Precambrian blueschist terranes of the world
Precambrian blueschist terranes of the world
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DOI：
10.1016/0040-1951(90)90010-6
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发表时间：
1990-09
期刊：
影响因子：
2.9
通讯作者：
J. Liou;S. Maruyama;X. Wang;S. Graham
中科院分区：
文献类型：
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作者：
J. Liou;S. Maruyama;X. Wang;S. Graham
蓝片岩在后古生代造山带最为常见；前寒武纪较高的地温梯度和较薄的岩石圈被广泛认为不利于蓝片岩的形成。然而，欧亚大陆已经报道了几条前寒武纪蓝片岩带。根据已建立的地质和地质年代学约束，这些带被归类为可疑的、可能的和确定的前寒武纪蓝片岩。可疑的包括阿拉斯加、安格尔西(英国)、斯堪的纳维亚喀里多尼底山脉和云南(中国)。可能的前寒武纪蓝片岩带有德里(印度)、中国中部、黑龙江(中国)、乌拉尔和阿拉泰-萨彦天山(苏联)。明确的前寒武纪蓝片岩地体是西非和阿克苏地区的蓝片岩地体(中国)。对这些蓝片岩带进行了描述，强调了年龄限制。以新近研究的阿克苏蓝片岩地体为例。阿克苏高P/T一致的蓝片岩-绿片岩地体不整合地被厚厚的、未变质的沉积序列所覆盖，其年龄范围从晚震旦世(约600-800 Ma)到始寒武纪至古生代。基底部震旦系砾岩中含有下伏片岩的近圆形至圆形碎屑，清楚地显示了前震旦纪的蓝片岩相变质作用。上述蓝片岩带的特征包括：(1)蓝片岩地体连贯，原生成分-岩性层保存完好；混杂岩带不常见；(2)原岩包括泥灰岩、砂岩、双峰式火山岩、石灰岩和少量枕状玄武岩和浅海沉积；超基性岩罕见；(3)原岩经历了多次变形和过渡的蓝片岩-绿片岩相变质作用。典型的前寒武纪蓝片岩组合包括交错角闪石+绿帘石+绿泥石+钠长石+石英±阳起岩)；稀有的劳钠闪石、硬玉辉石和文石分布于少数带；(4)变质相系属于宫城式高压中间型(1961)。前寒武纪的欧亚大陆前寒武纪蓝片岩和其他前中生代欧亚陆内蓝片岩形成于两个大陆之间的洋盆最终闭合之前，而不是在此期间。它们通常不标志末端缝合的位置，而是早期通过俯冲、底侵和包括海洋和大陆来源的弹性岩在内的增生楔体的叠置作用而形成的大陆增长的结果。前寒武纪蓝片岩的罕见保存是由于热流和地温梯度高时绿片岩的叠加，而不是反映了产量的缺乏。它们在中国和非洲的存在表明，至少在700 Ma前就存在蓝片岩相变质的必要条件。
Blueschists are most common in post-Paleozoic orogenic belts; higher geothermal gradients and a thinner lithosphere in the Precambrian are widely thought to have disfavored blueschist formation. However, several Precambrian blueschist belts have been reported from Eurasia. On the basis of established geologic and geochronologic constraints, these belts are grouped as suspect, possible and definite Precambrian blueschists. Suspect ones include those in Alaska, in Anglesey (U.K.), in the Scandinavian Caledonides, and in Yunnan (China). Possible Precambrian blueschist belts are those in Delhi (India), in central China, in Heilongjiang (China) and in the Urals and Altai-Sayan-Tien Shan (U.S.S.R.). Definite Precambrian blueschist terranes are those in western Africa and in Aksu (China). These blueschist belts are described with emphasis on the age constraints. The recently investigated Aksu blueschist terrane is presented as an example. This high-P/Tcoherent blueschist-greenschist terrane at Aksu is unconformably overlain by a thick, unmetamorphosed sedimentary sequence ranging in age from Late Sinian (ca. 600–800 Ma) through Eocambrian to Paleozoic. The basal Sinian conglomerates contain subrounded to rounded clasts of the underlying schist, clearly demonstrating the pre-Sinian age of blueschist facies metamorphism.Characteristic features of the above-mentioned blueschist belts include:1.(1) the blueschist terranes are coherent and primary compositional-lithological layers are well preserved; mèlange belts are not common;2.(2) protoliths include pelite, sandstone, bimodal volcanics, limestone and small quantities of pillow basalt and pelagic sediment; ultramafics are rare;3.(3) the protoliths were subjected to multiple deformation and transitional blueschist-greenschist facies metamorphism. Typical Precambrian blueschist contains the assemblage crossitic amphibole + epidote + chlorite + albite + quartz ± actinolite); rare lawsonite, jadeitic pyroxene and aragonite have been described in few belts;4.(4) the metamorphic facies series belongs to the high-pressure intermediate type of Miyashiro (1961).The Precambrian blueschist and other pre-Mesozoic intracontinental blueschist of Eurasia formed before, rather than during, final closure of intervening ocean basins between pairs of continents. They do not usually mark the location of terminal sutures, but are the result of earlier accretion and continental growth by subduction, underplating and imbrication of accretionary wedges including both oceanic and continent-derived elastics. The rare preservation of Precambrian blueschist is due to greenschist overprinting when the heat flow and geothermal gradient was high, rather than reflecting lack of production. Their existence in China and Africa demonstrates that requisite conditions for blueschist facies metamorphism existed at least 700 Ma ago.