Melt segregation and transfer mechanisms in the lower crust
4-9 July 2004
Kaapvaal Craton, South Africa
| Co-sponsored by
Geological Society of America
- and -
Geological Society of South Africa
Conveners and Reporters:
Wolf Uwe Reimold, Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa
Carl R. Anhaeusser, Economic Geology Research Institute, School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa
Ken A. Eriksson, Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA
Axel Hofmann, School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa
Roger L. Gibson, Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa
Christian Koeberl, Department of Geological Sciences, University of Vienna, Vienna, Austria
Bruce M. Simonson, Geology Department, Oberlin College, Oberlin, Ohio, USA
Frances Westall, Centre de Biophysique Moléculaire, CNRS, Orléans, France
The first-ever Field Forum hosted jointly by GSA and the Geological Society of South Africa was held in South Africa July 4-9, 2004. The 42 participants from 13 countries gathered to investigate and debate recent developments dealing with "Processes on the Early Earth." Themes within the forum included: accretionary processes; the age of the Earth and the oldest rocks on Earth; early heavy impact on the Earth and Moon; Archean crust formation and geodynamics; impact flux through time; traces of impact events in Archean strata; significance of large impact events for development of early life; earliest formation of life on Earth; early sedimentary processes; and Archean oceans. Discussion was facilitated by two superlative Archean field localities in the Kaapvaal Craton: the Barberton Mountain Land and the Vredefort Dome — the central uplift part of the world's largest and oldest meteorite impact structure. Together, these terrains provided an opportunity to examine events spanning at least 1000 m.y. of Archean history and crustal environments ranging from the surface to the deep crust.
Participants included experts in several of these fields, but also a number of newcomers and postgraduate students, thus providing an opportunity for multilevel exchange. Invited lectures set the scene, and field examinations involved both traditional introductory talks and short reviews by experts on specific subjects, such as impact spherule layers and the record of the early atmosphere, inter alia. A mixture of evening overview talks and presentations in the field, with poster presentations by all participants, was chosen for this event. Highlights follow.
An overview of the geology (including stratigraphy, chronology, geodynamic models, and historical aspects) of the Barberton greenstone belt (BGB) by Carl Anhaeusser (University of Witwatersrand) set the scene for the first day in the field. Field observations in the vicinity of the Komati Formation type section in the Komati river valley followed, as did a series of stops in surrounding granitoids, providing an introduction to granite-greenstone relationships and 3.5-3.2 Ga tectonics in the southern parts of the belt. This included a detailed discussion of high-grade metamorphic (possible core complex) granitoids in close proximity to low-grade mafic-ultramafic rocks by Annika Dziggel (University of Aachen). Nick Arndt (University of Grenoble) gave an in-depth overview of the current understanding of the petrogenesis of Archean mafic-ultramafic volcanic sequences. New evidence for high-grade, low-geothermal gradient metamorphism, presented by Annika Dziggel, challenged the view that BGB metamorphism was shallow-level and associated with high geothermal gradients. This view also suggests that the lowermost part of the greenstone succession is, in fact, a distinct structural unit juxtaposed by tectonics.
The second day involved the study of the Komati river gorge section in the Songimvelo Nature Reserve (led by Axel Hofmann, University of Witwatersrand) through 3.45-3.3 Ga metasedimentary and metavolcanic rocks of the Kromberg Formation, which included aspects related to early surface processes, earliest microfossil evidence and hydrothermal activity, as well as pyroclastic ultramafic magmatism. The visit of the Komati River gorge in the Songimvelo Nature Reserve yielded several surprises: elephants took a fleeting interest in the group, and a serious case of geo-vandalism was observed. Many prime exposures along this exceptional traverse had been recently drilled for paleomagnetic and other analysis, thus defacing a series of what were once photogenic exposures. In the afternoon, Carl Anhaeusser led the excursion to exposures of spinifex-textured komatiite and pillowed basaltic komatiite exposed in some of the xenoliths in the area south of the BGB. In addition, outstanding river sections exposing TTG (trondhjemite-tonalite-granodiorite) gneisses and migmatites were examined.
The evening session comprised an overview by Ken Eriksson (Virginia Polytechnic Institute, Blacksburg) of the intriguing 3.3-3.2 Ga Fig Tree and Moodies Group metasediments (including the earliest tidalites) from the northern part of the BGB, which was followed by Bruce Simonson's (Oberlin College) presentation on impact spherule layers as the only available witnesses of Archean and Paleoproterozoic impact activity and sedimentological processes related to their deposition, types of meteoritic projectiles, and the nature of the early target regions.
Day 3 began with a field presentation on Archean atmospheric and seawater conditions by David Catling (University of Washington) at an impressive outcrop of jaspilitic banded iron formation. An impact spherule layer was visited in the Barite Valley Syncline area, before heading off to the northern part of the belt to study the oldest known tidal deposits of the Moodies Group in Sheba Creek, Eureka Syncline (Fig. 1). A stop on the basal Moodies conglomerate at Izzy's Pass completed the field program for the day. The evening session began with a presentation by Patricia Corcoran (University of Western Ontario, London), who linked modern sedimentological processes with observations on Archean metasediments of the Slave Province, Canada, followed by poster viewing and debate. The quality of posters presented was of a high standard and covered a wide spectrum, from regional geological, to geochemical, to geodynamic issues, mostly on Archean and early Proterozoic topics.
The next day involved the transfer from Barberton to the Vredefort Dome (a distance of approximately 400 km), after break-out groups visited either the Msauli Chert type section with microfossil-bearing carbonaceous cherts and accretionary lapilli beds (in contrast to the impact spherule layer site visited earlier), the Stolzburg layered ultramafic complex, or the giant stromatolites of the Malmani Group in the Transvaal Supergroup. All participants reconvened in the evening near Vredefort for a feast of local (Free State) cuisine (which must also be considered as the coldest barbecue most of us had ever attended). Despite the late hour, Roger Gibson (University of Witwatersrand) then presented a review of the various geodynamic models for the evolution of the Kaapvaal Craton, with Cristiano Lana (Imperial College) adding the specific Vredefort basement-derived version. The field stops during the ensuing two days examined various aspects of the Meso- and Late Archean evolution of the central Kaapvaal Craton, a period that was marked by the change from granite-greenstone tectonics to the development of broad supracrustal basins that have experienced comparatively little subsequent disturbance. Central to this transition were events at ca. 3.1 Ga that are manifested by voluminous postorogenic granitoid plutonism across large parts of the craton; however, the Vredefort region shows a somewhat anomalous pattern, with intense polyphase deformation, high-grade metamorphism and TTG plutonism at this time, suggesting that it represents a 3.1 Ga craton margin. Delegates had the opportunity to examine the evidence for magmatic and tectonic thickening of the granite-greenstone crust at Vredefort during this event, and to examine new evidence for syn- to late-orogenic extensional collapse that exhumed the high-grade terrane and facilitated the deposition of the basal units of the supracrustal sequence as early as 3.074 Ga. Excellent quarry and road exposures proved particularly informative in examining the complex and unusual deformation features (shatter cones, breccias) that accompanied the dome-forming impact event at 2.02 Ga.
The final evening session was devoted to early impact processes and geochemistry, with Christian Koeberl (University of Vienna) providing a seminal talk on the earliest impact record of the solar system and especially Earth, followed by a talk on the early geochemical record by Jan Kramers (University of Bern). If anything, the excellent record of interaction and debate that was characteristic of the entire Field Forum was surpassed that evening, with questions regarding the apparent paucity of the impact record during the first half of the Earth's existence being animatedly debated.
The week after the Field Forum, the Geoscience Africa 2004 conference was held at the University of the Witwatersrand in Johannesburg. This conference also entailed a weeklong symposium on "Birth and Growth of Continents," with much attention to Archean and early Proterozoic geodynamics.
National Research Foundation of South Africa, a significant sponsorship which allowed the participation of a number of postgraduate students from several countries.
Council for Geoscience (Geological Survey of South Africa), which supported the participants with relevant publications.
Carl R. Anhaeusser
Sjoukje de Vries
Wolf Uwe Reimold