The Geochronology of the frontal zones of the Namaqua-Natal mobile belt

Abstract

Abstract provided by author:

The Namaqua-Natal Mobile Belt, a polymetamorphic gneissic terrane, extending across southern Africa from the Atlantic to the Indian Ocean, is masked in the centre by younger sedimentary rocks. Three areas chosen for intensive study were two marginal zones of the mobile belt against the Archaean Kaapvaal Craton (Upington and Natal Geotraverses) and one marginal zone against the mid-Proterozoic Richtersveld Province (Namaqualand Geotraverse). In addition, two units from the Kaapvaal Craton were investigated, the Marydale Group basic volcanic rocks and banded iron formation and the Nselini granitoid gneiss

Forty-four rock units were analysed by one or more of the following isotopic methods: Rb-Sr, Pb-Pb, Th-Pb (total-rock), U-Pb (zircon) and Rb-Sr (mineral) age measurements. The polyphase deformation and meta-morphism experienced by the rocks in the mobile belt have disturbed the isotope systematics to various degrees. The resulting large uncertainties in the radiometric ages precluded the establishment of a detailed chronology of the separate deformation events in the mobile belt. However, the concept of the Namaqua-Natal Mobile Belt, hitherto based largely on the approximately 1000 m. y. mineral ages on gneissic rocks, is corroborated in this study by the total-rock ages of approximately 1300 to 1050 m. y. obtained for volcanic and granitoid rocks in both the Namaqua and Natal sectors of the mobile belt

The Orange River and Namaqua-Natal orogenies are bracketed in the intervals of approximately 2000 to 1700 m. y. and 1300 to 1050 m. y. respectively. The early Orange River event, not recognised in the Natal sector, is represented in the west by the emplacement and meta-morphism of the volcanic rocks of the Haib Subgroup (approximately 200 m. y. ago) and the emplacement of granitoid rocks of the Vioolsdrif and Gladkop Suites in the Richtersveld Province and adjacent portions of the mobile belt (approximately 1900 to 1730 m. y. ago). The later Namaqua-Natal event is characterised mainly by extensive granitoid emplacement and locally high grade metamorphism to granulite facies at approximately 1200 m. y. ago. Volcanic activity during the Namaqua-Natal event appears to be mainly confined to the marginal regions adjacent to the craton. This volcanic activity, dated at approximately 1300 m. y., preceded the high grade metamorphism of the Namaqua-Natal Orogeny. The extensive nature of the approximately 2000 m. y. -old crustal protolith is confirmed by the recognition of approximately 1800 m. y. -old rocks in the mobile belt. Furthermore, isotopic and chemical evidence supports the contention that reworking of the approximately 2000 m. y-old crust took place during the Namaqua-Natal Orogeny

The deformation fabrics associated with the Orange River and Namaqua-Natal orogenies are bracketed by the time of emplacement of syn-and post-tectonic granitic rocks. On this basis the regional fabric observed in two adjacent regions in the mobile belt was found to have not been developed synchronously and this fact emphasises the dubious usefulness of lithological correlation on the basis of fabric

Profound age differences are confirmed at the boundaries of the late-Proterozoic Namaqua-Natal Mobile Belt with the Archaean Kaapvaal Craton. The Nselini granitoid gneiss is dated at approximately 3200 m. y. (Rb-Sr total-rock) and a banded iron formation unit within the Marydale Group is dated at approximately 2900 m. y. (Pb-Pb total-rock)

The banded iron formation within the Marydale Group is metamorphosed under conditions ranging from greenschist to amphibolite facies. The preservation in the greenschist grade banded iron formation of an approximately 2900 m. y. age (interpreted to reflect the time of emplacement) is in marked contrast with an age of approximately 2200 m. y. obtained for the amphibolite facies banded iron formation. The younger age is interpreted to indicate the facility of Pb-isotopic homogenisation in banded iron formation during metamorphism

The polymetamorphism associated with both the Orange River and Namaqua-Natal orogenies was manifested by crustal reworking and the disruption of isotopic systems on total-rock and mineral scales. However, on the basis of ages and initial isotopic ratios, some constraints on crustal evolution are suggested. The Sr- and Pb-isotopic data for felsic rocks are compatible with interpretations of crustal reworking. These rocks have high initial 87 Sr/ 86 Sr and 238 U/204 Pb ratios, diagnostic of crustal derivation. However, these data are interpreted to preclude the derivation of these rocks by reworking of Archaean crust

For certain mafic volcanic rocks, presumably of mantle derivation, the Sr-isotopic results indicate mantle source regions slightly enriched in Rb relative to Sr in comparison to bulk earth evolution. The Pb-isotopic-data for these same rocks indicate high initial 238 U/ 204 Pb ratios. These relationships are considered compatible with an interpretation of a "mantle" source into which some recycling of crustal material (not exclusive of Archaean crust) has taken place. However, it is equivocal whether this apparent enrichment reflects a long-term heterogeneity of a mantle source or a short-term -enriched magma source distinct from the mantle, that may have been initiated in a subduction environment

In terms of geodynamic processes, crustal recycling can occur at active continental plate margins. On the basis of the isotopic data as well as other geological evidence, a magmatic arc environment is envisaged for the rocks generated during the Orange River Orogeny. A similar accreting continental margin is envisaged to have existed at the mobile belt-craton interface during the Namaqua-Natal Orogeny. The isotopic data are interpreted to not indicate reworking or isotopic resetting of pre-existing crust older than approximately 2000 m. y. in the Namaqua-Natal Mobile Belt. By implication, the apparent absence of Archaean crust within the mobile belt and the evidence suggestive of extensive crustal reworking and isotopic resetting are interpreted to reflect a continent-continent convergence during the Namaqua-Natal Orogeny about 1200 m. y. ago, of the Kaapvaal Craton with an exotic continental fragment containing the Richtersveld Province and the rocks generated and deformed during the Orange River Orogeny