the granites deformed homogenously with the schist belt matrix or whether they were deformed by block sliding.

Bimodal tholeiitic—dacitic magmatism and the Early Precambrian crust

Interlayered plagioclase-quartz gneisses and amphibolites from 2.7 to more than 3.6 b.y. old form much of the basement underlying Precambrian greenstone belts of the world; they are especially well-developed and preserved in the Transvaal and Rhodesian cratons. We postulate that these basement rocks are largely a metamorphosed, volcanic, bimodal suite of tholeiite and high-silica low-potash dacite—compositionally similar to the 1.8-b.y.-old Twilight Gneiss — and partly intrusive equivalents injected into the lower parts of such volcanic piles. We speculate that magmatism in the Early Precambrian involved higher heat flow and more hydrous conditions than in the Phanerozoic. Specifically, we suggest that the early degassing of the Earth produced a basaltic crust and pyrolitic upper mantle that contained much amphibole, serpentine, and other hydrous minerals. Dehydration of the lower parts of a downgoing slab of such hydrous crust and upper mantle would release sufficient water to prohibit formation of andesitic liquid in the upper part of the slab. Instead, a dacitic liquid and a residuum of amphibole and other silica-poor phases would form, according to Green and Ringwood's experimental results. Higher temperatures farther down the slab would cause total melting of basalt and generation of the tholeiitic member of the suite. This type of magma generation and volcanism persisted until the early hydrous lithosphere was consumed. An implication of this hypothesis is that about half the present volume of the oceans formed before about 2.6 b.y. ago.