Formation of pseudotachylytes under granulite facies conditions in the UHT In Ouzzal terrane (Western Hoggar, Algeria): Witnesses of paleoseism in the Paleoproterozoic lower continental crust at 0.82 Ga

Abstract

Archaean charnockitic orthogneisses transected by 2.0 Ga high-grade exhumed ductile shear zones form part of the ultra-high temperature In Ouzzal Terrane, located in the western Hoggar, within the Neoproterozoic Pan-African Trans-Saharan belt of north-west Africa. Faults and injection veins of pseudotachylytes exhibit sharp contacts with their host charnockitic orthogneisses and are composed of a dark, and sometimes banded matrix, containing numerous clasts of pyroxene, feldspar, quartz, and rarely biotite. Recrystallized mineral assemblages in the matrix are indicative of granulitic facies conditions and consist of very fine-grained crystals of hypersthene, phlogopite, oligoclase, orthoclase, quartz, and accessory Mn-ilmenite and Cr-magnetite. This assemblage differs from that of the charnockitic orthogneiss host notably by the absence of the mesoperthitic K-feldspar. The chemical compositions of the pseudotachylytes are in general similar to those of the charnockitic orthogneisses suggesting a non-selective melting process consistent with in situ and total recrystallization. The chemical variations are observed in Tin Tchik Tchik pseudotachylytes and shear planes, including slight enrichments of calcium in orthopyroxene, plagioclase, and K-feldspar, as well as iron in biotite. These variations are attributed to recrystallization under decreasing temperature during faster cooling. The newly formed granulitic-facies mineral phases within the pseudotachylytes are synchronous with brittle deformation, lining fractures parallel to major shear zones and occupying spaces created by T-fractures, or those between Riedel R and T fractures, or P and R'. This Riedel fracture network indicates NW-SE to WNW-ESE shortening.

The ⁴⁰Ar/³⁹Ar dating of the pseudotachylyte matrix indicates its formation at 820.5 ± 6.6 Ma, providing the first evidence of early Neoproterozoic tectonism in the In Ouzzal terrane. Regionally, this event corresponds to the convergence period between the West African Craton and East Saharan Craton leading to subduction and the development of oceanic/continental arcs in western Hoggar. Earlier polyphase dehydration of charnockitic orthogneiss, during Archaean partial melting events (2.65 and 2.5 Ga) and Eburnean (2.0 Ga) ultra-high temperature metamorphism, suggests a reactivation under brittle coseismic conditions of Paleoproterozoic ductile main shear zones at 0.82 Ga in a nearly anhydrous lower crust and provides a natural example of earthquakes in the lower continental crust, controlled by a brittle rheology.