Processes Operating during the Initial Stage of Magma Chamber Evolution: Insights from the Marginal Reversal of the Imandra Layered Intrusion, Russia

Abstract

The 13 m thick marginal zone of the 3·5 km thick mafic Imandra Layered Intrusion (ILI) consists of fine-grained pigeonite gabbros that are chilled against the underlying host andesitic basalts but reveals a non-chilled, highly irregular upper contact with orthopyroxenites of the overlying Layered Series. The marginal zone shows pronounced reverse compositional trends from the bottom to the very top and can therefore be referred to as a marginal reversal. In particular, the Mg-number of pyroxenes and the An content of plagioclase, as well as the whole-rock MgO, Mg-number and normative An content show a systematic upward increase, whereas all incompatible components (e.g. TiO2, K2O, P2O5, Rb, Ba, Zr, Y, REE) reveal an upward decrease. Similar reverse compositional trends are characteristic of intratelluric phenocrysts of plagioclase and inverted pigeonite that occur throughout the entire marginal zone. The marginal zone is abruptly terminated by the overlying coarse-grained orthopyroxenites, which are interlayered with massive chromitite of the Layered Series. The boundary between these two major units of the ILI represents a sharp break in terms of grain size, chemical composition and crystallization sequence. It is noteworthy that the fine-grained pigeonite gabbros of the marginal zone are distinguished by much lower concentrations of all incompatible elements than the coarse-grained orthopyroxenite of the Layered Series. The data indicate that filling of the chamber started with evolved, phenocryst-bearing liquids that probably represent the leading fractionates of parental basaltic magma that crystallized against the sidewalls of a deep conduit system.The formation of the marginal zone from these evolved liquids was interrupted by the emplacement of a large batch of parental magma that reached the chamber without much fractionation en route to crystallize into the Layered Series. The origin of the marginal reversal is best explained by the ‘three-increase model’ , which implies that rocks becomes more primitive inwards in response to (1) an inward increase in the extent of primitivity of successively intruding magma pulses, (2) an inward increase in the degree of chemical equilibrium among the phases and (3) an inward increase in the proportion of crystallizing cumulus minerals. The notable distinction in incompatible element abundances between the marginal zone and the Layered Series is attributed to fundamentally different regimes of magma flow during their formation.The crystallization of the marginal zone was dominated by fast-flowing (0·5^5 m s ) intruding magmas that favoured the effective removal of evolved liquids from in situ growing crystals and the generation of incompatible element-depleted mesoto adcumulates (fine-grained pigeonite gabbro). In contrast, the formation of the Layered Series was associated with a slow-flowing (0·5^5 km a ), thermally convecting magma that was much less effective in stripping evolved liquid from the crystallizing minerals and therefore gave rise to incompatible element-enriched orthocumulates (coarse-grained orthopyroxenites). The proposed sequence of magma replenishment and rock-forming processes may be common during the initial stage of the development of basaltic magma chambers.