古亚洲洋俯冲引起的壳—幔作用与沉积碳酸盐岩地幔再循环

发布时间：2018-04-16 10:07

  本文选题：深部碳循环 + 碳酸岩质包体　； 参考：《中国地质大学》2016年博士论文

【摘要】：古亚洲洋的俯冲闭合形成了显生宙最大的增生造山带。在此过程中大量的地壳物质伴随着洋壳俯冲返回到地幔,引发大规模的壳-幔作用。由此引起的熔体-橄榄岩反应可能也是诱发华北克拉通破坏的重要因素。另外,俯冲板块上的沉积碳酸盐岩也可随着俯冲的进行返回地幔。作为地球表层碳返回地球深部的唯一方式,沉积碳酸盐岩的俯冲再循环在深部碳循环研究中起着不可替代的作用。本论文通过兴蒙造山带南缘达里湖地区玄武岩中的碳酸岩质包体及华北克拉通北缘汉诺坝地区玄武岩中的单斜辉石巨晶的系统研究,揭示了古亚洲洋俯冲引起的熔体-橄榄岩反应及沉积碳酸盐岩深部地幔再循环。古亚洲洋俯冲引起的沉积碳酸盐岩地幔再循环作用被兴蒙造山带南部达里湖玄武岩中的碳酸岩质包体所记录。达里湖碳酸岩质包体主要由碳酸钙组成,其稀土元素配分模式及微量元素类型均和沉积碳酸盐岩非常相似,都具有明显的Ce负异常和Sr正异常。而且其亦具有演化的Sr同位素组成(87Sr/86Sr=0.70893-0.71054),表明此碳酸岩质包体为沉积碳酸盐岩通过俯冲作用深部地幔再循环的产物。达里湖碳酸岩质包体内存在着原位金刚石,证明沉积碳酸盐岩曾俯冲到至少120 km的深度。沉积碳酸盐岩通过底劈作用脱离俯冲板块,进入热的地幔楔。其后通过脱气过程快速上升到浅部岩石圈地幔。达里湖碳酸岩质包体的Ni和Fe的含量远远超过沉积碳酸盐岩,暗示了碳酸岩质包体从俯冲板块脱离入地幔后可能受到了地幔物质的影响。达里湖碳酸岩质包体中上地幔矿物斑晶可能为碳酸岩质熔体侵入到上地幔辉石岩中而成。达里湖碳酸岩质包体中存在着一个以SiC为主的超还原矿物组合,包括SiC、TiC、自然金属(Si、Fe、Ni)和硅化物。SiC大小为20-50 gm,蓝色到无色透明,常发现于碳酸岩质包体的微空洞中。SiC有四种多型,以β-SiC (3C)和4H型为主,其后为15R型和6H型。达里湖SiC具有明显’3C亏损的碳同位素组成(813C=-13.2‰～-22.8‰,平均值为-17.7‰),而且具有明显的空间变化。数值模拟结果表明达里湖SiC的低δ13C值及碳同位素的空间变化可由石墨与硅酸盐矿物的脱气反应形成。达里湖SiC主要产出于微空洞中,其中充填的CO或CO2可有效的维持局部超还原环境,将SiC和外部氧化环境分开。寄主岩石的快速冷却在一定程度上也有利于SiC的保存。古亚洲洋的南向俯冲可能引发华北克拉通北缘广泛的熔体-橄榄岩反应。这一过程被华北克拉通北部汉诺坝玄武岩中的单斜辉石巨晶所记录。汉诺坝玄武岩中产出大量单斜辉石巨晶,根据其物理化学特征,可将其分为两类。类型一单斜辉石巨晶比类型二拥有更高的Cr、Nb含量和Mg#(82.9)以及更演化的Sr-Nd同位素组成(87Sr/86Sr=0.704520,143N/144Nd=0.512350).这些特点表明类型一单斜辉石可能由熔体-橄榄岩反应生成,而且熔体可能来自于俯冲古亚洲洋壳于金红石不稳定域的熔融。类型二单斜辉石巨晶的Mg#和主微量元素间具有很好的相关性,而且其Sr-Nd同位素组成落在了寄主玄武岩的最不演化端元。这些数据表明类型二单斜辉石巨晶由寄主岩浆于高压下结晶而成。类型一单斜辉石巨晶中含有大量平行的不透光条带及斜方辉石出溶体。不透光条带由微晶斜长石、橄榄石组成。其总体化学成分和石榴子石非常相似,而且和寄主单斜辉石具有相同的Sr同位素组成。表明不透光条带是类型一单斜辉石巨晶中石榴子石出溶体裂解的产物。石榴子石出溶的温压条件为2.75 GPa、1290℃。实验结果表明单斜辉石在增压或者降温的条件下可以出溶石榴子石。虽然在玄武岩喷发过程温度会降低,但是急剧的降压过程会抑制石榴子石的出溶。因此,类型一单斜辉石巨晶中的石榴子石出溶体可能形成于古亚洲洋俯冲造成的华北克拉通北缘岩石圈加厚过程中。
[Abstract]:The closure of the paleo Asian Ocean subduction formed the largest Phanerozoic accretionary orogenic belt. A large number of crustal material in the process with the subduction of oceanic crust returned to the mantle, causing massive crust mantle interaction. The important factors of peridotite melt reaction may be caused by the destruction of the North China Craton is induced. In addition, the deposition of carbonate rocks in subduction the plate can also be with the subduction of the mantle. The only way to return to the earth's surface as carbon return in deep earth, recirculation subduction of the carbonate rock plays an irreplaceable role in the carbon cycle of the deep. This paper through the system research and the North China Craton basalts from Hannuoba in monoclinic pyroxene megacrysts carbonate rock basalt pack up in the lake area of xing'anling-mongolian orogenic belt on the southern margin of the peridotite melt reaction reveals subduction of the paleo Asian Ocean caused by carbonate rocks and sedimentary The deep mantle recycling. Recycling effect caused by the subduction of the paleo Asian Ocean sedimentary carbonate mantle is xing'anling-mongolian orogenic belt of Southern Dali Lake basalt in carbonate enclaves recorded. Up in the lake carbonate rock xenoliths mainly composed of calcium carbonate, the REE patterns and trace elements and types the deposition of carbonate rocks is very similar, have obvious Ce negative anomaly and Sr positive anomaly. But it is also composed of Sr isotope evolution (87Sr/86Sr=0.70893-0.71054), showed that the product of the carbonate rock inclusions for carbonate rocks by the subduction of the deep mantle recycling. Up in the lake carbonate rock package exists in vivo in situ diamond, proof of sedimentary carbonate rocks have subducted to at least 120 km. The depth of sedimentary carbonate rocks from the subducting plate through the diapir action, into the mantle wedge. Then through the heat degassing process quickly rose to the shallow 宀╃煶鍦堝湴骞，

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