Multi-scale magnetic stratification of an ultramafic-mafic complex: Example of the Great Dyke of Zimbabwe and implications for magmatic differentiation.

摘要

Layered mafic intrusions represent an important aspect of magmatism on earth and have occurred from Archean to present times. Literature on the geochemistry and petrology of these intrusions abounds but their physical properties, which could provide significant constraints on their formation, have seldom been investigated. Classic petrological methods such as whole-rock geochemistry, textural analysis and mineral chemistry have been applied to several intrusions of various ages. Most of these methods are relatively expensive or time intensive which limits high resolution studies. On the contrary, magnetic methods are typically inexpensive and fast and have been successfully applied to various occurrences of mafic rocks. In this study, several magnetic methods have been applied to a 600 m-long continuous borehole core drilled through one of the world's largest layered mafic intrusion, the Great Dyke of Zimbabwe. The main goal of this study is to constrain the magmatic history of the intrusion. More specifically, it is important to determine if the intrusion functioned as an open system, characterized by multiple magma pulses, or as a closed system, undergoing differentiation after a single magmatic pulse. The magnetic methods have also been validated by other independent approaches including image analysis, and electron microprobe. This study demonstrates that magnetic methods can be used to rapidly obtain critical information on the internal structure of this type of intrusion before applying more costly chemical analyses. The main scientific result of this study is to document the closed system nature of the Great Dyke of Zimbabwe, at least throughout the sequence investigated.