Granite-greenstone Terrain Research Articles

Structural testing of tectonic hypotheses by field-based analysis of distributed tangential shear: examples from major high-strain zones in the Grenville Province and other parts of the southern Canadian Shield

The Grenville Province and other parts of the Canadian Shield contain major (>100 km long) high-strain zones, also called shear belts or ductile shear zones, that are hosted by heterogeneously deformed gneisses and schists. In well-exposed segments of three nontabular zones whose dip angle is known locally, at the erosion level and (or) in the shallow subsurface, we investigate the tangential shear strain (better called the tangential unit shear or TUS) without assuming that mineral-shape lineations, common varieties of stretching lineations, are effectively parallel to the local TUS direction. Employing a graphic technique that copes with the geometric conditions of general triaxial strain, we approximate the actual direction and find the sense of local TUS in parts of (i) the Parry Sound shear zone, Grenville Province; (ii) the South Range shear zone, Southern Province; and (iii) the Uchi – English River subprovince boundary zone, Superior Province. Information thus obtained for individual high-strain zones in Ontario confirms the validity of published hypotheses: (i) 1020–970 Ma, normal-sense distributed shearing in the Grenvillian thrust stack; (ii) northwest- directed thrusting of Huronian rocks over Archean basement; and (iii) north-directed thrusting of English River metasediments and associated migmatites onto the Uchi granite–greenstone terrain, under peak metamorphic conditions.

GEOLOGIA E GEOCRONOLOGIA DOS GRANITÓIDES ARQUEANOS DA REGIÃO DE XINGUARA-PA E SUAS IMPLICAÇÕES NA EVOLUÇÃO DO TERRENO GRANITO-GREENSTONE DE RIO MARIA, CRÁTON AMAZÔNICO

The Xinguara area, located in the Rio Maria Granite-Greenstone Terrain - Pará, is composed of several Archaean granitoids. Structures and fabric elements visible at outcrop and microscopic scales, and zircon Pb-Pb ages suggest that these granitoids were emplaced in two distinct episodes of growth of the Archaean crust (i) 2.948 ± 5 to 2.924 ± 2 Ma, genesis, emplacement and deformation of Caracol tonalitic complex, related to Arco Verde tonalite, wich outcrops in Marajoara area, and to TTG rocks of the São Félix do Xingu region. This period was marked by the formation of thick greenstone sequences and TTG plutons, forming dome-and-keel structures, related to a vertical tectonics, like those reported in the east Pilbara (Autralia) and Dharwar (India) cratons; (ii) 2.878 ± 4 to 2.865 ± 1 Ma, genesis, emplacement and deformation of the Rio Maria granodiorite, Água Fria trodhjemite and Xinguara granite. In this period, some 50 Ma after formation of the last TTG, distinct magmatic series were formed in a short time span (10- 15 Ma). In this stage, plate convergence and subduction processes were operative.

The Archaean sanukitoid series of the Baltic Shield: geological setting, geochemical characteristics and implications for their origin

Archaean high-Mg granitoids (sanukitoids) occur in the Karelian granite–greenstone terrain in the Baltic Shield in two distinct zones. In the west of the Shield sanukitoid intrusions formed between 2700 and 2720 Ma and consist of a single igneous phase that varies in composition from diorite to granite. In the Eastern part of the Shield, sanukitoid intrusions formed between 2730 and 2745 Ma and are strongly differentiated, varying in composition from ultramafic to felsic. All the sanukitoids are enriched in light rare earth elements (LREE: La≤80 ppm, Ce≤150 ppm, La N /Yb N ≤30–40), Sr≤2000 ppm, Ba≤2500 ppm, P 2O 5≤1.5%, alkalis (Na 2O+K 2O=5–10%), possess high mg# values (0.50–0.65), and show a negative Nb–Ta anomaly. They are spatially and temporally related to syenite intrusions and lamprophyre dykes. Sanukitoid intrusions in the Western and Eastern zones differ in composition. In the west, they have higher SiO 2 (mainly>60%) and lower alkalis, Sr, Ba, LREE than in the Eastern zone intrusions. The most differentiated intrusion, the Panozero intrusion in the Eastern zone, was formed in two magmatic cycles separated by ductile deformation. In the first cycle, ultramafic to monzonitic rocks formed, whereas, in the second cycle, the magmas were monzodioritic to quartz monzonite. Ultramafic and mafic rocks make up about 10% of the outcrop and occur as enclaves in monzonites and monzodiorites, and, as dykes, implies a number of discrete magmatic events. All rocks of the Panozero intrusion have high K 2O, and the composition of the initial melt, calculated from the weighted average of the first cycle magmas is monzodiorite (SiO 2=52%, mg#=0.55, Na 2O+K 2O∼6%). The presence of magmatic hornblende and biotite, a high carbonate content, widely distributed explosive breccias and evidence of liquid immiscibility are consistent with a high H 2O–CO 2 content in the sanukitoid melt. The geodynamic model which most satisfactorily explains our geological and geochemical observations is the formation of the Karelian sanukitoids in a cratonic environment adjacent to the Belomorian–Karelian suture.

On the formation of continental silicic melts in thermochemical mantle convection models: implications for early Earth

Important constituents of Archean cratons, formed in the early and hot history of the Earth, are Tonalite–Trondhjemite–Granodiorite (TTG) plutons and greenstone belts. The formation of these granite–greenstone terrains is often ascribed to plate-tectonic processes. Buoyancy considerations, however, do not allow plate tectonics to take place in a significantly hotter Earth. We therefore propose an alternative mechanism for the coeval and proximate production of TTG plutons and greenstone-like crustal successions. That is, when a locally anomalously thick basaltic crust has been produced by continued addition of extrusive or intrusive basalts due to partial melting of the underlying convecting mantle, the transition of a sufficient amount of basalt in the lower crust to eclogite may trigger a resurfacing event, in which a complete crustal section of over 1000 km long sinks into the mantle in less than 2 million years. Pressure release partial melting in the complementary upwelling mantle produces large volumes of basaltic material replacing the original crust. Partial melting at the base of this newly produced crust may generate felsic melts which are added as intrusives and/or extrusives to the generally mafic crustal succession, adding to what resembles a greenstone belt. Partial melting of metabasalt in the sinking crustal section produces a significant volume of TTG melt which is added to the crust directly above the location of ‘subduction’, presumably in the form of a pluton. This scenario is self-consistently produced by numerical thermochemical mantle convection models, presented in this paper, including partial melting of mantle peridotite and crustal (meta)basalt. The metamorphic p, T conditions under which partial melting of metabasalt takes place in this scenario are consistent with geochemical trace element data for TTGs, which indicate melting under amphibolite rather than eclogite facies. Other geodynamical settings which we have also investigated, including partial melting in small scale delaminations of the lower crust, at the base of a anomalously thick crust and due to the influx of a lower mantle diapir fail to reproduce this behavior unequivocally and mostly show melting of metabasalt in the eclogite stability field instead.

SHRIMP U-Pb zircon ages of granitoids from the Western Domain of the Kaapvaal Craton, Southeastern Botswana: implications for crustal evolution

SHRIMP U-Pb zircon dating of granitoids from borehole cores from the Amalia-Kraaipan granite-greenstone terrain of southeast Botswana yielded dates of: 2939±9 Ma for the biotite granite gneiss, 2943±9 Ma for the pink granite and 2782±6 Ma for the red granite. These dates define two main granite magmatic events in the Western Domain of the Kaapvaal Craton at ~2.96 to ~2.94 Ga and ~2.78 Ga. In the Kaapvaal Shield the main magmatic events are dated at ~3.5 to ~3.3, ~3.2 to ~3.1, ~2.96 to ~2.94 and ~2.78 Ga. The two domains were therefore affected by coeval igneous events at ~2.96 to ~2.94 Ga and ~2.78 Ga and therefore the age of ~2.96 to ~2.94 Ga is taken to date their amalgamation along the Colesberg lineament. The ages of ~2.78 Ga correspond to the emplacement of the Gaborone Igneous Complex of southeast Botswana, which is possibly related to the reactivation of or renewed igneous activity along the Colesberg lineament.

Temporal evolution of the Neoarchean Central Hearne supracrustal belt: rapid generation of juvenile crust in a suprasubduction zone setting

New U–Pb zircon ages are reported for 17 volcanic, plutonic and sedimentary rocks from the Neoarchean Central Hearne supracrustal belt in the Western Churchill Province of the Canadian Shield. The Central Hearne supracrustal belt, historically referred to as the Rankin-Ennadai greenstone belt, is a major granite-greenstone terrain with a strike length of over 700 km. This study, focused on the central and eastern segments of the belt in the Kaminak Lake area, represents the first detailed chronological study of this extensive greenstone belt. The geochronological data indicate rapid formation of crust between 2711 and 2667 Ma. Rocks of the oldest assemblage (I) formed between 2711 and 2691 Ma and consist of mixed tholeiitic and calc-alkaline mafic and felsic, submarine to subaerial volcanic rocks. Minor plutonic rocks (Group 1) are associated with this interval. A younger assemblage (II) of volcanic rock with significant calc-alkaline plutonic rocks (Group 2) developed between 2686 and 2679 Ma. A period of deformation (D1) locally separates the two assemblages. Major regional, penetrative deformation and metamorphism developed during the latter stages of the plutonic event at ca. 2680 Ma. An extensive belt of metasedimentary rocks and associated iron formation was deposited after 2681 Ma, based on ages of detrital zircon. The youngest events include intrusion of 2666.4±1.1 Ma post-deformational granite (Group 3), 2659 Ma carbonatite [Can. J. Earth Sci. 29 (1992) 896], and deposition of post-2660 Ma conglomerates and arenites of possible “Timiskiming”-type. There is no indication of contributions from Mesoarchean crust either in inherited grains in igneous rocks, or as detrital grains in sedimentary rocks. Although the evolutionary sequence has parallels to greenstone belts such as the Abitibi belt, in terms of the rock types and relative timing of events, there are some differences. We envision development of the belt in an extensional, oceanic suprasubduction environment.

Extensional detachment faulting and core-complex formation in the southern Barberton granite–greenstone terrain, South Africa: evidence for a 3.2 Ga orogenic collapse

Abstract The Barberton greenstone belt in South Africa is an Early- to Mid-Archaean, very low-grade metamorphic supracrustal belt that is bordered in the south by a mid- to lower crustal gneiss terrain. Detailed mapping of the contacts between the supracrustal and gneiss domains along the southern margin of the greenstone belt shows that the supracrustal rocks are separated from the high-grade metamorphic gneiss terrain by an extensional detachment that is situated at and close to the base of the belt. The extensional detachment is approximately 1-km wide and its location corresponds with the heterogeneous, melange-like rocks of the Theespruit Formation. Within the detachment zone, two main strain regimes can be distinguished. Amphibolite-facies rocks at and below the granite–greenstone contacts are characterized by rodded gneisses and strongly lineated amphibolite-facies mylonites. The bulk constrictional deformation at these lower structural levels records, in a subhorizontal orientation, the vertical shortening and horizontal, NE–SW directed stretching of the mid-crustal rocks. The prolate, coaxial fabrics are overprinted by greenschist-facies mylonites at higher structural levels that cut progressively deeper into the underlying high-grade basement rocks. These mylonites have developed during non-coaxial strain and kinematic indicators consistently point to a top-to-the-NE sense of movement of the greenstone sequence with respect to the lower structural levels. This relationship between bulk coaxial NE–SW stretching of mid-crustal basement rocks and non-coaxial, top-to-the-NE shearing along retrograde mylonites at upper crustal levels is consistent with an extensional orogenic collapse of the belt and the concomittant exhumation of deeper crustal levels. The exhumation was initiated under amphibolite-facies conditions at depths of approximately 18 km. The extensional collapse is coeval with or shortly follows the main D2 collisional event in the Barberton greenstone belt at ca. 3230–3220 Ma. Voluminous plutonism at ca. 3225 Ma along the northern margin of the belt is possibly related to the orogenic collapse and associated decompression melting of lower crustal rocks. The extensional collapse coincides with the onset of the coarse-clastic Moodies Group sedimentation which suggests that the small, isolated Moodies basins formed as supradetachment basins in the collapsing hanging wall of the detachment. The steepening of lithologies and fabrics to their present-day vertical attitudes is ascribed to a late-stage solid-state diapiric component of the exhumed hot and buoyant basement gneisses that underlie the relatively cool and dense mafic and ultramafic supracrustal succession.

Yarrabubba - a large, deeply eroded impact structure in the Yilgarn Craton, Western Australia

Yarrabubba is a newly discovered impact structure situated within the complex granite–greenstone terrain of the Yilgarn Craton. Shock-metamorphic effects including shatter cones, planar deformation features in quartz grains, and pseudotachylites, were found in deeply eroded Archean granites near Yarrabubba station, southeast of Meekatharra, Western Australia. Aeromagnetic images reveal arcuate demagnetization features at diameters between 11 and 25 km, which roughly correspond to the outcropping of the Yarrabubba Granite, and are centered on a magnetic-high halo around the Barlangi Granophyre (119°50′E, 27°10′S). Rapid-quench textures in the Barlangi Granophyre and its inter-fingering relationships with pseudotachylites suggest that it is an impact melt that was injected into the Yarrabubba Granite and spread along fault discontinuities. Both the potassic Yarrabubba Granite and the felsic Barlangi Granophyre are atypical in the northern Yilgarn, as are the abundant fracturing and frictional melting within the local granitoids. These anomalous geological features associated with shock-metamorphic effects are indicative of a hypervelocity impact origin. The age of the impact is uncertain, but geological and geophysical relationships suggest that the Yarrabubba structure was formed during the early Proterozoic.

Ore fluids associated with the Wynad gold mineralization, southern India: evidence from fluid inclusion microthermometry and gas analysis

In southern India, major gold deposits occur in the Archean granite–greenstone terrain of Kolar, Hutti, Ramagiri, and within Proterozoic granulite terrain in Wynad. The Wynad Gold Field (WGF) which is located within the Moyar–Bhavani lineament—a Proterozoic transcrustal shear zone hosts gold within quartz veins, which are emplaced along the mesoscopic faults and shears in the region. The quartz veins cut the regional fabric of the country rocks, suggesting that the veins were formed in post-tectonic and post-metamorphic episode of crustal shearing. Gold as visible specks or interstitial grains and veinlets are seen within quartz and associated sulfide minerals, indicative of multiple mineralizing events. Fluid inclusion studies carried out on gold–quartz veins showed the frequent occurrence of bi-phase primary inclusions of H 2O–CO 2, pseudosecondary inclusions of H 2O–CO 2 and aqueous secondary inclusions. Microthermometric studies of different samples revealed a near pure CO 2 composition, associated with the aqueous phase, with melting temperatures close to −56.6 °C. Clathrate melting temperatures of primary and pseudosecondary inclusions show that the fluids were low to moderately saline (2–14 wt% NaCl equiv.). Homogenization temperatures of primary and pseudosecondary inclusions range between 250–330 °C and 230–300 °C, respectively. The secondary aqueous inclusions have much lower homogenization temperatures between 130 and 220 °C. Fluid densities calculated from the microthermometric data were between 0.8 and 1.0 g/cm 3. The isochores pertaining to these fluid densities intercept, the trapping temperatures, within pressure ranges of 0.5 and 2.9 kbars. Fluid inclusion gas analyses were carried out on selected samples using a quadrupole mass spectrometer. The analysis shows that the fluid inclusions were mainly of H 2O and CO 2 with minor quantities of N 2 and CH 4, which was in close correlation with our visual analytical data. We envisage that the fluids responsible for the Wynad gold mineralization evolved from an H 2O–CO 2 fluid and precipitated gold at temperatures of 300–350 °C, due to the sudden pressure and temperature decrease as the fluids entered the shear zone.

U–Pb single zircon geochronology of granitoids in the Vumba granite–greenstone terrain (NE Botswana):: Implications for the evolution of the Archaean Zimbabwe craton

U–Pb SHRIMP single zircon crystallization ages were determined on representative samples of various granitoids exposed in the Vumba granite–greenstone terrain in NE Botswana. Based on structural and petrographic features, these granitoids were previously divided into five chronological groups: G1 (oldest) up to G5 (youngest). This chronology is not confirmed by U–Pb zircon dating, except for G5, which is the youngest granite. Our data show that igneous crystallization ages in the Vumba granite–greenstone terrain fall in the range 2696±4 and 2647±4 Ma. This time span is similar to the range of U–Pb zircon ages of 2710±19–2646±3 Ma for the Matsitama granite–greenstone terrain in NE Botswana. The oldest zircon age (2733±5 Ma) in the Vumba granite–greenstone terrain was obtained on a mafic xenolith from G2 gneiss/migmatite. This xenolith is inferred to represent a fragment of the Lower mafic–ultramafic volcanic unit of the Vumba greenstone belt. The minimum estimate of the protolith age of this gneiss is 2690±4 Ma and is within error of the age of the G1 tonalitic gneiss (2686±6 Ma). A crystallization age of 2696±4 Ma is yielded by the G4 dioritic granitoid, which was previously assumed to be part of the ca. 2.56 Ga post-kinematic granitoids. Field relationships and textural features suggest that this granitoid represent a subvolcanic intrusion probably coeval to the Upper intermediate to felsic volcanic units of the Vumba greenstone belt. A comparison of crystallization ages suggests that the Neoarchaean granitoids from NE Botswana might be correlated to the Sesombi granitoids and Upper Bulawayan volcanic sequence of the Zimbabwe craton. The G5 granitoids in the Vumba granite–greenstone terrain were emplaced at the same time as the Razi suite granitoids in Zimbabwe. The discrete shear zones documented in the Vumba granite–greenstone terrain developed between ca. 2686±6 and 2647±4 Ma. They are correlated to the D1-accretion-related deformation event in the Zimbabwe craton.

The Timing of Mineralization in the Archean North Pilbara Terrain, WesternAustralia

Mineralizing events in the North Pilbara terrain of Western Australia occurred between 3490 and 2700 Ma and include the oldest examples in the world of many ore deposit types. These events were pulsed and associated with major volcano-plutonic episodes (volcanic-hosted massive sulfide, porphyry Cu, Sn-Ta pegmatite, mafic-ultramafic-hosted Ni-Cu-PGE, and epithermal deposits) and deformation events (lode Au deposits). In many cases, the mineralizing events are associated with extension, either in rifts, pull-apart or back-arc basins, or as the consequence of doming associated with granitoid diapirism. Although mineralizing events occurred throughout the evolution of the North Pilbara terrain, the largest deposits are related to the development of the Mallina basin, which is located between the older East and West Pilbara granite-greenstone terranes. Four volcano-plutonic and three deformation events occurred in and around the Mallina basin between 2950 and 2840 Ma. Mineralization in the East and West Pilbara granite-greenstone terranes is less intense and occurred over a much longer period. Compared to other Archean granite-greenstone terrains, the North Pilbara terrain is poorly endowed; the only known world-class deposit in this region is the Wodgina Ta-Sn pegmatite deposit, but large iron ore deposits are being mined in the northeastern North Pilbara terrain. The long history of crustal development and slow rate of crustal growth may be responsible for the diversity of mineral deposits and could account for the apparent poor endowment of the North Pilbara terrain.

U–Pb geochronology of Archean metasedimentary rocks in the Pontiac and Abitibi subprovinces, Quebec, constraints on timing, provenance and regional tectonics

U–Pb isotopic ages of single detrital zircons have been obtained from metasandstones in the northern part of the Pontiac subprovince and from the Cadillac, Kewagama and Lac Caste Groups within the southern Abitibi subprovince. Youngest detrital zircon ages from these sequences are 2685, 2686, 2687, and 2693 Ma, respectively. The Lac Fournière pluton, which intrudes Pontiac Group metasedimentary rocks, is dated at 2682±1 Ma. A similar age was previously measured from a pluton that cuts the Kewagama Group. These data bracket deposition of greywacke in the Pontiac and Abitibi subprovinces to 2685±3 Ma. In contrast, the youngest zircon found in a volcaniclastic unit interbedded with Timiskaming Group metawacke is 2673±2 Ma, while the age of a pluton that intrudes the Timiskaming is 2672±1 Ma. Thus, the Timiskaming Group is distinctly younger than the other metasedimentary rocks. The normalized probability distributions of detrital zircon ages from the Pontiac and the Abitibi metasedimentary rocks, as well as of dated igneous rocks from the southern Abitibi subprovince, all show that their major zircon populations formed in the age range 2685–2710 Ma. The distributions for the Abitibi metaturbidites and dated igneous rocks are very similar, suggesting that metasedimentary rocks in the Abitibi belt were mostly derived from the surrounding granite–greenstone terrain. The largest peak of the Pontiac distribution is a few million years younger than that from the Abitibi metasedimentary rocks but there is a correlation between major peaks and both groups show similar age ranges for older, pre-Abitibi components (2750–2850 and 3020–3030 Ma). Deposition of the ca. 2685 Ma sediments was virtually syn-deformational. Based on previous field, geochemical and isotopic studies, as well as the detrital zircon age data, the Pontiac Group may represent a foreland basin that may have developed from an accretionary prism. The source of much of the sediment may have been an uplifted extinct continental arc to the north. The arc was mostly eroded into the sediment basin and perhaps tectonically overridden by a southward advancing nappe complex comprised of its back arc basin, now partly preserved in the southern volcanic zone of the Abitibi subprovince. Metaturbidites exposed between metavolcanic assemblages within the greenstone belt may represent overturned unconformities at the structural base of the greenstones. These were brought up by high angle faults, which also localized deposition of non-marine (Timiskaming-type) sediments. It is tentatively suggested that Pontiac-type metasedimentary rocks extend most of the way beneath the Abitibi subprovince, the oldest part of the basin emerging as the Quetico Group in the western Superior province.

Interaction of flexural shear, S– C fabrics, and oblique shear during folding of micaceous quartzite

This paper investigates the geometry, microstructure, and c-axis fabrics of an outcrop scale, micaceous quartzite fold produced under greenschist facies metamorphic conditions in the Moeda quartzite, Quadrilátero Ferrı́fero granite–greenstone terrain, southeastern Brazil. The fold limbs show development of opposed S– C fabrics and asymmetric quartz c-axis fabrics compatible with flexural slip along the fold surface. Towards the fold hinge, there is an increasing presence of oblique shear bands (here named S-bands) which gradually change to crenulations within the hinge zone. The oblique S-bands are interpreted to have formed through connection of several S-planes, increasing accommodation of antithetical shear along these S-planes and offset of the initial C-planes at intermediate stages of folding. This mechanism represents a kinematic inversion in the role played by the two sets of foliations in S– C structures. Our observations support flexural slip for early stages of folding. However, with progressive closure of the fold, the flexural slip mechanism involves increasing contributions from oblique shear on the S-bands, thus approximating an intermediate situation between flexural slip and passive folding (shear parallel to the axial plane).

Archaean tectonics: a review, with illustrations from the Slave craton

Abstract The tectonic evolution of Archaean granite-greenstone terranes remains controversial. Here this subject is reviewed and illustrated with new data from the Slave craton. These data show that a thick, c. 2.7Ga, pillow basalt sequences extruded across extended sialic basement of the Slave craton at a scale comparable with that of modern large igneous provinces. The pillow basalts do not represent obducted oceanic allochthons. Basement-cover relationships argue for autochthonous to parautochthonous development of the basaltic greenstone belts of the west-central Slave craton, an interpretation that is further supported by geochemical and geochronological data. Similar data exist for several other cratons and granite-greenstone terrains, including the Abitibi greenstone belt of the Superior craton, where stratigraphic and subtle zircon inheritance data are equally incompatible with accretion of oceanic allochthons. Many classical granite-greenstone terrains, including most well-documented komatiite occurrences, thus appear to have formed in extensional environments within or on the margins of older continental crust. Closest modern analogues for such basalt-komatiite-rhyolite-dominated greenstone successions are rifts, marginal basins and volcanic rifted margins. Indeed, these environments have high preservation potential compared with fully oceanic settings. Collapse and structural telescoping of these highly extended volcano-sedimentary basins would allow for the complex structural development seen in granite-greenstone terrains while maintaining broadly autochthonous to parautochthonous tectonostratigraphic relationships. Seismic reflection profiles cannot discriminate between these telescoped autochthonous to parautochthonous settings and truly allochthonous accretionary complexes. Only carefully constructed structural-stratigraphic cross-sections, allowing some degree of palinspastic reconstruction, and underpinned by sufficient U-Pb zircon dating, can address the degree of allochthoneity of greenstone packages. Furthermore, seismic reflection profiles are essentially blind for the steep structures produced by multiple phases of upright folding and buoyant rise of mid- to lower-crustal, composite, granitoid and gneiss domes. Such structures are ubiquitous in granite-greenstone terrains and, indeed, most of these terrains appear to have experienced at least one phase of convective overturn to re-establish a stable density configuration, irrespective of the complexities of the pre-doming structural history. Buoyant rise of mid- to lower-crustal granitoid and gneiss domes can explain the typical size and spacing characteristics of such domes in granite-greenstone terranes, and the coeval deposition of late-kinematic, ‘Timiskaming-type’ conglomerate-sandstone successions in flanking basins. The extensional and subsequent contractional evolution of granite-greenstone terrains may have occurred in the overall context of a plate tectonic regime (e.g. volcanic rifted margins, back-arc basins) but highly extended, intraplate, rift-like settings seem equally plausible. Explaining the evolution of the latter in terms of Wilson cycles is misguided. Periods of intense rifting and flood volcanism (e.g. 2.73–2.70 Ga) may have been related to increased mantle plume activity or perhaps catastrophic mantle overturn events. Although there is evidence for plate-like lateral movement in late Archaean time (e.g. lateral heterogeneity of cratons, arc-like volcanism, cratonscale deformation patterns, strike-slip faults, etc.), the details of how these plate-like crustal blocks interacted and how they responded to rifting and collision appear to have differed significantly from those in Phanerozoic time. The most productive approach for Archaean research is probably to more fully understand and quantify these differences rather than the common emphasis on the superficial similarities with modern plate tectonics.

Polyoxometallate bleaching catalyst

An outline is given of the Narryer Gneiss Complex that forms the northwest part of the Yilgarn Craton. It consists of two major groups of quartzo-feldspathic gneiss: Meeberrie gneiss mainly derived from ∼ 3680 Ma monzogranite, and Dugel gneiss derived from ∼ 3400 Ma syenogranite and monzogranite. The gneisses contain inclusions of an ∼ 3730 Ma layered gabbro-anorthosite intrusion called the Manfred Complex and are tectonically interleaved with metasedimentary rocks, mainly quartzite and banded iron formation. The complex was deformed and metamorphosed in granulite facies about 3350-3300 Ma ago.The gneiss complex is in tectonic contact with a granite-greenstone terrain that forms most of the Yilgarn Craton. These terrains were juxtaposed, intruded by sheets of granite, intensely deformed and metamorphosed about 27002600 Ma ago.The Narryer Gneiss Complex is a substantial addition to known areas of early Archaean crust. In contrast to most previously described terrains of this antiquity, it is largely derived from granite, it is devoid of metavolcanic rocks, and metasedimentary rocks are predominantly siliceous. The latter contain detrital zircons with U-Pb ages of 4275-4000 Ma, far older than any known terrestrial rocks. These features all suggest the former presence of a substantial amount of very old sialic crust (> 3700 Ma) and that remnants of it may still survive in this vast complex.

Structural evolution of the Warrawoona Greenstone Belt and adjoining granitoid complexes, Pilbara Craton, Australia: implications for Archaean tectonic processes

The Pilbara granite–greenstone terrain (3.6–2.8 Ga), Australia, is quoted as the key area formed by passive, gravity-driven solid-state diapirism. But, contrasting views have been reported recently, which led us to re-examine the key area: the Warrawoona Greenstone Belt and the adjoining Mt Edgar and Corunna Downs Granitoid Complexes. We found: (i) an asymmetric, non-synclinal distribution of metamorphic grade and deformation intensity across the Warrawoona Greenstone Belt; (ii) a major shear zone on the margin of the Mt Edgar granitoid complex with a unidirectional stretching lineation pattern; (iii) a NE-trending transcurrent shear zone within the central gneiss complex of the Mt Edgar Granitoid Complex; (iv) coeval magmatic activity and doming of the granitoid complexes; and (v) the loss of about 9 km of stratigraphic units in the Warrawoona Greenstone Belt. The area has had a prolonged history with at least four deformation phases between ca. 3.47 and 2.8 Ga. The main domal geometry of the Mt Edgar granitoid complex was obtained during active NE–SW extension at 3.31 Ga (D x+2 ), which appears to been in response to E–W compression. Possibly, the E–W compression resulted in crustal thickening and partial melting, probably of a 3.46–3.43 Ga granitic basement, causing crustal weakening, collapse and extrusion of the 3.31 Ga granitoids. Extension at 3.31 Ga was accommodated by the Mt Edgar Shear Zone, a mid-crustal detachment, and by the associated Beaton Well Zone, which acted as a transfer zone within the gneissic basement. Doming of the Mt Edgar Granitoid Complex is interpreted to have occurred in an active, extensional core complex-type mode. An indication for deformation prior to 3.32 Ga comes from syntectonically intruded gabbro/diorite and dolerite sills within the Mt Edgar Shear Zone, which intruded possibly as early as 3.46–3.45 Ga (D x+1 ). Ultramafic schistose xenoliths within the gabbro/diorite intrusions are the oldest observed deformed rocks, formed during D x . Post-doming deformation involved NE–SW compression, D x+3 , followed by dextral transcurrent deformation, D x+4 , during NW–SE compression. The latter may be related to the regional phase of deformation at 2.93 Ga, during which the eastern and western Pilbara were amalgamated. Comparison of this deformation sequence to others within the Marble Bar Domain suggests a common tectonic history from 3.46 Ga onwards. Our structural and kinematic data, especially the 3.31 Ga unidirectional stretching lineation pattern, lead us to reject solid-state diapirism due to convectional overturn as the main cause for doming of the Mt Edgar Granitoid Complex. We suggest that the cause lies in a plate tectonic process.

Variation of radiogenic heat production rate in 2.8–1.8 Ga old rocks in the central Fennoscandian shield

We present results of a geochemical survey based on 1150 outcrop samples for heat production studies in a E–W oriented band ( 120 km×500 km) located approximately between 62 and 63°N in Finland, the central Fennoscandian shield. The results provide representative averages of heat production rate in the studied tectonic units, but also demonstrate the heterogeneity and spatial variation in radiogenic heat production rate of the Precambrian lithosphere. The study area covers formations from the Archaean granite-greenstone terrain in the east via a Palaeoproterozoic mobile belt to a major granitoid complex surrounded by schist belts in central and western Finland. In this study we review the heat production rates as calculated from the U, Th and K total analyses and densities of the samples, and the relations of heat production rate values with major tectonic setting, lithological types, major composition and petrophysical properties of rocks. The data is further correlated with heat flow values measured in boreholes and applied in a lithospheric heat production model in the central part of the shield. Generally, heat production rate increases from Archaean to Proterozoic rocks in E–W direction but this trend is relatively weak and often overrun by lithological variations. The arithmetic means are 1.1±1.3 μW m −3 in the Archaean domain, 1.3±0.6 μW m −3 in the Höytiäinen autochthonous Proterozoic domain, 1.2±0.5 μW m −3 in the Suvasvesi domain characterized by allochthonous Proterozoic cover, 1.0±0.6 μW m −3 in the Proterozoic Raahe–Ladoga Mobile belt, 1.3±1.1 μW m −3 in the Proterozoic Rantasalmi–Haukivuori domain, 1.6±0.8 μW m −3 in the Central Finland Granitoid Complex and 1.4±0.8 μW m −3 in the Bothnian Schist Belt. The standard deviations of the mean values are considerable and reflect the heterogeneity of heat production. All distributions overlap. Heat production rate shows spatial variations which range in scale from kilometres to tens of kilometres. Well-defined systematic variations of heat production rate values with either SiO 2 content, density or P-wave velocity could not be found, although the correlations are statistically significant at 1% risk level. The correlations are weak and scattered, and sometimes opposite in sign in the major rock groups (plutonic, metavolcanic and metasedimentary). Heat flow density measured in boreholes increases with increasing heat production rate, but the present data do not support a strict linear dependence between the variables, and the regression line parameters vary depending on the applied heat production values (drill core analyses versus hole site averages from the present study). This indicates that the heat production rate is spatially heterogeneously distributed in the scale of the boreholes and their formations. A lithospheric heat production model was constructed using data from the present study for the upper crust, literature data for lower crustal xenoliths in the study area and a xenolith-derived heat flow density value for the mantle. The total crustal contribution to the surface heat flow density (37 mW m −2) from radiogenic heat sources is calculated as 26 mW m −2, and the contributions from the lower, middle and upper crust are 4, 11 and 11 mW m −2, respectively.

Nd, Pb and Sr isotopes in the Identidade Belt, an Archaean greenstone belt of the Rio Maria region (Carajás Province, Brazil): implications for the Archaean geodynamic evolution of the Amazonian Craton

The Rio Maria granite–greenstone terrain (RMGGT), located in the Amazonian craton at about 250 km south of the Carajás Ridge, is a well preserved Mesoarchaean domain composed of metavolcanic rocks and several suites of granitoids ranging in age from ca. 2.96 to 2.87 Ga. The metavolcanics comprise a basal unit of komatiites and low-K tholeiites overlain by sodic metadacites. A Sm–Nd whole-rock reference isochron for the metabasalts, metagabbros (tholeiites) and metadacites gives an age of 3046±32 Ma (MSWD of 1.6, 1 σ). Nd model ages ( T DM) of the metadacites range between 3.04 and 3.11 Ga. The ε Nd calculated at 3.0 Ga varies from +1.70 to +2.60 for the tholeiites and +0.30 to +3.22 for the metadacites. A Pb–Pb whole-rock linear array for the metabasalts yielded an anomalous older age of 3.41 Ga. The Sm–Nd age is coincident, within analytical error, with a Pb–Pb whole-rock errorchron age of 2944±88 Ma (1 σ) obtained for the metadacites. Rb–Sr systematic for the metadacites yielded an isochron age that is ca. 400 Ma youngest. The isotopic, geochemical and geological data suggest that oceanic lithosphere was the probable source of the ultramafic and mafic rocks in such a way that partial melting of the depleted mantle generated komatiites and low-K tholeiites. Partial melting of oceanic crust transformed into garnet amphibolite or eclogite generated the parental magmas of the metadacites. Isotopic and trace element characteristics of both metavolcanic rocks and associated plutonics suggest intra-oceanic island arcs as the most probable tectonic setting of the RMGGT. Two main events of crustal accretion are recognised at about 3.04–2.96 Ga and 2.87 Ga. Geological correlation indicates that the oceanic island arcs were completely amalgamated at around 2.76 Ga and acted as sources for the overlying sediments of the Rio Fresco Group.

Development of the Archaean Mallina Basin, Pilbara Craton, northwestern Australia; a study of detrital and inherited zircon ages

SHRIMP U–Pb zircon dates are combined with an examination of the age distribution patterns and provenance of both detrital zircons and of zircon xenocrysts in granites to investigate the development of the Archaean Mallina Basin, in the granite–greenstone terrain of the Pilbara Craton, northwestern Australia. The oldest dated components of the basin are c. 3010 Ma volcaniclastic rocks in the western part of the area. New data indicate that siliciclastic turbidites that dominate the southern and eastern part of the basin were deposited at or after c. 2970 Ma but before c. 2955 Ma. Linking both the detrital zircon populations as well as zircon xenocrysts from granites that intruded the Mallina Basin to well-dated areas of the Pilbara granite–greenstone terrane indicates that the sediment was derived from the south, north, northwest, and east. The basin probably evolved primarily in an intracontinental setting between two elevated land masses to the southeast and northwest. Most of the rocks within the basin were folded before intrusion of granites, the oldest of which has been dated at 2954±4 Ma . Evidence of a second depositional cycle is provided by a maximum depositional age of 2941±9 Ma , indicated by a detrital zircon population from a sample of wacke from the southeast part of the Mallina Basin. This second depositional phase may have been related to renewed extension, and recycling of sedimentary rocks within the basin.

Deformation partitioning during folding of banded iron formation

Domainal electron and optical microscopy and c-axis fabric analysis are utilized to document microstructures and the associated deformation partitioning between crystal plastic, brittle and fluid-assisted deformation mechanisms during folding of Proterozoic banded iron formation (a hematite–quartz–calcite multilayer) from the Quadrilátero Ferrı́fero granite–greenstone terrain (southeastern Brazil). The operation of different mechanisms was partially determined by the contrasting rheologic response of these three minerals at greenschist facies metamorphic conditions. Fracturing was the main deformation process in hematite, while solution-transfer accounted for part of the deformation in both calcite and quartz. Notably, there is strong interaction among the different deformation mechanisms, which have influenced each other in various ways. Fracturing of hematite in hinge zones of folds caused opening of gaps, which were sealed by direct precipitation of silica from the fluid phase. As a consequence, a chemical potential gradient between crystals and fluid phase was produced, and quartz was dissolved to restore the thermodynamical crystal–fluid equilibrium. Thus, brittle deformation of hematite partially controlled solution-precipitation creep in quartz. Heterogeneous access of fluid into the deforming medium also affected the deformation processes. Inhomogeneous deformation in the quartz–calcite aggregates generated intergranular porosity and increased fluid access, with solution-transfer becoming dominant in these domains. In contrast, the relatively more homogeneous deformation in the pure quartz aggregates served to maintain well-adjusted grain boundaries and reduced fluid access into the intergranular space, such that these domains deformed uniformly by crystal plastic processes, at relatively dry conditions.