Simon C. Milner

AGE OF ETENDEKA FLOOD VOLCANISM AND ASSOCIATED INTRUSIONS IN SOUTHWESTERN AFRICA

Detailed 40Ar/39Ar laser step-heating analyses of mineral separates from five volcanic units in Namibia and Angola and five intrusions in Namibia yield important geochronological data for the Etendeka igneous province. Ten plateau dates on plagioclase, hornblende, and biotite between 131.7 ± 0.7 and 132.3 ± 0.7 Ma were obtained, and a late syenite from the Messum intrusive complex yielded a slightly younger hornblende plateau date of 129.3 ± 0.7 Ma. Magnetostratigraphy of the volcanic rocks in three sections up to 700 m thick, laterally spanning more than 100 km, suggests that the flows record only two geomagnetic polarity reversals. Precise temporal coincidence with the Parana flood volcanic province in South America indicates that Etendeka volcanism does not represent a significantly younger phase of magmatism that migrated from northwest to southeast over 10 m.y., as has recently been proposed. The duration of intrusive activity was at least 2–3 m.y. longer than recorded by volcanism, and its total duration awaits further constraints.

Isotope characteristics of the Okenyenya igneous complex, northwestern Namibia: constraints on the composition of the early Tristan plume and the origin of the EM 1 mantle component

Sr, Nd and Pb isotope data are presented for a variety of intrusive rocks from the Mesozoic age Okenyenya igneous complex, which is temporally and spatially associated with the Etendeka Group volcanic rocks in northwestern Namibia. On the basis of bulk rock geochemistry the Okenyenya intrusions can be subdivided into tholeiitic and alkaline suites. The tholeiitic suite has a wide range in isotope composition; for example, initial ϵSr (ϵSr(i)) from 1.2 to 150 with decrease in initial ϵNd (ϵNd(i)) from 4.8 to −3.9. In contrast, the undersaturated rock types show a more restricted range and, in terms of ϵSr(i) (− 11.0–15.1) and ϵNd(i) (0.3–5.0), plot within the mantle array and close to Bulk Earth values. The range in isotope composition shown by the Okenyenya intrusions is similar to that shown by the Etendeka Group volcanic rocks. The tholeiitic suite is comparable in isotope composition to the Etendeka low TiZr (LTZ) basalts and defines a trend towards continental crust, whereas the alkaline suite is similar to the Etendeka Tafelkop basalts. The Etendeka high TiZr (HTZ) basalts do not have an isotopic equivalent amongst the Okenyenya intrusions, but are indistinguishable from basalts in DSDP Hole 525A on the Walvis Ridge; both are strongly displaced towards enriched mantle (EM 1) sources. The large variation in ϵSr(i) shown by the tholeiitic suite and Etendeka LTZ basalts appears to reflect extensive crustal contamination of the magmas, whereas the HTZ basalts, which trend towards EM 1, owe their isotope composition to melting of ancient continental lithospheric mantle. The alkaline gabbros and the Tafelkop basalts have compositions similar to the present-day composition of the Tristan plume and are interpreted as direct melts of the upwelling Tristan mantle plume at the time of continental break-up. An analogous relationship is observed between the Marion plume, Madagascan Upper Cretaceous basalts, and MORB erupted at the intersection between the Southwest Indian Ridge (SWIR) and the Madagascar Ridge. The strong EM 1 signature of basalts found in DSDP Hole 525A on the Walvis Ridge, and at 39–41°E on the SWIR is attributed to melting at shallow depth of ancient continental lithospheric mantle thermally eroded and rafted into the surrounding asthenosphere at the time of continental breakup.

Age of Mesozoic igneous rocks in northwesternNamibia, and their relationship to continental breakup

Mesozoic igneous rocks of northwestern Namibia comprise continental flood basalts and quartz latites of the Paraná–Etendeka province, and a suite of central sub-volcanic intrusions, the Damaraland complexes. Conventional K–Ar ages published for these rocks range from 377–88 Ma, with the majority of samples yielding ages of 150–110 Ma. This large spread of ages is interpreted as being due to disturbance of the K–Ar system by low-temperature alteration, which is ubiquitous among the volcanic rocks. Rb–Sr and 40Ar/39Ar dating methods, which are less susceptible to such disturbance, yield a narrower range of ages (132–124 Ma, with one outlier at 149 Ma). New Rb–Sr internal isochron ages of 129.8 ± 3.8 Ma, 126.8 ± 1.3 Ma and 126.1 ± 7.3 Ma have been determined for an Etendeka quartz latite, and nepheline syenite intrusions in the Messum and Okorusu complexes, respectively. In addition, 40Ar/39Ar release spectrum ages of 137.0 ± 0.7 Ma, 135.0 ± 0.7 Ma and 126.9 ± 0.6 Ma have been determined for samples from the Paresis (comendite), Cape Cross (nepheline syenite) and Okorusu (nepheline syenite) complexes, respectively. A 13 Ma period between 137 and 124 Ma is considered to be the best estimate of the timing of Mesozoic igneous activity in northwestern Namibia. The continental flood basalt volcanism was probably limited to the early part of this period and is inferred to have been relatively short-lived (1–2 Ma). Magmatic activity among the Damaraland complexes was contemporaneous with the onset of flood basalt volcanism, but continued for approximately 15 Ma and terminated after the onset of sea floor spreading in this area (130–125 Ma). Both the flood basalt volcanism and the magmatic activity of the Damaraland complexes are thought to have occurred in response to rifting across the upwelling Tristan mantle plume. The linear distribution of intrusions in Damaraland is interpreted as being due to magmatism focused along a structural discontinuity between the Pan-African Damara terrain to the south and Proterozoic cratonic basement to the north.

Magma flow inferred from anisotropy of magnetic susceptibility in the coastal Paraná-Etendeka igneous province: Evidence for rifting before flood volcanism

The Parana-Etendeka igneous province is one of the largest flood volcanic provinces in the world; peak magmatic activity at 132 Ma is believed to have occurred about 5 m.y. before the birth of south Atlantic sea floor and development of rift basins along the Brazilian coastal margin. Anisotropy of magnetic susceptibility (AMS) measurements on 283 samples (28 flows and 3 sills) from the Etendeka igneous province of Namibia and 180 samples (21 flows) from the Parana province in Brazil reveal remarkably consistent fabric orientations with maximum susceptibility (K1) axes subhorizontal and parallel to the rifted margin. The AMS results are most likely due to shape anisotropy reflecting magma flow directions, suggesting that lava flows and intrusive conduits near the eventual rifted margin were controlled by structures having topographic expression in existence at the time of peak flood volcanism. These results imply that rifting preceded flood volcanism, at least in the portion of the magmatic province within 100 km of the nascent Mid-Atlantic Ridge.

Oxygen isotope geochemistry of the silicic volcanic rocks of the Etendeka-Paraná province: Source constraints

Oxygen isotope ratios of pyroxene phenocrysts in the silicic volcanic rocks from the Cretaceous Etendeka-Parana flood basalt province (Namibia, South America) are believed to reflect the {delta}{sup 18}O values of the original magmas. The authors recognize a high {delta}{sup 18}O value type ({delta}{sup 18}O pyroxene {approximately} +10{per thousand}) found in the south of both regions, and a low {delta}{sup 18}O value type ({delta}{sup 18}O pyroxene {approximately} +6.5{per thousand}) found in the north. Other differences between thee two rhyolite types include higher concentrations of incompatible elements and lower initial {sup 87}Sr/{sup 86}Sr ratios in the low {delta}{sup 18}O value type. The authors suggest that the regional distribution of rhyolite types reflects differences in source composition, which can best be explained if the sources are lower crustal, Late Proterozoic mobile belt material (high {delta}{sup 18}O) and Archean lower crust (low {delta}{sup 18}O).

Rb-Sr age determinations of rocks from the Okenyenya igneous complex, northwestern Namibia

The Okenyenya igneous complex is one of a suite of intrusions which define a prominent northeast-trending linear feature in Damaraland, northwestern Namibia. Precise Rb–Sr internal isochron ages range from 128.6 ± 1 to 123.4 ± 1.4 Ma for the major phases of intrusion identified within the complex. The tholeiitic gabbros forming the outer rings of the complex, and the later alkali gabbros which form the central hills, cannot be distinguished in terms of Rb–Sr ages, although field relations clearly indicate the younger age of the latter. The intrusionsof nepheline-syenite and essexite comprising the mountain of Okenyenya Bergon the northern edge of the complex give ages of 123.4 ± 1.4 and 126.3 ± 1 Ma, respectively, and form the final major phase of intrusion. The ages obtained for early and late intrusive phases define a minimum magmatic ‘life-span’ of approximately 5 Ma for the complex. The determined age of the Okenyenya igneous complex (129–123 Ma), when taken together with the few reliable published ages for other Damaraland complexes (130–134 Ma), suggests that these sub-volcanic complexes were emplaced contemporaneously with the widespread Etendeka volcanics (˜ 130 Ma), and relate to magmatism associated with the breakup of southern Africa and South America with the opening of the South Atlantic Ocean. The linear distributionof intrusions in Damaraland is interpreted to be due to magmatism resultingfrom the upwelling Tristan plume being focused along a structural discontinuity between the Pan-African, Damaran terrain to the south, and Proterozoiccratonic basement to the north.

Oxygen isotope geochemistry of the Mesozoic volcanics of the Etendeka Formation, Namibia

The Etendeka Formation volcanics consist of a bimodal association of basalts and quartz latites. Forty three new whole rock oxygen isotope analyses are reported for all the major magma types. All the rocks except a minor suite of dolerites have higher δ18O values than normal mantle. The basic rocks (average of 29=8.8‰) have significantly different δ18O to the acid rocks (average of 10=14.4‰) These data are apparently consistent with previously published petrogenetic models, which propose that the basalts were affected by crustal contamination and that the quartz latites are crustally derived. However, mineral oxygen data show that there is significant oxygen isotopic disequilibrium between phenocryst and whole rock, the latter being significantly higher in most cases. One of the basic magma types (the Tafelberg basalts) shows mutual positive correlations between δ18O, SiO2 and ɛSr. If these correlations are due to crustal contamination, then as much as 45% contamination is required by material having a δ18O value of 15‰ which is the maximum observed value in the Damaran basement rocks. In the absence of pyroxene phenocryst δ18O data for the high ɛSr Tafelberg basalts (they are aphyric), it is not possible to confirm that contamination has taken place. An alternative explanation is that the correlation between ɛSr and SiO2 resulted from assimilation coupled with fractional crystallization (AFC) (before emplacement). Post-eruption alteration resulted in a correlation between SiO2 δ18O because the material with the most Si-O bonds was able to concentrate 18O more effectively. The limited mineral data for the quartz latites suggests that there is some source heterogeneity. A pyroxene δ18O value of 10% for a southern Etendeka quartz latite is consistent with a crustal source.

OXYGEN ISOTOPE EVIDENCE FOR EXTENSIVE CRUSTAL CONTAMINATION IN THE OKENYENYA IGNEOUS COMPLEX, NAMIBIA

Abstract Oxygen isotope data are presented for silicate minerals separated from a variety of rock types from the 130 Ma Okenyenya igneous complex of northwestern Namibia. These rock types include a tholeiitic suite ranging from olivine gabbro to quartz monzodiorite and syenite, and an alkaline suite including gabbro, syenite, nepheline syenite, and essexite. In general, the difference in δ 18 O values between coexisting feldspar, pyroxene, and biotite are consistent with equilibrium at high temperatures and preclude significant interaction with magmatic or external fluids. The δ 18 O values of pyroxene can be used to model magmatic processes and show a systematic increase from 7.1–9.0‰ along a transect through a tholeiitic olivine gabbro-quartz monzodioritic body, corresponding to a systematic decrease in Mg#. Pyroxene δ 18 O values also show a strong correlation with initial strontium and neodymium isotope ratios, with the alkaline suite having lower δ 18 O values, lower initial strontium-isotope ratios, and higher initial neodymium isotope ratios than the tholeiitic suite. Simple oxygen mass balance calculations suggest that the extremes of isotope composition resulted from at least 60% contamination by material of similar isotope composition to the nearby southern Etendeka quartz latites. Assimilation/crystal fractionation models of oxygen, strontium, and neodymium isotope data suggest that the rate of assimilation was high and may have approached simple mixing. The contaminated tholeiitic olivine gabbro-quartz monzodioritic body was the earliest to crystallize in the complex and this was followed by relatively uncontaminated silica-undersaturated magmas.