Event Markers: Proterozoic versus Archean - a Contrast
Proterozoic event markers in cratonized, thickened Archean continental crust consist of plume-related, generally radiating, diabase dike swarms and non-plume related development of epi-cratonic and continental margin basins containing sediments and minor volcanics with events therein characterized by deposition and with gaps manifested by development of unconformities. Over the interval ~2.7-1.2 Ga the Archean Superior Province contains at least 6 diabase swarms and the major (Huronian) epi-cratonic basin contains at least 4 unconformity-bounded sequences. However, the protracted evolution of substantial parts of the Superior Province during the Archean (e.g. ~3.1-2.70 Ga) is sufficiently long to show evidence of multiple plume events and major depositional gaps manifested by unconformities. Conventional wisdom would state that Archean plume activity does not yield dike swarms and that there are limited depositional gaps in greenstone sequences save the subaerial unconformities between uninterrupted evolution of Archean volcanic sequences (Keewatin) and overlying successor basin sequences (Timiskaming). However, for example during the interval ~3.7-2.7 Ga, the west Greenland high grade gneisses are cut by multiple generations of amphibolite dikes, but the ~3.1-2.7 Ga evolution of the North Caribou terrane of the Superior Province is marked by limited evidence of as yet undated deformed amphibolite dike swarms in older tonalities (e.g. Steep Rock area). An additional manifestation of plumes in this region is the plume-related komatiite-tholeiite sequences typifying 3-2.7 Ga greenstones themselves. But are the depositional gaps of the Proterozoic history found in Archean development? Recent work indicates that most Archean greenstone belt assemblages representing as much as 250 My of deposition are marked by iron formation between assemblages. For example, in the Abitibi greenstone belt, the inter-assemblage boundaries, including an important 20 My depositional gap, are marked by iron formation. The iron formations are alternating iron-rich bands and chert beds with the chert geochemistry indicating either hydrothermal or seawater or silicification of precursor rock types ranging from komatiite to rhyolite. It is concluded that the iron formation intervals represent pre-diagenetic silicification during extended depositional gaps representing submarine unconformities.
Modeling Archean Plate Collision Under an Aggressive Atmosphere and Buoyant Mantle Lithosphere
It is still uncertain whether plate tectonics and plate boundary evolution were appreciably different during the Archean owing to different thermal/chemical/environmental conditions then. For example, it has been suggested that a highly depleted mantle lithosphere in the Archean may have meant a more buoyant/stable sub-crustal lithosphere compared to today. Also, based on inferences that the Archean Earth had a much warmer and more humid atmosphere as a result of increased or highly fluctuating levels of CH4 and CO2, it has been suggested that weathering and erosion may have been far more intense during the Archean. These two factors (lithospheric buoyancy and surface erosion) have a significant influence on present-day tectonics. Here, we explore how the geodynamics of lithospheric collision is modified with varying 'Archean-like' mantle lithosphere buoyancy and surface erosion rates. Specifically, computational forward modeling based on the arbitrary Lagrangian-Eulerian finite element technique is used to investigate the structural evolution of the crust and mantle lithosphere. Results suggest that continental collision with buoyant Archean mantle lithosphere may have been accommodated by very shallow, underplating subduction-type behaviour. In certain cases, the mantle lithosphere structure correlates to deformational features in the crust. The high surface erosion leads to enhanced strain localization and exhumation of the crust as well as modification of the sub-crustal tectonics. The modeling results are reconciled with first order interpretations of lithospheric assembly interpreted from Archean rocks.
The Serrinha Nucleus Granite-Greenstone (NE Bahia, Brazil): The Precambrian History of a Gold Mineralized Terrane by U-Pb Geochronology
U-Pb single zircon crystallization ages were determined using TIMS and SHRIMP on rocks exposed in the Serrinha nucleus granite-greenstone terrane in NE Brazil. Our data show that the granitoid plutons can be divided into 3 distinct groups that formed over a 1.5 b.y. time span. Group 1 Mesoarchaean intrusions, consisting of gneisses and elongated plutons with gneissic borders, form a major component of the basement. The gneiss-migmatitic rocks (ca. 3200 Ma) of the Uauá complex are the oldest known unit. Shortly afterwards, partial melting of mafic material produced a medium-K calc-alkaline melt, the younger Santa Luz complex (ca. 3100), to the south. Subsequent melts intruded in different phases now exposed as N-S elongated plutons such as Ambrósio (3162 Ma). These rocks have typical features of Archaean TTGs, implying varying degrees of partial melting of underplated crust during a magmatic event at ~3070 Ma. Some of the plutons have what appear to be intrusive, but are probably remobilized, contacts with the Transamazonian Itapicuru greenstone belt and enclaves of older gneissic rocks. Thus, serial additions of juvenile material over a period of several hundred m.y. led to the formation of a stable micro-continent by 2.9 Ga. Evidence for Neoarchaean activity is found in the inheritance pattern of only one sample, the group 2 Euclides pluton. Group 2 is represented by 2.16 to 2.13 Ga pretectonic TTG and calc-alkaline plutons that are less deformed than group 1, and were emplaced in a continental arc environment floored by the Mesoarchaean crust. This Transamazonian orogeny can be explained as a consequence of ocean closure followed by collision and slab break-off. It resulted in formation of the Itapicuru Greenstone Belt (~2.15 to 2.12 Ga), one of the most important gold areas of Brazil. The group 3 geochronology, geochemical, spatial associations and lithostratigraphy indicate two stages of alkaline magmatism: a first stage (2114-2097 Ma), with intrusion of shoshonite, syenite and ultrapotassic lamprophyric rocks; and a second stage (ca. 2080-2070 Ma), with intrusion of small semi-circular peraluminous isotropic K-granites and dykes. These were the probable heat source driving Palaeoproterozoic metamorphism at ~ 2070 Ma. The only subsequent magmatism was kimberlitic, probably emplaced during the Neoproterozoic Braziliano event, which sampled older zircon (~2.15 Ga) from the basement. The earliest evidence of crust formation appears as rare Paleoarchaean (3.6 Ga) zircon xenocrysts are among the oldest yet recorded within the São Francisco craton, were found in the group 3 Euclides shoshonite and in the group 2 Quijingue trondhjemite, indicating the presence of Paleoarchaean sialic basement. If these interpretations are correct, evidence for at least two major phases of regional deformation and metamorphism (Archaean and Transamazonian) should be present in at least some of the group 1 plutons, whereas only the Transamazonian events should have affected group 2 plutons.
From Extension to Transtension: the Telemark Supracrustals, ca. 400 Ma Sedimentary Record of Variable Tectonics Before and During the Sveconorwegian (Grenvillian) Orogeny
The Mesoproterozoic volcanic-sedimentary Telemark supracrustal rocks in southern Norway make up an important succession of strata within the Fennoscandian Shield. A model of the geological evolution of the region comprising the Telemark supracrustals has been made by the combination of field records of structural geology, stratigraphy, sedimentology, Lu-Hf isotopes and U-Pb age determinations of detrital zircons, and whole rock geochemistry of magmatic rocks. The Telemark succession was formed in two different large-scale tectonic regimes: first in a large rift basin when the crust experienced only extension, and later in smaller extensional basins created by transpression. Tectonic movements along faults bounding the basins and within the basins affected the depositional patterns by creating local sites of uplift and erosion, controlling sediment transport routes and accommodation space for deposition. These basin fills provide a nearly continuous sedimentary record spanning from the pre- Sveconorwegian time to the onset of the orogeny and assembly of the supercontinent Rodinia. The formation of the Telemark supracrustals started with establishment of the ca. 1500 Ma Rjukan Rift Basin, which in its early rift stage was featured by voluminous bimodal volcanism. The rift basin was subsequently filled with coarse, locally derived alluvial sediments and within-plate basalts. The magmatic and tectonic evolution was characterized by deep intraplate melting combined with crustal contamination. The post-rift stage, commenced by cooling of the lithosphere, resulted in regional slow subsidence rates, marine incursion, exhumation of rift-shoulders and filling of the rift basin with sediments from mainly extrabasinal source rocks. After the rift basin was filled, at least 2 km of mature sediments accumulated in a tidal - shallow marine environment, in a type of a peri-cratonic basin, welding the rocks of the Rjukan Group to the western margin of Fennoscandia. This cratonization lasted at least 100 Ma. During the onset of the Sveconorwegian orogeny at ca. 1170 Ma, the underlying rocks were folded and eroded, forming a major regional unconformity. The crust entered a transpressional regime where various smaller fault-bounded pull-apart basins developed and were filled with alluvial sediments and bimodal volcanic rocks. Volcanism ceased and the basins were later filled only with siliciclastic sediments. Detrital zircons and lithological variations show rapidly changing provenances and vertical crustal movements. The lithosphere was first penetrated by faults resulting in upwelling of the asthenosphere and crustal melting and bimodal magmatism. Later the tectonic regime was transformed into thin-skinned tectonics before the culmination of the orogeny at ca. 1050 Ma. The Sveconorwegian evolution of the Telemark supracrustal rocks has a possible structural analogue setting in California where compressional tectonics between the North American Plate and the Pacific Plate are overprinted or combined with strike-slip tectonics and formation of a series of pull-apart basins.
A Paleomagnetic Study of the Lewisian Foreland, NW Scotland: Precambrian Chronicle Revisited
A paleomagnetic study of the central Lewisian foreland, located in NW Scotland, is presented. The Lewisian foreland is predominantly comprised by late Archean tonalitic-trondhjemitic gneisses that are intruded by a suite of Paleoproterozoic (ca. 2.0 -2.4 Ga) mafic-ultramafic dykes, commonly referred to as the Scourie dykes. In particular, dykes pertaining to the Lewisian's central zone exhibit the least degree of reworking during subsequent (ca. 1.7 Ga) Laxfordian events, thereby making the terrain applicable for obtaining ancient, pre- Laxfordian, paleomagnetic directions. Although previous studies for the area have concluded no such directions, this study aims to reevaluate the area in light of the new geochronologic and geochemical data that has become available. For instance, it is now known that the Scourie dyke intrusion records several asynchronous magmatic events spanning as much as ca. 400 Ma. As such, the paleomagnetic data from this study is interpreted considering that each dyke (or site) may reflect a distinct tectonic history. In addition, this study uniquely presents directions acquired from the hosting gneissic terrains allowing for contact tests and the proper discernment of directions recorded during metamorphism. It is proposed that not all directions have been acquired during post-Laxfordian events, and that several sites may present relict directions acquired during earlier times (~2.0 Ga and ~2.4 Ga). The resulting paleomagnetic poles are used to formulate suppositions concerning Precambrian continental assembly from the Lewisian perspective.
Towards a Dyke Swarm Map of Venus
There remain some fundamental questions regarding the nature and origin of magmatism on Venus: 1) Did global magmatic resurfacing occur at about 750 Ma ago, or did resurfacing occur at a low, steady-state rate over a long time period? 2) What is the origin of coronae and are there terrestrial analogues? 3) Does Venus correspond to an early or late stage Earth (i.e. pre, or post-plate tectonics)? A new approach for addressing some of these major questions is the study of graben-fissure systems that are widespread on Venus, and in many cases are the surface expression of dyke swarms. Long narrow extensional lineaments (grabens, fissures and fractures) group into radiating, linear and circumferential systems. Radiating Systems: Those that radiate have been the most intensely studied. Early research by Grosfils and Head (e.g. GRL, v 21, pp. 701- 704, 1994) established the importance of radiating graben-fissure systems on Venus as an indication of subsurface dyke swarm distributions and as a sensitive tool for locating centres of intraplate magmatism (mantle plumes?). Detailed mapping in the Guinevere Planitia region (14 Mkm2) by Ernst and others (Icarus, v. 164, pp. 282-316, 2003) indicated that GIS analysis using 75 m/pixel Magellan data increases the number of systems detected, relative to earlier 225 m/pixel reconnaissance mapping, by ~6x. Extrapolated globally, we predict ~900 radiating systems, with ~360 >300 km in radius and ~160 with radii >1000 km. Linear Systems: Detailed mapping also reveals that linear graben-fissure systems are widespread and numerous on Venus. Linear systems could be purely tectonic features (rift zone related) or be underlain by dykes, or a combination of the two. Regional-scale mapping has already helped show that some linear systems are distal parts of gigantic radial systems linked to magmatic centres. Circumferential Systems: Arcuate systems which surround volcanic/tectonic features) have been mainly studied in the context of coronae. Assessment of which are underlain by dykes and which are purely tectonic has not been completed. Global Graben-Fissure (Dyke Swarm) Map for Venus: We are undertaking an effort to produce the first comprehensive global map of graben- fissure systems on Venus, and to attempt to determine which are underlain by dyke swarms, using the highest-resolution Magellan radar data. Specifically, we are integrating published information with our own detailed mapping in order to provide a resource that will complement existing catalogues of other tectonic and magmatic features (e.g., the distribution of volcanoes, coronae, wrinkle ridges) and should result in a more comprehensive understanding of the distribution in time and space of intraplate magmatism on Venus. This global graben-fissure (dyke swarm) map for Venus will also be compared with the dyke swarm distributions mapped on Earth, and Mars, and newly identified on Mercury. This effort to produce a dyke swarm map of Venus represents part of the legacy of Prof. Henry Halls, who launched the modern study of dyke swarms with a key paper in 1982 (in Geoscience Canada, v. 9, pp. 145-154) and with a series of International Dyke Swarm conferences (and associated edited volumes) starting in 1985, and occurring every 5 years, with the next scheduled for India in February 2010.