The evolution of layered, basic plutons - evidence from small-scale structures

Abstract

Many basic and ultrabasic plutons are commonly lithologically, texturally and compositionally layered on several scales. Associated with and defined by, the layering are a variety of small-scale structures, considered analogous to structures found in sedimentary rocks, and capable of interpretation using similar techniques. Observations on these small-scale structures are vised to infer the "depositional" and "diagenetic" processes that operated during the solidification of a number of' plutons. Erosion structures and structures associated with lithic fragments indicate that physical redistribution of solids, crystals and rock fragments, was an important process in the evolution of these bodies and that plagioclase was deposited at the floor even where it was less dense than the contemporary magma. From their relationships with lithic fragments normally graded layers are demonstrated to be the solidified remnants of crystal-laden density currents flowing across the transient floor of the magma chamber and are inferred to be the best approximation to truly chronostratigraphic horizons in layered plutons identified to date. The proposition that crystal- and rock-laden density currents can transport material vertically through the magma body is investigated experimentally by examining the nature of such flows in a small tank, Experiments and calculations indicate that such density currents straddle the range of conditions from laminar to fully turbulent. Laminar flows do not mix with the contemporary magma and will transport crystals to the transient chamber floor. It is suggested that such a process may have given rise to lensoid mafic layers of limited areal extent in the Kiglapait intrusion. Structures inferred to have formed during the "diagenetic" stage of the formation of the layered rocks include deformation structures, layers and replacement bodies. Examples of the latter two sets of structures are shown to have textures identical to those in rocks interpreted as cumulates and it is concluded that those textures alone are not sufficient basis on which to infer crystal growth from the contemporary magma. Many of the structures testify to the former presence of mobile pore liquids and the contemporaneity of pore liquids of different compositions. Movement of pore liquid is considered to have been driven by density differences due to variations in pore magma composition and thus the structures can be considered as evidence for interstitial compositional convection. Under certain circumstances pore liquids may be expelled from the crystal mush and mix with the contemporary magma. The chemical consequences of such mixing events are discussed and it is proposed that chromitite layers in the Eastern Layered Series of the Rum intrusion record the operation of the process. Several features of the Rum intrusion suggest that the magma chamber was thermally and compositionally zoned at times during its active history and this leads to the formulation of a new scheme for the formation of cyclic stratigraphy in the Rum layered intrusion, based on the progradation and regression of a liquid/liquid interface and two environments of accumulation.