The Coastal Terrane of the Kaoko Belt, Namibia: Outboard arc-terrane and tectonic significance
The Coastal Terrane of the Kaoko Belt, Namibia: Outboard arc-terrane and tectonic significance
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DOI：
10.1016/j.precamres.2007.01.008
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发表时间：
2007-05
影响因子：
3.8
通讯作者：
B. Goscombe;D. Gray
中科院分区：
文献类型：
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作者：
B. Goscombe;D. Gray
滨海地体，即三棕榈糜棱岩带外侧考古带最西端，具有明显的长石砂岩沉积序列，无基底，除太古代外具有明显的ɛ沉积特征和中元古代-新元古代物源特征，I型花岗岩类具有原始的弧状地球化学特征。它包含了∼650-645 Ma的更古老的变质事件的证据，在考古带的任何其他部分都不存在。该变质作用(M1)为高T/低P麻粒岩至上角闪岩相，并包括与I型花岗岩岩浆作用有关的混合岩化作用。650-640 Ma的弧形生长发生在埃达马斯洋的外侧，而斯瓦科普群相的浊积岩沉积则继续沿被动边缘向内侧沉积。叠加的M1和M2组构和变质组合限制了对接发生在650-580 Ma之间。沿三掌糜棱岩带缺乏蛇绿岩碎屑和中-高磷变质作用的证据表明，它不是缝合，更有可能是弧后弧后扳动-剪切系统的一部分，该系统发育在E-倾角俯冲系统的内侧，在该系统中，埃达马斯海被俯冲到削弱的刚果克拉通的前缘之下。650 Ma和580 Ma之间的斜碰撞，由于这个外侧的沿海地体与弧形亲缘对接，导致：(I)地壳规模倾斜地盖过刚果克拉通被动边缘的弧形地体；(Ii)地壳规模的左旋剪切分割成两个主要的韧性剪切带；(Iii)挤压的剪切带边界的核中的高T麻粒岩相变质作用；以及(Iv)向外或向克拉通边缘的基底岩心褶皱推覆体。
The Coastal Terrane, or westernmost part of the Kaoko Belt outboard of the Three Palms Mylonite Zone, has distinct feldspathic arenite sedimentary sequences, no basement, distinct ɛNdsediment signatures excluding the Archaean and suggesting Mesoproterozoic–Neoproterozoic provenance sources, and primitive arc-like geochemical signatures for I-type granitoids. It contains evidence for an older metamorphic event at ∼650–645Ma not present in any other part of the Kaoko Belt. This metamorphism (M1) was of high-T/low-P granulite to upper-amphibolite facies and includes migmatisation associated with I-type granitic magmatism of arc affinity. Arc growth at 650–640Ma took place outboard within the Adamastor Ocean while Swakop Group facies turbidite sedimentation continued inboard along the passive margin. Overprinting M1 and M2 fabrics and metamorphic assemblages constrain the docking to have occurred between 650 and 580Ma. Lack of ophiolite fragments and evidence of intermediate- to high-P metamorphism along the Three Palms Mylonite Zone suggest that it is not a suture and more likely to be part of an arc–backarc wrench–shear system that developed inboard of an E-dipping subduction system where the Adamastor Ocean was subducted beneath the leading edge of the attenuated Congo Craton. Oblique collision between 650 and 580Ma, due to docking of this outboard Coastal Terrane with arc affinities, caused: (i) crustal scale oblique-overriding of the arc terrane over the passive margin of the Congo Craton; (ii) crustal scale sinistral shear partitioned into two major ductile shear zones; (iii) high-T granulite facies metamorphism in an extruded, shear zone-bounded core; and (iv) outwards- or cratonwards-verging basement-cored fold nappes.