Autochthonous and allochthonous strata of the El Callao greenstone belt: Implications for the nature of the Paleoproterozoic Trans-Amazonian orogeny and the origin of gold-bearing shear zones in the El Callao mining district, Guayana shield, Venezuela
Autochthonous and allochthonous strata of the El Callao greenstone belt: Implications for the nature of the Paleoproterozoic Trans-Amazonian orogeny and the origin of gold-bearing shear zones in the El Callao mining district, Guayana shield, Venezuela
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DOI：
10.1016/j.precamres.2005.09.009
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发表时间：
2005-12
影响因子：
3.8
通讯作者：
R. Hildebrand
中科院分区：
文献类型：
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作者：
R. Hildebrand
瓜亚那盾构古元古代跨亚马逊河造山运动的构造样式被前人认为是非现实构造，在该构造中，Supamo杂岩的花岗质岩浆块的上升使2.1-2.0Ga的火山岩和沉积岩表壳带变形，形成绿岩带。随后与麻粒岩级为主的伊马塔卡杂岩的碰撞使绿岩带变形。然而，绿岩带的底部到处都有一个主要的糜糜岩剪切带，它将绿岩带的岩石置于一个区域石英岩单元之上，而石英岩单元不整合地坐落在伊马塔卡和苏帕莫杂岩上。对这些关系最简单的解释是，绿岩带与Imataca和Supamo复合体是异域的。由于石英岩超越了Imataca和Supamo杂岩之间的接触，两个杂岩的并置必须早于石英岩的沉积和跨亚马逊河造山运动的变形。以石英岩和逆冲断层为古水平标志，揭示了El Callao地区的绿岩带，该地区的含金剪切带明显早于逆冲构造，呈现出反向运动的特征。当恢复到褶皱前的方位时，它们是正断层。这与拉斑岩-钙碱性火山包裹体相结合，成矿作用和火山-沉积层序均与伸展弧前-弧背景相适应。同种异体和原生异体的东-东北走向褶皱，加上石英岩的北增厚，表明同种异体是以前位于北方的未知克拉通的一部分。第二个区域褶皱组，向北走向，重新折叠早期的构造。这可能与圣卡洛斯地体与合并的伊马塔卡-苏帕莫-帕斯托拉地体的碰撞有关。上覆1.98Ga Cuchivero弧地体的不整合为这两次碰撞事件提供了最小年龄。
The tectonic style of the Paleoproterozoic Trans-Amazonian orogeny in the Guayana shield was considered by previous workers to represent non-actualistic tectonics in which rising masses of granitoid magma of the Supamo complex deformed 2.1–2.0Ga supracrustal belts of volcanic and sedimentary rocks to form greenstone belts. Subsequent collision with the dominantly granulite-grade Imataca complex deformed the greenstone belts. However, the base of the greenstone belts is everywhere marked by a major mylonitic shear zone that placed rocks of the greenstone belts atop a regional unit of quartzite that sits unconformably upon both the Imataca and Supamo complexes. The simplest interpretation of these relationships is that the greenstone belts are allochthonous with respect to the Imataca and Supamo complexes. As the quartzite oversteps the contact between the Imataca and Supamo complexes, juxtaposition of the two complexes must predate deposition of the quartzite and deformation of the Trans-Amazonian orogeny. The quartzite and the thrust fault are used as paleohorizontal markers to unfold the greenstone belt in the El Callao area where auriferous shear zones, which clearly predate the thrusting, show reverse sense of movement. When restored to their pre-folding orientation they are normal faults. This, and the tholeiitic to calc-alkaline volcanic packages, are compatible with an extensional forearc to arc setting for both the mineralization and the volcano-sedimentary sequences. East–northeasterly trending folds of both the allochthon and the autochthon, coupled with the northerly thickening of the quartzite, suggest that the allochthon was part of an unknown craton that formerly lay to the north. A second regional fold set, which trends northerly, refolds the earlier structures. It is possibly related to collision of the San Carlos terrane with the amalgamated Imataca–Supamo–Pastora terranes. The unconformably overlying 1.98Ga Cuchivero arc terrane provides a minimum age for the two collisional events.