Exhumation of Mesoarchean TTG gneisses from the middle crust: Insights from the Steynsdorp core complex, Barberton granitoid-greenstone terrain, South Africa

Abstract

The processes leading to the assembly of supracrustal greenstone sequences and plutonic rocks of the trondhjemite-tonalite-granodiorite (TTG) suite, the two main building blocks of all Archean cratons, form a central aspect of our understanding of early continental growth. Key metamorphic and structural data are presented from granitoid-greenstone contacts in the Steynsdorp dome, the oldest and one of the best preserved parts of the Mesoarchean Barberton granitoid-greenstone in South Africa. Pressure and temperature estimates of P = 10–13 kbar and T = 640–660 °C from supracrustal rocks of the Steynsdorp dome indicate the burial of these rocks to depths >30–40 km during the main phase of collisional (D2) tectonics recorded in the terrane. Peak metamorphic assemblages define flattening-type fabrics, indicating crustal stacking of cool, rigid crust. The subsequent retrogression of rocks is associated with constrictional fabrics that are interpreted to have formed in response to the orogen-parallel extrusion and exhumation of rocks. The progressive retrogression of rocks and associated fabric development, unidirectional lineation and fold plunges, and consistent granitoid-up, greenstone-down kinematic indicators point to the exhumation of the TTG gneisses along an extensional detachment below the low-grade Barberton greenstone belt. The results are consistent with findings along the granitoid-greenstone contacts some 40 km west of the Steynsdorp area, and they indicate the regional extent of this metamorphic core complex and the allochthonous nature of the high-pressure, low-temperature terrane with respect to the rest of the greenstone belt. This also implies that the 3.45 Ga suite of TTG rocks south of the Barberton greenstone belt is unlikely to have represented the source of similar-aged volcanic rocks in the belt. More voluminous and larger extent of 3.45 Ga TTG plutonism represents a significant early crust-forming event in the Barberton terrain, the evidence of which has been obliterated during later episodes of tectonomagmatic recycling. The existence of the high-pressure, low-temperature terrane in the southern Barberton granitoid-greenstone terrain indicates the presence of a cool and rigid continental nucleus in the Mesoarchean around which plate-tectonic processes could initiate.