Tectonic evolution of a greenstone sequence in northern Zimbabwe: Sequential early stacking and pluton diapirism

Abstract

Structural-metamorphic relationships in the 2.71-2.64 Ga Harare-Shamva greenstone belt in northern Zimbabwe demonstrate that the Archean stratigraphy of the Zimbabwe craton can not be regarded as a coherent and nonrepetitive, autochthonous succession of (ultra)mafic and felsic greenstones. The greenstone belt shows an early set of amphibolite-grade D 1 /M 1 structures and associated synkinematic metamorphic assemblages that were formed during an episode of layer-parallel shearing at about 2.64 Ga. This resulted in sequential west directed imbricate stacking and recumbent folding of distinct tectonostratigraphic greenstone sequences and includes the development of a syntectonic sedimentary succession. D 2 /M 2 fabrics, strain patterns, and contact aureoles overprint D 1 geometries and are related to the diapiric emplacement of the Chinamora batholith. The structural data indicate that granite-gneiss terrains can be subdivided into two types: (1) structurally emplaced terrains (the Madziwa gneiss terrain) and (2) diapiric domes (the Chinamora batholith). The history of the Harare-Shamva greenstone belt indicates that the late Archean (2.7-2.6 Ga) evolution of the Zimbabwe craton must allow for accretion of diverse crustal fragments including oceanic and back arc mafic crust, volcanic arc felsic crust, continental crust, and related sedimentary sequences. The process of accretion was probably diachronous across the craton and may have involved concomitant diapirism, strike-slip faulting, magma intrusion, and sedimentation. Copyright 2000 by the American Geophysical Union.