The early siliceous component of planetary crusts: experimental petrology of the tonalite-trondhjemite rock series

Abstract

There are two approaches to understanding the processes that lead to the formation of planetary crusts. The first is to determine the geology and geochemistry of rocks in the field and try to develop a consistent petrogenetic scheme to account for their occurrence. This approach has been extensively pursued on Moon and Earth, and remotely so on Mars. Another method is to establish the phase relationships of these rocks and their possible sources under various conditions of pressure, temperature, volatile content, etc., and apply the results to a petrogenetic model that is also consistent with isotope and trace-element geochemistry. This technique has been applied to the relationship between planetary mantles and basalts, and the formation of andesites and more siliceous rocks in subduction zones. Observations of rocks from Archean terranes on Earth reveal that the most prominent types are komatiites, tholeiites, tonalites and trondhjemites (grey gneisses), potassic granites (pink gneisses), and rare syenites. Early magmatic activity was dominated by the production of basalts which were subsequently followed, and intruded by, large volumes of the tonalite-trondhjemite series magmas. Approximately 80% of nine Archean cratons are composed of rocks that are chemically of tonalite-trondhjemite affinity (1).