中国台湾东部利吉混杂岩中蛇纹岩角砾碎屑岩及伴生碳酸盐岩脉的地球化学特征及成因

Serpentinite mud volcanoes at seafloor with associated fluid seepage activities and chemosynthetic biocommunity, occurring in forearc regions of subduction zones, are increasingly getting focused owing to their close relationships with abiogenic methane, the origin of life on Earth, and deep ocean environment. The typical modern active examples at seafloor are Mariana forearc serpentinite mud volcanoes. Their ancient counterparts have been found in many places around the world. However, there are no such reports in China. Numerous serpentinite breccias with sedimentatary characteristics occurred in the forearc Lichi Mélange of eastern Taiwan, China. Carbonates usually occurred as veins in these serpentinites have extremely negative values of δ13C (as low as -54.5‰ VPDB). But, their formation mechanisms are unknown at present. This proposal intends to study the geology, petrography and geochemistry of serpentinite breccias and associated carbonate veins occurred in the Lichi Mélange, to constrain the characteristics of the serpentinite breccias and the parent fluids and the carbon sources of the carbonate veins. Authigenic deposits of fluid seepage and chemosynthetic biocommunity fossils would be investigated, as well. Finally, this proposal will answer the key scientific questions that whether the serpentinite breccias are originated from serpentinite mud volcanoes, and whether the carbonate veins are related to the anaerobic microbial oxidation of abiogenic methane formed during serpentinization.

海底蛇纹岩泥火山及其流体活动和自养生物群发育于俯冲带弧前海底，因其与无机成因甲烷、地球生命起源和深海环境等重大科学问题有关，而受到越来越多的关注。马里亚纳海沟弧前蛇纹岩泥火山是现代海底的实例，古代的记录在全球多处被发现，但我国尚未见报导。前人在中国台湾东部利吉混杂岩中发现有大量具有沉积特征的蛇纹岩角砾碎屑岩，其中普遍发育有δ13C值极低的碳酸盐岩脉(δ13C低达-54.5‰ VPDB)，但其成因至今还没有定论。本申请项目拟对利吉混杂岩中蛇纹岩角砾碎屑岩及其伴生碳酸盐岩脉开展野外地质和室内岩石学与地球化学研究，确定蛇纹岩角砾碎屑岩的地质和地球化学特征、脉状碳酸盐岩形成的流体特征及碳源，寻找蛇纹岩泥火山海底流体渗漏形成的自生沉积和自养生物化石，回答蛇纹岩角砾碎屑岩是否由海底蛇纹岩泥火山作用形成、伴生碳酸盐岩脉是否与蛇纹岩化过程中无机成因甲烷的微生物氧化有关等关键科学问题。

随着全球气候变暖，生命元素碳在地球各系统中的循环越来越多地受到了关注，俯冲带是地表碳和深源碳循环的重要场所，蛇纹岩化是非生物成因甲烷形成的主要作用，发育于俯冲带的蛇纹岩与伴生碳酸盐岩和渗漏流体，对生命的起源及全球碳循环具有重要意义，是地球科学研究中的热点之一。在台湾东部海岸山脉新生代利吉混杂岩中，发育有蛇纹岩碎屑角砾岩，通过野外地质观察、岩石和矿物学及地球化学的研究，发现该类蛇纹岩具有沉积成因的碎屑结构特征，部分角砾碎屑磨圆度较高。蛇纹岩的Al2O3含量低（0.61-1.86%），CaO含量（0.08-0.71%）也偏低，指示可能来自于俯冲带的上盘地幔楔。蛇纹岩富集As、Sb、Pb元素，表明蛇纹岩化的上盘地幔楔有富集来自俯下盘冲板片脱水的中低温流体。因此，利吉蛇纹岩碎屑角砾岩很可能具有马里亚纳弧前蛇纹岩泥火山相似的成因。而且蛇纹岩碎屑角砾岩伴有碳酸盐岩脉，并且碳酸盐岩脉发育有泥微晶团粒、凝块、以及放射纤维状方解石和草莓状黄铁矿，总有机碳含量较高（0.11~0.25%），脉状碳酸盐岩的δ13CV-PDB为-17.8～2.9‰，表明碳很可能是包括无机成因甲烷与海水源碳的混合。方解石脉大多具有平坦型REE配分模式和Eu异常特征，显示Ce负异常与Ce无异常特征，表明了指示其形成于蛇纹岩泥火山的碱性的、氧化与还原复杂多变的流体环境。