Volume loss, fluid flow, and coaxial versus noncoaxial deformation in retrograde, amphibolite facies shear zones, northern Malawi, east-central Africa

Authors
Citation
U. Ring, Volume loss, fluid flow, and coaxial versus noncoaxial deformation in retrograde, amphibolite facies shear zones, northern Malawi, east-central Africa, GEOL S AM B, 111(1), 1999, pp. 123-142
Citations number
55
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Earth Sciences
Journal title
GEOLOGICAL SOCIETY OF AMERICA BULLETIN
ISSN journal
0016-7606 → ACNP
Volume
111
Issue
1
Year of publication
1999
Pages
123 - 142
Database
ISI
SICI code
0016-7606(199901)111:1<123:VLFFAC>2.0.ZU;2-9
Abstract
Two retrograde, amphibolite facies shear zones were studied to explore the relationship between retrograde mineral reactions, volume strain, fluid flo w, mylonitization, and coaxial versus noncoaxial deformation. The two shear zones are the contractional Mafwewu Hills shear zone and the transcurrentl y displacing Mkamasa River shear zone of northern Malawi. In general, shear -zone formation is characterized by the breakdown of feldspar and biotite a nd the formation of sillimanite, quartz, and water. Silica, alkali, and alk ali earth elements were mobile. Mass-balance calculations, based on major- and trace-element geochemistry, indicate as much as 50%-60% volume loss in mylonite. Fluid to rock ratios estimated from the calculated depletions of silica are as much as 200-400, indicating that the initiation and activity of the shear zones were accompanied by large amounts of fluids that infiltr ated the shear zones. These fluids caused an almost complete loss of alkali es within the shear zone, leading to the development of almost "dry" and sh eet-silicate-free mylonite. It is proposed that the associated destabilizat ion of micas destroyed the pathways for fluid circulation and ultimately ca used the cessation of shearing. In the Mafwewu Hills shear zone, shear-zone-related stretching lineations a re oriented parallel to tectonic transport. In the Mkamasa River shear zone , the stretching lineations curve from a subhorizontal attitude in the wall rock into a subvertical attitude in mylonite characterized by large volume loss. Shear bands in the Mkamasa River shear zone indicate a sinistral sen se of shear. A synoptic analysis of mesoscale S and Z asymmetric folds indi cates that the orientation of the stretching lineations in mylonite charact erized by large volume loss is subperpendicular to the regional tectonic tr ansport direction. Aspect ratios of feldspar and quartz in XZ sections (X g reater than or equal to Y greater than or equal to Z, principal axes of fin ite-strain ellipsoid) show oblate grain shapes and show a positive correlat ion with volume strain. Mean kinematic vorticity numbers are 0.26-0.44 in t he wall rock and 0.57-0.79 in mylonite. In mylonite, small kinematic vortic ity numbers correlate with large volume loss. The transport-perpendicular o rientation of the stretching lineations in strongly volume-deficient myloni te is thought to be the result of a pronounced component of shortening norm al to the shear zone; the resulting higher degree of coaxial deformation co ntrolled the final orientation of the stretching lineations.