Transcurrent shearing, granite sheeting and the incremental construction of the tabular 3.1 Ga Mpuluzi batholith, Barberton granite-greenstone terrane, South Africa

Journal of the Geological Society, Mar 2005 by Westraat, Janus D, Kisters, Alexander F M, Poujol, Marc, Stevens, Gary

Abstract:

Structural, petrographic and geochronological studies show that the tabular 3.1 Ga Mpuluzi batholith in the Barberton granite-gneiss terrane in South Africa was emplaced via a combination of external and internal processes. External structural controls are indicated by systematic variations in intrusive relationships and strain along the margins of the Mpuluzi batholith and are consistent with an emplacement of the granite in a dilational jog within a NE-ENE-trending system of dextral transcurrent synmagmatic shear zones. Internally, the Mpuluzi batholith is essentially made up of granite sheets. The structurally higher parts of the granite are made up of shallowly dipping sheets that are underlain by an anastomosing network of steeply dipping, variably deformed dykes and sheets. These granite sheets at lower structural levels intruded either into the actively deforming shear zones or into extensional sectors between and along the bounding shear zones. Multiple intrusive relationships and geochronological evidence suggests that granite sheeting and the assembly of the pluton occurred over a period of 3-13 Ma. The spatial and temporal relationship between deformation and magma emplacement reflects episodes of incremental dilation related to deformation along the bounding shear zones and granite sheeting. The transition to the mainly subhorizontal granite sheets at higher structural levels of the tabular Mpuluzi batholith indicates the intrusion of the granites during subhorizontal regional shortening, where the reorientation of the minimum normal stress to vertical attitudes at the shallow levels of emplacement allowed for vertical dilation and subhorizontal emplacement of the granite sheets.

Keywords: Archaean, Mpuluzi batholith, granites, shear zones, absolute age.

The petrogenesis of granites is almost invariably linked to active erogenic settings and the transport and emplacement of granitic magmas is now widely recognized to be aided and/or controlled by regional-scale structures such as fault and shear zones, fold structures or regional fabric patterns (e.g. Hutton 1988; Paterson & Fowler 1993; Collins & Sawyer 1996; Clemens et al. 1997; Petford et al. 2000). The intrusion of granitoids along and into actively deforming wall rocks presents an elegant solution to the so-called space problem of granite emplacement in that deformation potentially creates regions of localized dilation in a variety of kinematic scenarios, including extensional, convergent and wrench-tectonic environments (e.g. Guineberteau et al. 1987; Hutton & Ingram 1992; Tikoff & Teyssier 1992; Vauchez et al. 1997; Brown & Solar 1998). One of the most widely used approaches to decipher the actual mechanisms of granite emplacement is the structural analysis of wall-rock strains in the strain aureole of granites and within the granites themselves (Paterson et al. 1989; Ramsay 1989). However, a distinction between regional strains related to, for example, shear zones or regional foliation patterns that may have controlled granite emplacement, and emplacement-related strains caused by the granitoids themselves, such as granite ballooning and the displacement of wall rocks, is commonly difficult (Cruden 1998). In both cases, the superimposition of regional and intrusion-induced strains is common, and granite emplacement is, in most cases, achieved through multiple mechanisms that can be both of a regional and a more local nature (Paterson & Fowler 1993).

By far the most prolific period of granite production and crustal differentiation was the Archaean, when significant parts of the present continents were formed during accretionary tectonic events and associated short-lived but voluminous episodes of granitoid magmatism. The actual nature of events that prompted the production of these vast amounts of granitoids is as ambiguous and controversial as the modes of emplacement of the granitoid magmas. It is, thus, not surprising that Archaean cratons and their granite-greenstone terranes have often been at the centre of the debate about granite ascent and emplacement mechanisms (e.g. Ramsay 1989; Jelsma et al. 1993; Ridley et al. 1997; Van Kranendonk et al. 2004). The Palaeo- to Mesoarchaean Barberton granite-greenstone terrane in the Kaapvaal Craton in South Africa (Fig. 1) has featured very prominently in this debate (Viljoen & Viljoen 1969; Anhaeusser 1973; De Wit et al. 1992). This composite granite-greenstone terrane was assembled during several tectonomagmatic episodes between c. 3.5 and 3.1 Ga (e.g. Anhaeusser & Robb 1980; Robb & Anhaeusser 1983; Armstrong et al. 1990; De Ronde & De Wit 1994; Kamo & Davis 1994). Earlier, c. 3.5-3.2 Ga plutonic suites are characterized by trondhjemites, tonalites and granodiorites. These rocks, collectively referred to as the TTG suite, form typically relatively small (

This study focuses on laterally extensive granite plutons of a subsequent magmatic episode associated with the intrusion of vast amounts of granodiorites, monzogranites and syenites, the GMS suite, at c. 3.1 Ga. Rocks of the GMS suite are found not only in the Barberton granite-greenstone terrane, but also over large parts of the Kaapvaal Craton, and their emplacement coincides with the first stabilization of the central parts of the craton (De Wit et al. 1992; Kamo & Davis 1994; Poujol & Anhaeusser 2001). The GMS suite in the Barberton granite-greenstone terrane shows very different internal and external characteristics from the earlier TTG suite. Individual plutons may cover several thousand square kilometres and these composite granitoid bodies have traditionally been referred to as batholiths, alluding to their compositionally and texturally heterogeneous nature and enormous areal extent (e.g. Anhaeusser et al. 1981). For the most part, the plutons appear undeformed, intrusion-related wall-rock strains are only locally recorded, and intrusive relationships with wall rocks are commonly sharply discordant (e.g. Hunter 1973; Anhaeusser & Robb 1983; Robb et al. 1983). Regional studies have demonstrated that most of these granitoids represent subhorizontal, sheet-like intrusions. The tabular granites are commonly underlain by so-called migmatite terranes and dyke complexes that have tentatively been interpreted as the feeders to the overlying granite sheets (e.g. Hunter 1957, 1973; Anhaeusser et al. 1981; Anhaeusser & Robb 1983; Robb et al. 1983). The sum of these features has traditionally been interpreted to indicate a 'passive', post-tectonic and anorogenic emplacement of the granitoids (e.g. Anhaeusser & Robb 1983). This interpretation has not remained unchallenged, and Robb et al. (1983) and Jackson & Robertson (1983) described the presence of regional-scale gneiss belts within and along the margins of the batholiths. The multiphase intrusive relationships between basement gneisses and the GMS suite and deformation of the potassic granitoids suggests that the emplacement of the 3.1 Ga granitoids is, at least partly, structurally controlled. As a result of these contrasting views on the contact relationships and the lack of detailed structural work on the large batholiths, the emplacement and tectonic setting of the cratonwide plutonic suite have remained somewhat enigmatic.