[Trumbull] How was the Bushveld complex assembled? Pulsed intrusions, crystallization conditions and the origin of layering in the Main Zone
German Title: How was the Bushveld complex assembled? Pulsed intrusions, crystallization conditions and the origin of layering in the Main Zone
Current Status: approved
Main Applicant:Dr. Robert Trumbull
Begin: 1 April, 2020
Conveyor End: 30 September, 2022
Conveyor Duration: 30
The Bushveld complex in South Africa is the largest igneous intrusion on earth and the richest in terms of mineral wealth. Apart from its size,
the most striking feature of the complex is the strong lithologic layering. Although studied for decades, the origin of layering is still debated. The aim of this project is to test a promising recent concept of multiple intrusions of a crystal-rich magma "slurry" from an external magma chamber. This was inspired by detailed study of vertical drillcores in the Main Zone of the Bushveld north limb. The present project will provide the first detailed and nearly- continuous vertical profile (2700 meters) of the same horizon in the eastern Limb, based on drill cores that were donated to the ICDP project by Implats mining company. The overarching objective is to test if the layering patterns discovered in the northern limb are a general feature of the Main Zone, and if so, whether differences in the number of layers and/or variations in their thickness and composition can help reveal how the layering formed. The working hypothesis is that the Main Zone was assembled by repeated, separate magma injections from a deeper “staging” chamber. By analogy with the northern limb, evidence of the new injections should be preserved in physical properties of the rock succession (density variations) and in compositional features of the minerals involved (An-content of plagioclase, Mg# of pyroxenes) and in the bulk-rock geochemistry and Sr-isotope ratios. In addition to these parameters we will apply new approaches to the problem of the Main Zone assembly (and cooling) by determining the equilibration temperatures of REE and Ca-Mg in coexisting minerals and evaluating of degree of textural maturity (grain boundary geometries).
These methods will be applied systematically along the vertical profile. If evidence for repeated intrusions is not found and the Main Zone is deduced to have formed from just one or two intrusive pulses, the results will still provide a record of the subsequent magma differentiation and crystallization of the Main Zone. The research proposed will focus on several complementary topics:
- Testing for evidence of one or more intrusive pulses in the Main Zone by density and mineral-chemistry variations;
- Evaluate the degree of chemical and textural re-equilibration of the rocks and the vertical variations in those properties, taking account lithologic layering and (if present) intrusive cycles;
- Calculate equilibration temperatures of rockforming minerals and estimate cooling rates at different levels in the 2700 meter-thick sequence.