Structural, kinematic and metallogenic evolution of selected domains of the Pilbara granitoid-greenstone terrain : implications for mid Archean tectonic regimes

Abstract

The Archean period (4000-2500 Ma) is one of the more enigmatic periods of the Earth's evolution. The oldest preserved rocks on Earth stem from this period, as does the oldest evidence of life and water on Earth. Much about the Archean is, however, still unknown. One of the main questions that has dominated studies in Archean terrains is when plate tectonic processes, as we know them in the present Earth, started to operate and whether or not they were active in the Archean (Condie, 1981; Kroner, 1981; Windley, [984; Nisbet, [987; Condie, [994). In geological studies of the Phanerozoic, the uniformitarian approach is usually adopted when interpreting geological data. The plate tectonic setting is commonly used as a key to understand geological processes that occurred in the past in a particular terrain. For example, an ophiolite complex is a reliable indication for oceanization, thick turbidite sequences usually form in trenches associated with subduction and at continental margins and calc-alkaline volcanism is a good indication for a volcanicarc setting with subduction of oceanic crust. This uniformitarian approach can not be used as rigorously in Archean terrains because there still is no consensus on whether the current style of plate tectonics was actually active. The reason for this uncertainty is not only based on theoretical considerations such as an expected higher heat production in the Archean, but also on differences observed in the Archean rock record. In many respects, including rock types, geochemistry of rocks, degree of alteration, intensity of deformation and largescale geometry, Archean terrains are different from modern terrains (see Chapter 2). The question of whether or not present day style plate tectonic processes operated in the (mid) Archean is one of the central themes of modem Archean geology. As a consequence it is the topic that underlies much of the research presented in this thesis.