B.M. Eglington

Precambrian evolution of the Sirwa Window, Anti-Atlas Orogen, Morocco

Abstract We present the results of a field, geochemical and geochronological study of a ∼5000 km2 area of the Sirwa Window of the Anti-Atlas Orogen of Morocco. The region includes the northern edge of the Palaeoproterozoic (Eburnean) West African Craton (Zenaga Complex) and the southern margin of the Neoproterozoic (Pan-African) Anti-Atlas Orogen. The Zenaga Complex comprises medium grade supracrustal schists and intrusive granitoid orthogneisses, three of which gave within-error U–Pb SHRIMP zircon dates of ∼2035 Ma. The Anti-Atlas Orogen contains a vast thickness of volcano-sedimentary rocks known collectively as the Anti-Atlas Supergroup. The oldest of these comprises three, probably coeval, sequences collectively known as the Bleida Group. The Bleida Group includes tectonic inliers of schists and orthogneisses which gave a SHRIMP date of 743±14 Ma; medium-grade ophiolitic rocks in the central part of the area (Khzama and Nqob fragments) and a low-grade clastic-chemical volcano-sedimentary sequence (Taghdout Subgroup) along the northern margin of the Zenaga Complex. These rocks are interpreted as representing island-arc, fore-arc basin ocean-floor, and rifted continental margin sequences, respectively. The rocks developed north of the West African Craton during Neoproterozoic subduction of oceanic crust and the development of an arc/fore-arc complex. The Bleida Group is overlain by the thick flysch-like volcano-sedimentary rocks of the Sarhro Group which were deposited before 615 Ma, according to the SHRIMP dates obtained from the oldest granitic bodies intruding them. The presence of glaciogenic diamictite units suggests a possible depositional age of ∼700 Ma. It is thought that the Sarhro Group was deposited in the fore-arc basin which developed between the island arc and the cratonic continental margin to the south. A reversal of plate movement vectors during Sarhro Group times led to a change from turbidite to coarse clastic deltaic deposition. This culminated in closure of the fore-arc basin and collision of the island arc with the Craton margin, ophiolite emplacement and deformation of the Sarhro Group and older rocks during the Pan-African Orogeny, which is probably dated at ∼660 Ma, the SHRIMP age given by metamorphic overgrowths on zircons from the arc rocks. The Pan-African collision event was associated with widespread late- to post-orogenic magmatism, including granitoids intruded at ∼614 Ma (Mzil Granite, Ida-ou-Illoun batholith) and a huge gabbro-diorite-granodiorite-granite I-type granitoid batholith at ∼580 Ma (Askaoun batholith). The granitoids are post-dated by the extensive post-orogenic volcano-sedimentary molasse sequences of the Ouarzazate Group and coeval polyphase granite plutons between 575 and 560 Ma. The Ouarzazate Group comprises immature, coarse clastic sedimentary rocks (conglomerates, arkoses, reworked volcanic rocks) acid to intermediate volcaniclastic rocks (lapilli and crystal tuffs, volcanic breccias, ignimbrites, etc.) and lavas (minor basalt, andesite and voluminous rhyolite). The volcanic component of this extensive succession was extruded from at least five separate interfingering volcanic centres, each with characteristic stratigraphies (designated subgroups), typically in fault-bound settings. The volcanic centres include calderas containing coeval high-level granites and quartz porphry bodies, along with rhyolitic plugs, domes and dykes. Post-orogenic magmatism terminated with the emplacement of a number of potassic leucogranite bodies, one of which has been dated at ∼560 Ma. This plutonic phase is recorded in the Zenaga Complex by Rb–Sr mica ages of ∼580–525 Ma. The Ouarzazate Group is conformably to disconformably overlain by a typical foreland basin succession (Tata Group). A sporadically-developed basal conglomerate unit is overlain by two marine carbonate-clastic/shale cycles. Stable isotope studies have indicated that the base of the Cambrian Era (∼544 Ma) lies near the top of the lowermost dolomitic unit. The evolution of the Sirwa Window may serve as a model for the entire Anti-Atlas Orogen, recording a cycle of cratonic rifting and ocean basin formation (∼800 Ma), subduction and island-arc formation (∼750 Ma), flysch deposition and volcanism in a fore-arc basin. Plate movement reversal from extension to convergence in Sarhro Group times led to eventual arc-continent collision and ophiolite obduction (∼660 Ma), late- to post-orogenic plutonism (∼615–580 Ma), post-orogenic extension, collapse, exhumation and molasse sedimentation, volcanism and plutonism (580–560 Ma) and marine foreland basin deposition (550 Ma to Palaeozoic).

The Kaapvaal Craton and adjacent orogens, southern Africa: a geochronological database and overview of the geological development of the craton

Geochronological comparisons of large datasets are facilitated by the use of structured databases. Data for the Precambrian of South Africa, Swaziland, Lesotho and Botswana have been compiled in a DateView database and linked to chronostratigraphy and GIS databases to produce a series of ‘time-slice’ maps illustrating the development of the Kaapvaal Craton. Linking geochronological data to GIS coverages provides a valuable visual perspective on the development of the southern African lithosphere. The oldest preserved rock formation dates occur south of the Barberton Greenstone Belt in South Africa and Swaziland. Subsequent scattered development of new crust occurred in the south eastern, eastern and northern Kaapvaal Craton before being ‘stitched’ together by extensive granitoid intrusions at ~3.25 Ga and ~3.1 Ga. Coeval development of new crust occurred in what would later become the central zone of the Limpopo Belt. The patterns of igneous activity from ~3.1 Ga to ~2.8 Ga, outboard of major cratonic lineaments (Colesberg lineament in the west and Thabazimbi-Murchison lineament in the north) may indicate that these lineaments represent suture zones along which the younger domains were accreted during formation of the Kaapvaal Craton. By ~3 Ga the lithosphere was sufficiently rigid to support development of the Dominion, Witwatersrand and Pongola sedimentary basins, followed by extensive volcanism during the Ventersdorp and concomitant granitoid activity throughout the Craton. Subsequent geological activity, not necessarily evident in the available geochronological record, was concentrated on craton with the development of the widespread Transvaal Supergroup followed by essentially coeval extrusion of the Rooiberg felsites and intrusion of the Bushveld Complex at ~2.06 Ga. Deposition of sediments comprising the Waterberg and Soutpansberg Groups followed. Igneous activity along the south-western edge of the Kaapvaal Craton terminated at ~1.93 Ga with formation of the Hartley basalts, Olifantshoek Supergroup. Post-Olifantshoek Supergroup and pre-Volop Group tectonism has been reported from the western margin of the Kaapvaal Craton. There is currently no geochronological evidence for major igneous or metamorphic activity post-dating formation of the Olifantshoek Supergroup until the early stages of the Namaqua-Natal Belt subsequent to ~1.4 Ga i.e . there is no geochronological evidence for a major late-Palaeoproterozoic ‘Kheisian orogeny’. Off-craton, new crust formed in the Richtersveld Sub-province at ~1.8 Ga but was presumably only accreted to the Kaapvaal Craton some 700 million years later during the Namaqua-Natal orogenesis.

New isotope data from a neoproterozoic porphyritic garnitoid-charnockite suite from Natal, South Africa

Abstract The tectonic setting and age of emplacement of the Oribi Gorge Suite, voluminous rapakivi-textured granites and charnockites (“megacrystic granites”) from the ∼ 1 Ga (Kibaran) Natal Metamorphic Province has been controversial. Isotopic dates of ∼ 1.0 Ga have been obtained fromfive plutons (various isotopic systems), but two of these (Fafa and Oribi Gorge plutons) have also given ∼ 0.89 Ga RbSr whole-rock isochrons. The age problem has been exacerbated by apparently equivocal field and structural relationships, in which both syn- and post-tectonic settings have been argued. New UPb isotopic analyses for zircon fractions from a sample of the Fafa pluton give a concordia data of 1029±1010Ma. Single zircon evaporation analyses of selected grains from the Oribi Gorge pluton (Bomela locality) give a data of 1092 ± 2Ma RbSr model dates of biotite separates from three plutons range between 970 and 880 Ma, generally similar to the RbSr whole-rock dates. It is significant that no Pan-African (∼ 500 Ma) dates were obtained. The zircon data strongly support a > 1.0 Ga age for the emplacement of the Oribi Gorge Suite, an age similar to both the main regional tectogenesis and syntectonic granites of the Margate Suite. The Oribi Gorge Suite can thus be regarded as late (syn)tectonic and not partly post-tectonic as previously envisaged. Consequently, the need to invoke two periods of identical A-type magmatism is removed. The younger RbSr whole-rock and biotite dates may represent the time when the various plutons or the entire terrane cooled through the relevant blocking temperatures, though this was not a simple, homogeneous process.

Isotope and geochemical constraints on Proterozoic crustal evolution in south-eastern Africa

Proterozoic exposures south of the imbricate thrust terrane in the eastern sector of the Namaqua-Natal Belt of southern Africa are dominated by granitoid plutons intrusive into older supracrustal gneisses. Many of these granites have geochemical characteristics similar to “A-type” and rapakivi suites, as previously described in Australia and the northern hemisphere. Ages obtained from the Natal units range from ∼1.1 to ∼0.88 Ga. Nd TDM values cluster about ∼1.4 Ga. The oldest granitoids have “volcanic arc” chemical signatures but, subsequent to ∼1.1 Ga, “A-type” granites are common. This suggests a change in tectonic environment or melting process at about this time. Initial 87Sr/86Sr ratios and ϵNd values for the older suites approach values normally associated with a depleted mantle reservoir whilst μ values are similar to those found in “orogen” environments. These data clearly indicate that the Proterozoic crust of south-eastern Africa was not derived by extensive reactivation of Archaean or Early Proterozoic protoliths. Low 87Sr/86Sr ratios in metasedimentary carbonates and ∼1.8 Ga Nd TDM ages for metapelites from the Belt support this model and preclude the formation of the Belt in an intracratonic setting. All the data thus support models involving substantial crustal accretion and differentiation, primarily subsequent to ∼1.5 Ga.

Geochronological and isotopic constraints on the Mesoproterozoic Namaqua–Natal Belt: evidence from deep borehole intersections in South Africa

Mesoproterozoic lithologies of the Namaqua–Natal Belt are exposed in the west and east of South Africa but the central section of the belt is obscured by younger sedimentary cover. This belt is one of several Grenvillian-aged belts world-wide and is of importance in reconstructions of the Meso- to Neo-Proterozoic supercontinent of Rodinia. Zircons from granite and biotite gneiss in two deep boreholes which penetrated the Phanerozoic cover provide SHRIMP U–Pb dates of 1053 +29 /−25 Ma and 1134 +15 /−15 Ma. Sm–Nd model dates for whole-rock samples from these and two other boreholes range from ∼2400 to 1200 Ma, unlike model dates from the eastern (Natal) sector of the Namaqua–Natal Belt which are all younger than ∼1400 Ma. Model dates are similar to those noted in the Bushmanland and Richtersveld Sub-provinces of the western (Namaqua) sector of the Belt. The results are interpreted to indicate that this Proterozoic crust may be an easterly extension of the Bushmanland Sub-province or some equivalent terrane and that there must be a terrane boundary between the eastern-most borehole studied and the surface outcrops further east.

Dating the cessation of Kibaran magmatism in Natal, South Africa

Abstract Published U-Pb and Pb-Pb zircon dates from pre- to late-tectonic granitoids of the Kibaran (Grenvillian) Natal Metamorphic Province fall within the range ≈1200 to ≈1040 Ma. A major problem regarding the duration and timing of magmatism in the Natal belt has been the age of the youngest intrusive events. This is important within a Gondwana context, especially with respect to the nature and distribution of the Pan-African event in this area. It is known that the youngest magmatic rocks in Natal are certain undeformed, cross-cutting microgranite dykes (Mbizana Microgranite) from the Margate Terrane. These dykes have previously given a Rb-Sr whole-rock date of ≈950 Ma, showing them to be broadly late Kibaran in age. This paper presents new U-Pb data from four zircon fractions from the Mbizana Microgranite, which give a date of emplacement of 1026 ± 3 Ma. Consequently, this date is considered to define the latest magmatic event in the Natal Metamorphic Province and points to the timing of the cessation of Kibaran magmatism in this area. The data supports recent suggestions that there was no Pan-African magmatic activity in Natal. The ≈950 Ma Rb-Sr dates previously obtained for these rocks are re-interpreted as representing the time at which the dykes cooled through the Rb-Sr biotite and K-feldspar blocking temperatures.

Metallogeny and its link to orogenic style during the Nuna supercontinent cycle

Abstract The link between observed episodicity in ore deposit formation and preservation and the supercontinent cycle is well established, but this general framework has not, however, been able to explain a lack of deposits associated with some accretionary orogens during specific periods of Earth history. Here we show that there are intriguing correlations between styles of orogenesis and specific mineral deposit types, in the context of the Nuna supercontinent cycle. Using animated global reconstructions of Nunas assembly and initial breakup, and integrating extensive databases of mineral deposits, stratigraphy, geochronology and palaeomagnetism we are able to assess spatial patterns of deposit formation and preservation. We find that lode gold, volcanic-hosted-massive-sulphide and nickel–copper deposits peak during closure of Nunas interior ocean but decline during subsequent peripheral orogenesis, suggesting that accretionary style is also important. Deposits such as intrusion-related gold, carbonate-hosted lead-zinc and unconformity uranium deposits are associated with the post-assembly, peripheral orogenic phase. These observations imply that the use of plate reconstructions to assess orogenic style, although challenging for the Precambrian, can be a powerful tool for mineral exploration targeting. Supplementary material: Supplementary material including (1) tables (S1–S3) of Euler poles and palaeopoles used, summary of Nuna orogens; (2) a figure (S1) of modelled plate velocities; (3) mp4 files (S1 & S2) of the model with age data; ore deposits and VGPs; and (4) a zip file (S1) of the Gplates model is available at http://www.geolsoc.org.uk/SUP18822.

A review of the evolution of the Mozambique Belt and implications for the amalgamation and dispersal of Rodinia and Gondwana

Abstract Geochronological, isotopic, lithological and structural data from the Mozambique Belt, and its extensions in Antarctica, Sri Lanka, India and Madagascar, are summarized and reviewed within a Gondwana framework. Much of the southern Mozambique Belt is dominated by Rodinian-Grenvillian-age juvenile magmatism and crustal genesis, with a strong metamorphic overprint during the Pan-African. Magmatism at c. 800 Ma, possibly related to Rodinian fragmentation, is restricted to the Zambesi Belt, a few examples in northern Mozambique, southern Malawi, Tanzania, Madagascar and the Rayner Complex in Antarctica. Significantly, no crust of this age is recognized in western Dronning Maud Land. Amalgamation of Gondwana initially involved the closure of the Mozambique Ocean between the Tanzanian-Congo Cratons and the Madagascar-India-Enderby Cratons forming the East African Orogeny between c. 580 and c. 800 Ma. This was followed by the c. 500–580 Ma transpressional sinistral collision of the combined Kalahari-East Antarctic Cratons and associated Grenville-age belts in the south, with the northern block comprising the combined Tanzanian-Congo-Madagascar-India-Enderby Block.

Lead isotope variations within the Bushveld complex, Southern Africa: a reconnaissance study

Abstract Variations in initial 87 Sr 86 Sr through the mafic cumulates of the Bushveld complex have long been used to trace the nature and relative influence of different parental liquids in the development of the magma chamber. The authors report the results of the first systematic investigation of the lead isotopic variation in the complex and provide information on both silicate and sulphide phases through the cumulate succession. Marginal and cumulate lithologies indicate that the initial lead isotopic composition (represented by model source 238 U 204 Pb ratios) varies systematically in a manner analogous to that detected with strontium isotopic ratios. Low μ2 ratios are evident in the Lower Zone and increase up the cumulate pile to the Main Zone, which apparently had homogeneous initial lead isotopic ratios. Unlike 87 Sr 86 Sr , however, considerable μ2 variation is detected in the Upper Zone. Cumulate lithologies also exhibit a greater range of μ2 values than those noted in the Marginal suite representatives, comparable to the situation for strontium isotopes. Most of the model μ2 results are higher than values expected for mantle-derived magmas, suggesting the incorporation of material with a crustal pre-history. Analyses of a detailed sampling of the Merensky and Bastard cyclic units at the Atok mine, eastern Bushveld are also presented and those reveal extreme values of μ2 while the variation in μ2 do not mirror the 87 Sr 86 Sr variations reported for the same samples by Lee and Butcher (1990). Elevated μ2 in some cumulate samples are impossibly high for a magmatic origin and presumably reflect the introduction of extra-magmatic components during sub-solidus hydrothermal alteration.

Metamorphic history and U-Pb Zircon (SHRIMP) geochronology of the Glenmore Granite: Implications for the tectonic evolution of the Natal Metamorphic Province

A new SHRIMP U-Pb zircon age of 1091 ± 9 Ma has been acquired for the gneissic, S-type Glenmore Granite from the Margate Terrane of the Natal Metamorphic Province. The Glenmore Granite contains two foliations and mineral textures indicate it underwent two metamorphic episodes separated by a period of retrogression. The presence of a folded S1 foliation in the Glenmore Granite indicates that it was either pre- or syntectonic relative to D1, thus providing a maximum age constraint of 1091 ± 9 Ma on the D1 event in the Margate Terrane. This is ~60 Ma later than the completion of the main tectonism (D1–3) documented from the Tugela Terrane, suggesting diachronous arc accretion. Syntectonic granitoids with ages of ~1090 Ma have also been documented from the Mzumbe terrane, as well as from Western Dronning Maud Land (east Antarctica), and the Cape Meredith Complex in the Falklands, which on reconstruction of Gondwanaland, lie adjacent to the Natal Metamorphic Province.