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

Melt Generation and Fluid Flow in the Thermal Aureole of the Bushveld Complex 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 Ba 2+ , and to a lesser extent, Sr 2+ and Eu 2+ , 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 ( 87 Sr/ 86 Sr) 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 δ 18 O 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. Key words http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Petrology Oxford University Press

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

Loading next page...
 
/lp/oxford-university-press/melt-generation-and-fluid-flow-in-the-thermal-aureole-of-the-bushveld-vcSYyXylK0
Publisher
Oxford University Press
Copyright
Copyright © 2015 Oxford University Press
ISSN
0022-3530
eISSN
1460-2415
D.O.I.
10.1093/petrology/44.6.1031
Publisher site
See Article on Publisher Site

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 Ba 2+ , and to a lesser extent, Sr 2+ and Eu 2+ , 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 ( 87 Sr/ 86 Sr) 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 δ 18 O 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. Key words

Journal

Journal of PetrologyOxford University Press

Published: Jun 1, 2003

There are no references for this article.

You’re reading a free preview. Subscribe to read the entire article.


DeepDyve is your
personal research library

It’s your single place to instantly
discover and read the research
that matters to you.

Enjoy affordable access to
over 18 million articles from more than
15,000 peer-reviewed journals.

All for just $49/month

Explore the DeepDyve Library

Search

Query the DeepDyve database, plus search all of PubMed and Google Scholar seamlessly

Organize

Save any article or search result from DeepDyve, PubMed, and Google Scholar... all in one place.

Access

Get unlimited, online access to over 18 million full-text articles from more than 15,000 scientific journals.

Your journals are on DeepDyve

Read from thousands of the leading scholarly journals from SpringerNature, Elsevier, Wiley-Blackwell, Oxford University Press and more.

All the latest content is available, no embargo periods.

See the journals in your area

DeepDyve

Freelancer

DeepDyve

Pro

Price

FREE

$49/month
$360/year

Save searches from
Google Scholar,
PubMed

Create lists to
organize your research

Export lists, citations

Read DeepDyve articles

Abstract access only

Unlimited access to over
18 million full-text articles

Print

20 pages / month

PDF Discount

20% off