February 2019 LIP of the Month

Hekpoort Formation, South Africa, ca. 2.223 Ma

Wladyslaw Altermann,

Exxaro Chair, Department of Geology, University of Pretoria, RSA.

altermannw@gmail.com

This is based on the following:

Humbert et al., 2018, Petrology, physical volcanology and geochemistry of a Paleoproterozoic large igneous province: The Hekpoort Formation in the southern Transvaal sub-basin (Kaapvaal craton) Precambrian Research, 315, 232–256;

And

Humbert et al., 2019, Neoarchaean to Early Palaeoproterozoic Within-Plate Volcanism of the Kaapvaal Craton: Comparing the Ventersdorp Supergroup and the Ongeluk and Hekpoort Formations (Transvaal Supergroup), In Kröner, A. and Hofmann, A. (eds): The Archaean Geology of the Kaapvaal Craton, Southern Africa, Publisher: Springer Nature Switzerland AG, March 2019. 277-302. DOI: 10.1007/978-3-319-78652-0_11,

The Archaean-Palaeoproterozoic Kaapvaal craton (South Africa) records several successive voluminous intra-plate magmatic events. Here we concentrate on the early Palaeoproterozoic Hekpoort Formation (Fm.) which represents an episode of widespread continental flood volcanism and localized rift-related sedimentation at about 2223 Ma in the Transvaal sub-basin, South Africa. The Hekpoort volcanism predates the emplacement of the ∼2.055 Ga LIP of the Bushveld Igneous Complex (BIC), the world’s largest layered intrusion and an endless source of chromium and Platinum Group Elements (PGEs).

Based on its areal extent of ca. 100,000 km² and maximum thickness of up to 1100 m (Lenhardt et al., 2012), the Hekpoort Fm. can be classified as a Large Igneous Province (LIP). The Hekpoort LIP consists of a thick subaerial volcanic sequence in which volcanoclastic rocks occur mainly at the base. Localized hyaloclastites and variolitic rocks record the presence of ponded water in lacustrine and adjacent environments, while interbedded sedimentary rocks and paleoweathered flow tops suggest prolonged time-breaks in volcanic activity. The Hekpoort Fm. is the youngest of a trinity of Neoarchean to Paleoproterozoic subaerial large igneous provinces of the Kaapvaal craton, encompassing the Ventersdorp Supergroup (2.78-2.71 Ga) and the Ongeluk Fm. (2.426 Ga).

The sedimentary and tectonic setting of the Hekpoort and Ongeluk formations within the Transvaal basin and their geochemical character are so similar that both formations were long viewed as lateral equivalents of each other, but representing different subbasins. Only the dating of dykes in the Ongeluk Fm. (Gumsley et al., 2017) revealed that it is actually ca. 200 myr older than the Hekpoort Fm. Thus, the two formations do not correlate and there is a large age discrepancy between the Pretoria Group in the Transvaal subbasin (Hekpoort Fm.) and the Postmasburg Group in Griqualand West (comp. Eriksson et al., 2006). At the same time Humbert et al. (2017) showed that the paleomagnetic poles for the Ongeluk and Hekpoort formations differ from each other (the Ongeluk paleopole being characterized by Evans et al., 1997).

The age of the Hekpoort Fm. is not precisely constrained. Dates of 2236 ± 38 Ma (whole rock Pb–Pb isochron; Cornell et al., 1996) and 2224 ± 21 Ma (whole-rock Rb–Sr isochron; Burger and Coertze, 1973) result directly from the lavas. U-Pb SHRIMP dating of detrital zircons from palaeosols or interbedded sedimentary units in the formation yielded a maximum age of 2247 ± 10 Ma (Schröder et al., 2016). Sedimentary rocks directly overlying the Hekpoort Fm. contain detrital zircons of 2233 ± 15 Ma (Dorland, 2004 – U-Pb SHRIMP ages).

The Hekpoort Fm. is composed mainly of thick successive lava flows interpreted as originating from a single magma pulse (i.e., similar to ‘tabular-classic flows’, Jerram, 2002). The lava flows have thicknesses of between 8 and 60 m, and are often characterized by amygdaloidal flow tops with thicknesses between 0.5 and 1 m. Abundant amygdales are coalesced, with anastomosing geometries and are partially or completely filled with secondary minerals: mostly quartz, but also clinochlore, zeolite and calcite. The flow tops can be highly altered. These apparent weathering horizons are more common near the base of the formation, which may suggest a lower initial eruption rate. However, all outcrops have a reddish hydroxide coating due to weathering. Locally developed, several meters-thick flow top breccias were described and volcanoclastic materials occur in the formation (e.g., Button, 1973; Sharpe et al., 1983; Oberholzer, 1995; Oberholzer and Eriksson, 2000; Coetzee, 2001).

Manifold additional physical volcanology phenomena can be observed in these ancient lavas (Humbert et al., 2018b), such as:

1. Shear induced outgassing or ‘second order’ lava flows
2. Vesicle cylinders of amygdale concentrations in the form of oblique columns or pipes
3. Pahoehoe lava or ropy lava flow tops
4. Hyaloclastites
5. Variolitic lava flows
6. Stratified and massive lapilli tuffs and lapilli tuff breccias
7. Pyroclastic breccias
8. Lahars and related mass flow deposits

Like most Archean to Paleoproterozoic LIPs, the Hekpoort rocks underwent at least greenschist facies metamorphism but also episodes of metasomatism and silicification. Preserved primary magmatic minerals are clinopyroxene (pigeonite, augite and diopside), and rarely plagioclase (labradorite).

Lavas of the Hekpoort Fm. are mostly basalts, with widespread silicification responsible for increased SiO₂ content. The variable whole rock Mg# (evolving from 69 to 50) and the changes in clinopyroxene composition attest to magmatic fractionation. The Hekpoort Fm. displays remarkably similar trace elements patterns to those of the 200 myr older Ongeluk Fm. in the west of the Kaapvaal Craton; such a compositional match is remarkable and unique among the Precambrian mafic magmatic units of the Kaapvaal Craton.

Similar to the mafic rocks of the Ventersdorp Supergroup (2.78-2.71 Ga) and the basalts of the Ongeluk Fm. (2.426 Ga), the Hekpoort basalts show an arc-like trace element signature, mainly represented by negative Nb-Ta anomalies (in normalized trace element patterns). The Hekpoort and Ongeluk also exhibit relatively high contents of Th and U and high Th/Nb ratios, which sharply contrast with Archean units. The Hekpoort magmas thus seem to represent a stage in a progressive differentiation series from a single parental magma with variable degrees of crustal assimilation, strongly inconsistent with their extrusion order on the craton, and which can be related to metasomatism of the subcontinental lithospheric mantle by past subduction(s). This suggests that a subduction process marked the Archean-Proterozoic boundary on the Kaapvaal craton.

The Hekpoort Fm. was described as having been largely deposited in a subaerial setting (e.g., Button, 1973; Sharpe et al., 1983; Engelbrecht, 1986; Eriksson and Twist, 1986; Oberholzer, 1995; Reczko et al., 1995) and is seen as a major continental flood basalt event on the Kaapvaal craton (e.g., Reczko et al., 1995; Cornell et al., 1996). Notable is the absence of wide-spread pillow lavas, and the presence of rare hyaloclastites and of pahoehoe flow tops with uniform amygdales layers. In a small area east of Pretoria a bedding plane with preserved syn-depositional raindrop imprints has been identified (Altermann et al., 2018), clearly evidencing subaerial deposition.

Localized ponds and lakes (Button, 1973) can explain the hyaloclastites and possible pillows at the top of the Hekpoort Fm., and the variolitic lava flows (Humbert et al., 2018a) and volcanoclastic rocks, which mostly result from phreatomagmatism. Nevertheless, lavas are preponderant in the whole Hekpoort Fm. (Button, 1973; Sharpe et al., 1983; Engelbrecht, 1986; Schreiber, 1991). The rare presence of large amount of volcanoclastic rocks in some localities (e.g., the Hekpoort River Valley, suggest that the Hekpoort magma chamber was rich in volatiles and/or that the lava encountered fluids at shallow surface (e.g., Fisher and Schmincke, 1984), thus promoting a more explosive style of volcanism and allowing the formation of varying proportions of lava flows and pyroclastic rocks.

The Hekpoort river area is in a way anomalous because it contains mainly coarse volcanoclastic deposits and mass flow deposits of lahars and coarse tuffs (Oberholzer and Eriksson, 2000; Humbert et al., 2018b). In places, the NE striking strata are oriented partly sub-vertical, with evidence for folding and faulting, but without developed schistosity. Areas with subhorizontal bedding appear to be separated by normal faults from steeply dipping blocks of regional extent. Humbert et al. (2018b), envisage therefore the development of a paleo-graben valley during the subsidence of the Kaapvaal craton, which experienced stretching perpendicular to graben shoulders (Eriksson et al., 2006). Mass flows were triggered by earthquakes, accompanied by volcanic eruptions which led to the accumulation of thick and laterally extensive fans of coarse to fine, proximal to distal volcanoclastic deposits. This extensional event was likely an underlying cause for Hekpoort magmatism. Similar processes were discussed for e.g., the Ventersdorp LIP lavas by Van der Westhuizen et al. (2006), Altermann and Lenhardt (2012) and Humbert et al. (2019).