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Petrology and microthermometry of aluminous rocks in the Botswanan Limpopo Central Zone: evidence for isothermal decompression and isobaric cooling
Author(s) -
HISADA K.,
MIYANO T.
Publication year - 1996
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1046/j.1525-1314.1996.05857.x
Subject(s) - isobaric process , geology , isothermal process , metamorphism , geochemistry , cordierite , mineralogy , quartz , thermodynamics , materials science , metallurgy , paleontology , ceramic , physics
Phase analysis in the model K 2 O‐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–X Mg space were constructed to investigate the isobaric cooling event. The results are in good agreement with the hydration P–T path. Further, the T–X Mg 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.