INTRODUCTION

Early in the development of petrology, it was recognized that certain rock types are commonly associated, whereas others never occur together. Moreover, the common associations were seen to correlate with certain geologic settings. Today, with the insight provided by plate tectonic theory, most igneous rocks can be assigned to particular plate tectonic environments, each of which has its own distinctive thermal regime, magma source region, and crustal stress pattern. But not all rock associations can be explained through plate tectonics. Some magmatism in the Archean and even the Proterozoic was different from that of Phanerozoic time, and distinctive rock associations were formed that were never again repeated in later times.

Seismic evidence indicates that the lithosphere and upper mantle are essentially solid, although a small amount of liquid may exist in the low velocity layer. The formation of large magma chambers and volcanic edifices is therefore a rare occurrence that requires special conditions. Yet, the majority of crustal rocks are of igneous origin, and thus these conditions must, on occasion, be met. The steady-state geotherm beneath a continent or ancient ocean floor (Section 1.6) does not come near the dry beginning of melting curve for mantle peridotite, at least not at the depths at which we believe magmas are generated. Therefore, either the geotherm must be raised or the beginning of melting curve lowered if magmas are to form.