南美地台中生代活化的火成岩记录(英文)

在冈瓦纳大陆裂解的同时及其后,出现了岩浆作用过程,它影响到远离安第斯山链的南美地台的大部分。这个岩浆作用过程构成了几种类型的记录:(1)拉斑玄武质侵入岩和喷出岩,如沉积盆地中的熔岩流、岩床、岩墙和基底中的岩墙。最大活动强度出现于中侏罗世至前阿普第阶早白垩世。在大陆的北部,出现局部的二叠—三叠纪拉斑玄武质岩墙。(2)碱性和过碱性镁铁质至长英质岩浆活动和共生的碳酸盐岩,它们出现于巴西Serra do Mar、巴拉那盆地周边以及在亚马孙地盾周边和玻利维亚,呈孤立的小面积出现,时代上大部分属后阿尔必阶。由于此期岩浆作用发生于南美和非洲移开之时,所以一直延续至中新世。少量的侵入岩是与侏罗纪—始自垩纪火山活动同时的。(3)在前寒武纪末期结束造山作用的地区以及在时代为中元古代的巴西克拉通区,已找到了几个金伯利岩体,少量的同位素资料指出其年龄属中生代。 地质构造影响了中新生代岩浆活动。玄武质熔岩流和岩床在克拉通内盆地中下沉较大的内部更常见,厚度也较大。许多前寒武纪断裂受到辉绿岩墙的影响,这些岩墙可以构成岩墙群。其它岩墙明显与古老断裂无关,但是它们位于大陆分离过程中遭受张应力的地区。巴拉那盆地周边的碱性岩浆作用通常受穹窿和断开的单褶的控制。相似的机制——也发现

THE IGNEOUS RECORD OF THE MESOZOIC ACTIVATION OF SOUTH AMERICAN PLATFORM

The breaking of Gondwana was accompanied and succeeded by a magmatic process that affected most part of South American Platform far from the Andean Chain. This process comprised several types of records: (1) Tholeiitic intrusions and effusions such as lava flows, sills and dykes in the sedimentary basins, as well as dykes in the basement, with maximum intensity from Middle Jurassic to pre-Aptian Lower Cretaceous: it has been locally preceded by Permian-Triassic tholeiitic dykes in the northern part of the continent. (2)Alkaline and peralkaline magmatism, mafic to felsic and associated carbonatites, at the Brazilian Serra do Mar, around the Parana Basin border, as well as in isolated spots in the border of Amazon Shield, in Bolivia. Mostly post-Albian, because this magmatism took place when South America and Africa were moving apart, it lasted until Miocene.A few alkaline intrusions are syn chronous with the Jurassic-Eocretaceous volcanism. (3)Several kimberlite intrusions recognized in Brazilian cratonic areas of Middle Proterozoic age, as well as in areas submitted to oro genic processes until the end of the Precambrian. A few isotopic dates suggest a Mesozoic age for these rocks.Geological structures have influenced the Mesozoic-Cenozoie magmatism: basaltic lava flows and sills are more frequent and thicker in the inner more subsident interior of the intracratonic basins.Many Precambrian faults and fractures have been affected by diabase dykes, which may form swarms. Other dykes are apparently independent of older fractures: they belong, however, to areas submitted to tensional stresses during the continental partition. Thealkaline magmatism of Parana Basin borders is generally controlled by arches and faulted flex ures.A similar mechanism, also found in Bolivia, has led to associations between ancientfault zones and intrusions.The relationship between oceanic fracture zones and the paleogenic magmatism in the con tinental crust is clearly displayed at least in two sites, corresponding to projections of the Romanche and Fernando de Noronha fracture zones.