Petrology and microthermometry of aluminous rocks in the Botswanan Limpopo Central Zone: evidence for isothermal decompression and isobaric cooling

Abstract

Phase analysis in the model K2O‐poor aluminous rock system (FMASH) illustrates the following sequence of reactions during retrograde metamorphism in the Botswanan Limpopo Central Zone: Opx+Sil+Qtz=Crd , Opx+Sil=Spr+Crd , Grt+Qtz=Opx+Crd ; Opx+Crd+W=Ged+Qtz , Grt+Opx+Crd+W=Ged ; and Grt+Qtz+W=Ged+Crd . A quantitative petrogenetic grid with phase relations shows that sapphirine results from nearly isothermal decompression in the quartz‐undersaturated portions of the grid, and that gedrite formation by reactions (4)–(6) records isobaric cooling from high temperature (c. 800° C) after the decompression. Conditions for hydration in the western part of the area were 700–800° C and c. 6 kbar, based on microthermometric data and the available garnet–cordierite geothermometer. On the basis of these conditions and predicted thermodynamic properties of gedrite, phase relations in T–XMg space were constructed to investigate the isobaric cooling event. The results are in good agreement with the hydration P–T path. Further, the T–XMg topologies show that hydration of orthopyroxene in the central part of the area (reaction 4) occurred at about 800° C and c. 5 kbar. Therefore, we conclude that the Botswanan Limpopo Central Zone has suffered isothermal decompression, similar to the Central Zone in South Africa and Zimbabwe, followed by isobaric cooling. The isobaric cooling event in the western (at c. 6 kbar) and central (at c. 5 kbar) parts of the area commenced at nearly the same temperature (c. 800° C), and appear to be consistent with a tectonic model that involved westward movement (thrusting) of the Central Zone.

Conflicting P–T paths within the Central Zone of the Limpopo Belt: A consequence of different thermobarometric methods?

“…6A-D) are consistent with the modes, compositions and zonations in the observed mineral assemblages, which define a P-T path that traverses from peak conditions of 8-9 kbar and 840 +/À 10°C along a decompression cooling path to 4.5-5.5 kbar and $650°C. The peak P-T conditions from this study are in agreement with those determined by Watkeys et al (1983), Windley et al (1984), Droop (1989), Hisada and Miyano (1996) and Perchuk et al (2000). The retrograde path, characterized by decompression cooling, is in agreement with the paths defined by Zeh et al (2004), Van Reenen et al (2004), and Boshoff et al (2006) and the DC1 path of Perchuk et al (2008).…”

“…The retrograde path, characterized by decompression cooling, is in agreement with the paths defined by Zeh et al (2004), Van Reenen et al (2004), and Boshoff et al (2006) and the DC1 path of Perchuk et al (2008). The path is, however, inconsistent with isothermal decompression (Droop, 1989) followed by isobaric cooling (Hisada and Miyano, 1996). Retrograde paths characterized by ITD (Droop, 1989) have been used as evidence to support a 'pop-up' style of exhumation for the CZ (e.g.…”

“…2). The peak metamorphic P-T conditions agree well with the peak P-T estimates obtained by Droop (1989) and Hisada and Miyano (1996). However, the retrograde path, defined by matrix assemblages, garnet resorption textures, mineral zoning and diffusion patterns (Zeh et al, 2004) requires a simultaneous P-T decrease, in agreement with the DC paths defined by Perchuk et al (2000), Van Reenen et al (2004), Hisada et al (2005) and Rigby et al (2008b) but in stark contrast to retrograde paths defined by ITD (e.g.…”

“…However, the retrograde path, defined by matrix assemblages, garnet resorption textures, mineral zoning and diffusion patterns (Zeh et al, 2004) requires a simultaneous P-T decrease, in agreement with the DC paths defined by Perchuk et al (2000), Van Reenen et al (2004), Hisada et al (2005) and Rigby et al (2008b) but in stark contrast to retrograde paths defined by ITD (e.g. Droop, 1989;Hisada and Miyano, 1996) and subsequent IBC (Hisada and Miyano, 1996).…”

“…Whereas, the relative timing, style and extent of metamorphic activity in CZ has, over the years, come into question by numerous workers (e.g. Harris and Holland, 1984;Droop, 1989;Hisada and Miyano, 1996;Jaeckel et al, 1997;Holzer et al, 1998;Kröner et al, 1998Kröner et al, , 1999Smit et al, 2001;Van Reenen et al, 2004;Zeh et al, 2004;Zeh et al, 2005a,b;Hisada et al, 2005;Boshoff et al, 2006;Zeh et al, 2007;Rigby et al, 2008b;Gerdes and Zeh, 2009;Van Reenen et al, 2008;Perchuk et al 2008) who have defined different P-T paths. This has ultimately led to a great deal of controversy surrounding the tectonic evolution of the Limpopo Belt (Rigby et al, 2008a).…”