Most of the bedrock in the Wallace quadrangle belongs to the Belt Supergroup, a thick (about 18,000 m) sequence of generally fine-grained clastic and carbonate rocks of Middle Proterozoic age. Regional metamorphism prior to Cambrian time prograded the Belt rocks to greenschist facies, and some metal-bearing veins were emplaced in fractures. The Belt rocks were intruded in Late Proterozoic time by basic dikes and sills.
New U‐Pb zircon ages and geochemical data for felsic intrusive and extrusive rocks from the Richtersveld Igneous Complex (RIC) and related rocks in the westernmost part of the 1.03–1.06‐Ga Namaqua‐Natal metamorphic belt, South Africa, indicate that this complex is not related to post‐Namaqua orogenic collapse but is the product of mantle‐derived alkaline magmatism in an extensional stress field that led to the breakup of a Neoproterozoic supercontinent. The oldest age obtained for the crystallization of granitic to syenitic melts is $$833\pm 2$$ Ma. Continued thinning of the crust is reflected by the intrusion of bostonite dikes and a related extrusive phase dated at $$801\pm 8$$ Ma. Magmatism in this igneous province, which stretches for about 200 km along a southwest‐northeast linear trend, lasted at least until $$771\pm 6$$ Ma, which is now the best constraint on the beginning of rifting. Subsequent rift sediment deposition was accompanied by the emplacement of regionally extensive mafic dikes and bimodal, predominantly felsic volcanism along growth faults in the evolving rift basin at $$741\pm 6$$ Ma. Consistent lower intercept ages on concordia diagrams (mean: $$282\pm 28$$ Ma) suggest uplift and possible exposure to groundwater flow in response to late Paleozoic tectonism in the Cape Fold Belt farther south. The genesis of the RIC is in accordance with crustal thinning above a mantle plume that contributed to a mantle‐derived magma addition to the crust over a prolonged period of some 100 m.yr. Comparison with existing geochronological and petrological/geochemical data from elsewhere invites speculation as to the existence of a superplume stretching from southeastern Africa to South China in the heart of a supercontinent Palaeopangea.
It is commonly accepted that beneath the continental crust lies a keel of lithospheric mantle, which extends 50–200 kilometres downward to a transition zone into the asthenosphere. The chemical and physical properties of this reservoir are best known through studies of the basalts and xenoliths that provide samples of the subcrustal mantle. Although sharing many characteristics with oceanic island basalts, some continental basalts become increasingly distinct isotopically as crustal age increases, strongly supporting a permanent association between crust and mantle. Consequently, the distinctive trace element and isotope composition of the lithospheric mantle is able to give important clues to its origin and evolution. The mantle under newly‐created crust is typified by a radiogenic isotope variability that emphasizes the materials from which the continental lithosphere is assembled. Old lithospheric mantle, on the other hand, exhibits more evolved isotopic patterns that attest to the existence of long‐lived, chemically complex systems. A comparison of the Pb, Sr and Nd isotopes in alkalic to sub‐alkalic basalt derived from Phanerozoic (Patagonia) and Middle Archaean to Early Proterozoic (eastern China) subcrustal mantle is useful for identifying 'end‐member' components of the lithosphere. One component, having an isotopic composition close to PREMA, either continues to evolve virtually unchanged after incorporation into the lithosphere or is, itself, a relatively new addition even to old lithosphere. Another component, beginning with the isotopic composition of BSE, undergoes significant reduction in U/Pb and Sm/Nd (but not Rb/Sr) upon incorporation into the lithosphere and, with time, shows an increasingly retarded evolution of 206Pb/204Pb and negative εNd‐values approaching the isotopic composition of EMI. Five models are discussed that relate the isotopic composition of the continental lithospheric mantle to that of other parts of the terrestrial system, which may be involved in its origin and evolution. The potential locations of the contributing components and the mechanisms and timing of their assembly into lithosphere are considered. Current knowledge, however, does not allow us to distinguish unequivocally among the various scenarios for the creation and evolution of this reservoir. Key words: basaltscontinental lithosphereeastern ChinaPatagoniaSrNd and Pb isotopes
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