Revisiting the age and paleomagnetism of the Modipe Gabbro of South Africa

Highlights

• Tectonic styles and geophysical processes were different in the Archean.

• The 2.78 Ga Modipe Gabbro of South Africa offers a window into the Archean.

• We present new paleomagnetic and geochronological data for the Modipe Gabbro.

• We link the Modipe Gabbro to the oldest fully differentiated magmatic event in the world.

• The Modipe Gabbro's well-preserved magnetization offers insights into cratonic configuration in the Archean.

Abstract

The Modipe Gabbro is a Precambrian intrusive unit hosted by the Kaapvaal craton of South Africa and provides one of the oldest records of geomagnetic field behavior in the late Archean. Earlier paleomagnetic research into the Modipe Gabbro provided valuable insight into its magnetic properties and mineralogy, but predate the development of paleomagnetic tools that are important in unraveling potentially complex histories of Precambrian rocks. Further, a lack of information about the precise age of the intrusion prevented this earlier paleomagnetic study from being used to evaluate the tectonic history of the Kaapvaal craton. Here we present a comprehensive paleomagnetic and geochronologic study of the Modipe Gabbro that complements and builds upon the existing work. The emplacement age of the gabbro (2784.0 ± 1.0 Ma), established using the U–Pb ID-TIMS method on baddeleyite, links the intrusion to nearby igneous bodies in the Kaapvaal craton, establishing one of the oldest fully differentiated magmatic events in the geologic record. The precise age of the Modipe Gabbro and field observations about its strike allow us to perform a tilt correction using the both the attitude and paleomagnetism of the nearby Derdepoort basalts. The tilt-corrected data allow a comparison of Kaapvaal craton's paleolatitude with that of the Pilbara craton at 2.78 Ga, with implications for the existence of the supercraton Vaalbara.

Introduction

The transition from the Archean to the Paleoproterozoic (3.0–2.5 Ga) is a key period in Earth's geophysical and geochemical evolution. In particular, this period encompasses an episode of substantial continental crustal growth and preservation, thought to be related to periodic mantle overturn events and extensive plume volcanism (Condie, 2001). This reconstitution of the upper mantle by deeper hotter mantle was followed by, or was synchronous with, the assembly and breakup of some of the planet's earliest supercontinents or supercratons (Nance et al., 1986, Williams et al., 1991, Young et al., 2001, Cheney, 1996, Aspler and Chiarenzelli, 1998, Condie, 2001, Barley et al., 2005), and the development of oxygenated conditions in the atmosphere. Establishing links between these major environmental changes and the suspected mantle-plume tectonic regime that characterized the late Archean is an ongoing research focus that is often hampered by the absence of reliable paleomagnetic and geochronologic constraints.

One of the earliest and most influential paleomagnetic studies on Archean rocks was that of Evans and McElhinny (1966), who examined the Modipe Gabbro in the Kaapvaal craton. This intrusive unit outcrops as a chain of low rising hills, or inselbergs, approximately 15 km long extending from southern Botswana into northern South Africa (Fig. 1). The Modipe Gabbro was emplaced as a sub-horizontal, sill-like body and the low-lying areas between the gabbro inselbergs consist of soils derived from the Gaborone Granite (McElhinny, 1966). Although exposures between the gabbro and the granite are exceedingly rare, Evans and McElhinny (1966) reported a sheared contact between the two lithologies that suggested that the gabbro was intruded by the granite. Importantly, they reported a positive baked contact test between the two lithologies. They also noted exceptionally stable magnetizations in the gabbro with high coercivities (≥180 mT) and high unblocking temperatures (>500 °C) and identified elongate magnetite inclusions exsolved within clinopyroxene as the main carrier of remanence (Evans et al., 1968). Subsequent work on similar occurrences of magnetite exsolved from clinopyroxenes has shown that they are robust recorders of the geomagnetic field (e.g., Renne et al., 2002, Feinberg et al., 2005, Muxworthy et al., 2013). However, these exsolved magnetite crystals are not ubiquitous throughout the Modipe Gabbro, and identical, stable remanences are found in those parts of the intrusion that do not contain them. There is, therefore, at least one other primary magnetic phase present, and we have identified it as part of this study. There are no clear paleohorizontal indicators within the Modipe Gabbro, which precludes evaluating whether a tilt-correction to paleomagnetic directions is needed. Evans and McElhinny (1966) established a paleolatitude of 80° for the Modipe Gabbro without using any form of tilt correction, and the corresponding virtual geomagnetic pole (VGP) calculated from the mean of their sampling sites (33.3° S and 31.1° E) is still used when discussing the tectonic history of the Kaapvaal craton. The exact position of the Kaapvaal craton relative to that of the Pilbara craton is especially important when considering the configurational history of the supercontinent Vaalbara.

Two years after their paleomagnetic analysis of the Modipe Gabbro, McElhinny and Evans (1968) estimated the strength of the Earth's ancient geomagnetic field, or paleointensity, at the time the unit was emplaced. The Earth's magnetic field is generated by convection of the liquid iron outer core and recordings of the strength of the field through time have been highly sought after to address questions about the stand-off distance of the solar wind (Tarduno et al., 2010) and the timing of the nucleation of solid inner core (Smirnov et al., 2003). This study has been updated by the recent work of Muxworthy et al. (2013) to indicate that magnetic field strength at the time of emplacement was approximately 80% of the modern field. However, the usefulness of this paleointensity estimate has been blunted due to ambiguity surrounding the Modipe Gabbro's precise paleolatitude and age.

Until now, the age of the Modipe Gabbro has been roughly constrained by Rb–Sr and K–Ar ages determined by McElhinny (1966). Rb–Sr total rock measurements yielded an age of 2630 ± 470 Ma. K–Ar measurements of pyroxene separates from three sites yielded ages ranging from ∼2600 to ∼3000 Ma with uncertainties of ±100 Ma. The average of these pyroxene K–Ar dates is 2758 ± 126 Ma when recalculated with the calibration of Renne et al. (2011) (Supplemental Data File). K–Ar measurements on plagioclase separates yielded anomalously young ages of 1983–1934 Ma, which were interpreted to possibly reflect thermal resetting by the nearby Bushveld Complex. The average K–Ar plagioclase age of McElhinny (1966), recalculated per Renne et al. (2011), is 1961 ± 52 Ma, which overlaps with integrated ⁴⁰Ar/³⁹Ar cooling ages for plagioclase from a Bushveld gabbro reported by Cassata et al. (2009). Thus, while portions of the paleomagnetic analysis of the Modipe Gabbro have long been known to be of high quality, it has been difficult to make use of these analyses owing to the poor geochronologic constraints on the unit.

Here we report the results of a new paleomagnetic and geochronologic study of the Modipe Gabbro. We make use of modern paleomagnetic protocols, including least-squares fitting and great circle analyses, to examine and refine the earlier paleomagnetic work of Evans and McElhinny (1966). Rock magnetic techniques are used to better characterize the extent of mineralogic alteration experienced by the magnetic oxides within the gabbro since their formation in the late Archean. The precise emplacement age of Modipe Gabbro is determined using U–Pb TIMS geochronological methods using baddeleyite as a geochronometer. These improved paleomagnetic and geochronologic data allow us to better constrain the precise position of the Kaapvaal craton at the time of the Modipe Gabbro's emplacement and to use the data from the Modipe Gabbro in reconstructions and evaluations of the supercontinent Vaalbara.