Layered intrusions: Fundamentals, novel observations and concepts, and controversial issues

Layered intrusions are fossilized natural laboratories that historically have constrained many fundamental principles of igneous petrology. Layered intrusions are typically stratiform, usually sill-like bodies of cumulate rocks, at least a few hundred metres to as much as 10 km thick, characterized by the presence of a variety of different types of layering over a range of length scales. They are the solid record of crystallization, differentiation and solidification processes of mainly basaltic magmas. The importance of layered intrusions also lies in hosting a significant proportion of the world's known reserves and resources of important critical metals: particularly, the majority of the global resource of platinum-group elements (PGE), chromium (Cr) and vanadium (V) and also very large resources of nickel (Ni), copper (Cu) and cobalt (Co). This paper summarizes the progress that has been made in the study of layered intrusions during the last three decades. The progress is marked by a number of novel observations from layered intrusions. Among them are: (1) draping of igneous layering over a few-km-high sloping step in the chamber floor; (2) development of igneous layering on the overturned to undercutting portions of a chamber floor; (3) magmatic karstification of the floor cumulates, (4) existence of threedimensional framework of crystals in (oxide) cumulates; (5) systematic variations in dihedral angles between touching grains, and other microtextural features; (6) Cr-rich structures at the base of magnetitite layers; (7) coexistence of melt inclusions of contrasting composition in minerals; (8) thermal and chemical histories recorded by plagioclase; (9) textural and chemical features of minerals revealed by X-ray microscopy, (10) intrusion-scale to mineral-scale isotopic heterogeneity; (11) out-of-sequence zircon ages; and (12) skeletal/dendritic growth of minerals revealed by minor element zonation. The progress is also evident from development of several new concepts and refinement of some established ones. These include: (1) time and length scales in layered intrusion processes, (2) catastrophically fast growth of magma chambers, (3) out-of-sequence emplacement in layered intrusions, (4) large-scale slumping and mineral sorting in layered intrusions, (5) production of monomineralic cumulates from single phase-saturated melts, (6) origin of non-cotectic cumulate by in situ growth, (7) the arrival of new phases on the liquidus, (8) inward propagation of solidification fronts, (9) mushy and hard chamber floor, (10) absence of roof sequences due to their disruption, (11) basal reversals and chilled margins, (12) adcumulus growth theory, (13) compositionally stratified magma chambers, (14) melt-sediment interactions during magma chamber growth, (15) lateral reactive infiltration in a crystal mush, (16) reactions involving conjugate immiscible liquids in crystal mushes, and (17) constraints on subsolidus processes from non-traditional Fe-Mg-Cr stable isotopes. Finally, we show that the major controversies regarding layered intrusions currently revolve around whether: (a) the microstructure of igneous rocks are primary or secondary , (b) compaction in layered in-trusions is pervasive or non-existent (c) large, long-lived and entirely-molten magma chambers exist or not. The review shows that layered intrusions provide ground-truth information on the processes of magma crystalliza-tion, differentiation , solidification in crustal chambers as well as on mechanisms of ore-forming elements concentration into economically viable mineral deposits. We propose a few lines for future research that may potentially raise igneous petrology to a new level of understanding of the processes that govern the evolution of terrestrial magmatic systems.