Introduction: basic magmatism and crustal evolution

Abstract

The bulk of the crust of the Earth comprises, and probably always has comprised, igneous rocks which are basic or basic derivative in character, and a vast amount of scientific effort has gone into understanding their petrogenesis over the past fifty years. Why, then, distinguish the basic magmatism of the early Precambrian for particular attention? In a sense the reasons are obvious and simple. At that time, the Earth was a hotter body than now, more capable of producing higher temperature magmas. The brittle plate tectonics which control modern magma genesis were possibly not so prevalent, and entirely different processes may have operated. The present differentiation between oceanic and continental crust and their attendant lithospheres was probably less well defined. All of these factors influenced and contributed to differences in the composition, style and volume of basic magmatism in the early Precambrian compared with what we see today. Understanding the mechanisms and details of the various processes must bring a clearer insight into the evolution of the mantle and thus to the processes which produce basic magmas — both old and modern. It also contributes to our knowledge of how and when continental crust differentiated and developed its modern characteristics, and to how and why much of the metallogenesis of the early Earth took place.