The Rustenburg Layered Suite formed as a stack of mush with transient magma chambers

Zhuosen Yao,M Christopher Jenkins,James E Mungall

doi:10.1038/s41467-020-20778-w

Abstract

The Rustenburg Layered Suite of the Bushveld Complex of South Africa is a vast layered accumulation of mafic and ultramafic rocks. It has long been regarded as a textbook result of fractional crystallization from a melt-dominated magma chamber. Here, we show that most units of the Rustenburg Layered Suite can be derived with thermodynamic models of crustal assimilation by komatiitic magma to form magmatic mushes without requiring the existence of a magma chamber. Ultramafic and mafic cumulate layers below the Upper and Upper Main Zone represent multiple crystal slurries produced by assimilation-batch crystallization in the upper and middle crust, whereas the chilled marginal rocks represent complementary supernatant liquids. Only the uppermost third formed via lower-crustal assimilation–fractional crystallization and evolved by fractional crystallization within a melt-rich pocket. Layered intrusions need not form in open magma chambers. Mineral deposits hitherto attributed to magma chamber processes might form in smaller intrusions of any geometric form, from mushy systems entirely lacking melt-dominated magma chambers.

Highlights

The Rustenburg Layered Suite of the Bushveld Complex of South Africa is a vast layered accumulation of mafic and ultramafic rocks
Layered mafic intrusions represent portions of the plumbing systems of many large igneous provinces and are principal repositories of several critically important ore elements, including Cr, Ti, V, and the platinum-group elements (PGE)[1]. Layered mafic intrusions, such as the iconic Rustenburg Layered Suite (RLS) of South Africa, have historically been considered to represent the solidified remnants of vast liquid-dominated reservoirs of magma called magma chambers where crystallization-differentiation has occurred by fractional crystallization[2,3,4]
The spectrum of bulk cumulate macrolayer compositions observed in the RLS can be described as the first-order products of magma evolution during processes ranging from simple ABC in the upper crust for the ultramafic rocks of the Lower Zone and Critical Zone, through a two-stage ABC process in the mid-crust to generate the mafic rocks of the Upper Critical and Main Zones, to classical assimilation–fractional crystallization (AFC) in the lower crust to form the parental melt for the Upper and Upper Main zones, which evolved by fractional crystallization in an essentially closed magma chamber affected by a small number of recharge events

Summary

Introduction

The Rustenburg Layered Suite of the Bushveld Complex of South Africa is a vast layered accumulation of mafic and ultramafic rocks. 1234567890():,; Layered mafic intrusions represent portions of the plumbing systems of many large igneous provinces and are principal repositories of several critically important ore elements, including Cr, Ti, V, and the platinum-group elements (PGE)[1] Layered mafic intrusions, such as the iconic Rustenburg Layered Suite (RLS) of South Africa, have historically been considered to represent the solidified remnants of vast liquid-dominated reservoirs of magma called magma chambers where crystallization-differentiation has occurred by fractional crystallization[2,3,4]. When the concept of fractional crystallization is applied to layered intrusions, with or without a prior episode of assimilation, the conventional view is that magmatic evolution occurs within large, liquid-dominated melt reservoirs in the crust (i.e., magma chambers)[2,4] This viewpoint has previously driven petrologists to search for the existence of the solidified remnants of such bodies in the rock record. We infer that the internal differentiation of macrolayers into subsidiary layers of different modal proportions, including monomineralic units like chromitites and anorthosites, resulted from second-order effects like crystal sorting during emplacement

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Conclusion