PETROGRAPHY AND CHEMISTRY OF PSEUDO-TACHYLITIC BRECCIAS FROM LARGE IMPACT STRUCTURES

Breccias in the Vredefort Dome (VD) include pseudotachylitic breccia (PTB) [1], impact melt breccia (Granophyre), and a single location with lithic breccia resembling the Footwall Breccia of Sudbury. PTB are the most common type of breccia in the VD [2, 3] and occur as microscopic veins and large breccia zones. Controversy remains about the genesis of these impact-induced melt breccias, with the most popular hypotheses being genesis by (1) shearing (friction melt = pseudotachylite); (2) shock compression melting; (3) decompression melting immediately after shock propagation through the target; (4) combinations of these processes, or (5) intrusion of allochthonous impact melt.

Tectonic pseudotachylite is widely known from fault/shear zones. The violent genesis of the dome would suggest that faulting/shearing related movement zones could occur in abundance, but the enormous breccia volumes observed in the dome seemingly can not be produced along fault/shear zones [4]. In this study, no textural evidence for a purely frictional origin of melt breccias was found in the basement rocks of the VD which is in agreement with the geological/petrographic findings of [5] for melt breccias in the core of the Araguinha impact structure, Brazil. Further, our results exclude PTB generation by intrusion of impact melt. As in the core of the VD, the thin melt veins in the Araguainha crystalline target rocks record compelling evidence of selective melting of minerals. First microchemical investigations of PTBs in the VD have indicated at the grain scale variable chemical composition in very thin PTB veinlets (< 1mm) - best explained by local shock melting (plus/minus friction) origin. However, for larger melt breccia occurrences it is likely that some melt was mobilized into dilational sites where pooling could occur. Another possible origin for the PTBs includes sudden decompression (e.g. [6]) following the rapid rise of the crust (decompression melting during the modification phase).

References: [1] Reimold, W.U., 1998. E.-Sci. Rev. 43:25-47; [2] Dressler B.O. & Reimold W.U. 2004. E.-Sci. Rev. 67: 1–60. [3] Reimold W.U. & Gibson R.L. 2006. GSA SP 405: pp. 407. [4] Melosh H.J., 2005. In: Impact Tectonics, Springer, Berlin, p.55-80. [5] Machado A. et al., GCA (rev. MS). [6] Martini, J.E.J., 1991. EPSL, 103 : 285-300.