EARTHS EARLIEST CONTINENTAL LITHOSPHERE, HYDROTHERMAL FLUX AND CRUSTAL RECYCLING

The Kaapvaal craton in southern Africa and the Pilbara craton of north western Australia are the largest regions on Earth to have retained re latively, pristine mid-Archaean rocks (3.0-4.0 Ga). The Kaapvaal crato n covers about 1.2 X 10(6) km(2), and varies in lithospheric thickness between 170 and 350 km. At surface, the craton can be subdivided into a number of Archaean sub-domains; some of the subdomains are also wel l defined at depth, and local variations in tomography of the lithosph ere correspond closely with subdomain boundaries at surface. The Archa ean history of the Kaapvaal craton spans about 1 Gyr and can be conven iently subdivided into two periods, each of about the same length as t he Phanerozoic. The first period, from circa 3.7-3.1 Ga, records the i nitial separation of the cratonic lithosphere from the asthenosphere, terminating with a major pulse of accretion tectonics between 3.2 and 3.1 Ga, which includes the formation of ''paired metamorphic belts''. This period of continental growth can be compared to plate tectonic pr ocesses occurring in modern-day oceanic basins. However, the differenc e is that in the mid-Archaean, these oceanic processes appear to have occurred in shallower water depths than the modern ocean basins. The s econd period, from circa 3.1-2.6 Ga, records intra-continental and con tinental-edge processes: continental growth during this period occurre d predominantly through a combination of tectonic accretion of crustal fragments and subduction-related igneous processes, in much the same way as has been documented along the margins of the Pacific and Tethys oceans since the Mesozoic. The intra-oceanic processes resulted in sm all, but deep-rooted continental nucleii; the first separation of this early continental lithosphere could only have occurred when the mean elevation of mid-oceanic-ridges sank below sea-level. Substantial recy cling of continental lithosphere into the mantle must have occurred du ring this period of Earth history. During the second period, at least two large continental nucleii amalgamated during collisional processes which, together with internal chemical differentiation processes, cre ated the first stable continental landmass. This landmass, which is kn own to have been substantially bigger than its present outline, may ha ve been part of the Earth's first supercontinent. The oldest known sub domains of the craton include the oceanic-like rocks of the Barberton greenstone belt. The comagmatic mafic-ultramafic rocks (3.48-3.49 Ga) of this belt represent a remnant of very early oceanic-like lithospher e (known as the Jamestown Ophiolite Complex), which was obducted, appr oximately 45 Ma after its formation, onto a volcanic arc-like terrain by processes similar to those which have emplaced modern ophiolites at convergent margins of Phanerozoic continents. The early metamorphic h istory, metamorphic mineralogy, oxygen isotope profiles and degree of hydration of the 3.49 Ga Jamestown Ophiolite Complex are similar to pr esent day subseafloor hydrothermal systems. The ratio of Delta Mg to D elta Si for hydrothermally altered igneous rocks, both present day and Archaean, are remarkably uniform at - 5( +/- 0.9) and the same as tha t of hydrothermal fluids venting on the present-day East Pacific Rise. This observation suggests that the process of Mg exchange for Si in h ydrothermal systems was commonplace throughout Earth's history.