Proterozoic crustal evolution of the Awasib mountain terrain, southern Namibia, with special reference to the volcanic Haiber Flats formation

Abstract

Abstract provided by author:

The oldest sediments in the late-stage crust are represented by immature and poorly sorted clastic sediments of the Urusib Formation. Clasts (granitic and volcanic) are locally derived and sedimentary features suggest the intercalation of braided streamflow fans with lacustrine deposits. This typical rift succession reaches a maximum thickness of 2400 m

The predominantly volcanic Haiber Flats Formation (HFF) overlies the Urusib Formation disconformably and has a minimum thickness of 3000 m. This bimodal succession occurs at three localities, or centres, all of which are dominated by rhyolites. Basaltic andesites are volumetrically important at an early stage, but andesites, dacites and rhyodacites are poorly represented. While porphyritic textures are characteristic of all volcanic types, pyroclastic textures are most common in the rhyolites. Stratified tuffites and volcaniclastic and clastic rocks typically make up less than 10 percent of the HFF. Barby Formation basaltic andesites and tuffites are lithologically and compositionally similar to their counterparts in the HFF, but only occur in the eastern part of the AMT and are not in contact with the latter. Eruptive rocks of the HFF therefore constitute the most westerly occurrence of Sinclair volcanism and appear to be situated close to a palaeomargin

Despite the presence of metamorphic biotite, compositions of relict plagioclase, pyroxene and titanomagnetite phenocrysts in HFF basaltic andesites are similar to their counterparts in Orogenic basaltic andesites. Whole-rock compositions likewise show a strong similarity to calc-alkaline or high-K (not shoshonitic) volcanic suites from active continental margins such as western North America and Chile. These distinctive compositional features include high abundances of K, Rb, LREE, Th and Zr (and sometimes Nb and Y), and low abundances of the high field strength elements (except Nb and Y in the rhyolites). Rhyolites reveal typical A-type characteristics which are compatible with a significant crustal contribution in their petrogenesis

HFF basaltic andesite yields an age of 1086 ± 44 Ma and Ro of 0. 7030 ± 2 indicating derivation from a mantle source region which is slightly depleted relative to "Bulk Earth". However, a relatively high 238U/204Pb ratio (u.) of 10. 2 suggests a crustal (slab?) component undetected by the Rb-Sr system. An errorchron age of 1038 ± 74 Ma for rhyolite porphyry is within error of that deduced for the basaltic andesite, while a high Ro of 0. 718 ± 15 is consistent with the inferred crustal origin. Petrogenetic modelling suggests that basaltic andesite (including high-Mg types) has evolved from a mantle-derived basaltic parent by fractional crystallisation and has subsequently undergone assimilation-fractional crystallisation and mixing to form andesite. Rhyolites represent near-minimum partial melts of a basic to intermediate source in the lower to middle crust which have undergone fractional crystallisation, volatile transfer involving late-stage magmatic fluids, and possibly thermogravitational diffusion. Rhyodacite magmas have a similar origin to the rhyolites, but possibly also underwent mixing with basic magmas

The post-HFF calc-alkaline Haisib Intrusive Suite (HIS) and slightly peralkaline Awasib Granite show close spatial and compositional associations with the volcanics, despite relative enrichment of the former in K, Rb, LREE, Th, Zr, Nb, Ni and Crat intermediate silica levels. The HIS-Awasib "suite" compositional range reflects the increasing influence of both subcontinental within-plate mantle lithosphere and continental crust at an active continental margin. The A-type Awasib Granite yields an errorchron age of 957 ± 50 Ma and Ro of 0. 717 ± 8, both features being compatible with field relationships and an inferred crustal source. The Saffier Intrusive Suite (formerly intrusive rocks of the "Barby Formation" and "Spes Bona Syenite") and Bushman Hill Quartz Diorite are broadly contemporaneous with the HIS, while the Chowachasib Granite Suite represents the youngest pre-dyke plutonism. The change in composition from calc-alkaline to alkali-calcic of the late-stage granitoids is compatible with an overall increase in arc maturity

Crustal evolution of the AMT took place in two major tectonomagmatic events. Formation of the early-stage crust started around 1460 Ma ago as a primitive oceanic arc or continental back-arc and ended as a relatively mature arc or active continental margin about 1270 Ma ago. Continental collision (coinciding with D2, and possibly D3, in the Central Zone of the NMC) initiated the second event by causing the formation of a new subduction zone. Subduction, oblique to the new continental margin, resulted in both compressional ("ice-floe") and tensional ("pull-apart") tectonics close to 1200 Ma ago. While mafic and felsic magmatism were related to the alternation of compressional and tensional regimes, overall development formed part of three major pulses of sedimentation, magmatism and tectonic activity in the Sinclair Sequence. During back-arc development prior to 957 Ma old granite plutonism in the AMT, synchronous transcurrent movements resulted in the slicing-up and removal of part of the arc and accretionary complex. Extensional tectonics enabled the preservation of thick volcano-sedimentary successions and the intrusion of bimodal dyke swarms, provisionally dated at 844 ± 35 Ma (Ro 0. 7064 ± 2). relationships and an inferred crustal source. The Saffier Intrusive Suite (formerly intrusive rocks of the "Barby Formation" and "Spes Bona Syenite") and Bushman Hill Quartz Diorite are broadly contemporaneous with the HIS, while the Chowachasib Granite Suite represents the youngest pre-dyke plutonism. The change in composition from calc-alkaline to alkali-calcic of the late-stage granitoids is compatible with an overall increase in arc maturity. Crustal evolution of the AMT took place in two major tectonomagmatic events. Formation of the early-stage crust started around 1460 Ma ago as a primitive oceanic arc or continental back-arc and ended as a relatively mature arc or active continental margin about 1270 Ma ago. Continental collision (coinciding with D2, and possibly D3, in the Central Zone of the NMC) initiated the second event by causing the formation of a new subduction zone. Subduction, oblique to the new continental margin, resulted in both compressional ("ice-floe") and tensional ("pull-apart") tectonics close to 1200 Ma ago. While mafic and felsic magmatism were related to the alternation of compressional and tensional regimes, overall development formed part of three major pulses of sedimentation, magmatism and tectonic activity in the Sinclair Sequence. During back-arc development prior to 957 Ma old granite plutonism in the AMT, synchronous transcurrent movements resulted in the slicing-up and removal of part of the arc and accretionary complex. Extensional tectonics enabled the preservation of thick volcano-sedimentary successions and the intrusion of bimodal dyke swarms, provisionally dated at 844 ± 35 Ma (Ro 0. 7064 ± 2)

Crustal evolution of the AMT took place in two major tectonomagmatic events. Formation of the early-stage crust started around 1460 Ma ago as a primitive oceanic arc or continental back-arc and ended as a relatively mature arc or active continental margin about 1270 Ma ago. Continental collision (coinciding with D2, and possibly D3, in the Central Zone of the NMC) initiated the second event by causing the formation of a new subduction zone. Subduction, oblique to the new continental margin, resulted in both compressional ("ice-floe") and tensional ("pull-apart") tectonics close to 1200 Ma ago. While mafic and felsic magmatism were related to the alternation of compressional and tensional regimes, overall development formed part of three major pulses of sedimentation, magmatism and tectonic activity in the Sinclair Sequence. During back-arc development prior to 957 Ma old granite plutonism in the AMT, synchronous transcurrent movements resulted in the slicing-up and removal of part of the arc and accretionary complex. Extensional tectonics enabled the preservation of thick volcano-sedimentary successions and the intrusion of bimodal dyke swarms, provisionally dated at 844 ± 35 Ma (Ro 0. 7064 ± 2). Relationships and an inferred crustal source. The Saffier Intrusive Suite (formerly intrusive rocks of the "Barby Formation" and "Spes Bona Syenite") and Bushman Hill Quartz Diorite are broadly contemporaneous with the HIS, while the Chowachasib Granite Suite represents the youngest pre-dyke plutonism. The change in composition from calc-alkaline to alkali-calcic of the late-stage granitoids is compatible with an overall increase in arc maturity