The petrogenesis of Merensky Reef potholes at the Western Platinum Mine, Bushveld Complex: Sr-isotopic evidence for synmagmatic deformation

Citation
Hw. Carr et al., The petrogenesis of Merensky Reef potholes at the Western Platinum Mine, Bushveld Complex: Sr-isotopic evidence for synmagmatic deformation, MIN DEPOSIT, 34(4), 1999, pp. 335-347
Citations number
32
Categorie Soggetti
Earth Sciences
Journal title
MINERALIUM DEPOSITA
ISSN journal
00264598 → ACNP
Volume
34
Issue
4
Year of publication
1999
Pages
335 - 347
Database
ISI
SICI code
0026-4598(199905)34:4<335:TPOMRP>2.0.ZU;2-S
Abstract
Potholes represent areas where the normally planar PGE-rich Merensky Reef o f the upper Critical Zone of the Bushveld Complex transgresses its footwall , such geometric relationships being unusual in layered intrusions. The rec ognition of vertical dykes of Merensky pyroxenite in the footwall suggests downward collapse of crystal mush into pull-apart sites resulting from tens ional deformation due to the loading effects of major new magma additions. In contrast, crosscutting anorthosite veins display physical and isotopic e vidence of upward emplacement. The Merensky Reef and its footwall have dist inct initial Sr-isotope ratios (R-0 > 0.7066 and < 0.7066, respectively), w hich may be used to constrain these processes related to pothole formation. Merensky Reef in potholes (R-0 = 0.7069-0.7078) shows no isotopic evidence of assimilation of, or reaction with, footwall material. Discrete, discord ant replacement bodies of anorthosite extend from the footwall lithologies to cross-cut the Merensky Reef and its hanging wall. The initial Sr-isotope ratio in these replaced rocks is totally reset to footwall values (R-0 = 0 .7066), and immediately adjacent stratiform lithologies are slightly modifi ed towards footwall values. In contrast, Neptunian pyroxenitic (Merensky) d ykes cross-cutting the footwall lithologies: with a large surface area to v olume ratio, and low Sr content, do not display footwall-like Sr-isotope in itial-ratios (R-0 = 0.7077), and thus show no evidence for assimilation of or reaction with footwall material. Furthermore, pegmatoidal replacement py roxenite ("replacement pegmatoid"), at the base of the Merensky Reef within potholes, has a high initial-ratio (R-0 > 0.7071), and so models of pervas ive metasomatism by footwall material are not applicable. This isotopic evi dence indicates that there was no active interaction of footwall material w ith the overlying magma during, or after, the formation of Merensky Reef po tholes, a basic tenet of existing pothole formation hypotheses involving fo otwall mass-transfer. In contrast, the isotopic data are entirely consisten t with an extensional model for pothole formation, with the more radiogenic Merensky magma migrating laterally to fill extensional zones in the footwa ll layers.