129-4 Petrology of Quartzo-Feldspathic Suspect Pseudotachylyte from East Africa

Abstract:

Pseudotachylytes serve as unique markers of ancient and modern rheology. While historically thought to have primarily mafic matrix composition, we propose pseudotachylytes cross-cutting quartzo-feldspathic granitic host rock, especially when forming in fluid-rich environments, can exhibit primarily felsic matrices. We used EPMA to conduct EDS and BSE imaging on four samples of suspect pseudotachylyte collected along exhumed fault rocks in the Kgomodikae Shear Zone in Kaapvaal Craton, Botswana, and the Kilosa border fault in Tanzania. The host rocks along these fault zones lack common mafic minerals such as biotite and amphibole, which melt at significantly lower temperatures than quartz and feldspars and typically contribute to the composition of pseudotachylyte matrices. The Tanzania suspect pseudotachylyte matrix is composed primarily of quartz with streaked albite and oxides containing iron, titanium, and magnesium. The pseudotachylytes cut hydrothermal veins of kaolinites and rocks containing chloritized biotite. Younger iron oxide veins cut the pseudotachylytes. The Botswana suspect pseudotachylytes have a similar matrix of quartz and muscovite with Ti-silicates. The pseudotachylytes exploit the boundaries of and cut through quartz veins. Survivor clasts in both pseudotachylytes are composed primarily of 50-150 µm quartz and feldspars with embayments and semi-rounded edges. The pseudotachylyte matrices are likely recrystallized from a finer-grained precursor. The unusual felsic matrix suggests extremely high temperatures of shearing: up to 900C hotter than necessary to begin melting mafic biotites and amphiboles. Hydrothermal veins and hydrothermally altered minerals throughout the host rocks suggest that there may have been fluid present at the time of melt, lowering mineral melting points and mediating the necessary temperature increase to melt the host rock. Ongoing work examines titanium in quartz to obtain a better understanding of the temperature of formation of the host rocks.

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