The Open UniversitySkip to content
 

Melt Generation and Fluid Flow in the Thermal Aureole of the Bushveld Complex

Harris, Nigel; McMillan, Andy; Holness, Marian; Uken, Ron; Watkeys, Mike; Rogers, Nick and Fallick, Anthony (2003). Melt Generation and Fluid Flow in the Thermal Aureole of the Bushveld Complex. Journal of Petrology, 44(6) pp. 1031–1054.

DOI (Digital Object Identifier) Link: http://dx.doi.org/10.1093/petrology/44.6.1031
Google Scholar: Look up in Google Scholar

Abstract

Granite sheets emplaced into the migmatite zone of the eastern contact aureole of the Bushveld Complex resulted from fluid-enhanced, incongruent biotite melting of the underlying Silverton Formation shales during prograde metamorphism. Ba concentrations are extreme in both the sheets (>1000 ppm) and the hornfels (>800 ppm) into which they have been emplaced. We conclude that a Ba-rich, hydrothermal fluid induced melting in the aureole, and that fluid transport of Ba2+, and to a lesser extent, Sr2+ and Eu2+, persisted in the melt zones under subsolidus conditions. Sr-isotope systematics from high-Ba localities define an errorchron of 2161 ± 106 Ma with an initial (87Sr/86Sr) ratio of 0·705 ± 0·001. Metasedimentary rocks unaffected by fluid infiltration were homogenized at the same time but with an increased initial ratio, suggesting that whereas isotope homogenization was achieved between outcrops permeated by fluids, there is no evidence of regional homogenization. Oxygen-isotope compositions of psammitic metasediments in the aureole are uncorrelated with distance from the contact, suggesting the infiltrating fluid equilibrated isotopically with the metasediments. Their elevated 18O values (11·3–12·1) are consistent with a fluid source from devolatilization of the sedimentary lithologies. Textural analysis at both outcrop and thin-section scale of arkose and psammites in the aureole (Lakenvalei and Magaliesberg Formations) shows that the extent of melting was highly heterogeneous, even on the grain scale, and resulted from heterogeneously distributed infiltration of aqueous fluid on dilatant cracks and grain boundaries. Cathodoluminescence imaging of quartz shows a marked difference in the amount of fine structure, with that in the melted rocks having uniform luminescence in contrast to that in rocks containing little or no melt, which preserve textures inherited from the regionally metamorphosed protolith. Simple finite-difference thermal modelling of the aureole suggests that the width of the melt zone (>500 m) is inconsistent with conductive heat transfer, and hence that the thermal structure has been modified by fluid advection.

Item Type: Journal Article
ISSN: 0022-3530
Extra Information: Some of the symbols may not have transferred correctly into this bibliographic record and/or abstract.
Keywords: thermal aureole; crustal melting; fluid infiltration; isotope homogenization
Academic Unit/Department: Science > Environment, Earth and Ecosystems
Interdisciplinary Research Centre: Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR)
Item ID: 3964
Depositing User: Nigel Harris
Date Deposited: 04 Jul 2006
Last Modified: 02 Dec 2010 19:50
URI: http://oro.open.ac.uk/id/eprint/3964
Share this page:

Actions (login may be required)

View Item
Report issue / request change

Policies | Disclaimer

© The Open University   + 44 (0)870 333 4340   general-enquiries@open.ac.uk