Title

Proterozoic transpressive deformation in the Northern Marginal Zone, Limpopo Belt, Zimbabwe

Publication Title

The Journal of Geology

Publication Date

9-1995

Document Type

Article

DOI

10.1086/629772

Disciplines

Geochemistry | Geology

Abstract

A major tectono-metamorphic event affected the southern half of the Northern Marginal Zone of the Limpopo Belt at 2.0 Ga. Structural analysis along a 43 km-long profile perpendicular to strike shows extension inclined toward the northeast, extension parallel to axes of folds that uplift the southeast of the section (i.e., the Central Zone of the Limpopo Belt), and dextral shear. These deformation patterns developed coevally. The lineations form distinct, tightly clustered groups that occur together in the profile: there is no evidence for superimposed deformation or deformed lineations, nor are lineations observed cross-cutting each other. This single episode of deformation can therefore be called transpression. Petrographic and thermobarometric evidence support this kinematic picture, in that decreases of both pressure (2.5 kb) and temperature (150°C) are documented from southeast to northwest across strike. 39Ar40Ar step-heating of metamorphic hornblende from amphibolites has yielded ages of 2.00 Ga in the northwest; based on a redefinition of closure temperatures in amphiboles, we propose that this is the age of the peak conditions (6.5 kb, 600-650°C) of the transpressional episode. This age is in agreement with existing Pb/Pb and Sm/Nd garnet ages for the dextral shear in the southeast. In contrast, biotite in the northwest yields an Rb/Sr age of 1.86 Ga and coupled with existing garnet and sphene data, this suggests a post-transpressional cooling rate of 1.0 ± 0.5K/m.y. We obtained five near-plateau ages between 2.00 and 1.94 Ga in chemically diverse hornblendes from adjacent amphibolites. Because the hornblendes show negligible evidence for mineralogical and microstructural recrystallization, this age spread probably reflects chemically dependent diffusivity differences upon uniform slow cooling; it can be quantitatively modeled using the concept of "ionic porosity," Z. In our samples, Z ranges from 36.7% to 37.8%; the extreme values correspond to a 41 ± 9K difference in the model closure temperature. This translates into a model cooling rate of 0.7 ± 0.9 0.3 K/m.y . Within error this agrees with the independent cooling rate from garnet, sphene, and biotite. Biotites in the southeastern block yield an Rb/Sr age of 1.97 Ga, suggesting faster cooling than in the northeast. Contrasting cooling histories north and south of the studied Transition Zone following a 2.00 Ga tectonometamorphism entirely independent of and superimposed on the late Archean event negates the hypothesis of slow cooling after a supposed 2.65 Ga Tibetan-style orogeny. Vital parts of the evidence for such late Archean continental collision models for the Limpopo Belt can now be shown to be early Proterozoic in age.

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