Abstract
The Archean crust in the Barberton area, on the eastern side of the Kaapvaal Craton, South Africa, consists of a variety of granitic rocks ranging in age from ca. 3600 to 2500 Ma. A suite of tonalite-trondhjemite-granodiorite (TTG) intrusions has been divided into an older group (ca. 3400 Ma) and a younger group (ca. 3200 Ma) and form a set of domal plutons or “cells” that have similar petrological, geochemical, and structural characteristics. The Theespruit Pluton, one of the older trondhjemite cells, dated between 3443 and 3437 Ma, displays a number of unique features that distinguishes it from others in the region. These features include transgressive as well as concordant contact relationships with the enveloping ca. 3530 Ma amphibolite-facies metavolcanic rocks of the lower part of the Onverwacht Group (Sandspruit and Theespruit formations) of the Barberton greenstone belt. The trondhjemites further display a variety of textures ranging from homogeneous cumulate rocks to foliated, lineated and banded gneissic granitoids. Amphibolite xenoliths occur commonly throughout the pluton and in many cases show little or no effects of strain, thereby contributing to the interpretation that these rocks represent roof pendants stoped from the overlying supracrustal volcanic rocks. Some xenoliths and pluton contact areas have experienced hydraulic fracturing accompanied by brittle deformation, brecciation and agmatite development. Locally, the breccias have experienced additional transformation to dioritic rocks following assimilation, metasomatism and hybridization resulting from trondhjemite magma injection and late-stage hydrothermal activity in the apical portion of what is interpreted to be a high-level granitic intrusion. At the level exposed, the pluton is geochemically and mineralogically uniform throughout, consisting of plagioclase-quartz-biotite rocks (trondhjemite) with only minor plagioclase-quartz-biotite-hornblende rocks (tonalites, diorites) developed near assimilated amphibolite xenoliths. Views on the emplacement style of the pluton are considered and processes akin to diapirism are favored over alternate views suggesting that the Barberton TTG plutons formed as a consequence of horizontal thrust-accretionary processes.
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